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GaAs superlattices grown by low-temperature MBE
Journal of Crystal Growth 201/202 (1999) 260}262
Ordered arrays of arsenic clusters coincided with InAs/GaAs superlattices grown by low-temperature MBE V.V. Chaldyshev *, N.N. Faleev , N.A. Bert , Yu.G. Musikhin , A.E. Kunitsyn , V.V. Preobrazhenskii, M.A. Putyato, B.R. Semyagin, P. Werner Iowe Physico-Technical Institute, 194021 St. Petersburg, Russia Institute of Semiconductor Physics, 630090 Novosibirsk, Russia Max-Planck-Institute of Microstructure Physics, Halle, Germany
Abstract The InAs/GaAs superlattices with up to 30 periods were grown by molecular-beam epitaxy at 2003C. The thickness of the GaAs layers was varied from 20 to 60 nm. The nominal thickness of the InAs layers was either 1 or 0.5 monolayers. High-resolution transmission electron microscopy study revealed indium containing layer to be as thick as 4 monolayer in both cases. This was attributed to the roughness of the growth surface at the low substrate temperature. The concentration of As antisite defects in the as-grown samples was evaluated as (0.5}2);10 cm\. In spite of such a high concentration of point defects, the X-ray rocking curves were found to be very close to theoretical ones with a large number of interference patterns originated from periodical structure. Upon annealing the excess arsenic precipitated at the InAs delta-layers and in between them. Appropriate annealing conditions were found which allow us to dissolve the clusters in the GaAs spacers and accumulate the majority of clusters in two-dimensional sheets. As a result, arti"cially ordered superlattices of As cluster sheets were produced. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.46.#w; 68.55.Bd Keywords: Molecular-beam epitaxy; Low-temperature grown GaAs; As clusters
1. Introduction GaAs grown by molecular-beam epitaxy (MBE) at low temperature (LT) has attracted much atten-
* Corresponding author. Fax: #7 812 2471017; e-mail: [email protected]!e.rssi.ru.
tion during the last few years due to a number of interesting properties [1}4]. The major feature of this material is a high arsenic excess (up to 1.5 at%) incorporated into the growing "lm during LT MBE. Post-growth annealing leads to formation of As clusters built in the GaAs matrix. The clusters in the conventional LT GaAs are randomly dispersed over the "lm bulk. However, it has been shown that two-dimensional arrays of As clusters can be
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 3 5 - 9
V.V. Chaldyshev et al. / Journal of Crystal Growth 201/202 (1999) 260}262
produced in LT GaAs delta-doped with isovalent indium impurity [5}7]. In this paper we report the results on lowtemperature MBE of InAs/GaAs superlattices and demonstrate arti"cially ordered arrays of As clusters produced by subsequent anneals.
2. Experimental procedure The InAs/GaAs superlattices were grown in a dual-chamber `Katuna MBE system on undoped semi-insulating 2 in GaAs(0 0 1) substrates which were prepared for the growth procedure in the conventional manner. A 85 nm thick bu!er layer of undoped GaAs was grown on the substrate at 5803C, after that the substrate temperature was lowered to 2003C, and an LT-GaAs "lm was deposited at the growth rate of 1 lm/h under As pressure of 7;10\ Pa. During the growth, indium deltalayers were inserted in the "lm by interrupting the Ga beam and using an In beam instead for 4 or 8 s that produced approximately 0.5 or 1 monolayer (ML) of InAs, accordingly. The distance between the In delta-layers was varied from 20 to 60 nm. The samples grown were cleaved into four pieces of which one was kept as-grown, while the other three were subjected to annealing in the MBE chamber under As overpressure for 15 min at three di!erent temperatures: 5003C, 6003C or 7003C. The samples were characterized using transmission electron microscopy (TEM), high-resolution X-ray di!raction (HRXRD), and near-infrared optical absorption (NIRA). The TEM specimens were prepared using wet etching for plan-view observation or conventional route of mechanical dimpling followed by Ar ion-beam milling for cross-sectional one and were studied in Philips EM 420 or JEM 4000 instruments. The HRXRD study was carried out using a double-crystal Ge monochromator adjusted to (0 0 4) re#ection of Cu K radiation. a
261
were as thick as 4 ML, while the indium deposition was equivalent to 0.5 or 1 ML. Since the segregation and intermixing e!ects are strongly suppressed at the low growth temperature, the observed thickness of InAs layers may be attributed to a shortrange roughness of the growth surface. The lateral size of this roughness can be estimated from TEM data as (10 nm. The NIRA study of the as-grown superlattices revealed a strong absorption related to arsenic antisite defects. Using Martin's calibration [8] the antisite defect concentration was evaluated as (0.5}2);10 cm\ for di!erent samples. Fig. 1 shows X-ray rocking curve for as-grown InAs/GaAs superlattice with 30 periods. Each period is as thick as 30 nm. Two main peaks in the rocking curve correspond to the GaAs substrate and InAs/GaAs superlattice (0SL). In addition, a large number of interference patterns are seen, which originated from the periodical structure. In spite of the very high concentration of point defects, the experimental rocking curve was found to be close to the theoretical one calculated for an ideal InAs/GaAs superlattice. Upon annealing at 5003C and higher temperature the lattice mismatch between the substrate and the "lm decreased as a result of precipitation of excess arsenic. The arsenic in excess was deduced from the lattice relaxation using calibration by Liu et al. [9]. Its value appeared to be well consistent with the NIRA data. When the annealing temperature was 5003C, thin and #at cluster sheets were observed by TEM.
3. Results and discussion The high-resolution TEM study of the as-grown superlattices revealed that the In-containing layers
Fig. 1. X-ray rocking curve for InAs/GaAs superlattice grown at 2003C. Re#ection (0 0 4), Cu K radiation. a
262
V.V. Chaldyshev et al. / Journal of Crystal Growth 201/202 (1999) 260}262
Fig. 2. Cross-sectional TEM image of LT InAs/GaAs superlattice annealed at 6003C. The majority of As clusters are located at InAs d-layers.
to be very close to theoretical ones with a large number of interference patterns originated from periodical structure. High-resolution TEM study showed that the real thickness of the InAs layers was 4 ML. This value was found to be independent of nominal In deposition (0.5 or 1 ML). It was attributed to a short-range roughness of the growth surface. The post-growth annealing caused precipitation of the excess arsenic at the InAs layers and in between. Annealing conditions were found to produce arti"cially ordered superlattices of twodimensional cluster sheets which accumulate the majority of As clusters.
Acknowledgements However, many As clusters were detected in between two-dimensional sheets. Fig. 2 shows a TEM micrograph of the sample annealed at 6003C. Twodimensional sheets of As clusters are clearly seen at InAs delta-layers. These sheets accumulate the majority of As clusters. Annealing at higher temperature (7003C) resulted in random-like cluster distribution. Such an evolution of the cluster system is a speci"c feature of precipitation in inhomogeneous media. Heterogeneous nucleation provides bigger As clusters at InAs layers when compared to those in GaAs spacers. Under Ostwald ripening stage the bigger clusters grow, while their small neighbors dissolve. However, upon long-term or high-temperature annealing, the cluster sheets become thick and interact with each other. As a result, the cluster system transforms to random.
4. Conclusions InAs/GaAs superlattices with up to 30 periods were grown by MBE at 2003C. NIRA study showed that the concentration of As antisite defects in the as-grown samples was as high as (0.5}2); 10 cm\. In spite of such a high concentration of point defects, the X-ray rocking curves were found
The research was carried out within the framework of Russian National Programs: `Physics of Solid State Nanostructuresa and `Fullerenes and Atomic Clustersa. It was also supported by the Russian Foundation for Basic Research and DFG.
References [1] F.W. Smith, A.R. Calawa, C.L. Chen, M.J. Mantra, L.J. Mahoney, Electron Dev. Lett. 9 (1988) 77. [2] M. Kaminska, Z. Liliental-Weber, E.R. Weber, T. George, J.B. Kortright, F.W. Smith, B.Y. Tsaur, A.R. Calawa, Appl. Phys. Lett. 54 (1989) 1831. [3] M.R. Melloch, K. Mahalingam, N. Otsuka, J.M. Woodall, A.C. Warren, J. Crystal Growth 111 (1991) 39. [4] N.A. Bert, A.I. Veinger, M.D. Vilisova, S.I. Goloshchapov, I.V. Ivonin, S.V. Kozyrev, A.E. Kunitsyn, L.G. Lavrentieva, D.I. Lubyshev, V.V. Preobrazhenskii, B.R. Semyagin, V.V. Tretyakov, V.V. Chaldyshev, M.P. Yakubenya, Phys. Solid State 35 (1993) 1289. [5] T.M. Cheng, C.V. Chang, A. Chin, M.F. Huang, J.H. Huang, Appl. Phys. Lett. 64 (1994) 2517. [6] N.A. Bert, V.V. Chaldyshev, D.I. Lubyshev, V.V. Preobrazhenskii, B.R. Semyagin, Semiconductors 29 (1995) 1170. [7] N.A. Bert, V.V. Chaldyshev, N.N. Faleev, A.E. Kunitsyn, D.I. Lubyshev, V.V. Preobrazhenskii, B.R. Semyagin, V.V. Tretyakov, Semicond. Sci. Technol. 12 (1997) 51. [8] G.M. Martin, Appl. Phys. Lett. 39 (1981) 747. [9] X. Liu, A. Prasad, J. Nishio, E.R. Weber, Z. Liliental-Weber, W. Walukiewicz, Appl. Phys. Lett. 67 (1995) 279.
Ordered arrays of arsenic clusters coincided with InAs/GaAs superlattices grown by low-temperature MBE V.V. Chaldyshev *, N.N. Faleev , N.A. Bert , Yu.G. Musikhin , A.E. Kunitsyn , V.V. Preobrazhenskii, M.A. Putyato, B.R. Semyagin, P. Werner Iowe Physico-Technical Institute, 194021 St. Petersburg, Russia Institute of Semiconductor Physics, 630090 Novosibirsk, Russia Max-Planck-Institute of Microstructure Physics, Halle, Germany
Abstract The InAs/GaAs superlattices with up to 30 periods were grown by molecular-beam epitaxy at 2003C. The thickness of the GaAs layers was varied from 20 to 60 nm. The nominal thickness of the InAs layers was either 1 or 0.5 monolayers. High-resolution transmission electron microscopy study revealed indium containing layer to be as thick as 4 monolayer in both cases. This was attributed to the roughness of the growth surface at the low substrate temperature. The concentration of As antisite defects in the as-grown samples was evaluated as (0.5}2);10 cm\. In spite of such a high concentration of point defects, the X-ray rocking curves were found to be very close to theoretical ones with a large number of interference patterns originated from periodical structure. Upon annealing the excess arsenic precipitated at the InAs delta-layers and in between them. Appropriate annealing conditions were found which allow us to dissolve the clusters in the GaAs spacers and accumulate the majority of clusters in two-dimensional sheets. As a result, arti"cially ordered superlattices of As cluster sheets were produced. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.46.#w; 68.55.Bd Keywords: Molecular-beam epitaxy; Low-temperature grown GaAs; As clusters
1. Introduction GaAs grown by molecular-beam epitaxy (MBE) at low temperature (LT) has attracted much atten-
* Corresponding author. Fax: #7 812 2471017; e-mail: [email protected]!e.rssi.ru.
tion during the last few years due to a number of interesting properties [1}4]. The major feature of this material is a high arsenic excess (up to 1.5 at%) incorporated into the growing "lm during LT MBE. Post-growth annealing leads to formation of As clusters built in the GaAs matrix. The clusters in the conventional LT GaAs are randomly dispersed over the "lm bulk. However, it has been shown that two-dimensional arrays of As clusters can be
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 3 5 - 9
V.V. Chaldyshev et al. / Journal of Crystal Growth 201/202 (1999) 260}262
produced in LT GaAs delta-doped with isovalent indium impurity [5}7]. In this paper we report the results on lowtemperature MBE of InAs/GaAs superlattices and demonstrate arti"cially ordered arrays of As clusters produced by subsequent anneals.
2. Experimental procedure The InAs/GaAs superlattices were grown in a dual-chamber `Katuna MBE system on undoped semi-insulating 2 in GaAs(0 0 1) substrates which were prepared for the growth procedure in the conventional manner. A 85 nm thick bu!er layer of undoped GaAs was grown on the substrate at 5803C, after that the substrate temperature was lowered to 2003C, and an LT-GaAs "lm was deposited at the growth rate of 1 lm/h under As pressure of 7;10\ Pa. During the growth, indium deltalayers were inserted in the "lm by interrupting the Ga beam and using an In beam instead for 4 or 8 s that produced approximately 0.5 or 1 monolayer (ML) of InAs, accordingly. The distance between the In delta-layers was varied from 20 to 60 nm. The samples grown were cleaved into four pieces of which one was kept as-grown, while the other three were subjected to annealing in the MBE chamber under As overpressure for 15 min at three di!erent temperatures: 5003C, 6003C or 7003C. The samples were characterized using transmission electron microscopy (TEM), high-resolution X-ray di!raction (HRXRD), and near-infrared optical absorption (NIRA). The TEM specimens were prepared using wet etching for plan-view observation or conventional route of mechanical dimpling followed by Ar ion-beam milling for cross-sectional one and were studied in Philips EM 420 or JEM 4000 instruments. The HRXRD study was carried out using a double-crystal Ge monochromator adjusted to (0 0 4) re#ection of Cu K radiation. a
261
were as thick as 4 ML, while the indium deposition was equivalent to 0.5 or 1 ML. Since the segregation and intermixing e!ects are strongly suppressed at the low growth temperature, the observed thickness of InAs layers may be attributed to a shortrange roughness of the growth surface. The lateral size of this roughness can be estimated from TEM data as (10 nm. The NIRA study of the as-grown superlattices revealed a strong absorption related to arsenic antisite defects. Using Martin's calibration [8] the antisite defect concentration was evaluated as (0.5}2);10 cm\ for di!erent samples. Fig. 1 shows X-ray rocking curve for as-grown InAs/GaAs superlattice with 30 periods. Each period is as thick as 30 nm. Two main peaks in the rocking curve correspond to the GaAs substrate and InAs/GaAs superlattice (0SL). In addition, a large number of interference patterns are seen, which originated from the periodical structure. In spite of the very high concentration of point defects, the experimental rocking curve was found to be close to the theoretical one calculated for an ideal InAs/GaAs superlattice. Upon annealing at 5003C and higher temperature the lattice mismatch between the substrate and the "lm decreased as a result of precipitation of excess arsenic. The arsenic in excess was deduced from the lattice relaxation using calibration by Liu et al. [9]. Its value appeared to be well consistent with the NIRA data. When the annealing temperature was 5003C, thin and #at cluster sheets were observed by TEM.
3. Results and discussion The high-resolution TEM study of the as-grown superlattices revealed that the In-containing layers
Fig. 1. X-ray rocking curve for InAs/GaAs superlattice grown at 2003C. Re#ection (0 0 4), Cu K radiation. a
262
V.V. Chaldyshev et al. / Journal of Crystal Growth 201/202 (1999) 260}262
Fig. 2. Cross-sectional TEM image of LT InAs/GaAs superlattice annealed at 6003C. The majority of As clusters are located at InAs d-layers.
to be very close to theoretical ones with a large number of interference patterns originated from periodical structure. High-resolution TEM study showed that the real thickness of the InAs layers was 4 ML. This value was found to be independent of nominal In deposition (0.5 or 1 ML). It was attributed to a short-range roughness of the growth surface. The post-growth annealing caused precipitation of the excess arsenic at the InAs layers and in between. Annealing conditions were found to produce arti"cially ordered superlattices of twodimensional cluster sheets which accumulate the majority of As clusters.
Acknowledgements However, many As clusters were detected in between two-dimensional sheets. Fig. 2 shows a TEM micrograph of the sample annealed at 6003C. Twodimensional sheets of As clusters are clearly seen at InAs delta-layers. These sheets accumulate the majority of As clusters. Annealing at higher temperature (7003C) resulted in random-like cluster distribution. Such an evolution of the cluster system is a speci"c feature of precipitation in inhomogeneous media. Heterogeneous nucleation provides bigger As clusters at InAs layers when compared to those in GaAs spacers. Under Ostwald ripening stage the bigger clusters grow, while their small neighbors dissolve. However, upon long-term or high-temperature annealing, the cluster sheets become thick and interact with each other. As a result, the cluster system transforms to random.
4. Conclusions InAs/GaAs superlattices with up to 30 periods were grown by MBE at 2003C. NIRA study showed that the concentration of As antisite defects in the as-grown samples was as high as (0.5}2); 10 cm\. In spite of such a high concentration of point defects, the X-ray rocking curves were found
The research was carried out within the framework of Russian National Programs: `Physics of Solid State Nanostructuresa and `Fullerenes and Atomic Clustersa. It was also supported by the Russian Foundation for Basic Research and DFG.
References [1] F.W. Smith, A.R. Calawa, C.L. Chen, M.J. Mantra, L.J. Mahoney, Electron Dev. Lett. 9 (1988) 77. [2] M. Kaminska, Z. Liliental-Weber, E.R. Weber, T. George, J.B. Kortright, F.W. Smith, B.Y. Tsaur, A.R. Calawa, Appl. Phys. Lett. 54 (1989) 1831. [3] M.R. Melloch, K. Mahalingam, N. Otsuka, J.M. Woodall, A.C. Warren, J. Crystal Growth 111 (1991) 39. [4] N.A. Bert, A.I. Veinger, M.D. Vilisova, S.I. Goloshchapov, I.V. Ivonin, S.V. Kozyrev, A.E. Kunitsyn, L.G. Lavrentieva, D.I. Lubyshev, V.V. Preobrazhenskii, B.R. Semyagin, V.V. Tretyakov, V.V. Chaldyshev, M.P. Yakubenya, Phys. Solid State 35 (1993) 1289. [5] T.M. Cheng, C.V. Chang, A. Chin, M.F. Huang, J.H. Huang, Appl. Phys. Lett. 64 (1994) 2517. [6] N.A. Bert, V.V. Chaldyshev, D.I. Lubyshev, V.V. Preobrazhenskii, B.R. Semyagin, Semiconductors 29 (1995) 1170. [7] N.A. Bert, V.V. Chaldyshev, N.N. Faleev, A.E. Kunitsyn, D.I. Lubyshev, V.V. Preobrazhenskii, B.R. Semyagin, V.V. Tretyakov, Semicond. Sci. Technol. 12 (1997) 51. [8] G.M. Martin, Appl. Phys. Lett. 39 (1981) 747. [9] X. Liu, A. Prasad, J. Nishio, E.R. Weber, Z. Liliental-Weber, W. Walukiewicz, Appl. Phys. Lett. 67 (1995) 279.