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LP-MOVPE growth of InAs quantum dots using tertiarybutylarsine (TBA) in pure N2 ambient
ARTICLE IN PRESS
Journal of Crystal Growth 268 (2004) 444–448
LP-MOVPE growth of InAs quantum dots using tertiarybutylarsine (TBA) in pure N2 ambient G.S. Huang*, X.H. Tang, B.L. Zhang, Y.C. Zhang, S.C. Tjin Photonics Research Center, Microelectronics Division, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
Abstract The InAs quantum dots on GaAs (0 0 1) substrate were first grown by LP-MOVPE using tertiarybutylarsine (TBA) in nitrogen ambient. Samples were characterized using atomic force microscope (AFM) and photoluminescence (PL) spectroscopy at 77 K. The average diameter of the dots increases with the group III source trimethylindium (TMIn) inlet flow. The shape of dots also changes with the TMIn inlet flow. The shape of dots changes from oval to circle, respectively, when the TMIn inlet flow was changed from 60 to 80 sccm. When TMI was set at 100 sccm, the shape of InAs quantum dots is tetrahedron with two {1 4 14} facets and one {1 1 14} facet and their edges were parallel to [1 4 0] and [1 1 0] directions, respectively. The dot density is (1.5–2.5) 1010 cm 2. The PL emission from the quantum dots is centered at 1071 nm wavelength and has a full width at half maximum PL spectrum of 55 meV. r 2004 Elsevier B.V. All rights reserved. PACS: 68.35.Bs; 68.65.+g; 78.66.Fd Keywords: A1. Low dimensional structures; A3. Metalorganic chemical vapor deposition; B1. Arsenates; B2. Semiconducting III–V materials
1. Introduction Quantum dots (QD) have been the focus of numerous experimental and theoretical studies in recent years. Because of the ability its strong confinement with individual charge carriers, quantum dots have been proposed for a wide variety of applications. Possible uses include devices for applications in quantum computing, biological, *Corresponding author. Tel.: +65-6790-6319; fax: +656790-4161. E-mail address: [email protected] (G.S. Huang).
and optoelectronics. Metal organic vapor-phase epitaxy (MOVPE) for the growth of III–V semiconductors has been a rapidly developing technology. MOVPE is recognized as a key technology for manufacturing optoelectronic devices due to its high-quality growth, high throughput, short downtimes, and scalability to large reactors. In general, MOVPE process uses high toxic hydride AsH3 as a group V-precursor. For safety reasons therefore several alternative, less toxic group V-sources have been investigated. Recently, organo-arsines, such as tertiarybutylarsine (TBA) [1–4], have attracted much attention.
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.070
ARTICLE IN PRESS G.S. Huang et al. / Journal of Crystal Growth 268 (2004) 444–448
Devices made from structures grown with TBA exhibit state of the art performance [1]. However, all these former studies were performed in a hydrogen ambient, which is explosive and thus a safety hazard. To improve the process safety, an inert gas such as nitrogen used as carrier gas in MOVPE growths has been investigated by several groups. Using of nitrogen as the carrier gas in the MOVPE growths of III/V materials has been proved to be advantageous in term of obtaining highly homogeneous and pure layers [5]. The increased layer homogeneity was achieved in nitrogen carrier gas compared to hydrogen [6,7]. In this paper, LP-MOVPE growth of InAs quantum dots grown on GaAs substrate by using TBA as group V precursor in pure nitrogen ambient was first studied. The effect of different growth rate and amounts of deposited InAs material on the InAs islands has been investigated.
2. Experimental procedure All the samples were grown in a low-pressure horizontal MOVPE reactor (Aixtron, AIX200) using gas foil rotation of the susceptor. The precursors were TBA, trimethylgallium (TMGa) and TMIn. The nitrogen carrier gas was purified by a MonoTorr phase II Getter column. The pressure for all the growths was set at 20 mbar. The growth temperature varied in the range of 500–600 C. The V/III ratio was set at 25 and 20 for GaAs and InAs, respectively. The total flow of the gas in the reactor was Qtot=3.1 slm. GaAs epiready semi-insulating substrates with normally oriented in (0 0 1) 70.1 direction were used for all the growths. A 0.2 mm GaAs buffer layer was first grown on (0 0 1) GaAs substrate at 680 C after thermal annealing, which was performed at 700 C for 5 min under the protection of TBA. The growth temperature was then reduced to 550 C for depositing a 6 nm GaAs layer. Then the TMIn inlet flow of 60, 80, 100 sccm for sample A, sample B and sample C, respectively, was used for growing the InAs layer for 10 s. After 15 s interruption, samples were then rapidly cooled down under the protection of TBA. The growth rate of InAs was determined by measuring the
445
thickness of single thin InAs layer. The surface morphology of the samples was studied by Nanoscope III atomic force microscopy (AFM). For photoluminescence (PL) measurement, the samples were excited by a 488 nm Ar+ laser and the PL signal was detected by a cooled Ge photodetector.
3. Result and discussion Fig. 1 shows the AFM images of the QDs formed with different TMIn inlet flow in MOVPE growth. The thickness of InAs was 2ML, 2.8 ML and 3.5ML, respectively. It shows the growth rate of InAs does not change linearly to the TMIn inlet flow at the growth temperature of growing the InAs dots. As shown in Fig. 1(a), two types of InAs islands of sample A, classified by size, are observed in the AFM images. The type-A islands are large, oval-shaped, whose long axis and short axis are about 45 and 36 nm, respectively. The type-B islands are small islands, round- and ovalshaped, whose diameter is about 20 nm. The
Fig. 1. The AFM images of different TMIn inlet flow and the micrograph shows a 0.5 0.5 mm2 area. (a) 60 sccm (b) 80 sccm (c)100 sccm (1 1 mm2).
ARTICLE IN PRESS 446
G.S. Huang et al. / Journal of Crystal Growth 268 (2004) 444–448
density of islands is about 1.52 1010 cm 2, which is not very high. It can be seen that the surface is not very flat. The undulations are along direction [1 1 0]. These undulations originate from the surface energy. Fig. 1(b) shows the AFM image of sample B. As shown in this figure, the type-A islands are large, bar-shaped, which may originate from the coalescence of islands arranged along this direction. The type-B islands are small islands, round-shaped, whose diameter is about 32 nm.The density of islands is about 2.32 1010 cm 2. From this figure, the islands arrange mainly along direction [ 1% 1 0]. This corresponds to the characteristic of the surface morphology of sample A. The undulations on the surface are along with direction [1 1 0]. Fig. 1(c) shows the AFM image of sample C. In this sample, the type-A islands are large and domeshaped. The type-B islands in this sample are small islands and triangle-shaped. The lateral base of the island is about 48 nm.The mean density of the islands is about 1.8 1010 cm 2. The angles of most islands point to direction [ 1% 1 0]. Since the numbers of type-A and coalesced islands are small, the changes of the small islands with the growth conditions will be investigated. Comparing the three samples (Fig. 1. (a)–(c)), we can see that the total areas covered by InAs islands are almost same with different growth conditions. The density of InAs dots increases when the TMIn flow increases from 60 to 80 sccm, The increase of density with large TMIn flow in the growth is due to the thickness of InAs layer deposited was increased. However, the density of InAs dots decreases while the TMIn flow increases from 80 to 100 sccm. The density of InAs dots of sample C is smaller than that of sample B due to the increase of the island base area and the formation of island coalescence. The shape of the islands is different for different thickness of InAs layer. At lower TMIn inlet flow, thinner InAs layer was deposited. The shape is same as that grown by MOVPE in hydrogen ambient [8]. Because the thermal conductivity and heat capacity of nitrogen are higher than that of hydrogen, the cooling rate of MOVPE reactor in nitrogen ambient is faster. It was measured to be about 35 C/min. The annealing effect on island size is very small. It has been found
that island base size of small islands is nearly independent of the annealing time [9]. The small island base is triangle-shaped as shown in Fig. 1(c). The angle of this triangle points to [ 1% 1 0] direction between the base edges is about 62 . One edge of the triangle aligned parallel to [1 4% 0], while other two edges aligned parallel to [4 1% 0] and [1 1 0], respectively. Based upon the observations, we propose a structure model depicted in Fig. 2 to explain the observed AFM image of the island formed. The three-dimensional island shape of sample C is tetrahedron as shown in Fig. 3. The side view profile of island along [1 1 0] direction of sample C is shown in Fig. 4. The island a, b, c are three dots along [1 1 0]. The angle between the high edges of island a and b are smaller than that of island c because the cross-section cuts through the center of island c. Fig. 4 shows the center crosssection of island c. The angle between the high edges of island also calculated to be about 150 . The height of this dot is about 4 nm.According the angle of height profile and height, the surface index of z-axis is calculated to 14. Then tetrahedron was calculated to consist of two {1 4 14} facets and one {1 1 14} facet. This model is
_ [410]
(1 4 14)
61.9°
_ (1 1 14)
__ ( 4 1 14)
_ [1 4 0] [100]
_ [1 10] [010]
Fig. 2. A schematic diagram showing the crystallographic geometry of the InAs QD on GaAs.
ARTICLE IN PRESS G.S. Huang et al. / Journal of Crystal Growth 268 (2004) 444–448
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5000
Intensity (a.u.)
4000 3000 2000 1000 0 950
1000
1050
1100
1150
1200
Wavelength (nm)
Fig. 5. The PL spectrum of InAs QDs of sample C at 77 K. Fig. 3. Three representative surface morphology of sample C and the shape is tetrahedron.
nm 10 a
b
electronic structure calculations may be readily apprehended by considering that the symmetry of the quantum dot is P rather than C4v as has been assumed in recent theoretical calculations [9]. Fig. 5 Shows the 77 K PL spectrum of sample C. The peak position lies at 1071 nm and the full width at half maximum (FWHM) is 55 nm.
c
4. Conclusions
0
-10 0.0
0.2
0.4
0.6
0.8
µm
Fig. 4. 1 1 mm2 top-view AFM image for InAs islands of sample C height profile along [1 1 0] direction.
supported by AFM images of InAs quantum dots reported by Jin and co-workers [10]. The model depicted in Fig. 4 is physically reasonable when considered from the standpoint of the energetics and kinetics of island formation. The {1 4 14} and {1 1 14} planes are vicinal surfaces which contain a high density of steps and kinks that are expected to favor strain accommodation [11]. Note also that some quantum dots grows very fast. The implications of our result to
We observed that the morphology of the InAs self-assembled quantum dots grown on GaAs by LP-MOVPE using TBA in nitrogen ambient changed significantly with TMIn inlet flow and the thickness of InAs layer deposited. The deposition thickness plays an important role in the base shape of self-assembled quantum dots. The base shape of InAs quantum dots changes from oval to circle, when TMIn inlet flow was changed from 60 to 80 sccm. The shape of InAs quantum dots is tetrahedron when the TMIn inlet flow was set at 100 sccm. The base shape is triangle, which edges aligned parallel to [1 4 0] and [1 1 0] direction, respectively. A model based on the shape of InAs quantum dots was proposed to describe the structure formed. The tetrahedron island formed with the TMIn flow rate of 100 sccm consists of two {1 4 14} facets and one {1 1 14} facet. The PL spectrum shows good quality of InAs quantum dots.
ARTICLE IN PRESS 448
G.S. Huang et al. / Journal of Crystal Growth 268 (2004) 444–448
Acknowledgements The authors would like to thank the Defense Science and Technology Agency of Singapore for supporting this project. We also acknowledge Mdm Neo Bee Geok for AFM measurement and Dr. Qu Yi for Photoluminescence measurement and Dr. S. R. Natarajan for his helpful suggestions.
References [1] A. Sasaki, M. Kasu, T. Yamamoto, S. Noda, Japan J. Appl. Phys. 28 (1989) L1249. [2] S. Chichibu, A. Iwai, S. Matsumoto, H. Higuchi, Appl. Phys. Lett. 60 (4) (1992) 489.
[3] S. Chichibu, A. Iwai, Y. Nakahara, S. Matsumoto, H. Higuchi, L. Wei, S. Tangigawa, Appl. Phys. Lett. 73 (8) (1993) 3880. [4] T. Kikkawa, H. Tanaka, J. Komeno, J. Appl. Phys. 67 (1990) 7576. [5] H. Hardtdegen, Proceedings of the Ninth International Workshop on the Physics of the Semiconductor Devices (IWPSD), Delhi, India, 1997, p. 244. [6] M. Dauelsberg, H. Hardtdegen, L. Kadinski, A. Kaluza, P. Kaufmann, J. Crystal Growth 223 (2001) 21. [7] G.S. Huang, X.H. Tang, B.L. Zhang, S.C. Tjin, J. Appl. Phys. 94 (2003) 4890. [8] T. Chung, G. Walter, N. Holonyak Jr., Appl. Phys. Lett. 79 (2001) 4501. [9] H. Lee, R. Lowe-Webb, W. Yang, P.C. Sercel, Appl. Phys. Lett. 71 (1997) 2325. [10] Z. Jin, S. Yang, B. Liu, M. Li, X. Wang, Z. Li, G. Du, S. Liu, Opt. Mater. 14 (2000) 214. [11] C. Ratsch, P. Smilauer, D.D. Vvdensky, A. Zangwill, J. Phys. I 6 (1996) 575.
Journal of Crystal Growth 268 (2004) 444–448
LP-MOVPE growth of InAs quantum dots using tertiarybutylarsine (TBA) in pure N2 ambient G.S. Huang*, X.H. Tang, B.L. Zhang, Y.C. Zhang, S.C. Tjin Photonics Research Center, Microelectronics Division, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
Abstract The InAs quantum dots on GaAs (0 0 1) substrate were first grown by LP-MOVPE using tertiarybutylarsine (TBA) in nitrogen ambient. Samples were characterized using atomic force microscope (AFM) and photoluminescence (PL) spectroscopy at 77 K. The average diameter of the dots increases with the group III source trimethylindium (TMIn) inlet flow. The shape of dots also changes with the TMIn inlet flow. The shape of dots changes from oval to circle, respectively, when the TMIn inlet flow was changed from 60 to 80 sccm. When TMI was set at 100 sccm, the shape of InAs quantum dots is tetrahedron with two {1 4 14} facets and one {1 1 14} facet and their edges were parallel to [1 4 0] and [1 1 0] directions, respectively. The dot density is (1.5–2.5) 1010 cm 2. The PL emission from the quantum dots is centered at 1071 nm wavelength and has a full width at half maximum PL spectrum of 55 meV. r 2004 Elsevier B.V. All rights reserved. PACS: 68.35.Bs; 68.65.+g; 78.66.Fd Keywords: A1. Low dimensional structures; A3. Metalorganic chemical vapor deposition; B1. Arsenates; B2. Semiconducting III–V materials
1. Introduction Quantum dots (QD) have been the focus of numerous experimental and theoretical studies in recent years. Because of the ability its strong confinement with individual charge carriers, quantum dots have been proposed for a wide variety of applications. Possible uses include devices for applications in quantum computing, biological, *Corresponding author. Tel.: +65-6790-6319; fax: +656790-4161. E-mail address: [email protected] (G.S. Huang).
and optoelectronics. Metal organic vapor-phase epitaxy (MOVPE) for the growth of III–V semiconductors has been a rapidly developing technology. MOVPE is recognized as a key technology for manufacturing optoelectronic devices due to its high-quality growth, high throughput, short downtimes, and scalability to large reactors. In general, MOVPE process uses high toxic hydride AsH3 as a group V-precursor. For safety reasons therefore several alternative, less toxic group V-sources have been investigated. Recently, organo-arsines, such as tertiarybutylarsine (TBA) [1–4], have attracted much attention.
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.070
ARTICLE IN PRESS G.S. Huang et al. / Journal of Crystal Growth 268 (2004) 444–448
Devices made from structures grown with TBA exhibit state of the art performance [1]. However, all these former studies were performed in a hydrogen ambient, which is explosive and thus a safety hazard. To improve the process safety, an inert gas such as nitrogen used as carrier gas in MOVPE growths has been investigated by several groups. Using of nitrogen as the carrier gas in the MOVPE growths of III/V materials has been proved to be advantageous in term of obtaining highly homogeneous and pure layers [5]. The increased layer homogeneity was achieved in nitrogen carrier gas compared to hydrogen [6,7]. In this paper, LP-MOVPE growth of InAs quantum dots grown on GaAs substrate by using TBA as group V precursor in pure nitrogen ambient was first studied. The effect of different growth rate and amounts of deposited InAs material on the InAs islands has been investigated.
2. Experimental procedure All the samples were grown in a low-pressure horizontal MOVPE reactor (Aixtron, AIX200) using gas foil rotation of the susceptor. The precursors were TBA, trimethylgallium (TMGa) and TMIn. The nitrogen carrier gas was purified by a MonoTorr phase II Getter column. The pressure for all the growths was set at 20 mbar. The growth temperature varied in the range of 500–600 C. The V/III ratio was set at 25 and 20 for GaAs and InAs, respectively. The total flow of the gas in the reactor was Qtot=3.1 slm. GaAs epiready semi-insulating substrates with normally oriented in (0 0 1) 70.1 direction were used for all the growths. A 0.2 mm GaAs buffer layer was first grown on (0 0 1) GaAs substrate at 680 C after thermal annealing, which was performed at 700 C for 5 min under the protection of TBA. The growth temperature was then reduced to 550 C for depositing a 6 nm GaAs layer. Then the TMIn inlet flow of 60, 80, 100 sccm for sample A, sample B and sample C, respectively, was used for growing the InAs layer for 10 s. After 15 s interruption, samples were then rapidly cooled down under the protection of TBA. The growth rate of InAs was determined by measuring the
445
thickness of single thin InAs layer. The surface morphology of the samples was studied by Nanoscope III atomic force microscopy (AFM). For photoluminescence (PL) measurement, the samples were excited by a 488 nm Ar+ laser and the PL signal was detected by a cooled Ge photodetector.
3. Result and discussion Fig. 1 shows the AFM images of the QDs formed with different TMIn inlet flow in MOVPE growth. The thickness of InAs was 2ML, 2.8 ML and 3.5ML, respectively. It shows the growth rate of InAs does not change linearly to the TMIn inlet flow at the growth temperature of growing the InAs dots. As shown in Fig. 1(a), two types of InAs islands of sample A, classified by size, are observed in the AFM images. The type-A islands are large, oval-shaped, whose long axis and short axis are about 45 and 36 nm, respectively. The type-B islands are small islands, round- and ovalshaped, whose diameter is about 20 nm. The
Fig. 1. The AFM images of different TMIn inlet flow and the micrograph shows a 0.5 0.5 mm2 area. (a) 60 sccm (b) 80 sccm (c)100 sccm (1 1 mm2).
ARTICLE IN PRESS 446
G.S. Huang et al. / Journal of Crystal Growth 268 (2004) 444–448
density of islands is about 1.52 1010 cm 2, which is not very high. It can be seen that the surface is not very flat. The undulations are along direction [1 1 0]. These undulations originate from the surface energy. Fig. 1(b) shows the AFM image of sample B. As shown in this figure, the type-A islands are large, bar-shaped, which may originate from the coalescence of islands arranged along this direction. The type-B islands are small islands, round-shaped, whose diameter is about 32 nm.The density of islands is about 2.32 1010 cm 2. From this figure, the islands arrange mainly along direction [ 1% 1 0]. This corresponds to the characteristic of the surface morphology of sample A. The undulations on the surface are along with direction [1 1 0]. Fig. 1(c) shows the AFM image of sample C. In this sample, the type-A islands are large and domeshaped. The type-B islands in this sample are small islands and triangle-shaped. The lateral base of the island is about 48 nm.The mean density of the islands is about 1.8 1010 cm 2. The angles of most islands point to direction [ 1% 1 0]. Since the numbers of type-A and coalesced islands are small, the changes of the small islands with the growth conditions will be investigated. Comparing the three samples (Fig. 1. (a)–(c)), we can see that the total areas covered by InAs islands are almost same with different growth conditions. The density of InAs dots increases when the TMIn flow increases from 60 to 80 sccm, The increase of density with large TMIn flow in the growth is due to the thickness of InAs layer deposited was increased. However, the density of InAs dots decreases while the TMIn flow increases from 80 to 100 sccm. The density of InAs dots of sample C is smaller than that of sample B due to the increase of the island base area and the formation of island coalescence. The shape of the islands is different for different thickness of InAs layer. At lower TMIn inlet flow, thinner InAs layer was deposited. The shape is same as that grown by MOVPE in hydrogen ambient [8]. Because the thermal conductivity and heat capacity of nitrogen are higher than that of hydrogen, the cooling rate of MOVPE reactor in nitrogen ambient is faster. It was measured to be about 35 C/min. The annealing effect on island size is very small. It has been found
that island base size of small islands is nearly independent of the annealing time [9]. The small island base is triangle-shaped as shown in Fig. 1(c). The angle of this triangle points to [ 1% 1 0] direction between the base edges is about 62 . One edge of the triangle aligned parallel to [1 4% 0], while other two edges aligned parallel to [4 1% 0] and [1 1 0], respectively. Based upon the observations, we propose a structure model depicted in Fig. 2 to explain the observed AFM image of the island formed. The three-dimensional island shape of sample C is tetrahedron as shown in Fig. 3. The side view profile of island along [1 1 0] direction of sample C is shown in Fig. 4. The island a, b, c are three dots along [1 1 0]. The angle between the high edges of island a and b are smaller than that of island c because the cross-section cuts through the center of island c. Fig. 4 shows the center crosssection of island c. The angle between the high edges of island also calculated to be about 150 . The height of this dot is about 4 nm.According the angle of height profile and height, the surface index of z-axis is calculated to 14. Then tetrahedron was calculated to consist of two {1 4 14} facets and one {1 1 14} facet. This model is
_ [410]
(1 4 14)
61.9°
_ (1 1 14)
__ ( 4 1 14)
_ [1 4 0] [100]
_ [1 10] [010]
Fig. 2. A schematic diagram showing the crystallographic geometry of the InAs QD on GaAs.
ARTICLE IN PRESS G.S. Huang et al. / Journal of Crystal Growth 268 (2004) 444–448
447
5000
Intensity (a.u.)
4000 3000 2000 1000 0 950
1000
1050
1100
1150
1200
Wavelength (nm)
Fig. 5. The PL spectrum of InAs QDs of sample C at 77 K. Fig. 3. Three representative surface morphology of sample C and the shape is tetrahedron.
nm 10 a
b
electronic structure calculations may be readily apprehended by considering that the symmetry of the quantum dot is P rather than C4v as has been assumed in recent theoretical calculations [9]. Fig. 5 Shows the 77 K PL spectrum of sample C. The peak position lies at 1071 nm and the full width at half maximum (FWHM) is 55 nm.
c
4. Conclusions
0
-10 0.0
0.2
0.4
0.6
0.8
µm
Fig. 4. 1 1 mm2 top-view AFM image for InAs islands of sample C height profile along [1 1 0] direction.
supported by AFM images of InAs quantum dots reported by Jin and co-workers [10]. The model depicted in Fig. 4 is physically reasonable when considered from the standpoint of the energetics and kinetics of island formation. The {1 4 14} and {1 1 14} planes are vicinal surfaces which contain a high density of steps and kinks that are expected to favor strain accommodation [11]. Note also that some quantum dots grows very fast. The implications of our result to
We observed that the morphology of the InAs self-assembled quantum dots grown on GaAs by LP-MOVPE using TBA in nitrogen ambient changed significantly with TMIn inlet flow and the thickness of InAs layer deposited. The deposition thickness plays an important role in the base shape of self-assembled quantum dots. The base shape of InAs quantum dots changes from oval to circle, when TMIn inlet flow was changed from 60 to 80 sccm. The shape of InAs quantum dots is tetrahedron when the TMIn inlet flow was set at 100 sccm. The base shape is triangle, which edges aligned parallel to [1 4 0] and [1 1 0] direction, respectively. A model based on the shape of InAs quantum dots was proposed to describe the structure formed. The tetrahedron island formed with the TMIn flow rate of 100 sccm consists of two {1 4 14} facets and one {1 1 14} facet. The PL spectrum shows good quality of InAs quantum dots.
ARTICLE IN PRESS 448
G.S. Huang et al. / Journal of Crystal Growth 268 (2004) 444–448
Acknowledgements The authors would like to thank the Defense Science and Technology Agency of Singapore for supporting this project. We also acknowledge Mdm Neo Bee Geok for AFM measurement and Dr. Qu Yi for Photoluminescence measurement and Dr. S. R. Natarajan for his helpful suggestions.
References [1] A. Sasaki, M. Kasu, T. Yamamoto, S. Noda, Japan J. Appl. Phys. 28 (1989) L1249. [2] S. Chichibu, A. Iwai, S. Matsumoto, H. Higuchi, Appl. Phys. Lett. 60 (4) (1992) 489.
[3] S. Chichibu, A. Iwai, Y. Nakahara, S. Matsumoto, H. Higuchi, L. Wei, S. Tangigawa, Appl. Phys. Lett. 73 (8) (1993) 3880. [4] T. Kikkawa, H. Tanaka, J. Komeno, J. Appl. Phys. 67 (1990) 7576. [5] H. Hardtdegen, Proceedings of the Ninth International Workshop on the Physics of the Semiconductor Devices (IWPSD), Delhi, India, 1997, p. 244. [6] M. Dauelsberg, H. Hardtdegen, L. Kadinski, A. Kaluza, P. Kaufmann, J. Crystal Growth 223 (2001) 21. [7] G.S. Huang, X.H. Tang, B.L. Zhang, S.C. Tjin, J. Appl. Phys. 94 (2003) 4890. [8] T. Chung, G. Walter, N. Holonyak Jr., Appl. Phys. Lett. 79 (2001) 4501. [9] H. Lee, R. Lowe-Webb, W. Yang, P.C. Sercel, Appl. Phys. Lett. 71 (1997) 2325. [10] Z. Jin, S. Yang, B. Liu, M. Li, X. Wang, Z. Li, G. Du, S. Liu, Opt. Mater. 14 (2000) 214. [11] C. Ratsch, P. Smilauer, D.D. Vvdensky, A. Zangwill, J. Phys. I 6 (1996) 575.