Surface electronic properties of clean and S-terminated InSb and 001 111

JOURNAL OF APPLIED PHYSICS 104, 083709 共2008兲
Surface electronic properties of clean and S-terminated InSb„001…
and „111…B
P. D. C. King, T. D. Veal, M. J. Lowe, and C. F. McConvillea兲
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
共Received 27 June 2008; accepted 29 August 2008; published online 24 October 2008兲
The electronic properties of clean and sulfur-terminated surfaces of InSb共001兲 and 共111兲B are
investigated using x-ray photoemission spectroscopy and high-resolution electron energy loss
spectroscopy. The clean surfaces exhibit upward band bending 共electron depletion兲 consistent with
the charge neutrality level in InSb lying at the valence band maximum. The surface Fermi level to
valence band maximum separation is increased for the S terminated compared with the clean
surface, leading to flat bands and downward band bending 共electron accumulation兲 for the 共001兲 and
共111兲B surfaces, respectively. This is discussed in terms of compensation of native acceptor surface
states. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3000567兴
I. INTRODUCTION
Following the discovery that the electronic properties of
devices can be improved by coating the surface with sulfurcontaining compounds,1 chemical and electronic passivation
of III–V semiconductors by sulfur has been intensively investigated. Chemically, sulfur passivation aims to provide
and maintain a surface free from atmospheric contaminants.
This is important for device applications as the native oxides
of III–V materials are generally considered unsuitable
dielectrics.2 However, potentially more important is the electronic passivation. The breaking of the translational crystal
symmetry at a surface or interface allows evanescent states
to exist within the semiconductor band gap.3 These states act
to pin the Fermi level at the surface/interface, largely independent of the bulk doping level or the properties of a contact made to the semiconductor. This Fermi level pinning
prevents control of the barrier height of metal-semiconductor
contacts, and the high density of surface states leads to high
surface recombination. Electronic passivation aims to reduce
共ideally to zero兲 the density of surface states within the semiconductor band gap, hence unpinning the surface Fermi level
so that it occurs near to the value found in the bulk.
Sulfur can be chemisorbed at the surface by treating the
samples in aqueous sulfur containing compounds,1,4,5 or in
vacuo by gas-based methods,6 ultraviolet photosulphidation,7
or by depositing sulfur from an electrochemical cell.8 The
influence of sulfur on the surface electronic properties of
III–V materials to date has largely focused on GaAs,9–11
InP,12,13 and InAs.14,15 Due to their small band gaps, InSb
and related alloys are favorable materials for use in long
wavelength optoelectronic devices. Despite this technological importance, few investigations have been performed into
sulfur passivation of these materials. Weiguo16 grew a thick
共⬃300 Å兲 sulphide-oxide film on InSb and reported almost
flat-band conditions to result. Ichikawa et al.17 observed a
6–7 monolayer 共ML兲 sulphide layer formed following immersion of InSb in 共NH4兲2Sx solution for 60 min. Annealing
a兲
Electronic mail: [email protected].
0021-8979/2008/104共8兲/083709/8/$23.00
was observed to result in a breaking of Sb–S bonds 共310 ° C兲
and In–S bonds 共400 ° C兲 with a reduction in the unoccupied
surface state density for S terminated in comparison to clean
InSb inferred from inverse photoemission spectroscopy measurements.
In this paper, the surface chemical and electronic properties of InSb共001兲 and 共111兲B surfaces, following initial
preparation by atomic hydrogen cleaning 共AHC兲 and after S
dosing from an electrochemical cell and various annealing
treatments, are investigated. X-ray photoemission spectroscopy 共XPS兲 provides information on the bonding of the sulfur atoms at the surface. High resolution electron energy loss
spectroscopy 共HREELS兲 is utilized to investigate the band
bending at the surface of the material by probing the variation in near-surface conduction band plasmon energy. For the
clean surfaces, depletion layers were found to occur with flat
bands 关共001兲 surface兴 and electron accumulation 关共111兲B surface兴 resulting following deposition of sulfur and annealing
to break the Sb–S bonds. Upon desorption of all sulfur from
the surface, depletion layers were recovered, restoring the
upward band bending present for the clean surface prepared
by AHC.
II. EXPERIMENTAL DETAILS
n-type Te-doped InSb共001兲 and undoped InSb共111兲B
wafers obtained from Wafertech, U.K. were grown by the
Czochralski method and mechanically polished and chemically etched before being loaded into the vacuum chambers.
The carrier concentration and electron mobility measured by
the single-field Hall effect were 5.3⫻ 1017 cm−3 and
12 800 cm2 V−1 s−1, respectively, for the InSb共001兲 and
1.9⫻ 1016 cm−3 and 49 900 cm2 V−1 s−1, respectively, for
the InSb共111兲B samples.
Room temperature XPS measurements were performed
using a VG Scientific ESCALAB Mk I spectrometer. Al K␣
x-rays of energy h␯ = 1486.6 eV were produced using a nonmonochromated dual anode x-ray source. The ejected photoelectrons were analyzed by a 100 mm mean radius 150°
spherical-sector electron energy analyzer. The effective instrumental resolution is ⬃1.1 eV derived from the Gaussian
104, 083709-1
© 2008 American Institute of Physics
Downloaded 25 Oct 2008 to 137.205.202.120. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
083709-2
(a)
In 3d5/2 (b)
In-Sb
O 1s
Sb 3d5/2
(c)
In-Sb
C 1s
Sb-In
In 3d5/2
(a)
(b)
Sb 3d5/2
Sb-O
Sb-In
(d)
(e)
S-In
(f)
In-S (c)
447
444
441 534 531 528 525 288 285 282
Binding energy (eV)
FIG. 1. In 3d5/2, Sb 3d5/2, and C 1s XPS core level spectra from InSb共001兲
before 关共a兲–共c兲兴 and after 关共d兲–共f兲兴 surface preparation by AHC. The In 3d5/2
and Sb 3d5/2 peaks have been fitted using a Shirley background and Voigt
共mixed Lorentzian–Gaussian兲 line shapes, shown vertically offset for clarity.
convolution of the analyzer broadening and the natural linewidth of the x-ray source 共⬃0.85 eV兲. Satellite peaks due to
the nonmonochromatic nature of the x-ray source are also
observed in the spectra. The predominant satellite observed
is from Al K␣3 x-rays, giving rise to an additional peak located at a binding energy of 9.7 eV below each Al K␣1,2
peak with an intensity of 7.3% of that of the main peak.18
This was accounted for in the peak fitting.
Room temperature HREELS measurements were performed using a VSW HREELS spectrometer in the specular
scattering geometry with incident electron energies in the
range of 7–60 eV. The spectrometer consisted of a fixed
monochromator and rotatable analyzer, both of the 180°
hemispherical deflector design with four-element entrance
and exit lens systems.
Initial surface preparation was achieved via AHC. The
AHC consisted of annealing the sample at ⬃125 ° C under
exposure to a 5 kilo-Langmuir 共kL兲 dose of molecular hydrogen passed through a thermal gas cracker with a cracking
efficiency of approximately 50%, followed by a further 20
kL hydrogen dose, while the sample was annealed to
⬃200 ° C. XPS core level spectra for as-loaded and AHC
treated InSb共001兲 are shown in Fig. 1. Before AHC, two
components are required to fit the In 3d5/2 peak, attributed to
In–Sb and In–O bondings, in addition to satellite features
arising from the In 3d3/2 peak. Three main components were
required to fit the Sb 3d5/2 region of the spectrum, attributed
to Sb–In and Sb–O bondings, as well as an O 1s core-level
component 共⬃531 eV兲, which overlaps the Sb 3d5/2 region
of the spectrum. Additionally, a large C 1s peak was observed. Upon AHC, the O- and C-related components are
quenched.
An electrochemical sulfur cell based on the design of
Heegemann et al.19 held at a temperature of ⬃275 ° C was
used for deposition of sulfur on the clean InSb共001兲 and
共111兲B surfaces. Dosing times were 30 min. Following dosing, the 共001兲 关共111兲 B兴 sample was annealed successively
for 1 h at 200, 300, 350, and 400 ° C 共200, 300, 400, and
450 ° C兲.
Intensity (arb. units)
Intensity (arb. units)
In-O
J. Appl. Phys. 104, 083709 共2008兲
King et al.
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
530 528 526
165
447
444
441
S 2p
162
159
Binding energy (eV)
FIG. 2. In 3d5/2, Sb 3d5/2, and S 2p XPS core level spectra from InSb共001兲
after AHC 关共a兲 and 共b兲兴, sulfur dosing and annealing to 300 ° C at normal
emission 关共c兲–共e兲兴 and at an emission angle of 30° 关共f兲–共h兲兴, and at normal
emission after sulfur dosing and annealing to 400 ° C 关共i兲–共k兲兴. All peaks
have been fitted using a Shirley background and Voigt 共mixed Lorentzian–
Gaussian兲 line shapes, shown vertically offset for clarity. Peak assignments
are indicated by dashed vertical lines.
III. RESULTS, ANALYSIS, AND DISCUSSIONS
A. InSb„001…
Core level XPS spectra from InSb共001兲, following surface preparation by AHC and after S dosing and annealing to
300 and 400°C, are shown in Fig. 2. All peaks have been
fitted using a Shirley background and Voigt 共mixed
Lorentzian–Gaussian兲 line shapes.
After AHC, a single component due to In–Sb bonding
was observed in the In 3d5/2 and Sb 3d5/2 core levels at 444.3
and 527.9 eV, respectively. Additionally, satellite peaks due
to the In 3d3/2 and Sb 3d3/2 core levels were observed. The
lack of oxide or carbon components in the XPS spectra indicates the preparation of a clean surface, as discussed
above.
HREEL spectra for a variety of incident electron energies from the clean surface are shown in Fig. 3共a兲. The
Fuchs–Kliewer surface phonon mode occurs at ⬃23 meV in
InSb 共Refs. 20–22兲 and, for the bulk carrier concentration of
the InSb共001兲 sample used in this work, a bulk conduction
band plasmon frequency of ⬃40 meV would be expected.
Due to the proximity of these energies, the phonon and plasmon couple resulting in a plasmaron mode, which is ob-
Downloaded 25 Oct 2008 to 137.205.202.120. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
083709-3
J. Appl. Phys. 104, 083709 共2008兲
King et al.
TABLE I. Material parameters 兵zero temperature band gap 共Eg兲 and Varshni
parameters 共␣ , ␤兲, band edge effective mass 共mⴱ0兲, spin-orbit splitting 共⌬so兲,
dielectric constants 关␧共0兲 , ␧共⬁兲兴, and TO phonon frequency 共␻TO兲其 for
InSb.
(a)
60 eV
Eg 共0 K兲 共meV兲
Intensity (arb. units)
30 eV
15 eV
7 eV
␣ 共meV/K兲
␤ 共K兲
mⴱ0 共m0兲
⌬so 共meV兲
␧共0兲
␧共⬁兲
␻TO 共meV兲
(b)
60 eV
30 eV
15 eV
7 eV
␻p =
60 eV
15 eV
7 eV
0
25
50
Energy loss (meV)
75
冉
ne2
␧0␧共⬁兲具mⴱ共E兲典
100
具mⴱ共E兲典 =
FIG. 3. 共Color online兲 InSb共001兲 HREEL spectra 共open circles兲 normalized
to the elastic peak intensity after 共a兲 AHC, 共b兲 sulfur dosing and annealing to
350°C, and 共c兲 sulfur dosing and annealing to 400 ° C for a variety of excitation energies as shown in the figure and corresponding semiclassical dielectric theory simulations 共solid lines兲. The spectra for different excitation
energies are vertically offset for clarity.
冕
served in the HREEL spectra with a loss energy of
⬃35 meV. A slight dispersion of the plasmaron mode to
higher loss energies is observed with increasing incident
electron energy, indicating an increase in the carrier concentration with increasing depth below the surface, suggesting
the presence of a depletion layer at the surface, as has previously been observed by HREELS at clean InSb共001兲
surfaces.23–25
To obtain a more quantitative analysis of the variation in
carrier concentration in the near-surface region, the HREEL
spectra were simulated using semiclassical dielectric theory,
as developed by Lambin et al.26 A multilayered dielectric
function 共DF兲 model is utilized to describe the variation in
electronic properties with depth normal to the surface, with
the DF of each layer described, in the hydrodynamic model
used here by
冋
−
2
关␧共0兲 − ␧共⬁兲兴␻TO
2
␧共⬁兲关␻TO
− ␻2 − i⌫␻兴
册
␻2p
,
␻2 − 共i␻/␶兲 − ␤2q2
1/2
共2兲
,
共1兲
where q and ␻ are the wave vector and frequency of the
excitation, ␧共0兲 关␧共⬁兲兴 is the static 关high frequency兴 dielectric constant of the material, ␻TO 共⌫兲 is the frequency 共damping兲 of the transverse optical phonon, ␻ p 共␶兲 is the frequency
共lifetime兲 of the conduction band plasmon, and ␤ is the spatial dispersion coefficient. The plasma frequency is related to
⬁
0
g共E兲mⴱ共E兲f共E兲dE
冕
,
⬁
共3兲
g共E兲f共E兲dE
0
where g共E兲 is the density of states and f共E兲 is the Fermi–
Dirac factor and the momentum effective mass
mⴱ共E兲 = ប2k
␧共q, ␻兲 = ␧共⬁兲 1 +
冊
where 具mⴱ共E兲典 denotes the density of states averaged momentum effective mass
30 eV
-25
0.336
241
0.0135
810
17.88
15.68
22.9
the carrier concentration, n, and effective mass via
(c)
-50
235
冏 冏
dE共k兲
dk
−1
.
共4兲
The effective mass is energy dependent due to the distinct
nonparabolicity of the conduction band, which is described
here in the Kane formalism.27 The InSb material parameters
used are listed in Table I.
A two-layer DF model consisting of a carrier-free layer
atop a semi-infinite layer representing the bulk of the semiconductor was found to be sufficient to reproduce the measured HREEL spectra. The dielectric theory simulations resulting from such a two layer model are shown in Fig. 3共a兲
and exhibit good agreement with the experimental HREEL
spectra for all excitation energies. The plasma frequency
layer profile is converted into a carrier concentration histogram profile, shown in Fig. 4共a兲, using Eqs. 共2兲–共4兲 and consists of a carrier-free layer of 185 Å thickness, followed by a
semi-infinite layer of carrier concentration 4.6⫻ 1017 cm−3,
very close to the bulk carrier concentration measured by the
single field Hall effect. The large thickness of the carrier-free
layer required to simulate the HREEL spectra confirms the
presence of a depletion layer at the semiconductor surface.
Realistic charge profiles calculated by solving Poisson’s
equation within a modified Thomas–Fermi approximation
共MTFA兲 共Refs. 28 and 29兲 as described elsewhere30 were
compared to the histogram charge profiles in order to determine the band bending, position of the Fermi level at the
surface, and the surface state density for the clean InSb共001兲
surface. From this, an upward band bending of
0.15⫾ 0.05 eV is determined at the clean surface, corre-
Downloaded 25 Oct 2008 to 137.205.202.120. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
083709-4
17
-3
n(z) (10 cm )
6
Atomic Hydrogen Cleaning
Sulphur dosing + 350°C anneal
Sulphur dosing + 400°C anneal
5
4
3
2
1
0
Energy (eV)
J. Appl. Phys. 104, 083709 共2008兲
King et al.
(a)
(b)
(c)
(d)
(e)
(f)
0.0
EF
-0.1
CBM
-0.2
VBM
-0.3
0
200 400 600
0
200 400 600
0
200 400 600
Depth, z (Å)
FIG. 4. 共Color online兲 InSb共001兲 carrier concentration histogram profiles
used for simulating the HREEL spectra following 共a兲 AHC, 共b兲 sulfur dosing
and annealing to 350 ° C, and 共c兲 sulfur dosing and annealing to 400 ° C
with the calculated MTFA charge profiles. The corresponding band bending
profiles are also shown 关共d兲–共f兲兴.
sponding to a surface state density of Nss = 共−9.9⫾ 2.4兲
⫻ 1011 cm−2, leading to a depletion of electrons at the surface. The charge profile and band bending are shown in Figs.
4共a兲 and 4共d兲.
The overriding mechanism driving the depletion of electrons at the surface can be understood within the theory of
virtual gap states 共ViGS兲 共Ref. 3兲, evanescent surface states
that occur within the band gap region whose wave functions
decay exponentially into the vacuum. The ViGS are derived
from the bulk band structure and so their character changes
from predominantly donorlike close to the valence band to
predominantly acceptorlike close to the conduction band.
The position where they have equal donor and acceptorlike
character is termed the charge neutrality level 共CNL兲 or
branch point energy, which is located close to the average
midgap energy across the Brillouin zone.
Theoretical estimates of the CNL position in InSb range
from 0.34 eV below to 0.22 eV above the VBM,2 a range
over twice the size of the band gap in InSb. However, the
experimental Schottky barrier height of Au on InSb of 0.00
eV relative to the VBM 共Ref. 31兲 and the observed stabilization of the Fermi level at the VBM upon irradiation32 indicate that the CNL lies at the VBM in InSb.
If the Fermi level is located above the CNL at the surface, a number of acceptor ViGS will be occupied, and hence
negatively charged, although the exact density of acceptor
ViGS is specific to a given surface reconstruction and is affected by the presence of any adatoms on the surface. In the
presence of a negative surface charge, the bands must bend
upward at the surface leading to a depletion of electrons in
the near surface region, maintaining charge neutrality—the
negatively charged acceptor ViGS are compensated by the
background positively charged donor ions in the electron
depletion region. From the bulk Fermi level and the upward
band bending present, the surface Fermi level is determined
to lie 0.11⫾ 0.05 eV above the VBM, and hence below the
conduction band minimum 共CBM兲. The depletion layer observed here indicates that the Fermi level at the surface must
be pinned slightly above the CNL and is consistent with the
CNL lying at the VBM. This rather low location of the CNL
relative to the band edges in InSb can be understood by
considering the high energy of the Sb 5p atomic orbital, p
− d repulsion due to the occupied In 4d orbitals and the large
spin-orbit splitting, which all push the p-like VBM to high
energies on an absolute energy scale.33
Sulfur was deposited on the clean surface. XPS corelevel spectra 共not shown兲 revealed the formation of In–S and
Sb–S bonds, in addition to In–Sb bonds, and also some carbon contamination 共attributed to residual contamination from
the electrochemical sulfur source兲. C–H vibrational modes
and a broadening of the elastic peak were also observed in
HREEL spectra of the S-dosed surface 共not shown兲, indicating some carbon contamination and a reduction in the degree
of surface order, respectively, following S dosing.
Annealing the sample to 200 ° C resulted in the breaking
of Sb–S bonds, revealed by core-level XPS spectra 共not
shown兲, although some carbon was still present on the surface. Further annealing to 300 ° C was sufficient to remove
this carbon and also resulted in a slight increase in the In:Sb
ratio determined from the XPS core levels, attributed to the
formation of In–S bonds at the expense of In–Sb bonds at the
surface. The XPS core-level spectra following this annealing
treatment are shown in Fig. 2.
A clear S 2p peak is evident, indicating the presence of S
at the surface. The In 3d5/2 core-level peak is well described
by two components in addition to satellite features from the
In 3d3/2 core-level peak. The lower binding energy component at 444.6 eV is attributed to In–Sb bonding. The higher
binding energy component at 445.3 eV is attributed to In–S
bonding. The separation of these components is of the order
of that previously observed for In–Sb and In–S bondings.34,35
Additionally, the area of the In–S component increases relative to that of the In–Sb component on moving to more grazing 共and hence more surface sensitive兲 emission angles 关Figs.
2共c兲 and 2共f兲兴, supporting the assignment of In–S bonding at
the surface. The Sb 3d5/2 peak consists of only a single Sb–In
component at 528.3 eV binding energy 共and a satellite of the
Sb 3d3/2 peak兲, indicating that no S is bonded to Sb after a
300 ° C anneal. The In–Sb bonding components of the
In 3d5/2 and Sb 3d5/2 core-level peaks are shifted to higher
binding energies by ⬃0.3 eV following the S dosing and
300 ° C annealing compared with after the AHC treatment.
This suggests that the surface Fermi level is located higher
above the VBM following the S dosing and annealing than
for the clean surface, although accurate quantification of the
shift via XPS would require high resolution valence band
photoemission to directly determine the VBM to surface
Fermi level separation in each case.
After a further annealing treatment at 350 ° C, very similar XPS spectra were obtained although the elastic peak of
the HREEL spectra showed a reduction in width with successive annealing treatments, indicating an improvement in
surface order. The sulfur coverage was estimated as 1.4 ML
using the inelastic mean free path of the photoelectrons calculated using the TPP-2M predictive formula of Tanuma et
al.36
HREEL spectra following the 350° anneal are shown in
Fig. 3共b兲. Very little dispersion is observed in the plasmaron
Downloaded 25 Oct 2008 to 137.205.202.120. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
083709-5
J. Appl. Phys. 104, 083709 共2008兲
King et al.
peak as a function of incident electron energy, indicating that
the charge profile varies much less in the near surface region
than for the clean surface. Three-layer dielectric theory simulations were required to simulate the HREEL spectra. The
first layer is a thin 共2 Å兲 carrier-free layer with the dielectric
constants modified to represent the differing properties of an
InS surface layer. A further 20 Å carrier-free layer 共with
InSb properties兲 followed by a semi-infinite InSb layer with
a carrier concentration n = 4.87⫻ 1017 cm−3 were required to
simulate the spectra. This carrier concentration is slightly
higher than the bulk carrier concentration used to simulate
the clean surface spectra, and this may be due to a small
amount of sulfur diffusing into the subsurface region and
acting as an n-type dopant.
The carrier-free layer thickness is much smaller than for
the clean surface, as shown in Fig. 4, indicating a drastic
reduction in the band bending compared with the clean surface. Calculated charge profiles and band bending at the surface for a small band bending of ⫾0.01 eV are shown in
Figs. 4共b兲 and 4共e兲, respectively. As the surface acts as an
almost infinite potential barrier, the wave functions of the
carriers must have zero amplitude at the surface, and hence
the carrier concentration tends smoothly to zero, requiring a
“dead layer” at the surface regardless of the type of spacecharge layer.3 It is therefore difficult to distinguish between
the flat band condition 共where there is no band bending at the
surface兲 and small amounts of accumulation or depletion at
the surface, as evident from Fig. 4共b兲. However, the band
bending is clearly very small in this case resulting in approximately flat band conditions with a band bending at the
surface of 0.00⫾ 0.01 eV. Thus, the sulfur effectively passivates the electron depletion.
The Fermi level is located significantly higher at the surface 共0.09⫾ 0.01 eV above the CBM兲 and is thus located
further above the CNL than for the clean surface. As acceptorlike ViGS are the dominant species above the CNL, this
would be expected to lead to a higher density of occupied
共and hence negatively charged兲 ViGS than for the clean surface, and hence a higher 共more negative兲 surface state density resulting in a greater depletion width. However, the band
bending is reduced to zero here, resulting in an approximately zero space charge. Two mechanisms are possible to
maintain charge neutrality. First, sulfur bonded to indium at
the surface acts as a donor donating an electron into the
depletion layer, hence reducing the depletion. A positive S
ion is therefore left at the surface, which passivates the negative surface charge. Second, the intrinsic surface state distribution may be modified by the dosing of S on the clean
surface. A diffuse 共1 ⫻ 1兲 low-energy electron diffraction
共LEED兲 pattern was observed here at 45 eV incident electron
energy on the S-terminated surface, which may be a slightly
disordered version of the 共2 ⫻ 1兲 surface reconstruction,
which has previously been observed on an ammonium sulphide treated InSb共001兲 surface following annealing to
310 ° C.17 This is in contrast to the 共4 ⫻ 2兲 / c共8 ⫻ 2兲 reconstruction commonly observed at clean InSb共001兲 surfaces,
which was also observed by LEED here following AHC and
following the desorption of all sulfur from the surface. A
reduction in the density of unoccupied dangling bond states
was reported from inverse photoemission measurements for
the 共2 ⫻ 1兲 reconstruction;17 such a change could be partly
responsible for the reduction in band bending at the surface
of S-terminated InSb共001兲.
Further annealing of the sample to 400 ° C breaks the
In–S bonds, and the S is completely desorbed from the surface, indicated by the quenching of the S 2p and In–S component of the In 3d XPS core level peaks 关Figs. 2共k兲 and
2共i兲兴. The In–Sb components are also located at very similar
binding energies as for the AHC surface 共444.3 and 528.0 eV
for the In 3d5/2 and Sb 3d5/2 core levels, respectively兲, suggesting the surface electronic properties are like those of the
AHC surface. Indeed, the HREEL spectra following this anneal 关Fig. 3共c兲兴 are very similar to those of the AHC surface
and are well simulated assuming a carrier-free layer of thickness 160 Å at the surface and a semi-infinite layer with a
carrier concentration n = 5.27⫻ 1017 cm−3. The slightly
higher “bulk” carrier concentration compared with after the
previous treatments is again attributed to additional subsurface diffusion of S. The slightly smaller depth of the carrierfree layer compared with the AHC surface may also be due
to this higher bulk carrier concentration reducing the screening length for the space-charge region.
Again, realistic smooth charge profiles were calculated
to determine the band bending at the surface, and good
agreement with the HREEL histogram charge profile was
achieved for an upward band bending of 0.15⫾ 0.05 eV as
for the AHC surface, corresponding to a surface state density
of 共−10.5⫾ 2.6兲 ⫻ 1011 cm−2, very similar to that of the AHC
surface. Once the sulfur is desorbed from the surface, the
surface states will no longer be compensated by the sulfur
ions, leading to the reformation of a depletion layer as for the
AHC surface. Additionally, a 共4 ⫻ 2兲 / c共8 ⫻ 2兲 surface reconstruction was observed by LEED after desorption of the sulfur as for the AHC surface, and so any reconstruction related
change in the distribution of intrinsic acceptor surface states
between the clean surface and the surface following desorption of the sulfur would not be expected.
B. InSb„111…B
The chemical nature of the 共111兲B surface after AHC, S
dosing, and various annealing stages was very similar to the
共001兲 surface, revealed by core-level XPS measurements 共not
shown兲, although In–S bonds remained on the surface with
annealing treatments up to 400 ° C. This indicates that the
S-terminated surface is slightly more thermally stable for the
共111兲B surface than the 共001兲 surface with a breaking of the
In–S bonds and desorption of sulfur from the surface being
achieved only after annealing at 450 ° C. After annealing to
400 ° C, the sulfur coverage was estimated to be ⬃2 ML.
HREEL spectra for an excitation energy of 10 eV after
AHC, S dosing, and annealing to 400 ° C and 450 ° C are
shown in Fig. 5. The bulk carrier concentration for the
共111兲B sample was low, resulting in the loss-related features
in the HREEL spectra consisting of a shoulder on the elastic
peak. The shoulder becomes much more pronounced after S
dosing and annealing at 400 ° C, indicating the presence of a
higher electron concentration in the near surface region for
Downloaded 25 Oct 2008 to 137.205.202.120. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
083709-6
J. Appl. Phys. 104, 083709 共2008兲
King et al.
AHC
S-dosed + 400°C anneal
S-dosed + 450°C anneal
(c) S-dosed +
450°C anneal
(b) S-dosed +
400°C anneal
17
-3
n(z) (10 cm )
Intensity (arb. units)
2.0
1.5
1.0
0.5
(a)
(a) AHC
-50
-25
0
25
50
75
0.0
100
CBM
Energy loss (meV)
the S-terminated surface, although further qualitative analysis of the HREEL spectra is limited due to the low carrier
densities involved. Dielectric theory simulations of the
HREEL spectra have been performed for five excitation energies ranging from 8 to 60 eV, enabling quantitative analysis.
The HREEL spectra for the clean 共111兲B surface prepared by AHC were well reproduced using a two-layer
model: a 160 Å carrier-free layer followed by a semi-infinite
layer with a carrier concentration n = 2.4⫻ 1016 cm−3, similar
to the bulk carrier concentration determined by the single
field Hall effect. Although lower than for the 共001兲 sample,
the bulk Fermi level in the 共111兲B sample still lies above the
CNL. Consequently, the surface Fermi level will also lie
above the CNL and a number of acceptor ViGS will still be
occupied, hence negatively charged, leading to an upward
bending of the bands and electron depletion at the surface, as
for the 共001兲 surface. Poisson-MTFA charge profiles have
again been calculated, and those showing the best agreement
with the HREELS histogram charge profiles are shown in
Fig. 6. For the clean 共111兲B surface, an upward band bending
共surface state density兲 of 0.03⫾ 0.02 eV 共−8.0⫾ 3.6
⫻ 1010 cm−2兲 has been determined, resulting in the Fermi
level at the surface being pinned above the CNL, as required
for acceptor ViGS to be occupied. The smaller amount of
band bending in the depletion layer observed here than for
the 共001兲 sample is predominantly due to the lower bulk
Fermi level in this case.
Sulfur dosing and annealing to 400 ° C resulted in a
S-terminated surface with S–In bonds but no S–Sb bonds
present, as identified by XPS measurements. Dielectric
theory simulations of the HREEL spectra following this
treatment required a four-layer model consisting of a 3 Å
InS carrier-free layer, a 15 Å InSb carrier-free 共dead兲 layer, a
225 Å layer of carrier concentration n = 2.3⫻ 1017 cm−3,
and a semi-infinite layer of carrier concentration n = 2.4
⫻ 1016 cm−3. The third layer having a significantly higher
carrier density than that of the bulk indicates the presence of
an electron accumulation layer at the S–InSb共111兲B surface.
Energy (eV)
FIG. 5. 共Color online兲 InSb共111兲B HREEL spectra 共open circles兲 normalized to the elastic peak intensity after 共a兲 AHC, 共b兲 sulfur dosing and annealing to 400 ° C, and 共c兲 sulfur dosing and annealing to 450 ° C for an
excitation energy of 10 eV and corresponding semiclassical dielectric theory
simulations 共solid lines兲. The spectra are vertically offset for different S
dosing and annealing treatments for clarity.
0.0 EF
-0.1
VBM
-0.2
-0.3
(b)
0
300
600
900
1200
Depth, z (Å)
FIG. 6. 共Color online兲 共a兲 InSb共111兲B Poisson-MTFA carrier concentration
profiles and 共b兲 corresponding band bending profiles following AHC, sulfur
dosing and annealing to 400 ° C, and sulfur dosing and annealing to 450 ° C.
Comparison of Poisson-MTFA charge profiles with the
HREELS histogram charge profile indicates a downward
band bending of 0.12⫾ 0.02 eV for the S-terminated
InSb共111兲B surface.
This pronounced downward band banding results in the
Fermi level at the surface being located substantially above
the CBM and a large accumulation of electrons in the near
surface region as shown in Fig. 6. The absolute shift in the
surface Fermi level position between the AHC and S-treated
surface after annealing to 400 ° C is similar to that for the
共001兲 surface between AHC and S dosing and annealing to
350 ° C 共⬃0.15 eV兲, suggesting a similar mechanism is responsible for the shift in both cases. Considering the difference in bulk Fermi levels, a similar shift in the surface Fermi
level as for the S–InSb共001兲 surface is sufficient to induce a
pronounced electron accumulation at the S–InSb共111兲B surface. The Fermi level lies significantly above the CNL at the
S-terminated 共111兲B surface suggesting that acceptor ViGS
will be occupied and hence negatively charged. These will be
compensated by S donors at the surface, as was proposed for
the 共001兲 surface. This mechanism is supported by the plasmon lifetime values required in the dielectric theory simulations of the HREEL spectra, which were higher for the clean
surface 共both AHC and after sulfur desorption兲 than for the
S-terminated surface. This suggests that the presence of the
sulfur increases the carrier scattering; the sulfur is charged,
acting as an ionized impurity at the surface. The ionized
sulfur therefore acts to compensate the acceptor surface
states. However, as for the 共001兲 surface, a change in the
intrinsic distribution of surface states with surface recon-
Downloaded 25 Oct 2008 to 137.205.202.120. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
083709-7
J. Appl. Phys. 104, 083709 共2008兲
King et al.
FIG. 7. 共Color online兲 共a兲 LEED image and 共b兲 inverse image of InSb共111兲B
prepared by AHC obtained with an incident electron energy of 43 eV showing a 共冑3 ⫻ 冑3兲R30° reconstruction and 共c兲 LEED image and 共d兲 inverse
image of InSb共111兲B after S dosing and annealing to 450 ° C obtained with
an incident electron energy of 49 eV showing a 共3 ⫻ 3兲 reconstruction. The
red boxes indicate the unit cell.
struction for the S-terminated surface cannot be ruled out as
a possible cause of some changes in surface Fermi level pinning position.
Annealing of the sample to 450 ° C to desorb the sulfur
resulted in a depletion layer very similar to that of the AHC
surface. A slightly higher bulk Fermi level was required to
simulate the HREEL spectra 共2.9⫻ 1016 cm−3 as opposed to
2.4⫻ 1016 cm−3 for the AHC preparation兲, which is attributed to the diffusion of a small amount of sulfur into the
near-surface region, acting as an n-type dopant, as was observed for the 共001兲 surface.
The HREELS analysis presented above for the 共111兲B
surface is consistent with the positions of the XPS core level
peaks, which show, as for the 共001兲 surface, a shift of
⬃0.3 eV to higher binding energies for the S terminated
compared with the clean surface. This indicates that the
VBM to surface Fermi level separation is greater for the S
terminated than the clean surface, as found from the analysis
of the HREELS spectra.
Following preparation of the 共111兲B surface by AHC, a
共冑3 ⫻ 冑3兲R30° reconstruction was observed by LEED, as
shown in Fig. 7共a兲. This reconstruction is believed to be Sbrich by analogy with the As-rich GaAs共111兲B 共冑3
⫻ 冑3兲R30° reconstruction37 and is likely observed here due
to the low-temperature AHC procedure used to prepare the
clean surface. In contrast, following desorption of sulfur
from the surface, a 共3 ⫻ 3兲 reconstruction was observed 关Fig.
7共c兲兴. The very similar surface electronic properties observed
for the two different clean-surface reconstructions indicate
that the distribution of intrinsic surface states may not be
especially sensitive to the surface reconstruction in this case.
IV. CONCLUSIONS
The clean and sulfur-terminated surfaces of InSb共001兲
and 共111兲B have been investigated using x-ray photoemission spectroscopy and high-resolution electron energy loss
spectroscopy. Depletion layers were observed at the clean
共001兲 and 共111兲B surfaces prepared by AHC. This was explained within the theory of ViGS and is consistent with the
CNL in InSb lying at the VBM.
Dosing with sulfur from an electrochemical cell resulted
in In–S and Sb–S bond formation, although after annealing
to 350 ° C 共400 ° C兲 for the 共001兲 关共111兲B兴 surface, an ordered S-terminated surface remains with only In–S bonds
present. The S termination caused an increase in the surface
Fermi level for both surface orientations of ⬃0.15 eV, resulting in approximately flat band conditions for the 共001兲
surface and a pronounced electron accumulation for the
共111兲B surface. This was attributed to compensation of intrinsic acceptor surface states above the CNL by ionized sulfur. Annealing to 400 ° C 共450 ° C兲 resulted in desorption of
sulfur from the 共001兲 关共111兲B兴 surface. Depletion layers very
similar to those found on the clean surface prepared by AHC
were recovered, even for the 共111兲B surface where a 共冑3
⫻ 冑3兲R30° reconstruction was observed following the AHC
treatment, in comparison to a 共3 ⫻ 3兲 reconstruction for the
S-desorbed surface.
1
C. J. Sandroff, R. N. Nottenburg, J.-C. Bischoff, and R. Bhat, Appl. Phys.
Lett. 51, 33 共1987兲.
2
J. Robertson and B. Falabretti, J. Appl. Phys. 100, 014111 共2006兲.
3
W. Mönch, Semiconductor Surfaces and Interfaces 共Springer, Berlin,
2001兲.
4
M. S. Carpenter, M. R. Melloch, B. A. Cowans, Z. Dardas, and W. N.
Delgass, J. Vac. Sci. Technol. B 7, 845 共1989兲.
5
V. N. Bessolov, M. V. Lebedev, and E. B. Novikov, MRS Symposia Proceedings No. 315 共Materials Research Society, Pittsburgh, 1993兲, p. 175.
6
C.-H. Chung, S. I. Yi, and W. H. Weinberg, J. Vac. Sci. Technol. A 15,
1163 共1997兲.
7
K. R. Zavadil, C. I. H. Ashby, A. J. Howard, and B. E. Hammons, J. Vac.
Sci. Technol. A 12, 1045 共1994兲.
8
H. Sugahara, M. Oshima, R. Klauser, H. Oigawa, and Y. Nannichi, Surf.
Sci. 242, 335 共1991兲.
9
C. J. Spindt, R. S. Besser, R. Cao, K. Miyano, C. R. Helms, and W. E.
Spicer, J. Vac. Sci. Technol. A 7, 2466 共1989兲.
10
H. Sugahara, M. Oshima, H. Oigawa, H. Shigekawa, and Y. Nannichi, J.
Appl. Phys. 69, 4349 共1991兲.
11
M. V. Lebedev and M. Aono, J. Appl. Phys. 87, 289 共2000兲.
12
V. N. Bessolov and M. V. Lebedev, Semiconductors 33, 416 共1999兲.
13
G. J. Hughes, P. Ryan, P. Quinn, and A. A. Cafolla, Surf. Sci. 431, 1
共1999兲.
14
S. Ichikawa, N. Sanada, N. Utsumi, and Y. Fukuda, J. Appl. Phys. 84,
3658 共1998兲.
15
M. J. Lowe, T. D. Veal, A. P. Mowbray, and C. F. McConville, Surf. Sci.
544, 320 共2003兲.
16
S. Weiguo, Appl. Phys. A: Solids Surf. 52, 75 共1991兲.
17
S. Ichikawa, Y. Suzuki, N. Sanada, N. Utsumi, T. Yamaguchi, X. Y. Gong,
and Y. Fukuda, J. Vac. Sci. Technol. A 17, 421 共1999兲.
18
M. O. Krause and J. G. Ferreira, J. Phys. B 8, 2007 共1975兲.
19
W. Heegemann, K. H. Meister, E. Bechtold, and K. Hayek, Surf. Sci. 49,
161 共1975兲.
20
A. Ritz and H. Lüth, Phys. Rev. Lett. 52, 1242 共1984兲.
21
S. D. Evans, L. L. Cao, R. G. Egdell, R. Droopad, S. D. Parker, and R. A.
Stradling, Surf. Sci. 226, 169 共1990兲.
22
T. S. Jones, M. Q. Ding, N. V. Richardson, and C. F. McConville, Surf.
Sci. 247, 1 共1991兲.
23
T. S. Jones, M. O. Schweitzer, N. V. Richardson, G. R. Bell, and C. F.
McConville, Phys. Rev. B 51, 17675 共1995兲.
24
G. R. Bell, C. F. McConville, C. P. A. Mulcahy, and T. S. Jones, J. Phys.:
Condens. Matter 9, 2903 共1997兲.
25
G. R. Bell, T. S. Jones, and C. F. McConville, Surf. Sci. 405, 280 共1998兲.
26
P. Lambin, L. Henrard, P. Thiry, C. Silien, and J. P. Vigneron, J. Electron
Spectrosc. Relat. Phenom. 129, 281 共2003兲.
27
E. O. Kane, J. Phys. Chem. Solids 1, 249 共1957兲.
28
G. Paasch and H. Übensee, Phys. Status Solidi B 113, 165 共1982兲.
29
J.-P. Zöllner, H. Übensee, G. Paasch, T. Fiedler, and G. Gobsch, Phys.
Downloaded 25 Oct 2008 to 137.205.202.120. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
083709-8
Status Solidi B 134, 837 共1986兲.
P. D. C. King, T. D. Veal, and C. F. McConville, Phys. Rev. B 77, 125305
共2008兲.
31
J. Tersoff, Phys. Rev. B 32, 6968 共1985兲.
32
V. N. Brudnyi, S. N. Grinyaev, and N. G. Kolin, Semiconductors 37, 537
共2003兲.
33
P. D. C. King, T. D. Veal, P. H. Jefferson, S. A. Hatfield, L. F. J. Piper, C.
F. McConville, F. Fuchs, J. Furthmüller, F. Bechstedt, H. Lu, and W. J.
30
J. Appl. Phys. 104, 083709 共2008兲
King et al.
Schaff, Phys. Rev. B 77, 045316 共2008兲.
H. Iwasaki, Y. Mizokawa, R. Nishitani, and S. Nakamura, Surf. Sci. 86,
811 共1979兲.
35
G. E. McGuire, G. K. Schweitzer, and T. A. Carlson, Inorg. Chem. 12,
2450 共1973兲.
36
S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal. 21, 165
共1994兲.
37
J. G. Ping and H. E. Ruda, J. Appl. Phys. 75, 5332 共1994兲.
34
Downloaded 25 Oct 2008 to 137.205.202.120. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp