- Email: [email protected]
ZnSSe quantum islands
Journal of Crystal Growth 195 (1998) 569—573
MOCVD of vertically stacked CdSe/ZnSSe quantum islands U.W. Pohl*, R. Engelhardt, V. Tu¨rck, D. Bimberg Institut fu( r Festko( rperphysik, Technische Universita( t Berlin, PN 5-2, Hardenbergstr. 36, 10623 Berlin, Germany
Abstract Stacks of nominally one monolayer thick CdSe sheets, separated by ZnSSe barriers, were grown by metallorganic chemical vapor deposition. Cadmium interdiffusion and interface roughening was minimized at a VI/II ratio close to stoichiometry. CdSe quantum islands formed in the stacked sheets show a strong electronic coupling for barrier thicknesses below 50 A> . The excitonic luminescence of coupled islands is red-shifted with respect to the emission of uncoupled islands. Evidence for rather weak vertical, structural correlation of island coupling is found. Plastic relaxation of larger stacks can be suppressed by strain-compensating barriers. Such stacked CdSe/ZnSSe structures are particularly interesting for lateral excitonic waveguide structures. 1998 Elsevier Science B.V. All rights reserved. PACS: 78.55.Et; 78.60.Hk; 78.66.Hf Keywords: CdSe/ZnSSe; Quantum dots; MOCVD
1. Introduction Semiconductor quantum dot (QD) structures are attracting much interest at present. Self-organized InAs/GaAs QDs formed by the Stranski— Krastanow growth mode are found to be superior to quantum wells for lasers in many respects [1]. Stacking of QD sheets which are separated by a barrier of suitable thickness, induces a vertical coupling of the QDs resulting in an increased carrier capture efficiency and modal gain. For II—VI semiconductors, observation of Stranski— Krastanow growth was claimed until now for * Corresponding author. Tel.: #49 30 314 24239; fax: #49 30 314 22064; e-mail: [email protected].
CdSe/ZnSe depositions with thicknesses exceeding the critical value for dislocation-free coherent growth [2,3]. Formation of coherently grown CdSe quantum islands acting as electronic quantum dots in a ZnSe-based matrix was definitely achieved for ultrathin depositions in the monolayer and submonolayer range [4,5]. MBE-grown structures with stacked CdSe sheets have proved to enable efficient optically pumped excitonic lasing [6]. Low threshold intensities for lasing are induced by resonant waveguiding which results from a resonant oscillation of the refractive index at the energy of excitons localized in the CdSe quantum islands [6,7]. In the present paper, the effect of the ZnSSe spacer thickness on the coupling of the stacked CdSe sheets is studied.
0022-0248/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 7 0 9 - X
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U.W. Pohl et al. / Journal of Crystal Growth 195 (1998) 569–573
2. Experimental procedure Epitaxy was performed in a horizontal MOCVD reactor at 100 mbar total pressure using Me Zn Et N, tBu Se, tBu S, tBuSH, Me Cd and Cp Mg precursors. All structures were grown on GaAs(0 0 1) and 5 nm thick ZnSe buffers. Rocking curves were recorded using Cu K -radiation in a a high resolution double crystal diffractometer equipped with a double reflection Si(2 2 0) monochromator. Low temperature photoluminescence was excited by a HeCd laser at 3.81 eV with a power density of about 1 W/cm, dispersed by a 85 cm double-monochromator and recorded by a cooled GaAs PM tube. Cathodoluminescence [8] was used to selectively excite different positions of a structure. The diameter of the luminescent area at 3 kV acceleration voltage is approximately 350 nm, limited by the carrier diffusion length in the sample.
Fig. 1. X-ray rocking curves of the (0 0 4) reflection of tenfold ZnCdSe/ZnSSe superlattices which were grown at different Se/Zn partial pressures. Optimal interfaces are achieved at a near-stoichiometric ratio. The grey spectrum represents a simulation according to dynamical theory.
3. Growth conditions for abrupt interfaces 4. Stacked CdSe quantum islands A crucial problem in ZnCdSe-related structures is the growth of abrupt interfaces [9,10]. This is due to CdSe diffusion which occurs via lattice sites and which is strongly enhanced in the presence of cation vacancies [10]. ZnCdSe-related samples with abrupt interfaces can therefore not be grown with a high VI/II ratio — a behavior which we did not observe for ZnSSe/ZnSe structures. On the other hand, a low VI/II ratio results in a rough interface morphology. We therefore first optimized the Se/Zn ratio using ZnCdSe/ZnSSe superlattice test structures. The strain-compensated superlattices consisted of 10 periods of Zn Cd Se (3.0 or 1.6 nm) wells and ZnS Se (16 nm) barriers. A series of structures was grown at 350°C using different VI/II ratios ranging from Se-rich to Znrich. Rocking curves of the (0 0 4) reflection show only weak superlattice satellites at high and low Se/Zn ratios, respectively, see Fig. 1. At values near unity, well resolved satellites up to the third order appear. Using this value, a second series of superlattices was grown varying the growth temperature. Most pronounced superlattice satellites were found for structures grown for a growth temperature of 350°C.
Earlier experiments [5] had demonstrated that an average CdSe deposition of one monolayer (ML) in a ZnSSe matrix results in a bright luminescence of three-dimensionally confined excitons. At this thickness, the full width at half maximum (FWHM) of the luminescence is quite small, and no defect-related deep emission is observed. Furthermore, we now found that ripening effects during growth interruptions after the CdSe deposition and prior to the growth of the subsequent ZnSSe layer have only a minor effect on the optical properties of the structures. This is in contrast to distinct effects which we observed for thick (&3 ML) CdSe depositions essentially occurring within the first three seconds. For the stacked quantum islands with one ML CdSe deposition, we used a growth interruption of 6 s under selenium stabilization, since a slight increase of the photoluminescence intensity was found for these structures. The stacked structures consisted of superlattices composed of nominally one monolayer thick CdSe sheets, separated by ZnS Se barriers being lattice-matched to GaAs. To reduce the loss of carriers after excitation, the stacks were symmetrically
U.W. Pohl et al. / Journal of Crystal Growth 195 (1998) 569–573
Fig. 2. Normalized photoluminescence spectra of threefold stacked CdSe sheets being separated by ZnSSe barriers of different thicknesses d , recorded at room temperature. The inset 811 shows the energy of the peak maxima depending on the barrier thickness.
cladded by lattice matched 80 nm ZnSSe and 10 nm ZnMgSSe layers, and an additional 300 nm ZnS Se buffer was introduced. The small vertical size of the CdSe-induced confinement potential results in a large extent of the exciton wavefunction into the barrier. This leads to a strong tendency for coupling of quantum islands in the vertical direction. The photoluminescence spectra of a series of threefold stacks with different barrier thicknesses are shown in Fig. 2. The coupling induces a red-shift of the CdSe-related luminescence band. While a weak effect of coupling can already be recognized for a barrier thickness of 200 A> , strong coupling of the islands starts below 50 A> . The resulting luminescence consists of two superimposed bands, one exhibiting an apparent red-shift and a weaker, essentially not shifting band which appears as a high energy shoulder. The shifted and the unshifted bands are assigned to the simultaneous occurrence of coupled and uncoupled quantum islands, respectively. The luminescence originating from the unshifted band decreases in intensity with decreasing spacer thickness. This effect is assigned to a gradually increasing amount of
571
coupled islands and to a loss of carriers of the uncoupled islands to the deeper lying electronic states of the coupled islands. The high efficiency of carrier trapping at the coupled islands is also demonstrated by the decrease of the luminescence which originates from the ZnSSe barriers. Lateral selective cathodoluminescence of coupled islands shows the same features of three-dimensionally confined excitons like in the single CdSe sheets reported in Ref. [5]. Despite stacking and the resulting electronic coupling, a luminescence band of uncoupled islands is still detected. This can be an indication for a weak structural correlation of stacked quantum islands. Transmission electron microscopy (TEM) investigations of some samples did not provide a significant indication for structural correlation of CdSe islands as well [11]. This effect is supposed to originate from a weak strain-field in the ZnSSe barriers induced by the thin CdSe depositions. Such a weak strain field should not be able to mediate an effective coupling between quantum islands of subsequent CdSe layers. If the structure is to be used as the active region of a laser, the number of CdSe sheets in a stack must be increased to attain a thickness of about half an optical wavelength. We therefore grew stacks with increased numbers of CdSe sheets and different coupling strengths. The photoluminescence spectra of two series of stacks with increasing numbers of CdSe sheets for both, weak and strong coupling are shown in Fig. 3. The series of weakly coupled stacks show a slight red-shift of the emission maxima accompanied by a slight decrease of the intensity. The small red-shift originates from the coupling which leads to a gradually reduced effective quantum confinement energy. The intensity reduction essentially results from the onset of plastic relaxation. At high excitation density, an additional minor contribution to the decrease may occur by weak lateral excitonic waveguiding [7], because the guided intensity is emitted at the edge facet of the sample and is thus missing in the detected surface emission. In the case of strongly coupled islands shown in Fig. 3b, the red-shift is more pronounced, since the contribution of each additionally coupled island gives a stronger contribution to the decrease of the confinement energy of the
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U.W. Pohl et al. / Journal of Crystal Growth 195 (1998) 569–573
Fig. 4. Surface (grey) and edge emission of an optically pumped laser structure with strain-compensated stacked CdSe/ZnSSe quantum islands, excited at 2.88 eV with 100 kW/cm.
Fig. 3. Photoluminescence spectra of CdSe/ZnSSe structures with different numbers of stacked CdSe sheets at a fixed barrier thickness d of (a) 50 A> and (b) 20 A> , recorded at RT. 811
coupled wave function. A pronounced intensity drop is found only for larger stacks. This is induced by the occurrence of misfit dislocations which accommodate the strain of the highly lattice-mismatched coupled CdSe sheets. The accumulated strain in the stack can be compensated by reducing the lattice constant in the ZnSSe barriers. Stacks with an increased sulfur content of x"0.08 in the barrriers allow the coherent growth of 16 fold stacks which have a sufficient thickness (&80 nm) for optical waveguiding. To prove the efficiency of the stacked CdSe/ ZnSSe quantum island structures for lasers, we embedded a 16 fold CdSe stack with strain-compensating 50 A> ZnSSe barriers into symmetrical 100 nm ZnSSe and 10 nm ZnMgSSe cladding
layers. A preliminary study on optically pumped lasing clearly demonstrates optical waveguiding as shown in Fig. 4. The broad surface emission of the quantum islands is accompanied by a luminescence of the barriers with a comparable intensity. The corresponding edge emission is much brighter and shows a distinct line narrowing of the quantum island emission. The emission occurs at the low energy side of the excitonic absorption which was measured by reflection spectra. This provides a clear indication for excitonic waveguiding. The efficiency of waveguiding leads to a low threshold intensity for edge emitting lasing in the order of 10 kW/cm. Further details will be published elsewhere.
5. Conclusion Ultrathin CdSe sheets can be inserted into ZnSSe barriers without strong CdSe interdiffusion or interface roughening at a VI/II ratio close to stoichiometry. The excitonic luminescence of CdSe quantum islands which are formed in the stacked sheets prove a vertical coupling at barrier thicknesses below 200 A> . Strong coupling starts below
U.W. Pohl et al. / Journal of Crystal Growth 195 (1998) 569–573
50 A> and results in a luminescence which is redshifted with respect to the simultaneously appearing emission of uncoupled islands. To avoid plastic relaxation, stacks must be strain-compensated by reducing the lattice constant of the barriers. Lateral excitonic waveguiding and low threshold intensity for coupled island-related lasing was observed for strain-compensated stacks of sufficient thickness. Acknowledgements The authors like to thank I.L. Krestnikov and N.N. Ledentsov for valuable discussions. R.E. gratefully acknowledges a NaFo¨G fellowship granted by Berliner Senat. References [1] D. Bimberg, N. Kirstaedter, N.N. Ledentsov, Zh.I. Alferov, P.S. Kop’ev, V.M. Ustinov, IEEE J. Selected Topics in Quantum Electron. 3 (1997) 196.
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[2] I. Suemune, K. Uesugi, H. Suzuki, H. Nashiki, M. Arita, Phys. Stat. Sol. (b) 202 (1997) 845. [3] M. Arita, A. Avramescu, K. Uesugi, I. Suemune, T. Numai, H. Machida, N. Shimoyama, Jpn. J. Appl. Phys. 36 (1997) 4097. [4] M. Lowisch, M. Rabe, B. Stegemann, F. Henneberger, M. Grundmann, V. Tu¨rck, D. Bimberg, Phys. Rev. B 54 (1996) R11074. [5] R. Engelhardt, V. Tu¨rck, U.W. Pohl, D. Bimberg, J. Crystal Growth 184/185 (1998) 311. [6] M. Strassburg, V. Kutzer, U.W. Pohl, A. Hoffmann, I. Broser, N.N. Ledentsov, D. Bimberg, A. Rosenauer, U. Fischer, D. Gerthsen, I.L. Krestnikov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, Appl. Phys. Lett. 72 (1998) 942. [7] I.L. Krestnikov, M.V. Maximov, A.V. Sakharov, P.S. Kop’ev, Zh.I. Alferov, N.N. Ledentsov, D. Bimberg, C.M. Sotomayor Torres, J. Crystal Growth 184/185 (1998) 545. [8] J. Christen, M. Grundmann, D. Bimberg, J. Vac. Sci. Technol. B 9 (1991) 2358. [9] P.J. Parbrook, A. Kamata, T. Uemoto, Jpn. J. Appl. Phys. 32 (1993) 669. [10] M. Kuttler, M. Strassburg, V. Tu¨rck, R. Heitz, U.W. Pohl, D. Bimberg, E. Kurtz, G. Landwehr, D. Hommel, Appl. Phys. Lett. 69 (1996) 2647. [11] D. Litvinov, private communication.
MOCVD of vertically stacked CdSe/ZnSSe quantum islands U.W. Pohl*, R. Engelhardt, V. Tu¨rck, D. Bimberg Institut fu( r Festko( rperphysik, Technische Universita( t Berlin, PN 5-2, Hardenbergstr. 36, 10623 Berlin, Germany
Abstract Stacks of nominally one monolayer thick CdSe sheets, separated by ZnSSe barriers, were grown by metallorganic chemical vapor deposition. Cadmium interdiffusion and interface roughening was minimized at a VI/II ratio close to stoichiometry. CdSe quantum islands formed in the stacked sheets show a strong electronic coupling for barrier thicknesses below 50 A> . The excitonic luminescence of coupled islands is red-shifted with respect to the emission of uncoupled islands. Evidence for rather weak vertical, structural correlation of island coupling is found. Plastic relaxation of larger stacks can be suppressed by strain-compensating barriers. Such stacked CdSe/ZnSSe structures are particularly interesting for lateral excitonic waveguide structures. 1998 Elsevier Science B.V. All rights reserved. PACS: 78.55.Et; 78.60.Hk; 78.66.Hf Keywords: CdSe/ZnSSe; Quantum dots; MOCVD
1. Introduction Semiconductor quantum dot (QD) structures are attracting much interest at present. Self-organized InAs/GaAs QDs formed by the Stranski— Krastanow growth mode are found to be superior to quantum wells for lasers in many respects [1]. Stacking of QD sheets which are separated by a barrier of suitable thickness, induces a vertical coupling of the QDs resulting in an increased carrier capture efficiency and modal gain. For II—VI semiconductors, observation of Stranski— Krastanow growth was claimed until now for * Corresponding author. Tel.: #49 30 314 24239; fax: #49 30 314 22064; e-mail: [email protected].
CdSe/ZnSe depositions with thicknesses exceeding the critical value for dislocation-free coherent growth [2,3]. Formation of coherently grown CdSe quantum islands acting as electronic quantum dots in a ZnSe-based matrix was definitely achieved for ultrathin depositions in the monolayer and submonolayer range [4,5]. MBE-grown structures with stacked CdSe sheets have proved to enable efficient optically pumped excitonic lasing [6]. Low threshold intensities for lasing are induced by resonant waveguiding which results from a resonant oscillation of the refractive index at the energy of excitons localized in the CdSe quantum islands [6,7]. In the present paper, the effect of the ZnSSe spacer thickness on the coupling of the stacked CdSe sheets is studied.
0022-0248/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 7 0 9 - X
570
U.W. Pohl et al. / Journal of Crystal Growth 195 (1998) 569–573
2. Experimental procedure Epitaxy was performed in a horizontal MOCVD reactor at 100 mbar total pressure using Me Zn Et N, tBu Se, tBu S, tBuSH, Me Cd and Cp Mg precursors. All structures were grown on GaAs(0 0 1) and 5 nm thick ZnSe buffers. Rocking curves were recorded using Cu K -radiation in a a high resolution double crystal diffractometer equipped with a double reflection Si(2 2 0) monochromator. Low temperature photoluminescence was excited by a HeCd laser at 3.81 eV with a power density of about 1 W/cm, dispersed by a 85 cm double-monochromator and recorded by a cooled GaAs PM tube. Cathodoluminescence [8] was used to selectively excite different positions of a structure. The diameter of the luminescent area at 3 kV acceleration voltage is approximately 350 nm, limited by the carrier diffusion length in the sample.
Fig. 1. X-ray rocking curves of the (0 0 4) reflection of tenfold ZnCdSe/ZnSSe superlattices which were grown at different Se/Zn partial pressures. Optimal interfaces are achieved at a near-stoichiometric ratio. The grey spectrum represents a simulation according to dynamical theory.
3. Growth conditions for abrupt interfaces 4. Stacked CdSe quantum islands A crucial problem in ZnCdSe-related structures is the growth of abrupt interfaces [9,10]. This is due to CdSe diffusion which occurs via lattice sites and which is strongly enhanced in the presence of cation vacancies [10]. ZnCdSe-related samples with abrupt interfaces can therefore not be grown with a high VI/II ratio — a behavior which we did not observe for ZnSSe/ZnSe structures. On the other hand, a low VI/II ratio results in a rough interface morphology. We therefore first optimized the Se/Zn ratio using ZnCdSe/ZnSSe superlattice test structures. The strain-compensated superlattices consisted of 10 periods of Zn Cd Se (3.0 or 1.6 nm) wells and ZnS Se (16 nm) barriers. A series of structures was grown at 350°C using different VI/II ratios ranging from Se-rich to Znrich. Rocking curves of the (0 0 4) reflection show only weak superlattice satellites at high and low Se/Zn ratios, respectively, see Fig. 1. At values near unity, well resolved satellites up to the third order appear. Using this value, a second series of superlattices was grown varying the growth temperature. Most pronounced superlattice satellites were found for structures grown for a growth temperature of 350°C.
Earlier experiments [5] had demonstrated that an average CdSe deposition of one monolayer (ML) in a ZnSSe matrix results in a bright luminescence of three-dimensionally confined excitons. At this thickness, the full width at half maximum (FWHM) of the luminescence is quite small, and no defect-related deep emission is observed. Furthermore, we now found that ripening effects during growth interruptions after the CdSe deposition and prior to the growth of the subsequent ZnSSe layer have only a minor effect on the optical properties of the structures. This is in contrast to distinct effects which we observed for thick (&3 ML) CdSe depositions essentially occurring within the first three seconds. For the stacked quantum islands with one ML CdSe deposition, we used a growth interruption of 6 s under selenium stabilization, since a slight increase of the photoluminescence intensity was found for these structures. The stacked structures consisted of superlattices composed of nominally one monolayer thick CdSe sheets, separated by ZnS Se barriers being lattice-matched to GaAs. To reduce the loss of carriers after excitation, the stacks were symmetrically
U.W. Pohl et al. / Journal of Crystal Growth 195 (1998) 569–573
Fig. 2. Normalized photoluminescence spectra of threefold stacked CdSe sheets being separated by ZnSSe barriers of different thicknesses d , recorded at room temperature. The inset 811 shows the energy of the peak maxima depending on the barrier thickness.
cladded by lattice matched 80 nm ZnSSe and 10 nm ZnMgSSe layers, and an additional 300 nm ZnS Se buffer was introduced. The small vertical size of the CdSe-induced confinement potential results in a large extent of the exciton wavefunction into the barrier. This leads to a strong tendency for coupling of quantum islands in the vertical direction. The photoluminescence spectra of a series of threefold stacks with different barrier thicknesses are shown in Fig. 2. The coupling induces a red-shift of the CdSe-related luminescence band. While a weak effect of coupling can already be recognized for a barrier thickness of 200 A> , strong coupling of the islands starts below 50 A> . The resulting luminescence consists of two superimposed bands, one exhibiting an apparent red-shift and a weaker, essentially not shifting band which appears as a high energy shoulder. The shifted and the unshifted bands are assigned to the simultaneous occurrence of coupled and uncoupled quantum islands, respectively. The luminescence originating from the unshifted band decreases in intensity with decreasing spacer thickness. This effect is assigned to a gradually increasing amount of
571
coupled islands and to a loss of carriers of the uncoupled islands to the deeper lying electronic states of the coupled islands. The high efficiency of carrier trapping at the coupled islands is also demonstrated by the decrease of the luminescence which originates from the ZnSSe barriers. Lateral selective cathodoluminescence of coupled islands shows the same features of three-dimensionally confined excitons like in the single CdSe sheets reported in Ref. [5]. Despite stacking and the resulting electronic coupling, a luminescence band of uncoupled islands is still detected. This can be an indication for a weak structural correlation of stacked quantum islands. Transmission electron microscopy (TEM) investigations of some samples did not provide a significant indication for structural correlation of CdSe islands as well [11]. This effect is supposed to originate from a weak strain-field in the ZnSSe barriers induced by the thin CdSe depositions. Such a weak strain field should not be able to mediate an effective coupling between quantum islands of subsequent CdSe layers. If the structure is to be used as the active region of a laser, the number of CdSe sheets in a stack must be increased to attain a thickness of about half an optical wavelength. We therefore grew stacks with increased numbers of CdSe sheets and different coupling strengths. The photoluminescence spectra of two series of stacks with increasing numbers of CdSe sheets for both, weak and strong coupling are shown in Fig. 3. The series of weakly coupled stacks show a slight red-shift of the emission maxima accompanied by a slight decrease of the intensity. The small red-shift originates from the coupling which leads to a gradually reduced effective quantum confinement energy. The intensity reduction essentially results from the onset of plastic relaxation. At high excitation density, an additional minor contribution to the decrease may occur by weak lateral excitonic waveguiding [7], because the guided intensity is emitted at the edge facet of the sample and is thus missing in the detected surface emission. In the case of strongly coupled islands shown in Fig. 3b, the red-shift is more pronounced, since the contribution of each additionally coupled island gives a stronger contribution to the decrease of the confinement energy of the
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U.W. Pohl et al. / Journal of Crystal Growth 195 (1998) 569–573
Fig. 4. Surface (grey) and edge emission of an optically pumped laser structure with strain-compensated stacked CdSe/ZnSSe quantum islands, excited at 2.88 eV with 100 kW/cm.
Fig. 3. Photoluminescence spectra of CdSe/ZnSSe structures with different numbers of stacked CdSe sheets at a fixed barrier thickness d of (a) 50 A> and (b) 20 A> , recorded at RT. 811
coupled wave function. A pronounced intensity drop is found only for larger stacks. This is induced by the occurrence of misfit dislocations which accommodate the strain of the highly lattice-mismatched coupled CdSe sheets. The accumulated strain in the stack can be compensated by reducing the lattice constant in the ZnSSe barriers. Stacks with an increased sulfur content of x"0.08 in the barrriers allow the coherent growth of 16 fold stacks which have a sufficient thickness (&80 nm) for optical waveguiding. To prove the efficiency of the stacked CdSe/ ZnSSe quantum island structures for lasers, we embedded a 16 fold CdSe stack with strain-compensating 50 A> ZnSSe barriers into symmetrical 100 nm ZnSSe and 10 nm ZnMgSSe cladding
layers. A preliminary study on optically pumped lasing clearly demonstrates optical waveguiding as shown in Fig. 4. The broad surface emission of the quantum islands is accompanied by a luminescence of the barriers with a comparable intensity. The corresponding edge emission is much brighter and shows a distinct line narrowing of the quantum island emission. The emission occurs at the low energy side of the excitonic absorption which was measured by reflection spectra. This provides a clear indication for excitonic waveguiding. The efficiency of waveguiding leads to a low threshold intensity for edge emitting lasing in the order of 10 kW/cm. Further details will be published elsewhere.
5. Conclusion Ultrathin CdSe sheets can be inserted into ZnSSe barriers without strong CdSe interdiffusion or interface roughening at a VI/II ratio close to stoichiometry. The excitonic luminescence of CdSe quantum islands which are formed in the stacked sheets prove a vertical coupling at barrier thicknesses below 200 A> . Strong coupling starts below
U.W. Pohl et al. / Journal of Crystal Growth 195 (1998) 569–573
50 A> and results in a luminescence which is redshifted with respect to the simultaneously appearing emission of uncoupled islands. To avoid plastic relaxation, stacks must be strain-compensated by reducing the lattice constant of the barriers. Lateral excitonic waveguiding and low threshold intensity for coupled island-related lasing was observed for strain-compensated stacks of sufficient thickness. Acknowledgements The authors like to thank I.L. Krestnikov and N.N. Ledentsov for valuable discussions. R.E. gratefully acknowledges a NaFo¨G fellowship granted by Berliner Senat. References [1] D. Bimberg, N. Kirstaedter, N.N. Ledentsov, Zh.I. Alferov, P.S. Kop’ev, V.M. Ustinov, IEEE J. Selected Topics in Quantum Electron. 3 (1997) 196.
573
[2] I. Suemune, K. Uesugi, H. Suzuki, H. Nashiki, M. Arita, Phys. Stat. Sol. (b) 202 (1997) 845. [3] M. Arita, A. Avramescu, K. Uesugi, I. Suemune, T. Numai, H. Machida, N. Shimoyama, Jpn. J. Appl. Phys. 36 (1997) 4097. [4] M. Lowisch, M. Rabe, B. Stegemann, F. Henneberger, M. Grundmann, V. Tu¨rck, D. Bimberg, Phys. Rev. B 54 (1996) R11074. [5] R. Engelhardt, V. Tu¨rck, U.W. Pohl, D. Bimberg, J. Crystal Growth 184/185 (1998) 311. [6] M. Strassburg, V. Kutzer, U.W. Pohl, A. Hoffmann, I. Broser, N.N. Ledentsov, D. Bimberg, A. Rosenauer, U. Fischer, D. Gerthsen, I.L. Krestnikov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, Appl. Phys. Lett. 72 (1998) 942. [7] I.L. Krestnikov, M.V. Maximov, A.V. Sakharov, P.S. Kop’ev, Zh.I. Alferov, N.N. Ledentsov, D. Bimberg, C.M. Sotomayor Torres, J. Crystal Growth 184/185 (1998) 545. [8] J. Christen, M. Grundmann, D. Bimberg, J. Vac. Sci. Technol. B 9 (1991) 2358. [9] P.J. Parbrook, A. Kamata, T. Uemoto, Jpn. J. Appl. Phys. 32 (1993) 669. [10] M. Kuttler, M. Strassburg, V. Tu¨rck, R. Heitz, U.W. Pohl, D. Bimberg, E. Kurtz, G. Landwehr, D. Hommel, Appl. Phys. Lett. 69 (1996) 2647. [11] D. Litvinov, private communication.