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InAs interfaces
Applied Surface Science 162–163 Ž2000. 425–429 www.elsevier.nlrlocaterapsusc
Growth and optimization of InAsrGaSb and GaSbrInAs interfaces A. Tahraoui b, P. Tomasini a , L. Lassabatere ` a, J. Bonnet a,) a
Laboratoire d’Analyse des Interfaces et de Nanophysique (LAIN) UPRESA CNRS 5011, UM II-STL, cc 082, Place E. Bataillon, 34095 Montpellier Cedex 5, France b Center for Quantum deÕices, ECE Department, Northwestern UniÕersity, 2145 Sheridan Road, EÕanston, IL 60208, USA
Abstract In order to optimize the molecular beam epitaxy growth of indium arsenide–gallium antimonide structures, we study the effects of the stacking sequence of the interfacial monolayer and of the growth temperature. To this end, GaSbrInAs and InAsrGaSb heterojunctions involving ultra-thin epilayers have been fabricated at 3508C, 4008C and 4508C with In–Sb- or Ga–As-like interfaces. The structures have been investigated by reflection high-energy electron diffraction, Auger electron microscopy and atomic force microscopy. The best growth condition for InAsŽ100. as well as GaSbŽ100. have been obtained with a substrate temperature of 4008C and an In–Sb-like InAs–GaSb interface. q 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Hi Keywords: InAs; GaSb; MBE; Epitaxy; Interface; Semiconductor
1. Introduction Indium arsenide and gallium antimonide are promising candidates for a variety of device applications including infrared detectors, mid-wave infrared lasers and high-speed electronics w1x. Despite of a significant progress in device fabrication, the performances are still below one might expect. Since a device performance is a function of the structural quality of the heterojunctions, our contribution to the
E-mail addresses: [email protected] ŽA. Tahraoui., [email protected] ŽJ. Bonnet.. ) Corresponding author. Tel.: q33-46-714-3200; fax: q33-46714-3774.
device improvement focuses on the optimization of the GaSb–InAs interface. In this paper, we investigate the effects of the nature of the interfacial monolayer ŽIn–Sb- or Ga–As-like. and of the growth temperature on the quality of interface heterostructures.
2. Experiments The samples were grown by solid-source molecular beam epitaxy ŽMBE. as described previously w2,3x. The GaSb and InAs substrates were n-type Ž100.. The epilayers were grown at 3508C, 4008C and 4508C on a buffer layer w4,5x. The interface bonds, i.e. In–Sb-like or Ga–As-like were controlled
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 2 2 7 - 0
A. Tahraoui et al.r Applied Surface Science 162–163 (2000) 425–429
426
by migration enhanced epitaxy ŽMEE.. The layerby-layer growth mode was controlled by reflection high-energy electron diffraction ŽRHEED.. The InAs and GaSb growth rates were 0.3 molecular monolayer per second ŽMLrs. and 0.5 MLrs, respectively. The epilayer thicknesses are typically 1.5 nm. The room temperature Auger electron spectroscopy ŽAES. analysis involved the Ga1070, In404, As1228 and Sb454 peaks w1,6x. Atomic force microscopy ŽAFM. images were scanned at atmospheric pressure in a contact mode.
RHEED pattern changes to a cŽ2 = 6. reconstruction when growing GaSb. Table 1 gives the AES SbrGa peak ratio of the GaSb epilayer surfaces. The values span from 0.81 to 0.99, while the peak ratio for a GaSb buffer is typically 1 w1x. The AFM pictures Ž7. to Ž12. show typical GaSb epilayers surfaces. The samples grown at 4008C exhibit the best RMS, typically 0.3 nm for the sample Ž11. and 0.6 nm for the sample Ž8.. 4. Discussion
3. Results The deposition of 1 ML of In on a cŽ2 = 6. GaSbŽ100. surface leads to a Ž1 = 3. pattern, while a Ž2 = 3. pattern is seen with 1 ML of Ga. However, the RHEED pattern changes to a Ž2 = 4. when opening the As 4 shutter. AES results are summarized in Table 1. The AsrIn peak ratio ranges from 2.82 to 3.61 depending on the growth conditions. Our reference is the peak ratio of thick InAs epilayers grown by MBE that is found typically close to 3 w1x. AFM views of InAs surfaces are given by Fig. 1Ž1. to Ž6.. The samples Ž1. to Ž3. show wavy surfaces with a large density of islands and pitches, while Fig. 1Ž4. to Ž6. shows smooth surfaces. Especially, the InAs epilayers grown at 4008C with an In–Sb-like interface show atomically stepped surfaces with a root mean square ŽRMS. of 0.3 nm Ž; 1 ML. Žsee Fig. 1Ž5... The deposition of 1 ML of In or Ga on an InAsŽ100. surface leads to a Ž4 = 2. pattern. The
The epilayer composition is expected to be closer to the binary composition as the AES peak ratio of the epilayers is closer to the bulk one. The AES investigation shows that the best InAs epilayers are fabricated at 4008C. It seems at this point that the interface bonds does not matter too much since the two types of interfaces lead to similar AES results Žsee Table 1.. The AFM observations allows us to select the In–Sb-like interface as definitively leading to the best InAs epilayers, and therefore, the best InAsrGaSb interface Žsee Fig. 1Ž5... A similar analysis applied to the GaSb epilayers observation leads to a similar conclusion. The growth optimum for the epitaxy of InAs on GaSb is similar to the one of GaSb on InAs. However, we observe that the flatness of the two epilayers is not exactly comparable: molecular steps are more clearly visible for InAs than GaSb despite similar RMS values Žsee Fig. 1Ž5. and Ž11... The film topology might be a consequence of the larger mobility of the In adatoms comparatively to the Ga ones.
Table 1 AES measurements of several samples grown at different temperatures with controled interfaces Temperature Ž8C.
350 400 450
Interface type
In–Sb Ga–As In–Sb Ga–As In–Sb Ga–As
InAsrGaSb
GaSbrInAs
AsrIn ratio
RHEED pattern
GarSb ratio
RHEED pattern
2.82 3.56 2.90 3.11 3.61 3.31
sharp Ž2 = 4. weak Ž2 = 4. sharp Ž2 = 4. weak Ž2 = 4. sharp Ž2 = 4. weak Ž2 = 4.
0.81 0.87 0.98 0.96 0.91 0.92
sharp weak sharp weak sharp weak
cŽ2 = 6. cŽ2 = 6. cŽ2 = 6. cŽ2 = 6. cŽ2 = 6. cŽ2 = 6.
A. Tahraoui et al.r Applied Surface Science 162–163 (2000) 425–429 Fig. 1. AFM top view of ultra-thin pseudomorphic epilayers grown at 3508C, 4008C and 4508C, respectively, Ž1. to Ž3. InAs with a Ga–As-like interface, Ž4. to Ž6. InAs with an In–Sb-like interface; Ž7. to Ž9. GaSb with a Ga–As-like interface, Ž10. to Ž12. GaSb with an In–Sb-like interface. 427
A. Tahraoui et al.r Applied Surface Science 162–163 (2000) 425–429
Fig. 1 Ž continued ..
428
A. Tahraoui et al.r Applied Surface Science 162–163 (2000) 425–429
In addition, this work gives evidence that the GaSb and InAs growth do not depend exactly on similar parameters. The AES results show that the InAs stoichiometry depends on the growth temperature and the interface type, while the GaSb one depends mainly on the growth temperature. However, the AFM results allow a fine-tuning of the analysis: the pictures of samples with a Ga–As-like interface show clearly poorer surfaces and this analysis is consistent with the RHEED observations Žsee Table 1..
5. Conclusion To the end to improve experimental device fabrication, this study focused on the InAsrGaSb and GaSbrInAs interface growth. The influence of the substrate temperature and interface bond type on the epilayers roughness and composition has been investigated by RHEED, AES and AFM. Our results show that the interface bond play a key role for the growth of high-quality InAs epilayers on GaSb, while the leading parameter for the growth of GaSb epilayers on InAs is the growth temperature. The best InAs and GaSb epilayers are grown with an In–Sb inter-
429
face at 4008C. An InAsrGaSb heterojunction with a MEE controlled In–Sb-like interface monolayer grown at 4008C allows a layer by layer growth mode. Therefore, an atomically abrupt interface that should lead to a device improvement is expected.
Acknowledgements A. Tahraoui is deeply indebted to Pr. G. Leveque for many valuable discussions on AFM measurements and to J. Touret for technical assistance on the MBE machine.
References w1x A. Tahraoui, These ` de doctorat, Univ. de Montpellier II-STL, France, 1998. w2x G. Leveque, M. Nouaoura, Eur. Phys. J.: Appl. Phys. a Ž1998. 227. w3x N. Bertru, M. Nouaoura, J. Bonnet, L. Lassabatere, ` J. Vac. Sci. Technol., A 15 Ž1997. 2043. w4x N. Bertru, Y. Bacquet, M. Nouaoura, L. Lassabatere, ` Appl. Surf. Sci. Technol. 74 Ž1994. 331. w5x F.W.O. Da Silva, M. Silga, C. Raisin, L. Lassabatere, ` J. Vac. Sci. Technol., B 8 Ž1990. 75.
Growth and optimization of InAsrGaSb and GaSbrInAs interfaces A. Tahraoui b, P. Tomasini a , L. Lassabatere ` a, J. Bonnet a,) a
Laboratoire d’Analyse des Interfaces et de Nanophysique (LAIN) UPRESA CNRS 5011, UM II-STL, cc 082, Place E. Bataillon, 34095 Montpellier Cedex 5, France b Center for Quantum deÕices, ECE Department, Northwestern UniÕersity, 2145 Sheridan Road, EÕanston, IL 60208, USA
Abstract In order to optimize the molecular beam epitaxy growth of indium arsenide–gallium antimonide structures, we study the effects of the stacking sequence of the interfacial monolayer and of the growth temperature. To this end, GaSbrInAs and InAsrGaSb heterojunctions involving ultra-thin epilayers have been fabricated at 3508C, 4008C and 4508C with In–Sb- or Ga–As-like interfaces. The structures have been investigated by reflection high-energy electron diffraction, Auger electron microscopy and atomic force microscopy. The best growth condition for InAsŽ100. as well as GaSbŽ100. have been obtained with a substrate temperature of 4008C and an In–Sb-like InAs–GaSb interface. q 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Hi Keywords: InAs; GaSb; MBE; Epitaxy; Interface; Semiconductor
1. Introduction Indium arsenide and gallium antimonide are promising candidates for a variety of device applications including infrared detectors, mid-wave infrared lasers and high-speed electronics w1x. Despite of a significant progress in device fabrication, the performances are still below one might expect. Since a device performance is a function of the structural quality of the heterojunctions, our contribution to the
E-mail addresses: [email protected] ŽA. Tahraoui., [email protected] ŽJ. Bonnet.. ) Corresponding author. Tel.: q33-46-714-3200; fax: q33-46714-3774.
device improvement focuses on the optimization of the GaSb–InAs interface. In this paper, we investigate the effects of the nature of the interfacial monolayer ŽIn–Sb- or Ga–As-like. and of the growth temperature on the quality of interface heterostructures.
2. Experiments The samples were grown by solid-source molecular beam epitaxy ŽMBE. as described previously w2,3x. The GaSb and InAs substrates were n-type Ž100.. The epilayers were grown at 3508C, 4008C and 4508C on a buffer layer w4,5x. The interface bonds, i.e. In–Sb-like or Ga–As-like were controlled
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 2 2 7 - 0
A. Tahraoui et al.r Applied Surface Science 162–163 (2000) 425–429
426
by migration enhanced epitaxy ŽMEE.. The layerby-layer growth mode was controlled by reflection high-energy electron diffraction ŽRHEED.. The InAs and GaSb growth rates were 0.3 molecular monolayer per second ŽMLrs. and 0.5 MLrs, respectively. The epilayer thicknesses are typically 1.5 nm. The room temperature Auger electron spectroscopy ŽAES. analysis involved the Ga1070, In404, As1228 and Sb454 peaks w1,6x. Atomic force microscopy ŽAFM. images were scanned at atmospheric pressure in a contact mode.
RHEED pattern changes to a cŽ2 = 6. reconstruction when growing GaSb. Table 1 gives the AES SbrGa peak ratio of the GaSb epilayer surfaces. The values span from 0.81 to 0.99, while the peak ratio for a GaSb buffer is typically 1 w1x. The AFM pictures Ž7. to Ž12. show typical GaSb epilayers surfaces. The samples grown at 4008C exhibit the best RMS, typically 0.3 nm for the sample Ž11. and 0.6 nm for the sample Ž8.. 4. Discussion
3. Results The deposition of 1 ML of In on a cŽ2 = 6. GaSbŽ100. surface leads to a Ž1 = 3. pattern, while a Ž2 = 3. pattern is seen with 1 ML of Ga. However, the RHEED pattern changes to a Ž2 = 4. when opening the As 4 shutter. AES results are summarized in Table 1. The AsrIn peak ratio ranges from 2.82 to 3.61 depending on the growth conditions. Our reference is the peak ratio of thick InAs epilayers grown by MBE that is found typically close to 3 w1x. AFM views of InAs surfaces are given by Fig. 1Ž1. to Ž6.. The samples Ž1. to Ž3. show wavy surfaces with a large density of islands and pitches, while Fig. 1Ž4. to Ž6. shows smooth surfaces. Especially, the InAs epilayers grown at 4008C with an In–Sb-like interface show atomically stepped surfaces with a root mean square ŽRMS. of 0.3 nm Ž; 1 ML. Žsee Fig. 1Ž5... The deposition of 1 ML of In or Ga on an InAsŽ100. surface leads to a Ž4 = 2. pattern. The
The epilayer composition is expected to be closer to the binary composition as the AES peak ratio of the epilayers is closer to the bulk one. The AES investigation shows that the best InAs epilayers are fabricated at 4008C. It seems at this point that the interface bonds does not matter too much since the two types of interfaces lead to similar AES results Žsee Table 1.. The AFM observations allows us to select the In–Sb-like interface as definitively leading to the best InAs epilayers, and therefore, the best InAsrGaSb interface Žsee Fig. 1Ž5... A similar analysis applied to the GaSb epilayers observation leads to a similar conclusion. The growth optimum for the epitaxy of InAs on GaSb is similar to the one of GaSb on InAs. However, we observe that the flatness of the two epilayers is not exactly comparable: molecular steps are more clearly visible for InAs than GaSb despite similar RMS values Žsee Fig. 1Ž5. and Ž11... The film topology might be a consequence of the larger mobility of the In adatoms comparatively to the Ga ones.
Table 1 AES measurements of several samples grown at different temperatures with controled interfaces Temperature Ž8C.
350 400 450
Interface type
In–Sb Ga–As In–Sb Ga–As In–Sb Ga–As
InAsrGaSb
GaSbrInAs
AsrIn ratio
RHEED pattern
GarSb ratio
RHEED pattern
2.82 3.56 2.90 3.11 3.61 3.31
sharp Ž2 = 4. weak Ž2 = 4. sharp Ž2 = 4. weak Ž2 = 4. sharp Ž2 = 4. weak Ž2 = 4.
0.81 0.87 0.98 0.96 0.91 0.92
sharp weak sharp weak sharp weak
cŽ2 = 6. cŽ2 = 6. cŽ2 = 6. cŽ2 = 6. cŽ2 = 6. cŽ2 = 6.
A. Tahraoui et al.r Applied Surface Science 162–163 (2000) 425–429 Fig. 1. AFM top view of ultra-thin pseudomorphic epilayers grown at 3508C, 4008C and 4508C, respectively, Ž1. to Ž3. InAs with a Ga–As-like interface, Ž4. to Ž6. InAs with an In–Sb-like interface; Ž7. to Ž9. GaSb with a Ga–As-like interface, Ž10. to Ž12. GaSb with an In–Sb-like interface. 427
A. Tahraoui et al.r Applied Surface Science 162–163 (2000) 425–429
Fig. 1 Ž continued ..
428
A. Tahraoui et al.r Applied Surface Science 162–163 (2000) 425–429
In addition, this work gives evidence that the GaSb and InAs growth do not depend exactly on similar parameters. The AES results show that the InAs stoichiometry depends on the growth temperature and the interface type, while the GaSb one depends mainly on the growth temperature. However, the AFM results allow a fine-tuning of the analysis: the pictures of samples with a Ga–As-like interface show clearly poorer surfaces and this analysis is consistent with the RHEED observations Žsee Table 1..
5. Conclusion To the end to improve experimental device fabrication, this study focused on the InAsrGaSb and GaSbrInAs interface growth. The influence of the substrate temperature and interface bond type on the epilayers roughness and composition has been investigated by RHEED, AES and AFM. Our results show that the interface bond play a key role for the growth of high-quality InAs epilayers on GaSb, while the leading parameter for the growth of GaSb epilayers on InAs is the growth temperature. The best InAs and GaSb epilayers are grown with an In–Sb inter-
429
face at 4008C. An InAsrGaSb heterojunction with a MEE controlled In–Sb-like interface monolayer grown at 4008C allows a layer by layer growth mode. Therefore, an atomically abrupt interface that should lead to a device improvement is expected.
Acknowledgements A. Tahraoui is deeply indebted to Pr. G. Leveque for many valuable discussions on AFM measurements and to J. Touret for technical assistance on the MBE machine.
References w1x A. Tahraoui, These ` de doctorat, Univ. de Montpellier II-STL, France, 1998. w2x G. Leveque, M. Nouaoura, Eur. Phys. J.: Appl. Phys. a Ž1998. 227. w3x N. Bertru, M. Nouaoura, J. Bonnet, L. Lassabatere, ` J. Vac. Sci. Technol., A 15 Ž1997. 2043. w4x N. Bertru, Y. Bacquet, M. Nouaoura, L. Lassabatere, ` Appl. Surf. Sci. Technol. 74 Ž1994. 331. w5x F.W.O. Da Silva, M. Silga, C. Raisin, L. Lassabatere, ` J. Vac. Sci. Technol., B 8 Ž1990. 75.