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Formation and characterization of coupled quantum dots (CQDs) by selective area metalorganic vapor phase epitaxy
......... CRYSTAL
GROWTH
ELSEVIER
Journal of Crystal Growth 170 (1997) 700-704
BSPA submission
Formation and characterization of coupled quantum dots (CQDs) by selective area metalorganic vapor phase epitaxy Kazuhide Kumakura *, Junichi Motohisa, Takashi Fukui Research Center for Interface Quantum Electronics (RCIQE), Hokkaido UniversiO,, N13 W8, Sapporo 060, Japan
Abstract
We have demonstrated novel GaAs quantum dot arrays coupled to quantum wire networks, that is, coupled quantum dots (CQDs) formed by selective area metalorganic vapor phase epitaxy (SA-MOVPE). First, GaAs buffer layers are grown on GaAs(001) substrates with SiN~ square masks in 400 nm periodicity to [100] and [010] directions. GaAs cross-wire structures with pyramids at the corners are obtained. Next, GaAs/A1GaAs quantum wells are overgrown on top of these structures. Quantum dots (QDs) and quantum wires (QWRs) are formed at the top portions of the pyramids and at the ridges of wires, respectively. The cathodoluminescence (CL) image shows strong emission from the top portions of the pyramids, which suggests that high-quality CQD structures are formed by SA-MOVPE.
1. I n t r o d u c t i o n
Semiconductor quantum dots (QDs) have attracted much attention for new optical device applications, such as quantum dot lasers. Many researchers have reported GaAs and In(Ga)As QD structures fabricated using self-organized growth [13] and selective area metalorganic vapor phase epitaxy (SA-MOVPE) [4-6]. High density In(Ga)As QDs were automatically formed on GaAs by the strain effects, and QD lasers were reported using this self-organized growth mode [1,2]. However, it is very hard to control the dot sizes and positions on the substrates. SA-MOVPE is one of the useful techniques for fabrication of QDs, and has been applied to the fabrication of optical integrated circuits [7]. Using * Corresponding author. Fax: + 81 I I 716 6004.
this selective growth, uniform QD structures can be formed by adjusting the growth conditions as well as substrate orientation and mask pattern, for example, GaAs(001) [5,6,8] and GaAs(ll I)B [4] substrates. Thus, SA-MOVPE is the promising formation method for future optical devices and optoelectronic integrated circuits (OEIC). On the other hand, the application of QDs to electron transport devices, especially single electron devices, is also a very important research field for future electron devices. For fabrication of these devices, these well-arranged QDs have to be connected to each other. In this paper, we have proposed and demonstrated novel CQDs connected to QWRs by SA-MOVPE for the first time. An idealized CQD structure is shown in Fig. la. Firstly, growth behavior of GaAs on SiN, square masked substrates was investigated by scanning electron microscope (SEM) observation. On the basis of these structures, GaAs CQDs were formed
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 6 4 1-0
K. Kumakura et al. / Journal of C~'stal Growth 170 (1997) 700-704
(a)
41bQuantum Dot ~
Quantum Wire
cet
(b) 2oo (001 iGaAs sub.-'7"-----L/"[010]~-~ [100]
(c)
Fig. 1. Schematicviews of (a) idealized CQD structures connected with cross quantum wires, (b) SiNx maskpattern for selective area MOVPE, and (c) SEM image of A1GaAsgrowth on a masked substrate consisting of pyramids and cross wires.
701
underneath pyramids and wire growth, and 5.0 X 10 .4 atm for CQDs formation, respectively. Growth rates of GaAs and A10.35Ga0.65As were 0.5 and 0.8 /~m/h for planar substrates, respectively. The growth temperature was 750°C. The substrates were semi-insulating GaAs(001) coated with 40 nm thick SiN x layers. The mask pattern for SA-MOVPE is schematically shown in Fig. lb. SiN x square mask patterns, 200 nm × 200 nm, were aligned in the [100] and [010] directions, and were periodically defined with a pitch of 400 nm in the [100] and [010] directions formed by electron beam lithography and wet chemical etching. First, thick GaAs and A1GaAs multi-layered structures were grown on the masked substrates to clarify the growth behavior of CQDs. Cleaved cross sections of the epitaxial wafers were observed by SEM. Next, GaAs/A1GaAs quantum wells (QWs) were grown on GaAs buffer structures with pyramids and wires to form CQDs. The temperature dependence of CL spectra of CQDs was measured from 4 to 200 K, and spatially resolved CL images were also observed at 4 K. The acceleration voltage of the electron beam was 20 kV, and the beam current was 300 pA. The excitation area was 2 /~m X 2 /~m, which corresponds to about 25 CQDs.
3. Results and discussion 3.1. G r o w t h behaL, ior on m a s k e d s u b s t r a t e s
and buried by A1GaAs. Clear CL peak with high luminescence efficiency was observed from QDs.
2. Experimental procedure Selective area MOVPE growth was carried out using a low-pressure horizontal, RF-heated, quartz MOVPE reactor system. The working pressure of 76 Torr was automatically controlled. Purified hydrogen (H 2) was used as a carrier gas. The source materials were trimethylgallium (TMGa), triethylaluminum (TEAl) and 20% arsine (AsH 3) in H 2. The partial pressures of TMGa and TEA1 were kept constant at 1.9 × l0 6 and 6.7 × 10 - 7 atm, respectively. The partial pressures of AsH 3 were 6.7 X 10 -5 atm for
Fig. lc shows an SEM image of AIGaAs selective area growth on the masked substrate. Highly uniform pyramids connected with wire structures were observed. The sidewalls of these structures were {111}A and {lll}B facets at the bottom of the pyramids, {114}A and {114}B facets at the top of the pyramids, and {011} facets for wires. Similar SEM images were observed for both GaAs and GaAs/A1GaAs multilayer growth. Fig. 2a shows a cleaved cross-sectional SEM image of the A1GaAs/GaAs multi-layered structure at the pyramid part, and Fig. 2b shows a schematic illustration. The growth thicknesses of the GaAs and A1GaAs layers were about 8.4 and 25 nm on the planar substrate, respectively. In the SEM image, dark parts are GaAs and bright parts are A1GaAs,
702
K. Kumakura et a l . / Journal of Coastal Growth 170 (1997) 700-704
(a)
be realized under lower growth temperature a n d / o r higher AsH 3 partial pressure growth conditions, because of a high growth rate on the {011} plane [9].
3.2. Formation of CQDs and CL obseruations
(b)
Fig. 2. SEM image of (a) cross section of the pyramid part, and (b) schematic illustration for A1GaAs/GaAs growth on the masked substrate.
respectively. For the pyramid parts, {lll}A and {lll}B facet sidewalls preferentially appear at the bottom of the pyramids for both GaAs and AIGaAs growth, while {113} and {114} facets appear at the top portion of the pyramids for GaAs, and AlGaAs growth, respectively. For the wire parts connected to the pyramids, {011} facets appear as the sidewalls of the trapezoids for both GaAs and A1GaAs growth. After the completion of the pyramid and triangular shape wire formations, the shape of the top portion of the pyramids and the wires remains unchanged, although the growth proceeds on each sidewall. Therefore, uniform GaAs QDs surrounded by {113} and {114} facets can be formed at the top portion of the A1GaAs pyramids. GaAs ridge QWRs connected to GaAs QDs can also be formed at the ridge of AIGaAs triangular wire structures. This indicates that CQD structures composed of QD arrays directly connected to QWR networks, can be realized using this technique. From cleaved cross-sectional SEM observation, the thickness of GaAs on the pyramids is 1.8-2.1 times thicker than that on {111} planes. On the other hand, the growth thickness of GaAs on {011} sidewalls of QWRs is much thinner under this growth condition. QWRs with enough thickness can
In order to obtain GaAs CQD structures, first thin GaAs layers were grown on SiN, masked substrates as buffer layers. Next, A10.35Ga065As was grown to form the pyramid and wire structures. Then, AIG a A s / G a A s QWs were grown on these AIGaAs pyramid and wire structures, The thicknesses of GaAs QWs were 2.8 nm for planar substrates. A10.35Ga0.65As lower barrier layers have {111} and {114} facets on the pyramids, and {011} facets on the wire structures. For GaAs growth, {113} facets appear on the top portion of the pyramids because the growth rates of the {113} planes are slower than those of other low-index surfaces at the top portion of the pyramids. A1GaAs upper barrier layers and GaAs cap layers were grown under higher AsH 3 partial pressure conditions of 5.0 X 10 4 atm to bury QDs and QWRs. Fig. 3 shows CL spectra of CQDs at 4 to 200 K. The insert shows a schematic view of a QD on the pyramid. About 25 CQDs at 2 ~ m X 2 /zm area
k
________1
J
J
~
4K
CQDs S-QW i
1.5
1.6
~
i
1.7
1.8
Photon Energy [eV] Fig. 3. CL spectra of CQDs. Measured temperatures are from 4 to 200 K.
703
K. Kumakura et al./ Journal of Crystal Growth 170 (1997) 700-704
were excited by a 20 kV electron beam with a 300 pA current. A peak and a shoulder are seen at 1.62 and 1.67 eV, respectively. In order to identify the origins of this luminescence peak and shoulder, spatially resolved CL images were observed. Fig. 4a and Fig. 4b show CL images of CQDs at 4 K at the photon energies of (a) 1.62 eV and (b) 1.67 eV, respectively. It is difficult to identify their origins from CL images, although these two emissions come from pyramid areas. However, we can estimate from detailed observation of growth behavior by SEM that the origins of the peak at 1.62 eV comes from QDs on the top portion of the pyramids surrounded by {113} and {114} facets, and that at 1.67 eV comes from sidewall QWs on {111} planes of the pyramids. For this sample, no luminescence was observed from QWRs because the thickness of GaAs on triangular wire structures was estimated to less than 1 nm from the growth rate on {011} sidewalls. Next, we estimate the size of QDs on the top portion of the pyramids and sidewall QWs on {111} planes of the pyramids from CL measurement and SEM observation. The shoulder at 1.67 eV from
(a)
(b)
4~1U3
I
. . . .
I
. . . .
~
. . . .
o CQDs
"-2.
•
~'~ 102
SidewallQWs
~Oooo o o
"E
101
.d
100
~eoe
e
0
o
•
1N-1
. . . .
0
~
50
. . . .
I
. . . .
1O0
~
I
150
,
I
,
200
Temperature [K] Fig. 5. Temperature dependence of CL peak intensities for CQDs on the top portion of the pyramids(O), and sidewall QWs on the {111} planes of the pyramids(0).
sidewall QWs corresponds to a well thickness of about 3.5 nm for a normal GaAs QW on a planar substrate. Therefore, the GaAs well thickness of sidewall QWs on {111} planes of the pyramid is estimated to be about 3.5 nm, because there is no lateral confinement effect for sidewall QWs. Then, the peak at 1.62 eV for QDs on the top portion of the pyramids corresponds to that from GaAs QW with a well thickness of 5 nm. From the shape of the QDs, the quantum confinement effect of QDs (blue shift of 100 meV) mainly comes from the vertical confinement. Therefore, the effective thickness of QDs is a little less than 5 nm. To investigate the quality of CQDs, the temperature dependence of the CL peak intensities for QDs on the top portion of the pyramids and sidewall QWs on the {111} planes is shown in Fig. 5. The slight reduction of these two CL intensities indicate that high-quality CQD structures were realized.
4. Conclusion
500nm Fig. 4. Spatially resolved CL images of CQDs at 4 K at the photon energies of (a) 1.62 and (b) 1.57 eV, respectively.
We demonstrated novel GaAs coupled quantum dots formed by SA-MOVPE. First, GaAs buffer layers were grown on GaAs(001) substrates with SiN, square masks in 400 nm periodicity to the [100] and [010] directions. The cross-wire structures with pyramids at the comers were obtained. Next, GaAs/A1GaAs QWs were overgrown on top of
704
K. Kumakura et al. / Journal of Crystal Growth 170 (1997) 700-704
these structures. QDs and QWRs were formed at the top portions of the pyramids and at the ridges of the wires. CL spectra were observed and their origins were identified by spatially resolved CL images. Two emissions came from QDs on the top portion of the pyramids and QWs on the {111} facet sidewalls of the pyramids. Temperature dependence of the CL peak intensity suggests that high-quality CQD structures were formed. Using these CQDs formed by SA-MOVPE, novel single electron devices and electron wave interference devices as well as high density matrix of QD memory or QD network can be realized by further optimization of crystal growth conditions for narrower periodicity of CQDs.
Acknowledgements The authors wish to thank Professor H. Hasegawa for fruitful discussions and encouragement, and greatly acknowledge S. Hara, K. Nakakoshi, M. Sakuma and T. Umeda for technical assistance in
MOVPE growth. One of the authors (K.K.) would like to thank the Japan Society for the Promotion of Science for the partial financial support.
References [1] J. Temmyo, E. Kuramochi, M. Sugo, T. Nishiya, R. N~Atzel and T. Tamamura, Electron. Lett. 31 (1995) 209. [2] D. Bimberg, N.N. Ledentsov, M. Grundmann, N. Kirstaedter, O.G. Schmidt, M.H. Mao, V.M. Ustinov, A.Yu. Egorov, A.E. Zhukov, P.S. Kopev, Zh.I. Alferov, S.S. Ruvimov, U. Gtisele and J. Heydenreich, Jpn. J. Appl. Phys. 35 (1996) 1311. [3] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars and P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203. [4] T. Fukui, S. Ando, Y. Tokura and T. Toriyama, Appl. Phys. Lett. 58 (1991) 2018. [5] Y. Nagamune, S. Tsukamoto, M. Nishioka and Y. Arakawa, J. Crystal Growth 126 (1993) 707. [6] K. Kumakura, K. Nakakoshi, J. Motohisa, T. Fukui and H. Hasegawa, Jpn. J. Appl. Phys. 34 (1995) 4387. [7] M. Aoki, H. Sano, M. Suzuki, M. Takahashi, K. Uomi and A. Takai, Electron. Lett. 27 (1991) 2138. [8] J.A. Lebens, C.S. Tsai, K.J. Vahala and T.F. Kuech, Appl. Phys. Lett. 56 (1990) 2642. [9] T. Fukui, S. Ando and Y.K. Fukai, Appl. Phys. Lett. 57 (1990) 1209.
GROWTH
ELSEVIER
Journal of Crystal Growth 170 (1997) 700-704
BSPA submission
Formation and characterization of coupled quantum dots (CQDs) by selective area metalorganic vapor phase epitaxy Kazuhide Kumakura *, Junichi Motohisa, Takashi Fukui Research Center for Interface Quantum Electronics (RCIQE), Hokkaido UniversiO,, N13 W8, Sapporo 060, Japan
Abstract
We have demonstrated novel GaAs quantum dot arrays coupled to quantum wire networks, that is, coupled quantum dots (CQDs) formed by selective area metalorganic vapor phase epitaxy (SA-MOVPE). First, GaAs buffer layers are grown on GaAs(001) substrates with SiN~ square masks in 400 nm periodicity to [100] and [010] directions. GaAs cross-wire structures with pyramids at the corners are obtained. Next, GaAs/A1GaAs quantum wells are overgrown on top of these structures. Quantum dots (QDs) and quantum wires (QWRs) are formed at the top portions of the pyramids and at the ridges of wires, respectively. The cathodoluminescence (CL) image shows strong emission from the top portions of the pyramids, which suggests that high-quality CQD structures are formed by SA-MOVPE.
1. I n t r o d u c t i o n
Semiconductor quantum dots (QDs) have attracted much attention for new optical device applications, such as quantum dot lasers. Many researchers have reported GaAs and In(Ga)As QD structures fabricated using self-organized growth [13] and selective area metalorganic vapor phase epitaxy (SA-MOVPE) [4-6]. High density In(Ga)As QDs were automatically formed on GaAs by the strain effects, and QD lasers were reported using this self-organized growth mode [1,2]. However, it is very hard to control the dot sizes and positions on the substrates. SA-MOVPE is one of the useful techniques for fabrication of QDs, and has been applied to the fabrication of optical integrated circuits [7]. Using * Corresponding author. Fax: + 81 I I 716 6004.
this selective growth, uniform QD structures can be formed by adjusting the growth conditions as well as substrate orientation and mask pattern, for example, GaAs(001) [5,6,8] and GaAs(ll I)B [4] substrates. Thus, SA-MOVPE is the promising formation method for future optical devices and optoelectronic integrated circuits (OEIC). On the other hand, the application of QDs to electron transport devices, especially single electron devices, is also a very important research field for future electron devices. For fabrication of these devices, these well-arranged QDs have to be connected to each other. In this paper, we have proposed and demonstrated novel CQDs connected to QWRs by SA-MOVPE for the first time. An idealized CQD structure is shown in Fig. la. Firstly, growth behavior of GaAs on SiN, square masked substrates was investigated by scanning electron microscope (SEM) observation. On the basis of these structures, GaAs CQDs were formed
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 6 4 1-0
K. Kumakura et al. / Journal of C~'stal Growth 170 (1997) 700-704
(a)
41bQuantum Dot ~
Quantum Wire
cet
(b) 2oo (001 iGaAs sub.-'7"-----L/"[010]~-~ [100]
(c)
Fig. 1. Schematicviews of (a) idealized CQD structures connected with cross quantum wires, (b) SiNx maskpattern for selective area MOVPE, and (c) SEM image of A1GaAsgrowth on a masked substrate consisting of pyramids and cross wires.
701
underneath pyramids and wire growth, and 5.0 X 10 .4 atm for CQDs formation, respectively. Growth rates of GaAs and A10.35Ga0.65As were 0.5 and 0.8 /~m/h for planar substrates, respectively. The growth temperature was 750°C. The substrates were semi-insulating GaAs(001) coated with 40 nm thick SiN x layers. The mask pattern for SA-MOVPE is schematically shown in Fig. lb. SiN x square mask patterns, 200 nm × 200 nm, were aligned in the [100] and [010] directions, and were periodically defined with a pitch of 400 nm in the [100] and [010] directions formed by electron beam lithography and wet chemical etching. First, thick GaAs and A1GaAs multi-layered structures were grown on the masked substrates to clarify the growth behavior of CQDs. Cleaved cross sections of the epitaxial wafers were observed by SEM. Next, GaAs/A1GaAs quantum wells (QWs) were grown on GaAs buffer structures with pyramids and wires to form CQDs. The temperature dependence of CL spectra of CQDs was measured from 4 to 200 K, and spatially resolved CL images were also observed at 4 K. The acceleration voltage of the electron beam was 20 kV, and the beam current was 300 pA. The excitation area was 2 /~m X 2 /~m, which corresponds to about 25 CQDs.
3. Results and discussion 3.1. G r o w t h behaL, ior on m a s k e d s u b s t r a t e s
and buried by A1GaAs. Clear CL peak with high luminescence efficiency was observed from QDs.
2. Experimental procedure Selective area MOVPE growth was carried out using a low-pressure horizontal, RF-heated, quartz MOVPE reactor system. The working pressure of 76 Torr was automatically controlled. Purified hydrogen (H 2) was used as a carrier gas. The source materials were trimethylgallium (TMGa), triethylaluminum (TEAl) and 20% arsine (AsH 3) in H 2. The partial pressures of TMGa and TEA1 were kept constant at 1.9 × l0 6 and 6.7 × 10 - 7 atm, respectively. The partial pressures of AsH 3 were 6.7 X 10 -5 atm for
Fig. lc shows an SEM image of AIGaAs selective area growth on the masked substrate. Highly uniform pyramids connected with wire structures were observed. The sidewalls of these structures were {111}A and {lll}B facets at the bottom of the pyramids, {114}A and {114}B facets at the top of the pyramids, and {011} facets for wires. Similar SEM images were observed for both GaAs and GaAs/A1GaAs multilayer growth. Fig. 2a shows a cleaved cross-sectional SEM image of the A1GaAs/GaAs multi-layered structure at the pyramid part, and Fig. 2b shows a schematic illustration. The growth thicknesses of the GaAs and A1GaAs layers were about 8.4 and 25 nm on the planar substrate, respectively. In the SEM image, dark parts are GaAs and bright parts are A1GaAs,
702
K. Kumakura et a l . / Journal of Coastal Growth 170 (1997) 700-704
(a)
be realized under lower growth temperature a n d / o r higher AsH 3 partial pressure growth conditions, because of a high growth rate on the {011} plane [9].
3.2. Formation of CQDs and CL obseruations
(b)
Fig. 2. SEM image of (a) cross section of the pyramid part, and (b) schematic illustration for A1GaAs/GaAs growth on the masked substrate.
respectively. For the pyramid parts, {lll}A and {lll}B facet sidewalls preferentially appear at the bottom of the pyramids for both GaAs and AIGaAs growth, while {113} and {114} facets appear at the top portion of the pyramids for GaAs, and AlGaAs growth, respectively. For the wire parts connected to the pyramids, {011} facets appear as the sidewalls of the trapezoids for both GaAs and A1GaAs growth. After the completion of the pyramid and triangular shape wire formations, the shape of the top portion of the pyramids and the wires remains unchanged, although the growth proceeds on each sidewall. Therefore, uniform GaAs QDs surrounded by {113} and {114} facets can be formed at the top portion of the A1GaAs pyramids. GaAs ridge QWRs connected to GaAs QDs can also be formed at the ridge of AIGaAs triangular wire structures. This indicates that CQD structures composed of QD arrays directly connected to QWR networks, can be realized using this technique. From cleaved cross-sectional SEM observation, the thickness of GaAs on the pyramids is 1.8-2.1 times thicker than that on {111} planes. On the other hand, the growth thickness of GaAs on {011} sidewalls of QWRs is much thinner under this growth condition. QWRs with enough thickness can
In order to obtain GaAs CQD structures, first thin GaAs layers were grown on SiN, masked substrates as buffer layers. Next, A10.35Ga065As was grown to form the pyramid and wire structures. Then, AIG a A s / G a A s QWs were grown on these AIGaAs pyramid and wire structures, The thicknesses of GaAs QWs were 2.8 nm for planar substrates. A10.35Ga0.65As lower barrier layers have {111} and {114} facets on the pyramids, and {011} facets on the wire structures. For GaAs growth, {113} facets appear on the top portion of the pyramids because the growth rates of the {113} planes are slower than those of other low-index surfaces at the top portion of the pyramids. A1GaAs upper barrier layers and GaAs cap layers were grown under higher AsH 3 partial pressure conditions of 5.0 X 10 4 atm to bury QDs and QWRs. Fig. 3 shows CL spectra of CQDs at 4 to 200 K. The insert shows a schematic view of a QD on the pyramid. About 25 CQDs at 2 ~ m X 2 /zm area
k
________1
J
J
~
4K
CQDs S-QW i
1.5
1.6
~
i
1.7
1.8
Photon Energy [eV] Fig. 3. CL spectra of CQDs. Measured temperatures are from 4 to 200 K.
703
K. Kumakura et al./ Journal of Crystal Growth 170 (1997) 700-704
were excited by a 20 kV electron beam with a 300 pA current. A peak and a shoulder are seen at 1.62 and 1.67 eV, respectively. In order to identify the origins of this luminescence peak and shoulder, spatially resolved CL images were observed. Fig. 4a and Fig. 4b show CL images of CQDs at 4 K at the photon energies of (a) 1.62 eV and (b) 1.67 eV, respectively. It is difficult to identify their origins from CL images, although these two emissions come from pyramid areas. However, we can estimate from detailed observation of growth behavior by SEM that the origins of the peak at 1.62 eV comes from QDs on the top portion of the pyramids surrounded by {113} and {114} facets, and that at 1.67 eV comes from sidewall QWs on {111} planes of the pyramids. For this sample, no luminescence was observed from QWRs because the thickness of GaAs on triangular wire structures was estimated to less than 1 nm from the growth rate on {011} sidewalls. Next, we estimate the size of QDs on the top portion of the pyramids and sidewall QWs on {111} planes of the pyramids from CL measurement and SEM observation. The shoulder at 1.67 eV from
(a)
(b)
4~1U3
I
. . . .
I
. . . .
~
. . . .
o CQDs
"-2.
•
~'~ 102
SidewallQWs
~Oooo o o
"E
101
.d
100
~eoe
e
0
o
•
1N-1
. . . .
0
~
50
. . . .
I
. . . .
1O0
~
I
150
,
I
,
200
Temperature [K] Fig. 5. Temperature dependence of CL peak intensities for CQDs on the top portion of the pyramids(O), and sidewall QWs on the {111} planes of the pyramids(0).
sidewall QWs corresponds to a well thickness of about 3.5 nm for a normal GaAs QW on a planar substrate. Therefore, the GaAs well thickness of sidewall QWs on {111} planes of the pyramid is estimated to be about 3.5 nm, because there is no lateral confinement effect for sidewall QWs. Then, the peak at 1.62 eV for QDs on the top portion of the pyramids corresponds to that from GaAs QW with a well thickness of 5 nm. From the shape of the QDs, the quantum confinement effect of QDs (blue shift of 100 meV) mainly comes from the vertical confinement. Therefore, the effective thickness of QDs is a little less than 5 nm. To investigate the quality of CQDs, the temperature dependence of the CL peak intensities for QDs on the top portion of the pyramids and sidewall QWs on the {111} planes is shown in Fig. 5. The slight reduction of these two CL intensities indicate that high-quality CQD structures were realized.
4. Conclusion
500nm Fig. 4. Spatially resolved CL images of CQDs at 4 K at the photon energies of (a) 1.62 and (b) 1.57 eV, respectively.
We demonstrated novel GaAs coupled quantum dots formed by SA-MOVPE. First, GaAs buffer layers were grown on GaAs(001) substrates with SiN, square masks in 400 nm periodicity to the [100] and [010] directions. The cross-wire structures with pyramids at the comers were obtained. Next, GaAs/A1GaAs QWs were overgrown on top of
704
K. Kumakura et al. / Journal of Crystal Growth 170 (1997) 700-704
these structures. QDs and QWRs were formed at the top portions of the pyramids and at the ridges of the wires. CL spectra were observed and their origins were identified by spatially resolved CL images. Two emissions came from QDs on the top portion of the pyramids and QWs on the {111} facet sidewalls of the pyramids. Temperature dependence of the CL peak intensity suggests that high-quality CQD structures were formed. Using these CQDs formed by SA-MOVPE, novel single electron devices and electron wave interference devices as well as high density matrix of QD memory or QD network can be realized by further optimization of crystal growth conditions for narrower periodicity of CQDs.
Acknowledgements The authors wish to thank Professor H. Hasegawa for fruitful discussions and encouragement, and greatly acknowledge S. Hara, K. Nakakoshi, M. Sakuma and T. Umeda for technical assistance in
MOVPE growth. One of the authors (K.K.) would like to thank the Japan Society for the Promotion of Science for the partial financial support.
References [1] J. Temmyo, E. Kuramochi, M. Sugo, T. Nishiya, R. N~Atzel and T. Tamamura, Electron. Lett. 31 (1995) 209. [2] D. Bimberg, N.N. Ledentsov, M. Grundmann, N. Kirstaedter, O.G. Schmidt, M.H. Mao, V.M. Ustinov, A.Yu. Egorov, A.E. Zhukov, P.S. Kopev, Zh.I. Alferov, S.S. Ruvimov, U. Gtisele and J. Heydenreich, Jpn. J. Appl. Phys. 35 (1996) 1311. [3] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars and P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203. [4] T. Fukui, S. Ando, Y. Tokura and T. Toriyama, Appl. Phys. Lett. 58 (1991) 2018. [5] Y. Nagamune, S. Tsukamoto, M. Nishioka and Y. Arakawa, J. Crystal Growth 126 (1993) 707. [6] K. Kumakura, K. Nakakoshi, J. Motohisa, T. Fukui and H. Hasegawa, Jpn. J. Appl. Phys. 34 (1995) 4387. [7] M. Aoki, H. Sano, M. Suzuki, M. Takahashi, K. Uomi and A. Takai, Electron. Lett. 27 (1991) 2138. [8] J.A. Lebens, C.S. Tsai, K.J. Vahala and T.F. Kuech, Appl. Phys. Lett. 56 (1990) 2642. [9] T. Fukui, S. Ando and Y.K. Fukai, Appl. Phys. Lett. 57 (1990) 1209.