- Email: [email protected]
Si-multiple quantum wells using molecular beam epitaxy
Thin Solid Films 318 Ž1998. 158–162
Growth of Si 1yyC yrSi- and Si 1yxyyGe xC yrSi-multiple quantum wells using molecular beam epitaxy R. Hartmann
a,)
a a , D. Grutzmacher , E. Muller , U. Gennser a , A. Dommann b, P. Schroter ¨ ¨ ¨ b, P. Warren c a
Paul-Scherrer-Institut, CH-5232 Villigen, Switzerland Neutechnikum Buchs, CH-9470 Buchs, Switzerland Swiss Federale Institute of Technology, CH-1015 Lausanne, Switzerland b
c
Abstract SirSiC- and SirSiGeC-multiple quantum well ŽMQW. structures of different well thicknesses and C-concentrations up to 2.7% have been grown pseudomorphically on SiŽ001. using molecular beam epitaxy. Near bandedge photoluminescence ŽPL. is observed for SirSiC for certain growth parameters. Substrate temperature and Si growth rate strongly influence the structural and optical properties of the samples, in particular for samples with high C-content. The thermal stability of the SirSiC-layers was investigated by using PL, high resolution X-ray diffraction ŽHRXRD. and transmission electron microscopy ŽTEM.. At anneal temperatures of 9508C and below the structures relax only by interdiffusion but not by defect or SiC-formation. PL data on a nearly strainless SirSiGeC-MQW indicate a band alignment of type-I character. q 1998 Elsevier Science S.A. Keywords: Multiple quantum wells; Molecular beam epitaxy; Photoluminescence
1. Introduction The incorporation of C into substitutional sites offers a new degree of freedom in the bandgap, band offset and strain engineering of Si-based materialsw1–4x. However, the solubility of C in Si is only 3 = 10 17 atoms cmy3 at the melting point w5x. At higher concentrations the thermodynamically favoured phase are silicon carbide ŽSiC. clusters distributed in Si rather than the random Si 1y x C x alloy. Although low-temperature nonequilibrium epitaxy can produce films with substitutional C-contents of up to 7%, the window for growth temperature and rate is quite narrow. Twinning may lead to amorphous growth if the substrate temperature is chosen too low andror the growth rate is chosen too high. On the other hand, at substrate temperatures of above 6008C a mixed alloy carbide phase is produced.
)
Corresponding author. Micro- and Nanostructures Laboratory, Paul Scherrer Institute, CH-5234 Villigen, PSI. Tel.: q41-056-310-28-50; fax: q41-056-310-26-46; e-mail: [email protected]. 0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 5 7 - 7
In the following we critically review the growth conditions for high quality SiC alloys. The quality of the samples is directly reflected on the photoluminescence ŽPL. spectra, since defects provide competing recombination mechanisms which suppress the bandgap signal. A narrow growth window for the observation of bandgap PL in SirSiC- multiple quantum wells ŽMQWs. is found. The structural characterization of the samples is done by high resolution X-ray diffraction ŽHRXRD. and transmission electron microscopy ŽTEM. studies. The behaviour of the PL energy of SirSiGeC-MQWs at variations of the Si and SiGeC widths verifies the type-I band alignment with the SiGeC layer as the quantum well w2x.
2. Experimental Several Si 1y y C yrSi- and Si 1yxyyGe x C yrSi-MQWs of ˚ and 400 A˚ and C-concentraperiod lengths between 90 A tion of up to 2.7% have been grown on Ž001.Si substrates of a resistivity of 500 V cm. A solid source Si molecular beam epitaxy ŽMBE. was used with electron guns for Si
R. Hartmann et al.r Thin Solid Films 318 (1998) 158–162
and Ge evaporation and a directly heated pyrolithic graphite filament for C sublimation. Besides the variation of layer thickness and layer composition the growth temperature was varied between 4008C and 6008C and the growth rate ˚ sy1 and 0.3 A˚ sy1 . After the MBE growth between 0.1 A the samples were thermally annealed for between 4 min and 10 min at temperatures between 6508C and 9508C in a N2r4%H 2 gas mixture. In order to test the effect of growth temperature and rate on the structural and optical properties of the layers double crystal HRXRD, cross-sectional TEM and PL measurements have been performed.
3. Results and discussion Fig. 1 shows the PL spectra of a SirSiC-MQW grown at 4808C and subsequently annealed for 4 min at different temperatures ranging from 6508C to 9508C. The MQW ˚ Si and 48 A˚ SiC with a consists of six periods of 184 A concentration of substitutionally incorporated C of 0.4%. Without the RTA process only the Si-substrate TO-phonon line at 1.09 eV is clearly resolved. The PL of the SiC quantum well is only indicated by weak signals at 1.132 eV ŽNP. and 1.074 eV ŽTO.. After the thermal treatment this SiC duplet consisting of NP- and TO Si – Si-phonon lines is strongly enhanced. The PL-signal of the pseudomorphic Si 0.996 C 0.004-layer is approximately 18 meV lower in energy than the corresponding Si-line despite the larger
Fig. 1. 2.2 K-PL spectra of a 6 period SirSi 0.996 C 0.004-MQW grown at ˚ sy1 and subsequently annealed at different temperatures 4808C at 0.3 A by RTA.
159
bandgaps of b-SiC Ž2.2 eV. and diamond Ž5.5 eV.. The 8508C RTA opens two additional recombination paths with transition energies of about 1.06 eV and 1.12 eV. Both lines are not yet identified but their simultaneous appearance after the 8508C RTA and intensity increase after the 9508C anneal as well as their separation by the characteristic Si-TO-phonon energy indicate that they are related to each other. Partially locally relaxed layers with higher carbon concentrations or localized excitons bound to defects or isoelectronic traps in Si may account for these signals. An additional line at an energy of 1.11 eV in the spectrum of the 9508C sample is also still unclear. The dominance of the SiC-bandgap PL line in the spectra of the annealed sample reflects the good epitaxial quality. The RTA at the different temperatures shifts the SiC-PL slightly to higher energies. This is interpreted in terms of the outdiffusion of C from the quantum wells into the barriers. While below 8508C no significant energy shift is detected, the intermixing of adjacent layers seems to be enhanced for the 9508C RTA. TEM micrographs show a vanishing strain contrast between the Si- and SiC-layers for the annealed samples, confirming the C-outdiffusion. Within the accuracy of the TEM, no defect formation or SiC precipitation can be seen, that could have led to a thermally induced strain relaxation of the MQW. Therefore the overall strain seems to remain unchanged in the sample. This is verified by HRXRD measurements of the as grown and annealed sample ŽFig. 2.. The shoulder on the right hand side of the Si-substrate peak is unaffected by RTA, i.e., C remains dissolved in the Si-matrix. These results are consistent with observations that at annealing temperatures of more than 9008C SirSiC-MQWs relax by interdiffusion and that at temperatures of 10008C and above SiC-precipitation dominates w6,7x. The C-concentration in the SiC-layers can be increased either by increasing the power throughput of the C-sublimation source or by decreasing the Si-growth rate. Because of the big distance of approximately 0.5 m between the C-sublimation source and the substrate in our MBEchamber w8x the maximum substitutional C-content is ap˚ sy1 . The proximately 0.8% at a growth rate of 0.3 A growth temperature was varied between 4008C and 6008C. SiC-related PL can be observed only for a growth temperature of 5008C. TEM pictures ŽFig. 3. of the 5008C sample show flat SiC layers with sharp interfaces, while at the growth temperature of 4008C strong surface roughness is created. For the sample grown at the high temperature of 6008C no defined quantum well layers are visible by TEM. Including the above RTA results into our considerations it appears that the SiC layers, once deposited as a random alloy at a relatively low temperature of about 5008C, are able to withstand anneals up to significantly higher temperatures. This shows the importance of a kinetic stabilization of the metastable phase during the growth. In order to incorporate more C into the samples the ˚ sy1 . Several SirSiCgrowth rate was reduced to 0.1 A
160
R. Hartmann et al.r Thin Solid Films 318 (1998) 158–162
Fig. 2. HRXRD-measurements of the SirSi 0.996 C 0.004-MQW of Fig. 1 for the as grown and annealed sample. The shoulder on the right side of the Si Ž004. peak giving the average strain over a sample period remains unchanged by RTA. The simulation curve is also illustrated showing a good agreement with the experiment.
MQWs with C-concentration ranging between 0.3% and 2.7% were grown, but none of these samples show SiCbandgap luminescence. Fig. 4 shows the PL-spectra of a ˚ Si-barriers and 45 A˚ SirSiC-MQW of 10 periods of 112 A ˚ sy1 . Here SiC-quantum wells grown at 4808C with 0.1 A the C-content is only 0.3% C, i.e., less than in the luminescent sample of Fig. 1. In the spectrum of the as grown sample a broad band luminescence signal at approximately 0.8 eV appears which is known for samples grown by MBE at low temperatures w8,9x. After the RTA several lines at approximately 1.03 eV, 1.07 eV and 1.09 eV emerge which are similar to the unidentified PL-lines of Fig. 1. However, SiC-bandgap PL is not detected. We interpret this as a consequence of the deteriorated sample quality. Below certain Si growth rates C is known to hinder, respectively disrupt the epitaxy w10x. Furthermore, the reduction of the growth rate needs to be accompanied by a reduction of the growth temperature. This is necessary to keep the surface mobility low precluding islanding. Consequently the temperature window for the epitaxial C-growth is further shrunk. The TEM pictures of SirSiC-MQWs consisting of 6 ˚ thick Si-barriers and 30 A˚ thick SiC-wells periods of 60 A ˚ sy1 reveal that the structural with 2.7% C grown at 0.1 A qualities dramatically change within a temperature variation of only 608C. The best quality with relatively flat SiC
Fig. 3. Influence of changes of the growth temperature on the structural quality of a SirSi 0.992 C 0.008-MQW.
R. Hartmann et al.r Thin Solid Films 318 (1998) 158–162
Fig. 4. 2.2 K-PL spectra of a six period SirSi 0.997 C 0.003-MQW grown at ˚ sy1 . SiC-bandgap PL is not observed for the as grown 4808C at 0.1 A and RTA treated sample.
layers and sharp interfaces is achieved at 4708C. A growth temperature of 5008C together with the high C-content of 2.7% already leads to highly islanded morphology. The Si
161
layers are not always uninterrupted, but often cut off by the neighbouring SiC-films. At 4408C the flatness of the SiC-layers suffers compared to the growth temperature of 4708C. Because of the opposite lattice mismatch of Ge and C on Si, the growth window for epitaxial SiGeC broadens compared to SiC. Several SirSiGeC-MQWs with wGex s 5.75% and wCx s 0.4% were grown pseudomorphically on Si at a substrate temperature of 5008C and growth rates of ˚ sy1 and 0.3 A˚ sy1 . The layer thickness of Si and 0.1 A ˚ and 180 A. ˚ Since 1 at.% of C SiGeC varied between 27 A compensates the strain of 8.3 at.% Ge w11x the layers are slightly compressively strained as has been verified by X-ray diffraction. After a 10 min RTA at 8008C all ˚ sy1 show strong SiGeC-PL ŽFig. samples grown at 0.3 A 5.. ˚ to A variation of the SiGeC layer thickness from 27 A ˚ leads to a shift of the NP PL line from 1.132 eV to 180 A ˚ 1.115 eV. Keeping the SiGeC thickness constant at 45 A and varying the Si layer thickness instead has no effects on the energetic position of the NP PL lines. This can be understood as a proof that carriers are confined in the SiGeC layers and no type of carriers is confined within the Si layers indicating a type-I band alignment. In addition the shift of the PL line with the SiGeC well widths proves their origin from quantum confined states.
4. Conclusion Bandedge PL from pseudomorphic SirSiC and SirSiGeC-MQWs are observed after RTA treatments at temperatures between 650–9508C. No strain relaxation by defect formation or SiC precipitation occurs in the SirSiC-MQWs at this anneal temperatures indicating a good thermal stability of the samples. However diffusion plays a role for anneal temperatures above 8008C. Substrate temperatures and growth rate are shown to be crucial parameters for the growth of samples of high structural and optical quality. A high C-flux strongly limits the window for growth temperature and rate. PL-measurements on nearly strain compensated SirSiGeC-MQWs give evidence for a band-alignment of type-I with at least one carrier type efficiently confined.
References
Fig. 5. 2.2 K-PL spectra of nearly strain compensated SirSiGeC-MQWs with wCx s 0.4% and wGex s 5.75%. The SiGeC-PL shifts with variations of the SiGeC-width and does not move when the Si-layer width is varied. ˚ Sir48 A˚ SiC-MQW is also shown. The PL-spectrum of the 184 A
w1x K. Brunner, K. Eberl, W. Winter, Phys. Rev. Lett. 76 Ž1996. 303. w2x K. Eberl, K. Brunner, W. Winter, Thin Solid Films 294 Ž1997. 98. w3x K. Eberl, S.S. Iyer, S. Zollner, J.C. Tsang, F.K. LeGoues, Appl. Phys. Lett. 60 Ž1992. 3033. w4x A.St. Amour, C.W. Liu, J.C. Sturm, Appl. Phys. Lett. 67 Ž1995. 3915. w5x A.R. Bean, R.C. Newman, J. Phys. Chem. Solids 33 Ž1972. 255.
162
R. Hartmann et al.r Thin Solid Films 318 (1998) 158–162
w6x M.S. Goorsky, S.S. Iyer, K. Eberl, F. Legoues, J. Angilello, F. Cardone, Appl. Phys. Lett. 60 Ž1992. 2758. w7x A.R. Powell, K. Eberl, B.A. Ek, S.S. Iyer, J. Cryst. Growth 127 Ž1993. 425. w8x D. Grutzmacher, R. Hartmann, E. Muller, U. Gennser, A. Dom¨ ¨ mann, to be published.
w9x J.-P. Noel, ¨ N.L. Rowell, D.C. Houghton, D.D. Perovic, Appl. Phys. Lett. 57 Ž1990. 1037. w10x S.S. Iyer, K. Eberl, M.S. Goorsky, F.K. LeGoues, J.C. Tsang, F. Cardone, Appl. Phys. Lett. 60 Ž1992. 356. w11x J.L. Regolini, F. Gisbert, G. Dolino, P. Boucaud, Mater. Lett. 18 Ž1993. 57.
Growth of Si 1yyC yrSi- and Si 1yxyyGe xC yrSi-multiple quantum wells using molecular beam epitaxy R. Hartmann
a,)
a a , D. Grutzmacher , E. Muller , U. Gennser a , A. Dommann b, P. Schroter ¨ ¨ ¨ b, P. Warren c a
Paul-Scherrer-Institut, CH-5232 Villigen, Switzerland Neutechnikum Buchs, CH-9470 Buchs, Switzerland Swiss Federale Institute of Technology, CH-1015 Lausanne, Switzerland b
c
Abstract SirSiC- and SirSiGeC-multiple quantum well ŽMQW. structures of different well thicknesses and C-concentrations up to 2.7% have been grown pseudomorphically on SiŽ001. using molecular beam epitaxy. Near bandedge photoluminescence ŽPL. is observed for SirSiC for certain growth parameters. Substrate temperature and Si growth rate strongly influence the structural and optical properties of the samples, in particular for samples with high C-content. The thermal stability of the SirSiC-layers was investigated by using PL, high resolution X-ray diffraction ŽHRXRD. and transmission electron microscopy ŽTEM.. At anneal temperatures of 9508C and below the structures relax only by interdiffusion but not by defect or SiC-formation. PL data on a nearly strainless SirSiGeC-MQW indicate a band alignment of type-I character. q 1998 Elsevier Science S.A. Keywords: Multiple quantum wells; Molecular beam epitaxy; Photoluminescence
1. Introduction The incorporation of C into substitutional sites offers a new degree of freedom in the bandgap, band offset and strain engineering of Si-based materialsw1–4x. However, the solubility of C in Si is only 3 = 10 17 atoms cmy3 at the melting point w5x. At higher concentrations the thermodynamically favoured phase are silicon carbide ŽSiC. clusters distributed in Si rather than the random Si 1y x C x alloy. Although low-temperature nonequilibrium epitaxy can produce films with substitutional C-contents of up to 7%, the window for growth temperature and rate is quite narrow. Twinning may lead to amorphous growth if the substrate temperature is chosen too low andror the growth rate is chosen too high. On the other hand, at substrate temperatures of above 6008C a mixed alloy carbide phase is produced.
)
Corresponding author. Micro- and Nanostructures Laboratory, Paul Scherrer Institute, CH-5234 Villigen, PSI. Tel.: q41-056-310-28-50; fax: q41-056-310-26-46; e-mail: [email protected]. 0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 5 7 - 7
In the following we critically review the growth conditions for high quality SiC alloys. The quality of the samples is directly reflected on the photoluminescence ŽPL. spectra, since defects provide competing recombination mechanisms which suppress the bandgap signal. A narrow growth window for the observation of bandgap PL in SirSiC- multiple quantum wells ŽMQWs. is found. The structural characterization of the samples is done by high resolution X-ray diffraction ŽHRXRD. and transmission electron microscopy ŽTEM. studies. The behaviour of the PL energy of SirSiGeC-MQWs at variations of the Si and SiGeC widths verifies the type-I band alignment with the SiGeC layer as the quantum well w2x.
2. Experimental Several Si 1y y C yrSi- and Si 1yxyyGe x C yrSi-MQWs of ˚ and 400 A˚ and C-concentraperiod lengths between 90 A tion of up to 2.7% have been grown on Ž001.Si substrates of a resistivity of 500 V cm. A solid source Si molecular beam epitaxy ŽMBE. was used with electron guns for Si
R. Hartmann et al.r Thin Solid Films 318 (1998) 158–162
and Ge evaporation and a directly heated pyrolithic graphite filament for C sublimation. Besides the variation of layer thickness and layer composition the growth temperature was varied between 4008C and 6008C and the growth rate ˚ sy1 and 0.3 A˚ sy1 . After the MBE growth between 0.1 A the samples were thermally annealed for between 4 min and 10 min at temperatures between 6508C and 9508C in a N2r4%H 2 gas mixture. In order to test the effect of growth temperature and rate on the structural and optical properties of the layers double crystal HRXRD, cross-sectional TEM and PL measurements have been performed.
3. Results and discussion Fig. 1 shows the PL spectra of a SirSiC-MQW grown at 4808C and subsequently annealed for 4 min at different temperatures ranging from 6508C to 9508C. The MQW ˚ Si and 48 A˚ SiC with a consists of six periods of 184 A concentration of substitutionally incorporated C of 0.4%. Without the RTA process only the Si-substrate TO-phonon line at 1.09 eV is clearly resolved. The PL of the SiC quantum well is only indicated by weak signals at 1.132 eV ŽNP. and 1.074 eV ŽTO.. After the thermal treatment this SiC duplet consisting of NP- and TO Si – Si-phonon lines is strongly enhanced. The PL-signal of the pseudomorphic Si 0.996 C 0.004-layer is approximately 18 meV lower in energy than the corresponding Si-line despite the larger
Fig. 1. 2.2 K-PL spectra of a 6 period SirSi 0.996 C 0.004-MQW grown at ˚ sy1 and subsequently annealed at different temperatures 4808C at 0.3 A by RTA.
159
bandgaps of b-SiC Ž2.2 eV. and diamond Ž5.5 eV.. The 8508C RTA opens two additional recombination paths with transition energies of about 1.06 eV and 1.12 eV. Both lines are not yet identified but their simultaneous appearance after the 8508C RTA and intensity increase after the 9508C anneal as well as their separation by the characteristic Si-TO-phonon energy indicate that they are related to each other. Partially locally relaxed layers with higher carbon concentrations or localized excitons bound to defects or isoelectronic traps in Si may account for these signals. An additional line at an energy of 1.11 eV in the spectrum of the 9508C sample is also still unclear. The dominance of the SiC-bandgap PL line in the spectra of the annealed sample reflects the good epitaxial quality. The RTA at the different temperatures shifts the SiC-PL slightly to higher energies. This is interpreted in terms of the outdiffusion of C from the quantum wells into the barriers. While below 8508C no significant energy shift is detected, the intermixing of adjacent layers seems to be enhanced for the 9508C RTA. TEM micrographs show a vanishing strain contrast between the Si- and SiC-layers for the annealed samples, confirming the C-outdiffusion. Within the accuracy of the TEM, no defect formation or SiC precipitation can be seen, that could have led to a thermally induced strain relaxation of the MQW. Therefore the overall strain seems to remain unchanged in the sample. This is verified by HRXRD measurements of the as grown and annealed sample ŽFig. 2.. The shoulder on the right hand side of the Si-substrate peak is unaffected by RTA, i.e., C remains dissolved in the Si-matrix. These results are consistent with observations that at annealing temperatures of more than 9008C SirSiC-MQWs relax by interdiffusion and that at temperatures of 10008C and above SiC-precipitation dominates w6,7x. The C-concentration in the SiC-layers can be increased either by increasing the power throughput of the C-sublimation source or by decreasing the Si-growth rate. Because of the big distance of approximately 0.5 m between the C-sublimation source and the substrate in our MBEchamber w8x the maximum substitutional C-content is ap˚ sy1 . The proximately 0.8% at a growth rate of 0.3 A growth temperature was varied between 4008C and 6008C. SiC-related PL can be observed only for a growth temperature of 5008C. TEM pictures ŽFig. 3. of the 5008C sample show flat SiC layers with sharp interfaces, while at the growth temperature of 4008C strong surface roughness is created. For the sample grown at the high temperature of 6008C no defined quantum well layers are visible by TEM. Including the above RTA results into our considerations it appears that the SiC layers, once deposited as a random alloy at a relatively low temperature of about 5008C, are able to withstand anneals up to significantly higher temperatures. This shows the importance of a kinetic stabilization of the metastable phase during the growth. In order to incorporate more C into the samples the ˚ sy1 . Several SirSiCgrowth rate was reduced to 0.1 A
160
R. Hartmann et al.r Thin Solid Films 318 (1998) 158–162
Fig. 2. HRXRD-measurements of the SirSi 0.996 C 0.004-MQW of Fig. 1 for the as grown and annealed sample. The shoulder on the right side of the Si Ž004. peak giving the average strain over a sample period remains unchanged by RTA. The simulation curve is also illustrated showing a good agreement with the experiment.
MQWs with C-concentration ranging between 0.3% and 2.7% were grown, but none of these samples show SiCbandgap luminescence. Fig. 4 shows the PL-spectra of a ˚ Si-barriers and 45 A˚ SirSiC-MQW of 10 periods of 112 A ˚ sy1 . Here SiC-quantum wells grown at 4808C with 0.1 A the C-content is only 0.3% C, i.e., less than in the luminescent sample of Fig. 1. In the spectrum of the as grown sample a broad band luminescence signal at approximately 0.8 eV appears which is known for samples grown by MBE at low temperatures w8,9x. After the RTA several lines at approximately 1.03 eV, 1.07 eV and 1.09 eV emerge which are similar to the unidentified PL-lines of Fig. 1. However, SiC-bandgap PL is not detected. We interpret this as a consequence of the deteriorated sample quality. Below certain Si growth rates C is known to hinder, respectively disrupt the epitaxy w10x. Furthermore, the reduction of the growth rate needs to be accompanied by a reduction of the growth temperature. This is necessary to keep the surface mobility low precluding islanding. Consequently the temperature window for the epitaxial C-growth is further shrunk. The TEM pictures of SirSiC-MQWs consisting of 6 ˚ thick Si-barriers and 30 A˚ thick SiC-wells periods of 60 A ˚ sy1 reveal that the structural with 2.7% C grown at 0.1 A qualities dramatically change within a temperature variation of only 608C. The best quality with relatively flat SiC
Fig. 3. Influence of changes of the growth temperature on the structural quality of a SirSi 0.992 C 0.008-MQW.
R. Hartmann et al.r Thin Solid Films 318 (1998) 158–162
Fig. 4. 2.2 K-PL spectra of a six period SirSi 0.997 C 0.003-MQW grown at ˚ sy1 . SiC-bandgap PL is not observed for the as grown 4808C at 0.1 A and RTA treated sample.
layers and sharp interfaces is achieved at 4708C. A growth temperature of 5008C together with the high C-content of 2.7% already leads to highly islanded morphology. The Si
161
layers are not always uninterrupted, but often cut off by the neighbouring SiC-films. At 4408C the flatness of the SiC-layers suffers compared to the growth temperature of 4708C. Because of the opposite lattice mismatch of Ge and C on Si, the growth window for epitaxial SiGeC broadens compared to SiC. Several SirSiGeC-MQWs with wGex s 5.75% and wCx s 0.4% were grown pseudomorphically on Si at a substrate temperature of 5008C and growth rates of ˚ sy1 and 0.3 A˚ sy1 . The layer thickness of Si and 0.1 A ˚ and 180 A. ˚ Since 1 at.% of C SiGeC varied between 27 A compensates the strain of 8.3 at.% Ge w11x the layers are slightly compressively strained as has been verified by X-ray diffraction. After a 10 min RTA at 8008C all ˚ sy1 show strong SiGeC-PL ŽFig. samples grown at 0.3 A 5.. ˚ to A variation of the SiGeC layer thickness from 27 A ˚ leads to a shift of the NP PL line from 1.132 eV to 180 A ˚ 1.115 eV. Keeping the SiGeC thickness constant at 45 A and varying the Si layer thickness instead has no effects on the energetic position of the NP PL lines. This can be understood as a proof that carriers are confined in the SiGeC layers and no type of carriers is confined within the Si layers indicating a type-I band alignment. In addition the shift of the PL line with the SiGeC well widths proves their origin from quantum confined states.
4. Conclusion Bandedge PL from pseudomorphic SirSiC and SirSiGeC-MQWs are observed after RTA treatments at temperatures between 650–9508C. No strain relaxation by defect formation or SiC precipitation occurs in the SirSiC-MQWs at this anneal temperatures indicating a good thermal stability of the samples. However diffusion plays a role for anneal temperatures above 8008C. Substrate temperatures and growth rate are shown to be crucial parameters for the growth of samples of high structural and optical quality. A high C-flux strongly limits the window for growth temperature and rate. PL-measurements on nearly strain compensated SirSiGeC-MQWs give evidence for a band-alignment of type-I with at least one carrier type efficiently confined.
References
Fig. 5. 2.2 K-PL spectra of nearly strain compensated SirSiGeC-MQWs with wCx s 0.4% and wGex s 5.75%. The SiGeC-PL shifts with variations of the SiGeC-width and does not move when the Si-layer width is varied. ˚ Sir48 A˚ SiC-MQW is also shown. The PL-spectrum of the 184 A
w1x K. Brunner, K. Eberl, W. Winter, Phys. Rev. Lett. 76 Ž1996. 303. w2x K. Eberl, K. Brunner, W. Winter, Thin Solid Films 294 Ž1997. 98. w3x K. Eberl, S.S. Iyer, S. Zollner, J.C. Tsang, F.K. LeGoues, Appl. Phys. Lett. 60 Ž1992. 3033. w4x A.St. Amour, C.W. Liu, J.C. Sturm, Appl. Phys. Lett. 67 Ž1995. 3915. w5x A.R. Bean, R.C. Newman, J. Phys. Chem. Solids 33 Ž1972. 255.
162
R. Hartmann et al.r Thin Solid Films 318 (1998) 158–162
w6x M.S. Goorsky, S.S. Iyer, K. Eberl, F. Legoues, J. Angilello, F. Cardone, Appl. Phys. Lett. 60 Ž1992. 2758. w7x A.R. Powell, K. Eberl, B.A. Ek, S.S. Iyer, J. Cryst. Growth 127 Ž1993. 425. w8x D. Grutzmacher, R. Hartmann, E. Muller, U. Gennser, A. Dom¨ ¨ mann, to be published.
w9x J.-P. Noel, ¨ N.L. Rowell, D.C. Houghton, D.D. Perovic, Appl. Phys. Lett. 57 Ž1990. 1037. w10x S.S. Iyer, K. Eberl, M.S. Goorsky, F.K. LeGoues, J.C. Tsang, F. Cardone, Appl. Phys. Lett. 60 Ž1992. 356. w11x J.L. Regolini, F. Gisbert, G. Dolino, P. Boucaud, Mater. Lett. 18 Ž1993. 57.