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Orientation dependent adsorption of oxygen on a cylindrical Ge sample
408
Surface Science 201 (1988) 408-418 North-Holland, Amsterdam
ORIENTATION DEPENDENT ADSORPTION OF OXYGEN ON A CYLINDRICAL Ge SAMPLE H.J. KUHR and W. RANKE Fritz-Haber-lnstitut der Max.Pianck-Geseilschaft, Faradayweg 4-6, D.IO00 Berlin 33, Germany Received 17 February 1988; accepted for publication 29 March 1988
The dependence of oxygen adsorption on the crystallographic surface orientation was studied on a cylindrical Ge sample with [110] as cylinder axis by Auger electron and photoelectron spectroscopy. At 300-340 K, adsorption is slowest at (110), (111) and (113) and increases in the vicinity of these orientations due to step-enhanced adsorption. At 650 K, adsorption is faster, especially at (111). The analysis of chemical shifts associated with the Ge 3d emission on six orientations yields discrete shifts towards higher binding energy for Ge atoms with 1, 2 or 3 oxygen ligands (Ge 1+, Ge 2+, Ge 3+ ). At 340 K, the Ge 1+ and Ge 2+ components have similar intensities. Their average positions are at about - 0 . 7 and - 1 . 7 eV with respect to the bulk emission (towards higher binding energy). At 650 K, The Ge 2+ component becomes more prominent, corresponding to the formation of predominantly GeO. Surprisingly, the positions of Ge 1+ and Ge 2+ become strongly orientation dependent, varying between - 0 . 5 6 eV (331) and -1.17 eV (001) for Gei + and - 1 . 7 2 eV (331) and - 2 . 0 3 eV (001) for Ge 2+. Also a weak Ge 3+ component at about - 2 . 8 eV but no Ge 4+ component was observed. A shift of the O 2s orbital from - 2 1 . 9 to -22.4 eV is observed when going from predominantly edsorbed oxygen to a GeO-like configuration.
1. Introduction Compared to the oxygen-silicon system, only few investigations have addressed the system oxygen on clean, single crystal germanium surfaces. Surface conductivity and LEED measurements were performed by Henzler [1], the electron loss structure was investigated by Ludeke and Koma [2], and sticking coefficients and the oxygen-uptake kinetics were measured using different techniques by the same authors [2] on (001) and (111), by Frantsuzov and Makrushin [3] and by Surnev [4] on G e ( l l l ) and by Surnev and Tikhov [5] on Ge(001) with in part strongly varying numerical results. Core-level shifts of the Ge 3d emission were investigated by Garner et al. [6] on G e ( l l l ) and by Schmeisser et al. [7] on G e ( l l l ) and (100). The latter demonstrated that, similar to silicon, also on germanium distinct resolvable shifts corresponding to Ge atoms with one, two, three or four oxygen ligands (Ge 1+, Ge 2+, Ge 3+, Ge 4÷) could be observed. In contrast to Si, however, adsorption at low 0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
H.J. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
409
pressure yielded Ge 2+ as the most stable species, in agreement with the stability of GeO (monoxide) at low pressure. Also in thermal desorption experiments, GeO had been observed earlier [2-5] and thus shown its high stability. In this paper, we present Auger electron spectroscopy results on the dependence of the amount of adsorbed oxygen on 2he crystallographic orientation. Adsorption curves and initial sticking coefficients are compared for (001) and (111). Six orientations were further investigated by photoelectron spectroscopy and the Ge 3d core-level shifts analyzed.
2. Experimental 2.1. Experimental setup As described in detail before [8], a cylindrically shaped Ge single crystal was used to study the dependence of surface properties on the crystallographic orientation in the region (110)-(111)-(001). Auger electron spectroscopy (AES) was performed with a double-pass CMA with integral electron gun. The same analyzer type was used to determine the corresponding 3d core-level photoemission spectra excited with 70 eV synchrotron radiation using the plane grating monochromator SX-700 at the Berlin storage ring (BESSY). Alternatively, 40.8 eV radiation was used from a monochromatized He II source. The orientation resolution was limited by the primary beam diameter to 1 ° (AES) and 3 ° (UPS), respectively. Oxygen was admitted at sample temperatures of 300-340, 550 and 650 K with the filament of the ie~ gauge on. In our exJcdments, the ion gauge was out of direct sight of the sample in the bottom part of the chamber. No precautions were however taken to exclude an influence of oxygen excitation. Such influences are known e.g. for GaAs. To our knowledge, the effect of oxygen excitation by hot filaments was not yet studied on Ge. At 650 K, the time constant of desorption is about 300 s [3]. The exposing time was therefore kept shorter than 30 s and after adsorption the sample was immediately cooled down to room temperature. The oxygen ~oses are given in Langmuir units (1 L = 1.33 × 10 -6 mbar s). On the clean surface, the substrate Ge (1147 eV) Auger peak intensity varies with orientation over a range of 20% due to diffraction effects, even when using a CMA which averages over a relatively large range of emission directions. This effect appears to be weak for adsorbates because the main part of Auger emission does not pass through the crystal. As a measure for the adsorbed amount of oxygen we use therefore directly the oxygen OKLL peak intensity and not the O / G e intensity ratio. Absolute calibration was then obtained for the main orientations from the reduction of substrate Auger peaks by adsorption and from the intensity of
410
H.J. Kuhr, IV. Ranke / Orientation dependent adsorption of 0 on Ge
chemically shifted contributions in photoemission as described in ref. [8]. We estimate the coverage error to be + 15%. Our coverage value at saturation on G e ( l l l ) agrees with the value of Frantsuzov and Makrushin [3]. Z2. Core-level fitting procedure The 3d core-level photoemission peak consists of two branches with different spin-orbit quantum numbers. We regard them as equally shaped but energetically separated by the spin-orbit sprit Aso- The intensities are determined by the branching ratio 13/2/15/2. Nyholm and M~rtensson [9] have shown that these parameters are sufficient to decompose a core-level spectrum yielding only contributions of one branch (bulk, chemical shift, surface corelevel shift). For illustration see e.g. ref. [10]. In the case of the Ge 3d core-level, we found a spin-orbit sprit of 0.585 + 0.005 eV and a branching ratio of 0.62 + 0.002 (70 eV excitation energy) which agrees with values from other authors [7,11,12]. The orientation dependent variation is within the error limits. After spin-orbit decomposition and subtraction of the 3d3/2 contribution, the remaining 3d5/2 emission was decomposed into bulk and shifted contributions. The different contributions turned out to be so broad that a fit by only Gaussians could be used for this procedure. In close agreement with peak widths in refs. [7,11,12], a F W H M of 0.53 + 0.03 eV was determined for the bulk peak which is much larger than the experimental resolution (0.25 eV). After oxygen adsorption, we determined up to three chemically shifted contributions at higher binding energies which were much broader than the bulk peak (FWHM or 0.7-0.9 eV). We attribute this brc~adening to a varying chemical environment of the substrate atoms due to an amo~:rphous oxide growth. On each of the selected orientations the FWHM of all chemically shifted peaks was kept constant to get reproducible results for the peak positions and intensities. We consider the decomposition procedure as reliable if the different components are separated by more than half of their FWHM. Despite of the broadening this condition was always fulfilled.
3. Results and discussion
3.1. Auger electron spectroscopy Fig. 1 shows the oxygen coverages on Ge(001) and (111) deduced from the OKLL Auger intensity for exposures up to 5000 L at 300 and 550 K, respectively. On both orientations, the high temperature exposure leads to larger coverages. The initial sticking coefficients is about a factor of 2.5 larger than for oxygen adsorption at 300 K. For comparison, sticking coefficients
H.J. Kuhr, W. Ranke /
Orientationdependent adsorptionof 0 on Ge
411
2
(0Ol) .--I
(111)
550 K
550 K
IJ.I
> 0
,
1
10z
,
103
I,
I
10"
10
I
10z
m
,
103
0
10~
EXPOSURE (LANGMUIR UNITS) Fig. 1. Oxygen coverage in monolayers versus exposure for oxygen o n Ge(001) and (111). 1 Langmuir unit = 1.33 x 10 - ~ mbar s.
from flat Ge samples prepared similarly by ion bombardment and annealing are listed in table 1. They show a similar temperature dependence. However, the absolute values vary strongly, for 300 K over one order of magnitude. In part, this could be due to a varying defect density on the surface of the used samples. Frantsuzov and Makrushin [3] proposed a preferential oxygen adsorption on morphological defects (e.g. steps) at 300 K and on thermal defects (e.g. surface vacancies) above 500 K. The sticking coefficients could also be influenced by the existence of hot filaments (e.g. from the ionisation gauge). On both orientations, the coverage exceeds a monolayer for high temperature adsorption indicating oxygen penetration into the bulk. The orientation dependent oxygen coverage is shown in fig. 2 for different exposures and sample temperatures. The absolute coverage is given in adsorbed oxygen atoms per square centimeter (right scale). (:or (110), (111) and Table 1 Initial sticking coefficient for oxygen (values from literature were taken from diagrams) Orientation
Initial sticking coefficient
Ref.
(K) (001)
300 550 300 520 300
9 × 10 - 3 22 X 1 0 - 3 1.5 X 1 0 - 3 3.6 × 1 0 - 3 13 × 1 0 - 3
[This work] [This work] [5] [5] [2]
,'1
~
11~
t~l,!
Temperature
550 300 560 300 550 300
~
7 9 22 2 5 30
v
1N -3
× 10 - 3 x 10- 3 X 10- 3 X 10-3 XIO - 3 X 10 - 3
[Th;e
~rl~l
[This work] [4] [4]
[31 [3] [2]
412
H I . Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
16 33 hl lh ;z hl s 02/Ge(CYL.)
10 8
>,. I-03 z 1.1,1 In z I
t3
l,¢>
v
o
2 I
0
I I
I,
I
I
30 60 90 ORIENTATION = (deg.)
Fig. 2. Orientation dependence of the OKLL Auger intensity, calibrated into oxygen coverage (right scale) after adsorption of oxygen on the cylindrical Ge sample. The surface orientations are indicated on top. Exposures and sample temperatures are indicated.
(001), the coverage corresponding to one monolayer is additionally marked in the figure. A strong coverage variation with orientation is visible in all curves. The shape of the curves is very similar for exposures of 10 and 100 L oxygen at 300 K. For this sample temperature coverage minima are at (110), (111) and (113), whereas maxima appear at (331), around (001) and in the vicinity of (112). On germanium and silicon cylinders, a coverage minimum at (113) was observed for several gases before [8,13,14] and indicates a high stability of this orientation. At 650 K, the variation has dramatically changed in the region between (331) and (113) where the coverage has got a striking maximum on (111). The coverage increase in the vicinity of the minima at (110), (111) and (113) at 300 K and at (110) and (113) and 650 K indicates enhanced adsorption at steps or edges between facets. However, the curves are not as sharply structured as was observed for adsorption of H 2 0 [8] and H2S [13]. This is probably due to a destructive interaction with oxygen. Bond breaking and oxygen penetration slowly destroy the particular structural features of the different orientations. In the ~ase of Si, it was even observed that orientationdependent structural features were unimportant at low coverage and that the orientation-dependent penetration capability determines adsorption for higher exposure [14]. The orientation-dependent coverage observed here for Ge varies
H.J. Kuhr, W. Ranke / Orie~tation dependent adso~tion of 0 on Ge
413
more strongly and has sharper features indicating that surface structure is more important. The mentioned result of Frantsuzov and Makrushin [3] that defects on (111) increase adsorption at 300 K is in agreement with our result that (111) represents a minimum and that deviations from (111) increase adsorption. Whether the faster adsorption at 650 K is due to thermally induced defects as proposed in ref. [3] or to thermally activated dissociation and incorporation of oxygen cannot be decided from our experiments. Around (001), the coverage curves display a plateau-like shape with a width of about + 7 ° which was not seen for the adsorption of water and H2S, respectively [15]. The plateau width ~orresponds to the orientation range where the cylinder surface is stepped with (001) terraces (refs. [16,17], and our own observations). That the curve is flat in this region means that (001) terrace sites and step sites adsorb equally fast. Beyond this region, the surface forms facets of the (117), (115) and finally (113) type (hill and valley structure) which have a different adsorption behaviour. 3.2. Photoelectron spectra Until now, the valence band (VB) and the 3d core level of the oxygen-covered Ge surface were only investigated for the low-index orientations (111) and (001) [7]. We show here the spectra from the six orientations which display striking features (maxima or minima) in the coverage curves of fig. 2, i.e. from (001), (113), (112), (111), (331) and (110). The photoelectron spectra of the valence band and O 2s region are shown for clean and oxygen-covered G e ( l l l ) in fig. 3. Upon exposure, an O 2p-derived emission peak appears at - 5 e'~{with a shoulder towards higher binding energy (BE), very similar to published spectra [7]. The shape of the spectrum does not change significantly with increasing exposure at 340 K. Also at 650 K, it is very similar with only slightly sharper structures in the region of the high BE shoulder. The spectra from other orientations look the same, only the O 2p intensity follows the total adsorbed amount. The oxygen 2s emission appears at - 2 1 . 9 eV for low exposures at 340 K. With increasing exposure, it shifts slightly towazds - 2 2 . 4 eV, the position where it is also found after adsorption at 650 K. We attribute this shift to the transition from mainly adsorbed oxygen to a mixture of adsorbed and penetrated oxygen. Also the O 2s binding energies were the same on all six investigated orientations. Fig. 4 shows the G e ( l l l ) 3d5/2 spectrum after exposure to 3000 L ~2 at 480 K, taken with monochromatized He II radiation. The spectrum is decomposed into a residual surface co;nponent (S) and three chc~,-n/callyshifted peaks which are attributed to Ge bound to 1, 2 or 3 oxygen atoms, or Ge "+, Ge 2+ or Ge 3+, respectively. The second component is strongest. It corresponds to the
414
H.J. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
r
)I-Z Iii I"-"
z
!
!
-2's -2'0 -is -lo -~ ENERGYBELOWVB-MAXteV) Fig. 3. Valence band and O 2s photoelectron spectra for clean G e ( l l l ) and after exposure to the indicated amounts of oxygen at 340 and 650 K.
Ge(111). 3dsl 2 T=480K hv=40.8eV
i
Ii
I
z
t
~..,,.
j ./
~_W....~--=
-3
-2
.-
-1 E-EBULK(eV)
.
0
Fig. 4 Ge 3d photoelectron spectrum after subtraction of the 3d3/2 component for Ge(lll) after exposure to 3000 L O: at 480 K, taken with monochromatized He II radiation. The energy axis is referenced to the 3ds/2 bulk component (B) which is at -29.3 eV [18] below the valence band edge. The decomposition into bulk (B), surface (S) and chemically shifted (CS) components is demonstrated.
H.J. Kuhr, IV, Ranke / Orientation dependent adsorption of 0 on Ge
-i
-i
6
E - EOULK {eV)
i
i
-i
-i 6
415
i
E - EBULI( (eV}
Fig. 5, Ge 3ds/2 spectra for six different orientations after exposure to 100 L oxygen at 340 and 650 K. The position of chemically shifted components after curve fitting is indicated by dashes. The energy axis is references to the bulk peak position.
low pressure oxide stoichiometry GeO (monoxide) as has also been observed as the most stable form upon adsorption at low pressure by Schmeisser et al. [7]. However, Schmeisser et al. have observed an intense Ge 3+ component on G e ( l l l ) for an exposure to 3000 L. We were not able to reproduce this result at any of the used temperatures or orientations, and the spectrum of fig. 4 shows the most intense 3 + component which we ever observed. The Ge 3d5/2 spectra for six orientations are compared in fig. 5 for an exposure to 100 L 0 2 at 340 and 650 K. The spectra are normalized to equal total peak area. In agreement with the orientation-depend¢nt total amounts of adsorbed oxygen, the chemically shifted intensities vary with orientation and are generally higher at 650 than at 340 K. In contrast to the 340 K spectra, well resolved peaks appear at 650 K for the components corresponding to Ge 1+ and Ge 2+. On (!!0), (331), ( ! ! ! ) and (001), the Ge 2+ intensity is higher than the Ge 1+ intensity. On (113) and (001), also the Ge 1+ shift is well resolved and the long tail towards higher binding energy on (111) and (001) indicates the existence of a Ge 3+ component. The dashes indicate the position of the chemically shifted components after curve fitting.
416
H.J. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge I
I
110 331
O.
l
I
111
|
I
112 113
001
Bulkposition
-0.5-
m r--1
r-1
- - -1.0-1.5m
m" g=m
..a.
, m "m
r'~
tu -2.0-
I
I
LU
-25-3.0, I 0
'
,31.0K i I 3O
650K =m 'l' 60
I 90
ORIENTATION= (deg) Fig. 6. Positions and relative intensities (given by the length of the bars) of chemically shifted Ge 3d c o m p o n e n t s after exposure to 100 L oxygen at 340 a n d 650 K from the fit of the curves in fig. 5.
The absolute positions and relative intensities of I + , 2 + and 3 + components are orientation dependent and change with temperature. Positions and intensities are represented more quantitatively in fig. 6 and in table 2. At (111), Schmeisser et al. [7] have found shift values of -0.8, - 1 . 8 and - 2 . 7 eV Table 2 Chemical shift (CS) values in eV (negative values mean increase of binding energy) a n d relative shift intensities Jcs/Jto t (in parentheses) for adsorption of 100 L oxygen at 340 a n d 650 K on cylindrical G e (spectra taken with hp = 70 eV) Orien-
CSt
ration
340 K
650 K
340 K
650 K
650 K
(001)
(iii)
-0.77 (0.15) -0.62 (0.13) -0.65 (0.14) -0.6i - 0.69
- 1.77 (0.11) - 1.68 (0.08) - 1.72 (0.06) - 1.73 ~0.06) - 1.77
- 2.03 (0.22) - 1.82 (0.17) - 1.70 (0.22) - 1.77 (0.34,x - 1.72
-2.86 (0.02) - 2.68 ~3.03)
(331)
- 1.17 (0.17) -0.85 (0.23) -0.83 (0.22) -0.59 (0.18) - 0.56
(0.18)
(0.20)
(0.10)
(0.31)
-
(110)
- 0.70 (0.11)
- 0.61 (0.15)
- 1.66 (0.07)
- 1.82 (0.16)
-
Error
+ 0.10
:1:0.08
+ 0.06
+ 0.06
(113) (112)
(C 14)
CS 2
CS3
-
+ 0.10
H.J. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
417
which agree well with our data, apart from the Ge 1+ shift which we find to be - 0 . 6 eV. At 340 K, the position variation of the chemically shifted components with orientation is comparatively small with mean values of - 0 . 7 and - 1 . 7 eV for Ge 1+ and Ge 2+. At 650 K, however, their position varies much more with Ge 1+ between - 0 . 5 8 eV (331) and - 1 . 0 8 eV (001) and Ge 2+ shifts between -1.71 eV (331) and - 2 . 0 4 eV (001). We suppose that these differences have to do with structural changes during exposure at higher temperatures. The higher intensities of the Ge 2+ shifts and the existence of Ge 3+ shifts at 650 K are an indication that oxygen has penetrated into the substrate lattice. The differences of the shift positions are most evident at (001), although the orientation dependence of the total adsorbed amount in fig. 2 has a similar shape around (001) for both temperatures and gives no indication for a generally different adsorption mechanism. At (111), where the curves in fig. 2 have strongly different shapes, the chemical shifts have almost identical positions for both temperatures but the intensity ratio of Gel+/Ge 2+ is different. Probably, the adsorption mechanism is different on G e ( l l l ) for both temperatures. The higher Ge 2+ component indicates oxygen penetration, and the faster adsorption at 650 K indicates an activation barrier. The different values of the shift positions, especially at 650 K, indicate a local arrangement of Ge and O atoms which is orientation dependent. This is a surprising result, especially if we assume that oxygen has penetrated into the lattice. Obviously, not only the penetration capability depends on orientation and thus on surface structure, but even the local arrangement of the oxygen atoms in the lattice and theii~c,ha~-ging state.
4. Summary
The adsorption of oxygen on Ge depends strongly on the crystallographic orientation and on the temperature. Compared to 300-340 K, adsorption at 550 K to 650 K is faster at all orientations. This enhancement is particularly strong around (111), where the absolute minimum of the adsorbed amount of oxygen is observed at 300 K and the absolute maximum at 650 K. Two chemically shifted components of the Ge 3¢ emission are observed after adsorption at 340 K, corresponding to Ge atoms with one or two oxygen ligands (Ge 1+, Ge2+). At 650 K, the chemical shift peaks become more pronounced. For all orientations the Ge 2+ intensity has strongly increased. A weak Ge 3+ emission is observable for the orientations with highest coverage, (111) and (001). The increase of the Ge 2+ and the appearance of a Ge a+ component indicate penetration of the oxygen. Simultaneously, the oxygen 2s emission is shifted from -21.9 to - 2 2 . 4 eV. A Ge 4+ emission corresponding to GeO 2 was never observed consistent with the stability of GeO (monoxide) at low pressure. The position of chemically shifted components varies with
418
H,I. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
orientation, especially at 650 K. This surprising result indicates that the configuration of oxygen atoms (geometric and electronic) in the early stage of oxidation varies with orientation, even if the oxygen atoms are at least in part penetrated into the lattice.
Acknowledgements We thank Professors G. Ertl and K. Jacobi for helpful discussions, P. Geng for technical assistance and the Staff of the Berlin Synchrotron Radiation Source (BESSY) for their help. The work was in part supported by the Sonderforschungsbereich 6 der Deutschen Forschungsgemeinschaft.
References [1] [2] [3] [4] [5] [6]
M. Henzler, Surface Sci. 24 (1971) 209. R. Ludeke an~ A. Koma, Phys. Rev. B 13 (1976) 739. A.A. Frantsuzov and NJ. Makrushin, Surface Sci. 40 (1973) 320. L. Sumev, Sure,ace Sci. 110 (1981) 439. L. Sumev and M. Til,.hov, Surface Sci. 123 (1982) 505. C.M. Garner, !. Lindau, J.N. Miller, P. Pianetta and W.E. Spicer, J. Vacuum Sci. Technol. 14 (1977) 372, [7] D. Schmeissen R.D. Schneli, A. Bogen, F.J. Himpsel, D. Rieger, G. Landgren and J.F. Morar, Surface Sci. 172 (1986) 455. [8] H.J. Kuhr and W. Ranke, Surface Sci. 187 (1987) 98. [9] R. Nyholm and N. Mkrtensson, Chem. Phys. Letters 74 (1980) 337. [10] W. Ranke, J. Finster and H.J. Kuhr, Surface Sci. 187 (1987) 112. [11] R.D. Schnell, F.J. Himpsel, A. Bagcn, D Rieder and W. Steinmann, Phys. Rev. B 32 (1985) 8052. [12] T. Weser, A. Bogen, B. Konrad, R.D. Schne[1, C.A. Schug and W. Steinmann, Phys. Rev. B 35 (1987) 8184. [13] H.J. Kuhr, W. Ranke and J. Finster, Surface Sci. 178 (1986) 171. [14] W. Ranke and Y.R. Xing, Surface Sci. 157 (1985) 353. [15] H.J. Kuhr and W. Ranke, Surface Sci. 189/190 (1987) 420. [16] B.Z. Olshanetsky, S.M. Repinsky and A.A. Shklyaev, Surface Sci. 69 (1977) 205. [17] B.Z. Olshanetsky, V.I. Mashanov and A.I. Nikoforov, Surface Sci. 111 (1981) 429. [18] H.J. Kuhr and W. Ranke, Solid State Commun. 61 (1987) 285.
Surface Science 201 (1988) 408-418 North-Holland, Amsterdam
ORIENTATION DEPENDENT ADSORPTION OF OXYGEN ON A CYLINDRICAL Ge SAMPLE H.J. KUHR and W. RANKE Fritz-Haber-lnstitut der Max.Pianck-Geseilschaft, Faradayweg 4-6, D.IO00 Berlin 33, Germany Received 17 February 1988; accepted for publication 29 March 1988
The dependence of oxygen adsorption on the crystallographic surface orientation was studied on a cylindrical Ge sample with [110] as cylinder axis by Auger electron and photoelectron spectroscopy. At 300-340 K, adsorption is slowest at (110), (111) and (113) and increases in the vicinity of these orientations due to step-enhanced adsorption. At 650 K, adsorption is faster, especially at (111). The analysis of chemical shifts associated with the Ge 3d emission on six orientations yields discrete shifts towards higher binding energy for Ge atoms with 1, 2 or 3 oxygen ligands (Ge 1+, Ge 2+, Ge 3+ ). At 340 K, the Ge 1+ and Ge 2+ components have similar intensities. Their average positions are at about - 0 . 7 and - 1 . 7 eV with respect to the bulk emission (towards higher binding energy). At 650 K, The Ge 2+ component becomes more prominent, corresponding to the formation of predominantly GeO. Surprisingly, the positions of Ge 1+ and Ge 2+ become strongly orientation dependent, varying between - 0 . 5 6 eV (331) and -1.17 eV (001) for Gei + and - 1 . 7 2 eV (331) and - 2 . 0 3 eV (001) for Ge 2+. Also a weak Ge 3+ component at about - 2 . 8 eV but no Ge 4+ component was observed. A shift of the O 2s orbital from - 2 1 . 9 to -22.4 eV is observed when going from predominantly edsorbed oxygen to a GeO-like configuration.
1. Introduction Compared to the oxygen-silicon system, only few investigations have addressed the system oxygen on clean, single crystal germanium surfaces. Surface conductivity and LEED measurements were performed by Henzler [1], the electron loss structure was investigated by Ludeke and Koma [2], and sticking coefficients and the oxygen-uptake kinetics were measured using different techniques by the same authors [2] on (001) and (111), by Frantsuzov and Makrushin [3] and by Surnev [4] on G e ( l l l ) and by Surnev and Tikhov [5] on Ge(001) with in part strongly varying numerical results. Core-level shifts of the Ge 3d emission were investigated by Garner et al. [6] on G e ( l l l ) and by Schmeisser et al. [7] on G e ( l l l ) and (100). The latter demonstrated that, similar to silicon, also on germanium distinct resolvable shifts corresponding to Ge atoms with one, two, three or four oxygen ligands (Ge 1+, Ge 2+, Ge 3+, Ge 4÷) could be observed. In contrast to Si, however, adsorption at low 0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
H.J. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
409
pressure yielded Ge 2+ as the most stable species, in agreement with the stability of GeO (monoxide) at low pressure. Also in thermal desorption experiments, GeO had been observed earlier [2-5] and thus shown its high stability. In this paper, we present Auger electron spectroscopy results on the dependence of the amount of adsorbed oxygen on 2he crystallographic orientation. Adsorption curves and initial sticking coefficients are compared for (001) and (111). Six orientations were further investigated by photoelectron spectroscopy and the Ge 3d core-level shifts analyzed.
2. Experimental 2.1. Experimental setup As described in detail before [8], a cylindrically shaped Ge single crystal was used to study the dependence of surface properties on the crystallographic orientation in the region (110)-(111)-(001). Auger electron spectroscopy (AES) was performed with a double-pass CMA with integral electron gun. The same analyzer type was used to determine the corresponding 3d core-level photoemission spectra excited with 70 eV synchrotron radiation using the plane grating monochromator SX-700 at the Berlin storage ring (BESSY). Alternatively, 40.8 eV radiation was used from a monochromatized He II source. The orientation resolution was limited by the primary beam diameter to 1 ° (AES) and 3 ° (UPS), respectively. Oxygen was admitted at sample temperatures of 300-340, 550 and 650 K with the filament of the ie~ gauge on. In our exJcdments, the ion gauge was out of direct sight of the sample in the bottom part of the chamber. No precautions were however taken to exclude an influence of oxygen excitation. Such influences are known e.g. for GaAs. To our knowledge, the effect of oxygen excitation by hot filaments was not yet studied on Ge. At 650 K, the time constant of desorption is about 300 s [3]. The exposing time was therefore kept shorter than 30 s and after adsorption the sample was immediately cooled down to room temperature. The oxygen ~oses are given in Langmuir units (1 L = 1.33 × 10 -6 mbar s). On the clean surface, the substrate Ge (1147 eV) Auger peak intensity varies with orientation over a range of 20% due to diffraction effects, even when using a CMA which averages over a relatively large range of emission directions. This effect appears to be weak for adsorbates because the main part of Auger emission does not pass through the crystal. As a measure for the adsorbed amount of oxygen we use therefore directly the oxygen OKLL peak intensity and not the O / G e intensity ratio. Absolute calibration was then obtained for the main orientations from the reduction of substrate Auger peaks by adsorption and from the intensity of
410
H.J. Kuhr, IV. Ranke / Orientation dependent adsorption of 0 on Ge
chemically shifted contributions in photoemission as described in ref. [8]. We estimate the coverage error to be + 15%. Our coverage value at saturation on G e ( l l l ) agrees with the value of Frantsuzov and Makrushin [3]. Z2. Core-level fitting procedure The 3d core-level photoemission peak consists of two branches with different spin-orbit quantum numbers. We regard them as equally shaped but energetically separated by the spin-orbit sprit Aso- The intensities are determined by the branching ratio 13/2/15/2. Nyholm and M~rtensson [9] have shown that these parameters are sufficient to decompose a core-level spectrum yielding only contributions of one branch (bulk, chemical shift, surface corelevel shift). For illustration see e.g. ref. [10]. In the case of the Ge 3d core-level, we found a spin-orbit sprit of 0.585 + 0.005 eV and a branching ratio of 0.62 + 0.002 (70 eV excitation energy) which agrees with values from other authors [7,11,12]. The orientation dependent variation is within the error limits. After spin-orbit decomposition and subtraction of the 3d3/2 contribution, the remaining 3d5/2 emission was decomposed into bulk and shifted contributions. The different contributions turned out to be so broad that a fit by only Gaussians could be used for this procedure. In close agreement with peak widths in refs. [7,11,12], a F W H M of 0.53 + 0.03 eV was determined for the bulk peak which is much larger than the experimental resolution (0.25 eV). After oxygen adsorption, we determined up to three chemically shifted contributions at higher binding energies which were much broader than the bulk peak (FWHM or 0.7-0.9 eV). We attribute this brc~adening to a varying chemical environment of the substrate atoms due to an amo~:rphous oxide growth. On each of the selected orientations the FWHM of all chemically shifted peaks was kept constant to get reproducible results for the peak positions and intensities. We consider the decomposition procedure as reliable if the different components are separated by more than half of their FWHM. Despite of the broadening this condition was always fulfilled.
3. Results and discussion
3.1. Auger electron spectroscopy Fig. 1 shows the oxygen coverages on Ge(001) and (111) deduced from the OKLL Auger intensity for exposures up to 5000 L at 300 and 550 K, respectively. On both orientations, the high temperature exposure leads to larger coverages. The initial sticking coefficients is about a factor of 2.5 larger than for oxygen adsorption at 300 K. For comparison, sticking coefficients
H.J. Kuhr, W. Ranke /
Orientationdependent adsorptionof 0 on Ge
411
2
(0Ol) .--I
(111)
550 K
550 K
IJ.I
> 0
,
1
10z
,
103
I,
I
10"
10
I
10z
m
,
103
0
10~
EXPOSURE (LANGMUIR UNITS) Fig. 1. Oxygen coverage in monolayers versus exposure for oxygen o n Ge(001) and (111). 1 Langmuir unit = 1.33 x 10 - ~ mbar s.
from flat Ge samples prepared similarly by ion bombardment and annealing are listed in table 1. They show a similar temperature dependence. However, the absolute values vary strongly, for 300 K over one order of magnitude. In part, this could be due to a varying defect density on the surface of the used samples. Frantsuzov and Makrushin [3] proposed a preferential oxygen adsorption on morphological defects (e.g. steps) at 300 K and on thermal defects (e.g. surface vacancies) above 500 K. The sticking coefficients could also be influenced by the existence of hot filaments (e.g. from the ionisation gauge). On both orientations, the coverage exceeds a monolayer for high temperature adsorption indicating oxygen penetration into the bulk. The orientation dependent oxygen coverage is shown in fig. 2 for different exposures and sample temperatures. The absolute coverage is given in adsorbed oxygen atoms per square centimeter (right scale). (:or (110), (111) and Table 1 Initial sticking coefficient for oxygen (values from literature were taken from diagrams) Orientation
Initial sticking coefficient
Ref.
(K) (001)
300 550 300 520 300
9 × 10 - 3 22 X 1 0 - 3 1.5 X 1 0 - 3 3.6 × 1 0 - 3 13 × 1 0 - 3
[This work] [This work] [5] [5] [2]
,'1
~
11~
t~l,!
Temperature
550 300 560 300 550 300
~
7 9 22 2 5 30
v
1N -3
× 10 - 3 x 10- 3 X 10- 3 X 10-3 XIO - 3 X 10 - 3
[Th;e
~rl~l
[This work] [4] [4]
[31 [3] [2]
412
H I . Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
16 33 hl lh ;z hl s 02/Ge(CYL.)
10 8
>,. I-03 z 1.1,1 In z I
t3
l,¢>
v
o
2 I
0
I I
I,
I
I
30 60 90 ORIENTATION = (deg.)
Fig. 2. Orientation dependence of the OKLL Auger intensity, calibrated into oxygen coverage (right scale) after adsorption of oxygen on the cylindrical Ge sample. The surface orientations are indicated on top. Exposures and sample temperatures are indicated.
(001), the coverage corresponding to one monolayer is additionally marked in the figure. A strong coverage variation with orientation is visible in all curves. The shape of the curves is very similar for exposures of 10 and 100 L oxygen at 300 K. For this sample temperature coverage minima are at (110), (111) and (113), whereas maxima appear at (331), around (001) and in the vicinity of (112). On germanium and silicon cylinders, a coverage minimum at (113) was observed for several gases before [8,13,14] and indicates a high stability of this orientation. At 650 K, the variation has dramatically changed in the region between (331) and (113) where the coverage has got a striking maximum on (111). The coverage increase in the vicinity of the minima at (110), (111) and (113) at 300 K and at (110) and (113) and 650 K indicates enhanced adsorption at steps or edges between facets. However, the curves are not as sharply structured as was observed for adsorption of H 2 0 [8] and H2S [13]. This is probably due to a destructive interaction with oxygen. Bond breaking and oxygen penetration slowly destroy the particular structural features of the different orientations. In the ~ase of Si, it was even observed that orientationdependent structural features were unimportant at low coverage and that the orientation-dependent penetration capability determines adsorption for higher exposure [14]. The orientation-dependent coverage observed here for Ge varies
H.J. Kuhr, W. Ranke / Orie~tation dependent adso~tion of 0 on Ge
413
more strongly and has sharper features indicating that surface structure is more important. The mentioned result of Frantsuzov and Makrushin [3] that defects on (111) increase adsorption at 300 K is in agreement with our result that (111) represents a minimum and that deviations from (111) increase adsorption. Whether the faster adsorption at 650 K is due to thermally induced defects as proposed in ref. [3] or to thermally activated dissociation and incorporation of oxygen cannot be decided from our experiments. Around (001), the coverage curves display a plateau-like shape with a width of about + 7 ° which was not seen for the adsorption of water and H2S, respectively [15]. The plateau width ~orresponds to the orientation range where the cylinder surface is stepped with (001) terraces (refs. [16,17], and our own observations). That the curve is flat in this region means that (001) terrace sites and step sites adsorb equally fast. Beyond this region, the surface forms facets of the (117), (115) and finally (113) type (hill and valley structure) which have a different adsorption behaviour. 3.2. Photoelectron spectra Until now, the valence band (VB) and the 3d core level of the oxygen-covered Ge surface were only investigated for the low-index orientations (111) and (001) [7]. We show here the spectra from the six orientations which display striking features (maxima or minima) in the coverage curves of fig. 2, i.e. from (001), (113), (112), (111), (331) and (110). The photoelectron spectra of the valence band and O 2s region are shown for clean and oxygen-covered G e ( l l l ) in fig. 3. Upon exposure, an O 2p-derived emission peak appears at - 5 e'~{with a shoulder towards higher binding energy (BE), very similar to published spectra [7]. The shape of the spectrum does not change significantly with increasing exposure at 340 K. Also at 650 K, it is very similar with only slightly sharper structures in the region of the high BE shoulder. The spectra from other orientations look the same, only the O 2p intensity follows the total adsorbed amount. The oxygen 2s emission appears at - 2 1 . 9 eV for low exposures at 340 K. With increasing exposure, it shifts slightly towazds - 2 2 . 4 eV, the position where it is also found after adsorption at 650 K. We attribute this shift to the transition from mainly adsorbed oxygen to a mixture of adsorbed and penetrated oxygen. Also the O 2s binding energies were the same on all six investigated orientations. Fig. 4 shows the G e ( l l l ) 3d5/2 spectrum after exposure to 3000 L ~2 at 480 K, taken with monochromatized He II radiation. The spectrum is decomposed into a residual surface co;nponent (S) and three chc~,-n/callyshifted peaks which are attributed to Ge bound to 1, 2 or 3 oxygen atoms, or Ge "+, Ge 2+ or Ge 3+, respectively. The second component is strongest. It corresponds to the
414
H.J. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
r
)I-Z Iii I"-"
z
!
!
-2's -2'0 -is -lo -~ ENERGYBELOWVB-MAXteV) Fig. 3. Valence band and O 2s photoelectron spectra for clean G e ( l l l ) and after exposure to the indicated amounts of oxygen at 340 and 650 K.
Ge(111). 3dsl 2 T=480K hv=40.8eV
i
Ii
I
z
t
~..,,.
j ./
~_W....~--=
-3
-2
.-
-1 E-EBULK(eV)
.
0
Fig. 4 Ge 3d photoelectron spectrum after subtraction of the 3d3/2 component for Ge(lll) after exposure to 3000 L O: at 480 K, taken with monochromatized He II radiation. The energy axis is referenced to the 3ds/2 bulk component (B) which is at -29.3 eV [18] below the valence band edge. The decomposition into bulk (B), surface (S) and chemically shifted (CS) components is demonstrated.
H.J. Kuhr, IV, Ranke / Orientation dependent adsorption of 0 on Ge
-i
-i
6
E - EOULK {eV)
i
i
-i
-i 6
415
i
E - EBULI( (eV}
Fig. 5, Ge 3ds/2 spectra for six different orientations after exposure to 100 L oxygen at 340 and 650 K. The position of chemically shifted components after curve fitting is indicated by dashes. The energy axis is references to the bulk peak position.
low pressure oxide stoichiometry GeO (monoxide) as has also been observed as the most stable form upon adsorption at low pressure by Schmeisser et al. [7]. However, Schmeisser et al. have observed an intense Ge 3+ component on G e ( l l l ) for an exposure to 3000 L. We were not able to reproduce this result at any of the used temperatures or orientations, and the spectrum of fig. 4 shows the most intense 3 + component which we ever observed. The Ge 3d5/2 spectra for six orientations are compared in fig. 5 for an exposure to 100 L 0 2 at 340 and 650 K. The spectra are normalized to equal total peak area. In agreement with the orientation-depend¢nt total amounts of adsorbed oxygen, the chemically shifted intensities vary with orientation and are generally higher at 650 than at 340 K. In contrast to the 340 K spectra, well resolved peaks appear at 650 K for the components corresponding to Ge 1+ and Ge 2+. On (!!0), (331), ( ! ! ! ) and (001), the Ge 2+ intensity is higher than the Ge 1+ intensity. On (113) and (001), also the Ge 1+ shift is well resolved and the long tail towards higher binding energy on (111) and (001) indicates the existence of a Ge 3+ component. The dashes indicate the position of the chemically shifted components after curve fitting.
416
H.J. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge I
I
110 331
O.
l
I
111
|
I
112 113
001
Bulkposition
-0.5-
m r--1
r-1
- - -1.0-1.5m
m" g=m
..a.
, m "m
r'~
tu -2.0-
I
I
LU
-25-3.0, I 0
'
,31.0K i I 3O
650K =m 'l' 60
I 90
ORIENTATION= (deg) Fig. 6. Positions and relative intensities (given by the length of the bars) of chemically shifted Ge 3d c o m p o n e n t s after exposure to 100 L oxygen at 340 a n d 650 K from the fit of the curves in fig. 5.
The absolute positions and relative intensities of I + , 2 + and 3 + components are orientation dependent and change with temperature. Positions and intensities are represented more quantitatively in fig. 6 and in table 2. At (111), Schmeisser et al. [7] have found shift values of -0.8, - 1 . 8 and - 2 . 7 eV Table 2 Chemical shift (CS) values in eV (negative values mean increase of binding energy) a n d relative shift intensities Jcs/Jto t (in parentheses) for adsorption of 100 L oxygen at 340 a n d 650 K on cylindrical G e (spectra taken with hp = 70 eV) Orien-
CSt
ration
340 K
650 K
340 K
650 K
650 K
(001)
(iii)
-0.77 (0.15) -0.62 (0.13) -0.65 (0.14) -0.6i - 0.69
- 1.77 (0.11) - 1.68 (0.08) - 1.72 (0.06) - 1.73 ~0.06) - 1.77
- 2.03 (0.22) - 1.82 (0.17) - 1.70 (0.22) - 1.77 (0.34,x - 1.72
-2.86 (0.02) - 2.68 ~3.03)
(331)
- 1.17 (0.17) -0.85 (0.23) -0.83 (0.22) -0.59 (0.18) - 0.56
(0.18)
(0.20)
(0.10)
(0.31)
-
(110)
- 0.70 (0.11)
- 0.61 (0.15)
- 1.66 (0.07)
- 1.82 (0.16)
-
Error
+ 0.10
:1:0.08
+ 0.06
+ 0.06
(113) (112)
(C 14)
CS 2
CS3
-
+ 0.10
H.J. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
417
which agree well with our data, apart from the Ge 1+ shift which we find to be - 0 . 6 eV. At 340 K, the position variation of the chemically shifted components with orientation is comparatively small with mean values of - 0 . 7 and - 1 . 7 eV for Ge 1+ and Ge 2+. At 650 K, however, their position varies much more with Ge 1+ between - 0 . 5 8 eV (331) and - 1 . 0 8 eV (001) and Ge 2+ shifts between -1.71 eV (331) and - 2 . 0 4 eV (001). We suppose that these differences have to do with structural changes during exposure at higher temperatures. The higher intensities of the Ge 2+ shifts and the existence of Ge 3+ shifts at 650 K are an indication that oxygen has penetrated into the substrate lattice. The differences of the shift positions are most evident at (001), although the orientation dependence of the total adsorbed amount in fig. 2 has a similar shape around (001) for both temperatures and gives no indication for a generally different adsorption mechanism. At (111), where the curves in fig. 2 have strongly different shapes, the chemical shifts have almost identical positions for both temperatures but the intensity ratio of Gel+/Ge 2+ is different. Probably, the adsorption mechanism is different on G e ( l l l ) for both temperatures. The higher Ge 2+ component indicates oxygen penetration, and the faster adsorption at 650 K indicates an activation barrier. The different values of the shift positions, especially at 650 K, indicate a local arrangement of Ge and O atoms which is orientation dependent. This is a surprising result, especially if we assume that oxygen has penetrated into the lattice. Obviously, not only the penetration capability depends on orientation and thus on surface structure, but even the local arrangement of the oxygen atoms in the lattice and theii~c,ha~-ging state.
4. Summary
The adsorption of oxygen on Ge depends strongly on the crystallographic orientation and on the temperature. Compared to 300-340 K, adsorption at 550 K to 650 K is faster at all orientations. This enhancement is particularly strong around (111), where the absolute minimum of the adsorbed amount of oxygen is observed at 300 K and the absolute maximum at 650 K. Two chemically shifted components of the Ge 3¢ emission are observed after adsorption at 340 K, corresponding to Ge atoms with one or two oxygen ligands (Ge 1+, Ge2+). At 650 K, the chemical shift peaks become more pronounced. For all orientations the Ge 2+ intensity has strongly increased. A weak Ge 3+ emission is observable for the orientations with highest coverage, (111) and (001). The increase of the Ge 2+ and the appearance of a Ge a+ component indicate penetration of the oxygen. Simultaneously, the oxygen 2s emission is shifted from -21.9 to - 2 2 . 4 eV. A Ge 4+ emission corresponding to GeO 2 was never observed consistent with the stability of GeO (monoxide) at low pressure. The position of chemically shifted components varies with
418
H,I. Kuhr, W. Ranke / Orientation dependent adsorption of 0 on Ge
orientation, especially at 650 K. This surprising result indicates that the configuration of oxygen atoms (geometric and electronic) in the early stage of oxidation varies with orientation, even if the oxygen atoms are at least in part penetrated into the lattice.
Acknowledgements We thank Professors G. Ertl and K. Jacobi for helpful discussions, P. Geng for technical assistance and the Staff of the Berlin Synchrotron Radiation Source (BESSY) for their help. The work was in part supported by the Sonderforschungsbereich 6 der Deutschen Forschungsgemeinschaft.
References [1] [2] [3] [4] [5] [6]
M. Henzler, Surface Sci. 24 (1971) 209. R. Ludeke an~ A. Koma, Phys. Rev. B 13 (1976) 739. A.A. Frantsuzov and NJ. Makrushin, Surface Sci. 40 (1973) 320. L. Sumev, Sure,ace Sci. 110 (1981) 439. L. Sumev and M. Til,.hov, Surface Sci. 123 (1982) 505. C.M. Garner, !. Lindau, J.N. Miller, P. Pianetta and W.E. Spicer, J. Vacuum Sci. Technol. 14 (1977) 372, [7] D. Schmeissen R.D. Schneli, A. Bogen, F.J. Himpsel, D. Rieger, G. Landgren and J.F. Morar, Surface Sci. 172 (1986) 455. [8] H.J. Kuhr and W. Ranke, Surface Sci. 187 (1987) 98. [9] R. Nyholm and N. Mkrtensson, Chem. Phys. Letters 74 (1980) 337. [10] W. Ranke, J. Finster and H.J. Kuhr, Surface Sci. 187 (1987) 112. [11] R.D. Schnell, F.J. Himpsel, A. Bagcn, D Rieder and W. Steinmann, Phys. Rev. B 32 (1985) 8052. [12] T. Weser, A. Bogen, B. Konrad, R.D. Schne[1, C.A. Schug and W. Steinmann, Phys. Rev. B 35 (1987) 8184. [13] H.J. Kuhr, W. Ranke and J. Finster, Surface Sci. 178 (1986) 171. [14] W. Ranke and Y.R. Xing, Surface Sci. 157 (1985) 353. [15] H.J. Kuhr and W. Ranke, Surface Sci. 189/190 (1987) 420. [16] B.Z. Olshanetsky, S.M. Repinsky and A.A. Shklyaev, Surface Sci. 69 (1977) 205. [17] B.Z. Olshanetsky, V.I. Mashanov and A.I. Nikoforov, Surface Sci. 111 (1981) 429. [18] H.J. Kuhr and W. Ranke, Solid State Commun. 61 (1987) 285.