- Email: [email protected]
InAs quantum dots and dashes grown on (100), (211)B, and (311)B GaAs substrates
Physica E 2 (1998) 672—677
InAs quantum dots and dashes grown on (100), (211)B, and (311)B GaAs substrates S.P. Guo*,1, A. Shen2, Y. Ohno, H. Ohno Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-77, Japan
Abstract InAs self-organized quantum dots (QDs) and quantum dashes (QDHs) grown on GaAs (100), (211)B and (311)B substrates by molecular beam epitaxy at different growth temperatures (¹ ) have been investigated. QDs were observed 4 after deposition of 2ML (or 4ML ) of InAs on GaAs (100) (or (311)B) at ¹ ranging from 450°C to 530°C. The 100 311 4 average density decreases and the average size increases monotonically with increasing ¹ . QDs with bimodal size 4 distribution were formed when 6ML of InAs was deposited on GaAs (2 1 1)B at lower ¹ . When the same amount of 211 4 InAs was deposited at higher ¹ , however, QDHs were observed. The photoluminescence intensity of the QDs and 4 QDHs showed similar temperature dependence, whereas the excitation density dependence showed quite different behaviors. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Quantum dots; Quantum dashes; Molecular beam epitaxy; Photoluminescence
1. Introduction Based on the Stranski—Krastanov growth mode, various quantum dot (QD) systems have been grown successfully by molecular beam epitaxy (MBE) [1—6]. The study of the QDs has permitted
* Corresponding author. Tel. and fax: #81 22 2175553; email: [email protected]. 1 On leave from Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, People’s Republic of China. 2 Present address: Institute for microstructural Sciences, National Research Council, Ottawa K1A OR6, Canada.
identification of unique and interesting physical phenomena, with hopes for potential device applications. Up to now, most of the studies of selforganized QDs have been focused on the (100) substrates and only a few attempts were taken on non (100) substrates. In addition to the QD structure, the self-organized growth has led to the formation of quantum dash (QDH) and quantum wire-like structures, as demonstrated by Utzmeier et al. [7], who observed the formation of InSb QDHs by growing more than 3.2 monolayer (ML) InSb on InP (100) substrates, and by Vaccaro et al. [8], who observed InGaAs quantum wire-like structure by growing 6ML (6ML thickness in 100
1386-9477/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII: S 1 3 8 6 - 9 4 7 7 ( 9 8 ) 0 0 1 3 7 - 4
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(100) plane) InGaAs on GaAs (311)A substrates. Recently, we found that InAs QDHs can be formed on GaAs (211)B by growing 6ML InAs on GaAs 211 (211)B at 500°C or 510°C [9]. Considering the significant effect of the substrate orientation on the formation of the nanostructures, it is quite interesting to study the InAs nanostructures grown on substrates with different orientations. In this work we investigate the MBE growth and photoluminescence (PL) properties of InAs QDs and QDHs grown on GaAs (100), (211)B and (311)B substrates at various growth temperatures (¹ ). 4 2. Experiments All the samples were grown by MBE (ULVAC MBC-100) on semi-insulating GaAs (100), (211)B and (311)B epi-ready substrates. Growth rates were determined by reflection high energy electron diffraction (RHEED) oscillation measurements on (100) orientation and they were 0.6 lm/h (GaAs) and 0.5 lm/h (AlAs) for buffer layer growth, 0.15 lm/h for InAs nanostructure growth and 0.2 lm/h for GaAs caplayer growth. After removing surface oxide under an arsenic flux, a 0.5 lm GaAs buffer layer was grown at 600°C, then a 40 period GaAs/AlAs (2.3 nm/1.5 nm) superlattice was deposited, followed by a 10 nm GaAs layer. InAs nanostructures were deposited at ¹ ranging from 4 450°C to 530°C for (100) or (311)B orientation and from 400°C to 510°C for (211)B orientation with As to In beam equivalent pressure ratio of about 20. For PL measurement, a 30 nm GaAs caplayer was grown at ¹ 20°C lower than the InAs growth 4 temperature or at 480°C for high InAs growth temperature in order to avoid In segregation. The atomic force microscopy (AFM) measurements were carried out at room temperature using a Si N cantilever. The PL experiments were per3 4 formed at measurement temperature (¹ ) ranging . from 4.5 to 200 K in a closed-cycle He cryostat with a 514.5 nm line of Ar` laser for excitation. The resultant luminescence was collected by a 0.5 meter spectrometer before being detected by a cooled photomultiplier tube (detection range is 0.3— 1.6 lm) whose signal was processed by a lock-in amplifier.
673
3. Results and discussion When 2ML of InAs (or 4ML of InAs) was 100 311 deposited on GaAs (100) (or (311)B) in the temperature range from 450°C to 530°C the RHEED patterns changed from streaky to spotty patterns, indicating a transition from two dimensional growth mode to three dimensional growth mode and the formation of QDs. Fig. 1a and b shows the AFM images of 2ML of InAs on GaAs (100) and 100 4ML of InAs on GaAs (311)B grown at 500°C. 311 QD structures were observed. The average density decreases and the average size increases monotonically with increasing ¹ . Fig. 2 shows the average 4 density, diameter and height as a function of the inverse of ¹ for QDs with 2ML of InAs grown 4 100 on GaAs (100). The absence of a single straight line fit over the studied temperature range for QD density indicates that more than one kinetic process is likely competing to control the nucleation of QDs. On the other hand, the existence of a single straight line fit for QD diameter and height reveals that the diameter and the height of the QDs is probably controlled by a single kinetic process. Similar dependence was also observed for the QDs grown on GaAs (311)B. At high ¹ , however, the change was 4 much slower compared to the QDs grown on GaAs (100). The average density of QDs grown at 530°C is 1.2]1010 cm~2 with the average diameter of 45 nm and the average height of 7.5 nm. The higher density of QDs grown on GaAs (311)B at high ¹ (almost one order of magnitude higher than that 4 of QDs grown on GaAs (100) under the same growth conditions) might be due to the limited migration length of In adatoms on the (311)B surface. When 6ML of InAs was deposited on GaAs 211 (211)B at lower ¹ (from 400°C to 470°C), the 4 RHEED pattern changed from streaky to spotty, revealing the formation of QDs. When the same amount of InAs was deposited at higher ¹ (500°C 4 or 510°C), the RHEED pattern along the [11 11] azimuth was almost the same as that grown at lower ¹ , but there was a drastic change of the 4 RHEED pattern along the [0111 ] azimuth; it changed from streaky to spotty patterns right after deposition of 6ML of InAs just as ¹ "450°C, 211 4 and then the spotty pattern transformed to
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Fig. 2. The growth temperature dependence of the average density, the average diameter and the average height of QDs with 2ML of InAs grown on GaAs (100). 100
Fig. 1. AFM images of (a) 2ML of InAs deposited on GaAs 100 (100) at 500°C (1]1 lm2), (b) 4ML of InAs deposited on 311 GaAs (311)B at 500°C (1]1 lm2) and (c) 6ML of InAs 211 deposited on GaAs (211) at 500°C (5]5 lm2).
streaky pattern after several seconds of growth interruption, indicating the formation of a new structure. QDs with bimodal dot size distribution were observed by AFM measurements when 6ML of 211 InAs was deposited at ¹ ranging from 400°C to 4 470°C. The average dot density, the average lateral
size and the average height of small dots and large dots for QDs grown at 450°C are 1.9]1010 cm~2 and 1.6]109 cm~2, 13 and 47 nm, 3 and 10 nm, respectively. The average dot size increases and the average density decreases monotonically with increasing ¹ . This behavior is similar to that of QDs 4 grown on GaAs (100) and (311)B orientation. When 6ML of InAs was deposited at 500°C or 510°C, 211 however, a drastic change of InAs nanostructure from QDs to QDHs was observed. The QDHs were formed with no trace of the existence of small QDs. Fig. 1c shows the AFM image of 6ML of InAs grown on GaAs (211)B at 500°C. 211 The QDHs have asymmetric hut-like shape and self-aligned along [0111 ] direction. The average density of QDHs is 2.8]108 cm~2 with the average size of 163 nm]65 nm and 20 nm in height. The observation of the time evolution of RHEED images of InAs deposited on (211)B plane
S.P. Guo et al. / Physica E 2 (1998) 672—677
at higher ¹ suggests that an intermediate state of 4 QDs exists before the formation of the QDHs. RHEED observation indicated that when InAs was deposited on GaAs, InAs wetting layer was formed at first, and then the QDs developed on the wetting layer for further deposition of InAs. At lower ¹ the 4 QD structure is stable most probably due to the limited migration length of In adatoms. At higher ¹ , however, due to thermally enhanced In surface 4 migration, the QDs are metastable and coalesce each other, forming the QDH structure. The hutlike shape of the QDHs is believed to be related to the stress anisotropy on the (211)B plane and the metastability of the intermediate state of QDs [9]. Fig. 3 shows the low temperature (4.5 K) PL spectra of the QDs with 2ML of InAs grown on 100 GaAs (100) at various ¹ . When ¹ increased from 4 4 490°C to 510°C the peak position of QDs shifted to low energy, whereas the QD peak shifted to high energy when ¹ was 530°C. The red-shift of peak 4 energy is believed to be due to the effect of the quantum confinement which is related to the dot size. The unusual blue-shift of peak energy for QDs grown at 530°C might be due to the emission related to the excited electron states in big QDs. The blue-shift behavior was also observed in the sample with 4ML of InAs grown at 500°C (AFM meas100 urement showed the existence of the big QDs with the diameter of 62 nm and the height of 20 nm in such a structure). The other possible explanation of the blue-shift is the thermally enhanced intermixing of In and Ga atoms at the QD interface. The multi-peaks in PL spectra were attributed to the transitions from the ground level to the ground and the excited levels in QDs and the weak peak around 1.41 eV was attributed to the emission from the InAs wetting layer. The temperature dependence of PL spectra of InAs QDs and QDHs grown on GaAs (211)B is shown in Fig. 4. We attribute the two PL peaks in spectra of QDs (around 1.24 and 1.4 eV at low ¹ ) . to the bimodal size distribution in the QDs. The low PL efficiency of QDs is believed to be related to the poor quality of GaAs caplayer grown at low temperature. The PL peak intensity and position of QDs and QDHs showed similar temperature dependence: at low ¹ the peak intensity as well as .
675
Fig. 3. Low temperature PL spectra of QDs with 2ML of 100 InAs grown on GaAs (100) at various ¹ . 4
the peak position was almost independent of the ¹ , showing that the exciton is localized spatially in . a certain potential minimum. On the other hand, the peak intensity decreased drastically and peak position shifted to low energy monotonically with the increase of ¹ above a certain transition tem. perature (¹ ). This behavior is most probably due to 5 the dissociation of excitons into the electron-hole pairs which then escape from the QDs/QDHs. ¹ was 52 K for QDHs and 60 K for QDs. The 5 lower ¹ for the QDH structure might be due to the 5 weaker quantum confinement effect. The excitation density dependence of PL spectra of QDs and QDHs has also been studied (not shown). The PL intensity of QDs exhibited a linear behavior with respect to the excitation density and no shift in the peak position above experimental error was observed, whereas the PL intensity of QDHs exhibited a nonlinear behavior and the peak shifted to high energy by increasing the excitation density. The nonlinear dependence and the blue-shift of peak energy in QDH structures suggest that the mechanism of the radiative recombination in QDHs is different from that in QDs. One possible
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S.P. Guo et al. / Physica E 2 (1998) 672—677
Fig. 4. Temperature dependence of PL spectra of InAs nanostructures with 6ML of InAs grown on GaAs (211)B, (a) QDs grown at 211 400°C and (b) QDHs grown at 510°C.
explanation is the existence of piezo-electric field in QDHs. The temperature dependence of PL spectra as well as the excitation density dependence of PL spectra of InAs QDs grown on GaAs (100) and (311)B has also been investigated (figures not shown here). The behavior is similar to that of the QDs grown on GaAs (211)B at lower ¹ and is in 4 agreement with the results reported by other groups [10,11].
4. Conclusions InAs self-organized nanostructures grown on GaAs (100), (211)B and (311)B substrates by MBE at different ¹ have been investigated. QDs were 4 observed after deposition of 2ML (or 4ML ) 100 311 of InAs on GaAs (100) (or (311)B) at ¹ ranging 4 from 450°C to 530°C. The average density decreases and the average size increases monotonically with increasing ¹ . When 6ML of InAs was 4 211 deposited on GaAs (211)B at lower ¹ ranging from 4 400 to 470°C, QDs with bimodal size distribution were observed. When the same InAs was deposited at higher ¹ (500°C or 510°C), however, QDHs 4
were formed with no trace of small QDs. The Photoluminescence intensity of the QDs and the QDHs showed similar temperature dependence; at low temperatures the intensity was independent of the ¹ , whereas the intensity decreased rapidly . with increase of temperature above ¹ . The excita5 tion density dependence of the QDs and QDHs, however, was quite different. The PL intensity of QDs exhibited a linear behavior with the increase of excitation density and the peak position is almost invariable, whereas the intensity of QDHs exhibited a nonlinear behavior and the peak shifted to high energy by increasing the excitation density.
Acknowledgements The authors thank Professor S. Kawakami for the use of AFM and Dr. F. Matsukura for helpful discussions. Part of this work was supported by Research for the Future Program from the Japan Society for the Promotion of Science (JSPS-RFT F97 00202) and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (09244103).
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References [1] D. Leonard, M. Krishnamurthy, S. Fafard, J.M. Merz, P.M. Petroff, J. Vac. Sci. Technol. B 12 (1994) 1063. [2] A. Madhukar, Q. Xie, P. Chen, A. Konkar, Appl. Phys. Lett. 64 (1994) 2727. [3] S. Fafard, R. Leon, D. Leonard, J.L. Merz, P.M. Petroff, Phys. Rev. B 50 (1994) 8086. [4] F. Hatami, N.N. Ledentsov, M. Grundmann, J. Bohrer, F. Heinrichsdorff, M. Beer, D. Bimberg, S.S. Ruvimov, P. Werner, U. Gosele, J. Heydenreich, U. Richter, S.V. Ivanov, B.Y. Meltser, P.S. Kop’ev, Z. Alferov, Appl. Phys. Lett. 67 (1995) 656. [5] A. Kurtenbach, E. Eberl, T. Shitara, Appl. Phys. Lett. 66 (1995) 361.
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[6] S. Fafard, Z. Wasilewski, J. McCafrey, S. Raymond, S. Charbonneau, Appl. Phys. Lett. 68 (1996) 991. [7] T. Utzmeier, P.A. Postigo, J. Tamayo, R. Garcia, F. Briones, Appl. Phys. Lett. 69 (1996) 2674. [8] O. Vaccaro, K. Fujita, T. Watanabe, Jpn. J. Appl. Phys. 36 (1997) 1948. [9] S.P. Guo, H. Ohno, A. Shen, F. Matsukura, Y. Ohno, Appl. Phys. Lett. 70 (1997) 2738. [10] D.I. Labyshev, P.P. Gonza´lez-Borrero, E. Marega Jr., E. Petitprez, P. Basmaji, J. Vac. Sci. Technol. B14 (1996) 2212. [11] K. Kamath, P. Bhattacharya, J. Phillips, J. Crystal Growth 175/176 (1997) 1720.
InAs quantum dots and dashes grown on (100), (211)B, and (311)B GaAs substrates S.P. Guo*,1, A. Shen2, Y. Ohno, H. Ohno Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-77, Japan
Abstract InAs self-organized quantum dots (QDs) and quantum dashes (QDHs) grown on GaAs (100), (211)B and (311)B substrates by molecular beam epitaxy at different growth temperatures (¹ ) have been investigated. QDs were observed 4 after deposition of 2ML (or 4ML ) of InAs on GaAs (100) (or (311)B) at ¹ ranging from 450°C to 530°C. The 100 311 4 average density decreases and the average size increases monotonically with increasing ¹ . QDs with bimodal size 4 distribution were formed when 6ML of InAs was deposited on GaAs (2 1 1)B at lower ¹ . When the same amount of 211 4 InAs was deposited at higher ¹ , however, QDHs were observed. The photoluminescence intensity of the QDs and 4 QDHs showed similar temperature dependence, whereas the excitation density dependence showed quite different behaviors. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Quantum dots; Quantum dashes; Molecular beam epitaxy; Photoluminescence
1. Introduction Based on the Stranski—Krastanov growth mode, various quantum dot (QD) systems have been grown successfully by molecular beam epitaxy (MBE) [1—6]. The study of the QDs has permitted
* Corresponding author. Tel. and fax: #81 22 2175553; email: [email protected]. 1 On leave from Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, People’s Republic of China. 2 Present address: Institute for microstructural Sciences, National Research Council, Ottawa K1A OR6, Canada.
identification of unique and interesting physical phenomena, with hopes for potential device applications. Up to now, most of the studies of selforganized QDs have been focused on the (100) substrates and only a few attempts were taken on non (100) substrates. In addition to the QD structure, the self-organized growth has led to the formation of quantum dash (QDH) and quantum wire-like structures, as demonstrated by Utzmeier et al. [7], who observed the formation of InSb QDHs by growing more than 3.2 monolayer (ML) InSb on InP (100) substrates, and by Vaccaro et al. [8], who observed InGaAs quantum wire-like structure by growing 6ML (6ML thickness in 100
1386-9477/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII: S 1 3 8 6 - 9 4 7 7 ( 9 8 ) 0 0 1 3 7 - 4
S.P. Guo et al. / Physica E 2 (1998) 672—677
(100) plane) InGaAs on GaAs (311)A substrates. Recently, we found that InAs QDHs can be formed on GaAs (211)B by growing 6ML InAs on GaAs 211 (211)B at 500°C or 510°C [9]. Considering the significant effect of the substrate orientation on the formation of the nanostructures, it is quite interesting to study the InAs nanostructures grown on substrates with different orientations. In this work we investigate the MBE growth and photoluminescence (PL) properties of InAs QDs and QDHs grown on GaAs (100), (211)B and (311)B substrates at various growth temperatures (¹ ). 4 2. Experiments All the samples were grown by MBE (ULVAC MBC-100) on semi-insulating GaAs (100), (211)B and (311)B epi-ready substrates. Growth rates were determined by reflection high energy electron diffraction (RHEED) oscillation measurements on (100) orientation and they were 0.6 lm/h (GaAs) and 0.5 lm/h (AlAs) for buffer layer growth, 0.15 lm/h for InAs nanostructure growth and 0.2 lm/h for GaAs caplayer growth. After removing surface oxide under an arsenic flux, a 0.5 lm GaAs buffer layer was grown at 600°C, then a 40 period GaAs/AlAs (2.3 nm/1.5 nm) superlattice was deposited, followed by a 10 nm GaAs layer. InAs nanostructures were deposited at ¹ ranging from 4 450°C to 530°C for (100) or (311)B orientation and from 400°C to 510°C for (211)B orientation with As to In beam equivalent pressure ratio of about 20. For PL measurement, a 30 nm GaAs caplayer was grown at ¹ 20°C lower than the InAs growth 4 temperature or at 480°C for high InAs growth temperature in order to avoid In segregation. The atomic force microscopy (AFM) measurements were carried out at room temperature using a Si N cantilever. The PL experiments were per3 4 formed at measurement temperature (¹ ) ranging . from 4.5 to 200 K in a closed-cycle He cryostat with a 514.5 nm line of Ar` laser for excitation. The resultant luminescence was collected by a 0.5 meter spectrometer before being detected by a cooled photomultiplier tube (detection range is 0.3— 1.6 lm) whose signal was processed by a lock-in amplifier.
673
3. Results and discussion When 2ML of InAs (or 4ML of InAs) was 100 311 deposited on GaAs (100) (or (311)B) in the temperature range from 450°C to 530°C the RHEED patterns changed from streaky to spotty patterns, indicating a transition from two dimensional growth mode to three dimensional growth mode and the formation of QDs. Fig. 1a and b shows the AFM images of 2ML of InAs on GaAs (100) and 100 4ML of InAs on GaAs (311)B grown at 500°C. 311 QD structures were observed. The average density decreases and the average size increases monotonically with increasing ¹ . Fig. 2 shows the average 4 density, diameter and height as a function of the inverse of ¹ for QDs with 2ML of InAs grown 4 100 on GaAs (100). The absence of a single straight line fit over the studied temperature range for QD density indicates that more than one kinetic process is likely competing to control the nucleation of QDs. On the other hand, the existence of a single straight line fit for QD diameter and height reveals that the diameter and the height of the QDs is probably controlled by a single kinetic process. Similar dependence was also observed for the QDs grown on GaAs (311)B. At high ¹ , however, the change was 4 much slower compared to the QDs grown on GaAs (100). The average density of QDs grown at 530°C is 1.2]1010 cm~2 with the average diameter of 45 nm and the average height of 7.5 nm. The higher density of QDs grown on GaAs (311)B at high ¹ (almost one order of magnitude higher than that 4 of QDs grown on GaAs (100) under the same growth conditions) might be due to the limited migration length of In adatoms on the (311)B surface. When 6ML of InAs was deposited on GaAs 211 (211)B at lower ¹ (from 400°C to 470°C), the 4 RHEED pattern changed from streaky to spotty, revealing the formation of QDs. When the same amount of InAs was deposited at higher ¹ (500°C 4 or 510°C), the RHEED pattern along the [11 11] azimuth was almost the same as that grown at lower ¹ , but there was a drastic change of the 4 RHEED pattern along the [0111 ] azimuth; it changed from streaky to spotty patterns right after deposition of 6ML of InAs just as ¹ "450°C, 211 4 and then the spotty pattern transformed to
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S.P. Guo et al. / Physica E 2 (1998) 672—677
Fig. 2. The growth temperature dependence of the average density, the average diameter and the average height of QDs with 2ML of InAs grown on GaAs (100). 100
Fig. 1. AFM images of (a) 2ML of InAs deposited on GaAs 100 (100) at 500°C (1]1 lm2), (b) 4ML of InAs deposited on 311 GaAs (311)B at 500°C (1]1 lm2) and (c) 6ML of InAs 211 deposited on GaAs (211) at 500°C (5]5 lm2).
streaky pattern after several seconds of growth interruption, indicating the formation of a new structure. QDs with bimodal dot size distribution were observed by AFM measurements when 6ML of 211 InAs was deposited at ¹ ranging from 400°C to 4 470°C. The average dot density, the average lateral
size and the average height of small dots and large dots for QDs grown at 450°C are 1.9]1010 cm~2 and 1.6]109 cm~2, 13 and 47 nm, 3 and 10 nm, respectively. The average dot size increases and the average density decreases monotonically with increasing ¹ . This behavior is similar to that of QDs 4 grown on GaAs (100) and (311)B orientation. When 6ML of InAs was deposited at 500°C or 510°C, 211 however, a drastic change of InAs nanostructure from QDs to QDHs was observed. The QDHs were formed with no trace of the existence of small QDs. Fig. 1c shows the AFM image of 6ML of InAs grown on GaAs (211)B at 500°C. 211 The QDHs have asymmetric hut-like shape and self-aligned along [0111 ] direction. The average density of QDHs is 2.8]108 cm~2 with the average size of 163 nm]65 nm and 20 nm in height. The observation of the time evolution of RHEED images of InAs deposited on (211)B plane
S.P. Guo et al. / Physica E 2 (1998) 672—677
at higher ¹ suggests that an intermediate state of 4 QDs exists before the formation of the QDHs. RHEED observation indicated that when InAs was deposited on GaAs, InAs wetting layer was formed at first, and then the QDs developed on the wetting layer for further deposition of InAs. At lower ¹ the 4 QD structure is stable most probably due to the limited migration length of In adatoms. At higher ¹ , however, due to thermally enhanced In surface 4 migration, the QDs are metastable and coalesce each other, forming the QDH structure. The hutlike shape of the QDHs is believed to be related to the stress anisotropy on the (211)B plane and the metastability of the intermediate state of QDs [9]. Fig. 3 shows the low temperature (4.5 K) PL spectra of the QDs with 2ML of InAs grown on 100 GaAs (100) at various ¹ . When ¹ increased from 4 4 490°C to 510°C the peak position of QDs shifted to low energy, whereas the QD peak shifted to high energy when ¹ was 530°C. The red-shift of peak 4 energy is believed to be due to the effect of the quantum confinement which is related to the dot size. The unusual blue-shift of peak energy for QDs grown at 530°C might be due to the emission related to the excited electron states in big QDs. The blue-shift behavior was also observed in the sample with 4ML of InAs grown at 500°C (AFM meas100 urement showed the existence of the big QDs with the diameter of 62 nm and the height of 20 nm in such a structure). The other possible explanation of the blue-shift is the thermally enhanced intermixing of In and Ga atoms at the QD interface. The multi-peaks in PL spectra were attributed to the transitions from the ground level to the ground and the excited levels in QDs and the weak peak around 1.41 eV was attributed to the emission from the InAs wetting layer. The temperature dependence of PL spectra of InAs QDs and QDHs grown on GaAs (211)B is shown in Fig. 4. We attribute the two PL peaks in spectra of QDs (around 1.24 and 1.4 eV at low ¹ ) . to the bimodal size distribution in the QDs. The low PL efficiency of QDs is believed to be related to the poor quality of GaAs caplayer grown at low temperature. The PL peak intensity and position of QDs and QDHs showed similar temperature dependence: at low ¹ the peak intensity as well as .
675
Fig. 3. Low temperature PL spectra of QDs with 2ML of 100 InAs grown on GaAs (100) at various ¹ . 4
the peak position was almost independent of the ¹ , showing that the exciton is localized spatially in . a certain potential minimum. On the other hand, the peak intensity decreased drastically and peak position shifted to low energy monotonically with the increase of ¹ above a certain transition tem. perature (¹ ). This behavior is most probably due to 5 the dissociation of excitons into the electron-hole pairs which then escape from the QDs/QDHs. ¹ was 52 K for QDHs and 60 K for QDs. The 5 lower ¹ for the QDH structure might be due to the 5 weaker quantum confinement effect. The excitation density dependence of PL spectra of QDs and QDHs has also been studied (not shown). The PL intensity of QDs exhibited a linear behavior with respect to the excitation density and no shift in the peak position above experimental error was observed, whereas the PL intensity of QDHs exhibited a nonlinear behavior and the peak shifted to high energy by increasing the excitation density. The nonlinear dependence and the blue-shift of peak energy in QDH structures suggest that the mechanism of the radiative recombination in QDHs is different from that in QDs. One possible
676
S.P. Guo et al. / Physica E 2 (1998) 672—677
Fig. 4. Temperature dependence of PL spectra of InAs nanostructures with 6ML of InAs grown on GaAs (211)B, (a) QDs grown at 211 400°C and (b) QDHs grown at 510°C.
explanation is the existence of piezo-electric field in QDHs. The temperature dependence of PL spectra as well as the excitation density dependence of PL spectra of InAs QDs grown on GaAs (100) and (311)B has also been investigated (figures not shown here). The behavior is similar to that of the QDs grown on GaAs (211)B at lower ¹ and is in 4 agreement with the results reported by other groups [10,11].
4. Conclusions InAs self-organized nanostructures grown on GaAs (100), (211)B and (311)B substrates by MBE at different ¹ have been investigated. QDs were 4 observed after deposition of 2ML (or 4ML ) 100 311 of InAs on GaAs (100) (or (311)B) at ¹ ranging 4 from 450°C to 530°C. The average density decreases and the average size increases monotonically with increasing ¹ . When 6ML of InAs was 4 211 deposited on GaAs (211)B at lower ¹ ranging from 4 400 to 470°C, QDs with bimodal size distribution were observed. When the same InAs was deposited at higher ¹ (500°C or 510°C), however, QDHs 4
were formed with no trace of small QDs. The Photoluminescence intensity of the QDs and the QDHs showed similar temperature dependence; at low temperatures the intensity was independent of the ¹ , whereas the intensity decreased rapidly . with increase of temperature above ¹ . The excita5 tion density dependence of the QDs and QDHs, however, was quite different. The PL intensity of QDs exhibited a linear behavior with the increase of excitation density and the peak position is almost invariable, whereas the intensity of QDHs exhibited a nonlinear behavior and the peak shifted to high energy by increasing the excitation density.
Acknowledgements The authors thank Professor S. Kawakami for the use of AFM and Dr. F. Matsukura for helpful discussions. Part of this work was supported by Research for the Future Program from the Japan Society for the Promotion of Science (JSPS-RFT F97 00202) and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (09244103).
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