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Oxidation of Si1−x−yGexCy strained layers grown on Si: kinetics and interface properties
Microelectronics Reliability 40 (2000) 829±832
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Oxidation of Si1ÿxÿy Gex Cy strained layers grown on Si: kinetics and interface properties A. Cuadras a,*, B. Garrido a, C. Bonafos a, J.R. Morante a, L. Fonseca b, K. Pressel c a
Departament d'Electr onica, Universitat de Barcelona (EME-UB), Martõ i Franqu es 1, 08028 Barcelona, Spain b Institut de Microelectr onica de Barcelona (CNM-CSIC), Campus UAB, 08193 Bellaterra, Spain c Institute for Semiconductor Physics, Walter-Korsing-Strasse 2, 15230 Frankfurt (Oder), Germany
Abstract We have investigated the thermal oxidation of strained Si1ÿx Gex and Si1ÿxÿy Gex Cy layers and the in¯uence of the thermal process on the structure of the layers. Using XPS and SIMS depth pro®les, we have found a germanium pile-up in the epitaxial layer near the oxide-layer interface. Using transmission electron microscopy (TEM), we have observed that crystallinity is well conserved, but additionally, we have found SiC precipitates. A qualitative model for the oxidation of this kind of binary and ternary alloys is presented. The model is based on the strain development of the samples and depends on germanium and carbon compositions and on the temperature of the process. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction For mainstream semiconductor technologies, silicon is the natural choice. With the introduction of germanium and carbon in silicon, the properties of silicon technology can be signi®cantly enhanced. A Si1ÿx Gex BICMOS technology [1] and recently, a Si1ÿxÿy Gex Cy heterojunction bipolar transistors (HBT) technology have been developed [2]. Integrated circuits with HBT made of Si1ÿx Gex are meanwhile oered for applications in wireless systems. Si1ÿx Gex also presents promising results in the ®eld of modulated doped ®eld eect transistors (MODFETs). An increase of hole mobilities with respect to silicon devices has already been reported [3]. In CMOS technology, the mobility dierence between the n-MOSFET and p-MOSFET is still a drawback. Si1ÿx Gex MOSFET devices are meanwhile under experimental investigation. Although they can show improved electrical properties compared to silicon
*
Corresponding author. Tel.: +34-934021-141; fax: +34934021-148. E-mail address: [email protected] (A. Cuadras).
transistors, they still present structural instability problems. Moreover, it has been shown that the addition of a small quantity of carbon into the Si1ÿx Gex alloy makes it more stable against strain relaxation because carbon compensates the compressive strain introduced by germanium [4]. It is accepted that strained layers will oer better electrical properties than completely compensated ones [5]. Because of this, a MOSFET based on strained Si1ÿxÿy Gex Cy is interesting. An increase in hole mobility with respect to silicon and germanium is expected. This increase of hole mobilities is due to the splitting of the light hole band [6] and intrinsic high mobility in SiGe. But mainly due to oxidation problems of Si1ÿxÿy Gex Cy , as germanium piling up at the interface [7] and carbon leaving its substitutional position [8], a Si1ÿxÿy Gex Cy MOSFET is still missing. The purpose of this work is to get an understanding of dry oxidation of Si1ÿxÿy Gex Cy strained layers on silicon as the ®rst step towards de®ning their ability to become a suitable material for the fabrication of MOSgated HFET devices. We report a reliable dry oxidation process for the layer and the quality of the interface is assessed. High electrical performance, high yield and reliability are the main aims. Essential problems of the oxidation are (a) oxide growth and oxide composition,
0026-2714/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 9 ) 0 0 3 1 7 - 0
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A. Cuadras et al. / Microelectronics Reliability 40 (2000) 829±832
(b) in¯uence of the thermal process (i.e. relaxation and diusion).
2. Oxidation of IV±IV strained layers Previous reports concerning thermal oxidation of SiGe strained layers have found a germanium rejection of the oxidation front and a pile-up at the oxide/layer interface. At 800°C, germanium is trapped at the interface and does not diuse into the layer. It was a common observation that Ge acted as a catalyst of thermal wet oxidations. However, dry oxidation behavior was not completely understood; as it seemed to show no significant dierences with respect to silicon oxidation. Wet oxidation kinetics of Si1ÿy Cy does not dier signi®cantly with respect to silicon kinetics. In this case, some carbon is found in the oxide and at the interface. Germanium introduces a compressive strain in the layer, whereas carbon introduces a tensile one. According to VegardÕs law, they approximately compensate each other in the ratio Ge/C 10/1. Thermal eects on the layer may lead to strain relaxation in SiGe layers. This is a reason to introduce carbon in the structure, to compensate the strain due to Ge in the layer and minimize the risk of formation of dislocations.
3. Experimental results We performed dry oxidations on Si0:9 Ge0:1 and Si0:895 Ge0:1 C0:005 MBE grown samples in a rapid thermal furnace for temperatures between 900°C and 1100°C. The layers were 200 nm thick and were grown on 100 nm silicon buer layers. We worked with compressive strained layers (germanium dominated with the ratio Ge/C 20/1). Several oxidation times were tried in order to characterize the eects of oxidation time and temperature on the layers. Kinetics of the oxidation process and properties of the grown oxide were also obtained [9]. No traces of germanium and carbon were found in the oxide, so that silicon dioxide is formed [9]. As long as the layer is oxidised, germanium enriches the alloy layer. The pro®le of germanium after the oxidation process is plotted in Fig. 1. It is shown that the germanium pile-up clearly depends on temperature and also on the presence of carbon. This is also in agreement with the SIMS measurements. This accumulation can be described with two variables, a mean FWHM of the overshot peak and a mean maximum xGe . The pile-up of germanium evolves in two steps: (a) First, germanium enrichment increases quickly in a small region below the oxidation front with a rapid increase of xGe . (b) The maximum decreases and Ge diuses into the alloy layer. The
Fig. 1. (a) XPS germanium pro®les. The pile-up at the oxide/ layer interphase is clearly seen. Oxidation conditions were 3 in. at 1000°C and 1 in. at 1100°C. (b) SIMS pro®le for a long time oxidation (8.5 in. at 1100°C and 15 in. at 1000°C). Interdiusion of Ge into the substrate is signi®cant and the pile-up has completely diused into the epitaxial layer.
FWHM becomes wider due to the diusion process towards the inner layer. The diusion is a result of the concentration gradient and a high temperature process. On increasing the temperature, xGe increases and FWHM decreases. Contrarily, on increasing the oxidation time, xGe decreases but FWHM increases. Finally, carbon causes both the xGe and FWHM decrease. But by far, the most important fact is that SiGe crystalline structure is preserved for the oxidation temperatures studied (<1100°C). Also from XPS characterization, a depth pro®le study of silicon oxidised states shows an in¯uence of carbon on the oxidation process as well as on germanium diusion (Fig. 2). We found an evolution of suboxides concentration from metallic Si though Si1 , Si2 , Si3 up to Si4 . SiGe samples show a parallel behavior when compared with those of silicon. In both SiGe and SiGeC, the presence of Si2 is lower than the other states. The pro®les of Si0 and Si4 are more abrupt for SiGe than those containing carbon. It seems that carbon
A. Cuadras et al. / Microelectronics Reliability 40 (2000) 829±832
831
Fig. 2. Suboxide states, Si3 and Si4 . SiGe behaviour is very similar to silicon samples despite the carbon enlargement of the compositional interface.
allows suboxides to spread over a larger region (100 A) both in the oxide and in the layer. From the structural point of view, transmission electron microscopy (TEM) results show a nearly atomic-¯at interface (Fig. 3(a)). Roughness is characterized considering the square root of deviation of the interface with respect to an ideal ¯at line (mean inter deviation is found. face) limiting SiO2 /SiGeC. A 2 A This means that oxidation takes place uniformly across the whole surface. The crystallinity of the layer is well conserved for the whole temperature range of 900±1000°C. No mis®t dislocations are found in the carbon-containing samples, but in the Si1ÿx Gex layers at the layer±substrate interface. This con®rms the idea that carbon stabilizes the strain of the layer. The interaction of the oxidation front with carbon produces CO and CO2 , which outdiuses with a consequent loss of carbon. But this is only a surface eect. Metastable substitutional position makes carbon to be very sensitive to thermal processes. The main drawback is carbon precipitation with Si to form b-SiC. This conclusion is inferred from TEM micrographs in Fig. 3(b), where precipitates are found distributed throughout the layers that contain carbon. Precipitates do not introduce dislocations and crystallinity of the
Fig. 3. (a) TEM micrograph. Atomically ¯at interface of SiGeC sample, (b) b-SiC precipitates in the epitaxial matrix, (c) diameter distribution of b-SiC precipitates.
layer is preserved. Precipitates are also crystalline, as is inferred from interference fringes. These precipitates have a mean diameter of about 2.5±3 nm, depending on temperature. Distribution diameters are shown in the histogram in Fig. 3(c). FTIR analysis reveals an absorption mode at about 810 cmÿ1 , which we associate with the absorption of nanocrystalline structures of b-SiC. The evolution of b-SiC concentration obtained by FTIR is plotted in Fig. 4. For Tox up to 950°C, b-SiC concentration increases from zero, progressively. For Tox above 1000°C, a very fast formation of b-SiC occurs. This behavior might be related to the existence of an energy barrier, which is exceeded between 950°C and 1000°C, giving rise to the formation of b-SiC. At 1000°C, the carbon contained in b-SiC precipitates is estimated to be about 35% of the initial concentration and 15% is estimated to remain in the substitutional position after quanti®cation by FTIR. The other 50% should be in the interstitial position.
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A. Cuadras et al. / Microelectronics Reliability 40 (2000) 829±832
5. Conclusions
Fig. 4. SiC concentration evolution after FTIR analysis of the 810 cmÿ1 peak.
SiGeC and SiGe layers have been oxidized on a window temperature ranging between 900°C and 1100°C. Good oxides have been obtained on these layers. Layer characterization has shown that composition modi®cation can arise in two ways: (i) Ge segregates from oxide growth to pile up at the oxide/layer interface and then, it diuses into the layer. (ii) Substitutional carbon migrates to interstitial positions as well as precipitates in the form of b-SiC precipitates. It is concluded that oxidation temperature (T 900°C) must be kept low (900°C) to avoid layer degradation. Quanti®cation of carbon distribution after the thermal process has been found out.
4. Discussion
Acknowledgements
It was reported recently that strain has an important in¯uence on dry oxidation kinetics [10]. It is shown here that SiGe layers oxidize faster than silicon does but slower than b-SiC. This observation is in agreement with Ge pile-up in SiGe samples. The compressive strain in the layer, combined with a lower energy bond Si±Ge (3.12 eV) compared with Si±Si (3.30 eV), allows the oxygen to react faster with silicon. Oxygen reaction with silicon (ESi±O 8 eV) is more favorable than with germanium (EGe±O 6.8 eV). Thus, carbon does not allow chemical reactions with oxygen but diusion through the layer. Contrarily, the presence of carbon in SiGeC alleviates the strain, as tensile strain of carbon compensates the compressive strain due to germanium. In this case, oxygen diuses better into the layer but the reaction with silicon is more dicult, because of the more energetic bond, Si±C (4.51 eV). Moreover, silicon suboxides, XPS pro®les show that less strained layers (that is, compensated with carbon) make oxygen to diuse deeper into the layer. The reaction with silicon is not so favorable because carbon avoids a fast Si±O reaction, so oxygen can come further. Moreover, a less strained layer allows oxygen to diuse easily. But once carbon precipitates or leaves its substitutional position, suboxide states should evolve towards a common behavior in SiGe and SiGeC samples, i.e., suboxides should saturate to pure silicon dioxide. The observation that carbon makes diusion easier is also consistent with germanium behavior as the germanium in carbon-containing layers also diuses further and faster.
The authors are indebted to the MBE group of the Institute for Semiconductor Physics for providing SiGeC samples. References [1] Johnson RA, Zierak MZ, Outama KB, Bahn TC, Joseph AJ, Cordero CN, Malinowski J, Bard KA, Weeks T, Milliken R, Medve T, May G, Chong W, Walter K, Tempest SL, Chau B, Boenke M, Nelson M, Harame D. International Electronic Device Meeting Proceedings. 1998. p. 217. [2] Knoll D, Heinemann B, Osten HJ, Ehwald KE, Tillack B, Schley P, Barth R, Matthes M, Park KS, Kim Y, Winkler W. International Electronic Device Meeting Proceedings. 1998. p. 703. [3] K ock G, Gl uck M, Hackbarth T, Herzog H, Kohn E. Thin Solid Films 1998;336:141. [4] Osten HJ. Materials Science and Engineering 1996;B36:268. [5] Osten HJ, Lippert G, Gaworzewski P, Sorge R. Appl Phys Lett 1997;71:1522. [6] Ismail K, Chu JO, Meyerson BS. Appl Phys Lett 1994;64:3125. [7] Xiang J, Herbots N, Jacobson H, Ye PE, Whaley S. J Appl Phys 1996;80:1857. [8] Pressel K, Garrido B, Franz M, Kr uger D, Osten HJ, Morante JR. Journal of Vacuum Science and Technology B 1998;16:1757. [9] Cuadras A, Garrido B, Bonafos C, Morante JR, Fonseca L, Pressel K. E-MRS (Symp P). June 1999. [10] Garrido B, Cuadras A, Bonafos C, Morante JR, Fonseca L, Pressel K. INFOS. June 1999.
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Oxidation of Si1ÿxÿy Gex Cy strained layers grown on Si: kinetics and interface properties A. Cuadras a,*, B. Garrido a, C. Bonafos a, J.R. Morante a, L. Fonseca b, K. Pressel c a
Departament d'Electr onica, Universitat de Barcelona (EME-UB), Martõ i Franqu es 1, 08028 Barcelona, Spain b Institut de Microelectr onica de Barcelona (CNM-CSIC), Campus UAB, 08193 Bellaterra, Spain c Institute for Semiconductor Physics, Walter-Korsing-Strasse 2, 15230 Frankfurt (Oder), Germany
Abstract We have investigated the thermal oxidation of strained Si1ÿx Gex and Si1ÿxÿy Gex Cy layers and the in¯uence of the thermal process on the structure of the layers. Using XPS and SIMS depth pro®les, we have found a germanium pile-up in the epitaxial layer near the oxide-layer interface. Using transmission electron microscopy (TEM), we have observed that crystallinity is well conserved, but additionally, we have found SiC precipitates. A qualitative model for the oxidation of this kind of binary and ternary alloys is presented. The model is based on the strain development of the samples and depends on germanium and carbon compositions and on the temperature of the process. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction For mainstream semiconductor technologies, silicon is the natural choice. With the introduction of germanium and carbon in silicon, the properties of silicon technology can be signi®cantly enhanced. A Si1ÿx Gex BICMOS technology [1] and recently, a Si1ÿxÿy Gex Cy heterojunction bipolar transistors (HBT) technology have been developed [2]. Integrated circuits with HBT made of Si1ÿx Gex are meanwhile oered for applications in wireless systems. Si1ÿx Gex also presents promising results in the ®eld of modulated doped ®eld eect transistors (MODFETs). An increase of hole mobilities with respect to silicon devices has already been reported [3]. In CMOS technology, the mobility dierence between the n-MOSFET and p-MOSFET is still a drawback. Si1ÿx Gex MOSFET devices are meanwhile under experimental investigation. Although they can show improved electrical properties compared to silicon
*
Corresponding author. Tel.: +34-934021-141; fax: +34934021-148. E-mail address: [email protected] (A. Cuadras).
transistors, they still present structural instability problems. Moreover, it has been shown that the addition of a small quantity of carbon into the Si1ÿx Gex alloy makes it more stable against strain relaxation because carbon compensates the compressive strain introduced by germanium [4]. It is accepted that strained layers will oer better electrical properties than completely compensated ones [5]. Because of this, a MOSFET based on strained Si1ÿxÿy Gex Cy is interesting. An increase in hole mobility with respect to silicon and germanium is expected. This increase of hole mobilities is due to the splitting of the light hole band [6] and intrinsic high mobility in SiGe. But mainly due to oxidation problems of Si1ÿxÿy Gex Cy , as germanium piling up at the interface [7] and carbon leaving its substitutional position [8], a Si1ÿxÿy Gex Cy MOSFET is still missing. The purpose of this work is to get an understanding of dry oxidation of Si1ÿxÿy Gex Cy strained layers on silicon as the ®rst step towards de®ning their ability to become a suitable material for the fabrication of MOSgated HFET devices. We report a reliable dry oxidation process for the layer and the quality of the interface is assessed. High electrical performance, high yield and reliability are the main aims. Essential problems of the oxidation are (a) oxide growth and oxide composition,
0026-2714/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 9 ) 0 0 3 1 7 - 0
830
A. Cuadras et al. / Microelectronics Reliability 40 (2000) 829±832
(b) in¯uence of the thermal process (i.e. relaxation and diusion).
2. Oxidation of IV±IV strained layers Previous reports concerning thermal oxidation of SiGe strained layers have found a germanium rejection of the oxidation front and a pile-up at the oxide/layer interface. At 800°C, germanium is trapped at the interface and does not diuse into the layer. It was a common observation that Ge acted as a catalyst of thermal wet oxidations. However, dry oxidation behavior was not completely understood; as it seemed to show no significant dierences with respect to silicon oxidation. Wet oxidation kinetics of Si1ÿy Cy does not dier signi®cantly with respect to silicon kinetics. In this case, some carbon is found in the oxide and at the interface. Germanium introduces a compressive strain in the layer, whereas carbon introduces a tensile one. According to VegardÕs law, they approximately compensate each other in the ratio Ge/C 10/1. Thermal eects on the layer may lead to strain relaxation in SiGe layers. This is a reason to introduce carbon in the structure, to compensate the strain due to Ge in the layer and minimize the risk of formation of dislocations.
3. Experimental results We performed dry oxidations on Si0:9 Ge0:1 and Si0:895 Ge0:1 C0:005 MBE grown samples in a rapid thermal furnace for temperatures between 900°C and 1100°C. The layers were 200 nm thick and were grown on 100 nm silicon buer layers. We worked with compressive strained layers (germanium dominated with the ratio Ge/C 20/1). Several oxidation times were tried in order to characterize the eects of oxidation time and temperature on the layers. Kinetics of the oxidation process and properties of the grown oxide were also obtained [9]. No traces of germanium and carbon were found in the oxide, so that silicon dioxide is formed [9]. As long as the layer is oxidised, germanium enriches the alloy layer. The pro®le of germanium after the oxidation process is plotted in Fig. 1. It is shown that the germanium pile-up clearly depends on temperature and also on the presence of carbon. This is also in agreement with the SIMS measurements. This accumulation can be described with two variables, a mean FWHM of the overshot peak and a mean maximum xGe . The pile-up of germanium evolves in two steps: (a) First, germanium enrichment increases quickly in a small region below the oxidation front with a rapid increase of xGe . (b) The maximum decreases and Ge diuses into the alloy layer. The
Fig. 1. (a) XPS germanium pro®les. The pile-up at the oxide/ layer interphase is clearly seen. Oxidation conditions were 3 in. at 1000°C and 1 in. at 1100°C. (b) SIMS pro®le for a long time oxidation (8.5 in. at 1100°C and 15 in. at 1000°C). Interdiusion of Ge into the substrate is signi®cant and the pile-up has completely diused into the epitaxial layer.
FWHM becomes wider due to the diusion process towards the inner layer. The diusion is a result of the concentration gradient and a high temperature process. On increasing the temperature, xGe increases and FWHM decreases. Contrarily, on increasing the oxidation time, xGe decreases but FWHM increases. Finally, carbon causes both the xGe and FWHM decrease. But by far, the most important fact is that SiGe crystalline structure is preserved for the oxidation temperatures studied (<1100°C). Also from XPS characterization, a depth pro®le study of silicon oxidised states shows an in¯uence of carbon on the oxidation process as well as on germanium diusion (Fig. 2). We found an evolution of suboxides concentration from metallic Si though Si1 , Si2 , Si3 up to Si4 . SiGe samples show a parallel behavior when compared with those of silicon. In both SiGe and SiGeC, the presence of Si2 is lower than the other states. The pro®les of Si0 and Si4 are more abrupt for SiGe than those containing carbon. It seems that carbon
A. Cuadras et al. / Microelectronics Reliability 40 (2000) 829±832
831
Fig. 2. Suboxide states, Si3 and Si4 . SiGe behaviour is very similar to silicon samples despite the carbon enlargement of the compositional interface.
allows suboxides to spread over a larger region (100 A) both in the oxide and in the layer. From the structural point of view, transmission electron microscopy (TEM) results show a nearly atomic-¯at interface (Fig. 3(a)). Roughness is characterized considering the square root of deviation of the interface with respect to an ideal ¯at line (mean inter deviation is found. face) limiting SiO2 /SiGeC. A 2 A This means that oxidation takes place uniformly across the whole surface. The crystallinity of the layer is well conserved for the whole temperature range of 900±1000°C. No mis®t dislocations are found in the carbon-containing samples, but in the Si1ÿx Gex layers at the layer±substrate interface. This con®rms the idea that carbon stabilizes the strain of the layer. The interaction of the oxidation front with carbon produces CO and CO2 , which outdiuses with a consequent loss of carbon. But this is only a surface eect. Metastable substitutional position makes carbon to be very sensitive to thermal processes. The main drawback is carbon precipitation with Si to form b-SiC. This conclusion is inferred from TEM micrographs in Fig. 3(b), where precipitates are found distributed throughout the layers that contain carbon. Precipitates do not introduce dislocations and crystallinity of the
Fig. 3. (a) TEM micrograph. Atomically ¯at interface of SiGeC sample, (b) b-SiC precipitates in the epitaxial matrix, (c) diameter distribution of b-SiC precipitates.
layer is preserved. Precipitates are also crystalline, as is inferred from interference fringes. These precipitates have a mean diameter of about 2.5±3 nm, depending on temperature. Distribution diameters are shown in the histogram in Fig. 3(c). FTIR analysis reveals an absorption mode at about 810 cmÿ1 , which we associate with the absorption of nanocrystalline structures of b-SiC. The evolution of b-SiC concentration obtained by FTIR is plotted in Fig. 4. For Tox up to 950°C, b-SiC concentration increases from zero, progressively. For Tox above 1000°C, a very fast formation of b-SiC occurs. This behavior might be related to the existence of an energy barrier, which is exceeded between 950°C and 1000°C, giving rise to the formation of b-SiC. At 1000°C, the carbon contained in b-SiC precipitates is estimated to be about 35% of the initial concentration and 15% is estimated to remain in the substitutional position after quanti®cation by FTIR. The other 50% should be in the interstitial position.
832
A. Cuadras et al. / Microelectronics Reliability 40 (2000) 829±832
5. Conclusions
Fig. 4. SiC concentration evolution after FTIR analysis of the 810 cmÿ1 peak.
SiGeC and SiGe layers have been oxidized on a window temperature ranging between 900°C and 1100°C. Good oxides have been obtained on these layers. Layer characterization has shown that composition modi®cation can arise in two ways: (i) Ge segregates from oxide growth to pile up at the oxide/layer interface and then, it diuses into the layer. (ii) Substitutional carbon migrates to interstitial positions as well as precipitates in the form of b-SiC precipitates. It is concluded that oxidation temperature (T 900°C) must be kept low (900°C) to avoid layer degradation. Quanti®cation of carbon distribution after the thermal process has been found out.
4. Discussion
Acknowledgements
It was reported recently that strain has an important in¯uence on dry oxidation kinetics [10]. It is shown here that SiGe layers oxidize faster than silicon does but slower than b-SiC. This observation is in agreement with Ge pile-up in SiGe samples. The compressive strain in the layer, combined with a lower energy bond Si±Ge (3.12 eV) compared with Si±Si (3.30 eV), allows the oxygen to react faster with silicon. Oxygen reaction with silicon (ESi±O 8 eV) is more favorable than with germanium (EGe±O 6.8 eV). Thus, carbon does not allow chemical reactions with oxygen but diusion through the layer. Contrarily, the presence of carbon in SiGeC alleviates the strain, as tensile strain of carbon compensates the compressive strain due to germanium. In this case, oxygen diuses better into the layer but the reaction with silicon is more dicult, because of the more energetic bond, Si±C (4.51 eV). Moreover, silicon suboxides, XPS pro®les show that less strained layers (that is, compensated with carbon) make oxygen to diuse deeper into the layer. The reaction with silicon is not so favorable because carbon avoids a fast Si±O reaction, so oxygen can come further. Moreover, a less strained layer allows oxygen to diuse easily. But once carbon precipitates or leaves its substitutional position, suboxide states should evolve towards a common behavior in SiGe and SiGeC samples, i.e., suboxides should saturate to pure silicon dioxide. The observation that carbon makes diusion easier is also consistent with germanium behavior as the germanium in carbon-containing layers also diuses further and faster.
The authors are indebted to the MBE group of the Institute for Semiconductor Physics for providing SiGeC samples. References [1] Johnson RA, Zierak MZ, Outama KB, Bahn TC, Joseph AJ, Cordero CN, Malinowski J, Bard KA, Weeks T, Milliken R, Medve T, May G, Chong W, Walter K, Tempest SL, Chau B, Boenke M, Nelson M, Harame D. International Electronic Device Meeting Proceedings. 1998. p. 217. [2] Knoll D, Heinemann B, Osten HJ, Ehwald KE, Tillack B, Schley P, Barth R, Matthes M, Park KS, Kim Y, Winkler W. International Electronic Device Meeting Proceedings. 1998. p. 703. [3] K ock G, Gl uck M, Hackbarth T, Herzog H, Kohn E. Thin Solid Films 1998;336:141. [4] Osten HJ. Materials Science and Engineering 1996;B36:268. [5] Osten HJ, Lippert G, Gaworzewski P, Sorge R. Appl Phys Lett 1997;71:1522. [6] Ismail K, Chu JO, Meyerson BS. Appl Phys Lett 1994;64:3125. [7] Xiang J, Herbots N, Jacobson H, Ye PE, Whaley S. J Appl Phys 1996;80:1857. [8] Pressel K, Garrido B, Franz M, Kr uger D, Osten HJ, Morante JR. Journal of Vacuum Science and Technology B 1998;16:1757. [9] Cuadras A, Garrido B, Bonafos C, Morante JR, Fonseca L, Pressel K. E-MRS (Symp P). June 1999. [10] Garrido B, Cuadras A, Bonafos C, Morante JR, Fonseca L, Pressel K. INFOS. June 1999.