- Email: [email protected]
Scanning capacitance microscopy investigations of buried heterostructure laser structures
Applied Surface Science 144–145 Ž1999. 137–140
Scanning capacitance microscopy investigations of buried heterostructure laser structures O. Bowallius, S. Anand ) , M. Hammar, S. Nilsson, G. Landgren Department of Electronics, Laboratory of Semiconductor Materials, Royal Institute of Technology, Electrum 229, S-16440 Kista, Sweden
Abstract In this work, InP-based buried heterostucture lasers are used to demonstrate the utility of scanning capacitance microscopy ŽSCM. for characterising complex device structures. The lasers use p–n junctions formed by selective regrowth of p and n doped InP layers around a mesa for current confinement. For comparison, the regrowth was performed by liquid phase epitaxy ŽLPE. and metal organic vapour phase epitaxy ŽMOVPE.. Our investigations show that scanning capacitance microscopy is capable of detecting the p–n junctions formed at different regions of the device and thereby allows visualisation of the current confinement regions. Variations in the imaged depletion regions are attributed to doping variations due to modification of the regrowth process by the mesa. The SCM data show significant differences between the devices regrown by LPE and MOVPE and the results are consistent with the different regrowth mechanisms. Finally, the implications of the SCM data on device performance are discussed. q 1999 Elsevier Science B.V. All rights reserved. PACS: 85.60.Bt; 07.79.y v; 73.40.Kp; 85.30.De Keywords: SCM; p–n Junctions; Regrowth; Buried hetero-structure lasers; LPE; MOVPE
Scanning capacitance microscopy is a promising tool for high resolution two-dimensional doping profiling w1,2x. The sensitivity of the SCM technique to doping variations and its ability to delineate p–n junctions has resulted in several novel applications w3,4x. However, very few reports pertain to III–V materials in spite of their technological importance. The inherent advantages of the SCM make it an attractive tool to characterise selective regrowth which is a critical step in the fabrication of advanced optoelectronic devices such as buried heterostructure lasers. In these lasers, current confinement is an
)
Corresponding author. Tel.: q46-8-752-1470; Fax: q46-8752-1240; E-mail: [email protected]
important issue and it is normally provided by epitaxial regrowth of either a semi-insulating material or p–n junctions around a mesa Žcontaining the active region.. Due to orientation dependence, growth on non-planar substrates can induce modifications in morphology and dopant incorporation. These in turn could result in poorer current confinement. Further, since these devices involve several process steps Že.g., dry etching. the electrical character of the interfaces can be important. In this work, we explore the application of SCM to address these issues. Imaging the cross-section of the lasers Žafter stain etching. by scanning electron microscope ŽSEM. gives only the gross features and cannot provide electrical information. In contrast, the SCM technique not only provides useful electrical information
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 7 8 4 - 3
138
O. Bowallius et al.r Applied Surface Science 144–145 (1999) 137–140
but also does not require any special sample preparation such as stain etching. Here, SCM is used to investigate InPrGaInAsP buried heterostructures that form the gain section in vertical grating assisted co-directional coupler with a rear sampled grating reflector ŽGCSR. lasers w5x. In the lasers used in the present work, the current confinement is obtained by selective regrowth of p–n junction. For comparison, two different growth techniques, LPE and MOVPE, were used for the regrowths. Apart from the device perspective, it is interesting to see how the SCM images correlate with the evolution of growth in the two different methods. That the effect of topography on the distribution of sulphur in regrown InP layers can be visualised directly is demonstrated w6x. In the present context, we recognise that the doping distribution close to the mesa is not visualised in the same way, but indirectly via variations of the associated depletion regions. The SCM measurements were performed with a Digital Instruments Dimension 3000 SPM system operating in contact mode AFM using commercial metal-coated tips. The capacitance sensor operating at about 900 MHz is connected to the tip, which is connected to ground via an inductor that has a high impedance at the sensor frequency. The sample is biased with separately controlled ac and dc biases. For the measurements presented here the data was acquired in dCrdV mode and the ac bias frequency was 100 kHz. The lasers used here come from two fabrication batches and a few representative devices from each batch were investigated. The cross-sections of the devices studied here were prepared by cleaving the gain section of the GCSR lasers. The native oxide that is formed due to exposure to air serves as a thin insulator between the metal-coated tip and the sample. While this approach is quite appropriate for obtaining qualitative information, quantitative analysis is rather difficult since the quality of the native oxide is not known. A schematic cross-section showing the essential layer sequence of the gain section of the GCSR laser is shown in Fig. 1. Details of the device fabrication are given elsewhere w5x. Briefly, first the basic structure including the active layers is grown by MOVPE on a nq InP Ž100. substrate. With silicon nitride as the etch mask a w110x oriented mesa is formed by
Fig. 1. Schematic sketch of the cross-section of the gain section of the GCSR laser showing the essential layer sequence. The doping is about 4=10 18 cmy3 in the InP substrate. The buffer layer Žn-InP. is 500 nm and doped to 1=10 18 cmy3. The p-InP layer at the mesa top is Zn doped to 4=10 17 cmy3 . The first regrown layer p-InPŽ1. is doped with Zn to 9–10=10 17 cmy3 . The next regrown layer n-InPŽ2. is doped with S to 9–10=10 17 cmy3 . These n-regions occur above the mesa and on either side. The exact shape of these arms depends on the growth technique and conditions. The final layer p-InPŽ3. is doped with Zn to 1=10 18 cmy3 . Current confinement is obtained by the formed p–n junctions.
reactive ion etching. The mesas are typically 1.5 to 2 mm wide and 2 to 2.5 mm high. Retaining the silicon nitride layer, the regions around the mesa are regrown selectively, first by p-doped InP and then by n-doped InP. Subsequently the silicon nitride is removed and the structure is regrown by pqInP to complete the device structure. Either LPE or MOVPE was used for the above mentioned regrowths. The p–n junctions formed in the resulting structure define the current injection window. Therefore, their location and isolation properties are important. Besides, p–n junctions are also present at the mesa sidewalls, and, at the substrate and the first regrown p-layer. This is a rather complicated situation with p–n junctions in several different regions of the sample. In addition, doping variations can also be present in the upper n-doped arms ŽFig. 1.. Fig. 2 shows the SCM image obtained for the sample regrown by LPE. In our measurement configuration, the depletion region appears dark since there is no free carrier response. The p and n type character on either side of the junction is not distinguished since only the absolute value of dCrdV was recor-
O. Bowallius et al.r Applied Surface Science 144–145 (1999) 137–140
139
this point of view, both the techniques appear to be suitable. The variation of the SCM signal along the w100x direction and through the mesa is shown in Fig. 3b. The relative contrast levels corresponding to the substrate and the buffer layer Žlight band occurring below the depletion region at the mesa top. qualitatively agree with the doping differences. We note here that the relative contrast between n and p regions depends on the bias. However, comparing Fig. 2. Cross-sectional SCM image Ž16 mm=8 mm. of the device for which the regrowth was performed by LPE. The dc and ac biases were 0 V and 1 V, respectively. The topographical variations were less than 1 nm.
ded. The depletion regions are marked by the continuous dark contours tracking the etched pattern and those around the two arms. The opening between the depletion regions at the arms Žjust above the mesa. defines the current injection aperture and is approximately 1 mm. The undepleted p-regions near the mesa top edges are about 300 to 500 nm Žcomparatively more resistive. and therefore significant portion of the current will flow through the active region. In addition, the barriers present Ždue to the p–n junctions. at the sidewalls and at the substrate interface make the current confinement more effective. The dark region Ždepletion region. close to the Žvisible. mesa top roughly corresponds to the location of the active region. The lighter band Ž450 nm wide. just below the depletion region corresponds to the n-type buffer layer. The depletion regions at the bottom of the mesa walls and those at the side-walls of the etched trenches are significantly wider compared to other regions of the p–n junctions. This suggests that the dopant incorporation is lower in these regions due to a higher growth rate. The angles estimated from the left and right portions of the trench Žaway from the mesa. are close to the Ž111.B planes. Further, the shape of the depletion contour at the lower side of the n-doped arms clearly indicates that the growth rate is higher close to the mesa side wall compared to the w100x direction. As we will see below, the MOVPE case is quite different in several aspects. Fig. 3 shows a typical SCM image of a sample regrown by MOVPE. Here also, the dark contours represent the depletion regions. The current confinement properties are similar to the LPE case. From
Fig. 3. Ža. Cross-sectional SCM image Ž14 mm=7 mm. of the device for which the regrowth was performed by MOVPE. The dc and ac biases were 0 V and 1 V, respectively. The topographical variations were less than 1 nm. Žb. Variation of the SCM signal along B1–B2. Žc. Variation of the SCM signal in one of the n-doped arms along C1–C2. The labels 1 and 2 correspond to the respective regions marked on the image.
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O. Bowallius et al.r Applied Surface Science 144–145 (1999) 137–140
contrasts in the image between regions of the same doping is valid. Fig. 3c shows the SCM signal variation along one of the n-doped arms. In contrast to the LPE case, here, contrast variation Žregions marked 1 and 2. is clearly visible and is suggestive of doping variation along the arm. However, it does not appear to be significant enough to alter the depletion regions. It is also interesting to note that the depletion regions are similar irrespective of where they occur. Clearly, here the doping incorporation is less sensitive to orientations compared to LPE. Previous reports of regrowth studies performed under similar growth conditions show that the regrowth follows the underlying shape w7x. This argues for nearly uniform dopant incorporation close to the mesa. Similar findings are reported for MOVPE for a variety of dopants w8x. Further, the shape of the depletion contours around the n-doped arms follow the substrate, as was also seen by SEM Žnot shown.. The angles corresponding to the contours around the n-doped arms near the mesa top Žas in the image. correspond to Ž111.B planes in agreement with previously reported temporal growth studies w7x. The imaged depletion regions Žabout 200–500 nm wide. for both LPE and MOVPE cases ŽFigs. 2 and 3. appear to be much wider than the estimated values which are in the neighbourhood of 50–60 nm. Variation of dc bias within reasonable limits Ž"5 V. did not alter the situation very much. Resolution set by the tip radius Ž20–50 nm. together with lateral spreading cannot fully account for the imaged widths. Recently, it was shown that the metallurgical p–n junction could be located to within a depletion region width of about 60 nm w3x. It is likely that the wider depletion regions seen here are due to dopant diffusion during growth andror due to interface defects. Here, it is worthwhile to comment on the appearance of the depletion regions in the SCM images ŽFigs. 2 and 3a.. A consistent feature in both the images is the appearance of a bright line inside the depletion zones. This we believe is an artefact that is associated with the present measurement method. A likely cause for this artefact is a combined effect of the rectification from measuring the absolute magnitude of dCrdV and a possible offset in the SCM signal due to some undesirable coupling
between the capacitance sensor and the sample. Although the conclusions of the present work are not affected by this, we recognise that the phase information of the dCrdV signal could be important. This is particularly so if one needs to understand the bias dependence of SCM signal close to the p–n junction and for a rigorous analysis of the widths of the depletion regions. In conclusion, SCM was used to investigate InPrGaInAsP buried heterostructures that form the gain section in GCSR lasers. The current confinement properties were evaluated for two different regrowth techniques, namely LPE and MOVPE, and were similar. The SCM results were correlated to the growth related variations in dopant incorporation. In this aspect, LPE and MOVPE were significantly different. Such SCM analysis clearly has the potential to relate directly to device performance. Application of SCM to study the tuning sections of the GCSR lasers will be the subject matter of our future investigations.
Acknowledgements The authors thank S. Lourdudoss for fruitful discussions.
References w1x Y. Huang, C.C. Williams, M.A. Wendman, J. Vac. Sci. Technol. B 14 Ž1996. 1168. w2x J.J. Kopanski, J.F. Marchiando, J.R. Lowney, J. Vacc, Sci. Technol. B 14 Ž1996. 242. w3x H. Edwards, R. McGlothlin, R.S. Martin, U. Elisa, M. Gribelyuk, R. Mahaffy, C.K. Shih, R.S. List, V.A. Ukraintsev, Appl. Phys. Lett. 72 Ž1998. 698. w4x H. Tomiye, T. Yao, H. Kawami, T. Hayashi, Appl. Phys. Lett. 69 Ž1996. 4050. ¨ w5x M. Obergm, S. Nilsson, K. Struebel, J. Wallin, L. Backbom, ¨ T. Klinga, IEEE Photon. Technol. Lett. 5 Ž1993. 735. w6x M. Hammar, E. Rodriguez Messmer, M. Luzuy, S. Anand, S. Lourdudoss, G. Landgren, Appl. Phys. Lett. 72 Ž1998. 815. w7x N. Nordell, J. Borglind, J. Crystal Growth 114 Ž1991. 92. w8x M. Kondo, C. Anayama, N. Okada, H. Sekiguchi, K. Domen, T. Tanahashi, J. Appl. Phys. 76 Ž1994. 914.
Scanning capacitance microscopy investigations of buried heterostructure laser structures O. Bowallius, S. Anand ) , M. Hammar, S. Nilsson, G. Landgren Department of Electronics, Laboratory of Semiconductor Materials, Royal Institute of Technology, Electrum 229, S-16440 Kista, Sweden
Abstract In this work, InP-based buried heterostucture lasers are used to demonstrate the utility of scanning capacitance microscopy ŽSCM. for characterising complex device structures. The lasers use p–n junctions formed by selective regrowth of p and n doped InP layers around a mesa for current confinement. For comparison, the regrowth was performed by liquid phase epitaxy ŽLPE. and metal organic vapour phase epitaxy ŽMOVPE.. Our investigations show that scanning capacitance microscopy is capable of detecting the p–n junctions formed at different regions of the device and thereby allows visualisation of the current confinement regions. Variations in the imaged depletion regions are attributed to doping variations due to modification of the regrowth process by the mesa. The SCM data show significant differences between the devices regrown by LPE and MOVPE and the results are consistent with the different regrowth mechanisms. Finally, the implications of the SCM data on device performance are discussed. q 1999 Elsevier Science B.V. All rights reserved. PACS: 85.60.Bt; 07.79.y v; 73.40.Kp; 85.30.De Keywords: SCM; p–n Junctions; Regrowth; Buried hetero-structure lasers; LPE; MOVPE
Scanning capacitance microscopy is a promising tool for high resolution two-dimensional doping profiling w1,2x. The sensitivity of the SCM technique to doping variations and its ability to delineate p–n junctions has resulted in several novel applications w3,4x. However, very few reports pertain to III–V materials in spite of their technological importance. The inherent advantages of the SCM make it an attractive tool to characterise selective regrowth which is a critical step in the fabrication of advanced optoelectronic devices such as buried heterostructure lasers. In these lasers, current confinement is an
)
Corresponding author. Tel.: q46-8-752-1470; Fax: q46-8752-1240; E-mail: [email protected]
important issue and it is normally provided by epitaxial regrowth of either a semi-insulating material or p–n junctions around a mesa Žcontaining the active region.. Due to orientation dependence, growth on non-planar substrates can induce modifications in morphology and dopant incorporation. These in turn could result in poorer current confinement. Further, since these devices involve several process steps Že.g., dry etching. the electrical character of the interfaces can be important. In this work, we explore the application of SCM to address these issues. Imaging the cross-section of the lasers Žafter stain etching. by scanning electron microscope ŽSEM. gives only the gross features and cannot provide electrical information. In contrast, the SCM technique not only provides useful electrical information
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 7 8 4 - 3
138
O. Bowallius et al.r Applied Surface Science 144–145 (1999) 137–140
but also does not require any special sample preparation such as stain etching. Here, SCM is used to investigate InPrGaInAsP buried heterostructures that form the gain section in vertical grating assisted co-directional coupler with a rear sampled grating reflector ŽGCSR. lasers w5x. In the lasers used in the present work, the current confinement is obtained by selective regrowth of p–n junction. For comparison, two different growth techniques, LPE and MOVPE, were used for the regrowths. Apart from the device perspective, it is interesting to see how the SCM images correlate with the evolution of growth in the two different methods. That the effect of topography on the distribution of sulphur in regrown InP layers can be visualised directly is demonstrated w6x. In the present context, we recognise that the doping distribution close to the mesa is not visualised in the same way, but indirectly via variations of the associated depletion regions. The SCM measurements were performed with a Digital Instruments Dimension 3000 SPM system operating in contact mode AFM using commercial metal-coated tips. The capacitance sensor operating at about 900 MHz is connected to the tip, which is connected to ground via an inductor that has a high impedance at the sensor frequency. The sample is biased with separately controlled ac and dc biases. For the measurements presented here the data was acquired in dCrdV mode and the ac bias frequency was 100 kHz. The lasers used here come from two fabrication batches and a few representative devices from each batch were investigated. The cross-sections of the devices studied here were prepared by cleaving the gain section of the GCSR lasers. The native oxide that is formed due to exposure to air serves as a thin insulator between the metal-coated tip and the sample. While this approach is quite appropriate for obtaining qualitative information, quantitative analysis is rather difficult since the quality of the native oxide is not known. A schematic cross-section showing the essential layer sequence of the gain section of the GCSR laser is shown in Fig. 1. Details of the device fabrication are given elsewhere w5x. Briefly, first the basic structure including the active layers is grown by MOVPE on a nq InP Ž100. substrate. With silicon nitride as the etch mask a w110x oriented mesa is formed by
Fig. 1. Schematic sketch of the cross-section of the gain section of the GCSR laser showing the essential layer sequence. The doping is about 4=10 18 cmy3 in the InP substrate. The buffer layer Žn-InP. is 500 nm and doped to 1=10 18 cmy3. The p-InP layer at the mesa top is Zn doped to 4=10 17 cmy3 . The first regrown layer p-InPŽ1. is doped with Zn to 9–10=10 17 cmy3 . The next regrown layer n-InPŽ2. is doped with S to 9–10=10 17 cmy3 . These n-regions occur above the mesa and on either side. The exact shape of these arms depends on the growth technique and conditions. The final layer p-InPŽ3. is doped with Zn to 1=10 18 cmy3 . Current confinement is obtained by the formed p–n junctions.
reactive ion etching. The mesas are typically 1.5 to 2 mm wide and 2 to 2.5 mm high. Retaining the silicon nitride layer, the regions around the mesa are regrown selectively, first by p-doped InP and then by n-doped InP. Subsequently the silicon nitride is removed and the structure is regrown by pqInP to complete the device structure. Either LPE or MOVPE was used for the above mentioned regrowths. The p–n junctions formed in the resulting structure define the current injection window. Therefore, their location and isolation properties are important. Besides, p–n junctions are also present at the mesa sidewalls, and, at the substrate and the first regrown p-layer. This is a rather complicated situation with p–n junctions in several different regions of the sample. In addition, doping variations can also be present in the upper n-doped arms ŽFig. 1.. Fig. 2 shows the SCM image obtained for the sample regrown by LPE. In our measurement configuration, the depletion region appears dark since there is no free carrier response. The p and n type character on either side of the junction is not distinguished since only the absolute value of dCrdV was recor-
O. Bowallius et al.r Applied Surface Science 144–145 (1999) 137–140
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this point of view, both the techniques appear to be suitable. The variation of the SCM signal along the w100x direction and through the mesa is shown in Fig. 3b. The relative contrast levels corresponding to the substrate and the buffer layer Žlight band occurring below the depletion region at the mesa top. qualitatively agree with the doping differences. We note here that the relative contrast between n and p regions depends on the bias. However, comparing Fig. 2. Cross-sectional SCM image Ž16 mm=8 mm. of the device for which the regrowth was performed by LPE. The dc and ac biases were 0 V and 1 V, respectively. The topographical variations were less than 1 nm.
ded. The depletion regions are marked by the continuous dark contours tracking the etched pattern and those around the two arms. The opening between the depletion regions at the arms Žjust above the mesa. defines the current injection aperture and is approximately 1 mm. The undepleted p-regions near the mesa top edges are about 300 to 500 nm Žcomparatively more resistive. and therefore significant portion of the current will flow through the active region. In addition, the barriers present Ždue to the p–n junctions. at the sidewalls and at the substrate interface make the current confinement more effective. The dark region Ždepletion region. close to the Žvisible. mesa top roughly corresponds to the location of the active region. The lighter band Ž450 nm wide. just below the depletion region corresponds to the n-type buffer layer. The depletion regions at the bottom of the mesa walls and those at the side-walls of the etched trenches are significantly wider compared to other regions of the p–n junctions. This suggests that the dopant incorporation is lower in these regions due to a higher growth rate. The angles estimated from the left and right portions of the trench Žaway from the mesa. are close to the Ž111.B planes. Further, the shape of the depletion contour at the lower side of the n-doped arms clearly indicates that the growth rate is higher close to the mesa side wall compared to the w100x direction. As we will see below, the MOVPE case is quite different in several aspects. Fig. 3 shows a typical SCM image of a sample regrown by MOVPE. Here also, the dark contours represent the depletion regions. The current confinement properties are similar to the LPE case. From
Fig. 3. Ža. Cross-sectional SCM image Ž14 mm=7 mm. of the device for which the regrowth was performed by MOVPE. The dc and ac biases were 0 V and 1 V, respectively. The topographical variations were less than 1 nm. Žb. Variation of the SCM signal along B1–B2. Žc. Variation of the SCM signal in one of the n-doped arms along C1–C2. The labels 1 and 2 correspond to the respective regions marked on the image.
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O. Bowallius et al.r Applied Surface Science 144–145 (1999) 137–140
contrasts in the image between regions of the same doping is valid. Fig. 3c shows the SCM signal variation along one of the n-doped arms. In contrast to the LPE case, here, contrast variation Žregions marked 1 and 2. is clearly visible and is suggestive of doping variation along the arm. However, it does not appear to be significant enough to alter the depletion regions. It is also interesting to note that the depletion regions are similar irrespective of where they occur. Clearly, here the doping incorporation is less sensitive to orientations compared to LPE. Previous reports of regrowth studies performed under similar growth conditions show that the regrowth follows the underlying shape w7x. This argues for nearly uniform dopant incorporation close to the mesa. Similar findings are reported for MOVPE for a variety of dopants w8x. Further, the shape of the depletion contours around the n-doped arms follow the substrate, as was also seen by SEM Žnot shown.. The angles corresponding to the contours around the n-doped arms near the mesa top Žas in the image. correspond to Ž111.B planes in agreement with previously reported temporal growth studies w7x. The imaged depletion regions Žabout 200–500 nm wide. for both LPE and MOVPE cases ŽFigs. 2 and 3. appear to be much wider than the estimated values which are in the neighbourhood of 50–60 nm. Variation of dc bias within reasonable limits Ž"5 V. did not alter the situation very much. Resolution set by the tip radius Ž20–50 nm. together with lateral spreading cannot fully account for the imaged widths. Recently, it was shown that the metallurgical p–n junction could be located to within a depletion region width of about 60 nm w3x. It is likely that the wider depletion regions seen here are due to dopant diffusion during growth andror due to interface defects. Here, it is worthwhile to comment on the appearance of the depletion regions in the SCM images ŽFigs. 2 and 3a.. A consistent feature in both the images is the appearance of a bright line inside the depletion zones. This we believe is an artefact that is associated with the present measurement method. A likely cause for this artefact is a combined effect of the rectification from measuring the absolute magnitude of dCrdV and a possible offset in the SCM signal due to some undesirable coupling
between the capacitance sensor and the sample. Although the conclusions of the present work are not affected by this, we recognise that the phase information of the dCrdV signal could be important. This is particularly so if one needs to understand the bias dependence of SCM signal close to the p–n junction and for a rigorous analysis of the widths of the depletion regions. In conclusion, SCM was used to investigate InPrGaInAsP buried heterostructures that form the gain section in GCSR lasers. The current confinement properties were evaluated for two different regrowth techniques, namely LPE and MOVPE, and were similar. The SCM results were correlated to the growth related variations in dopant incorporation. In this aspect, LPE and MOVPE were significantly different. Such SCM analysis clearly has the potential to relate directly to device performance. Application of SCM to study the tuning sections of the GCSR lasers will be the subject matter of our future investigations.
Acknowledgements The authors thank S. Lourdudoss for fruitful discussions.
References w1x Y. Huang, C.C. Williams, M.A. Wendman, J. Vac. Sci. Technol. B 14 Ž1996. 1168. w2x J.J. Kopanski, J.F. Marchiando, J.R. Lowney, J. Vacc, Sci. Technol. B 14 Ž1996. 242. w3x H. Edwards, R. McGlothlin, R.S. Martin, U. Elisa, M. Gribelyuk, R. Mahaffy, C.K. Shih, R.S. List, V.A. Ukraintsev, Appl. Phys. Lett. 72 Ž1998. 698. w4x H. Tomiye, T. Yao, H. Kawami, T. Hayashi, Appl. Phys. Lett. 69 Ž1996. 4050. ¨ w5x M. Obergm, S. Nilsson, K. Struebel, J. Wallin, L. Backbom, ¨ T. Klinga, IEEE Photon. Technol. Lett. 5 Ž1993. 735. w6x M. Hammar, E. Rodriguez Messmer, M. Luzuy, S. Anand, S. Lourdudoss, G. Landgren, Appl. Phys. Lett. 72 Ž1998. 815. w7x N. Nordell, J. Borglind, J. Crystal Growth 114 Ž1991. 92. w8x M. Kondo, C. Anayama, N. Okada, H. Sekiguchi, K. Domen, T. Tanahashi, J. Appl. Phys. 76 Ž1994. 914.