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AlSb quantum wells grown on GaAs substrates
Applied Surface Science 159–160 Ž2000. 313–317 www.elsevier.nlrlocaterapsusc
Influence of interface bonds and buffer materials on optical properties of InAsrAlSb quantum wells grown on GaAs substrates K. Ohtani ) , A. Sato, Y. Ohno, F. Matsukura, H. Ohno Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communication, Tohoku UniÕersity, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received 1 December 1999
Abstract Optical properties of InAsrAlSb multiquantum wells ŽMQWs. epitaxially grown on GaAs substrates with a buffer layer are shown to be dependent on the type of the interface bond and the buffer layer material. Combinations of the two possible interface bond configurations ŽIn–Sb and Al–As. with the two buffer layer materials ŽInAs and AlSb. were prepared by molecular beam epitaxy. The photoluminescence intensity ŽPL. of MQWs was considerably higher for two kinds of structures: Ž1. the In–Sb bond with the AlSb buffer or Ž2. the Al–As bond with the InAs buffer. The two other possible combinations resulted in a drastically reduced PL intensity. X-ray diffraction ŽXRD. measurements revealed that lattice matching between the average lattice constant of MQW and the buffer layer plays a key role in determining the PL intensity. q 2000 Published by Elsevier Science B.V. PACS: 81.10; 61.43; 78.60; 78.66; 61.10.N Keywords: Interface bonds; Buffer layer; Photoluminescence; X-ray diffraction; InAsrAlSb multiquantum wells
1. Introduction The InAsrŽGa,Al.Sb system has attracted much attention because of its nearly matched lattice con˚ . and its unique properties. Due to the stants Ž; 6.1 A small effective mass and the large conduction band offset Ž; 1.35 eV., the structures involving InAsrŽGa,Al.Sb quantum wells ŽQWs. are promis-
)
Corresponding author. Tel.: q81-22-217-5555; fax: q81-22217-5555. E-mail addresses: [email protected] ŽK. Ohtani., [email protected] ŽH. Ohno..
ing systems for electronic devices such as high-speed field effect transistors w1,2x and resonant tunneling diodes w3x. Also, by making use of its unique band line-up, optoelectronic devices for the mid-infrared region w4,5x have been proposed and demonstrated. It has been commonly accepted that in order to obtain high-quality QWs it is crucial to have the In–Sb bonds at the InAsrŽGa,Al.Sb interface. For instance, Tuttle et al. w6x observed higher values of electron mobility for the In–Sb bonds than for the Al–As bonds in the case of single quantum wells of InAsrAlSb on the AlSb buffer layer. The optical and structural properties, especially the photoluminescence ŽPL. intensity, were shown to be superior
0169-4332r00r$ - see front matter q 2000 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 1 0 6 - 9
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K. Ohtani et al.r Applied Surface Science 159–160 (2000) 313–317
for InAsrAlSb multiquantum wells ŽMQWs. both on the GaSb buffer w7x and on the AlSb buffer w8–10x with the In–Sb bonds compared to those with the Al–As bonds. Here, we report that the optical properties depend not only on the type of the interface bond but also on the chosen combination of the buffer layer material ŽInAs or AlSb. and the interface bonds. By using different shutter sequences, four types of MQWs were coherently deposited onto GaAs substrates by molecular beam epitaxy ŽMBE.. They contained two different interface bonds ŽIn–Sb or Al–As. and were grown on two different buffer layers ŽInAs or AlSb.. Structural and optical properties of these samples were examined by both X-ray diffraction ŽXRD. and PL measurements. 2. Experimental All studied samples were grown on GaAs Ž100. substrates in a solid-source MBE chamber equipped with a compound As cell and a cracking Sb cell. In order to accommodate misfit dislocations between the MQWs and the GaAs substrate, a 600-nm buffer layer was grown at substrate temperature of 5708C for AlSb buffer layers and at 4308C in the case of the InAs buffer layers. Once the growth of the buffer layer was completed, a 20-period InAsrAlSb MQW was grown at substrate temperature of 4408C. The InAsrAlSb MQWs on the AlSb buffer layer consisted of six monolayers ŽML. of InAs and 20 ML of AlSb while in the case of the InAs buffer layer the thickness of the AlSb layers was reduced to 10 ML. The nominal growth rate was 0.2 mmrh for InAs and 0.5 mmrh for AlSb. The group V to group III beam equivalent pressure ratio was ; 5 for InAs and ; 9 for AlSb. Four different combinations of interface bonds and buffer layer materials were grown: ŽA. In–Sb bonds with AlSb buffer, ŽB. Al–As bonds with AlSb buffer, ŽC. In–Sb bonds with InAs buffer, and ŽD. Al–As bonds with InAs buffer. Fig. 1Ža. shows the shutter sequence for the MQW with the In–Sb bonds. The RHEED patterns from the Ž011. direction obtained during the formation of the In–Sb bonds are shown in Fig. 1Žb.. The patterns ŽŽa. – Žd.. correspond to the growth stages shown in Fig. 1Ža.. For samples A and C, only the In shutter was kept opened at the end of the InAs growth in order to
Fig. 1. Ža. Diagram of the shutter sequence for the formation of the In–Sb bond. Žb. RHEED patterns corresponding to the stages ŽŽa. – Žd.. of the shutter sequence shown in the upper panel.
supply 1 ML of the In. The RHEED pattern changed from the Ž2 = 4. As-stabilized InAs pattern to the Ž4 = 2. In-stabilized InAs pattern. This was followed by closing the In shutter and opening the Sb shutter to grow 1 ML of InSb. The RHEED pattern revealed the Ž1 = 3. Sb-stabilized InSb pattern after this process. Subsequently the AlSb was grown. The RHEED pattern showed the Ž1 = 3. reconstruction during the AlSb growth. For the opposite side of the interface, In and Sb were alternatively supplied to obtain 1 ML InSb at the interface. For samples B and D ŽAl–As interface bonds., only the As shutter was opened after the InAs growth. This was followed by closing the As shutter and opening the Al shutter to grow 1 ML of AlAs. For sample B, an additional 1 ML of In
K. Ohtani et al.r Applied Surface Science 159–160 (2000) 313–317
was supplied at the interface in order to keep the total amount of In supply in one period the same as
315
in the case of sample A. In the case of the sample D, no additional ML of In was supplied, so that the total
Fig. 2. XRD patterns of four InAsrAlSb MQW samples. Ža. Sample A ŽIn–Sb bond. and B ŽAl–As bond. were grown on the AlSb buffer layer. Žb. Sample C ŽIn–Sb bond. and D ŽAl–As bond. were grown on the InAs buffer layer.
316
K. Ohtani et al.r Applied Surface Science 159–160 (2000) 313–317
amount of the deposited In was smaller than that of sample C.
3. Results and discussion Fig. 2 shows ur2 u XRD patterns of the four types of InAsrAlSb MQWs around Ž002. symmetric reflection ŽFig. 2Ža.: Samples A and B, Fig. 2Žb.: Samples C and D.. For all the samples, the higherorder satellite peaks were clearly resolved. Sample A had the best structural quality with the narrower full width at half maximum ŽFWHM. of the higher-order satellite peaks. For samples A and D, the position of the 0th-order satellite peak, which represents the average lattice constant of MQWs, was almost equal to that of the peak of the buffer layer. The calculated average lattice constants of the MQWs taking the ˚ for sample A interface bonds into account, 6.151 A ˚ for sample D, are in a good agreement and 6.062 A ˚ for sample A with the experimental results, 6.147 A ˚ for sample D. The strain Ž; 1.3%. and 6.087 A between InAs and AlSb layer was found to be compensated by the strain Ž; 5–8%. between the interface bond and the MQWs. Samples B and C showed a large mismatch between the MQWs and the buffer layer. The calculated average lattice con˚ for sample B and 6.238 A˚ for sample stants, 6.028 A C, are in disagreement with the experimental results, ˚ for sample B and 6.122 A˚ for sample C. In 6.065 A samples B and C, the direction of strain in the MQWs and in the interface bonds have the same sign, so that no compensation is expected. The average lattice constant obtained from the experiment is closer to the calculated average lattice constant assuming that the MQWs are free-standing, not strained by the buffer layer. Fig. 3 shows PL spectra of the four samples measured at 4.3 K using an Arq ion laser as an excitation source. Fig. 3Ža. depicts data for samples A and B, while Fig. 3Žb. is for samples C and D. A high PL intensity was observed in the case of samples A and D, the FWHM being 46 meV for sample A and 49 meV for sample D, which are indicative of their good optical quality. At the same time, the PL intensity of samples B and C was much weaker and the FWHM was much broader than those corresponding to samples A and D. The higher energy of
Fig. 3. PL spectra of four InAsrAlSb MQW samples. Ža. Sample A ŽIn–Sb bond. and B ŽAl–As bond. grown on the AlSb buffer layer. Žb. Sample C ŽIn–Sb bond. and D ŽAl–As bond. grown on the InAs buffer layer.
the PL peak in samples A and D compared to that of samples B and C is due to the smaller thickness of the InAs QW. In the case of samples B and C, which showed the poor optical properties and the large lattice mismatch, pseudomorphic growth of InAsrAlSb MQWs seems to become disrupted at the interface. This results in the inferior optical quality because of the formation of misfit dislocations and interfacial defects. Our results show therefore, that
K. Ohtani et al.r Applied Surface Science 159–160 (2000) 313–317
the optical properties depend not on the choice of the interface bond but on the strain between the MQW and the buffer layer.
317
Promotion of Science ŽJSPS-RFTF 97P00202., and by grants-in-aid for the Scientific Research ŽA. ŽNo. 11355012. from the Ministry of Education, Science, Sports and Culture, Japan.
4. Conclusion References Four types of InAsrAlSb MQWs having two types of interface bonds were grown on two different buffer layers by MBE. From the XRD and PL measurements, correlation between the structural quality and the PL intensity was observed. It has been demonstrated that in order to obtain high PL intensity from MQWs, the interface bond and the buffer layer material have to be chosen in a way that minimizes the lattice mismatch between the MQW and the buffer layer.
Acknowledgements This work was partly supported by ‘‘Research for the Furture Program’’ from the Japan Society for the
w1x C.R. Bolognesi, E.J. Caine, H. Kroemer, IEEE Electron Device Lett. 15 Ž1994. 16. w2x B. Brar, H. Kroemer, IEEE Electron Device Lett. 16 Ž1994. 548. w3x D.H. Chow, J.H. Schulman, Appl. Phys. Lett. 64 Ž1994. 76. w4x R.H. Miler, D.H. Chow, J.N. Schulman, T.C. McGill, Appl. Phys. Lett. 57 Ž1990. 801. w5x R.Q. Yang, Superlattices Microstruct. 17 Ž1995. 77. w6x G. Tuttle, H. Kroemer, J.H. English, J. Appl. Phys. 67 Ž1990. 3032. w7x M. Seta, H. Asahi, S.G. Kim, K. Asami, S. Gonda, J. Appl. Phys. 74 Ž1993. 5033. w8x B. Brar, H. Kroemer, J. Ibbetson, S.J. English, Appl. Phys. Lett. 62 Ž1993. 3303. w9x M. Yano, M. Okuizumi, Y. Iwai, M. Inoue, J. Appl. Phys. 74 Ž1993. 7472. w10x A. Sato, K. Ohtani, R. Terauchi, Y. Ohno, F. Matsukura, H. Ohno, J. Cryst. Growth 201r202 Ž1999. 861.
Influence of interface bonds and buffer materials on optical properties of InAsrAlSb quantum wells grown on GaAs substrates K. Ohtani ) , A. Sato, Y. Ohno, F. Matsukura, H. Ohno Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communication, Tohoku UniÕersity, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received 1 December 1999
Abstract Optical properties of InAsrAlSb multiquantum wells ŽMQWs. epitaxially grown on GaAs substrates with a buffer layer are shown to be dependent on the type of the interface bond and the buffer layer material. Combinations of the two possible interface bond configurations ŽIn–Sb and Al–As. with the two buffer layer materials ŽInAs and AlSb. were prepared by molecular beam epitaxy. The photoluminescence intensity ŽPL. of MQWs was considerably higher for two kinds of structures: Ž1. the In–Sb bond with the AlSb buffer or Ž2. the Al–As bond with the InAs buffer. The two other possible combinations resulted in a drastically reduced PL intensity. X-ray diffraction ŽXRD. measurements revealed that lattice matching between the average lattice constant of MQW and the buffer layer plays a key role in determining the PL intensity. q 2000 Published by Elsevier Science B.V. PACS: 81.10; 61.43; 78.60; 78.66; 61.10.N Keywords: Interface bonds; Buffer layer; Photoluminescence; X-ray diffraction; InAsrAlSb multiquantum wells
1. Introduction The InAsrŽGa,Al.Sb system has attracted much attention because of its nearly matched lattice con˚ . and its unique properties. Due to the stants Ž; 6.1 A small effective mass and the large conduction band offset Ž; 1.35 eV., the structures involving InAsrŽGa,Al.Sb quantum wells ŽQWs. are promis-
)
Corresponding author. Tel.: q81-22-217-5555; fax: q81-22217-5555. E-mail addresses: [email protected] ŽK. Ohtani., [email protected] ŽH. Ohno..
ing systems for electronic devices such as high-speed field effect transistors w1,2x and resonant tunneling diodes w3x. Also, by making use of its unique band line-up, optoelectronic devices for the mid-infrared region w4,5x have been proposed and demonstrated. It has been commonly accepted that in order to obtain high-quality QWs it is crucial to have the In–Sb bonds at the InAsrŽGa,Al.Sb interface. For instance, Tuttle et al. w6x observed higher values of electron mobility for the In–Sb bonds than for the Al–As bonds in the case of single quantum wells of InAsrAlSb on the AlSb buffer layer. The optical and structural properties, especially the photoluminescence ŽPL. intensity, were shown to be superior
0169-4332r00r$ - see front matter q 2000 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 1 0 6 - 9
314
K. Ohtani et al.r Applied Surface Science 159–160 (2000) 313–317
for InAsrAlSb multiquantum wells ŽMQWs. both on the GaSb buffer w7x and on the AlSb buffer w8–10x with the In–Sb bonds compared to those with the Al–As bonds. Here, we report that the optical properties depend not only on the type of the interface bond but also on the chosen combination of the buffer layer material ŽInAs or AlSb. and the interface bonds. By using different shutter sequences, four types of MQWs were coherently deposited onto GaAs substrates by molecular beam epitaxy ŽMBE.. They contained two different interface bonds ŽIn–Sb or Al–As. and were grown on two different buffer layers ŽInAs or AlSb.. Structural and optical properties of these samples were examined by both X-ray diffraction ŽXRD. and PL measurements. 2. Experimental All studied samples were grown on GaAs Ž100. substrates in a solid-source MBE chamber equipped with a compound As cell and a cracking Sb cell. In order to accommodate misfit dislocations between the MQWs and the GaAs substrate, a 600-nm buffer layer was grown at substrate temperature of 5708C for AlSb buffer layers and at 4308C in the case of the InAs buffer layers. Once the growth of the buffer layer was completed, a 20-period InAsrAlSb MQW was grown at substrate temperature of 4408C. The InAsrAlSb MQWs on the AlSb buffer layer consisted of six monolayers ŽML. of InAs and 20 ML of AlSb while in the case of the InAs buffer layer the thickness of the AlSb layers was reduced to 10 ML. The nominal growth rate was 0.2 mmrh for InAs and 0.5 mmrh for AlSb. The group V to group III beam equivalent pressure ratio was ; 5 for InAs and ; 9 for AlSb. Four different combinations of interface bonds and buffer layer materials were grown: ŽA. In–Sb bonds with AlSb buffer, ŽB. Al–As bonds with AlSb buffer, ŽC. In–Sb bonds with InAs buffer, and ŽD. Al–As bonds with InAs buffer. Fig. 1Ža. shows the shutter sequence for the MQW with the In–Sb bonds. The RHEED patterns from the Ž011. direction obtained during the formation of the In–Sb bonds are shown in Fig. 1Žb.. The patterns ŽŽa. – Žd.. correspond to the growth stages shown in Fig. 1Ža.. For samples A and C, only the In shutter was kept opened at the end of the InAs growth in order to
Fig. 1. Ža. Diagram of the shutter sequence for the formation of the In–Sb bond. Žb. RHEED patterns corresponding to the stages ŽŽa. – Žd.. of the shutter sequence shown in the upper panel.
supply 1 ML of the In. The RHEED pattern changed from the Ž2 = 4. As-stabilized InAs pattern to the Ž4 = 2. In-stabilized InAs pattern. This was followed by closing the In shutter and opening the Sb shutter to grow 1 ML of InSb. The RHEED pattern revealed the Ž1 = 3. Sb-stabilized InSb pattern after this process. Subsequently the AlSb was grown. The RHEED pattern showed the Ž1 = 3. reconstruction during the AlSb growth. For the opposite side of the interface, In and Sb were alternatively supplied to obtain 1 ML InSb at the interface. For samples B and D ŽAl–As interface bonds., only the As shutter was opened after the InAs growth. This was followed by closing the As shutter and opening the Al shutter to grow 1 ML of AlAs. For sample B, an additional 1 ML of In
K. Ohtani et al.r Applied Surface Science 159–160 (2000) 313–317
was supplied at the interface in order to keep the total amount of In supply in one period the same as
315
in the case of sample A. In the case of the sample D, no additional ML of In was supplied, so that the total
Fig. 2. XRD patterns of four InAsrAlSb MQW samples. Ža. Sample A ŽIn–Sb bond. and B ŽAl–As bond. were grown on the AlSb buffer layer. Žb. Sample C ŽIn–Sb bond. and D ŽAl–As bond. were grown on the InAs buffer layer.
316
K. Ohtani et al.r Applied Surface Science 159–160 (2000) 313–317
amount of the deposited In was smaller than that of sample C.
3. Results and discussion Fig. 2 shows ur2 u XRD patterns of the four types of InAsrAlSb MQWs around Ž002. symmetric reflection ŽFig. 2Ža.: Samples A and B, Fig. 2Žb.: Samples C and D.. For all the samples, the higherorder satellite peaks were clearly resolved. Sample A had the best structural quality with the narrower full width at half maximum ŽFWHM. of the higher-order satellite peaks. For samples A and D, the position of the 0th-order satellite peak, which represents the average lattice constant of MQWs, was almost equal to that of the peak of the buffer layer. The calculated average lattice constants of the MQWs taking the ˚ for sample A interface bonds into account, 6.151 A ˚ for sample D, are in a good agreement and 6.062 A ˚ for sample A with the experimental results, 6.147 A ˚ for sample D. The strain Ž; 1.3%. and 6.087 A between InAs and AlSb layer was found to be compensated by the strain Ž; 5–8%. between the interface bond and the MQWs. Samples B and C showed a large mismatch between the MQWs and the buffer layer. The calculated average lattice con˚ for sample B and 6.238 A˚ for sample stants, 6.028 A C, are in disagreement with the experimental results, ˚ for sample B and 6.122 A˚ for sample C. In 6.065 A samples B and C, the direction of strain in the MQWs and in the interface bonds have the same sign, so that no compensation is expected. The average lattice constant obtained from the experiment is closer to the calculated average lattice constant assuming that the MQWs are free-standing, not strained by the buffer layer. Fig. 3 shows PL spectra of the four samples measured at 4.3 K using an Arq ion laser as an excitation source. Fig. 3Ža. depicts data for samples A and B, while Fig. 3Žb. is for samples C and D. A high PL intensity was observed in the case of samples A and D, the FWHM being 46 meV for sample A and 49 meV for sample D, which are indicative of their good optical quality. At the same time, the PL intensity of samples B and C was much weaker and the FWHM was much broader than those corresponding to samples A and D. The higher energy of
Fig. 3. PL spectra of four InAsrAlSb MQW samples. Ža. Sample A ŽIn–Sb bond. and B ŽAl–As bond. grown on the AlSb buffer layer. Žb. Sample C ŽIn–Sb bond. and D ŽAl–As bond. grown on the InAs buffer layer.
the PL peak in samples A and D compared to that of samples B and C is due to the smaller thickness of the InAs QW. In the case of samples B and C, which showed the poor optical properties and the large lattice mismatch, pseudomorphic growth of InAsrAlSb MQWs seems to become disrupted at the interface. This results in the inferior optical quality because of the formation of misfit dislocations and interfacial defects. Our results show therefore, that
K. Ohtani et al.r Applied Surface Science 159–160 (2000) 313–317
the optical properties depend not on the choice of the interface bond but on the strain between the MQW and the buffer layer.
317
Promotion of Science ŽJSPS-RFTF 97P00202., and by grants-in-aid for the Scientific Research ŽA. ŽNo. 11355012. from the Ministry of Education, Science, Sports and Culture, Japan.
4. Conclusion References Four types of InAsrAlSb MQWs having two types of interface bonds were grown on two different buffer layers by MBE. From the XRD and PL measurements, correlation between the structural quality and the PL intensity was observed. It has been demonstrated that in order to obtain high PL intensity from MQWs, the interface bond and the buffer layer material have to be chosen in a way that minimizes the lattice mismatch between the MQW and the buffer layer.
Acknowledgements This work was partly supported by ‘‘Research for the Furture Program’’ from the Japan Society for the
w1x C.R. Bolognesi, E.J. Caine, H. Kroemer, IEEE Electron Device Lett. 15 Ž1994. 16. w2x B. Brar, H. Kroemer, IEEE Electron Device Lett. 16 Ž1994. 548. w3x D.H. Chow, J.H. Schulman, Appl. Phys. Lett. 64 Ž1994. 76. w4x R.H. Miler, D.H. Chow, J.N. Schulman, T.C. McGill, Appl. Phys. Lett. 57 Ž1990. 801. w5x R.Q. Yang, Superlattices Microstruct. 17 Ž1995. 77. w6x G. Tuttle, H. Kroemer, J.H. English, J. Appl. Phys. 67 Ž1990. 3032. w7x M. Seta, H. Asahi, S.G. Kim, K. Asami, S. Gonda, J. Appl. Phys. 74 Ž1993. 5033. w8x B. Brar, H. Kroemer, J. Ibbetson, S.J. English, Appl. Phys. Lett. 62 Ž1993. 3303. w9x M. Yano, M. Okuizumi, Y. Iwai, M. Inoue, J. Appl. Phys. 74 Ž1993. 7472. w10x A. Sato, K. Ohtani, R. Terauchi, Y. Ohno, F. Matsukura, H. Ohno, J. Cryst. Growth 201r202 Ž1999. 861.