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Characterization of the interactions of hydrogen, nitrogen and oxygen with silicon and III-V semiconductor surfaces

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Title:
Characterization of the interactions of hydrogen, nitrogen and oxygen with silicon and III-V semiconductor surfaces
Creator:
Corallo, Cheryl Fox, 1959-
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[s.n.]
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ix, 95 leaves : ill. ; 28 cm.

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Subjects / Keywords:
Adsorption ( jstor )
Desorption ( jstor )
Deuterium ( jstor )
Energy ( jstor )
Hydrogen ( jstor )
Ions ( jstor )
Nitrogen ( jstor )
Oxides ( jstor )
Room temperature ( jstor )
Spectral energy distribution ( jstor )
Chemical Engineering thesis Ph. D
Dissertations, Academic -- Chemical Engineering -- UF
Electron-stimulated desorption ( lcsh )
Gases ( lcsh )
Silicon ( lcsh )
Thermal desorption ( lcsh )
Genre:
bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Bibliography: leaves 85-94.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Cheryl Fox Corallo.

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CHARACTERIZATION OF THE INTERACTIONS
OF HYDROGEN, NITROGEN AND OXYGEN WITH SILICON AND
III-V SEMICONDUCTOR SURFACES













By

CHERYL FOX CORALLO


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1987




CHARACTERIZATION OF THE INTERACTIONS
OF HYDROGEN, NITROGEN AND OXYGEN WITH SILICON AND
III-V SEMICONDUCTOR SURFACES
By
CHERYL FOX CORALLO
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1987


ACKNOWLEDGEMENTS
I would like to express my appreciation to Dr. Gar Hoflund for
his endless support and assistance. In his capacity as research
advisor he was outstanding. Beyond educational and financial support
he was instrumental in my growing appreciation of the "gourmet
experience." I would also like to thank Greg Corallo for the support
he provided and much, much more. Additionally, I would like to thank
my parents, Frank and Elaine Fox, for the myriad of things they have
done for me which go well beyond words.
Financial support for the research presented in this
dissertation was provided by the National Science Foundation, the
Petroleum Research Fund administered by the American Chemical
Society, the Air Force Office for Scientific Research, a Women
Entering Non-Traditional Fields fellowship administered by the
Graduate School and the Department of Chemcial Engineering. Thanks
go to Perkin-Elmer Physical Electronics in Eden Prairie, MN, at which
a portion of the data presented was obtained. I would also like to
thank Cerametals Inc. and Showa Denko K. K. for furnishing the
InP(111) samples through Sumitomo Electric U.S.A. Inc.
ii


TABLE OF CONTENTS
PAGE
ACKNOWLEDGEMENTS ii
LIST OF FIGURES v
ABSTRACT viii
CHAPTERS
I GENERAL INTRODUCTION 1
II AN ENERGY-RESOLVED, ELECTRON-STIMULATED
DESORPTION STUDY OF HYDROGEN FROM CLEANED
AND OXIDIZED Si (100) 5
Introduction 5
Experimental 7
Results and Discussion 8
Conclusions 22
III AN ION SCATTERING SPECTROSCOPY AND
TEMPERATURE-PROGRAMMED DESORPTION STUDY
OF THE INTERACTION OF N2 WITH Si (111) 23
Introduction 23
Experimental 24
Results and Discussion 26
Adsorption 26
Desorption 31
Conclusions 38
IV AN ELECTRON-STIMULATED DESORPTION STUDY OF
THE INTERACTION OF HYDROGEN, DEUTERIUM AND
OXYGEN WITH GaAs(IOO) 40
Introduction 40
Experimental 45
Results and Discussion 48
Conclusions 61
iii


PAGE
V A CHARACTERIZATION STUDY OF THE NATIVE OXIDE
LAYER FORMED ON CHEMICALLY ETCHED InP(111) 63
Introduction 63
Experimental 66
Results and Discussion 67
Conclusions 80
VI GENERAL CONCLUSIONS AND RECOMMENDATIONS FOR
FUTURE RESEARCH 82
REFERENCES 85
BIOGRAPHICAL SKETCH 95


LIST OF FIGURES
FIGURE PAGE
2-1 ISS spectra taken from (a) the cleaned sample
and (b) a sample exposed to 2000 L of O2 at
MOO C 10
2-2 ESD mass spectrum taken from the cleaned
Si(100) surface corresponding to figure 2-
Ka) 11
2-3 H+ energy distribution spectra taken from (a)
the cleaned sample and (b) the sample
following exposure to a 2M0 eV electron beam
for 20 minutes and a M00 eV beam for 15
minutes 13
2-M H+ energy distribution spectra taken from (a)
the electron-beam-exposed sample shown in
figure 2-3(b)f and (b) the beam-exposed
sample which had been annealed at 650 C for
15 minutes. The ordinates of (a) and (b) are
scaled relatively to one another 15
2-5 Angle-resolved, energy-resolved ESD spectra
obtained from a cleaned surface for (a) H+
desorbing normal to the surface and (b) H
desorbing at M50 with respect to the sample
surface plane 17
2-6 H+ energy distribution spectra obtained from
(a) a sample exposed to 2000 L of O2 at M00
C and (b) the cleaned sample (same as figure
2- 3 (a)) 19
2-7 The upper portions of the H+ energy
distribution spectra taken from (a) the
cleaned surface, (b) a surface exposed to a
2M0 eV electron beam for 30 minutes, (c) a
surface exposed as in (b) with an additional
exposure to a M00 eV beam for 25 minutes, (d)
the beam-exposed surface in (c) followed by a
M00 C anneal for 15 minutes and (e) the
beam-exposed surface followed by a 950 C
anneal for 10 minutes 21
v


PAGE
3-1 ISS spectrum taken from a Si(111) surface
which had been exposed to 75 L of N2 at room
temperature. For a scattering angle of 73
E-j/Eq is 0.70 for N and 0.79 for Si. All ISS
data were taken at room temperature using 1
keV helium ions 27
3-2 Ratio of N ISS peak heights to that of Si as
a function of room temperature N2 exposure 28
3-3 Typical TPD spectra for masses (a) 14, (b) 28
and (c) 29 for a sample which had been
exposed to 1000 L of N2 at room
temperature. The ordinates of (a), (b) and
(c) are not scaled relatively to one
another. The experimental peak area ratios
are (a(T1)):(a(T2)):(b):(c) = 1:3.5:44.3:6.1 32
3-4 TPD spectra for a sample which had been
exposed to 1000 L of N2 at room
temperature. Spectra labeled fa) refer to
the desorption of H2 and (b) refers to the
desorption of H. The data in (A) were taken
from a sample with a different history than
the sample in (B) (see text) 36
4-1 AES spectrum (a) of contaminated GaAs(100)
taken after solvent cleaning and (b) taken
after the IBA cleaning procedure 47
4-2 ESD mass spectrum taken from the contaminated
surface corresponding to figure 4-1(a). All
ESD spectra were taken using a primary beam
energy of 140 eV 50
4-3 ESD mass spectrum taken after exposing the
clean GaAs surface corresponding to figure 4-
1(b) to a dose of 2000 L of D2 while heating
the sample at 200 C 53
4-4 ESD spectra corresponding to the mass
spectrum shown in figure 4-3. (a) Total-ion
(H+ and D+) energy distribution spectrum and
ESDI ED spectra of (b) H+ and (c) D 55
4-5 AES spectrum taken from GaAs(100) after a
dose of 12,000 L of 02 at room temperature
57


PAGE
4-6 ESD spectra taken after exposing the oxidized
surface corresponding to figure 4-5 to 2000 L
of V>2 while heating the sample at 200 C. (a)
Total-ion (H+ and D+) energy distribution
spectrum and ESDIED spectra of (b) H+ and (c)
D + 59
4-7 ESD mass spectra taken (a) at an ion kinetic
energy of 0.8 eV and (b) at an ion kinetic
energy of 3.5 eV 60
5-1 ISS spectrum of air-exposed InP(111) 68
5-2 Angle-integrated AES spectrum of the same
surface shown in figure 5-1 70
5-3 AES spectra of the In eN^Nijc features
taken from the same surface shown in figure
5-2. Spectra (a) and (c) were taken at exit
angles of 10 and 85 respectively. Spectrum
(b) is angle integrated over all exit angles 71
5-4 XPS spectra of the (A) P 2p and (B) In 3d
peaks taken at exit angles of (a) 10 and
(b) 85 73
5-5 XPS spectra of the (A) C 1s and (B) 0 1s
peaks taken at exit angles of (a) 10 and
(b) 85 74
5-6 Angle-resolved valence band XPS spectra taken
at exit angles of (a) 10 and (b) 85 78
5-7 Angle-integrated AES compositional profile
obtained by argon-ion sputtering 79
vil


Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
CHARACTERIZATION OF THE INTERACTIONS
OF HYDROGEN, NITROGEN AND OXYGEN WITH SILICON AND
III-V SEMICONDUCTOR SURFACES
By
CHERYL FOX CORALLO
December, 1987
Chairman: Gar B. Hoflund
Major Department: Chemical Engineering
The interaction between gases and semiconductors has been
studied using ultrahigh vacuum (UHV) surface analytical techniques.
Electron-stimulated desorption (ESD) was used to study hydrogen
present on cleaned and oxidized Si(100). It is demonstrated by
kinetic energy analysis of the hydrogen ions emitted through ESD that
multiple states of adsorbed hydrogen are present. Electron beam
exposure, annealing and oxygen exposure dramatically alter the sites
or binding energy states of hydrogen present on the surface.
Ion scattering spectroscopy (ISS) and temperature-programmed
desorption (TPD) were used to investigate the interaction between N2
and a Si(111)(7x7) surface at room temperature. Adsorbed nitrogen is
detected using ISS for doses as low as 2 L (1L = 10-^ Torr*sec.) and
saturation occurs at 75 L. At least two chemisorbed states of
nitrogen are observed using TPD. Hydrogen, originating from the
sample bulk, associates with the surface nitrogen. The surface
viii


structure was monitored throughout the N2 adsorption experiments
using low energy electron diffraction (LEED).
The interaction of hydrogen with cleaned and oxidized GaAs(100)
was studied using ESD. Deuterium adsorbs on the cleaned and oxidized
surfaces for a radiantly heated sample in the range 200-400 C.
Different adsorbed states of hydrogen are observed by kinetic energy
analysis of the desorbing ions.
The native oxide layer of a chemically polished and etched
InP(111) surface was characterized using Auger electron spectroscopy
(AES), angle-resolved X-ray photoelectron spectroscopy (ARXPS) and
0
ISS. The oxide layer is about 30A thick and forms a sharp interface
between the oxide and the bulk InP. The outermost atomic layer is In
rich, and the region just below this is P rich. InP and InPO^ exist
in approximately equal concentrations in the oxide layer. A small
amount of In2C>2 is also detected. Carbon is present throughout the
oxide layer as graphite and/or hydrocarbons.
ix


CHAPTER I
GENERAL INTRODUCTION
The study of the interaction between gases and semiconductors is
very important for gaining fundamental knowledge of semiconductor
surface properties as well as in terms of understanding and
controlling the growth of epitaxial layers and passivating layers on
semiconductor surfaces. Numerous studies have been performed to
investigate these interactions. However, discrepancies continue to
exist and a considerable number of questions remain unanswered.
The focus of the experimental work which is presented here has
been to investigate the interaction of gases with Si and III-V (GaAs
and InP) semiconductor single crystals. In order to investigate
these interactions with minimal contamination effects, the studies
were done under ultra-high vacuum (UHV) conditions. The vacuum
system used in a majority of the studies presented here has been
described previously.1 (The study for which another vacuum system was
used is noted in the text.) Briefly, it consists of a stainless
steel chamber, typically at a base pressure of 1010 Torr, with
capabilities for performing a wide variety of experimental
techniques. Techniques such as X-ray photoelectron spectroscopy
(XPS) (also called electron spectroscopy for chemical analysis or
ESCA), Auger electron spectroscopy (AES), ion scattering spectroscopy
(ISS) and electron-stimulated desorption (ESD) were performed using a
double-pass cylindrical mirror analyzer (CMA) equipped with an
1


2
internal coaxial electron gun and an angularly resolving movable
aperture. Low energy electron diffraction (LEED) patterns were
observed using a hemispherical grid system and were recorded by
photographing the pattern through a window opposite the hemisphere.
Temperature-programmed desorption (TPD) data were obtained using a
quadrupole mass spectrometer by recording the intensity of signals of
as many as four masses simultaneously as a function of temperature.
The disseration consists of five chapters in addition to the
general introduction. Chapters II and III are concerned with the
interaction of gases with the (100) and (111) surfaces of Si,
respectively. Chapter IV is a study of the interaction of gases with
a GaAsOOO) surface, and Chapter V is an investigation of an InP(111)
surface. Each chapter is a complete individual study. The general
conclusions and recommendations for future research are presented in
Chapter VI.
More specifically, Chapter II presents results of an energy-
resolved ESD study of hydrogen from cleaned and oxidized Si(100).
One of the relatively few surface spectroscopies which detects hydro
gen directly is ESD. Therefore, it is very useful in studying the
interaction between hydrogen and semiconductors. This interaction
has both scientific and technological importance particularly due to
the fact that hydrogen has been shown to reduce electrical activity
P 1 P
in a number of semiconductors. Using ESD, it was found that
cleaning the surface by ion bombardment and annealing does not com
pletely remove hydrogen from a Si(100) surface. Kinetic energy


3
analysis of the hydrogen ions emitted through ESD is useful in
distinguishing between different adsorbed states of hydrogen.
Electron beam exposure, annealing and oxygen exposure dramatically
alter the kinetic energy distribution of the desorbing hydrogen
ions. Angle-resolved, energy-resolved ESD spectra of H+ from the
cleaned Si(100) surface are also presented which show an angular
dependence of the H+ energy distributions.
The interaction between nitrogen and silicon is also important,
particularly since silicon nitride is increasingly being used as an
insulatng layer in device production and nitrogen is sometimes
19
present as a background gas during device fabrication. In Chapter
III a study is presented in which ISS and TPD were used to study the
interaction between N2 and a Si(111)(7x7) surface at room
temperature. ISS provides an excellent means of monitoring the
amount of adsorbed species at the surface as a function of exposure
due to its high surface sensitivity. Using ISS, the adsorption of
nitrogen on Si(111) at room temperature was characterized and a
saturation dose was determined. The initial sticking probability was
also found. TPD results indicate that at least two chemisorbed
states of nitrogen exist. Hydrogen, which originates from the bulk
of the sample, associates with the surface nitrogen. LEED was used
to monitor surface structure throughout N2 adsorption and during
subsequent heating.
The interaction between GaAs and hydrogen is an important topic
which has been discussed in relatively few studies. The results of


4
an ESD study of the interaction of hydrogen and deuterium with
cleaned and oxidized GaAs(100) are presented in Chapter IV. Cleaning
the surface by ion bombardment and annealing does not completely
remove hydrogen from the surface so dosing was performed using
deuterium. It was verified that molecular deuterium does not adsorb
at significant rates at room temperature on the cleaned or oxidized
surface, but it does adsorb on both surfaces when the sample is
subjected to radiant heating in the range 200-400 C with a tungsten
filament. ESD gives a semi quantitative relative measure of the
amount of adsorbed deuterium, and kinetic energy analysis of the
desorbing ions is useful in distinguishing between different
adsorbed states of hydrogen.
InP is becoming an important III-V semiconductor material for
pQ-pO
the fabrication of semiconductor devices. J Chapter V consists of
a characterization study of the native oxide layer formed on
chemically polished and etched InP(111) by exposure to air. ISS was
used to determine the composition of the outermost surface layer, and
angle-resolved XPS and depth profiling using Ar+ and AES give the
composition of the oxide layer. The thickness of the oxide layer and
the relative amount of the species present in this layer were also
determined.


CHAPTER II
AN ENERGY RESOLVED, ELECTRON-STIMULATED DESORPTION
STUDY OF HYDROGEN FROM CLEANED AND OXIDIZED Si(100)
Introduction
An understanding of the interaction between hydrogen and
semiconductors has both technological and scientific importance.
P-1 P
Hydrogen has been shown to reduce electrical activity in silicon,
germanium 3 and gallium arsenide. For example, Benton et al.
achieved hydrogen passivation of electrically active defects created
during laser annealing of crystalline silicon by exposure to atomic
ji
hydrogen in a hydrogen plasma. Pearton and Tavendale showed that
both the deep donor and acceptor levels associated with gold are
passivated by exposing silicon samples to a hydrogen plasma. Mecha
nisms responsible for hydrogen passivation in silicon have been
K_q ii 24 2H
proposed by a number of authors.
In addition to the interest in the passivation effects of
hydrogen, the interaction of hydrogen and silicon has been studied in
order to gain a better understanding of the surface geometry of
reconstructed clean silicon surfaces.^"31 The ease with which
hydrogen induces surface reconstructions on the low-index planes of
silicon has been used to evaluate the various models for
reconstruction of the clean surface. For example, Norton^0 found
that changes in the diffraction pattern induced by adsorption of
hydrogen on Si(001) are consistent with the asymmetric dimer model as
5


6
opposed to the symmetric dimer model. Numerous calculations of
surface electronic properties have also been carried out for both
07 72-40
clean and hydrogen-exposed silicon surfaces, and a variety of
experimental studies of the silicon/hydrogen system have been
performed .2^ 31 ,41 -66 These studies show that several surface
species (including mono-, di- and trihydrides) can exist depending
upon sample history and exposure conditions.
Relatively few surface analytical techniques detect hydrogen
directly due to its light mass, small cross section for many
processes and lack of core-level electrons. However, ESD is highly
sensitive to hydrogen because it is a mass spectrometric technique.
ESD is powerful in that it can yield information on surface bonding
structure and/or mechanisms of excitation and desorption. Few ESD
studies of the silicon/hydrogen system have been published. Madden
et al. correlated the threshold for electron- and photon-stimulated
desorption with a localized Auger final state for atomically adsorbed
hydrogen on Si(100). ESD was also used in a study by Kazmerski^ to
investigate the interaction of hydrogen ions at grain boundaries in
polycrystalline silicon.
In the present study ESD is used to examine the binding states
of hydrogen present on Si(100). Hydrogen is found to be present on
both the cleaned and oxygen-exposed Si(100) surfaces and energy
distributions of desorbing H+ from cleaned and oxygen-exposed
surfaces are presented. The effects of electron beam exposure and
annealing on the H+ energy distributions are also discussed.


7
Additionally, angle-resolved and energy-resolved ESD spectra of H+
from the cleaned Si surface are presented.
Experimental
A polished Si(100) single crystal was used in this study. The
sample was ultrasonically cleaned in electronic grade
trichloroethylene, acetone and methanol and then rinsed with
deionized water prior to insertion into the vacuum chamber. The Si
was heated radiantly to 700 C by a tungsten filament placed directly
behind the sample, and heating above 700 C was accomplished using
electron bombardment. The temperature of the sample was monitored
using an optical pyrometer. The sample was cleaned using cycles of
ion bombardment and annealing (IBA) at 950 C until a distinct (2x1)
LEED pattern was obtained and the ISS oxygen signal was no longer
detected. The oxygen signal was below the AES detection limit before
it was below the ISS detection limit.
The vacuum system (base pressure of 2x10-1^ Torr) used in these
experiments has been described previously.1 ESD, AES and ISS were
performed using a double-pass cylindrical mirror analyzer (CMA) (PHI
Model 15-255 GAR) equipped with an internal coaxial electron gun and
an angularly resolving movable aperture. The sample was positioned
at an angle of 45 with respect to the CMA axis. AES and ISS data
were taken by operating the CMA in the nonretarding mode. The
details of the data collection are given elsewhere. LEED patterns
were observed using a PHI Model 15-180 hemispherical grid system.


8
ESD data were taken using the CMA as a time-of-flight mass
6Q
spectrometer. 7 Energy analysis of low-energy (less than 20 eV)
positive ions was accomplished by operating the CMA in the retarding
mode with a pass energy of 80 eV. The ions were first accelerated by
a negative potential placed on the second grid and inner cylinder.
The outer cylinder was maintained at a more positive potential (but
negative with respect to ground) than the inner cylinder. The
complete set of CMA operating voltages has been published pre-
70
viously. For mass analysis the electron beam was pulsed onto the
sample for 300 ns and pulses were counted for 300 ns after an
appropriate delay period (5.5 ps for H+). A primary beam energy of
240 eV and a primary beam current of approximately 500 nA were
used. The mass spectrum of the desorbing ions was obtained by
selecting an ion kinetic energy and scanning the delay time. An
electron-stimulated desorption-ion energy distribution (ESDIED)
spectrum was obtained by scanning the ion kinetic energy with the
CMA. H+ was the only species found to desorb by ESD in this study.
Therefore, time gating was not necessary to obtain the energy
distributions of the hydrogen ions.
Results and Discussion
In ESD electrons impact a surface creating localized excitations
which decay causing desorption of ions, neutrals and metastable
species. These low energy ions can desorb only from the top atomic
layer because ions which are created beneath the top monolayer have a


9
high probability of being neutralized during the desorption
process. However, excitations created beneath the surface can emit
secondary electrons which may create excitations at the surface
71
resulting in emission of ions. Thus, ESD is highly surface sensi
tive with respect to the emitted ionic signal but not as surface
specific with respect to the primary electron beam.
The ISS spectrum shown in figure 2-1(a) was taken from a sample
cleaned by IBA cycles followed by a 950 C anneal. Only a silicon
peak is observed in this ISS spectrum. Also, an AES spectrum (not
shown) and a distinct (2x1) LEED pattern characteristic of the clean
Si (100) surface were obtained from this surface. The ESD mass
spectrum corresponding to figure 2-1(a) is shown in figure 2-2. It
is obvious from this spectrum that hydrogen is present on the Si(100)
surface which was cleaned by IBA cycles. The amount of hydrogen
present on this silicon surface is not known because the total ESD
cross sections for hydrogen on Si(100) are not known and an
independent means of measuring surface hydrogen concentration was not
available. It is difficult to determine the surface concentration of
hydrogen since relatively few techniques directly detect hydrogen and
these techniques are difficult to quantify. For example, Madden^
has shown that changes in the lineshape on the low energy side of the
Si L2 3VV AES peak are induced by hydride formation. However, the
AES sensitivity factor for hydrogen is not known so quantifying the
amount present is not possible using AES. Although the amount of
hydrogen present is not known, ESD provides considerable information
about the adsorbed states of hydrogen.


N (E) (arb. units)
10
He+ ISS
0.6 0.7 0.8
E/E0
Figure 2-1. ISS spectra taken from (a) the cleaned sample and
(b) a sample exposed to 2000 L of O2 at 400 C.


N (E) (arb. units)
11
FLIGHT TIME (^s)
Figure 2-2. ESD mass spectrum taken from the cleaned Si(100)
surface corresponding to figure 2-1(a).


12
In most cases different peaks in an energy distribution spectrum
are due to different desorbing states. However, complexities such as
differing cross sections for desorption from different states or
desorption by more than one mechanism can make ESDIED spectra more
difficult to interpret. A review of ESDIED which covers the
experimental methods, ESD mechanisms and theory and addresses the
topics of ESD cross sections, desorption by more than one mechanism
and desorption from defect sites has been published by Hoflund.^0
These topics are not discussed in detail in this paper.
An interesting effect on the energy distribution of the
desorbing ions was found to occur upon exposure of the sample to the
electron beam. The total ion (H+) energy distributions taken from
(a) the cleaned sample and (b) the same sample following collection
of 33 ESD spectra (primary beam energy of 240 eV for 20 minutes) and
further exposure to a 400 eV beam for 15 minutes is shown in figure
2-3. Electron beam exposure results in a depopulation of the states
which desorb at higher kinetic energy implying that they have larger
desorption cross sections. This effect was also observed for less
severe beam exposures. However, the depopulation is less dramatic in
these cases. This implies that the features in the energy
distribution are due to desorption from different sites or binding
energy states with different desorption cross sections.
It was found that the higher energy states could be repopulated
with hydrogen by annealing. Energy distributions taken from (a) the
electron-beam-exposed sample shown in figure 2~3(b), and (b) the


N (E) (arb. units)
13
ION KINETIC ENERGY (eV)
Figure 2-3. H+ energy distribution spectra taken from (a)
the cleaned sample and (b) the sample following
exposure to a 240 eV electron beam for 20 minutes
and a 400 eV beam for 15 minutes.


14
beam-exposed sample following a 15 minute anneal at 650 C are shown
in figure 2-4. The energy-resolved ESD spectrum following annealing
closely resembles spectrum 2-3(a) except for the small features
present on the top of the energy distribution spectra which are
discussed in more detail below. For less severe beam exposures and
correspondingly lessened depopulation of the higher energy states,
annealing temperatures as low as 400 C are sufficient to repopulate
these states at reasonable rates.
An attempt was also made to adsorb deuterium on the Si (100)
surface under various conditions. In agreement with previous
OQ Kil
studies, D2 does not adsorb appreciably at room temperature on a
clean Si(100) surface. Furthermore, D+ was not detected by ESD
following D2 exposures of 1000 to 3000 L (1L = 10-^ Torr*sec.) at
dosing temperatures of 400, 500 or 600 C.
Both of the facts that annealing the silicon repopulates surface
hydrogen states and that D2 does not adsorb on Si(100) imply that the
source of the surface hydrogen detected by ESD is hydrogen in the
silicon bulk which diffuses to the surface during annealing. The
presence of molecular and atomic hydrogen in the Si lattice is
72
discussed in a paper by Corbett et al.
Electron-stimulated desorption-ion angular distribution (ESDIAD)
experiments are often capable of yielding bonding structural
information,^0 but it is usually not possible to obtain the energy
distributions of ions desorbing at particular angles using typical
ESDIAD experimental configurations. It is expected that ions which


N (E) (arb. units)
15
ION KINETIC ENERGY (eV)
Figure 2-4. H"1" energy distribution spectra taken from (a) the
electron-beam-exposed sample shown in figure 2-3
(b), and (b) the beam-exposed sample which had
been annealed at 650 C for 15 minutes. The
ordinates of (a) and (b) are scaled relatively
to one another.


16
desorb with different angular distributions from different sites or
binding states would also have different energy distributions. This
hypothesis was tested using the 12 aperture of the CMA to collect
ions which desorbed at different angles with respect to the
surface. The resulting angle-resolved, energy-resolved ESD spectra
are shown in figure 2-5. Spectrum (a) consists of H+ desorbing in
the direction normal to the surface, and spectrum (b) consists of
ions desorbing at approximately 45 with respect to the sample
surface plane (in the (TT1) direction relative to the bulk
silicon). The signal for the H+ desorbing normal to the surface is
similar in intensity to that of the H+ desorbing at an angle of
45. It is apparent from these spectra that H+ energy distributions
are angular dependent providing further evidence that multiple states
of adsorbed hydrogen are present on the cleaned Si(100) surface. In
fact, spectrum (a) appears to consist of at least two peaks while the
lower-energy feature is absent in spectrum (b). Hydrogen was
detected at all angles which were examined (approximately 15, ^5,
60 and 90 with respect to the sample surface plane). The signal
intensity for H+ desorbing at 60 is similar to that of H+ desorbing
at 45 and 90, but the signal intensity for H+ desorbing at 15 is
less than half of the signal intensity obtained at the other
angles. The energy distribution for hydrogen desorbing at 60 is
very similar to the distribution for normal desorption, while the
distribution for H+ desorbing at 15 resembles that of H+ desorbing
at 45.


N (E) (arb. units)
17
0 10
ION KINETIC ENERGY (eV)
Figure 2-5. Angle-resolved, energy-resolved ESD
spectra obtained from a cleaned surface
for (a) H+ desorbing normal to the
surface and (b) H+ desorbing at 45 with
respect to the sample surface plane.


18
In order to investigate the effect that the presence of oxygen
has on the surface hydrogen, the sample was annealed at 950 C for 5
minutes and then dosed with 2000 L of O2 at 400 C. The ISS spectrum
obtained from this oxygen-exposed surface is shown in figure 2-
1(b). Following the oxygen exposure, the 0 ISS signal is 1.3 times
that of the Si signal. From scattering cross section estimates,
there are more than twice as many 0 atoms on this surface as Si
atoms. An ESD mass spectrum taken after the oxygen exposure reveals
that H+ is the only ionic species which desorbs from this surface
(i.e., no oxygen or hydroxyl ions are detected). The ESD ion energy
distribution spectrum obtained from this surface is shown in figure
2-6(a). The spectrum shown in figure 2-6(b) is the same as that
shown in figure 2~3(a) taken from the cleaned, annealed surface. The
hydrogen ions desorb from the oxygen-dosed surface with a much
narrower energy distribution than from the cleaned surface. The
higher energy states are depopulated for the oxygen-exposed surface;
even more so than for the electron-beam-exposed surface. It is
possible that this narrow energy distribution is due to desorption
from one well-defined state such as hydrogen bonded in the form of
hydroxyl groups. The formation of hydroxyl groups has been shown to
occur upon hydrogen-ion exposure at oxygen-rich grain boundaries in
polycrystalline Si by Kazmerski^ using ESD, SIMS and TPD. Using
high resolution electron energy loss spectroscopy (HREELS), Nishijima
et al.^ determined that SiH and SiOH species form on the
Si(111)(7x7) surface following exposure to H20 at 300 K. However,


N (E) (arb. units)
19
ION KINETIC ENERGY (eV)
Figure 2-6. H+ energy distribution spectra obtained from (a)
a sample exposed to 2000 L of O2 at 400 C and
(b) the cleaned sample (same as figure 2-3(a)).


20
other possibilities exist which could explain the ESD data. It may
be that the hydrogen desorbing from the oxygen-exposed surface is
bonded as on the cleaned surface but that the higher energy
desorption states are no longer able to desorb by ESD due to an
interaction with oxygen or that the higher energy states are no
longer present on the surface. More information is required to
specify the forms of the hydrogen at the Si(100) surface.
The presence of small features on the top of the ESD I ED spectra
was observed during the course of this study. For a cleaned surface
which had been annealed at 950 C, there are four primary features on
the ESDIED spectra and a shoulder on the high energy side (figure 2-
7(a)). Electron beam exposure results in slight shifts in the energy
of these features and a slight depopulation of the higher energy
states relative to the lowest energy state. This can be seen in
figure 2-7(b) for a surface exposed to a 0.5 pA, 240 eV electron beam
for 30 minutes. The results of additional beam exposure are
illustrated in figure 2-7(c). This spectrum was obtained following
exposure to a 240 'eV beam for 30 minutes and a 400 eV beam for 25
minutes. There is a further depopulation of the higher energy
states. Annealing the beam-exposed surface results in repopulation
of the higher energy states as has been discussed previously.
However, the primary small features are not well defined for anneal
ing temperatures of 650 C and below. Figure 2-7(d) was obtained
from a sample which had been annealed to 400 C. Upon annealing to
950 C the well-defined small features are again present (figure 2-


N (E) (arb. units)
21
0 5
ION KINETIC ENERGY (eV)
Figure 2-7. The upper portion of the H+ energy distribution
spectra taken from (a) the cleaned surface, (b)
a surface exposed to a 240 eV electron beam for
30 minutes, (c) a surface exposed as in (b) with
an additional exposure to a 400 eV beam for 25
minutes, (d) the beam-exposed surface in (c)
followed by a 400 C anneal for 15 minutes and
(e) the beam-exposed surface followed by a 950
C anneal for 10 minutes.


22
7(e)). It is possible that the individual small features are due to
desorption from distinct adsorbed states of hydrogen on the surface.
Conclusions
ESD has been used to investigate the hydrogen adsorbed on a
cleaned and an oxidized Si(100) surface. IBA cleaning cycles do not
remove the surface hydrogen completely. The hydrogen detected using
ESD diffuses to the surface from the bulk of the Si sample during
annealing and is not due to adsorption of gas-phase hydrogen. Energy
distributions of the desorbing H+ provide information about the
adsorbed states of hydrogen on the surface. Electron beam exposure
results in a depopulation of the higher energy states which are
repopulated upon annealing. Exposure to oxygen results in a
narrowing of the energy distribution and a more marked depopulation
of the higher energy states. Angle-resolved, energy-resolved ESD
provides further evidence that multiple states of adsorbed hydrogen
are present on the cleaned Si(100) surface.


CHAPTER III
AN ION SCATTERING SPECTROSCOPY AND
TEMPERATURE-PROGRAMMED DESORPTION
STUDY OF THE INTERACTION OF N2 WITH Si(111)
Introduction
The importance of very thin insulating layers in the production
of VLSI (very large scale integration) devices is evident. Silicon
nitride is increasingly being used instead of the oxide for this
1 q
purpose. 7 As device down-scaling continues, a thorough
understanding of the chemical, physical and electronic properties of
insulating films becomes even more important. In addition, nitrogen
is sometimes present as a background gas during device fabrication.
For these reasons it is essential to understand the interaction
between nitrogen and silicon.
A number of studies have been done on films produced by chemical
vapor deposition (CVD) using SiHjj combined with NH^,^-^ NH^/N-jO,^
and NH^/NO.^^ Relatively high pressure (0.1-760 Torr) nitridation
80 88
of Si using N2 or NH^ has also been studied. However, in order
to study the initial stages of growth of thin nitride films on Si
with minimal contamination effects, the study must be done under
ultrahigh vacuum (UHV). Investigations have been reported in which
800*5 + Q1 Oil
Si is exposed to various nitriding gases such as: N, N 7
N2,95-97 NH^,92,95,98-100 n097,101,102 and N2o13 under UHV
conditions. The majority of these studies have focused on thermal
nitridiation and have shown that an (8x8) or a quadruplet surface is
23


24
formed on Si(111) depending upon the cleanliness and temperature of
the sample as well as the nitriding gas pressure. It has been
suggested that the quadruplet pattern is evidence of the formation of
a Si^Njj surface layer.8991 ,95,98,99
Relatively few room temperature studies have been reported,
particularly concerning the interaction of Si with N^0^ or n2>95,97
Heckingbottom^ found only a slight deterioration of the Si(111)(7x7)
pattern with a room temperature N2 exposure at 10~^ Torr while
97
Wiggins et al. were unable to detect any adsorbed nitrogen using
AES for N2 exposures at 10~^ Torr for 5 minutes and sample
temperatures below 600 C. However, it is demonstrated in the
present study using ISS and TPD that N2 does adsorb on Si(111) under
these conditions.
Experimental
A polished, n-type (P-doped) Si(111) single crystal was used in
this study. The resistivity of the sample was 5-30 ohm*cm. The
sample (17.5 x 9 x 0.5 mm) was ultrasonically cleaned in electronic
grade trichloroethylene, acetone and methanol and then rinsed with
deionized water prior to insertion into the vacuum chamber. The Si
was mounted in a tungsten frame and heated radiantly to 700 C by a
tungsten filament placed directly behind the sample. Heating above
700 C was accomplished using electron bombardment. The temperature
of the sample was monitored using an optical pyrometer. The sample
was cleaned using cycles of ion bombardment followed by annealing


25
(IBA) until a distinct (7x7) LEED pattern was obtained and the ISS
oxygen signal was minimized. The sample was cleaned by annealing at
920 C for at least five minutes between the repeated N2 exposures
during TPD experiments.
The vacuum system (base pressure of 10-1<^ Torr) used in these
experiments has been described previously.1 ISS and AES were
performed using a double-pass cylindrical mirror analyzer (CMA) (PHI
Model 15-255 GAR) equipped with an internal coaxial electron gun and
an angularly resolving movable aperture. ISS data were taken using a
sputter ion gun (PHI Model 04-161) positioned at an angle of 60 with
respect to the sample surface plane. The sample was positioned at an
angle of 45 with respect to the CMA axis, and the angle resolving
aperture was used to select He+ ions scattered at an angle of 13
with respect to the sample plane for energy analysis. A primary beam
of He+ ions with an energy of 1 keV was used with the current to the
sample being 0.6-0.74 pA over an area with an approximate diameter of
2 mm. LEED patterns were observed using a PHI Model 15-180
hemispherical grid system and a primary beam energy of 90 eV. TPD
data were obtained using a quadrupole mass spectrometer (EAI Series
Quad 250) by recording the intensity of the signals of as many as
four masses simultaneously as a function of temperature.


26
Results and Discussion
Adsorption
ISS provides an excellent means of monitoring the amount of
adsorbed species at a surface as a function of exposure. Due to the
high surface sensitivity of ISS, the composition of the outermost one
1 nt
or two surface layers can be determined if the scattering cross
1 os
sections and ion neutralization probabilities are known. J The
surface sensitivity of ISS is a result of the nearly 100$ probability
of neutralization or inealastic scattering for an ion which
penetrates beneath the outermost layer of atoms. A potential problem
with ISS is surface alteration due to sputtering. This was minimized
in this study by using a relatively low primary beam flux and
scanning over the N and Si peaks only. An ISS spectrum was taken
over the entire range (E-j/Eq of 0 to 1) preceeding the N2 dosing to
check surface cleanliness. Prior-to N2 dosing the ISS 0 signal was
reduced to less than 5% of the Si peak height by IBA. This was the
minimum 0 signal obtainable and is below the AES detection limit. In
AES the noise level was typically less than 0.005 of the Si LVV (91
eV) peak intensity.
An ISS spectrum taken from a sample exposed to 75 L of N2 at
room temperature is shown in figure 31, and the ratio of N-to-Si ISS
peak heights as a function of N2 dose is shown in figure 3~2. The
adsorption of N2 on Si(111) under these conditions is characterized
by an initial rapid linear uptake, a region of more gradual growth

and saturation at approximately 75 L. A very important point is that


(arbitrary units)
27
Figure 3-1. ISS spectrum taken from a Si(111) surface which
had been exposed to 75 L of N2 at room tempera
ture. For a scattering angle of 73 E^/Eg is
0.70 for N and 0.79 for Si. All ISS data were
taken at room temperature using 1 keV helium
ions.


N/Si
28
Figure 3-2. Ratio of N ISS peak heights to that of Si as a
function of room temperature N2 exposure.


29
at saturation coverage the amount of nitrogen present is below the
detection limit of AES. This explains why Wiggins et al.^ did not
detect nitrogen adsorption using AES.
According to Parilis,1^ assuming a potential of the screened
Coulomb Thomas-Fermi-Firsov form, a binary elastic collision and a
primary ion beam energy of approximately 1 keV or higher, the
differential cross section for scattering ^) can be found from
the expression:
dq( 6)
dfl
iv2(m1+ m2)aQ2 z^ 1 3.68) (it 0)
m (z 1/2+ z 1/2)2/3E (2ir 0)2 02 sin0
2 l 2 o
(3-D
0=0+ arcsin (sin 0/p)
where the subscript 1 refers to the incident ion and 2 refers to the
0
surface target atom, Z is the atomic number, aQ = 0.468 A EQ(eV) is
the primary beam energy, and 0 and 0 are the scattering angles of
the incident ion in center of mass and laboratory coordinates
respectively. From equation (31) the differential scattering cross
P O
section, for 0 = 73 is 1.2 x 10 for Si and 6.2 x 10 3 for N.
Then, the relative amounts of Si and N on the surface can be
estimated from ISS peak heights assuming that the neutralization
probabilities for He+ scattering off N and Si are the same. At
saturation the ratio of N-to-Si ISS peak heights is approximately 1.2
which yields a ratio of N-to-Si surface concentration of 2.2. This


30
implies that there are approximately 2 N atoms for each Si atom on
the surface or approximately 2/3 of a monolayer of nitrogen on the
Si (111) surface at saturation.
From the kinetic theory of gases, the rate of collision of N2
with the Si sample surface at room temperature and a N2 pressure of
10 ^ Torr is 6.0 x 101^ collisions/sec. The sticking probability (s
= rate of adsorption/rate of collision) can be found from the
107
following expression: '
s(0)
N
o de
vp dt
(3-2)
where 0 is the fractional coverage (number of molecules
adsorbed/maximum uptake), NQ is the number of surface sites, v is the
number of molecules colliding with the surface (number/areatime) at
room temperature, and p is the pressure. The initial sticking
probability (sQ) can be obtained from the initial slope in figure 32
assuming 0 is 1 at saturation and the number of surface sites
available for N2 adsorption is half the number of surface Si atoms (=
1/2 (7.8 x 1014)).107108 In this study sQ is calculated to be 4.7 x
10"2.
The LEED pattern was monitored throughout the room temperature
Ng adsorption steps. For doses up to saturation (75 L), a distinct
(7x7) pattern remains. For much higher exposures (1000 3000 L),
the (7x7) pattern deteriorates slightly (all spots become less
sharply defined). These results are in agreement with the findings


31
of Heckingbottom.^ The sample which had been exposed to 3000 L of
N2 at room temperature was then heated. The slightly deteriorated
pattern remains unchanged for temperatures up to 570 C. Upon
heating to 670 C the 1/7th order spots decrease in intensity
relative to the (1x1) spots. This intensity change continues with
further heating to 800 C. The distinct (7x7) pattern is recovered
by annealing the sample at 920 C for 5 minutes. No features due to
the (8x8) nor the quadruplet pattern were observed.
Desorption
TPD was performed in order to gain a better understanding of the
adsorbed nitrogen. The experiments were carried out by exposing the
sample to 1000 L of N2 and then heating to approximately 770 C. The
amount of each desorption product (assumed to be proportional to the
TPD peak area) varies with sample history. Nevertheless,
considerable information can be extracted from the TPD data.
Desorption peaks occur at only two temperatures 320(T^) and 510
C(T2). The masses of species which desorb at T1 are 1, 2 and 14
amu. At T2 masses of 1, 2, 14, 15, 16, 28 and 29 amu desorb. The
TPD peaks have very distinct and repeatable shapes (see figure 3~3)
1 no
which depend upon the kinetics of desorption. y The shape of the T1
peak is indicative of first order kinetics while the shape of the T2
peak probably suggests more complex desorption kinetics. Assuming
that the rate of desorption can be described by an Arrhenius
expression and that the desorption is first order, the relationship


(arbitrary units)
32
i 1 1 1 1 1 r
i i i i i i i
200 400 600
TEMPERATURE (C)
Figure 3-3. Typical TPD spectra for masses (a) 14, (b)
28 and (c) 29 for a sample which had been
exposed to 1000 L of N at room temperature.
The ordinates of (a), tb) and (c) are not
scaled relatively to one another. The
experimental peak area ratios are (a(T.)):
(a(T2)):(b):(c) = 1:3.5:44.3:6.1.


33
between the TPD peak temperature (Tp) and the desorption activation
energy (Ed) is given as:10^
-E
v exp d..
6 lR T j
P
(3-3)
where v is the frequency factor (assumed to be 1013 sec.-1) and 8 is
dT/dt. Using this expression gives an Ed(T-¡) of 3^.0 kcal/mol (1.5
eV/atom). This analysis probably does not apply to the T2 peak since
it probably is not first order.
The desorbing hydrogen is believed to come from the sample sur
face or near-surface region and is present prior to the adsorption of
nitrogen. In fact, ESD has been used to verify the presence of hy
drogen on the Si surface before and after exposure to nitrogen.11
The hydrogen detected on the surface is not believed to be due to
adsorption of background gases since attempts to adsorb D2 at room
temperature do not yeild TPD peaks for masses 3 or M. Additionally,
it is commonly believed that at room temperature molecular hydrogen
pq eh
does not adsorb appreciably on clean Si surfaces. Therefore, it
is most likely that hydrogen is present in the bulk of the Si and is
not removed by the cleaning procedure. Furthermore, the concen
tration of surface hydrogen varies considerably with sample pretreat
ment and history.
In order to interpret the TPD results the cracking patterns due
to the ionizer of the mass spectrometer and the isotope ratios must
be taken into account.111-11^ These figures as well as average


34
experimental peak area ratios are given in table 3~1. With the aid
of this table the TPD features are assigned as follows. The 28 amu
peak is due to the desorption of N2 and not Si for the following
reasons: the experimental peak area ratio of the 28 and 29 amu peaks
is not the same as the Si isotope ratio (see table 3-1); no peak is
found for 30 amu corresponding to the 30 amu Si isotope; and most
importantly, no desorption of masses 28, 29 or 30 is observed from a
clean sample. The 29 amu peak is due to desorption of primarily N2H
since the ratio of the amount of 29 to 28 is too great for the 29
peak to be due mostly to desorption of (see table 3-1). The
14 amu TPD peak is due to N desorption. The 15 amu peak is primarily
due to NH desorption since the peak area ratio of 14 to 15 amu does
not correspond with the ratio for the N isotopes (see table 3~1).
The 16 amu peak is believed to be due to NH2, and the 1 and 2 amu
peaks are due to H and H2, respectively. Species with masses greater
than 29 amu are not observed. Fragmentation or cracking of N H
x y
species does occur in the ionizer, but the cracking patterns are not
known for most of these species.
The majority of the atomic hydrogen detected is not due to the
cracking of H2 since for a number of trials H desorbs only at T2
while H2 desorbs only at T-j At other times for the same dosing
conditions but a slightly different sample history, H2 desorbs at
both temperatures (see figure 34). Factors which may affect the
temperature at which H2 desorbs as well as the TPD peak areas include
the number of TPD runs done prior to the exposure to N2 and


35
TABLE 3-1
Isotope, Cracking and Experimental TPD Area Information
Isotope
Ratios3)
Average
Experimental
TPD Peak Area Ratios
Species
mass(amu) relative amount($)
mass relative amount
Si:
28
92.21
14
80d)
29
4.70
15
7
30
3.09
16
13
N:
14
99.63
28
78e)
15
0.37
29
22
30
0
H:
1
99.985
2
0.015
Cracking
Patterns150)
H2:
1
2.6
14(^5
28(T2)
6f>
2
97.4
94
N2:
14
8.2
14(T2)
8g)
28
91.15
28(T2)
92
29
0.65
NH,:
14
1.1
J
15
3.7
16
42.3
17
52.9
Referenced relative
amounts were converted to
percentages.
a)
Reference 111, b) H2 and N2reference 112, c) NH^reference 113, d)
Taken from 1 TPD run, e) Ranged from 74-88$ mass 28, f) Ranged from
92-98$ mass 28, g) Ranged from 92-92.7$ mass 28.


(arbitrary units)
t 1 1 1 1 1 1 r
1 i i I i i i i
100 500 100 500
TEMPERATURE (C)
Figure 3-4. TPD spectra for a sample which had been exposed to 1000 L
of N2 at room temperature. Spectra labeled (a) refer to
the desorption of H2 and (b) refers to the desorption of
H. The data in (A) were taken from a sample with a dif
ferent history than the sample in (B) (see text).


37
subsequent TPD run, the heating and/or cooling rate for the 920 C
annealing step between doses and possibly the presence of nitrogen in
the sample near-surface region following the annealing step. The
hydrogen detected could be a result of the cracking of N H
a y
species. The facts that H and H2 do not desorb at temperatures other
than T1 or T2 and that their desorption peaks are the same shape as
the nitrogen peaks suggest that hydrogen is associated with or bonded
to the nitrogen on the surface.
TPD peak area comparisons reveal that the N desorbing at T2 may
be primarily due to the cracking of N2 (which only desorbs at T2) in
the mass spectrometer (see table 31). It is possible that the NH
and NH2, which desorb only at T2, are due to the cracking of NxHy
species. The presence of relatively more N2 on the surface than NxHy
would explain the large amount of mass 11* detected with respect to
the amounts of masses 15 and 16. Mass 17 was not detected, and
considering the ratio of mass 15 to mass 16, it is not likely that
any NH^ desorbs (see table 3-1).
Following N2 dosing and a thermal desorption experiment it is
found using ISS that nitrogen remains on the Si surface. This may be
due to nitrogen which is bound in a state which does not thermally
desorb. In order to investigate the effects of the thermal
desorption process on the adsorbed nitrogen, a series of repeated TPD
runs were done without redosing between the runs. As the number of
runs increase, there is a tendency for the TPD areas of species
detected at T2 to decrease while the areas of T.| peaks either


38
slightly decrease or increase. This indicates that a portion of the
species adsorbed in state 2 (T2) may be populating state 1 ()
during TPD runs. For the case of hydrogen the analysis is not as
straightforward. The complexity in the data suggests the coexistence
of a number of competing processes which increase or decrease the
aranount of hydrogen on the surface which is ultimately detected as H,
H2 or NxHy.
Conclusions
ISS and TPD have been used to study the interaction between N2
and the Si(111)(7x7) surface. Using ISS, the adsorption of nitrogen
as a function of dose was examined and a room temperature saturation
dose of 75 L is found. Following a saturation dose nitrogen cannot
be detected with AES. The initial sticking probability, sQ, of
nitrogen on Si(111) is 4.7 x 10 at room temperature.
A distinct (7x7) LEED pattern remains for the surface exposed to
75 L of N2 at room temperature. Additional exposures to N2 and
subsequent heating result in variations in this LEED pattern, but the
distinct (7x7) pattern is recovered by annealing at 920 C for 5
minutes. Features due to the (8x8) or the quadruplet pattern are not
observed.
Using TPD, evidence of at least two chemisorbed states of
nitrogen is found. These two states desorb at 320 and 510 C with
distinct and repeatable TPD peak shapes. Hydrogen also desorbs in
the forms of H, H2 and NxHy at the same temperatures and with the


39
same peak shapes as the desorbing nitrogen. This indicates that the
hydrogen is associated with or bonded to the adsorbed nitrogen prior
to thermal desorption. The hydrogen is believed to originate in the
bulk of the Si sample.


CHAPTER IV
AN ELECTRON-STIMULATED DESORPTION STUDY OF THE
INTERACTION OF HYDROGEN, DEUTERIUM AND OXYGEN
WITH GaAs (100)
Introduction
The interaction between GaAs and hydrogen is an important topic
which has been discussed in relatively few studies. Studies of this
interaction may provide information which is useful in the growth of
epitaxial films or passivating layers on semiconductor surfaces. It
has been shown that atomic hydrogen introduced by hydrogen plasma
exposure passivates certain electrically active deep-level defect
1 1} 1 K 1
sites J as well as certain shallow-level dopant sites in GaAs.
A similar effect is observed after incorporating hydrogen into GaAs
electrochemically in a phosphoric acid electrolyte.1^ Reduction in
electrical activity due to hydrogen passivation has been observed in
O-il 1 o
silicon and germanium as well. Studies of this type are partic
ularly important with regard t.o the growth of GaAs films using
1 1||
molecular beam epitaxy (MBE) and chemical vapor deposition
(CVD).115-118 jn £he qVD process hydrogen is typically present
1 1C
either as a carrier gas or in the compounds used to form the
GaAs;11^-11 e.g. AsH^ or GaCCH^)^. It has also been shown that
exposing GaAs surfaces to hydrogen plasmas provides an effective
means of removing contamination. y For these reasons the role
of hydrogen in the formation and preparation of GaAs and its effects
on the properties of GaAs should be considered.
40


41
1 PP
An early study by Pretzer and Hagstrum used ion neutral
ization spectroscopy (INS) to study the (111), (111) and (110) GaAs
surfaces before and after a room temperature exposure to different
gases including 02, N2, H2 and CO. A 3 x 10^ L exposure of a clean
GaAs surface to H2 causes no change in the INS spectrum. The authors
conclude that molecular hydrogen does not adsorb at significant rates
at room temperature on GaAs surfaces.
This finding was reaffirmed in the UV photoelectron spectroscopy
(UPS) study of Gregory and Spicer1who investigated the adsorption
of 02, CO and H2 on n- and p-type, single crystal, cleaved GaAs(110)
surfaces. They found that atomic hydrogen readily adsorbs on p-type
surfaces at room temperature causing a large change in the electron
energy distribution curve (EDC) between 1.6 and 5.5 eV below the
valence band maximum. For low exposures this adsorption is
reversible in that heating the sample to about 235 C results in an
EDC which is quite similar to that of clean GaAs(110). By comparing
1 ph
these spectra with results of a study of Cs-covered GaAs (110),
Gregory and Spicer assert that adsorbed atomic hydrogen behaves like
an alkali metal adsorbed on GaAs. Although room temperature
adsorption of molecular hydrogen is not observed, a 3 x 10^ L
exposure to H2 with the sample heated to about 415 C results in
adsorption. UPS shows that this adsorbed hydrogen is different than
the atomically adsorbed hydrogen. Heating the sample to about 700 C
does not result in an EDC equivalent to an EDC from a clean
surface. Gregory and Spicer suggest that a chemical compound may
form or that hydrogen diffuses into the GaAs.


42
1 pr
Mokwa et al. used TPD to investigate the interaction between
both molecular and atomic hydrogen and a cleaved GaAs(110) surface.
They found no interaction between H2 and the GaAs surface at 250 K.
However, atomic hydrogen adsorbs readily. Heating causes desorption
of AsH^ at 340 K, H2 at 450 K and Ga atoms above 700 K.
Luth and Matz D used high resolution electron energy loss
spectroscopy (HREELS) to study atomically adsorbed hydrogen and
deuterium on both n- and p-type GaAs(110). They assigned loss
features to H and D bonded to both Ga and As atoms and were able to
construct Morse functions which adequately describe the electronic
potential energy curves and a rate expression which describes thermal
desorption of molecular hydrogen. Another HREELS study by Dubois and
Schwartz also identifies hydrogen atoms bonded to both Ga and As
atoms on a (100) surface of chromium-doped GaAs. A lengthy anneal
forms a Ga-rich surface, and the HREELS peak due to H bonded to As
disappears. Electron energy loss spectroscopy (ELS) studies by
Bartels et al. and Moneh found that hydrogen atoms initially
bond to the As. Several studies11>128,129 agree that a dipole
or depletion layer forms at these surfaces upon adsorption of atomic
hydrogen.
The polar (100) surface of GaAs is more complex than the (110)
cleavage surface because it reconstructs in numerous ways resulting
in a variable ratio of surface Ga-to-As atoms. The relationship
between structure and composition has been examined1^*131 finding
that the As coverage varies from about 0.27 to 1.00 as the surface


43
reconstructs with LEED patterns: (4x6), (1x6), c(8x2), c(2x8) and
c(4x4). Bringans and Bachrach1^2~1^5 have USed angle-resolved UPS to
study atomically adsorbed hydrogen on GaAsOOO) and (111). They
found that this surface always exhibits the same LEED pattern
regardless of the original structure of the clean GaAs(IOO) and (111)
surfaces before adsorption. The hydrided structure of GaAs(IOO) lies
between the c(4x4) and c(2x8) structures, and annealing always yields
the c(2x8) pattern. The GaAs(lll) surface reverts to a (1x1)
symmetry after a saturation exposure to hydrogen. The authors
conclude that atomically adsorbed hydrogen on a GaAsOOO) surface
results in an As-rich surface which does not agree with the results
of Dubois and Schwartz.12"^ The UPS results show that adsorption of
atomic hydrogen removes states from near the top of the valence band
and introduces new states 4.3 to 5.2 eV lower. A surface state
exists on this surface, and the fact that it does not disperse in
the T -* K direction suggests that it does not participate in bonding
with the hydrogen.
Friedel and Gourrier 7 demonstrated that exposing a GaAs
surface to a hydrogen plasma is an effective way to remove carbon and
nitrogen contamination but is somewhat less effective for removing
1 ?0
oxygen. Chang and Darack have shown that a high frequency
hydrogen plasma can easily etch GaAs at a rate of about 20A/sec and
that this rate is proportional to both the hydrogen pressure and the
r.f. power of the discharge. A surface study of the effects of
exposing a GaAs(001) surface to a hydrogen plasma has been reported


44
1 21
by Friedel et al. In this study variations in the surface
structure, high lying core levels of As and Ga and valence band were
examined using angle-resolved UPS and reflective high energy electron
diffraction (RHEED) as a function of exposure to the plasma. By
curve resolving the As 3d and Ga 3d levels, they found that hydrogen
induces a new peak on the Ga spectrum and possibly two on the As
spectrum. They also believe that hydrogen initially bonds to the As
atoms and then to the Ga atoms at higher exposures. The Ga-to-As
ratio increases with plasma exposure, and the surface becomes more
disordered.
Many surface techniques including XPS, UPS, ELS and ISS are
relatively insensitive to hydrogen or can observe hydrogen only
indirectly through its influences on other spectral features. This
is usually due to its small cross section, light mass or lack of
core-level electrons. HREELS is an example of a technique in which
features due specifically to hydrogen are observed. Techniques which
rely upon a mass spectrometric determination of hydrogen are also
directly sensitive to hydrogen. This has been demonstrated in an
early study by Madey and Yates^6 in which they found ESD of H+ from
a cleaved, p-type GaAs(110) surface. H+ desorbs from both the
freshly cleaved surface and from a cesiated surface. They attributed
the presence of this hydrogen to adsorption of background hydrogen.
In the present study the usefulness of ESD in studying the
interaction of hydrogen with GaAs(100) and oxidized GaAs(100) was
investigated. This represents an initial attempt to use ESD to


45
examine hydrogen on GaAs surfaces in which the following questions
are considered. Under what conditions does hydrogen adsorb on GaAs
surfaces? Can ESD provide a measure of the amount of adsorbed
hydrogen? Is it possible to distinguish between different adsorbed
states of hydrogen using ESD? The preliminary results presented here
demonstrate that ESD is a useful technique for studying the
interaction of hydrogen with GaAs and, most probably, with other III-
V materials.
Experimental
A cut and polished, undoped single-crystal GaAs sample furnished
by MACOM was used in this study. It was produced using the liquid-
encapsulated Czochralski crystal growth method. A room temperature
chemical etch with a weak sodium hypochlorite solution was used to
produce a mirror finish of the (100) surface (2). After solvent
cleaning the sample in an ultrasonic bath of electronic grade
trichloroethylene followed by acetone and then methanol, the sample
(10 x 16 x 0.5 mm) was mechanically supported by a copper holder. A
0.5 mm diameter tungsten filament was mounted behind the sample for
radiant heating. The temperature of the sample was measured using an
iron-constantan thermocouple, which was mechanically pressed against
the sample, and an optical pyrometer was used over the range of 230
to 560 C. After inserting the sample, the vacuum system was baked
for 24 hours at 200 C and pumped down to a base pressure of 2 x 10~
Torr. The sample was cleaned using cycles of room temperature,


46
argon ion bombardment followed by annealing (IBA). Specific
conditions used were an Ar ion sputter at a beam energy of 1.0 keV
and a beam current of 1 pA over an area of approximately 2 mm in
diameter for 15 minutes, a 5 minute anneal at 500 C, an Ar ion
sputter at a beam energy of 1 .0 keV and a current of 1.5 pA for 30
minutes concluding with a 500 C anneal for 5 minutes. This
procedure results in a clean surface as determined by AES. AES
spectra taken from the contaminated and clean sample are shown in
figure 4-1, curves (a) and (b), respectively.
The vacuum system used in these experiments has been described
previously.1 AES and ESD were performed using a double-pass
cylindrical mirror analyzer (CMA)(PHI Model 15-255 GAR) equipped with
an internal coaxial electron gun and an angularly resolving movable
aperture. AES spectra were taken in the nonretarding mode using an
oscillation of 10 kHz and 0.5 V peak-to-peak, a primary beam energy
of 3 keV and a current of 25 pA over a spot size of approximately 0.1
mm in diameter.
ESD data were taken using the CMA as a time-of-flight mass
6Q
spectrometer as discussed by Traum and Woodruff. J Electronic
control and data collection were performed using a PDP 11/02 computer
system and a computer-interfaced digital pulse counting circuit
developed by Gilbert et al. In these experiments energy analysis
of low energy (less than 20 eV) positive ions was accomplished by
operating the CMA in the retarding mode with a pass energy of 80
eV. The ions were first accelerated by a negative potential placed


KINETIC ENERGY [eV]
Figure 4-1. AES spectrum (a) of contaminated GaAs(lOO) taken after
solvent cleaning and (b) taken after the IBA cleaning
procedure.


48
on the second grid and inner cylinder. The outer cylinder was
maintained at a more positive potential (but negative potential with
respect to ground) than the inner cylinder. The complete set of
operating voltages has been published previously.^0 For mass
analysis the electron beam was pulsed onto the sample for 300 ns
using the electron gun deflector plates in the CMA. After an
appropriate delay period (4.2 ys for H+, 5.8 ys for D+ or l6.8ys for
0+) pulses were counted for a 300 ns period. A primary beam energy
of 140 eV and primary beam current of about 500 nA were used. The
sample was biased at +10.0 V. It was mounted at an angle of 45 with
respect to the electron beam, and ions desorbing in the direction
normal to the sample were selected with the movable aperture.
Three different types of ESD spectra were taken in this study.
A mass spectrum of the desorbing ions was obtained by selecting an
ion kinetic energy and scanning the delay time. An ESDIED spectrum
was obtained by selecting a delay time appropriate to the ion of
interest and scanning the kinetic energy with the CMA. The total-ion
energy distribution was obtained by scanning ion kinetic energy
without time gating.
Results and Discussion
In ESD electrons impact a surface creating localized excitations
which can decay causing desorption of ions, neutrals and metastable
species. Thus ESD is a destructive process in that it alters the
composition and structure of the surface. Beam exposures were


49
minimized in these experiments to reduce surface damage while still
obtaining acceptable signal-to-noise ratios. In many cases a second
spectrum was taken, and since in each case this duplicates the
initial spectrum, it is assumed that effects due to beam damage are
negligible.
ESD is a complicated technique in that it can be performed in
many different ways to obtain various types of information about
bonding at a surface and/or the mechanism of excitation and
desorption. These topics have been discussed in a recent review
70
article' and are only considered when they are directly related to
the results presented here. Several points regarding ESD which do
relate to this study are its surface sensitivity, the isotope effect
and cross section determination.
In ESD a low energy ion can desorb only from the top mono-
layer. Ions which are created below the top monolayer have a nearly
100 probability of being neutralized during the desorption
process. However, excitations created beneath the surface can emit
secondary electrons which can create excitations at the surface
causing emission of ions.^1 Thus, ESD is highly surface sensitive
(outermost monolayer) with respect to the emitted ionic signal but
not so surface specific with respect to the primary electron beam.
The ESD mass spectrum shown in figure 4-2 was taken from the
contaminated surface corresponding to figure 4-1(a). The peak at 4.2
ys reveals the presence of hydrogen, but ESD peaks due to positive
ions containing carbon or oxygen do not appear although the surface


50
FLIGHT TIME [is]
Figure 4-2. ESD mass spectrum taken from the contaminated
surface corresponding to figure 4-1(a). All ESD
spectra were taken using a primary beam energy
of 140 eV.


51
is heavily contaminated with these species. After IBA cleaning the
ESD mass spectrum appears the same except that the total count due to
H+ is reduced by a factor of 8 using the same data collection
parameters. This indicates that hydrogen is present as a surface
species even on a "clean" GaAs surface. Drawing an analogy between
O-
GaAs and Si, which readily chemisorbs hydrogen and forms a hydride
li 72
' may be useful in understanding the interaction between hydrogen
and GaAs. GaAs is both isoelectronic and isostructural with Si
except GaAs has a slightly larger lattice constant and its bonding
structure is less covalent. These facts suggest that a hydride
compound of GaAs may form and support the supposition of Gregory and
Spicer J that hydrogen diffuses into GaAs forming a compound.
The surface concentration of hydrogen present in these
experiments is not known. In order to determine the surface
concentration of hydrogen using ESD, it is necessary to know the
total cross section for desorption. ESD cross sections of adsorbed
species vary over a large range but generally are orders of magnitude
smaller than the corresponding gas phase cross section due to the
high reneutralization probability of the escaping ions. However, an
independent measurement of the surface concentration is required to
determine the ESD cross section. As pointed out above, this is
difficult with hydrogen. An attempt was made to use TPD to estimate
the amount of hydrogen and deuterium present, but the sample mount
was too massive to perform high-quality TPD. A very crude estimate
from the TPD results suggests that tenths of a monolayer of deuterium


52
are adsorbed during the dosing procedure used. Efforts are being
made to improve the quantification of these results.
Regardless of the cleaning procedure used, hydrogen is always
present on the surface. It is possible that the surface hydrogen
originally present is not completely removed, that the surface
adsorbs background hydrogen during the cleaning process or that
hydrogen in the bulk diffuses to the surface during annealing. In
order to distinguish between surface hydrogen which is always present
and hydrogen which is adsorbed under particular conditions, deuterium
was used in these experiments. It was verified that molecular
deuterium does not adsorb on GaAs at significant rates at room
temperature. Heating the sample radiantly is known to dissociate D2
which effuses to the surface even when the filament is not in line-
of-sight of the surface.It is assumed in these studies that the
adsorbed deuterium is due to adsorption of dissociated deuterium but
the adsorption of some molecular deuterium cannot be ruled out. The
dose rate of deuterium atoms in these studies is only known in a
relative sense.
Figure 4-3 shows an ESD mass spectrum taken after exposing the
clean surface to a 2000 L dose of D2 while maintaining the sample at
200 C. In addition to the H+ peak, which is the typical height from
a clean surface, a peak due to D+ appears at a flight time of 5.8
ps. Determining the relative amounts of adsorbed hydrogen and
deuterium from this data is not straightforward because ESD exhibits
an isotope effect which is particularly large for hydrogen. The


53
(O

z
3
O
O
<
I-
o
H
5 10 15
FLIGHT TIME [ps]
20
Figure 4-3. ESD mass spectrum taken after exposing the clean
GaAs surface corresponding to figure 4-1(b) to a
dose of 2000 L of D2 while heating the sample at
200 C.


54
desorption cross section is proportional to exp (-c/M) where c is a
constant and M is the mass of the desorbing ion.1^ An isotopic ion
of higher mass leaves the surface with approximately the same kinetic
energy as a lower mass isotopic ion. Thus, the heavier ion desorbs
with a lower velocity and has a greater probability for
reneutralization. This has been verified experimentally. The ratio
of the ionic desorption cross sections for H+ and D+ desorbing from a
tungsten surface is greater than 100:1.1391By this reasoning
figure 4-3 suggests that the surface concentration of deuterium is
greater than the surface concentration of hydrogen by a factor of
approximately 50.
The total-ion energy spectrum and ESDIED spectra of H+ and D+
are shown in figures 4-4(a), (b) and (c), respectively. Due to the
similarities of figures 44(b) and (c), it is believed that hydrogen
and deuterium are desorbing from the same adsorbed state by the same
mechanism. Unfortunately, it is not known if hydrogen desorbs from
states bound to Ga or As atoms or both. The spectra were taken in
the order (c), (b), (a). Comparing the high energy tails of these-
spectra suggests that with increased exposure to the primary beam, a
higher kinetic energy state may appear or that hydrogen is initially
present in another state. This is seen most readily by comparing the
widths of the H+ and D+ spectra with the width of the total-ion
spectrum. This second state would have a lower desorption cross
section than the more prominent state or it would be less densely
populated and, therefore, would require a larger beam exposure to
build up a significant count.


ION KINETIC ENERGY [eV]
Figure 4-4. ESD spectra corresponding to the mass spectrum shown in figure 4-3
(a) Total-ion (H+ and D+) energy distribution spectrum and ESDIED
spectra of (b) H+ and (c) D+.
Ln
Ln


56
Following the 2,000 L D2 dose at 200 C, the sample was heated
to 500 C for 10 minutes. ESDI ED spectra taken before and after
heating are quite similar in shape. However, after heating the D+
signal of the ESD mass spectrum is reduced by a factor of 2 with
respect to the H+ signal which essentially remains unchanged. This
result agrees with the results of Gregory and Spicer1^3 which
adsorbed hydrogen could not be totally removed from the surface after
a 3 x 10' L exposure to molecular hydrogen with the sample heated at
415 C. As stated above, it is possible that both molecular and
atomic deuterium adsorb during dosing at elevated temperatures in
this study. It was also found that dosing the sample at 400 C
rather than 200 C results in spectra identical to those shown in
figures 4-3 and 4-4.
In order to examine the influence of surface oxygen on the
interaction between hydrogen and GaAs, the clean surface was oxidized
by dosing with 12,000 L of O2 at room temperature. The resulting AES
spectrum is shown in figure 4-5. Again, an ESD mass spectrum taken
after a 2,000 L dose of D2 at room temperature shows no evidence of
deuterium adsorption. However, heating the sample above 200 C while
dosing causes adsorption of deuterium. The H+ and D+ counts are
about 50% greater from the oxidized surface than from the clean
surface, and the ratio of D+ to H+ is similar for both surfaces. It
was also found that deuterium adsorbs more readily on a freshly
oxidized surface than on a vacuum-aged (1 week at 10-10 Torr)
oxidized surface. The normalized H+ count from a vacuum-aged surface


(arbitrary units)
57
I "P I 1 I 1 I 1 I 1 I 1T1 I 1 P 1 1 I 1 I 1 I 1
500 900 1300
KINETIC ENERGY [eV]
Figure 4-5. AES spectrum taken from GaAs(lOO) after a dose of
12,000 L of O2 at room temperature.


58
is 3 times as large as the count from a freshly oxidized surface.
Possible explanations for this behavior are that an oxidized surface
adsorbs water molecules during a long exposure to the background gas
in the UHV system or that the ion gauge tube filament dissociates
background hydrogen which then adsorbs at the surface.
ESDI ED spectra of the oxidized surface after exposure to 2,000 L
of D2 while heating the surface at 200 C are shown in figure 4-6.
The total-ion energy spectrum shown in figure 4-6(a) clearly reveals
the presence of a high kinetic energy shoulder. The presence of two
states is exibited by figures 4-6(b) and (c) for H+ and D+
respectively. However, the relative ratios of ions from the two
states are different for hydrogen and deuterium, and the ratios vary
as a function of beam exposure time. ESD mass spectra taken at ion
kinetic energies of 0.8 and 3.55 eV are shown in figures 4-7(a) and
(b) respectively. This figure shows that the deuterium-to-hydrogen
ratio is a function of ion kinetic energy. The difference in
concentration ratios is consistent with the interpretation that at
least two different binding states of hydrogen are present. The
precise nature of these states remains to be investigated.
1 4?
Vasquez et al. have suggested that hydroxyl groups on GaAs
surfaces decompose according to the reaction
2Ga(OH)3 -* Ga203 + 3H20
(4-1)


ION KINETIC ENERGY [eV]
ESD spectra taken after exposing the oxidized surface corresponding
to figure 4-5 to 2000 L of D2 while heating the sample at 200 C.
(a) Total-ion (H+ and D+) energy distribution spectrum and ESDIED
spectra of (b) H and (c) D+.
Figure 4-6.


. ESD mass spectra taken (a) at an ion kinetic energy of
0.8 eV and (b) at an ion kinetic energy of 3.5 eV.
O
Figure 4-7


61
at elevated temperature thereby reducing the oxygen content in the
surface region. This was considered in this study. It was found
that heating the deuterated, oxidized surface to 500 C results in a
very large decrease in the ESD D+ signal. Also, monitoring the
desorbing species with a quadrupole mass spectrometer while
increasing the sample temperature shows that D2, HD and H2 desorb and
that no H20 or other oxygen-containing species desorb. If hydroxyl
groups form under the dosing conditions used in this study, then
heating causes them to form molecular hydrogen leaving the oxygen on
the surface. This observation is also consistent with the fact that
surface oxygen on GaAs is more resistant to removal by a hydrogen
plasma 7 than other contaminants.
Conclusions
AES and ESD have been used to study the interaction between
deuterium and both clean and oxidized GaAs(100). Cleaning was
performed using IBA cycles, but it is not possible to remove the
surface hydrogen completely. Molecular deuterium does not adsorb on
either surface at significant rates at room temperature, but heating
the samples radiantly between 200 and 400 C with a hot filament
causes adsorption of deuterium. A partially successful attempt to
use TPD to quantify the amounts of adsorbed deuterium indicates that
a few tenths of a monolayer probably adsorb during the dosing
period. Taking the ESD isotope effect into account, it is estimated
that the amount of adsorbed deuterium is about 50 times the amount of


62
residual hydrogen. Energy analysis of the desorbing ions indicates
that at least one and possibly two adsorbed states are present on the
clean GaAs(100) surface, but the nature of the adsorption states is
not known. Heating the sample at 500 C reduces the amount of
adsorbed deuterium. It is presumed that most desorbs, but some may
diffuse into the bulk.
The ESD H+ and D+ signals from the oxidized surface are about
50% greater than those from the clean surface after dosing under the
same conditions. ESDIED clearly demonstrates that at least two
adsorption states of hydrogen are present on the oxidized surface.
Heating the sample to 500 C causes desorption of D2, HD and H2, but
no oxygen or oxygen-containing species desorb from the deuterium-
dosed, oxidized GaAs(100) surface.
Although much remains to be done in characterizing the
interaction of hydrogen with GaAs surfaces, and more generally with
III-V semiconductor surfaces, the results presented in this chapter
demonstrate that ESD provides a very useful technique for this type
of study. It is now important to quantify the ESD results more
accurately and to characterize the adsorbed states of hydrogen. Both
are possible through the combined use of other surface techniques
with ESD.


CHAPTER V
A CHARACTERIZATION STUDY OF THE NATIVE OXIDE
LAYER FORMED ON CHEMICALLY ETCHED InP(111)
Introduction
InP is becoming an important III-V semiconductor material for
fabrication of semiconductor devices.20-23 Small changes in the
composition of InP surfaces can significantly influence the
electronic structure at the surface.1^ Thus, the properties of
epitaxial layers and, consequently, of the final devices are
determined by the surface of the InP substrate and any oxide layer
which may be present. For this reason it is important to
characterize native oxide layers on InP substrates. Often the native
oxide layer is completely removed by polishing procedures or is
partially removed during chemical etching steps. Characterization of
the additional oxide formed upon exposure to air or formed by various
methods of oxidation is also important.
Numerous studies of the oxidation of InP have been performed in
order to investigate the growth mechanisms, surface compositions and
oxidation states of the surface species using various methods of
1 ill! 1 IK
oxidation such as thermal oxidation, high-pressure thermal
146 147
oxidation, photoenhanced thermal oxidation, photoenhanced
14 1 iiQ
oxidation, chemical oxidation 7 and electron-stimulated
1 SO
oxidation. ^ The effects of etchants on InP surfaces and oxide
1 Cl _1 Cl
layers have also been investigated J J as have the effects of
sputtering,155157 annealing1^_1and exposure to vacuum.1161
63


64
XPS, SIMS and X-ray diffraction were used by Nelson et al.1^ to
study the thermal oxides of p-type InP(100) formed in -1/3 atm of
02. They suggest that for a temperature in the range of 400 to 600
C an outer layer of In2C>2 and InPO^ is formed and an inner layer of
primarily InPOjj is formed. Elemental P is observed at the oxide/bulk
interface. For T>650 C the oxide is composed of InPO^. Yamaguchi
1 46
and Ando report that a polycrystalline oxide composed of InPOjj
(IngO^+PgO^) forms during a thermal oxidation (450-500 C) of an
undoped InP(100) surface in dry oxygen (0.2-2 atm). At higher
temperatures the oxide is mostly IngO^. Infrared absorption and
electron diffraction data suggest that for T>620 C the oxides become
In rich and lower oxides (possibly ln20) form due to the loss of
146
phosphorous. Wilmsen et al. find that bulk oxides consisting of
InPOi,, ln20^ and P20cj form at elevated temperature (450 C) and high
oxygen pressures (42-500 atm). Faithpour et al.1^ used XPS and SEM
to investigate the laser-enhanced thermal oxidation of n-type, semi-
insulating InP(100) in a N20 atmosphere. They found that In^^ and a
phosphate species form and that the growth rate is a function of the
light intensity, N20 pressure and temperature. The phosphate layer
thickness is believe to be limited by the outwardly diffusion of
144
phosphorus, which has also been suggested by Nelson et al. The
photon-stimulated oxidation of cleaved InP(110) studied by Koenders
148
et al. results in the formation of a depletion layer, but the fact
that no surface dipole is introduced suggests that oxygen is
incorporated into the lattice rather than adsorbed at the surface.


65
Exposure to oxygen with and without photon stimulation results in
depletion of phosphorus at the surface in addition to formation of
indium oxide. The chemical oxidation of n-type InP(100) in hot
nitric acid also reveals a depletion of surface phosphorus as
1 il Q
discussed by Korotohenkov et al. Annealing and exposure to vacuum
increases the amount of phosphorus in the surface region. Electron-
stimulated oxidation of n-type InP(100) was investigated by Oliver et
al. J using AES, LEED and ELS. ^20^ and a volatile phosphorus
oxide form in the electron irradiated region. Heating above 500 C
causes the formation of small droplets of elemental indium. Clark et
al. J used angle-resolved XPS to investigate the oxide layers and
hydrocarbon contamination of InP surfaces after numerous
treatments. The as-received samples were found to be phosphorus
rich, but various treatments result in substantial differences in the
composition of the surface region. Bertrand1^3 concludes that the
use of different etchants causes the amount of oxidized phosphorus on
n-type InP(100) surfaces to vary. However, significant amounts of
oxidized indium were found only on a sample which was heavily
oxidized in air. Oxides formed in air after chemical etching with
1 6?
Br-MeOH were studied by Wager et al. The as-etched surface is
only slightly oxidized and is primarily a phosphorus oxide.
Thus far, most of
the InP
studies have focused
on
the (100) and
(110) surfaces with
only
a few studies
of
the (111)
surface.'"5,5",62-65
This
may be due to
the
difficulties
encountered in obtaining a mirror finish using conventional etching


66
1 Sil
procedures. J In this study angle-resolved XPS, AES and ISS are
used to examine the oxide layer formed on chemically etched InP(111)
by exposure to air.
Experimental
An undoped InP(111) face cut to 1 was furnished by Sumitomo
Electric Industries, Ltd. for this study. The sample was grown by
the liquid encapsulated Czochralski method and was polished using a
mixture of ZrC^ and NaOCl and then a mixture of SiC>2 and NaOH. The
sample was then etched with H^PO^HBr (2:1) to produce a mirror
finish. The electrical properties and etch pit density are: carrier
concentration < 101^cm3 (n type), resistivity at room temperature =
1.8-1.9 x 101 ohm-cm, mobility = 3.94.0 x 10^ cm2V-^ s-1 and
c _p
E.P.D. < 1Crcm The as-received sample was cut using a wafer saw
to 21 x- 15 x 0.35 mm and was ultrasonically cleaned for 5 minutes in
each of the following electronic-grade solvents: trichloroethylene,
acetone, methanol and then rinsed in deionized water.
A Perkin-Elmer PHI model 570 analytical system with a base
pressure of 2 x lO-1^ Torr was used in this study. The data was
collected at Perkin-Elmer Physical Electronics in Eden Prairie, MN.
A double-pass cylindrical mirror analyzer (CMA) with an internal
electron gun and angularly resolving movable aperture was used to
collect AES, XPS and ISS data. AES spectra were taken using a
primary beam energy of 5 keV and a beam current of 0.3 pA rastered
over an area of 2 x 2 mm. XPS spectra were taken using a Mg anode at


67
a voltage of 14 kV and power of 300 watts. High resolution XPS
spectra were obtained using a pass energy of 25 eV which resulted in
a total energy resolution of 0.75 eV. The high resolution XPS
spectra were taken in the angle-integrated mode and angle-resolved
mode at take-off angles of 10 and 85 with respect to the surface
plane. ISS spectra were obtained using a 1 keV beam of ^He+, a
scattering angle of 144 and the movable aperture for improved
resolution. For all spectra taken, the sample was placed at an angle
of 50 with respect to the CMA normal.
In this study nondestructive techniques were used before
destructive techniques. Thus, the techniques were used in the
following order: XPS, angle-resolved XPS, AES, ISS and depth
profiling using argon-ion sputtering. The results are not presented
in this order in the following section.
Results and Discussion
Figure 5-1 shows the ISS spectrum taken from the air-exposed
InP(111) surface. Three peaks are discerned corresponding to In, P
and 0, and the In peak is much larger than the other two peaks. ISS
is essentially outermost layer sensitive for two reasons. The first
is that ions which penetrate beneath the top layer have a very high
probability of being neutralized, and the second is that even if they
are not neutralized, they usually scatter inelastically during escape
from the solid. In either case ions scattering off subsurface atoms
do not contribute to the ISS peaks. Inelastically scattered ions


N(E) (arbitrary units)
Figure 5-1. ISS spectrum of air-exposed InP(lll).


69
comprise the background and, hence, the magnitude of the background
depends upon the electron mobility in the near-surface region. Based
on previous ISS studies, it appears that electrons in the surface
region of this oxide layer are not highly mobile since the background
is fairly large. The elastic scattering cross section increases as
the mass of the surface atom increases. Even though In is heavier
than P, the size of the In peak indicates that the outermost layer
consists predominantly (>80) of In atoms with roughly equal amounts
of 0 and P atoms. No feature due to surface C appears.
Figure 5-2 shows an angle-integrated AES spectrum of the same
1 f. ii
surface. An intensity analysis using standard methods yields an
In/P atomic concentration ratio which is slightly greater than 1 but
much less than the In/P ratio obtained in ISS. This is due to the
fact that AES is much less surface sensitive than ISS. The Auger
signal is an exponentially weighted average from electrons
originating within about three mean-free paths of the sample
surface. Since ISS reveals that the outermost layer is mostly In and
AES shows that the outermost few layers contain approximately equal
amounts of In and P, the one- or two-layer region below the outermost
region must be P rich. Both C and 0 appear in this spectrum in
approximately equal concentrations even though no C is detected by
ISS at the surface. High resolution AES spectra of the dominant In
peaks at a kinetic energy of about 400 eV are shown in figure 5-3.
Spectrum (a) was taken at an exit angle of 10 and spectrum (c) was
taken at an exit angle of 85. Spectrum (b) is angle integrated.


(ARBITRARY UNITS)
400 1000 1600
KINETIC ENERGY (eV)
Figure 5-2. Angle-integrated AES spectrum of the same surface
shown in figure 5-1.


N(E) (ARBITRARY UNITS)
71
Figure 5-3. AES spectra of the In 11^ features taken from
the same surface shown i figure 5-2. Spectra (a)
and (c) were taken at exit angles of 10 and 85
respectively. Spectrum (b) is angle integrated over
all exit angles.


72
These data were taken in the pulse-counting mode and are
appropriately referenced so that the energy values are accurate. The
spectrum shown in (c) is quite similar in shape to that of In
I gc;
metal J and is taken to be representative of InP since most of these
electrons are originating deeply below the surface. The kinetic
energy of the transition is MOO.6 eV which is about 2.0 eV
smaller than values reported for In metal.1^1^ This value is also
about 0.6 eV larger than values reported for IngO^1^ and agrees with
the value found by Clark et al.1^1 for InP. Spectrum (a) is much
broader and less well defined in comparison with spectrum (c). Based
on mean-free path estimates, most of these electrons originate within
10A of the surface since the exit angle is only 10. It appears that
another set of peaks due to another In species is contained in
spectrum (a). The kinetic energy of the transition for this
species is about 397.1 eV. It is argued below that this species is a
phosphate and probably InPO^ rather than InPO^. Even though the
outermost layer is mostly In, it is probably bonded as InPO^ which is
present in an amount almost as large as the amount of InP in the
near-surface region. Spectrum (a) is similar to the high-resolution
AES spectrum of the primary In features of an InP(100) surface which
was etched in a 20? solution of Br2/HBr/H20 (1:17:35).1^1
Figures 5-4 and 5-5 show high resolution XPS spectra of the P
2p, In 3d, C 1s and 0 1s peaks at exit angles of 10 and 85. A very
large difference is seen between spectra taken at 10 and 85 for the
P 2p peaks. The major P 2p feature observed at 129 eV in figure 5-


N(E) ARBITRARY UNITS
BINDING ENERGY (eV)
Figure 5-4. XPS spectra of the (A) P 2p and (B) In 3d peaks taken at exit angles of
(a) 10 and (b) 85.


N(E) (ARBITRARY UNITS)
BINDING ENERGY (eV)
Figure 5-5. XPS spectra of the (A) C Is and (B) 0 Is peaks taken at
exit angles of (a) 10 and (b) 85.


75
(b) is assigned as phosphorous in InP. This is in agreement with
other studies '* J and the value assigned to P 2p in
GaP.1^ At the surface-sensitive exit angle of 10 shown in figure
5-4A(a), another P 2p peak appears at a binding energy of 133.7 eV
which is nearly as large as the P peak due to InP. This is assigned
as a phosphate species and probably is InPO^ (or InPOjj), but it is
difficult to rule out the presence of hydrogen-containing phosphate
species. The assignment as InPO^ seems somewhat more probable than
InPOjj based on an analogy with NaPO^ or (NaPO^)^ and Na^POjj which
have binding energies of 134, 133.5 and 132.2 eV respectively.1^ a
study of the In-P-0 phase diagram by Schwartz et al.1^ also suggests
that InPO^ forms at low temperatures while InPO^ is stable at
elevated temperatures 0650 C). Based on mean-free path arguments,
it is estimated that about 95% of the electrons collected are emitted
within 50A of the surface in the 85 exit-angle spectra and within
10A of the surface in the 10 exit-angle spectra. Thus, the spectra
shown in figure 5-4A imply that the near-surface region is composed
of InP and InPO^ in similar amounts and that the bulk is InP. The In
3d spectra shown in figure 5 hB support this hypothesis. The
spectrum shown in (b) is characteristic of In in InP at a binding
energy of 444.7 eV. The surface-sensitive spectrum shown in figure
5-4B(a) contains another In species which is assumed to be a
phosphate at a binding energy of about 445.5 eV. This is based on
the information about P contained in figure 5~4A(a). In in In202 has
a 3d binding energy which is about equal to that of In in InP so it


76
is not possible to determine if is present from the spectra
shown in figure 5-4.
The C 1s and 0 1s XPS features are shown in figure 5-5A and B
respectively for exit angles of (a) 10 and (b) 85. The C 1s peak
lies at a binding energy of 285.2 eV, in agreement with Hollinger et
al. DO and is most likely in the form of graphite or hydrocarbons.
It should be noted that this binding energy is about 0.6 eV higher
than the commonly reported reference values for these species. 0
Therefore, selecting carbon as the reference for peak energy
assignments at a binding energy of 284.6 eV would shift all the
values reported here to lower binding energies by 0.6 eV. The C 1s
peak shown in figure 5~5A(b) is broader than that shown in (a) so
more than one form of C may be present. It is possible that
discrepancies in the C 1s peak position are due to charging
effects. However, these are believed to be minimal for the sample
used in this study due to its relatively high conductivity and the
thinness of the oxide layer.
XPS 0 1s peaks are shown in figure 5~5B. These peaks are
discussed assuming that they are composed of a peak due to phosphate
at a binding energy of 532.0 eV and a peak due to ^20^ at a binding
energy of 530.5 eV. This seems reasonable with respect to both the
144 147 152
results of this analysis and previous studies. The
surface-sensitive spectrum shown in figure 5_5B(a) is almost entirely
due to phosphate. However, the presence of a very small shoulder on
the low binding energy side, which is difficult to discern in the


77
figure, may indicate that a very small amount of In202 is present in
the near-surface region. The more bulk-sensitive spectrum shown in
figure 5~5B(b) shows a very slight shift to lower binding energy.
This may indicate that the concentration of is greater with
respect to the phosphate at increasing depth below the surface, but
the majority of oxygen-containing species throughout the oxide layer
is phosphate.
Angle-resolved valence band XPS spectra are shown in figure 5-
6. The features of the bulk-sensitive spectrum shown in figure 5-
6(b) are in complete agreement with those shown in the spectrum
1 f¡Q
presented by Vesely and Kingston ^ for clean InP. Vesely and
Kingston found good agreement between the experimental spectrum and
the theoretical results of Stukel et al. and Kramer et al. 1 The
characters of the peaks are assigned as: 3 eV In 5s, 7 eV 0 2p
and 9.7 eV P 3p, and other peaks which are not shown are assigned
as: 14 eV P 3s, 18 eV In 4d, 23 eV 0 2s.166172 The surface-
sensitive spectrum shown in figure 5-6(a) is quite similar to the
spectrum shown in figure 5~6(b) in the 5 to 12 eV region. However,
the broad feature at about 3 eV due to In 5s electrons is absent in
the surface-sensitive spectrum. This implies that In is bonded
differently in InP than in InPO^.
Depth profiles for In, P, 0 and C are shown in figure 5-7.
These were obtained by performing angle-integrated AES while sputter
etching with argon ions. The atomic concentrations were determined
using Auger sensitivity factors. However, due to possible matrix


N(E) (ARBITRARY UNITS)
78
BINDING ENERGY (eV)
Figure 5-6. Angle-resolved valence band XPS
spectra taken at exit angles of
(a) 10 and (b) 85.


SPUTTER TIME (min)
Figure 5-7. Angle-integrated AES compositional profile obtained
by argon-ion sputtering.


80
effects and sputtering artifacts, this figure shows only trends in
the concentrations. The profiles are shown as a function of sputter
time, but from the angle-resolved XPS results, it is estimated that
the sputtering rate was about 50A/min. The In/P ratio is greater
than 1 at the surface, but the large ratio found using ISS is not
observed since angle-integrated AES is less surface sensitive than
ISS. The P concentration increases rapidly suggesting that the
region below the outermost layer is P rich. The In, P, C and 0
concentrations all change rapidly in the oxide layer, and a sharp
interface is observed at a sputtering time of 0.5 minutes. Past this
interface only In and P appear. The layer beneath the oxide is P
rich, but the concentration of In increases while the concentration
of P decreases. Although the In and P concentrations change slowly
in this region, it appears that a composition corresponding to InP
may be approached.
Conclusions
The oxide layer which forms on a chemically polished and etched
InP (111) surface upon exposure to air is very thin (estimated to be
about 30A thick) and nonhomogeneous. ISS shows that the outermost
layer is mostly In and AES combined with the ISS results suggests
that the region just below the surface layer is P rich. Indium is
believed to exist in the near-surface region as InPO^ and InP in
similar concentrations, but a very small amount of ^20^ is detected
in the subsurface region. Carbon is detected throughout the native


81
oxide layer, and XPS shows that it is present in graphitic or
hydrocarbon forms. Depth profiling using Ar+ and AES shows that the
oxide/InP interface is sharply defined. The region below the oxide
layer is P rich, but the composition appears to be approaching that
of InP as the sputtering time increases.


CHAPTER VI
GENERAL CONCLUSIONS AND
RECOMMENDATIONS FOR FUTURE RESEARCH
Hydrogen is found to be present on cleaned and oxidized Si(100)
and GaAsOOO) as well as cleaned and nitrogen-exposed Si (111). The
source of the surface hydrogen detected using ESD is the bulk of the
semiconductor which acquires hydrogen during growth, polishing and/or
cleaning processes. It would be helpful to substitute deuterium
where possible in the formation of the polished single crystal wafers
to determine the step(s) during which hydrogen is incorporated.
ESD is one of the relatively few surface analytical techniques
which detects hydrogen directly. Energy analysis of the desorbing
ions is very useful in distinguishing between different surface
binding sites of hydrogen. It was found that multiple states of
adsorbed hydrogen are present on cleaned and oxidized Si(100). The
ESD study of the Si(100) surface also revealed that the sample
treatment can dramatically alter the energy distribution of the
desorbing hydrogen ions. Electron beam exposure results in a
depopulation of higher energy desorption states which are repopulated
upon annealing. Oxygen exposure results in a more marked
depopulation of the higher energy desorption states. It was also
found from energy-resolved, angle-resolved ESD spectra that different
hydrogen bonding states desorb at different angles relative to the
surface plane with differing energy distributions. Further research
82


83
is necessary to more fully characterize the states of adsorbed
hydrogen on the cleaned and oxidized Si(100) surface. ESD threshold
studies would be helpful in this endeavor.
In the ISS and TPD study of the interaction between nitrogen and
a Si(111)(7x7) surface, ISS easily detects adsorbed nitrogen
following doses as low as 2 L and reveals that a saturation dose of
N2 at room temperature is 75 L. Using TPD, evidence of at least two
chemisorbed states of nitrogen is found. Hydrogen, which originates
from the bulk of the sample, is found to associate with the surface
nitrogen. No adsorbed nitrogen is detected using AES for doses as
high as 3000 L. Angle-resolved AES, a technique developed by
Hoflund1"^ which allows for greatly enhanced surface sensitivity
using AES, may be useful in determining whether the nitrogen is
present only on the surface or penetrates into the silicon.
From the ESD study of the GaAs(100), it was verified that
molecular deuterium does not adsorb at significant rates at room
temperature on the cleaned or oxidized surface, but that it does
adsorb on both surfaces when the sample is subjected to radiant
heating in the range 200-400 C. There appears to be two states of
hydrogen on oxidized GaAs(100). There also appears to be two states
on the cleaned GaAs(100) surface, but the higher energy state has a
lower desorption cross section or is less densely populated. ESD
gives a relative measure of the amount of adsorbed deuterium. Again,
further research is needed to fully characterize the states of
hydrogen on the surface.


84
The native oxide layer present on chemioallly poslished and
0
etched InP(111) is approximately 30A thick. A sharp interface
exists between the oxide and the bulk InP. ISS reveals that the
outermost atomic layer is In rich, and AES combined with the ISS
shows that the region beneath this is P rich. InP and InPO^ are
present in approximately equal concentrations, and a small amount of
In202 is detected. Carbon is present throughout the oxide layer as
graphite and/or hydrocarbons. Similar studies of native oxide
layers formed under different polishing and/or etching conditions
would be useful. Additionally, it is suspected that hydrogen is
present in the oxide layer. It would be interesting to perform an
ESD study to determine whether or not this is true and to
characterize the hydrogen.
The studies performed have been successful in obtaining an
understanding of a number of interactions between gases and
semiconductors. Further research would be helpful in gaining a more
thorough understanding of these interactions. Ultimately, it is also
very important to correlate the information gained using surface
analytical techniques with a characterization of the electronic
properties of the systems studied.


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UNIVERSITY OF FLORIDA
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