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X-ray photoemission study of the initial oxidation of the cleaved (110) surfaces of GaAs, GaP and InSb
Surface Science 0 North-IIolIand
86 (1979) 811-818 Publishing Company
X-RAY PHOTOEMISSION STUDY OF THE INITIAL OXIDATION CLEAVED (110) SURFACES OF GaAs, GaP AND InSb
OF THE
IWASAKI, Yusuke MIZOKAWA *, Ryusuke NISHITANI and Shogo NAKAMURA
Hiroshi
The Institute of Scientific and Irdmtrial Research, Osaka University, Suita, Osaka 565, Japan Manuscript
received
in final form 27 December
1978
The room temperature oxidation of GaAs, GaP and InSb upon exposure to atmospheric dry O2 has been studied by analyzing XP spectra quantitatively. The rate of oxidation decreases in the order InSb > GaP > GaAs. For CaAs and GaP, evidences of the formation of metastable surface complex are shown: the ratio of A”’ -0 bonds to B”-0 bonds are about 2 and 1, respectively. For I&b, it is shown that both elements are oxidized to comparable extent, giving rise to ln203 and Sb,03 probably, by using Auger spectra as well as photoemission spectra.
1. Introduction There has been considerable interest in the surface properties of the A”‘B” semiconducting compounds, due to the influence on electronic device characteristics. It has been shown that chemisorption of oxygen at room temperature gives a way to the formation of metastable surface complexes or surface oxides which are precursors to true oxide growth [ 11. In the present work the interaction of oxygen at atmospheric pressure with cleaved (110) surfaces of GaAs, GaP and InSb at room temperature was studied by X-ray photoelectron spectroscopy (XPS). Temperature effects on the initial oxidation were studied to gain insight into the mechanisms of the surface complex formation. Evidence is presented for the formation of metastable intermediate complexes for GaAs and GaP. The composition of the surface-oxygen complexes is analyzed quantitatively by decomposing the XP spectra into oxidized and nonoxidized peaks [ 21.
*Junior Japan.
College
of Engineering,
University
of Osaka Prefecture,
811
Katsuyama,
Ikuno,
Osaka 544,
812
II. Iwasaki et al. /Initial oxidation of‘ GaAs, GaP and In,%
2. Experimental Spectra were recorded using a DuPont-ESCA 6SOB spectrometer with Mg/Al Kcu radiation at 9 kV X 28 mA. The spectrometer base pressure was in the high 10m7 Torr range. To improve the surface sensitivity of the measurements, the ejection angle of the detected photoelectrons measured from the surface normal of the sample, 0, was taken to be 75”. The samples, of 5 mm diameter, were cut from single crystals and cleaved in dry Ar gas (tank Ar of 99.999% purity) and introduced into the spectrometer, without exposing them to air, by a double-valved transfer compartment. The samples that were studied in these experiments were Cr doped semi-insulating GaAs, S doped n-type GaP (n = 2 X I Or 7 cm-a) and non-doped n-type InSb (n = 2 X 1015 CITI-~) from Sumitomo Electric Industries Corporation. The room temperature oxidation was performed by exposing freshly cleaved sample surfaces to high pressure, dry oxygen (tank O2 of 99.99r%, purity at 1 atm) to reduce the effects of residual gases on the initial oxidation. It has been shown that the effect of water vapour was remarkable for the initial oxidation of GaAs [2]. Desiccation was achieved by passing gases over silica gel and phosphorous oxide and through a liquid N, trap. Energies were measured relative to the C Is line from the surface contamination. The energy of this line was taken to bc 285.0 eV.
3. Results The spectra from freshly cleaved GaAs surface did not show the oxide features [ 21. The spectra from GaP measured with 8 = 0” did not show the oxide features, while those with 0 = 7.5” showed slight oxide peaks. For a freshly cleaved InSb surface, the spectra measured with 0 = 0” showed slight oxide features. Photoelectron binding energies and Auger kinetic energies are summarized in table 1. Representative spectra of the In 3d 5/2 and Sb 3d 3/2 photoelectrons and In and Sb MNN Auger electrons from an InSb(1 IO) surface exposed to dry O2 for 10 min are shown in fig. 1. The spectrum of the Sb 3d 3/2 region is chosen rather than that of Sb 3d 5/2, as the latter is overlapped by the oxygen Is photoelectron, although the former is overlapped by the plasmon loss peak of the Sb 3d 5/2 peak. The spectra shown in fig. 1 were corrected by smoothing and subtracting the appropriate linear background from the experimental curves. The decompositions of spectra to non-shifted and shifted peaks shown in fig. 1 were performed by the iterative least-square method [3]. The peak shapes of the non-shifted peaks for GaAs and GaP were measured from freshly cleaved surfaces. These for InSb wet-e determined from a spectrum measured from an Ar ion-sputtered surface (1 kV at 5.5 mA for 5.min), taking into account the fact that peaks were broadened 1.04 times by Ar ion sputtering. Using the peak shape of Sb 3d 5/2 peak, a spectrum of Sb 3d 3/2 region is decomposed into the Sb 3d 3/2 clean peak and the plasmon loss
H. Iwasaki et al. /Initial oxidation of GaAs, GaP and InSb
Table I Photoelectron binding energies a, chemical shifts b aE and the ratios to the non-shifted peak area R upon oxygen exposures Compound
Bxposure c
AE(A”)
R(Am)
813
of the shifted
A.&R”)
peak area
R(B”)
10 min 2h
Ga 3d(19.2 f 0.1 eV) 0.28 + 0.03 1.1 + 0.1 ev 0.43 1.0
As 3d(41.2 t 0.1 eV) 0.13 * 0.03 2.9 f 0.1 eV 0.22 2.9
2 min 10 min 2h
Ga 3d(19.4 f: 0.1 eV) 0.29 * 0.03 1.3 * 0.1 eV 0.21 1.4 0.36 1.3
P 2p (129.0 f 1.0 eV) 0.22 ? 0.03 4.4 * 0.1 eV 0.26 4.6 0.42 4.6
10 min
In 3d S/2 (444.3 0.6 f 0.1 eV
Sb 3d 3/2(537.2 2.6 f 0.1 eV
10 min
In M4N4sN4s 2.2 f 0.2 eV
GaAs d
GaP
InSb
* 0.1 eV) 1.5 f 0.1 (401.4 f 0.1 eV) e 0.48 f + 0.05
Sb M4N4sN4s 3.6 f 0.2 eV
a Referenced to C 1s level at 285 .O eV. b Energy shifts with respect to compound semiconductors. c Freshly cleaved (110) surfaces were exposed to dry molecular oxygen room temperature. d Further measurements were performed after publication of ref. [ 21. e Auger kinetic energies a. f Auger electrons of InSb were measured with escape angle 0 = 0”.
?r 0.1 eV) 1.08 f 0.03 (451.1 f 0.1 eV) e 0.39 f + 0.05
at one atmosphere
at
peak of Sb 3d 5/2. The separation of the loss peak from Sb 3d S/2 peak is 14 eV and is smaller than the separation for metallic Sb (16 eV) [4]. The peak shapes of the well resolved shifted peaks (P 2p and Sb 3d photoelectrons) are determined from spectra, which contain well formed shifted peaks, by the least-square method. These shifted peaks for various oxygen exposures are similar to each other. Each peak shape of the shifted peaks of other photo and Auger electrons is assumed to be similar to the peak shape observed in a spectrum from a thermally oxidized surface which contains the single oxidized peak. The annealing temperatures, at which the single oxidized peak was observed by several minutes oxidation in air, were 520, 630 and 320°C for GaAs, GaP and InSb respectively. Synthetic spectra made from these component peaks are fitted to the experimental spectra very well, as shown in fig. 1, by iterative non-linear regression analysis [3]. The chemical shifts referenced to the non-shifted peak AE and the ratios of the shifted peak area to the non-shifted peak area R for photoelectron and Auger electron lines upon exposures to dry oxygen are summarized in table 1. By thermal oxidation of a cleaved (10) surface of GaAs in air at 35O”C, the new peak appeared on the high binding energy side of As 3d non-shifted peak (AIY = 3.4 eV), although this fact was not certain in the previous measurements with 8 = 0” [2]. For GaP, the chemical
814
II. Iwasaki et al. / Initial oxidatim
4
I
of C;aAs, GaP and lnSh
.. l
CLEAVED nSb(llO)
t
IOmin in 02(760Torrl
.
’
In 3d 512
t
.
’
’
, I
D
444
446
BINDING ENERGY (eV)
1" M4N45N45 CLEAVED InSbtllOl
l
IOmin in 02(760Torr)
.
388
.
*"',
,
:
406
402
410
KINET!C ENERGYfeV)
shifts
upon
formation
of
themally
activated
surface
oxides
increased
rather
continuously as the heating temperature increased; AE(P 2~) = 4.8 and 5.1 eV and aE(Ga 3d) = I .4 and 1.7 eV upon the thermal oxidation in air at 185 and 320°C. respectively. For InSb. there appeared no new peaks by thermal oxidation in air up to 320°C. The detailed results of the thermal oxidation will be given elsewhere [ 5 1
815
H. Iwasaki et al. /Initial oxidation of GaAs, GaP and InSb
CLEAVED InSb(ll0) 2
-
Sb 3d 312
IOmin in 02(760Torr)
,.%
.
’
540
542
538
536
BINDING ENERGY(eV)
CLEAVED InSb(ll0) IOmin in 02(760Tor:)
446
.* .a.. . 1'
450
’
454
458
462
KINETIC ENERGY(eV)
I:&. 1, Expanded spectra of In 3d 5/2 level (a), In M4N45N4s Auger line (b), Sb 3d 3/2 level (c) and Sb M4N4sN4s Auger line (d) of InSb(ll0) exposed to dry 02 (760 Torr) for 10 min at room temperature. The solid circles represent the experimental data points, while the open circles show a least-squares tit to the data. The solid circles are omtted when they are overlapped by the open circles. The decompositions are shown by the solid curves. The component on the high-binding energy side’or on the low-kinetic energy side represents the shifted component. The component peak Pin(c) represents the plasmon loss peak of Sb 3d S/2.
H. Iwasaki et al. /Initial oxidation of GaAs, GaP and InSh
816
4. Discussion
and conclusion
The binding
energies
listed
I are in agreement
in table
(within
0.3 eV) with the
values by Leonhardt et al. [6] for GaAs and with the values by Gudat et al. [7]. which have been corrected with 0.5 eV (see ref. [6]), for GaP and InSb. The binding energies
and the Auger electron
the chemically
etched
surface
energy
measured
for GaP are in agreement
with the same instrument
with those
for
[8] within
the
experimental errors. The energy shifts for Ga 3d photoelectron referred to metallic Ga are 1 .O f 0.1 and I .2 f 0.1 eV for GaAs and GaI’. respectively, in good agreement with the previously reported values 1 .I k 0.1 and 1 .3 2 0.1 eV, respectively [6]. The energy shift for As 3d electron for GaAs referred to elemental As is -0.4 2 0.3 eV
[9]
energy
shifts
and
the
previously
referred
reported
to elemental
values
materials
are
-0.6
f 0.1 eV
have been taken
from
[6],
Data of
the literature:
0.4 and -0.5 eV for the In and Sb 3d levels in InSb and -2.9 eV for the P 2p level in GaP [ 71, respectively. The binding states of As on the surface complex formed on GaAs( I 10) surfaces upon room temperature exposure to O2 and in thermal oxide are clearly different from
the
elemental the similar
chemical
shift
data;
Ga in the surface shift
chemical
state to As in As203, as the chemical
within the experimental errors, deficient state than As in As203. shifts
the
shifts
of
As are 2.5 ? 0.1 and 3.0 + 0.1 eV. respectively.
complex
of GaAs
to metallic
peak
referred
is probably
shift is equal to that for As203
to in [9 ]
and As in the surface complex is more oxygen It was difficult to discriminate the binding state of from
that
of Ga 3d level due to the low resolution of Ga 3d referred
As 3d
The latter
in thermal
oxide
of the measurements.
by the chemical The chemical
Ga is 2.1 + 0.1 eV and is nearly equal to that of
Ga203 (2.0 4 0.1 eV [b]). The chemical shift of P 2p level of GaP upon 0, exposure referred to elemental P (1.5~~ 1 .7 eV) is somewhat larger than that of P,O, (1.4 eV [lo]) and much less than that of P205 (4.9 eV [lo]). Th e chemical shift of Ga 3d (2.5 + 0.1 eV) is larger than that for Ga,03. These chemical shifts of P 3-p and Ga 3d levels are smaller than those upon thermal oxidation as shown in section 3. For InSb, lhe chemical shifts upon O2 exposures of more than IO’ 1 L (1 langmuir = 10e6 Torr set) and upon thermal oxidation are euqal within the experimental errors. The chemical shifts of In and Sb 3d levels referred to elemental rnaterials are I .O t 0.1 and 2.1 + 0.1 eV respectively. The reported values of the chemical shifts are I .O eV for Inz03 [IO], 1.6 [I I] or 2.0 eV [12] for Sb203 and 2.5 eV for Sb205 [IO,1 11. Judging from the binding energies, the surface oxides, which are formed on InSb (1 IO) surfaces by O2 exposures. are In,03 and probably Sb203. The chemical shift of In 3d level is small but, as has been found, the Auger energy shifts of In are larger than the core level shifts [ 131. One can see from the In MNN Auger spectrum in fig. 1 that In atoms are indeed oxidir.ed up011 02 exposure although In 3d peak seems to be just broadened. The chemical shifts of the core levels studied in the present work follow the periodic progression that the shifts increase within the
II. Iwasaki et al. f Initial oxidation of GaAs, GaP and InSh
817
periods but decrease within the groups [lo]. It is well known that the chemical shifts of core level peaks in XP spectra of substrates during chemisorption are very small compared to the shifts on oxidation [ 141. By virtue of the rather large chemical shifts upon room temperature oxidation, we attribute the emergence of the shifted peaks to the formation of the surface oxide nuclei rather than the sub-monolayer chemisorption without reconstitution. The effective thickness of the “oxygenated” layer is estimated based on the relative XPS signal intensities R assuming an exponential variation of electron escape probability with depth. The thickness is about 3 .8 for R = 0.4 for GaAs and GaP which corresponds to an overlayer formed on an exposure to atmospheric 02 for 2 h, by assuming an electron escape depth of 20 a for electron energy 1.2 keV and using the estimated value for the effective escape angle 0 of 66”. For GaAs and GaP, we conclude that the growth of the shifted peaks during room temperature as well as the energy shifts aE being oxidation with the values of [R(A’“)/R(B”)] kept constant, as seen in table 1, corresponds to the two dimensional island growth of the metastable surface complex which has a chemical and structural indentity of its own. The composition of the surface complex may be suggested by the values of [R(A”‘)/R(B”)] which are 2 and 1 for GaAs and GaP, respectively. Rosenberg has estimated the composition of the metastable configuration to be 1.5 oxygen atoms per surface atom for GaAs [ 11. Taking into account the estimated thickness of the nearly saturated overlayer for GaAs, it is probable that the surface complex is composed of two monolayers in which all Ga atoms arc bonded to oxygen while half the As atoms are bonded to oxygen. Pianctta et al. have observed that after 10 I2 L O2 exposure there was one shifted As 3d peak (&!? = 2.9 eV) of comparable strength to the non-shifted peak and there was an increase of 0.5 eV in the width (the full width at half maximum), of the Ga 3d peak [15]. As shown in the previous paper [2], the quantitative analysis of XP spectra have revealed that the broadened Ga 3d peak, after exposure to molecular oxygen heavily (-lo’* L O,), does contain the shifted peak and Ga atoms in the surface region are involved in the initial oxidation. By assuming that the escape depth for photoelectrons varies as EL/’ (Ek is the kinetic energy of excited electrons), the thicknesses of In oxide and Sb oxide formed by a 10 min exposure to atmospheric oxygen are estimated to be S.‘I’and 4.2 a on the basis of photoelectron spectra and 4.5 and 4.1 .&on the basis of Auger electron spectra, respectively. In consideration of the inaccuracies of the R value. of scarcely resolvable In 3d spectrum, and the value of the effective escape angle, the differences of the values estimated from the photoelectron and Auger data are rather small. Clearly InSb is oxidized faster than GaP and GaAs. The present results show that cleaved (1 10) surface of InSb is not oxidized selectively in the early stage of oxidation as Rosenberg has discussed; he evolved the mechanism of the hightemperature oxidation that Sb atoms are oxidized first and after evaporation of Sb oxide Sb203, remaining In atoms are oxidized [ 161. The behavior of the initial oxidation of III--V semiconductor surfaces has
XIX
tf. Iwpasaki et al. / Iiritial o.vidatiorl of (iail s, (;al’ aid IRISES
been studied and the composition of surface oxide was revealed by the quantitative analysis of XP spectra. The transition i‘rom the initial oxidation to the true oxide formation will be discussed elsewhere [j].
Acknowledgments Numerical computations for deconlpositions WI-C done at the Osaka University Computation Center. This work was partially supportccl by the Grant-Aid t‘o~Special Research Programme of Surface Electronics fi-oin lhe Ministry 01‘ Education of Japan.
References 1 I ] A.J. Rusenbcrg, J. Phys. C‘hem. Solids 14 ( 1960) 175. Y. hlizokaux, R. Nishitani and S. Nakamura. 3 15.
[ 21 II. Inasaki,
J;~lwn, J, ,Ippi.
I’hyh. 17 t 1978)
14 j
1t.A. Pollsk. L.. Lcy, I‘.R. Mcl‘eely, S.P. Ko\valc/.!.k and I).,\. Shirley. J. Iblectl-on Spectrl)‘;c. 3 (1974) 381. [S] II. Iwawki. Y. Mi~ukaua. R. Nishitmi and S. Nakumura. I:) Ix puhli\hcd. [ 6 1(i. Leonhardt, A. IIcrndtsson, J. Ilcdman, M. Klasson. R. Xilwm and C‘. Nordlinp, Php. Statu\ Solidi (II) 60 (I 973) 244.
( 101 R. Ilolm and s. storp. ,Appl. I’ll:.\. 9 11976) 217. 11I 1\+.I,‘. Rlorgan, W.J. Stec and J.R. Van Warcr. Inorg. (‘hem. I2 ( 1973) 953. 1I2 1K. Sic;bahn, C. Nordlinp. A. I~ahlman, Ii. Nordbcrs. K. Il;~merm. J. Ilctlman.
(;. Johan+ son, ‘I Bcrgmark, S.-I:. K;~rlsson, I. ILintl~ren and 13. l.intll~e~~. j\tomic, \lolccula~ x1d Solid State Structure Studied by Means ol’l lectron Spectroscopy (I’ppsala. 1967). [ 13 I (‘.I). \C’ap~cr and I’. IGlocn. Surface Sci. 35 ( 1973) X2. 114 1 Ihcuasion remarks. I sratlay Disc. Cllcrn. SW. 58 ( 1974) 175. [ IS I I’. Pianetta. I. Lindau, C’.hl. Grrncr and W.1,. Spicer. I%!,\. Rw. I ctterr 37 (1976) I 166. [ Ih] A.J. Koscnhcrg, in: Compound Semic~)ndu~t~)rs, Vol. I. Ibds. R.K. Willard\~~n and II.1 (;ocrm~ (Reinhold. New York, 1962) p. 436.
86 (1979) 811-818 Publishing Company
X-RAY PHOTOEMISSION STUDY OF THE INITIAL OXIDATION CLEAVED (110) SURFACES OF GaAs, GaP AND InSb
OF THE
IWASAKI, Yusuke MIZOKAWA *, Ryusuke NISHITANI and Shogo NAKAMURA
Hiroshi
The Institute of Scientific and Irdmtrial Research, Osaka University, Suita, Osaka 565, Japan Manuscript
received
in final form 27 December
1978
The room temperature oxidation of GaAs, GaP and InSb upon exposure to atmospheric dry O2 has been studied by analyzing XP spectra quantitatively. The rate of oxidation decreases in the order InSb > GaP > GaAs. For CaAs and GaP, evidences of the formation of metastable surface complex are shown: the ratio of A”’ -0 bonds to B”-0 bonds are about 2 and 1, respectively. For I&b, it is shown that both elements are oxidized to comparable extent, giving rise to ln203 and Sb,03 probably, by using Auger spectra as well as photoemission spectra.
1. Introduction There has been considerable interest in the surface properties of the A”‘B” semiconducting compounds, due to the influence on electronic device characteristics. It has been shown that chemisorption of oxygen at room temperature gives a way to the formation of metastable surface complexes or surface oxides which are precursors to true oxide growth [ 11. In the present work the interaction of oxygen at atmospheric pressure with cleaved (110) surfaces of GaAs, GaP and InSb at room temperature was studied by X-ray photoelectron spectroscopy (XPS). Temperature effects on the initial oxidation were studied to gain insight into the mechanisms of the surface complex formation. Evidence is presented for the formation of metastable intermediate complexes for GaAs and GaP. The composition of the surface-oxygen complexes is analyzed quantitatively by decomposing the XP spectra into oxidized and nonoxidized peaks [ 21.
*Junior Japan.
College
of Engineering,
University
of Osaka Prefecture,
811
Katsuyama,
Ikuno,
Osaka 544,
812
II. Iwasaki et al. /Initial oxidation of‘ GaAs, GaP and In,%
2. Experimental Spectra were recorded using a DuPont-ESCA 6SOB spectrometer with Mg/Al Kcu radiation at 9 kV X 28 mA. The spectrometer base pressure was in the high 10m7 Torr range. To improve the surface sensitivity of the measurements, the ejection angle of the detected photoelectrons measured from the surface normal of the sample, 0, was taken to be 75”. The samples, of 5 mm diameter, were cut from single crystals and cleaved in dry Ar gas (tank Ar of 99.999% purity) and introduced into the spectrometer, without exposing them to air, by a double-valved transfer compartment. The samples that were studied in these experiments were Cr doped semi-insulating GaAs, S doped n-type GaP (n = 2 X I Or 7 cm-a) and non-doped n-type InSb (n = 2 X 1015 CITI-~) from Sumitomo Electric Industries Corporation. The room temperature oxidation was performed by exposing freshly cleaved sample surfaces to high pressure, dry oxygen (tank O2 of 99.99r%, purity at 1 atm) to reduce the effects of residual gases on the initial oxidation. It has been shown that the effect of water vapour was remarkable for the initial oxidation of GaAs [2]. Desiccation was achieved by passing gases over silica gel and phosphorous oxide and through a liquid N, trap. Energies were measured relative to the C Is line from the surface contamination. The energy of this line was taken to bc 285.0 eV.
3. Results The spectra from freshly cleaved GaAs surface did not show the oxide features [ 21. The spectra from GaP measured with 8 = 0” did not show the oxide features, while those with 0 = 7.5” showed slight oxide peaks. For a freshly cleaved InSb surface, the spectra measured with 0 = 0” showed slight oxide features. Photoelectron binding energies and Auger kinetic energies are summarized in table 1. Representative spectra of the In 3d 5/2 and Sb 3d 3/2 photoelectrons and In and Sb MNN Auger electrons from an InSb(1 IO) surface exposed to dry O2 for 10 min are shown in fig. 1. The spectrum of the Sb 3d 3/2 region is chosen rather than that of Sb 3d 5/2, as the latter is overlapped by the oxygen Is photoelectron, although the former is overlapped by the plasmon loss peak of the Sb 3d 5/2 peak. The spectra shown in fig. 1 were corrected by smoothing and subtracting the appropriate linear background from the experimental curves. The decompositions of spectra to non-shifted and shifted peaks shown in fig. 1 were performed by the iterative least-square method [3]. The peak shapes of the non-shifted peaks for GaAs and GaP were measured from freshly cleaved surfaces. These for InSb wet-e determined from a spectrum measured from an Ar ion-sputtered surface (1 kV at 5.5 mA for 5.min), taking into account the fact that peaks were broadened 1.04 times by Ar ion sputtering. Using the peak shape of Sb 3d 5/2 peak, a spectrum of Sb 3d 3/2 region is decomposed into the Sb 3d 3/2 clean peak and the plasmon loss
H. Iwasaki et al. /Initial oxidation of GaAs, GaP and InSb
Table I Photoelectron binding energies a, chemical shifts b aE and the ratios to the non-shifted peak area R upon oxygen exposures Compound
Bxposure c
AE(A”)
R(Am)
813
of the shifted
A.&R”)
peak area
R(B”)
10 min 2h
Ga 3d(19.2 f 0.1 eV) 0.28 + 0.03 1.1 + 0.1 ev 0.43 1.0
As 3d(41.2 t 0.1 eV) 0.13 * 0.03 2.9 f 0.1 eV 0.22 2.9
2 min 10 min 2h
Ga 3d(19.4 f: 0.1 eV) 0.29 * 0.03 1.3 * 0.1 eV 0.21 1.4 0.36 1.3
P 2p (129.0 f 1.0 eV) 0.22 ? 0.03 4.4 * 0.1 eV 0.26 4.6 0.42 4.6
10 min
In 3d S/2 (444.3 0.6 f 0.1 eV
Sb 3d 3/2(537.2 2.6 f 0.1 eV
10 min
In M4N4sN4s 2.2 f 0.2 eV
GaAs d
GaP
InSb
* 0.1 eV) 1.5 f 0.1 (401.4 f 0.1 eV) e 0.48 f + 0.05
Sb M4N4sN4s 3.6 f 0.2 eV
a Referenced to C 1s level at 285 .O eV. b Energy shifts with respect to compound semiconductors. c Freshly cleaved (110) surfaces were exposed to dry molecular oxygen room temperature. d Further measurements were performed after publication of ref. [ 21. e Auger kinetic energies a. f Auger electrons of InSb were measured with escape angle 0 = 0”.
?r 0.1 eV) 1.08 f 0.03 (451.1 f 0.1 eV) e 0.39 f + 0.05
at one atmosphere
at
peak of Sb 3d 5/2. The separation of the loss peak from Sb 3d S/2 peak is 14 eV and is smaller than the separation for metallic Sb (16 eV) [4]. The peak shapes of the well resolved shifted peaks (P 2p and Sb 3d photoelectrons) are determined from spectra, which contain well formed shifted peaks, by the least-square method. These shifted peaks for various oxygen exposures are similar to each other. Each peak shape of the shifted peaks of other photo and Auger electrons is assumed to be similar to the peak shape observed in a spectrum from a thermally oxidized surface which contains the single oxidized peak. The annealing temperatures, at which the single oxidized peak was observed by several minutes oxidation in air, were 520, 630 and 320°C for GaAs, GaP and InSb respectively. Synthetic spectra made from these component peaks are fitted to the experimental spectra very well, as shown in fig. 1, by iterative non-linear regression analysis [3]. The chemical shifts referenced to the non-shifted peak AE and the ratios of the shifted peak area to the non-shifted peak area R for photoelectron and Auger electron lines upon exposures to dry oxygen are summarized in table 1. By thermal oxidation of a cleaved (10) surface of GaAs in air at 35O”C, the new peak appeared on the high binding energy side of As 3d non-shifted peak (AIY = 3.4 eV), although this fact was not certain in the previous measurements with 8 = 0” [2]. For GaP, the chemical
814
II. Iwasaki et al. / Initial oxidatim
4
I
of C;aAs, GaP and lnSh
.. l
CLEAVED nSb(llO)
t
IOmin in 02(760Torrl
.
’
In 3d 512
t
.
’
’
, I
D
444
446
BINDING ENERGY (eV)
1" M4N45N45 CLEAVED InSbtllOl
l
IOmin in 02(760Torr)
.
388
.
*"',
,
:
406
402
410
KINET!C ENERGYfeV)
shifts
upon
formation
of
themally
activated
surface
oxides
increased
rather
continuously as the heating temperature increased; AE(P 2~) = 4.8 and 5.1 eV and aE(Ga 3d) = I .4 and 1.7 eV upon the thermal oxidation in air at 185 and 320°C. respectively. For InSb. there appeared no new peaks by thermal oxidation in air up to 320°C. The detailed results of the thermal oxidation will be given elsewhere [ 5 1
815
H. Iwasaki et al. /Initial oxidation of GaAs, GaP and InSb
CLEAVED InSb(ll0) 2
-
Sb 3d 312
IOmin in 02(760Torr)
,.%
.
’
540
542
538
536
BINDING ENERGY(eV)
CLEAVED InSb(ll0) IOmin in 02(760Tor:)
446
.* .a.. . 1'
450
’
454
458
462
KINETIC ENERGY(eV)
I:&. 1, Expanded spectra of In 3d 5/2 level (a), In M4N45N4s Auger line (b), Sb 3d 3/2 level (c) and Sb M4N4sN4s Auger line (d) of InSb(ll0) exposed to dry 02 (760 Torr) for 10 min at room temperature. The solid circles represent the experimental data points, while the open circles show a least-squares tit to the data. The solid circles are omtted when they are overlapped by the open circles. The decompositions are shown by the solid curves. The component on the high-binding energy side’or on the low-kinetic energy side represents the shifted component. The component peak Pin(c) represents the plasmon loss peak of Sb 3d S/2.
H. Iwasaki et al. /Initial oxidation of GaAs, GaP and InSh
816
4. Discussion
and conclusion
The binding
energies
listed
I are in agreement
in table
(within
0.3 eV) with the
values by Leonhardt et al. [6] for GaAs and with the values by Gudat et al. [7]. which have been corrected with 0.5 eV (see ref. [6]), for GaP and InSb. The binding energies
and the Auger electron
the chemically
etched
surface
energy
measured
for GaP are in agreement
with the same instrument
with those
for
[8] within
the
experimental errors. The energy shifts for Ga 3d photoelectron referred to metallic Ga are 1 .O f 0.1 and I .2 f 0.1 eV for GaAs and GaI’. respectively, in good agreement with the previously reported values 1 .I k 0.1 and 1 .3 2 0.1 eV, respectively [6]. The energy shift for As 3d electron for GaAs referred to elemental As is -0.4 2 0.3 eV
[9]
energy
shifts
and
the
previously
referred
reported
to elemental
values
materials
are
-0.6
f 0.1 eV
have been taken
from
[6],
Data of
the literature:
0.4 and -0.5 eV for the In and Sb 3d levels in InSb and -2.9 eV for the P 2p level in GaP [ 71, respectively. The binding states of As on the surface complex formed on GaAs( I 10) surfaces upon room temperature exposure to O2 and in thermal oxide are clearly different from
the
elemental the similar
chemical
shift
data;
Ga in the surface shift
chemical
state to As in As203, as the chemical
within the experimental errors, deficient state than As in As203. shifts
the
shifts
of
As are 2.5 ? 0.1 and 3.0 + 0.1 eV. respectively.
complex
of GaAs
to metallic
peak
referred
is probably
shift is equal to that for As203
to in [9 ]
and As in the surface complex is more oxygen It was difficult to discriminate the binding state of from
that
of Ga 3d level due to the low resolution of Ga 3d referred
As 3d
The latter
in thermal
oxide
of the measurements.
by the chemical The chemical
Ga is 2.1 + 0.1 eV and is nearly equal to that of
Ga203 (2.0 4 0.1 eV [b]). The chemical shift of P 2p level of GaP upon 0, exposure referred to elemental P (1.5~~ 1 .7 eV) is somewhat larger than that of P,O, (1.4 eV [lo]) and much less than that of P205 (4.9 eV [lo]). Th e chemical shift of Ga 3d (2.5 + 0.1 eV) is larger than that for Ga,03. These chemical shifts of P 3-p and Ga 3d levels are smaller than those upon thermal oxidation as shown in section 3. For InSb, lhe chemical shifts upon O2 exposures of more than IO’ 1 L (1 langmuir = 10e6 Torr set) and upon thermal oxidation are euqal within the experimental errors. The chemical shifts of In and Sb 3d levels referred to elemental rnaterials are I .O t 0.1 and 2.1 + 0.1 eV respectively. The reported values of the chemical shifts are I .O eV for Inz03 [IO], 1.6 [I I] or 2.0 eV [12] for Sb203 and 2.5 eV for Sb205 [IO,1 11. Judging from the binding energies, the surface oxides, which are formed on InSb (1 IO) surfaces by O2 exposures. are In,03 and probably Sb203. The chemical shift of In 3d level is small but, as has been found, the Auger energy shifts of In are larger than the core level shifts [ 131. One can see from the In MNN Auger spectrum in fig. 1 that In atoms are indeed oxidir.ed up011 02 exposure although In 3d peak seems to be just broadened. The chemical shifts of the core levels studied in the present work follow the periodic progression that the shifts increase within the
II. Iwasaki et al. f Initial oxidation of GaAs, GaP and InSh
817
periods but decrease within the groups [lo]. It is well known that the chemical shifts of core level peaks in XP spectra of substrates during chemisorption are very small compared to the shifts on oxidation [ 141. By virtue of the rather large chemical shifts upon room temperature oxidation, we attribute the emergence of the shifted peaks to the formation of the surface oxide nuclei rather than the sub-monolayer chemisorption without reconstitution. The effective thickness of the “oxygenated” layer is estimated based on the relative XPS signal intensities R assuming an exponential variation of electron escape probability with depth. The thickness is about 3 .8 for R = 0.4 for GaAs and GaP which corresponds to an overlayer formed on an exposure to atmospheric 02 for 2 h, by assuming an electron escape depth of 20 a for electron energy 1.2 keV and using the estimated value for the effective escape angle 0 of 66”. For GaAs and GaP, we conclude that the growth of the shifted peaks during room temperature as well as the energy shifts aE being oxidation with the values of [R(A’“)/R(B”)] kept constant, as seen in table 1, corresponds to the two dimensional island growth of the metastable surface complex which has a chemical and structural indentity of its own. The composition of the surface complex may be suggested by the values of [R(A”‘)/R(B”)] which are 2 and 1 for GaAs and GaP, respectively. Rosenberg has estimated the composition of the metastable configuration to be 1.5 oxygen atoms per surface atom for GaAs [ 11. Taking into account the estimated thickness of the nearly saturated overlayer for GaAs, it is probable that the surface complex is composed of two monolayers in which all Ga atoms arc bonded to oxygen while half the As atoms are bonded to oxygen. Pianctta et al. have observed that after 10 I2 L O2 exposure there was one shifted As 3d peak (&!? = 2.9 eV) of comparable strength to the non-shifted peak and there was an increase of 0.5 eV in the width (the full width at half maximum), of the Ga 3d peak [15]. As shown in the previous paper [2], the quantitative analysis of XP spectra have revealed that the broadened Ga 3d peak, after exposure to molecular oxygen heavily (-lo’* L O,), does contain the shifted peak and Ga atoms in the surface region are involved in the initial oxidation. By assuming that the escape depth for photoelectrons varies as EL/’ (Ek is the kinetic energy of excited electrons), the thicknesses of In oxide and Sb oxide formed by a 10 min exposure to atmospheric oxygen are estimated to be S.‘I’and 4.2 a on the basis of photoelectron spectra and 4.5 and 4.1 .&on the basis of Auger electron spectra, respectively. In consideration of the inaccuracies of the R value. of scarcely resolvable In 3d spectrum, and the value of the effective escape angle, the differences of the values estimated from the photoelectron and Auger data are rather small. Clearly InSb is oxidized faster than GaP and GaAs. The present results show that cleaved (1 10) surface of InSb is not oxidized selectively in the early stage of oxidation as Rosenberg has discussed; he evolved the mechanism of the hightemperature oxidation that Sb atoms are oxidized first and after evaporation of Sb oxide Sb203, remaining In atoms are oxidized [ 161. The behavior of the initial oxidation of III--V semiconductor surfaces has
XIX
tf. Iwpasaki et al. / Iiritial o.vidatiorl of (iail s, (;al’ aid IRISES
been studied and the composition of surface oxide was revealed by the quantitative analysis of XP spectra. The transition i‘rom the initial oxidation to the true oxide formation will be discussed elsewhere [j].
Acknowledgments Numerical computations for deconlpositions WI-C done at the Osaka University Computation Center. This work was partially supportccl by the Grant-Aid t‘o~Special Research Programme of Surface Electronics fi-oin lhe Ministry 01‘ Education of Japan.
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