- Email: [email protected]
Interaction of oxygen with a Ge-covered Si(100) surface
Surface Science 282 (1993) lo-16 North-Holland
surface science
Interaction of oxygen with a Ge-covered Si(100) surface S. Surnev 1 Department of Solid State Physics, Sofia University, 5 James Boucher Blvd., Sofia 1126, Bulgaria
Received 27 July 1992; accepted for publication 27 October 1992
The interaction of oxygen with a Ge-covered Si(l~~2 X 1 surface has been studied by means of Auger electron spectroscopy &MS), electron energy loss spectroscopy (EELS), thermal desorption and work function WF) measurement. It has been found that the initial oxygen sticking coefficient of the Ge-covered (1 < 8, < 3) SitlOO)is almost independent of the Ge coverage, i&, and is close to that of a bare Si surface. Strong suppression of the oxygen uptake rate has been observed at 300 K for moderate oxygen coverages. This effect is weaker for the oxygen adsorption kinetics at 500 K. AES and EELS results show that short time annealing above 370 K can induce oxygen transfer from Ge to Si atoms. This is accompanied by diffusion of Si and formation of Si oxides over the Ge layer, as judged from the WF and AES data. The annealing experiments show that the shape of the GeO TPD curves is determined by complicated reduction-oxidation reactions in which diffusion of oxygen is involved.
1. Introduction The structural and electronic properties of Ge films grown on a Si surface have been subject of extensive studies in the recent years [l--8]. However, to my knowledge no work has been attempted on the interaction of the Ge-Si system with oxygen. In spite of the similar chemical behaviors of Ge and Si, the latter exhibits a much stronger bonding strength with oxygen. The oxygen sticking coefficient on Si [g-13] is also found to be l-2 orders of magnitude higher than on Ge [14-18,24,25]. This makes Ge-Si a very appropriate model system for clarifying the oxidation mechanism of the Si (Ge) surfaces. In our recent paper [19] it is shown that Ge coverages in the submonolayer range cause only a slight decrease of the oxygen initial sticking coefficient and a strong suppression of the oxidation rate at moderate oxygen coverages. These findings and the analysis of the adsorption kinetics reveal that
’ Present address: Institut fiir Experimentalphysik, A-8010 Graz, ~rl-Franzens-Universit~t Graz, Universitltplatz 5, Austria. ~39-~28/93/$~,~
oxygen adsorption on clean and Ge-dosed Si(100) surfaces is a precursor-mediated process. In the present work the oxygen adsorption kinetics are obtained for Ge coverages, BOe,above I ML, and the interaction of oxygen with a Gecovered Si(100) surface is studied within a wide temperature and Ge coverage (up to 3 ML in order to avoid 3D clustering [1,5]) ranges.
2. Experimental The experiments were performed in a conventional stainless steel UHV system (base pressure N 1 x lo-‘* Torr) described previously [ZO]. The UHV chamber was equipped with a cylindrical mirror analyzer for AES and EELS, QMS, a two-grid LEED system and a low energy electron gun for work function measurements. A clean Si(100) surface (5 1R* cm, boron-doped p-type, 24 x 5 x 0.4 mm3 target) was obtained after several cycles of Ar+ bombardment (500 eV, 3 PA/cm) and annealing at 1200 K (resistively heating), followed by slow cooling (0.5 K/s) to room temperature. A programmable temperature controller reproducibly provided the desired tem-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
11
S. Sumev / Interaction of oxygen with Ge-covered Si(100)
perature ramp. After this cleaning procedure, a sharp 2 X 1 LEED pattern was observed. The target temperature was measured by a Pt/PtlO%Rh thermocouple spot-welded on a small Ta clip attached to the lower end of the sample. Optical and IR pyrometers were used for thermocouple calibration. Ge was deposited from a well-outgassed Knudsen cell (pyrolytic boron nitride crucible) at rates of about 0.1 ML/min. The increase of pressure in the chamber was < 5 X lo-” Torr during the Ge deposition. Following this deposition, the carbon and oxygen signals were always below the detection limit of the AES analyzer. Pure oxygen (99.997%) was introduced into the UHV system via a leak valve. The oxygen exposure, 2, was performed at a constant pressure in the chamber (up to 5 X lo-* Torr) or by means of an effusion source when high exposures were needed. In the latter case the oxygen pressure in front of the effusor was about 10 times higher than the pressure in the UHV system. The oxygen coverage, 8,, was measured on the basis of the O(KLL)/Si(KLL) AES peak-to-peak ratios, Zo/Zsi. The Auger measurements were performed always in a new spot after each oxygen dose in order to prevent any electron beam effects. It is worth noting that the attenuation of the AES Si(KLL) transition at 1616 eV did not exceed 5% when 8,, reached 3 ML. For absolute oxygen coverage calibration we accepted that B. = 1 ML following 65 L 0, exposure at 300 K of clean Si(100). This 2 calibration value was obtained by Keim et al. [ll], in a comparative study of N,O and 0, adsorption on SXlOO). The uncertainty in 8, calibration should not influence the main results and conclusions obtained in this study. It was shown recently [l-5] that at 300 K the Ge overlayer grows on Si(100) uniformly, without 3D island formation or interdiffusion. An exponential decay of the Si(LW) AES transition intensity was reported as a function of the Ge coverage, e,, , in the range from 0 to 6 ML [5-71. Following these results, the Ge coverage of the Si(100) surface was evaluated by measuring the decrease in the peak-to-peak intensity of the Si(LW) Auger signal at 92 eV as a function of
the evaporation time. An exponential dependence is obtained. In order to calibrate the evaporation time scale the Ge coverage was determined by fitting the normalized Si Auger intensity, Z/Z,,, to the following formula: Z=Z, exp( --&/ah),
(1) where a = cos 42” is a constant characteristic to the geometry of the cylindrical mirror analyzer and A = 2.5 ML is the mean free path in Ge for the 92 eV Si(LW) electrons [6]. In spite of the fact that the formula (1) is not valid in the submonolayer coverage regime, the calculated Bee values are very close to those obtained using the evaporation time.
3. Results 3.1. Oxygen adsorption kinetics for BGe> 1 The curves in figs. la and lb show the Auger peak-to-peak height ratio O(KLL)/Si(KLL) as a 0.8
() a
0
0.0 0.8
(b)
T=500K
A 0
.~ 0.6 \ _o 0.4
Fig. 1. Normalized Auger intensity Io/Zsi as a function of oxygen exposure at 300 (a) and 500 K (b) recorded at various eGe.
12
S. Surnev /
Interaction of oxygen with Ge-covered Si(lO0)
function of oxygen exposure, Z, recorded for various 8, values at 300 and 500 K, respectively. For the sake of comparison the corresponding kinetics obtained for a bare Si(lOO)-2 x 1 surface are also given. As seen in fig. la, the initial oxygen sticking coefficient is almost independent Of @Ge and strong suppression of the oxygen uptake rate is observed for moderate oxygen coverages. The suppression effect increases with 0,. These observations are similar to those reported earlier in the submonolayer Ge coverage region [19]. A new feature is the plateau region of the adsorption kinetics, which develops at Z > *y5 L. The end of the plateau and the subsequent increase in the uptake rate occurs at 2 values which increase with 0,,. For the adsorption kinetics recorded at 500 K, the plateau is not so clearly expressed (see fig. lb). In this case, the Ge-induced suppression of the oxygen uptake rate in the moderate oxygen coverage regime is not so strong.
T=300K
GE?0
a-
500 600 700 800 9 0 Temperature (K) Fig. 3. GeO TPD spectra recorded at various Bo, and at the foilowing 6,: 0.16 (a), 0.32 (b) and OS ML Cc). 400
Figs. 2a and 2b present the oxygen-induced work function changes, A4, for clean and Gecovered Si(lOO), recorded at 300 and 500 K, respectively. Evidently, at 300 K the Ge dosing strongly reduces the saturated Al\b, values, the effect increasing with Ooe A similar result is reported before for the Ge submonolayer coverage regime [19]. On the contrary, upon oxygen adsorption at 500 K (fig. 2b), the saturated Ad values for Ge-dosed surfaces are very close to that for a Ge-free Si(100) surface. As is evident from fig. 2b, the higher eGe, the slower A#J changes. Curves, similar to those shown in fig. 2b, are obtained in the temperature range 400-550 R. It is found that at a constant 6oe, the rate of A4 changes increases with temperature. 3.2. Thermal desorption data
Fig. 2. Work function changes, AhQI,as a function of oxygen exposure at 300 (a> and 500 K (b) recorded at various 6,.
Fig. 3 displays the GeO TPD curves obtained at three Ge coverages and at various 8, values. The oxygen exposure is performed at 300 K following deposition of Ge. The main GeO TPD results can be summarized as follows: (i> for 8o, < 0.3 only one GeO TPD peak is observed at T = 740 K, (ii) above this Ge coverage a new TPD structure at N 680 K arises and becomes
S. Sunzev / Interaction
of oxygen with Ge-covered
SiO
00=0.5ML
A
a
@c&L)
3,’
0
I.35
ens
I.70 A
1.40 AI
1
850
I
I
900
950
Temperature
I
1000 (K)
l(
0
Fig. 4. SiO TPD spectra recorded at 0o = 0.5 ML and at various 13~~.
dominant; (iii) the shape, height and temperature position of the GeO TPD traces are independent of 0, and 8,, (for 0.4 < f3,, < 2.7). Fig. 4 shows a set of SiO TPD curves recorded at constant oxygen coverages (0, = 0.5) for increasing Ge coverage. It can be seen, that the area under the SiO TPD curves does not change with increasing Ge coverage. Only a slight downward temperature shift (up to 30 K) and broadening of the SiO TPD traces is observed with increasing e,,. 3.3. Effect of annealing Fig. 5 illustrates the changes of the intensities in Ge(MW), Si(LW) and oxygen (KLL) Auger transitions, recorded for two Ge coverages (e,, = 1.0 and 2.2) and oxygen-exposed (f3, = 0.5) Si(100) surface, after stepwise annealing at progressively increasing temperatures (30 s in each step). As can be seen, for both Ge coverages the amplitude of the Ge AES peak initially increases and, after passing through a maximum at - 550 K, diminishes upon further enhancement of temperature. On the other hand, at the same temperature (- 550 K) the Si Auger curves exhibit a mini-
13
Si(100)
mum. The oxygen AEB signals remain unchanged with increasing annealing temperature up to 850 K. Slight diminishing of the oxygen Auger intensity is observed above this temperature. The effect of annealing on the evolution of the EEL spectra in the Ge3d core level region is illustrated in fig. 6. In the top spectrum, obtained after deposition of a Ge layer (eo, = 2.2), the loss d (at 30.0 eV> was attributed to the transition involving the Ge3d core level as an initial state [8]. This peak disappears after a 500 L oxygen exposure at 300 K. The loss at 31.2 eV (labelled as d’), observed after oxygen exposure, could be related to the chemically shifted 3d core level of the oxidized Ge atoms. A mild annealing (30 s at - 370 K> results in the reappearence of the peak d, which is observed along with the shifted loss d’. The latter loss gradually diminishes in height with increasing annealing temperature and vanishes at T > 850 K. It is worth noting that this
1.5 ‘;; 1.3 \ = x 1.1 .% 3g 0.9 -c L 0.7 & 2 1.5 1.3 1.1 0.9
L
I
0e7300 400 500 600 700 800 900 Annealing Temperature (K) Fig. 5. Evolution of Ge(MVV) (I,,), S#LW), (I,,) and ONLL) (Is,,) Auger transitions as a function of the annealing temperature for (a) Ooe = 1.0 and (b) 0,, = 2.2 ML. The annealing time is 30 s.
14
S. Sumev
Ge 3d core
/ Interaction of oxygen with Ge-covered
d’
I:;&
2 annealed 370K \
s s /m
J
715K 85OK
27
Energy
I
Loss
I
(eV)
’
Fig. 6. EELS in the Ge3d core level region of Ge-covered Si(100) and after 500 L oxygen exposure (top curves). The other spectra refer to the evolution upon annealing at different temperatures; E, = 100 = 0.5 VP_,.
2.2 ML at 300 K of EELS eV, I/mod
temperature coincides with the end temperature of the GeO thermal desorption curves (see fig. 3).
4. Discussion The results presented in this work show that the oxidation of the Ge-covered SXlOO) surface is very complicated. The vanishing of the loss related to the transitions involving the 3d core level of unoxidized Ge (at N 30 eV) and the observation only of the shifted 3d core loss at 31.2 eV (alone) after a 500 L oxygen exposure at 300 K of the Ge-covered WOO) surface (see fig. 6), shows that Ge atoms in the layer are oxidized. The small value of the shift (u 1.2 eV) suggests a low oxidation state of Ge [21,22,24,25]. However, due to relatively low resolution of the EELS method used here (- 0.7 eV), the broad structure at 31.2 eV may consist of two unresolved peaks. Probably Ge atoms coordinated to one (Ge+) and two (Ge”) oxygen atoms coexist in the oxidized Ge films. It is worth noting that for Ge(100) this oxidation state (as judged by EELS [17]) is reached only upon oxygen exposure at elevated
Si(100)
temperatures (550-600 K), or after annealing at these temperatures of an oxygen-dosed surface at 300 K. The enhanced reactivity of the Ge layer, compared with that of the Ge(100) surface is in agreement with the results of Sampath Kumar et al. [22], where the easier oxidation of the Ge films is explained with the higher density of the dangling bonds in these structures. The high initial oxygen sticking coefficients, sO, observed for Ge-covered Si(100) is consistent with the enhanced oxidation rate of these Ge layers. Indeed, in the latter case (fig. la) the value of s0 is very close to that of a bare Si(100) surface, which is about two orders of magnitude higher than that of the Ge(100) surface [171. The same effect has been also observed on annealed Ge overlayers. At present it is difficult to explain unambiguously this interesting observation. Assuming that the first 2-3 Ge layers resemble, in a short range, the Si lattice (in spite of the fact that the Ge film growth on Si is amorphous [51), the only parameter, which is not changed after the Ge deposition, is the lattice constant (the lattice constant of Ge is 4% larger than that of Si). Since s0 is also not changed after the Ge deposition, it is suggested that the trapping probability of the oxygen molecule should strongly depend on the lattice constant of the substrate. Another interesting feature in the oxygen adsorption kinetics shown in fig. la, which is also difficult to explain, is the increase of the oxygen uptake rate, observed for exposures in the range 20-30 L. Supposing an oxygen-induced diffusion of Si and formation of Si oxides over the Ge film we have to expect work function changes close to those obtained after oxidation of a Ge-free Si(100) surface. This indeed is observed for oxygen adsorption at 500 K. As can be seen in fig. 2b, the work function changes approach the value for the oxidized Si surface, The diffusion mechanism of Si through the Ge layer is supported by the observations that the rate of work function changes increases with temperature and with the diminishing of 0,,. The much smaller Ad changes, observed upon oxygen adsorption at 300 K (see fig. 2a), do not favor the Si diffusion mechanism at this temperature. A reconstructing in the Ge film, which probably takes place upon
oxygen adsorption, may be the reason for the observed increase in the oxygen uptake rate. Possibly o~gen-induced clustering in the Ge layer takes pIace, leaving part of the Si surface Ge-free. The faster oxidation of these Si patches causes the enhancement of the oxidation rate observed in fig. la for Z > 50 L. Due to the higher work function of these patches of oxidized Si, they should not contribute substantially to the average Ahd,v&ue as determined by the retarding potential method used in this study 1231. The transfer of the oxygen from Ge to Si occurring at elevated temperatures is illustrated in figs. 5 and 6. As can be seen in fig. 6, the reappearence of the unshifted Ge3d core loss after short-time annealing at temperatures as low as 370 K, shows that part of the Ge atoms have lost their oxygen. An oxygen transfer to Si continues upon further increase of the annealing temperature (as judged from the diminishing of the intensity of the loss at 31.2 eV) and at 850 K ail oxygen is transferred to Si. The changes in intensities of the Ge(IvIW1 and the Si(LW1 Auger transitions, shown in fig. 5, are a resuft of the oxygen transfer from Ge to Si and the segregation of Si on tbe Ge layer. Tbe enhanced electron population of Ge and exhaustion of Si valence orbitais, which accompanies the oxygen transfer from Ge to Si, is reflected by the initial increase of the Ge AES intensity and the decrease of the amplitude of the Si peak at 92 eV. The changes with opposite sign observed at T > 500 K could be attributed to the dominating effect of the Si dif~sion across the Ge layer and fo~ation of Si oxides over the Ge layer. The transfer of oxygen from Ge to Si is an expected result taking into account the larger reaction free enthalpy of oxygen with Si compared with Ge. The constancy of the oxygen (KLLJ Auger transition intensity, observed up to 850 K, shows that a very small amount of Ge is desorbed as GeO in the TPD runs. Indeed, as was mentioned above, the transfer of oxygen from Ge to Si begins at 370 K and accelerates with increasing temperature. This process dominates over the GeO thermal desorption at this tempera~re range. Evidently the shape of the GeG TPD curves and the amount of the desorbed GeO are determined by a very
comphcated solid state redox reaction in which d~fusion of reactants is involved rather than a simple GeO thermal deletion. As can be seen from fig. 6, at the temperature corresponding to the end of the GeU TPD curves (w 850 K;) no oxidized Ge exists on the surface. At this temperature B. and 6o, keep their initial values. The explanation proposed above is consistent with the independence of the desorbed GeO amount, observed over a wide range of oxygen and Ge eoverages. The diminishing of the oxygen AES peak intensity, shown in fig. 5 at 900 K, is due to the begi~ing of the SiO thermal desertion. The independence of the desorbed SiG on 8, (see fig. 4), is consistent with the AES observations presented in fig. 5, i.e. a very small amount of oxygen is desorbed as GeO. The shift to the lower temperatures of the SiO TPD curves, recorded at constant B. (see fig, 41, could be explained with the presence of Ge atoms under the oxidized Si layer, which causes a reduction of the SiO-substrate bonding strength.
5. ConcIus~ons The interaction of oxygen with a Ge-covered Si(lOO) surface has been studied using AI%, EELS, TPD and WE measurements. The main conclusions are as follows: (1) Ge overlayers on Si(100) in the coverage regime 1 < 8, < 3 strongly reduce the mgen uptake at 300 EC.At 500 K the effect becomes weaker. (2) The initial oxygen sticking coefficient is found to be almost ~dependent of the Ge coverage. (3) At temperatures in the range 4fNMRM K transfer of the oxygen from Ge to Si atoms OGcurs, accompanied by Si diffusion across the Ge layer. Both processes are explained by the larger reaction free enthalpy of oxygen with Si compared to that with Ge. (4) The processes described in the preceding point determine the shape of the GeO TPD traces and the neghgible amount of oxygen desorbed as GeO.
16
S. Sumev / Interaction of oxygen with Ge-covered SiflOO)
Acknowledgement
I would like to acknowledge the hospitality of the Institute of General and Inorganic Chemistry at the Bulgarian Academy of Sciences where this work was completed.
References [l] K. Shoji, M. Hyodo, H. Ueba and C. Tatsuyama, Jpn. J. Appl. Phys. 22 (1983) 1482. [2] H.-J. Gossmann, J.C. Bean, L.C. Feldman and W.M. Gibson, Surf, Sci. 138 (1984) L175. [3] H.-J. Gossmann, L.C. Feldman and W.M. Gibson, Surf. Sci. 155 (1985) 413. [4] H.-J. Gossmann and L.C. Feldman, Appl. Phys. A 38 (1985) 171. [5] M. Asai, H. Ueba and C. Tatsuyama, J. Appl. Phys. 58 (1985) 2577. [6] P. Martensson, W.-X. Ni, G. Hansson, J.M. Nicholls and B. Reihl, Phys. Rev. B 36 (1987) 5974. [7] Y. Kataoka, H. Ueba and C. Tatsuyama, J. Appl. Phys. 63 (1988) 749. [8] P. Perfetti, S. Nannarone, F. Patella, C. Quaresima, F. Cerrina, M. Capozi, A. Savoia and I. Lindau, J. Vat. Sci. Technol. 19 (1981) 319.
[9] H. Ibach, K. Horn, R. Dom and H. Liith, Surf. Sci. 38 (1973) 433. [lo] P. Morgen, W. Wurth and E. Umbach, Surf. Sci. 152/153 (1985) 1086. [ll] E.G. Keim, L. Wolterbeek and A. van Silfhout, Surf. Sci. 180 (1987) 565. [12] P. Gupta, C.H. Mak, P.A. Coon and S.M. George, Phys. Rev. B 40 (1989) 39. [13] T. Miyake, S. Soeki, H. Kato, T. Nakamura, A. Namiki, H. Kamba and T. Suzaki, Phys. Rev. B 42 (1990) 11801. [14] G.A. Bootsma, Surf. Sci. 15 (1969) 340. [15] R.F. Lever, Surf. Sci. 9 (1868) 370. [16] A.A. Frantsuzov and N.I. Makrushin, Surf. Sci. 40 (1973) 320. [17] L. Surnev and M. Tikhov, Surf. Sci. 123 (1982) 505. [18] H.J.W. Zandvhet and A. van Silfhout, Solid State Commun. 65 (1988) 1425. [19] S. Sumev, Surf. Sci. 278 (1992) 375. [20] M. Tikhov, G. Boishin and L. Surnev, Surf. Sci. 241 (1991) 103. [21] C.M. Gamer, I. Lindau, J.N. Miller, P. Pianetts and W.E. Spicer, J. Vat. Sci. Technol. 14 (1977) 372. [22] T.S. Sampath Kumar, L. Kaameswara Rao and M.S. Hegde, Appl. Surf. Sci. 27 (1986) 255. [23] S. Sumev and M. K&nova, App]. Phys. A 46 (1988) 323. [24] D. Schmeisser, R.D. Schnell, A. Bogen, F.J. Himpsel, D. Rieger, G. Landgren and J.F. Morar, Surf. Sci. 172 (1986) 455. [25] H.J. Kuhr and W. Ranke, Surf. Sci. 187 (1987) 98.
surface science
Interaction of oxygen with a Ge-covered Si(100) surface S. Surnev 1 Department of Solid State Physics, Sofia University, 5 James Boucher Blvd., Sofia 1126, Bulgaria
Received 27 July 1992; accepted for publication 27 October 1992
The interaction of oxygen with a Ge-covered Si(l~~2 X 1 surface has been studied by means of Auger electron spectroscopy &MS), electron energy loss spectroscopy (EELS), thermal desorption and work function WF) measurement. It has been found that the initial oxygen sticking coefficient of the Ge-covered (1 < 8, < 3) SitlOO)is almost independent of the Ge coverage, i&, and is close to that of a bare Si surface. Strong suppression of the oxygen uptake rate has been observed at 300 K for moderate oxygen coverages. This effect is weaker for the oxygen adsorption kinetics at 500 K. AES and EELS results show that short time annealing above 370 K can induce oxygen transfer from Ge to Si atoms. This is accompanied by diffusion of Si and formation of Si oxides over the Ge layer, as judged from the WF and AES data. The annealing experiments show that the shape of the GeO TPD curves is determined by complicated reduction-oxidation reactions in which diffusion of oxygen is involved.
1. Introduction The structural and electronic properties of Ge films grown on a Si surface have been subject of extensive studies in the recent years [l--8]. However, to my knowledge no work has been attempted on the interaction of the Ge-Si system with oxygen. In spite of the similar chemical behaviors of Ge and Si, the latter exhibits a much stronger bonding strength with oxygen. The oxygen sticking coefficient on Si [g-13] is also found to be l-2 orders of magnitude higher than on Ge [14-18,24,25]. This makes Ge-Si a very appropriate model system for clarifying the oxidation mechanism of the Si (Ge) surfaces. In our recent paper [19] it is shown that Ge coverages in the submonolayer range cause only a slight decrease of the oxygen initial sticking coefficient and a strong suppression of the oxidation rate at moderate oxygen coverages. These findings and the analysis of the adsorption kinetics reveal that
’ Present address: Institut fiir Experimentalphysik, A-8010 Graz, ~rl-Franzens-Universit~t Graz, Universitltplatz 5, Austria. ~39-~28/93/$~,~
oxygen adsorption on clean and Ge-dosed Si(100) surfaces is a precursor-mediated process. In the present work the oxygen adsorption kinetics are obtained for Ge coverages, BOe,above I ML, and the interaction of oxygen with a Gecovered Si(100) surface is studied within a wide temperature and Ge coverage (up to 3 ML in order to avoid 3D clustering [1,5]) ranges.
2. Experimental The experiments were performed in a conventional stainless steel UHV system (base pressure N 1 x lo-‘* Torr) described previously [ZO]. The UHV chamber was equipped with a cylindrical mirror analyzer for AES and EELS, QMS, a two-grid LEED system and a low energy electron gun for work function measurements. A clean Si(100) surface (5 1R* cm, boron-doped p-type, 24 x 5 x 0.4 mm3 target) was obtained after several cycles of Ar+ bombardment (500 eV, 3 PA/cm) and annealing at 1200 K (resistively heating), followed by slow cooling (0.5 K/s) to room temperature. A programmable temperature controller reproducibly provided the desired tem-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
11
S. Sumev / Interaction of oxygen with Ge-covered Si(100)
perature ramp. After this cleaning procedure, a sharp 2 X 1 LEED pattern was observed. The target temperature was measured by a Pt/PtlO%Rh thermocouple spot-welded on a small Ta clip attached to the lower end of the sample. Optical and IR pyrometers were used for thermocouple calibration. Ge was deposited from a well-outgassed Knudsen cell (pyrolytic boron nitride crucible) at rates of about 0.1 ML/min. The increase of pressure in the chamber was < 5 X lo-” Torr during the Ge deposition. Following this deposition, the carbon and oxygen signals were always below the detection limit of the AES analyzer. Pure oxygen (99.997%) was introduced into the UHV system via a leak valve. The oxygen exposure, 2, was performed at a constant pressure in the chamber (up to 5 X lo-* Torr) or by means of an effusion source when high exposures were needed. In the latter case the oxygen pressure in front of the effusor was about 10 times higher than the pressure in the UHV system. The oxygen coverage, 8,, was measured on the basis of the O(KLL)/Si(KLL) AES peak-to-peak ratios, Zo/Zsi. The Auger measurements were performed always in a new spot after each oxygen dose in order to prevent any electron beam effects. It is worth noting that the attenuation of the AES Si(KLL) transition at 1616 eV did not exceed 5% when 8,, reached 3 ML. For absolute oxygen coverage calibration we accepted that B. = 1 ML following 65 L 0, exposure at 300 K of clean Si(100). This 2 calibration value was obtained by Keim et al. [ll], in a comparative study of N,O and 0, adsorption on SXlOO). The uncertainty in 8, calibration should not influence the main results and conclusions obtained in this study. It was shown recently [l-5] that at 300 K the Ge overlayer grows on Si(100) uniformly, without 3D island formation or interdiffusion. An exponential decay of the Si(LW) AES transition intensity was reported as a function of the Ge coverage, e,, , in the range from 0 to 6 ML [5-71. Following these results, the Ge coverage of the Si(100) surface was evaluated by measuring the decrease in the peak-to-peak intensity of the Si(LW) Auger signal at 92 eV as a function of
the evaporation time. An exponential dependence is obtained. In order to calibrate the evaporation time scale the Ge coverage was determined by fitting the normalized Si Auger intensity, Z/Z,,, to the following formula: Z=Z, exp( --&/ah),
(1) where a = cos 42” is a constant characteristic to the geometry of the cylindrical mirror analyzer and A = 2.5 ML is the mean free path in Ge for the 92 eV Si(LW) electrons [6]. In spite of the fact that the formula (1) is not valid in the submonolayer coverage regime, the calculated Bee values are very close to those obtained using the evaporation time.
3. Results 3.1. Oxygen adsorption kinetics for BGe> 1 The curves in figs. la and lb show the Auger peak-to-peak height ratio O(KLL)/Si(KLL) as a 0.8
() a
0
0.0 0.8
(b)
T=500K
A 0
.~ 0.6 \ _o 0.4
Fig. 1. Normalized Auger intensity Io/Zsi as a function of oxygen exposure at 300 (a) and 500 K (b) recorded at various eGe.
12
S. Surnev /
Interaction of oxygen with Ge-covered Si(lO0)
function of oxygen exposure, Z, recorded for various 8, values at 300 and 500 K, respectively. For the sake of comparison the corresponding kinetics obtained for a bare Si(lOO)-2 x 1 surface are also given. As seen in fig. la, the initial oxygen sticking coefficient is almost independent Of @Ge and strong suppression of the oxygen uptake rate is observed for moderate oxygen coverages. The suppression effect increases with 0,. These observations are similar to those reported earlier in the submonolayer Ge coverage region [19]. A new feature is the plateau region of the adsorption kinetics, which develops at Z > *y5 L. The end of the plateau and the subsequent increase in the uptake rate occurs at 2 values which increase with 0,,. For the adsorption kinetics recorded at 500 K, the plateau is not so clearly expressed (see fig. lb). In this case, the Ge-induced suppression of the oxygen uptake rate in the moderate oxygen coverage regime is not so strong.
T=300K
GE?0
a-
500 600 700 800 9 0 Temperature (K) Fig. 3. GeO TPD spectra recorded at various Bo, and at the foilowing 6,: 0.16 (a), 0.32 (b) and OS ML Cc). 400
Figs. 2a and 2b present the oxygen-induced work function changes, A4, for clean and Gecovered Si(lOO), recorded at 300 and 500 K, respectively. Evidently, at 300 K the Ge dosing strongly reduces the saturated Al\b, values, the effect increasing with Ooe A similar result is reported before for the Ge submonolayer coverage regime [19]. On the contrary, upon oxygen adsorption at 500 K (fig. 2b), the saturated Ad values for Ge-dosed surfaces are very close to that for a Ge-free Si(100) surface. As is evident from fig. 2b, the higher eGe, the slower A#J changes. Curves, similar to those shown in fig. 2b, are obtained in the temperature range 400-550 R. It is found that at a constant 6oe, the rate of A4 changes increases with temperature. 3.2. Thermal desorption data
Fig. 2. Work function changes, AhQI,as a function of oxygen exposure at 300 (a> and 500 K (b) recorded at various 6,.
Fig. 3 displays the GeO TPD curves obtained at three Ge coverages and at various 8, values. The oxygen exposure is performed at 300 K following deposition of Ge. The main GeO TPD results can be summarized as follows: (i> for 8o, < 0.3 only one GeO TPD peak is observed at T = 740 K, (ii) above this Ge coverage a new TPD structure at N 680 K arises and becomes
S. Sunzev / Interaction
of oxygen with Ge-covered
SiO
00=0.5ML
A
a
@c&L)
3,’
0
I.35
ens
I.70 A
1.40 AI
1
850
I
I
900
950
Temperature
I
1000 (K)
l(
0
Fig. 4. SiO TPD spectra recorded at 0o = 0.5 ML and at various 13~~.
dominant; (iii) the shape, height and temperature position of the GeO TPD traces are independent of 0, and 8,, (for 0.4 < f3,, < 2.7). Fig. 4 shows a set of SiO TPD curves recorded at constant oxygen coverages (0, = 0.5) for increasing Ge coverage. It can be seen, that the area under the SiO TPD curves does not change with increasing Ge coverage. Only a slight downward temperature shift (up to 30 K) and broadening of the SiO TPD traces is observed with increasing e,,. 3.3. Effect of annealing Fig. 5 illustrates the changes of the intensities in Ge(MW), Si(LW) and oxygen (KLL) Auger transitions, recorded for two Ge coverages (e,, = 1.0 and 2.2) and oxygen-exposed (f3, = 0.5) Si(100) surface, after stepwise annealing at progressively increasing temperatures (30 s in each step). As can be seen, for both Ge coverages the amplitude of the Ge AES peak initially increases and, after passing through a maximum at - 550 K, diminishes upon further enhancement of temperature. On the other hand, at the same temperature (- 550 K) the Si Auger curves exhibit a mini-
13
Si(100)
mum. The oxygen AEB signals remain unchanged with increasing annealing temperature up to 850 K. Slight diminishing of the oxygen Auger intensity is observed above this temperature. The effect of annealing on the evolution of the EEL spectra in the Ge3d core level region is illustrated in fig. 6. In the top spectrum, obtained after deposition of a Ge layer (eo, = 2.2), the loss d (at 30.0 eV> was attributed to the transition involving the Ge3d core level as an initial state [8]. This peak disappears after a 500 L oxygen exposure at 300 K. The loss at 31.2 eV (labelled as d’), observed after oxygen exposure, could be related to the chemically shifted 3d core level of the oxidized Ge atoms. A mild annealing (30 s at - 370 K> results in the reappearence of the peak d, which is observed along with the shifted loss d’. The latter loss gradually diminishes in height with increasing annealing temperature and vanishes at T > 850 K. It is worth noting that this
1.5 ‘;; 1.3 \ = x 1.1 .% 3g 0.9 -c L 0.7 & 2 1.5 1.3 1.1 0.9
L
I
0e7300 400 500 600 700 800 900 Annealing Temperature (K) Fig. 5. Evolution of Ge(MVV) (I,,), S#LW), (I,,) and ONLL) (Is,,) Auger transitions as a function of the annealing temperature for (a) Ooe = 1.0 and (b) 0,, = 2.2 ML. The annealing time is 30 s.
14
S. Sumev
Ge 3d core
/ Interaction of oxygen with Ge-covered
d’
I:;&
2 annealed 370K \
s s /m
J
715K 85OK
27
Energy
I
Loss
I
(eV)
’
Fig. 6. EELS in the Ge3d core level region of Ge-covered Si(100) and after 500 L oxygen exposure (top curves). The other spectra refer to the evolution upon annealing at different temperatures; E, = 100 = 0.5 VP_,.
2.2 ML at 300 K of EELS eV, I/mod
temperature coincides with the end temperature of the GeO thermal desorption curves (see fig. 3).
4. Discussion The results presented in this work show that the oxidation of the Ge-covered SXlOO) surface is very complicated. The vanishing of the loss related to the transitions involving the 3d core level of unoxidized Ge (at N 30 eV) and the observation only of the shifted 3d core loss at 31.2 eV (alone) after a 500 L oxygen exposure at 300 K of the Ge-covered WOO) surface (see fig. 6), shows that Ge atoms in the layer are oxidized. The small value of the shift (u 1.2 eV) suggests a low oxidation state of Ge [21,22,24,25]. However, due to relatively low resolution of the EELS method used here (- 0.7 eV), the broad structure at 31.2 eV may consist of two unresolved peaks. Probably Ge atoms coordinated to one (Ge+) and two (Ge”) oxygen atoms coexist in the oxidized Ge films. It is worth noting that for Ge(100) this oxidation state (as judged by EELS [17]) is reached only upon oxygen exposure at elevated
Si(100)
temperatures (550-600 K), or after annealing at these temperatures of an oxygen-dosed surface at 300 K. The enhanced reactivity of the Ge layer, compared with that of the Ge(100) surface is in agreement with the results of Sampath Kumar et al. [22], where the easier oxidation of the Ge films is explained with the higher density of the dangling bonds in these structures. The high initial oxygen sticking coefficients, sO, observed for Ge-covered Si(100) is consistent with the enhanced oxidation rate of these Ge layers. Indeed, in the latter case (fig. la) the value of s0 is very close to that of a bare Si(100) surface, which is about two orders of magnitude higher than that of the Ge(100) surface [171. The same effect has been also observed on annealed Ge overlayers. At present it is difficult to explain unambiguously this interesting observation. Assuming that the first 2-3 Ge layers resemble, in a short range, the Si lattice (in spite of the fact that the Ge film growth on Si is amorphous [51), the only parameter, which is not changed after the Ge deposition, is the lattice constant (the lattice constant of Ge is 4% larger than that of Si). Since s0 is also not changed after the Ge deposition, it is suggested that the trapping probability of the oxygen molecule should strongly depend on the lattice constant of the substrate. Another interesting feature in the oxygen adsorption kinetics shown in fig. la, which is also difficult to explain, is the increase of the oxygen uptake rate, observed for exposures in the range 20-30 L. Supposing an oxygen-induced diffusion of Si and formation of Si oxides over the Ge film we have to expect work function changes close to those obtained after oxidation of a Ge-free Si(100) surface. This indeed is observed for oxygen adsorption at 500 K. As can be seen in fig. 2b, the work function changes approach the value for the oxidized Si surface, The diffusion mechanism of Si through the Ge layer is supported by the observations that the rate of work function changes increases with temperature and with the diminishing of 0,,. The much smaller Ad changes, observed upon oxygen adsorption at 300 K (see fig. 2a), do not favor the Si diffusion mechanism at this temperature. A reconstructing in the Ge film, which probably takes place upon
oxygen adsorption, may be the reason for the observed increase in the oxygen uptake rate. Possibly o~gen-induced clustering in the Ge layer takes pIace, leaving part of the Si surface Ge-free. The faster oxidation of these Si patches causes the enhancement of the oxidation rate observed in fig. la for Z > 50 L. Due to the higher work function of these patches of oxidized Si, they should not contribute substantially to the average Ahd,v&ue as determined by the retarding potential method used in this study 1231. The transfer of the oxygen from Ge to Si occurring at elevated temperatures is illustrated in figs. 5 and 6. As can be seen in fig. 6, the reappearence of the unshifted Ge3d core loss after short-time annealing at temperatures as low as 370 K, shows that part of the Ge atoms have lost their oxygen. An oxygen transfer to Si continues upon further increase of the annealing temperature (as judged from the diminishing of the intensity of the loss at 31.2 eV) and at 850 K ail oxygen is transferred to Si. The changes in intensities of the Ge(IvIW1 and the Si(LW1 Auger transitions, shown in fig. 5, are a resuft of the oxygen transfer from Ge to Si and the segregation of Si on tbe Ge layer. Tbe enhanced electron population of Ge and exhaustion of Si valence orbitais, which accompanies the oxygen transfer from Ge to Si, is reflected by the initial increase of the Ge AES intensity and the decrease of the amplitude of the Si peak at 92 eV. The changes with opposite sign observed at T > 500 K could be attributed to the dominating effect of the Si dif~sion across the Ge layer and fo~ation of Si oxides over the Ge layer. The transfer of oxygen from Ge to Si is an expected result taking into account the larger reaction free enthalpy of oxygen with Si compared with Ge. The constancy of the oxygen (KLLJ Auger transition intensity, observed up to 850 K, shows that a very small amount of Ge is desorbed as GeO in the TPD runs. Indeed, as was mentioned above, the transfer of oxygen from Ge to Si begins at 370 K and accelerates with increasing temperature. This process dominates over the GeO thermal desorption at this tempera~re range. Evidently the shape of the GeG TPD curves and the amount of the desorbed GeO are determined by a very
comphcated solid state redox reaction in which d~fusion of reactants is involved rather than a simple GeO thermal deletion. As can be seen from fig. 6, at the temperature corresponding to the end of the GeU TPD curves (w 850 K;) no oxidized Ge exists on the surface. At this temperature B. and 6o, keep their initial values. The explanation proposed above is consistent with the independence of the desorbed GeO amount, observed over a wide range of oxygen and Ge eoverages. The diminishing of the oxygen AES peak intensity, shown in fig. 5 at 900 K, is due to the begi~ing of the SiO thermal desertion. The independence of the desorbed SiG on 8, (see fig. 4), is consistent with the AES observations presented in fig. 5, i.e. a very small amount of oxygen is desorbed as GeO. The shift to the lower temperatures of the SiO TPD curves, recorded at constant B. (see fig, 41, could be explained with the presence of Ge atoms under the oxidized Si layer, which causes a reduction of the SiO-substrate bonding strength.
5. ConcIus~ons The interaction of oxygen with a Ge-covered Si(lOO) surface has been studied using AI%, EELS, TPD and WE measurements. The main conclusions are as follows: (1) Ge overlayers on Si(100) in the coverage regime 1 < 8, < 3 strongly reduce the mgen uptake at 300 EC.At 500 K the effect becomes weaker. (2) The initial oxygen sticking coefficient is found to be almost ~dependent of the Ge coverage. (3) At temperatures in the range 4fNMRM K transfer of the oxygen from Ge to Si atoms OGcurs, accompanied by Si diffusion across the Ge layer. Both processes are explained by the larger reaction free enthalpy of oxygen with Si compared to that with Ge. (4) The processes described in the preceding point determine the shape of the GeO TPD traces and the neghgible amount of oxygen desorbed as GeO.
16
S. Sumev / Interaction of oxygen with Ge-covered SiflOO)
Acknowledgement
I would like to acknowledge the hospitality of the Institute of General and Inorganic Chemistry at the Bulgarian Academy of Sciences where this work was completed.
References [l] K. Shoji, M. Hyodo, H. Ueba and C. Tatsuyama, Jpn. J. Appl. Phys. 22 (1983) 1482. [2] H.-J. Gossmann, J.C. Bean, L.C. Feldman and W.M. Gibson, Surf, Sci. 138 (1984) L175. [3] H.-J. Gossmann, L.C. Feldman and W.M. Gibson, Surf. Sci. 155 (1985) 413. [4] H.-J. Gossmann and L.C. Feldman, Appl. Phys. A 38 (1985) 171. [5] M. Asai, H. Ueba and C. Tatsuyama, J. Appl. Phys. 58 (1985) 2577. [6] P. Martensson, W.-X. Ni, G. Hansson, J.M. Nicholls and B. Reihl, Phys. Rev. B 36 (1987) 5974. [7] Y. Kataoka, H. Ueba and C. Tatsuyama, J. Appl. Phys. 63 (1988) 749. [8] P. Perfetti, S. Nannarone, F. Patella, C. Quaresima, F. Cerrina, M. Capozi, A. Savoia and I. Lindau, J. Vat. Sci. Technol. 19 (1981) 319.
[9] H. Ibach, K. Horn, R. Dom and H. Liith, Surf. Sci. 38 (1973) 433. [lo] P. Morgen, W. Wurth and E. Umbach, Surf. Sci. 152/153 (1985) 1086. [ll] E.G. Keim, L. Wolterbeek and A. van Silfhout, Surf. Sci. 180 (1987) 565. [12] P. Gupta, C.H. Mak, P.A. Coon and S.M. George, Phys. Rev. B 40 (1989) 39. [13] T. Miyake, S. Soeki, H. Kato, T. Nakamura, A. Namiki, H. Kamba and T. Suzaki, Phys. Rev. B 42 (1990) 11801. [14] G.A. Bootsma, Surf. Sci. 15 (1969) 340. [15] R.F. Lever, Surf. Sci. 9 (1868) 370. [16] A.A. Frantsuzov and N.I. Makrushin, Surf. Sci. 40 (1973) 320. [17] L. Surnev and M. Tikhov, Surf. Sci. 123 (1982) 505. [18] H.J.W. Zandvhet and A. van Silfhout, Solid State Commun. 65 (1988) 1425. [19] S. Sumev, Surf. Sci. 278 (1992) 375. [20] M. Tikhov, G. Boishin and L. Surnev, Surf. Sci. 241 (1991) 103. [21] C.M. Gamer, I. Lindau, J.N. Miller, P. Pianetts and W.E. Spicer, J. Vat. Sci. Technol. 14 (1977) 372. [22] T.S. Sampath Kumar, L. Kaameswara Rao and M.S. Hegde, Appl. Surf. Sci. 27 (1986) 255. [23] S. Sumev and M. K&nova, App]. Phys. A 46 (1988) 323. [24] D. Schmeisser, R.D. Schnell, A. Bogen, F.J. Himpsel, D. Rieger, G. Landgren and J.F. Morar, Surf. Sci. 172 (1986) 455. [25] H.J. Kuhr and W. Ranke, Surf. Sci. 187 (1987) 98.