- Email: [email protected]
Polarization dependence of the Si K-edge X-ray absorption spectra of Si-Ge atomic layer superlattices
.:
,. ,..:.. ,:::. .::... .....:,.:
::‘,::‘:i ::.:: ::::: .,..,.:. .: ..: :: .:::.:.,,.:.: ,:.:, ...I
.I......:
...,: :,:.:.>. .,.: .,..,. ,.,.... .,...
: .,j...:...,,
:,
:i:,‘:j.,,:.,,,. ..’
‘surface science g:“;:;:::::;:.;:~..: :, ,, ,.,. ,,,:.:,,:* .j: :;,: .,.:.:;. ,.. :’.j ..>...y,,. .,.... ::,. .:.:.,.,.: ..,,,,: .“‘:.:‘.:.:.~.‘:::.:jj.::. ., ,., ,.:.:. ,:..,,...y ,.,.,. ...(...
ELSEVIER
Surface
Science 301 (1994) 260-272
Polarization dependence of the Si K-edge X-ray absorption spectra of Si-Ge atomic layer superlattices A.P. Hitchcock
*, T. Tyliszczak,
Institute for Materials Research, McMaster
P. Aebi ’
Uniuersity, Hamilton,
Ontario, Canada L8S 4MI
X.H. Feng CSRF, SRC, University of Wisconsin-Madison,
Z.H. Lu, J.-M. Baribeau, Institute for Microstructural (Received
Stoughton,
WI, USA
T.E. Jackman
Sciences, National Research Council, Ottawa, Canada
7 May 1993; accepted
for publication
19 August
1993)
Abstract The polarization dependence of the SiK-edge X-ray absorption spectra of several [(Si),(Ge),], atomic layer superlattice (ALS) materials grown on both SiflOO) and GetlOO) have been investigated using plane polarized synchrotron radiation. These spectra exhibit sharp, polarization dependent, Si 1s + conduction band (CB) resonance features which are absent in the spectrum of amorphous Si (a-Si). Subtraction of the spectrum of a-Si from that of the crystalline ALS materials is used to isolate the conduction band structure. A constrained curve fit analysis of up to eight data files simultaneously has been used to quantitatively analyze the signal. The CB structure is composed of a number of polarization independent components and several polarization dependent components. In [(Si),(Ge),],,,/Ge(lOO) the lowest energy transition at 1839.1 eV is polarized along the surface normal (the growth direction) while a doublet structure centred at 1841 eV is polarized in the surface plane (perpendicular to the growth direction). A similar spectral pattern is found in [(Si),(Ge),],s/Si(lOO) but the polarization effect is weaker and the sense of the polarization effect is reversed. The polarization dependent signal is attributed to anisotropic states associated with strain-induced tetragonal distortions in the strained-ALS materials.
1. Introduction
Strained superlattice from alternating layers
* Corresponding author. ’ Present address: Institut bourg,
Fribourg,
Switzerland,
structures constructed of pure Si, pure Ge or
Si-Ge alloys are currently of interest with regard to quantum engineered electronic structures adapted for specific device applications [l-3]. In addition atomic layer superlattice (ALS) structures grown by molecular are useful model systems
de Physique, CH-1700.
0039-6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(93)E0493-E
UniversitC
Science
de Fri-
beam epitaxy (MBE) for studying strained
interfaces and the dependence ture on growth and processing
B.V. All rights reserved
of material strucprocedures. The
A.P. Hitchcock et al. /Surface
strain in short period superlattice structures is the origin of the modified band structure. The band structure of [(Si),(Ge),l, ALS and strained layer superlattice structures composed of Si,Ge,_, alloys has been investigated by a number of groups [4-191. The question of whether or not the strain and/or the superlattice structure can lead to a direct band gap has been of particular interest. Optical techniques in the visible region have been used by several groups to characterize the effects of strain on the band structure of Si-Ge ALS materials [20-221. Recently, X-ray absorption has been applied to studies of strained semiconductors, typically using the extended fine structure signal. Both Ge K-edge [23-301 and Si K-edge spectroscopy [31,32] have been used to study Si-Ge ALS. The review by Woicik and Pianetta [32] is a very useful summary of this work. The SiK-edge spectra of c-Si, crystalline Si,Ge,_, alloys, and all of the Si-Ge ALS samples exhibit a characteristic double-peaked structure at the Si K-edge which is attributed to Si 1s + Si 3p conduction band (CB) transitions [31,32]. Both the average chemical composition and the nature of the superlattice structure appear to be involved in determining the energetics and intensity of the SiK-edge CB features [31]. X-ray absorption studies have been used to study other strained semiconductor systems. For example, Kavanagh and Cargill [33] have used GaK-shell EXAFS to investigate the local lattice distortions associated with strain in Si heavily doped with Ga. Tabor-Morris et al. [34] have used polarization dependent EXAFS to study strain effects in Ga,,~In,,~P grown by MOCVD on GaAs. In this paper we report the first observation of a polarization dependence in the SiK-edge spectra of single crystal strained layer superlattice materials and its interpretation in terms of the effect of strain-induced lattice distortions on the band structure.
2. Experimental The Si K-edge spectra were recorded using the double crystal beam line [35] funded by the Ontario Centre for Materials Research (OCMR)
Science 301 (1994) 260-272
261
and operated by the Canadian Synchrotron Radiation Facility (CSRF) at the Aladdin ring of the Synchrotron Radiation Centre (SRC) of the University of Wisconsin-Madison. The monochromator was equipped with InSb crystals. The resolution was 0.8 eV at the SiK-edge. Total electron yield (TEY) spectra were obtained using sample current detection. The counter electrode, a metal ring 4 cm in diameter mounted 2 cm in front of the sample, was biased by 130 V. Spectra were also measured using fluorescence yield (FY) detection, both to explore the relative detection sensitivity and to investigate the depth dependence of the total electron yield signal. SiKL fluorescence was detected using a multichannel plate mounted behind an aluminum film, which acts both to block direct detection of electrons and to convert X-ray photons into electrons. The incident photon flux (1,) was monitored using a gas ionization chamber (9.5 cm long; 1.05 Torr static N2) equipped with thin Be windows. All spectra reported are the ratio of the TEY or FY yield to this I, signal. The stability of the energy scale during measurements of the polarization dependence was monitored by recording the c-Si spectrum (normal incidence) before and after studies of a sequence of polar or azimuthal angle variations. The energy scale was stable to better than 0.02 eV over the ca. two hours required to record a sequence of spectra at 8 angles. Over longer periods, particularly between different fills of the electron storage ring, shifts of as much as 0.1 eV were noted. The excellent stability of both the SRC storage ring and the OCMR double crystal monochromator were critical in achieving these results. Absolute energy scales were established by setting the inflection point of the absorption edge of c-Si (as determined by the peak in the derivative) to 1839.2 eV [31]. This energy scale has been established by calibration to known gas phase spectral lines [31]. It also matches the Si 1s binding energy as determined by XPS [36]. Up to four samples were mounted simultaneously on a manipulator equipped with x-y-z and rotation motions. The polarization dependence (the variation of the spectrum with changes in the angle be-
A.P. Hitchcock et al. /Surface Science 301 (1994) 260-272
262 1
”
I
I
”
I
[(Si),(Ge),l,,/Ge(lOo)
PI
1 t 1830
1040
1850
1860
Energy (eV) Fig. 1. Total electron yield (TEY) spectra of [(Si),(Ge),l,, /Ge (#754) recorded at 10” intervals of 0, the angle of X-ray incidence relative to the surface. Offset plotting has not been used. The insert plots the intensity at 1835 eV versus sin-lo.
tween the electric vector of the incident radiation and the plane of the epitaxial layers) was measured by sample rotation. The angles quoted (k2”) are those measured between the direction of propagation of the X-rays and the (100) surface plane of the samples. For plane-polarized radiation this angle is identical to the angle between the electric vector of the incident radiation and the (001) sample normal, which is the growth direction of the epilayers (see sketch in Fig. 1). A careful search was made for azimuthal dependence of the normal-incidence SiK-edge spectrum of [(Si),(Ge),l,,/Ge(lOO) (#754). Within the precision of the measurements the spectra were independent of the azimuthal orientation. In particular, any variation with azimuthal angle
is less than 10% of the observed polar angle variation of the spectrum of [(Si),(Ge),],,,/ Ge(100). The epitaxial ALS and single crystal alloy samples were grown on a VG Semicon V80 MBE system on Czochralski-grown 100 mm Si(100) or 50 mm Ge(100) wafers [371. Prior to superlattice growth a thick (N 100 nm) epitaxial Si or Ge buffer layer was grown to ensure identical initial conditions. Growth procedures have been described in detail elsewhere [37]. X-ray diffraction [38], Raman scattering spectroscopy and crosssection transmission electron microscopy measurements have established the Ge and Si concentrations and nominal layer thicknesses [26,38]. These have been summarized in an earlier paper [31]. The thickness of these ALS samples (50 to 100 nm, depending on the values of m, n, p) was such that the SiK-edge spectrum of ALS grown on a Si substrate could be recorded with negligible contribution from the underlying Si substrate. A protective Si cap on ALS samples was removed by electrochemical oxidation followed by etching. A further oxide etch (using HF to remove SiO, and GeO, and, when required, NaOCl to remove Ge cap layers) was carried out just prior to insertion into the high vacuum measurement chamber (P < 2 x lo-’ Torr). The a-Si sample was prepared as described elsewhere [39]. It was HFetched just prior to insertion into the vacuum chamber.
3. Results and discussion 3.1. Polarization dependence of the SiK-edge near-edge spectrum of [(Si),(Ge),l,, / Ge(lO0) Fig. 1 presents a sequence of Si K-edge spectra of the [(Si>,(Ge),],,/Ge(lOO) sample (code #754), recorded at lo” intervals from 14” to 84” of the photon beam relative to the sample surface. As the angle of incidence decreases the total signal intensity increases dramatically. This is simply a consequence of depositing a greater proportion of the photon beam in the near surface region sampled by the current detection mode. The sampling depth is estimated to be about 20 nm, based
A.P. Hitchcock et al. /Surface
on comparison of the intensity of the oxide signal in the TEY and FY spectra of a partly oxidized c-Si sample. When there is a fixed signal sampling depth which is much less than the photon penetration depth, the signal intensity should vary with sin-l(0), where 8 is the angle between the incident radiation and the surface. The insert to Fig. 1 plots the intensity at 1835 eV (below the onset of Si 1s absorption) against sin-lo. Clearly the spectral intensity closely follows the relationship expected from the deposition of increasing photon dose in the depth sampled by the TEY detection technique. In addition to the gross intensity change, there are small but systematic changes in the shape of the CB structure between 1839 and 1842 eV. This is revealed more clearly by isolating the CB structure through subtraction of the spectrum of a-Si, as shown in Fig. 2. The SiK-edge spectrum of a-Si is very smooth between 1839 and 1842 eV. As Woicik and Pianetta [32] first noted, while the SiK-edge spectrum of a-Si does have a broad maximum, this is significantly less intense than that in c-Si. Furthermore, the a-Si spectrum does not exhibit any of the sharper CB structure observed in c-Si, c-Si,Ge,_, alloys and the ALS samples. Thus subtraction of an appropriately energy shifted (see below) and normalized a-Si spectrum should provide a means to isolate the structure associated with the CB states. In fact it is the featureless character of the CB structure in a-Si, along with comparisons to multiple scattering calculations that demonstrates that the CB signal is sensitive to the long range structure of the sample [31]. For the analysis described below each spectrum is first subjected to a linear background subtraction and then unit normalization of the continuum intensity at 1858 eV. This subtractive procedure for analysis of X-ray absorption data is not commonly used so it is reasonable to question its rationale. Since the short range order (first shell) of a-Si and c-Si are essentially identical whereas the long range order differs, the difference procedure should enhance differences in the electronic structure which are related to the long range order. In a scattering model it is multiple scattering from more distant shells that dominates the low energy range [31].
Science 301 (1994) 260-272
k 4
263
[(si),(Ge),l,,/Ge(loo)
I
1636
,
1836
Photon
I
1840
*
I
(
1842
1844
Energy
,
(eV)
Fig. 2. Overplot of the SiK-edge spectra of a-Si and #754, at 14” and 84” polar angles indicating the procedure used in generating difference spectra. The a-Si spectrum has been shifted + 0.42 eV from the as-recorded scale (see text).
In a-Si the absence of long range correlations means that this multiple scattering signal averages to zero, whereas in c-Si it is the origin of the conduction band structure. One possible complication is the contribution of multiple scattering in the first shell, which should be the same in both a-Si and c-Si and thus would be removed by the subtraction. However, this signal is believed to be slowly varying in the energy range of the conduction band maximum. Thus, an incorrect representation of this part of the signal will likely result in (at most) a background in the curve fits of the subtracted data (see Section 3.21, an aspect to which position and relative intensities of the fitted peaks are likely to be relatively insensitive.
264
A.P. Hitchcock et al./Surface
Very careful energy calibration through repetitive, sequential recording of spectra over short times has shown that the position of the Si K-edge, as measured by the inflection point, is 0.45(3) eV lower in energy in a-Si than in c-Si. The position of an absorption edge depends on both the energy of the core level and the onset of the conduction band (empty density of states). There may also be differences in the core excitonic contributions in c-Si and a-Si. However, this is considered to be a negligible factor at the Si 1s edge since the excitonic binding energy will be much smaller than the energy resolution (at the Si2p core edge the excitonic states of c-Si are bound by only 40 meV [40]). The Si 1s energy of c-Si and a-Si is expected to be identical since the electrostatic environment of Si in c-Si and a-Si is virtually identical. In contrast, it is well known that the conduction band onset is much lower in a-Si than in c-Si because of band tailing effects [41]. We conclude that the 0.45(3) eV shift to lower energy of the SiK-edge of a-Si relative to c-Si is a reflection of the lower energy conduction band onset in a-Si than in c-Si. The Si2p,,, absorption edge of a-Si is lower than that of c-Si, although the shift is only 0.05 eV [42]. The SiKedge rises much more slowly in c-Si than a-Si, as demonstrated by a much greater width of the peak in the first derivative spectrum of c-Si (1.45 eV FWHM) as compared to that of a-Si (0.95 eV FWHM). The greater breadth is associated with additional low-lying conduction band states which are not present in a-Si. The SiK-edge in the spectra of the ALS compounds is within 0.1 eV of that of c-Si. In order to minimize negative going signals in the difference spectra, we have shifted the a-Si spectrum by +0.42 eV to higher energy prior to subtraction. Analyses carried out without prior shift of the a-Si spectrum give the same qualitative results except that the first peak has consistently negative amplitude. The spectra presented in Figs. 1 and 2 were all determined with electron yield detection which has a rather shallow sampling depth. This raises the question as to whether the polarization effects could be a purely surface phenomena (e.g. a result of changing proportions of surface and
Suence 301 (1994) 260-272
1,
I
1838
f/I
1840
Photon Fig. 3. Comparison of the (Ge),l,, /Ge (#754) recorded electron yield.
1842
Energy
1,) 1844
184 6
(eV)
14” and 84” spectra of [(Si), by fluorescence yield and total
bulk transitions as the incident angle is changed). Surface-specific core excitation resonances have been identified in the Si K-edge spectrum of clean Si(ll1) [43]. In order to investigate this possibility the #754 ALS sample was also studied by fluorescence yield (FY) detection, while simultaneously recording the electron yield signal. FY samples the full X-ray penetration depth which is about 2 pm for 2 keV X-rays. This is much larger than the total thickness of the superlattice (about 60 nm in #754). The statistical precision of the FY data is much less than that of the TEY data and thus a detailed measurement of the angular dependence was not performed. Nevertheless, the polarization dependence, as monitored by a comparison of the normal and glancing incidence spectra, is very similar in FY and TEY (see Fig. 3). This clearly proves that the polarization dependence is a property of the bulk superlattice structure of the ALS material and not a surfacespecific phenomena.
265
A.P. Hitchcock et al. /Surface Science 301 (1994) 260-272
3.2. Multi-file curve-fit analysis of the polarization dependence
analysis procedure which has been described earlier [28]. In the present case fits to individual spectra had shown that the CB structure could be well reproduced by the sum of a minimum of six Gaussian peaks with widths similar to that estimated for the instrumental contribution and positions which were virtually independent of polar angle (i.e. changing by less than k 0.03 eV). Thus, the multi-file fit to the sequence of 8 spectra of #754 was carried out by constraining the position and width of the corresponding line in each spec-
In order to quantify the polarizaticn effect, the a-%-subtracted spectra were subjected to a curvefit analysis. To obtain more stable results we have developed a program (MGAUSS) to fit up to eight spectra simultaneously to a combination of lines (of Gaussian, Lorentzian or Voigt shape) with provision for constraints on the fit parameters. This is an extension of a multi-file EXAFS
I
’
I
’
123 Ill
1838
1840
I
I
A
multi-file
curve fit analysis
,
,
15
6
12345
II
I
III
1842
1844
”
Photon Fig. 4. Constrained details.
,
of the 14”-84” spectra
1838 Energy
1840
,
, 6
II
I
1842
1844
(eV)
of [(Si)2(Ge),],,/Ge
(#754) to 6 lines. See text for further
266
A.P. Hitchcock
et al. /Surface
Science 301 (1994) 260-272
trum to have the same value. The features at 1840.5 and 1841.5 were found to change in a very similar way through the series of spectra. A more stable fit was achieved by constraining these two components to have identical widths and amplitudes - i.e. to treat the two features as a doublet of two lines of equal intensity. The results of the constrained multi-file fit are presented graphically in Fig. 4 while the numerical values are listed in Table 1. In most cases there is good
Table 1 Results of curve fit analysis spectrum a Peak #
Energy (eV1
of Si 1s + CB features Width (eV)
in #754,
Intensity 14” c
9
agreement between the fit-generated curve and the experimental data. An exception to this is the leading edge of the 84” spectrum where there is considerable discrepancy between the fit and the experimental data because of the presence of negative signal. This discrepancy could be removed by a small, additional energy shift of the aSi spectrum when preparing the subtracted data file for the 84” data. However, we prefer to use the same data processing for all files. In addition,
#752,
#700
after a-Si subtraction
and #698
samples
isolated
by subtraction
of the a-Si
(arb. units)
24
34”
44”
54”
64”
74”
84
0.208 0.084 0.439 0.275 0.145
0.199 0.070 0.439 0.269 0.128
0.172 0.148 0.439 0.274 0.132
0.121 0.166 0.439 0.272 0.121
0.088 0.262 0.439 0.283 0.147
0.042 0.286 0.439 0.281 0.139
0.01s 0.328 0.439 0.282 0.134
~ 0.022 0.324 0.439 0.284 0.127
1.151
1.105
1.114
1.156
1.219
1.187
1.I 98
1.152
#754 [(Si~,(Ge),l,,/Ge(lOO) ;3,41 h 2 5 6
1840.50, 1841.57 1839.11 1839.86 1841.75 1843.15
0.93 1.11 1.11 1.11
xd #752 I(Si),(Ge),,],,/Ge(lOO) ;3,41 b 2 5 6
1839.13 1840.78, 1841.53 1839.93 1841.76 1843.25
0.945 0.936 1.077 1.077 1.077
xd #700 I(Si),(Ge),I,oo/Si(lOO) t>41 h 1839.70, 1841.82 1838.85
0.904 1.264
2 5 6
0.904 0.904 0.904
1840.74 1840.11 1841.46
Ed #698 f(Si),(Ge),l,, / Si(lO0) t1,4) h 1839.70, 1841.82 1838.85
0.904 1.264
2 5 6
0.904 0.904 0.904
Ed
1840.74 1840.11 1841.46
0.293 - 0.006 0.449 0.302 0.160
0.055 0.230 0.44’) 0.302 0.144
1.215
1.180
0.019 0.004 0.143 0.211 0.235
0.097 0.047 0.102 0.21 I 0.235
0.612
0.692
0.011 0.002 0.145 0.205 0.241
O.OY2 0.045 0.1 16 0.205 0.24 I
0.604
O.hYY
” Structure isolated by subtraction of the spectrum of a-Si (after an energy shift of the a-Si spectrum by +0.42 eV see text). h These two components were required to have the same intensity. The ntensity indicated is the sum of the areas of both peaks. ’ Angle between X-ray propagation direction and the surface plane. This is the same as the angle between the E-vector and the surface normal (see Fig. 5). d Sum of the peak areas for all components.
A.P. Hitchcock et al. /Swface
0
I
I
,
I
20
40
60
80
Polarization
Angle
(‘)
Fig. 5. Variation of the peak intensity (area) for the peak at 1839.1 eV and the doublet at 1840.5 and 1841.5 eV (sum of both peaks) as a function of 0, the angle between the E-vector and the sample normal. Note that the E-vector was rotated from the (001) to the (110) direction in the angular variation. The solid and dashed lines are cos% and sin28 functions scaled for best match to the intensity data.
there is good reason to believe that the conduction band profile has a more complex shape than one would obtain with the sum of a small number of Gaussian lines. Thus a perfect agreement is not expected. The curve-fit analysis shows that peak 1 and the {3,4} doublet are polarization dependent, whereas peaks 2, 5 and 6 are polarization independent. Fig. 5 plots the angular variation of the intensity (peak area) of the polarization dependent lines as derived by the multi-file analysis of the processed #754 data. Fig. 5 also plots sin28 (peak 1) and cos20 (doublet {3,4)) angular distributions whose amplitudes have been scaled to best match the measured angular dependence. The reasonable quality of the match to these expected angular dependencies is evidence in favour of a conventional polarization dependence
Science 301 (1994) 260-272
261
rather than band dispersion as the explanation of our observations. This conclusion should be treated cautiously because one could imagine certain types of band dispersion which could provide similar experimental results. In principle comparison of the measured angular dependent intensities with calculation could be used to deduce the spatial orientation of the transition moments. However, we do not believe the curve fit analysis of this data is sufficiently unique to warrant this procedure. Qualitatively, the polarization dependent intensities indicate that peak 1 corresponds to a transition whose moment (and thus the orientation of the conduction band state) is aligned along the growth direction while the doublet, (3,4} corresponds to states oriented in the planes of the layers. The polarization dependence for three other Si-Ge ALS materials (#752 - [W,(Gel,,l,,/ Ge(100); #700 - [(Si),(Ge),],,/Si(lOO); #698 was investigated by mea[(Si),(Ge),l,,/Si(lOO)l) suring spectra at 14” and 84” angles of X-ray incidence. These limiting spectra have been analyzed in the same fashion as the angular sequence of #754 data - i.e. a curve fit was performed on the CB structure isolated by subtraction of the a-Si spectrum. The fits for the 14 and 84” data of #752 and #700 are shown in Fig. 6, while numerical results for all three additional ALS samples are summarized in table 1. As in the analysis of the 754 data, the optimized widths of the individual lines are between 0.9 and 1.2 eV (see Table 11, similar to the instrumental resolution. The structure of the #752 sample is very similar to that of #754 (essentially “Si, in Ge”). Thus it is not surprising that the curve-fit analysis of the #752 data produces essentially the same result as found for #754 - i.e. a peak at 1839.1 eV with polarization along the growth direction and a doublet at (1840.7, 1841.5 eV> with in-plane polarization. However, the result for the #700 and #698 ALS samples - which are basically “Ge, in Si” structures - is dramatically different. The polarization dependence differs qualitatively from that of the #752 and #754 samples. The polarization dependence of the lowest energy transition (1838.9 eV> has the opposite sense. It is polarized in the plane of the layers in the #700
268
A.P. Hitchcock
et al. /Surface
and #698 samples rather than along the growth direction as was the case for the 1839 eV transition in the #752/754 samples. The polarization dependence at higher energy is also different. The changes in the region of the dip at 1841 eV in (#700, #698) are associated with a transition at 1840.7 eV which is partially polarized perpendicular to the layers. The 1840.7 eV peak also contains a polarization independent component which we associate with the Si atoms in the centre of the Si, layers which are likely in a much less strained environment than those at the Si-Ge interface. In order to obtain a satisfactory fit to the #698 and #700 data we have also introduced a doublet of peaks at 1839.7 and 1841.8 eV, on the outer sides of the two main components. This weak doublet signal appears to have an in-plane polarization. While there is no counterpart of this signal in the #752/754 samples, it is possible such a contribution exists but is being masked by the more intense polarization components in those samples. It is also noteworthy that the total intensity in the CB structure is considerably less in #700 and #698 than in the #752 and #754 samples (see the C column in table 1). We speculate that the reduced intensity may be the result of greater intermixing and thus reduced longer
Science 301 (1994) 260-272
range order in the ALS on Si samples. The a-Si subtraction and associated curve fit analysis shows there are large changes in the polarization effect for different Si-Ge ALS samples. Fig. 7 demonstrates these changes in a way which is more qualitative but which involves less data manipulation. Fig. 7 plots the limiting (14”, 84”) continuum normalized spectra and their differences for four different samples, two grown on Si and two grown on Ge. The top two curves (#754 and #752) are from [(Si),(Ge),,], samples grown on Ge. The lower two curves in Fig. 7 arc spectra of ALS samples grown on Si(100) (#69X and #700). These simple difference spectra clearly reveal that the polarization dependence is inverted between the on-Ge and on-Si ALS samples. This simple difference evaluation also shows that the polarization dependence is about three times weaker in the #698 and #700 samples relative to that in the #752 and #754 samples. The reduced polarization dependence evident in the “Ge, in Si” samples relative to that exhibited by the “Si, in Ge” samples is likely associated with the fact that only a portion of the Si atoms (those closest to the Ge layers) are in a locally distorted environment. It is important to note that the principal obserI
(D
/
’
I
#700 14O
I
I
1838 Photon
Fig. 6. Results (#700X
of fits to SiK edge spectra
(a-Si subtracted)
Energy
1840
,
I
1842
(eV)
for ALS samples
of (a) [(Si),(Ge),Zl,,
(#752)
and (b) [W),(Ge)Zl,,,,,
A.P. Hitchcock et al. /Surface
Science 301 (1994) 260-272
269
Difference
1835
1040
1845
1850 Photon
1835
1855 Energy
1840
1845
(eV)
Fig. 7. Left panel - 14”(solid) and 84” (dotted) spectra of [(Si)z(Ge),,],,/Ge (#752); [(Si)z(Ge),],,/Ge (#754); [(Si),(Ge),],,/Si (#698); [W,(Ge),],,/Si (#700). All spectra have been background subtracted and continuum normalized. The 14” and 84” spectra match each other outside the region plotted. Right panel - the difference in the normalized 14” and 84” spectra multiplied by 5 (for #698, #700) or by 2 (#752, #754).
vation is independent of the curve fits. The picture provided by the curve fits of all four ALS samples (754, 752, 700, 698) is fully consistent with the conclusions drawn from examination of the raw data. For example if one judges by the depth of the valley separating the two main peaks in the raw spectra, one concludes that the polarization dependence of the #698, #700 samples is reversed from that for #754 and #752 - i.e. the dip is greatest at 84” for #698 and #700, whereas it is greatest at 14” for the #752 and #754 samples.
3.3. Relationship of the polarization effect to the ALS structure In strained ALS samples grown on Ge, the Si-Si interatomic spacing perpendicular to the growth direction is stretched relative to the value in pure Si because of the commensurate epitaxy with the Ge substrate. This is accompanied by a decrease in Si-Si interatomic distance along the growth direction. X-ray diffraction shows clearly that the Si, layers in #754 and #752 are tetragonally distorted. There is an increased in-plane
270
A. P. Hitchcock
et al. /Surface
Si-Si spacing (to match the bulk Ge-Ge spacing1 and a decreased out-of-plane Si-Si spacing (to compensate for the reduced in-plane binding). This anisotropic strain field splits the conduction band and creates spatially-aligned states. In the #754 and #752 samples this results in CB features with a high degree of polarization because all of the Si atoms are in highly strained environments in these two samples. In contrast, the #698 and #700 samples consist of quite thin (n = 2) strained Ge layers separated by thicker, (Si), or (Sil, layers. Si grown on Si substrate would be expected to adopt a structure very similar to that of the Si(100) template: unstrained and bulk-like. However, the Si layers at the Si-Ge interface may actually be somewhat tetragonally distorted because of the influence of the adjacent strained Ge, layers. In addition there is inevitably some intermixing [26-281 and any Ge atoms present in the Si layers will introduce strain. The net result of these effects in ALS grown on Si is that, over the whole superlattice, the average in-plane spacing is compressed and the average out-of-plane spacing is increased relative to the Si substrate. The critical factor with regard to the polarization dependence is that the sense of the strain field in ALS on Si is inverted relative to that in strained-ALS materials grown on Ge. This is consistent with the fact that the polarization dependence for strained-layer superlattice samples grown on Si is opposite to that for strainedlayer superlattice samples grown on Ge. When strained-ALS samples are annealed the structure relaxes by intermixing. Measurements of the SiK-edge spectra show that the polarization dependence decreases with increasing amounts of annealing. Studies are presently in progress which aim to quantify the relationship between strain and the magnitude of the polarization dependence. The reduced polarization dependence in relaxed ALS samples or those with thicker Si layers is consistent with our interpretation that the polarization dependent states are associated with the strain induced structural anisotropy of the Si-Ge superlattice. Alloys which are grown epitaxially on a single crystal Si or Ge substrate contain an oriented strain field which is similar to that present in the ALS samples. Thus,
Science 301 (I 994) 260-272
if our interpretation of the polarization dependence of the SiK-edge spectra of the strainedALS samples is correct, we would expect strained alloys to exhibit a SiK-edge polarization dependence. Indeed this is the case [44]. Based solely on a consideration of the as-measured spectra one might consider that the CB structure consists of only 2 lines whose position changes with angle of X-ray incidence. Within this framework, the energies of these peaks might shift because of a dispersion dependence of the unoccupied band structure - i.e. the variation of incidence angle tracks shifts in the energy of an unoccupied band as a function of crystal direction, in a fashion similar to angle resolved photoemission (ARPES) mapping of occupied bands. However, the high quality of the Gaussian curvefit analysis of the CB structure (after isolation by subtraction of the a-Si signal) using ftiecl energy states of variable intensity, along with the absence of discernable azimuthal dependence, lead us to conclude that dispersion effects are smaller than our measurement sensitivity. While a detailed correlation of these results to published band structure will not be presented in this paper, it is worth pointing out a few aspects where the calculations support this work. Ghahramani et al. [16] have shown that the anisotropy introduced by the strain is more important than the superlattice periodicity in controlling the band structure and spectroscopic properties. Ma et al. [19], in an extensive study of strained atomic layer and alloy superlattices, have shown that the states at the conduction band minimum are aligned along the growth direction for epilayers grown on (1001 templates which are under tensile stress, such as [W,(Ge),,l,,/ Ge(100). These authors [19] have also presented a group theoretical description of the effect on the band structure of the reduction in symmetry from Ol(Fd?m) in pure Si or Ge to Diz (14,/amd) in strained epilayers grown on Si(100). In their analysis they predict the magnitude of conduction band splitting associated with the symmetry reduction and they discuss the polarization dependence of optical transitions. The calculations of Ma et al. [193 predict that the lowest energy conduction band states are aligned along the
A.P. Hitchcock et al. /Surface
growth direction for Si, grown on Ge and perpendicular to the growth direction for Ge, grown on Si. Among the relatively few reports of studies of SiK-edge polarization dependence, we note the recent work of Lagarde et al. [45] on quartz and densified silica, as well as that of McCrary et al. on poly(di-n-hexylsilane) [46]. The polarization dependence of the Si K-edge spectra of polysilane chains [46] is quite large because of the considerable anisotropy of the local structure around Si. However, in quartz or densified silica the polarization effects are rather small [451, of a similar magnitude to those observed in this work. We rationalize the weakness of the polarization dependence of the spectra of densified quartz/ silica and the Si-Ge ALS by noting that, in both species, the local symmetry of the Si atoms is only slightly perturbed from the normal, highly symmetric tetrahedral environment.
4. Summary The polarization dependence of the Si 1s spectra of strained Si-Ge atomic layer superlattice samples has revealed the existence of anisotropic low energy conduction band states which are sensitive to the magnitude and the orientation of the strain-induced tetragonal distortion. Theoretical study, in particular a detailed analysis of the anisotropy of the conduction band states in strained layer Si-Ge superlattices, is required to properly understand these observations. It seems likely that polarization dependent near edge spectroscopy at the SiK-edge and possibly other Si and Ge core edges will be particularly valuable for measuring band structure changes associated with quantum confinement and superlattice periodicity effects in Si-Ge materials.
Acknowledgements
This work is based upon research conducted at the Synchrotron Radiation Center, University of Wisconsin, which is supported by the NSF under award DMR-9212658. We thank the staff of SRC
Science 301 (1994) 260-272
271
for their expert operation of Aladdin. This research has been sponsored financially by the Ontario Centre for Materials Research and NSERC (Canada).
References
111Silicon
Molecular Beam Epitaxy, Eds. J.C. Bean and E. Kasper (CRC Press, Boca Raton, FL, 1988). Eds. G. 121Spectroscopy of Semiconductor Microstructure, Fasol, A. Fasolino and P. Lugli (Plenum, New York, 1989). [31 T.P. Pearsall, Mater. Sci. Eng. B 9 (1991) 225. [41 S. Froyen, D.M. Wood and A. Zunger, Phys. Rev. B 36 (1987) 4547. 151 L. Brey and C. Tejedor, Phys. Rev. Lett. 59 (1987) 1022. [61 M.A. Gell, Phys. Rev. B 38 (1988) 7535. 171 M.S. Hybertsen and M. Schliiter, Phys. Rev. B 36 (1987) 9683. M. Schliiter, R. People, S.A. Jackson, 181 M.S. Hybertsen, D.V. Lang, T.P. Pearsall, J.C. Bean, J.M. Vanderberg and J. Bevk, Phys. Rev. B 37 (1988) 10195. 191 D. Shen, F. Lu, K. Zhang and X. Xie, Appl. Phys. Lett. 52 (1988) 1599. 1101 I. Morrison and M. Jaros, Phys. Rev. B 37 (1988) 916. [ill K.B. Wong, M. Jaros, I. Morrison and J.P. Hagon, Phys. Rev. Lett. 60 (1988) 2221. [121 S. Circai and I.P. Batra, Phys. Rev. B 38 (1988) 1835. 1131 S. Froyen, D.M. Wood and A. Zunger, Thin Solid Films 183 (1989) 33. 1141 M.A. Gel1 and A.C. Churchill, Phys. Rev. B 39 (1989) 10449. and J.E. Sipe, Phys. Rev. B 40 (1989) El51 E. Ghahramani 1102. D.J. Moss and J.E. Sipe, Phys. Rev B 41 [161 E. Ghahramani, (1990) 5112. 1171 S. Circai, A. Baratoff and I.P. Batra, Phys. Rev. 41 (1990) 6069. 1181 J.E. Bernard and A. Zunger, Phys. Rev. B 44 (1991) 1663. 1191 Q.M. Ma, K.L. Wang and J.N. Schulman, Phys. Rev. B 47 (1993) 1936. [20] U. Schmid, F. Lukes, N.E. Christensen, M. Alouan, M. Cardona, E. Kasper, H. Kibbel and H. Presting, Mater. Sci. Eng. B 9 (1991) 233. [21] H. Okumura, K. Miki, S. Misawa, K. Sakamoto, T. Sakamoto, S. Yoshida, K. Asami and S. Gonda, Mater. Sci. Eng. B 9 (1991) 245. [221 F. Ferrieu, F. Beck and D. Dutartre, Solid State Commun. 82 (1992) 427. [231 H. Oyanagi, T. Sakamoto, K. Sakamoto, T. Matsushita, T. Yao and T. Isheguro, J. Phys. Sot. Jpn. 57 (1988) 2086; H. Oyanagi, T. Sakamoto, K. Sakamoto, H. Yamaguchi and T. Yao, Extended Abstracts of 21st Conference on Solid State Devices and Materials (Tokyo, 1989) p. 509;
272
I241 [25]
(261
[27] [28]
[29]
[30] [31]
I321
[33]
A.P. Hitchcock et al. /Surface H. Oyanagi, T. Sakamoto, K. Sakamoto, H. Yamaguchi, Y. Kuwahara, T. Matsushita and T. Yao, Proc. XAFS 6 (Oxford University Press, Oxford, 1990); H. Oyanagi, Appl. Surf. Sci. 60/61 (1992) 522. C. Bouldin, Physica B 158 (19891 596. J.C. Woicik, C.E. Bouldin, MI. Bell, J.O. Cross, D.J. Tweet, B.D. Swanson, T.M. Zhang, L.B. Sorensen, CA. King, J.L. Hoyt, P. Pianetta and J.F. Gibbons, Phys. Rev. B 43 (19911 2419. J.-M. Baribeau, D.J. Lockwood, T.E. Jackman, P. Aebi, T. Tyliszczak and A.P. Hitchcock, Can. J. Phys. 69 (1991) 246. P. Aebi, T. Tyliszczak, A.P. Hitchcock, T.E. Jackman and J.-M. Baribeau, J. Vat. Sci. Technol. A 9 (1991) 907. P. Aebi, T. Tyliszczak, A.P. Hitchcock, K.M. Baines, T.K. Sham, T.E. Jackman, J.-M. Baribeau and D.J. Lockwood, Phys. Rev. B 45 (1992) 13579; T.E. Jackman, J.-M. Baribeau, D.J. Lockwood, P. Aebi, T. Tyliszczak and A.P. Hitchcock, Phys. Rev. B 45 (1992) 13591. T. Tyliszczak, P. Aebi, A.P. Hitchcock, T.E. Jackman. J.-M. Baribeau and D.J. Lockwood, Jpn. J. Appl. Phys. (Suppl. 32-21 32 (19931 134. H. Oyanagi, K. Sakamoto and T. Sakamoto, Jpn. J. Appl. Phys. (Suppl. 32-l) 32 (1993) 119. A.P. Hitchcock, T. Tyliszczak, P. Aebi, J.Z. Xiong, T.K. Sham, K.M. Baines, K.A. Mueller, X.H. Feng, J.M. Chen, B.X. Yang, Z.H. Lu, J.-M. Baribeau and T.E. Jackman, Surf. Sci. 291 (19931 349. J.C. Woicik and P. Pianetta, Synchrotron Radiation Research: Advances in Surface and Interface Science, Vol. 2, Issues and Technology, Ed. R.Z. Bachrach (Plenum, New York, 19921 211. K.L. Kavanagh and G.S. Cargill III, Phys. Rev. B 45 (1992) 3323.
Science 301 (1994) 260-272 [34] A.E. Tabor-Morris, K.M. Kemner, B.A. Bunker and K.A. Bertness, Jpn. J. Appl. Phys. (Suppl. 32-2) 32 (1993) 404. [35] B.X. Yang, F. Middleton, B. Olssen, G.M. Bancroft, J.M. Chen, T.K. Sham, K.H. Tan and D. Wallace, Rev. Sci. Instrum. 63 (1992) 1355. [36] W. Huschka, D. Ross, M. Maier, E. Umbach, J. Electron Spectrosc. Relat. Phenom. 46 (1988) 273. [37] J.-M. Baribeau. D.J. Lockwood, M.W.C. Dharnawardana, N.L. Rowe11 and J.P. McCaffrey, Thin Solid Films 183 (1989) 17. [38] J.-M. Baribeau, Appl. Phys. Lett. 52 (1988) 105; 57 (19901 1748. 1391 Z.H. Lu, S. Poulin-Dandurand, E. Sacher and A. Yelon, Phys. Rev. B 42 (19901 5186. [40] R.D. Carson and S.E. Schnatterly, Phys. Rev. Lett. 59 (1972) 319; J.F. Morar, F.J. Himpsel, G. Hollinger, G. Hughes and J.L. Jordan, Phys. Rev. Lett. 54 (1985) 1960; H. Ma, S.H. Lin, R.W. Carpenter and O.F. Sankey, Phys. Rev. B 44 (1991) 13393. [41] J. Tam, Ed, Amorphous and Liquid Semiconductors (Plenum, London, 1974). I421 L. Ley. Semiconductors and Semimetals 21 (19871 385. [43] J.C. Woicik. B.B. Pate and P. Pianetta, Physica B 158 (I 9891 576. [44] A.P. Hitchcock, T. Tyliszczak, M.L.M. Rocco, J.T. Francis, S.G. Urquhart, X.N. Feng, Z.H. Lu, T.E. Jackman and J.-M. Baribeau, J. Vat. Sci. Technol. A, submitted. [45] P. Lagarde, A.M. Flank, G. Tourillon, R.C. Liebermann and J.P. Itie, J. Phys. I (France) 2 (1992) 1043. [46] V.R. McCrary, F. Sette, CT. Chen, A.J. Lovinger, M.B. Robin. J. Stohr and J.M. Zeigler, J. Chem. Phys. 88 (19881 5925.
,. ,..:.. ,:::. .::... .....:,.:
::‘,::‘:i ::.:: ::::: .,..,.:. .: ..: :: .:::.:.,,.:.: ,:.:, ...I
.I......:
...,: :,:.:.>. .,.: .,..,. ,.,.... .,...
: .,j...:...,,
:,
:i:,‘:j.,,:.,,,. ..’
‘surface science g:“;:;:::::;:.;:~..: :, ,, ,.,. ,,,:.:,,:* .j: :;,: .,.:.:;. ,.. :’.j ..>...y,,. .,.... ::,. .:.:.,.,.: ..,,,,: .“‘:.:‘.:.:.~.‘:::.:jj.::. ., ,., ,.:.:. ,:..,,...y ,.,.,. ...(...
ELSEVIER
Surface
Science 301 (1994) 260-272
Polarization dependence of the Si K-edge X-ray absorption spectra of Si-Ge atomic layer superlattices A.P. Hitchcock
*, T. Tyliszczak,
Institute for Materials Research, McMaster
P. Aebi ’
Uniuersity, Hamilton,
Ontario, Canada L8S 4MI
X.H. Feng CSRF, SRC, University of Wisconsin-Madison,
Z.H. Lu, J.-M. Baribeau, Institute for Microstructural (Received
Stoughton,
WI, USA
T.E. Jackman
Sciences, National Research Council, Ottawa, Canada
7 May 1993; accepted
for publication
19 August
1993)
Abstract The polarization dependence of the SiK-edge X-ray absorption spectra of several [(Si),(Ge),], atomic layer superlattice (ALS) materials grown on both SiflOO) and GetlOO) have been investigated using plane polarized synchrotron radiation. These spectra exhibit sharp, polarization dependent, Si 1s + conduction band (CB) resonance features which are absent in the spectrum of amorphous Si (a-Si). Subtraction of the spectrum of a-Si from that of the crystalline ALS materials is used to isolate the conduction band structure. A constrained curve fit analysis of up to eight data files simultaneously has been used to quantitatively analyze the signal. The CB structure is composed of a number of polarization independent components and several polarization dependent components. In [(Si),(Ge),],,,/Ge(lOO) the lowest energy transition at 1839.1 eV is polarized along the surface normal (the growth direction) while a doublet structure centred at 1841 eV is polarized in the surface plane (perpendicular to the growth direction). A similar spectral pattern is found in [(Si),(Ge),],s/Si(lOO) but the polarization effect is weaker and the sense of the polarization effect is reversed. The polarization dependent signal is attributed to anisotropic states associated with strain-induced tetragonal distortions in the strained-ALS materials.
1. Introduction
Strained superlattice from alternating layers
* Corresponding author. ’ Present address: Institut bourg,
Fribourg,
Switzerland,
structures constructed of pure Si, pure Ge or
Si-Ge alloys are currently of interest with regard to quantum engineered electronic structures adapted for specific device applications [l-3]. In addition atomic layer superlattice (ALS) structures grown by molecular are useful model systems
de Physique, CH-1700.
0039-6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(93)E0493-E
UniversitC
Science
de Fri-
beam epitaxy (MBE) for studying strained
interfaces and the dependence ture on growth and processing
B.V. All rights reserved
of material strucprocedures. The
A.P. Hitchcock et al. /Surface
strain in short period superlattice structures is the origin of the modified band structure. The band structure of [(Si),(Ge),l, ALS and strained layer superlattice structures composed of Si,Ge,_, alloys has been investigated by a number of groups [4-191. The question of whether or not the strain and/or the superlattice structure can lead to a direct band gap has been of particular interest. Optical techniques in the visible region have been used by several groups to characterize the effects of strain on the band structure of Si-Ge ALS materials [20-221. Recently, X-ray absorption has been applied to studies of strained semiconductors, typically using the extended fine structure signal. Both Ge K-edge [23-301 and Si K-edge spectroscopy [31,32] have been used to study Si-Ge ALS. The review by Woicik and Pianetta [32] is a very useful summary of this work. The SiK-edge spectra of c-Si, crystalline Si,Ge,_, alloys, and all of the Si-Ge ALS samples exhibit a characteristic double-peaked structure at the Si K-edge which is attributed to Si 1s + Si 3p conduction band (CB) transitions [31,32]. Both the average chemical composition and the nature of the superlattice structure appear to be involved in determining the energetics and intensity of the SiK-edge CB features [31]. X-ray absorption studies have been used to study other strained semiconductor systems. For example, Kavanagh and Cargill [33] have used GaK-shell EXAFS to investigate the local lattice distortions associated with strain in Si heavily doped with Ga. Tabor-Morris et al. [34] have used polarization dependent EXAFS to study strain effects in Ga,,~In,,~P grown by MOCVD on GaAs. In this paper we report the first observation of a polarization dependence in the SiK-edge spectra of single crystal strained layer superlattice materials and its interpretation in terms of the effect of strain-induced lattice distortions on the band structure.
2. Experimental The Si K-edge spectra were recorded using the double crystal beam line [35] funded by the Ontario Centre for Materials Research (OCMR)
Science 301 (1994) 260-272
261
and operated by the Canadian Synchrotron Radiation Facility (CSRF) at the Aladdin ring of the Synchrotron Radiation Centre (SRC) of the University of Wisconsin-Madison. The monochromator was equipped with InSb crystals. The resolution was 0.8 eV at the SiK-edge. Total electron yield (TEY) spectra were obtained using sample current detection. The counter electrode, a metal ring 4 cm in diameter mounted 2 cm in front of the sample, was biased by 130 V. Spectra were also measured using fluorescence yield (FY) detection, both to explore the relative detection sensitivity and to investigate the depth dependence of the total electron yield signal. SiKL fluorescence was detected using a multichannel plate mounted behind an aluminum film, which acts both to block direct detection of electrons and to convert X-ray photons into electrons. The incident photon flux (1,) was monitored using a gas ionization chamber (9.5 cm long; 1.05 Torr static N2) equipped with thin Be windows. All spectra reported are the ratio of the TEY or FY yield to this I, signal. The stability of the energy scale during measurements of the polarization dependence was monitored by recording the c-Si spectrum (normal incidence) before and after studies of a sequence of polar or azimuthal angle variations. The energy scale was stable to better than 0.02 eV over the ca. two hours required to record a sequence of spectra at 8 angles. Over longer periods, particularly between different fills of the electron storage ring, shifts of as much as 0.1 eV were noted. The excellent stability of both the SRC storage ring and the OCMR double crystal monochromator were critical in achieving these results. Absolute energy scales were established by setting the inflection point of the absorption edge of c-Si (as determined by the peak in the derivative) to 1839.2 eV [31]. This energy scale has been established by calibration to known gas phase spectral lines [31]. It also matches the Si 1s binding energy as determined by XPS [36]. Up to four samples were mounted simultaneously on a manipulator equipped with x-y-z and rotation motions. The polarization dependence (the variation of the spectrum with changes in the angle be-
A.P. Hitchcock et al. /Surface Science 301 (1994) 260-272
262 1
”
I
I
”
I
[(Si),(Ge),l,,/Ge(lOo)
PI
1 t 1830
1040
1850
1860
Energy (eV) Fig. 1. Total electron yield (TEY) spectra of [(Si),(Ge),l,, /Ge (#754) recorded at 10” intervals of 0, the angle of X-ray incidence relative to the surface. Offset plotting has not been used. The insert plots the intensity at 1835 eV versus sin-lo.
tween the electric vector of the incident radiation and the plane of the epitaxial layers) was measured by sample rotation. The angles quoted (k2”) are those measured between the direction of propagation of the X-rays and the (100) surface plane of the samples. For plane-polarized radiation this angle is identical to the angle between the electric vector of the incident radiation and the (001) sample normal, which is the growth direction of the epilayers (see sketch in Fig. 1). A careful search was made for azimuthal dependence of the normal-incidence SiK-edge spectrum of [(Si),(Ge),l,,/Ge(lOO) (#754). Within the precision of the measurements the spectra were independent of the azimuthal orientation. In particular, any variation with azimuthal angle
is less than 10% of the observed polar angle variation of the spectrum of [(Si),(Ge),],,,/ Ge(100). The epitaxial ALS and single crystal alloy samples were grown on a VG Semicon V80 MBE system on Czochralski-grown 100 mm Si(100) or 50 mm Ge(100) wafers [371. Prior to superlattice growth a thick (N 100 nm) epitaxial Si or Ge buffer layer was grown to ensure identical initial conditions. Growth procedures have been described in detail elsewhere [37]. X-ray diffraction [38], Raman scattering spectroscopy and crosssection transmission electron microscopy measurements have established the Ge and Si concentrations and nominal layer thicknesses [26,38]. These have been summarized in an earlier paper [31]. The thickness of these ALS samples (50 to 100 nm, depending on the values of m, n, p) was such that the SiK-edge spectrum of ALS grown on a Si substrate could be recorded with negligible contribution from the underlying Si substrate. A protective Si cap on ALS samples was removed by electrochemical oxidation followed by etching. A further oxide etch (using HF to remove SiO, and GeO, and, when required, NaOCl to remove Ge cap layers) was carried out just prior to insertion into the high vacuum measurement chamber (P < 2 x lo-’ Torr). The a-Si sample was prepared as described elsewhere [39]. It was HFetched just prior to insertion into the vacuum chamber.
3. Results and discussion 3.1. Polarization dependence of the SiK-edge near-edge spectrum of [(Si),(Ge),l,, / Ge(lO0) Fig. 1 presents a sequence of Si K-edge spectra of the [(Si>,(Ge),],,/Ge(lOO) sample (code #754), recorded at lo” intervals from 14” to 84” of the photon beam relative to the sample surface. As the angle of incidence decreases the total signal intensity increases dramatically. This is simply a consequence of depositing a greater proportion of the photon beam in the near surface region sampled by the current detection mode. The sampling depth is estimated to be about 20 nm, based
A.P. Hitchcock et al. /Surface
on comparison of the intensity of the oxide signal in the TEY and FY spectra of a partly oxidized c-Si sample. When there is a fixed signal sampling depth which is much less than the photon penetration depth, the signal intensity should vary with sin-l(0), where 8 is the angle between the incident radiation and the surface. The insert to Fig. 1 plots the intensity at 1835 eV (below the onset of Si 1s absorption) against sin-lo. Clearly the spectral intensity closely follows the relationship expected from the deposition of increasing photon dose in the depth sampled by the TEY detection technique. In addition to the gross intensity change, there are small but systematic changes in the shape of the CB structure between 1839 and 1842 eV. This is revealed more clearly by isolating the CB structure through subtraction of the spectrum of a-Si, as shown in Fig. 2. The SiK-edge spectrum of a-Si is very smooth between 1839 and 1842 eV. As Woicik and Pianetta [32] first noted, while the SiK-edge spectrum of a-Si does have a broad maximum, this is significantly less intense than that in c-Si. Furthermore, the a-Si spectrum does not exhibit any of the sharper CB structure observed in c-Si, c-Si,Ge,_, alloys and the ALS samples. Thus subtraction of an appropriately energy shifted (see below) and normalized a-Si spectrum should provide a means to isolate the structure associated with the CB states. In fact it is the featureless character of the CB structure in a-Si, along with comparisons to multiple scattering calculations that demonstrates that the CB signal is sensitive to the long range structure of the sample [31]. For the analysis described below each spectrum is first subjected to a linear background subtraction and then unit normalization of the continuum intensity at 1858 eV. This subtractive procedure for analysis of X-ray absorption data is not commonly used so it is reasonable to question its rationale. Since the short range order (first shell) of a-Si and c-Si are essentially identical whereas the long range order differs, the difference procedure should enhance differences in the electronic structure which are related to the long range order. In a scattering model it is multiple scattering from more distant shells that dominates the low energy range [31].
Science 301 (1994) 260-272
k 4
263
[(si),(Ge),l,,/Ge(loo)
I
1636
,
1836
Photon
I
1840
*
I
(
1842
1844
Energy
,
(eV)
Fig. 2. Overplot of the SiK-edge spectra of a-Si and #754, at 14” and 84” polar angles indicating the procedure used in generating difference spectra. The a-Si spectrum has been shifted + 0.42 eV from the as-recorded scale (see text).
In a-Si the absence of long range correlations means that this multiple scattering signal averages to zero, whereas in c-Si it is the origin of the conduction band structure. One possible complication is the contribution of multiple scattering in the first shell, which should be the same in both a-Si and c-Si and thus would be removed by the subtraction. However, this signal is believed to be slowly varying in the energy range of the conduction band maximum. Thus, an incorrect representation of this part of the signal will likely result in (at most) a background in the curve fits of the subtracted data (see Section 3.21, an aspect to which position and relative intensities of the fitted peaks are likely to be relatively insensitive.
264
A.P. Hitchcock et al./Surface
Very careful energy calibration through repetitive, sequential recording of spectra over short times has shown that the position of the Si K-edge, as measured by the inflection point, is 0.45(3) eV lower in energy in a-Si than in c-Si. The position of an absorption edge depends on both the energy of the core level and the onset of the conduction band (empty density of states). There may also be differences in the core excitonic contributions in c-Si and a-Si. However, this is considered to be a negligible factor at the Si 1s edge since the excitonic binding energy will be much smaller than the energy resolution (at the Si2p core edge the excitonic states of c-Si are bound by only 40 meV [40]). The Si 1s energy of c-Si and a-Si is expected to be identical since the electrostatic environment of Si in c-Si and a-Si is virtually identical. In contrast, it is well known that the conduction band onset is much lower in a-Si than in c-Si because of band tailing effects [41]. We conclude that the 0.45(3) eV shift to lower energy of the SiK-edge of a-Si relative to c-Si is a reflection of the lower energy conduction band onset in a-Si than in c-Si. The Si2p,,, absorption edge of a-Si is lower than that of c-Si, although the shift is only 0.05 eV [42]. The SiKedge rises much more slowly in c-Si than a-Si, as demonstrated by a much greater width of the peak in the first derivative spectrum of c-Si (1.45 eV FWHM) as compared to that of a-Si (0.95 eV FWHM). The greater breadth is associated with additional low-lying conduction band states which are not present in a-Si. The SiK-edge in the spectra of the ALS compounds is within 0.1 eV of that of c-Si. In order to minimize negative going signals in the difference spectra, we have shifted the a-Si spectrum by +0.42 eV to higher energy prior to subtraction. Analyses carried out without prior shift of the a-Si spectrum give the same qualitative results except that the first peak has consistently negative amplitude. The spectra presented in Figs. 1 and 2 were all determined with electron yield detection which has a rather shallow sampling depth. This raises the question as to whether the polarization effects could be a purely surface phenomena (e.g. a result of changing proportions of surface and
Suence 301 (1994) 260-272
1,
I
1838
f/I
1840
Photon Fig. 3. Comparison of the (Ge),l,, /Ge (#754) recorded electron yield.
1842
Energy
1,) 1844
184 6
(eV)
14” and 84” spectra of [(Si), by fluorescence yield and total
bulk transitions as the incident angle is changed). Surface-specific core excitation resonances have been identified in the Si K-edge spectrum of clean Si(ll1) [43]. In order to investigate this possibility the #754 ALS sample was also studied by fluorescence yield (FY) detection, while simultaneously recording the electron yield signal. FY samples the full X-ray penetration depth which is about 2 pm for 2 keV X-rays. This is much larger than the total thickness of the superlattice (about 60 nm in #754). The statistical precision of the FY data is much less than that of the TEY data and thus a detailed measurement of the angular dependence was not performed. Nevertheless, the polarization dependence, as monitored by a comparison of the normal and glancing incidence spectra, is very similar in FY and TEY (see Fig. 3). This clearly proves that the polarization dependence is a property of the bulk superlattice structure of the ALS material and not a surfacespecific phenomena.
265
A.P. Hitchcock et al. /Surface Science 301 (1994) 260-272
3.2. Multi-file curve-fit analysis of the polarization dependence
analysis procedure which has been described earlier [28]. In the present case fits to individual spectra had shown that the CB structure could be well reproduced by the sum of a minimum of six Gaussian peaks with widths similar to that estimated for the instrumental contribution and positions which were virtually independent of polar angle (i.e. changing by less than k 0.03 eV). Thus, the multi-file fit to the sequence of 8 spectra of #754 was carried out by constraining the position and width of the corresponding line in each spec-
In order to quantify the polarizaticn effect, the a-%-subtracted spectra were subjected to a curvefit analysis. To obtain more stable results we have developed a program (MGAUSS) to fit up to eight spectra simultaneously to a combination of lines (of Gaussian, Lorentzian or Voigt shape) with provision for constraints on the fit parameters. This is an extension of a multi-file EXAFS
I
’
I
’
123 Ill
1838
1840
I
I
A
multi-file
curve fit analysis
,
,
15
6
12345
II
I
III
1842
1844
”
Photon Fig. 4. Constrained details.
,
of the 14”-84” spectra
1838 Energy
1840
,
, 6
II
I
1842
1844
(eV)
of [(Si)2(Ge),],,/Ge
(#754) to 6 lines. See text for further
266
A.P. Hitchcock
et al. /Surface
Science 301 (1994) 260-272
trum to have the same value. The features at 1840.5 and 1841.5 were found to change in a very similar way through the series of spectra. A more stable fit was achieved by constraining these two components to have identical widths and amplitudes - i.e. to treat the two features as a doublet of two lines of equal intensity. The results of the constrained multi-file fit are presented graphically in Fig. 4 while the numerical values are listed in Table 1. In most cases there is good
Table 1 Results of curve fit analysis spectrum a Peak #
Energy (eV1
of Si 1s + CB features Width (eV)
in #754,
Intensity 14” c
9
agreement between the fit-generated curve and the experimental data. An exception to this is the leading edge of the 84” spectrum where there is considerable discrepancy between the fit and the experimental data because of the presence of negative signal. This discrepancy could be removed by a small, additional energy shift of the aSi spectrum when preparing the subtracted data file for the 84” data. However, we prefer to use the same data processing for all files. In addition,
#752,
#700
after a-Si subtraction
and #698
samples
isolated
by subtraction
of the a-Si
(arb. units)
24
34”
44”
54”
64”
74”
84
0.208 0.084 0.439 0.275 0.145
0.199 0.070 0.439 0.269 0.128
0.172 0.148 0.439 0.274 0.132
0.121 0.166 0.439 0.272 0.121
0.088 0.262 0.439 0.283 0.147
0.042 0.286 0.439 0.281 0.139
0.01s 0.328 0.439 0.282 0.134
~ 0.022 0.324 0.439 0.284 0.127
1.151
1.105
1.114
1.156
1.219
1.187
1.I 98
1.152
#754 [(Si~,(Ge),l,,/Ge(lOO) ;3,41 h 2 5 6
1840.50, 1841.57 1839.11 1839.86 1841.75 1843.15
0.93 1.11 1.11 1.11
xd #752 I(Si),(Ge),,],,/Ge(lOO) ;3,41 b 2 5 6
1839.13 1840.78, 1841.53 1839.93 1841.76 1843.25
0.945 0.936 1.077 1.077 1.077
xd #700 I(Si),(Ge),I,oo/Si(lOO) t>41 h 1839.70, 1841.82 1838.85
0.904 1.264
2 5 6
0.904 0.904 0.904
1840.74 1840.11 1841.46
Ed #698 f(Si),(Ge),l,, / Si(lO0) t1,4) h 1839.70, 1841.82 1838.85
0.904 1.264
2 5 6
0.904 0.904 0.904
Ed
1840.74 1840.11 1841.46
0.293 - 0.006 0.449 0.302 0.160
0.055 0.230 0.44’) 0.302 0.144
1.215
1.180
0.019 0.004 0.143 0.211 0.235
0.097 0.047 0.102 0.21 I 0.235
0.612
0.692
0.011 0.002 0.145 0.205 0.241
O.OY2 0.045 0.1 16 0.205 0.24 I
0.604
O.hYY
” Structure isolated by subtraction of the spectrum of a-Si (after an energy shift of the a-Si spectrum by +0.42 eV see text). h These two components were required to have the same intensity. The ntensity indicated is the sum of the areas of both peaks. ’ Angle between X-ray propagation direction and the surface plane. This is the same as the angle between the E-vector and the surface normal (see Fig. 5). d Sum of the peak areas for all components.
A.P. Hitchcock et al. /Swface
0
I
I
,
I
20
40
60
80
Polarization
Angle
(‘)
Fig. 5. Variation of the peak intensity (area) for the peak at 1839.1 eV and the doublet at 1840.5 and 1841.5 eV (sum of both peaks) as a function of 0, the angle between the E-vector and the sample normal. Note that the E-vector was rotated from the (001) to the (110) direction in the angular variation. The solid and dashed lines are cos% and sin28 functions scaled for best match to the intensity data.
there is good reason to believe that the conduction band profile has a more complex shape than one would obtain with the sum of a small number of Gaussian lines. Thus a perfect agreement is not expected. The curve-fit analysis shows that peak 1 and the {3,4} doublet are polarization dependent, whereas peaks 2, 5 and 6 are polarization independent. Fig. 5 plots the angular variation of the intensity (peak area) of the polarization dependent lines as derived by the multi-file analysis of the processed #754 data. Fig. 5 also plots sin28 (peak 1) and cos20 (doublet {3,4)) angular distributions whose amplitudes have been scaled to best match the measured angular dependence. The reasonable quality of the match to these expected angular dependencies is evidence in favour of a conventional polarization dependence
Science 301 (1994) 260-272
261
rather than band dispersion as the explanation of our observations. This conclusion should be treated cautiously because one could imagine certain types of band dispersion which could provide similar experimental results. In principle comparison of the measured angular dependent intensities with calculation could be used to deduce the spatial orientation of the transition moments. However, we do not believe the curve fit analysis of this data is sufficiently unique to warrant this procedure. Qualitatively, the polarization dependent intensities indicate that peak 1 corresponds to a transition whose moment (and thus the orientation of the conduction band state) is aligned along the growth direction while the doublet, (3,4} corresponds to states oriented in the planes of the layers. The polarization dependence for three other Si-Ge ALS materials (#752 - [W,(Gel,,l,,/ Ge(100); #700 - [(Si),(Ge),],,/Si(lOO); #698 was investigated by mea[(Si),(Ge),l,,/Si(lOO)l) suring spectra at 14” and 84” angles of X-ray incidence. These limiting spectra have been analyzed in the same fashion as the angular sequence of #754 data - i.e. a curve fit was performed on the CB structure isolated by subtraction of the a-Si spectrum. The fits for the 14 and 84” data of #752 and #700 are shown in Fig. 6, while numerical results for all three additional ALS samples are summarized in table 1. As in the analysis of the 754 data, the optimized widths of the individual lines are between 0.9 and 1.2 eV (see Table 11, similar to the instrumental resolution. The structure of the #752 sample is very similar to that of #754 (essentially “Si, in Ge”). Thus it is not surprising that the curve-fit analysis of the #752 data produces essentially the same result as found for #754 - i.e. a peak at 1839.1 eV with polarization along the growth direction and a doublet at (1840.7, 1841.5 eV> with in-plane polarization. However, the result for the #700 and #698 ALS samples - which are basically “Ge, in Si” structures - is dramatically different. The polarization dependence differs qualitatively from that of the #752 and #754 samples. The polarization dependence of the lowest energy transition (1838.9 eV> has the opposite sense. It is polarized in the plane of the layers in the #700
268
A.P. Hitchcock
et al. /Surface
and #698 samples rather than along the growth direction as was the case for the 1839 eV transition in the #752/754 samples. The polarization dependence at higher energy is also different. The changes in the region of the dip at 1841 eV in (#700, #698) are associated with a transition at 1840.7 eV which is partially polarized perpendicular to the layers. The 1840.7 eV peak also contains a polarization independent component which we associate with the Si atoms in the centre of the Si, layers which are likely in a much less strained environment than those at the Si-Ge interface. In order to obtain a satisfactory fit to the #698 and #700 data we have also introduced a doublet of peaks at 1839.7 and 1841.8 eV, on the outer sides of the two main components. This weak doublet signal appears to have an in-plane polarization. While there is no counterpart of this signal in the #752/754 samples, it is possible such a contribution exists but is being masked by the more intense polarization components in those samples. It is also noteworthy that the total intensity in the CB structure is considerably less in #700 and #698 than in the #752 and #754 samples (see the C column in table 1). We speculate that the reduced intensity may be the result of greater intermixing and thus reduced longer
Science 301 (1994) 260-272
range order in the ALS on Si samples. The a-Si subtraction and associated curve fit analysis shows there are large changes in the polarization effect for different Si-Ge ALS samples. Fig. 7 demonstrates these changes in a way which is more qualitative but which involves less data manipulation. Fig. 7 plots the limiting (14”, 84”) continuum normalized spectra and their differences for four different samples, two grown on Si and two grown on Ge. The top two curves (#754 and #752) are from [(Si),(Ge),,], samples grown on Ge. The lower two curves in Fig. 7 arc spectra of ALS samples grown on Si(100) (#69X and #700). These simple difference spectra clearly reveal that the polarization dependence is inverted between the on-Ge and on-Si ALS samples. This simple difference evaluation also shows that the polarization dependence is about three times weaker in the #698 and #700 samples relative to that in the #752 and #754 samples. The reduced polarization dependence evident in the “Ge, in Si” samples relative to that exhibited by the “Si, in Ge” samples is likely associated with the fact that only a portion of the Si atoms (those closest to the Ge layers) are in a locally distorted environment. It is important to note that the principal obserI
(D
/
’
I
#700 14O
I
I
1838 Photon
Fig. 6. Results (#700X
of fits to SiK edge spectra
(a-Si subtracted)
Energy
1840
,
I
1842
(eV)
for ALS samples
of (a) [(Si),(Ge),Zl,,
(#752)
and (b) [W),(Ge)Zl,,,,,
A.P. Hitchcock et al. /Surface
Science 301 (1994) 260-272
269
Difference
1835
1040
1845
1850 Photon
1835
1855 Energy
1840
1845
(eV)
Fig. 7. Left panel - 14”(solid) and 84” (dotted) spectra of [(Si)z(Ge),,],,/Ge (#752); [(Si)z(Ge),],,/Ge (#754); [(Si),(Ge),],,/Si (#698); [W,(Ge),],,/Si (#700). All spectra have been background subtracted and continuum normalized. The 14” and 84” spectra match each other outside the region plotted. Right panel - the difference in the normalized 14” and 84” spectra multiplied by 5 (for #698, #700) or by 2 (#752, #754).
vation is independent of the curve fits. The picture provided by the curve fits of all four ALS samples (754, 752, 700, 698) is fully consistent with the conclusions drawn from examination of the raw data. For example if one judges by the depth of the valley separating the two main peaks in the raw spectra, one concludes that the polarization dependence of the #698, #700 samples is reversed from that for #754 and #752 - i.e. the dip is greatest at 84” for #698 and #700, whereas it is greatest at 14” for the #752 and #754 samples.
3.3. Relationship of the polarization effect to the ALS structure In strained ALS samples grown on Ge, the Si-Si interatomic spacing perpendicular to the growth direction is stretched relative to the value in pure Si because of the commensurate epitaxy with the Ge substrate. This is accompanied by a decrease in Si-Si interatomic distance along the growth direction. X-ray diffraction shows clearly that the Si, layers in #754 and #752 are tetragonally distorted. There is an increased in-plane
270
A. P. Hitchcock
et al. /Surface
Si-Si spacing (to match the bulk Ge-Ge spacing1 and a decreased out-of-plane Si-Si spacing (to compensate for the reduced in-plane binding). This anisotropic strain field splits the conduction band and creates spatially-aligned states. In the #754 and #752 samples this results in CB features with a high degree of polarization because all of the Si atoms are in highly strained environments in these two samples. In contrast, the #698 and #700 samples consist of quite thin (n = 2) strained Ge layers separated by thicker, (Si), or (Sil, layers. Si grown on Si substrate would be expected to adopt a structure very similar to that of the Si(100) template: unstrained and bulk-like. However, the Si layers at the Si-Ge interface may actually be somewhat tetragonally distorted because of the influence of the adjacent strained Ge, layers. In addition there is inevitably some intermixing [26-281 and any Ge atoms present in the Si layers will introduce strain. The net result of these effects in ALS grown on Si is that, over the whole superlattice, the average in-plane spacing is compressed and the average out-of-plane spacing is increased relative to the Si substrate. The critical factor with regard to the polarization dependence is that the sense of the strain field in ALS on Si is inverted relative to that in strained-ALS materials grown on Ge. This is consistent with the fact that the polarization dependence for strained-layer superlattice samples grown on Si is opposite to that for strainedlayer superlattice samples grown on Ge. When strained-ALS samples are annealed the structure relaxes by intermixing. Measurements of the SiK-edge spectra show that the polarization dependence decreases with increasing amounts of annealing. Studies are presently in progress which aim to quantify the relationship between strain and the magnitude of the polarization dependence. The reduced polarization dependence in relaxed ALS samples or those with thicker Si layers is consistent with our interpretation that the polarization dependent states are associated with the strain induced structural anisotropy of the Si-Ge superlattice. Alloys which are grown epitaxially on a single crystal Si or Ge substrate contain an oriented strain field which is similar to that present in the ALS samples. Thus,
Science 301 (I 994) 260-272
if our interpretation of the polarization dependence of the SiK-edge spectra of the strainedALS samples is correct, we would expect strained alloys to exhibit a SiK-edge polarization dependence. Indeed this is the case [44]. Based solely on a consideration of the as-measured spectra one might consider that the CB structure consists of only 2 lines whose position changes with angle of X-ray incidence. Within this framework, the energies of these peaks might shift because of a dispersion dependence of the unoccupied band structure - i.e. the variation of incidence angle tracks shifts in the energy of an unoccupied band as a function of crystal direction, in a fashion similar to angle resolved photoemission (ARPES) mapping of occupied bands. However, the high quality of the Gaussian curvefit analysis of the CB structure (after isolation by subtraction of the a-Si signal) using ftiecl energy states of variable intensity, along with the absence of discernable azimuthal dependence, lead us to conclude that dispersion effects are smaller than our measurement sensitivity. While a detailed correlation of these results to published band structure will not be presented in this paper, it is worth pointing out a few aspects where the calculations support this work. Ghahramani et al. [16] have shown that the anisotropy introduced by the strain is more important than the superlattice periodicity in controlling the band structure and spectroscopic properties. Ma et al. [19], in an extensive study of strained atomic layer and alloy superlattices, have shown that the states at the conduction band minimum are aligned along the growth direction for epilayers grown on (1001 templates which are under tensile stress, such as [W,(Ge),,l,,/ Ge(100). These authors [19] have also presented a group theoretical description of the effect on the band structure of the reduction in symmetry from Ol(Fd?m) in pure Si or Ge to Diz (14,/amd) in strained epilayers grown on Si(100). In their analysis they predict the magnitude of conduction band splitting associated with the symmetry reduction and they discuss the polarization dependence of optical transitions. The calculations of Ma et al. [193 predict that the lowest energy conduction band states are aligned along the
A.P. Hitchcock et al. /Surface
growth direction for Si, grown on Ge and perpendicular to the growth direction for Ge, grown on Si. Among the relatively few reports of studies of SiK-edge polarization dependence, we note the recent work of Lagarde et al. [45] on quartz and densified silica, as well as that of McCrary et al. on poly(di-n-hexylsilane) [46]. The polarization dependence of the Si K-edge spectra of polysilane chains [46] is quite large because of the considerable anisotropy of the local structure around Si. However, in quartz or densified silica the polarization effects are rather small [451, of a similar magnitude to those observed in this work. We rationalize the weakness of the polarization dependence of the spectra of densified quartz/ silica and the Si-Ge ALS by noting that, in both species, the local symmetry of the Si atoms is only slightly perturbed from the normal, highly symmetric tetrahedral environment.
4. Summary The polarization dependence of the Si 1s spectra of strained Si-Ge atomic layer superlattice samples has revealed the existence of anisotropic low energy conduction band states which are sensitive to the magnitude and the orientation of the strain-induced tetragonal distortion. Theoretical study, in particular a detailed analysis of the anisotropy of the conduction band states in strained layer Si-Ge superlattices, is required to properly understand these observations. It seems likely that polarization dependent near edge spectroscopy at the SiK-edge and possibly other Si and Ge core edges will be particularly valuable for measuring band structure changes associated with quantum confinement and superlattice periodicity effects in Si-Ge materials.
Acknowledgements
This work is based upon research conducted at the Synchrotron Radiation Center, University of Wisconsin, which is supported by the NSF under award DMR-9212658. We thank the staff of SRC
Science 301 (1994) 260-272
271
for their expert operation of Aladdin. This research has been sponsored financially by the Ontario Centre for Materials Research and NSERC (Canada).
References
111Silicon
Molecular Beam Epitaxy, Eds. J.C. Bean and E. Kasper (CRC Press, Boca Raton, FL, 1988). Eds. G. 121Spectroscopy of Semiconductor Microstructure, Fasol, A. Fasolino and P. Lugli (Plenum, New York, 1989). [31 T.P. Pearsall, Mater. Sci. Eng. B 9 (1991) 225. [41 S. Froyen, D.M. Wood and A. Zunger, Phys. Rev. B 36 (1987) 4547. 151 L. Brey and C. Tejedor, Phys. Rev. Lett. 59 (1987) 1022. [61 M.A. Gell, Phys. Rev. B 38 (1988) 7535. 171 M.S. Hybertsen and M. Schliiter, Phys. Rev. B 36 (1987) 9683. M. Schliiter, R. People, S.A. Jackson, 181 M.S. Hybertsen, D.V. Lang, T.P. Pearsall, J.C. Bean, J.M. Vanderberg and J. Bevk, Phys. Rev. B 37 (1988) 10195. 191 D. Shen, F. Lu, K. Zhang and X. Xie, Appl. Phys. Lett. 52 (1988) 1599. 1101 I. Morrison and M. Jaros, Phys. Rev. B 37 (1988) 916. [ill K.B. Wong, M. Jaros, I. Morrison and J.P. Hagon, Phys. Rev. Lett. 60 (1988) 2221. [121 S. Circai and I.P. Batra, Phys. Rev. B 38 (1988) 1835. 1131 S. Froyen, D.M. Wood and A. Zunger, Thin Solid Films 183 (1989) 33. 1141 M.A. Gel1 and A.C. Churchill, Phys. Rev. B 39 (1989) 10449. and J.E. Sipe, Phys. Rev. B 40 (1989) El51 E. Ghahramani 1102. D.J. Moss and J.E. Sipe, Phys. Rev B 41 [161 E. Ghahramani, (1990) 5112. 1171 S. Circai, A. Baratoff and I.P. Batra, Phys. Rev. 41 (1990) 6069. 1181 J.E. Bernard and A. Zunger, Phys. Rev. B 44 (1991) 1663. 1191 Q.M. Ma, K.L. Wang and J.N. Schulman, Phys. Rev. B 47 (1993) 1936. [20] U. Schmid, F. Lukes, N.E. Christensen, M. Alouan, M. Cardona, E. Kasper, H. Kibbel and H. Presting, Mater. Sci. Eng. B 9 (1991) 233. [21] H. Okumura, K. Miki, S. Misawa, K. Sakamoto, T. Sakamoto, S. Yoshida, K. Asami and S. Gonda, Mater. Sci. Eng. B 9 (1991) 245. [221 F. Ferrieu, F. Beck and D. Dutartre, Solid State Commun. 82 (1992) 427. [231 H. Oyanagi, T. Sakamoto, K. Sakamoto, T. Matsushita, T. Yao and T. Isheguro, J. Phys. Sot. Jpn. 57 (1988) 2086; H. Oyanagi, T. Sakamoto, K. Sakamoto, H. Yamaguchi and T. Yao, Extended Abstracts of 21st Conference on Solid State Devices and Materials (Tokyo, 1989) p. 509;
272
I241 [25]
(261
[27] [28]
[29]
[30] [31]
I321
[33]
A.P. Hitchcock et al. /Surface H. Oyanagi, T. Sakamoto, K. Sakamoto, H. Yamaguchi, Y. Kuwahara, T. Matsushita and T. Yao, Proc. XAFS 6 (Oxford University Press, Oxford, 1990); H. Oyanagi, Appl. Surf. Sci. 60/61 (1992) 522. C. Bouldin, Physica B 158 (19891 596. J.C. Woicik, C.E. Bouldin, MI. Bell, J.O. Cross, D.J. Tweet, B.D. Swanson, T.M. Zhang, L.B. Sorensen, CA. King, J.L. Hoyt, P. Pianetta and J.F. Gibbons, Phys. Rev. B 43 (19911 2419. J.-M. Baribeau, D.J. Lockwood, T.E. Jackman, P. Aebi, T. Tyliszczak and A.P. Hitchcock, Can. J. Phys. 69 (1991) 246. P. Aebi, T. Tyliszczak, A.P. Hitchcock, T.E. Jackman and J.-M. Baribeau, J. Vat. Sci. Technol. A 9 (1991) 907. P. Aebi, T. Tyliszczak, A.P. Hitchcock, K.M. Baines, T.K. Sham, T.E. Jackman, J.-M. Baribeau and D.J. Lockwood, Phys. Rev. B 45 (1992) 13579; T.E. Jackman, J.-M. Baribeau, D.J. Lockwood, P. Aebi, T. Tyliszczak and A.P. Hitchcock, Phys. Rev. B 45 (1992) 13591. T. Tyliszczak, P. Aebi, A.P. Hitchcock, T.E. Jackman. J.-M. Baribeau and D.J. Lockwood, Jpn. J. Appl. Phys. (Suppl. 32-21 32 (19931 134. H. Oyanagi, K. Sakamoto and T. Sakamoto, Jpn. J. Appl. Phys. (Suppl. 32-l) 32 (1993) 119. A.P. Hitchcock, T. Tyliszczak, P. Aebi, J.Z. Xiong, T.K. Sham, K.M. Baines, K.A. Mueller, X.H. Feng, J.M. Chen, B.X. Yang, Z.H. Lu, J.-M. Baribeau and T.E. Jackman, Surf. Sci. 291 (19931 349. J.C. Woicik and P. Pianetta, Synchrotron Radiation Research: Advances in Surface and Interface Science, Vol. 2, Issues and Technology, Ed. R.Z. Bachrach (Plenum, New York, 19921 211. K.L. Kavanagh and G.S. Cargill III, Phys. Rev. B 45 (1992) 3323.
Science 301 (1994) 260-272 [34] A.E. Tabor-Morris, K.M. Kemner, B.A. Bunker and K.A. Bertness, Jpn. J. Appl. Phys. (Suppl. 32-2) 32 (1993) 404. [35] B.X. Yang, F. Middleton, B. Olssen, G.M. Bancroft, J.M. Chen, T.K. Sham, K.H. Tan and D. Wallace, Rev. Sci. Instrum. 63 (1992) 1355. [36] W. Huschka, D. Ross, M. Maier, E. Umbach, J. Electron Spectrosc. Relat. Phenom. 46 (1988) 273. [37] J.-M. Baribeau. D.J. Lockwood, M.W.C. Dharnawardana, N.L. Rowe11 and J.P. McCaffrey, Thin Solid Films 183 (1989) 17. [38] J.-M. Baribeau, Appl. Phys. Lett. 52 (1988) 105; 57 (19901 1748. 1391 Z.H. Lu, S. Poulin-Dandurand, E. Sacher and A. Yelon, Phys. Rev. B 42 (19901 5186. [40] R.D. Carson and S.E. Schnatterly, Phys. Rev. Lett. 59 (1972) 319; J.F. Morar, F.J. Himpsel, G. Hollinger, G. Hughes and J.L. Jordan, Phys. Rev. Lett. 54 (1985) 1960; H. Ma, S.H. Lin, R.W. Carpenter and O.F. Sankey, Phys. Rev. B 44 (1991) 13393. [41] J. Tam, Ed, Amorphous and Liquid Semiconductors (Plenum, London, 1974). I421 L. Ley. Semiconductors and Semimetals 21 (19871 385. [43] J.C. Woicik. B.B. Pate and P. Pianetta, Physica B 158 (I 9891 576. [44] A.P. Hitchcock, T. Tyliszczak, M.L.M. Rocco, J.T. Francis, S.G. Urquhart, X.N. Feng, Z.H. Lu, T.E. Jackman and J.-M. Baribeau, J. Vat. Sci. Technol. A, submitted. [45] P. Lagarde, A.M. Flank, G. Tourillon, R.C. Liebermann and J.P. Itie, J. Phys. I (France) 2 (1992) 1043. [46] V.R. McCrary, F. Sette, CT. Chen, A.J. Lovinger, M.B. Robin. J. Stohr and J.M. Zeigler, J. Chem. Phys. 88 (19881 5925.