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Semiconductor quantum dots for application in diode lasers
Thin Solid Films 318 Ž1998. 83–87
Semiconductor quantum dots for application in diode lasers M. Grundmann a,) , N.N. Ledentsov a , N. Kirstaedter a , F. Heinrichsdorff a , A. Krost a , D. Bimberg a , A.O. Kosogov b, S.S. Ruvimov b, P. Werner b, V.M. Ustinov c , P.S. Kop’ev c , Zh.I. Alferov c a
Institut fur Technische UniÕersitat ¨ Festkorperphysik, ¨ ¨ Berlin, D-10623 Berlin, Germany Max-Planck-Institut fur ¨ Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany c A.F. Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russian Federation
b
Abstract Recent progress in the epitaxy Žmolecular beam epitaxy and metal-organic chemical vapor deposition. of strained heterostructures and the use of the Stranski-Krastanow growth mode allows to create spontaneously ordered, defect-free and dense arrays of nano-size islands. Such islands act as electronic quantum dots. In superlattices the islands are ordered in vertical stacks. Using such self-ordered InGaAsrAlGaAs quantum dots we have fabricated diode lasers for which some properties are superior to those of current lasers based on quantum wells. In particular, we have demonstrated low laser threshold current and high temperature stability of the threshold. q 1998 Elsevier Science S.A. Keywords: Quantum dots; Self-organized epitaxy; Diode lasers
1. Introduction Quantum dots ŽQD. represent a novel gain medium in semiconductor diode lasers whose advantages like ultra-low threshold and reduced temperature dependence of the threshold have been predicted theoretically w1,2x in the 1980s. Only recently, the potential of QD could be assessed experimentally w3–5x due to the exploitation of layers and stacked layers of nanometer size islands which are self-ordering during Stranski–Krastanov growth w6–8x. In this paper, we review our experimental results on the growth of InŽGa.As self-organized QD structures on GaAs substrate using molecular beam epitaxy ŽMBE. and metal–organic chemical vapor deposition ŽMOCVD. ŽSection 2.. Both epitaxial techniques appear to be similarly suitable for the fabrication of QD diode lasers ŽSection 3.1.. Finally, a new theoretical approach to gain and threshold current in QD lasers is worked out ŽSection 3.2..
We emphasize that the use of QD for novel optoelectronic devices is a quite general concept not limited to the material system discussed here. QD can also be fabricated in other strained III–V heterostructures on GaAs and InP substrates w9x and in III–V nitride systems w10x, GerSi w11x and II–VI compounds w12x.
2. Epitaxy of quantum dots Stranski and Krastanov w13x have conjectured the formation of two-dimensional islands on initially flat layers. The first observation of island formation in III–V compound superlattices was reported in Ref. w14x but ignored until detailed exploration in the beginning and mid-1990s w6– 8,15–17x.
2.1. MOCVD
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 4 4 - 9
Initial attempts to grow InAs on GaAs by MOCVD w18x led to the formation of small QD with rectangular base
84
M. Grundmann et al.r Thin Solid Films 318 (1998) 83–87
shape similar to those grown by MBE Žsee Section 2.2., simultaneous to a large number of indium rich dislocated clusters ŽFig. 1.. In order to avoid the latter, a novel growth interruption scheme was introduced w19x employing switch-off of arsine flow allowing the fabrication of defect-free dense arrays of QD. In Fig. 2, a large area plan-view TEM image with an enlarged detail and a cross-sectional view are shown for a three-fold stack of MOCVD-grown InAs QD with intermediate GaAs barriers. No clusters or other defects appear in the plane view image; the dark stripes are only due to TEM contrast formation Žstrain.. The dot density is 4 = 10 10 cmy1 and the lateral dot size is 16–18 nm. The quantum dots arrange in vertical stacks. This effect is due to the strain imposed on the growth surface by buried QD w19,20x and possibly the surface corrugation present in the current sample. If the barrier thickness is chosen to be 18 nm or larger, the vertical correlation of individual dot positions is lost.
2.2. MBE The self-organizing mechanisms present in MBE grown samples are well documented by us w6,7x. Ordering of dot
size, shape and lateral as well as vertical arrangement are observed and theoretically modeled w20–22x. Multiple deposition of islands with thin barriers leads to the formation of strongly electronically coupled QD w6,7x. Stacks of up to 25 InGaAsrGaAs layers can be grown without structural degradation ŽFig. 3..
3. Quantum dot lasers 3.1. Experimental The first QD laser based on self-organized QD was reported in Refs. w3,4x. It exhibited much better performance than lasers based on etched QD. Our QD lasers now approach or excel the best values reported for quantum film lasers. Huge material gain of f 1.5 = 10 5 cmy1 was reported in Refs. w3,4x. A high T0-value of 350 K up to a temperature of 120 K was reported in Refs. w3,4x. Recently, T0 s 350 K is achieved up to 300 K for a three-fold MBE QD-stack, the internal efficiency amounts to up to 70% w23x. The lowest threshold currents reported are 62 Arcm2 for a MBE-grown four-side cleaved structure w6,7x and 160 Arcm2 for a 1-mm long MOCVD-grown InAsrGaAs
Fig. 1. TEM plan view images of MOCVD grown InGaAsrGaAs QD.
M. Grundmann et al.r Thin Solid Films 318 (1998) 83–87
85
Fig. 2. Plan view and cross-section TEM images of a three-fold stack of InAsrGaAs QD grown by MOCVD.
laser with InGaP cladding layers w19x. We emphasize that lasing at room temperature definitely occurs on the zerodimensional QD electronic states ŽFig. 4.. Up to 100 K, the threshold of that device is found to be only 12.7 Arcm2 , close to the theoretical limit Žsee Section 3.2.. 3.2. Theory The initial modeling approaches w1,2x assumed ideal QD ensembles with infinite barriers and without any size dispersion. Finite size dispersion was considered in Ref. w24x, where also a linear relation g A n q p of the gain g on the carrier density, where nŽ p . is the electron Žhole. carrier density, was found. However, the relation of the gain and
the threshold on the current density has been previously modeled inadequately. Essentially a bimolecular term j A np as for bulk material was used throughout for the recombination current w2,25x. In the framework of our novel stochastic theory of QD population w26x, the impact of the carrier capture mechanism on the gain–current relation was found w27x. No simple general relation exists between the carrier density and the current density for QD. Pure exciton capture Žonly neutral dots exist. leads to a transparency current density given by the monomolecular expression jtr s eND rt , where ND represents the total number of QD. Completely uncorrelated electron and hole capture Žcharged dots exist. leads to jtr s Ž5r8. eND rt ,
86
M. Grundmann et al.r Thin Solid Films 318 (1998) 83–87
Fig. 3. Cross-section TEM image of 25-fold stack of InGaAsrGaAs QD grown by MBE.
both being larger than the value jtr s Ž1r2. eND rt from the bimolecular ansatz ŽFig. 5a.. Assuming reasonable values for the size distribution and total optical loss as
given in the caption the dependence of the threshold current on the area coverage with dots is found ŽFig. 5b.. Values significantly smaller than 10 Arcm2 appear possible when the size homogeneity is improved further.
4. Conclusion Dramatic progress has been made in the fabrication of QD lasers which excel or are close to record values for quantum film based devices. The stochastic theory of QD ensembles and QD lasers has been developed. Very recent accomplishments like operation of a 1-W power QD laser w28x and realization of a QD-VCSEL w29x with performance comparable to the currently best quantum film VCSEL’s let QD appear to have the potential to be the semiconductor laser gain medium of the future.
Fig. 4. Ža. Optical power vs. injection current density at 77 K Žthreshold 12.7 Arcm2 . for MOCVD-grown InAsrGaAsrAlGaAs QD-laser. Žb. Laser emission at room temperature starts monomode on the lower energy side of the QD ground state visible in the photoluminescence spectrum Ž3=InAsrGaAsrAlGaAs QD’s..
Acknowledgements Parts of this work have been funded by Deutsche Forschungsgemeinschaft in the framework of Sfb 296,
M. Grundmann et al.r Thin Solid Films 318 (1998) 83–87
w6x
w7x
w8x
w9x w10x w11x w12x w13x w14x w15x w16x Fig. 5. Ža. Gain of a QD ensemble as a function of injection current for separate Ž eq h, solid line. and simultaneous ŽX, dashed line. capture of electron and holes. For comparison the result from mean field theory ŽMF, dotted line. is shown. The gain is given in units of the maximum gain. A barrier Žwetting layer. which provides an additional recombination channel is included. We have assumed identical recombination time constants t in the barrier and dot and a capture time into the dot of tc st r100. The inset shows the relation between gain and carrier density N Žin units of ND . in the dot ensemble, being identical and linear for all models. Žb. Threshold current for both capture models and MF theory as a function of coverage for a typical dot ensemble and a total loss of a tot s10 cmy1 as a function of area coverage.
w17x w18x w19x w20x
w21x w22x
Volkswagenstiftung and INTAS. One of us ŽNNL. is grateful to Alexander-von-Humboldt Stiftung.
w23x
References
w24x w25x w26x w27x
w1x Y. Arakawa, H. Sakaki, Appl. Phys. Lett. 40 Ž1982. 939. w2x M. Asada, Y. Miyamoto, Y. Suematsu, IEEE J. Quantum Electron. QE-22 Ž1986. 1915. w3x N. Kirstaedter, N.N. Ledentsov, M. Grundmann, D. Bimberg, V.M. Ustinov, S.S. Ruvimov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, U. Richter, P. Werner, U. Gosele, J. Heydenreich, Electron. Lett. 30 ¨ Ž1994. 1416. w4x N. Kirstaedter, O. Schmidt, N.N. Ledentsov, D. Bimberg, V.M. Ustinov, A.Yu. Egorov, A.E. Zhukov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, Appl. Phys. Lett. 69 Ž1996. 1226. w5x D. Bimberg, N. Kirstaedter, N.N. Ledentsov, Zh.I. Alferov, P.S.
w28x
w29x
87
Kop’ev, V.M. Ustinov, IEEE J. Selected Topics in Quantum Electron. Ž1997. Vol. 3, 1. M. Grundmann, N.N. Ledentsov, J. Christen, J. Bohrer, D. Bimberg, ¨ S.S. Ruvimov, P. Werner, U. Richter, U. Gosele, J. Heydenreich, ¨ V.M. Ustinov, A.Yu. Egorov, A.E. Zhukov, P.S. Kop’ev, Zh.I. Alferov, Phys. Stat. Sol B 188 Ž1995. 249. N.N. Ledentsov, V.A. Shchukin, M. Grundmann, N. Kirstaedter, J. Bohrer, O. Schmidt, D. Bimberg, V.M. Ustinov, A.Yu. Egorov, A.E. ¨ Zhukov, P.S. Kop’ev, S.V. Zaitsev, N.Yu. Gordeev, Zh.I. Alferov, A.I. Borovkov, A.O. Kosogov, S.S. Ruvimov, P. Werner, J. Heydenreich, Phys. Rev. B 54 Ž1996. 8743. G.E. Cirlin, G.M. Guryanov, A.O. Golubok, S.Ya. Tipissev, N.N. Ledentsov, P.S. Kop’ev, M. Grundmann, D. Bimberg, Appl. Phys. Lett. 67 Ž1995. 97. A. Kurtenbach, K. Eberl, T. Shirata, Appl. Phys. Lett. 66 Ž1995. 361. Sh. Nakamura, in: M. Scheffler, R. Zimmermann ŽEds.., Proc. ICPS-23, World Scientific, Singapore, 1996, p. 11. D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64 Ž1990. 1943. M. Lowisch, M. Rabe, B. Stegemann, F. Henneberger, M. Grundmann, V. Turck, D. Bimberg, Phys. Rev. B 54 Ž1996. R11074. ¨ I.N. Stranski, L. Krastanov, Sitzungsberichte d. Akad. d. Wissenschaften in Wien, Abt. IIb, Band 146 Ž1937. 797. L. Goldstein, F. Glas, J.Y. Marzin, M.N. Charasse, G. Le Roux, Appl. Phys. Lett. 47 Ž1985. 1099. S. Guha, A. Madhukar, K.C. Rajkumar, Appl. Phys. Lett. 57 Ž1990. 2110. J.M. Moison, F. Houzay, F. Barthe, L. Leprince, E. Andre, O. Vatel, Appl. Phys. Lett. 64 Ž1994. 196. D. Leonard, M. Krishnamurthy, S. Fafard, J.L. Merz, P.M. Petroff, J. Vac. Sci. Technol. B 12 Ž1994. 1063. F. Heinrichsdorff, M. Grundmann, A. Krost, D. Bimberg, A. Kosogov, P. Werner, Appl. Phys. Lett. 68 Ž1996. 3284. F. Heinrichsdorff, A. Krost, M. Mao, D. Bimberg, Appl. Phys. Lett. 71 Ž1997. 22. M. Grundmann, R. Heitz, N. Ledentsov, O. Stier, D. Bimberg, V.M. Ustinov, P.S. Kop’ev, Zh.I. Alferov, S.S. Ruvimov, P. Werner, U. Gosele, J. Heydenreich, Superlatt. Microstruct. 19 Ž1996. 81. ¨ V.A. Shchukin, N.N. Ledentsov, P.S. Kop’ev, D. Bimberg, Phys. Rev. Lett. 75 Ž1995. 2968. V.A. Shchukin, A.I. Borovkov, N.N. Ledentsov, P.S. Kop’ev, M. Grundmann, D. Bimberg, Phys. Low-Dim. Struct. 12 Ž1995. 43. V.M. Ustinov, A.Yu. Egorov, A.R. Kovsh, A.E. Zhukov, M.V. Maximov, A.F. Tsatsul’nikov, N.Yu. Grodeev, S.V. Zaitsev, Yu.M. Shernakov, N.A. Bert, P.S. Kop’ev, Zh.I. Alferov, N.N. Ledentsov, J. Bohrer, D. Bimberg, A.O. Kosogov, P. Werner, U. Gosele, J. ¨ ¨ Cryst. Growth, Ž1997. in print K.J. Vahala, IEEE J. Quantum Electron. QE-24 Ž1988. 523. L.V. Asryan, R.A. Suris, Semicond. Sci. Technol. 11 Ž1996. 1. M. Grundmann, D. Bimberg, Phys. Rev. B 55 Ž1997. 9740. M. Grundmann, D. Bimberg, Jpn. J. Appl. Phys. 6B Ž1997. 36, Part I and Phys. Stat. Sol. Ža. 164, Ž1997. 279. Yu.M. Shernyakov, A.Yu. Egorov, A.E. Zhukov, A.V. Zaitsev, A.R. Kovsh, I.L. Krestnikov, A.V. Lunev, N.N. Ledentsov, M.V. Maximov, A.V. Sakharov, V.M. Ustinov, Z. Zhen, P.S. Kop’ev, Zh.I. Alferov, D. Bimberg, Pis’ma v Zh. Tekhn. Fiz. 23 Ž1. Ž1997. 51, Tech. Phys. Lett. 23 Ž1997.. J.A. Lott, N.N. Ledentsov, V.M. Ustinov, A.Yu. Egorov, A.E. Zhukov, P.S. Kop’ev, Zh.I. Alferov, D. Bimberg, Electronics Letters 33 Ž1997. 1150.
Semiconductor quantum dots for application in diode lasers M. Grundmann a,) , N.N. Ledentsov a , N. Kirstaedter a , F. Heinrichsdorff a , A. Krost a , D. Bimberg a , A.O. Kosogov b, S.S. Ruvimov b, P. Werner b, V.M. Ustinov c , P.S. Kop’ev c , Zh.I. Alferov c a
Institut fur Technische UniÕersitat ¨ Festkorperphysik, ¨ ¨ Berlin, D-10623 Berlin, Germany Max-Planck-Institut fur ¨ Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany c A.F. Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russian Federation
b
Abstract Recent progress in the epitaxy Žmolecular beam epitaxy and metal-organic chemical vapor deposition. of strained heterostructures and the use of the Stranski-Krastanow growth mode allows to create spontaneously ordered, defect-free and dense arrays of nano-size islands. Such islands act as electronic quantum dots. In superlattices the islands are ordered in vertical stacks. Using such self-ordered InGaAsrAlGaAs quantum dots we have fabricated diode lasers for which some properties are superior to those of current lasers based on quantum wells. In particular, we have demonstrated low laser threshold current and high temperature stability of the threshold. q 1998 Elsevier Science S.A. Keywords: Quantum dots; Self-organized epitaxy; Diode lasers
1. Introduction Quantum dots ŽQD. represent a novel gain medium in semiconductor diode lasers whose advantages like ultra-low threshold and reduced temperature dependence of the threshold have been predicted theoretically w1,2x in the 1980s. Only recently, the potential of QD could be assessed experimentally w3–5x due to the exploitation of layers and stacked layers of nanometer size islands which are self-ordering during Stranski–Krastanov growth w6–8x. In this paper, we review our experimental results on the growth of InŽGa.As self-organized QD structures on GaAs substrate using molecular beam epitaxy ŽMBE. and metal–organic chemical vapor deposition ŽMOCVD. ŽSection 2.. Both epitaxial techniques appear to be similarly suitable for the fabrication of QD diode lasers ŽSection 3.1.. Finally, a new theoretical approach to gain and threshold current in QD lasers is worked out ŽSection 3.2..
We emphasize that the use of QD for novel optoelectronic devices is a quite general concept not limited to the material system discussed here. QD can also be fabricated in other strained III–V heterostructures on GaAs and InP substrates w9x and in III–V nitride systems w10x, GerSi w11x and II–VI compounds w12x.
2. Epitaxy of quantum dots Stranski and Krastanov w13x have conjectured the formation of two-dimensional islands on initially flat layers. The first observation of island formation in III–V compound superlattices was reported in Ref. w14x but ignored until detailed exploration in the beginning and mid-1990s w6– 8,15–17x.
2.1. MOCVD
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 4 4 - 9
Initial attempts to grow InAs on GaAs by MOCVD w18x led to the formation of small QD with rectangular base
84
M. Grundmann et al.r Thin Solid Films 318 (1998) 83–87
shape similar to those grown by MBE Žsee Section 2.2., simultaneous to a large number of indium rich dislocated clusters ŽFig. 1.. In order to avoid the latter, a novel growth interruption scheme was introduced w19x employing switch-off of arsine flow allowing the fabrication of defect-free dense arrays of QD. In Fig. 2, a large area plan-view TEM image with an enlarged detail and a cross-sectional view are shown for a three-fold stack of MOCVD-grown InAs QD with intermediate GaAs barriers. No clusters or other defects appear in the plane view image; the dark stripes are only due to TEM contrast formation Žstrain.. The dot density is 4 = 10 10 cmy1 and the lateral dot size is 16–18 nm. The quantum dots arrange in vertical stacks. This effect is due to the strain imposed on the growth surface by buried QD w19,20x and possibly the surface corrugation present in the current sample. If the barrier thickness is chosen to be 18 nm or larger, the vertical correlation of individual dot positions is lost.
2.2. MBE The self-organizing mechanisms present in MBE grown samples are well documented by us w6,7x. Ordering of dot
size, shape and lateral as well as vertical arrangement are observed and theoretically modeled w20–22x. Multiple deposition of islands with thin barriers leads to the formation of strongly electronically coupled QD w6,7x. Stacks of up to 25 InGaAsrGaAs layers can be grown without structural degradation ŽFig. 3..
3. Quantum dot lasers 3.1. Experimental The first QD laser based on self-organized QD was reported in Refs. w3,4x. It exhibited much better performance than lasers based on etched QD. Our QD lasers now approach or excel the best values reported for quantum film lasers. Huge material gain of f 1.5 = 10 5 cmy1 was reported in Refs. w3,4x. A high T0-value of 350 K up to a temperature of 120 K was reported in Refs. w3,4x. Recently, T0 s 350 K is achieved up to 300 K for a three-fold MBE QD-stack, the internal efficiency amounts to up to 70% w23x. The lowest threshold currents reported are 62 Arcm2 for a MBE-grown four-side cleaved structure w6,7x and 160 Arcm2 for a 1-mm long MOCVD-grown InAsrGaAs
Fig. 1. TEM plan view images of MOCVD grown InGaAsrGaAs QD.
M. Grundmann et al.r Thin Solid Films 318 (1998) 83–87
85
Fig. 2. Plan view and cross-section TEM images of a three-fold stack of InAsrGaAs QD grown by MOCVD.
laser with InGaP cladding layers w19x. We emphasize that lasing at room temperature definitely occurs on the zerodimensional QD electronic states ŽFig. 4.. Up to 100 K, the threshold of that device is found to be only 12.7 Arcm2 , close to the theoretical limit Žsee Section 3.2.. 3.2. Theory The initial modeling approaches w1,2x assumed ideal QD ensembles with infinite barriers and without any size dispersion. Finite size dispersion was considered in Ref. w24x, where also a linear relation g A n q p of the gain g on the carrier density, where nŽ p . is the electron Žhole. carrier density, was found. However, the relation of the gain and
the threshold on the current density has been previously modeled inadequately. Essentially a bimolecular term j A np as for bulk material was used throughout for the recombination current w2,25x. In the framework of our novel stochastic theory of QD population w26x, the impact of the carrier capture mechanism on the gain–current relation was found w27x. No simple general relation exists between the carrier density and the current density for QD. Pure exciton capture Žonly neutral dots exist. leads to a transparency current density given by the monomolecular expression jtr s eND rt , where ND represents the total number of QD. Completely uncorrelated electron and hole capture Žcharged dots exist. leads to jtr s Ž5r8. eND rt ,
86
M. Grundmann et al.r Thin Solid Films 318 (1998) 83–87
Fig. 3. Cross-section TEM image of 25-fold stack of InGaAsrGaAs QD grown by MBE.
both being larger than the value jtr s Ž1r2. eND rt from the bimolecular ansatz ŽFig. 5a.. Assuming reasonable values for the size distribution and total optical loss as
given in the caption the dependence of the threshold current on the area coverage with dots is found ŽFig. 5b.. Values significantly smaller than 10 Arcm2 appear possible when the size homogeneity is improved further.
4. Conclusion Dramatic progress has been made in the fabrication of QD lasers which excel or are close to record values for quantum film based devices. The stochastic theory of QD ensembles and QD lasers has been developed. Very recent accomplishments like operation of a 1-W power QD laser w28x and realization of a QD-VCSEL w29x with performance comparable to the currently best quantum film VCSEL’s let QD appear to have the potential to be the semiconductor laser gain medium of the future.
Fig. 4. Ža. Optical power vs. injection current density at 77 K Žthreshold 12.7 Arcm2 . for MOCVD-grown InAsrGaAsrAlGaAs QD-laser. Žb. Laser emission at room temperature starts monomode on the lower energy side of the QD ground state visible in the photoluminescence spectrum Ž3=InAsrGaAsrAlGaAs QD’s..
Acknowledgements Parts of this work have been funded by Deutsche Forschungsgemeinschaft in the framework of Sfb 296,
M. Grundmann et al.r Thin Solid Films 318 (1998) 83–87
w6x
w7x
w8x
w9x w10x w11x w12x w13x w14x w15x w16x Fig. 5. Ža. Gain of a QD ensemble as a function of injection current for separate Ž eq h, solid line. and simultaneous ŽX, dashed line. capture of electron and holes. For comparison the result from mean field theory ŽMF, dotted line. is shown. The gain is given in units of the maximum gain. A barrier Žwetting layer. which provides an additional recombination channel is included. We have assumed identical recombination time constants t in the barrier and dot and a capture time into the dot of tc st r100. The inset shows the relation between gain and carrier density N Žin units of ND . in the dot ensemble, being identical and linear for all models. Žb. Threshold current for both capture models and MF theory as a function of coverage for a typical dot ensemble and a total loss of a tot s10 cmy1 as a function of area coverage.
w17x w18x w19x w20x
w21x w22x
Volkswagenstiftung and INTAS. One of us ŽNNL. is grateful to Alexander-von-Humboldt Stiftung.
w23x
References
w24x w25x w26x w27x
w1x Y. Arakawa, H. Sakaki, Appl. Phys. Lett. 40 Ž1982. 939. w2x M. Asada, Y. Miyamoto, Y. Suematsu, IEEE J. Quantum Electron. QE-22 Ž1986. 1915. w3x N. Kirstaedter, N.N. Ledentsov, M. Grundmann, D. Bimberg, V.M. Ustinov, S.S. Ruvimov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, U. Richter, P. Werner, U. Gosele, J. Heydenreich, Electron. Lett. 30 ¨ Ž1994. 1416. w4x N. Kirstaedter, O. Schmidt, N.N. Ledentsov, D. Bimberg, V.M. Ustinov, A.Yu. Egorov, A.E. Zhukov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, Appl. Phys. Lett. 69 Ž1996. 1226. w5x D. Bimberg, N. Kirstaedter, N.N. Ledentsov, Zh.I. Alferov, P.S.
w28x
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