- Email: [email protected]
Optical observation of quantum-dot formation in sub-critical CdSe layers grown on ZnSe
Journal of Crystal Growth 214/215 (2000) 761}764
Optical observation of quantum-dot formation in sub-critical CdSe layers grown on ZnSe C.S. Kim , M. Kim , S. Lee , J.K. Furdyna , M. Dobrowolska *, H. Rho, L.M. Smith, Howard E. Jackson Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA Department of Physics, University of Cincinnati, Cincinnati, OH 45221-0011, USA
Abstract The evolution of CdSe quantum dot (QD) formation on ZnSe was investigated on a series of samples grown by MBE, with nominal CdSe coverages ranging from 1.1 to 2.6 monolayers (ML). Micro-PL data suggest strongly con"ned zero-dimensional excitons even for coverages as low as 1.1 ML. PL and micro-PL data show emission from the QDs as well as from an accompanying 2 D layer; the temperature dependence of these emissions is notably di!erent. Strong red-shifts of both emissions as the CdSe coverage increases are observed. 2000 Elsevier Science B.V. All rights reserved. PACS: 78.66.Hf; 78.66.!w; 78.55.Et; 68.65.#g; 81.15.Hi Keywords: II}VI semiconductors; Self-assembling; Quantum dots; Photoluminescence; MBE
Intense interest in the surface morphology of overlayers deposited on lattice-mismatched surfaces has been sparked by the interest in mechanisms leading to the formation of quantum dots (QDs). The epitaxial system most widely studied in this context is that of InAs overlayers on (0 0 1) GaAs. The growth of InAs QDs on GaAs substrates is usually described in terms of the Stranski}Krastanow (S}K) mechanism, where coherent islands form after deposition of 1 to 2 monolayers (ML) of InAs, referred to as the wetting layer [1}3]. The situation in II}VI materials is less clear. Here the * Corresponding author. Tel.: #1-219-631-6962; fax: #1219-631-5952. E-mail address: [email protected] (M. Dobrowolska).
most extensively explored materials combination is that of CdSe on (0 0 1) ZnSe [4}10]. In this paper we will present optical data which strongly suggest that in the case of CdSe grown on ZnSe, 3-D islands (QDs) begin to form well before the deposited layer reaches its critical thickness, indicating that the S}K mechanism is inadequate to describe the 2-D to 3-D transition that occurs in this system } atleast under conditions used for growing our CdSe/ZnSe specimens, described below. The samples were grown by molecular-beam epitaxy (MBE). A ZnSe bu!er was "rst grown at 3003C on a (1 0 0) GaAs substrate to a thickness of approximately 2 lm. For the growth of CdSe layers, the substrate temperature was raised to
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 2 2 1 - 9
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3503C. For the purposes of the present study we have grown four specimens, with nominal thicknesses of the CdSe layers of 1.1, 1.5, 1.9 and 2.6 ML. We observed the RHEED streaks transforming into spots only for the sample with 2.6 ML CdSe deposition. After deposition, the CdSe layers were capped by 50 nm of ZnSe. In our early work we performed AFM studies on both bare ZnSe and uncapped CdSe dots on ZnSe [4]. While AFM pictures taken on ZnSe sample show very smooth surface, the pictures taken on CdSe clearly show very well-developed dots. However, this study also revealed a ripening of the dots in room temperature. This rather quick ripening prevented us from using AFM method for systematic study of QD formation as a function of CdSe deposition. Therefore, in this paper we use the optical method to study the QD formation process. Systematic photoluminescence (PL) and microPL studies were carried out on the series of specimens described above. The micro-PL experiments were performed at 6 K using a Dilor spectrometer with a liquid-nitrogen-cooled CCD camera and a microscope. We recorded the spectrally dispersed luminescence from 1.7 lm-diameter regions of each sample. The samples were excited at 438 nm by a doubled mode-locked Ti-sapphire laser. Fig. 1 shows micro-PL spectra observed on four samples with di!erent CdSe coverages. One can see a clear progression of the broad PL spectrum towards lower energies as the CdSe coverage increases. Superimposed on the broad luminescence,
Fig. 1. Micro-photoluminescence spectra taken at 6 K on a series of CdSe/ZnSe QD samples with di!erent CdSe coverages (solid curves), together with best-"t two Gaussian simulations (dotted curves).
we observe many sharp lines, with a typical width of 200 leV. The most striking feature in Fig. 1 is that the sharp lines are seen on all of the measured samples, including the one with 1.1 ML CdSe coverage; the presence of these sharp spikes is more noticeable as coverage increases. The observation of the sharp peaks superimposed on the broad luminescence indicates the presence of 0}D excitons con"ned to quantum-dot regions. Another important feature of the data shown in Fig. 1 is the asymmetry of the broad PL lines, most obviously seen for samples with 1.5 and 1.9 ML coverages. Since the spectrum observed on the 1.5 ML sample suggests the presence of two lines, we model the broad luminescence as a superposition of two Gaussian lines. Dotted curves in Fig. 1 show two-line "ts to the data which provide a good description of the PL lineshape observed on samples with 1.1, 1.5, and 1.9 ML coverages. The luminescence observed on the sample with 2.6 ML coverage can be modeled satisfactorily with a single line. Three additional features emerge from the data displayed in Fig. 1. First, the intensity ratio of the lower-energy line (line B) to the higher-energy line (line A) increases with increasing coverage. Secondly, the sharp spikes superimposed on the broad emission are much more pronounced on the lower-energy side of each PL spectrum. Thirdly, we see in Fig. 1 that the PL energies of both lines A and B shift towards lower energies, and the energy separations between them systematically increase, as the thickness of the CdSe layer increases. In order to gain insight into the nature of both lines making up the broad spectrum, we performed a temperature study of the macro-PL. We separated the contributions from lines A and B by modeling all spectra as a superposition of two Gaussian lines, and followed the temperature dependence of the PL energy, the integrated intensity, and the FWHM of both lines. Fig. 2 shows the energy position of lines A and B as a function of temperature for samples with 1.1 and 1.9 ML deposition. The open symbols show the temperature behavior of line A, while full symbols represent line B. To facilitate comparison between the two lines, we approximated all curves by straight lines in the "gure.
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763
Fig. 2. Temperature dependence of the energy position of lines B (full symbols) and A (open symbols).
Fig. 3. Temperature dependence of full-width at half-maximum of line B (open circles) and of line A (full circles) observed on the CdSe/ZnSe sample with 1.9 ML CdSe deposition.
For a given coverage, the red shift of the PL energy of line B always increases faster than that of line A. For instance the slopes of the straightline "ts shown in Fig. 2 are as follows: for 1.1 ML coverage, the slopes are !1.6;10\ and !2.8;10\; for 1.9 ML, !2.4;10\ and !2.7;10\ for lines A and B, respectively. We "nd that the di!erence between the slopes of lines A and B increase with CdSe coverage. Note, for example, that for a coverage of 1.9 ML, the rate of energy shift with increasing temperature for line B is an order of magnitude larger than for line A. This di!erence in the temperature dependence of the PL emissions has been attributed to 0D and 2D con"ned excitons in both II}VI [11,12] and III}VI [13] systems, with the 0D or QD emission (line B) having a much more rapid temperature dependence of the energy shift than the 2D or QW (or wetting layer) emission (line A). In Fig. 3 we display the temperature dependence of the FWHM for the QD (line B) and QW (line A) for the 1.9 ML CdSe sample. The FWHM of the QD line shows signi"cant narrowing as the temperature increases. This narrowing as temperature increases has been observed in other QD systems and explained by a redistribution of carriers to the lower energy QD states. [11,13}15]. The QW line (line A), on the other hand, shows a behavior typical of the emission of excitons con"ned in a quantum well, where the FWHM increases monotonically with temperature. Thus, the temper-
ature dependence of the peak emission energies and the line width data are consistent with line B being associated with QDs or zero-dimensionally-con"ned excitons, and with line A being associated with more extended regions that are 2D in character (see also below). Since we observe line B in the PL spectrum for the sample with only a 1.1 ML coverage, we conclude that the precursors of the dots form at an early stage of CdSe deposition. This indicates that the onset of quantum dot formation in the case of CdSe layers grown on ZnSe is not abrupt as found in the InAs/GaAs case, but occurs more gradually. Based on our optical "ndings lead as to speculate on the following possible scenario for the early evolution of 2D and 0D growth. As we deposit CdSe, extended ZnCdSe 2-D islands form, with some graded composition containing local maxima of Cd concentration (and possibly accompanied by thickness #uctuations) [16]. The local maxima of Cd concentration provide localization for 0D excitons and give rise to the spikes associated with the lower-energy emission band (line B in Fig. 1). During CdSe deposition both the extended 2D layer and the localizing #uctuations evolve simultaneously; and as the amount of Cd delivered increases, both regions become richer in Cd, so that the two PL lines (A and B) shift to lower energies. Furthermore, because of strain energy considerations, the Cd-rich regions also preferentially accumulate more material, that may in turn lead to the
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development of island-like thickness #uctuations (which also relieves the signi"cant strain in the layer). This morphological picture is consistent with compositional nonuniformities driven by strain that stabilize "lm growth of lattice-mismatched alloys, as discussed by Guyer and Voorhees [17]. The same kind of model can also describe the relative intensity variation of lines A and B with increasing CdSe deposition. At the very early stage of dot formation (i.e., at coverages slightly exceeding 1 ML) the density of the dots themselves is very low, which gives rise to a very weak line B. Moreover, the microscopic regions which give rise to the spikes seen in line B can be easily saturated because of the characteristically low densities of states of the zero-dimensional objects, so that the states in the extended 2D ZnCdSe islands (which lie at a higher energy) can also be partially "lled. As the coverage increases from 1.5 to 1.9 ML, the density of the localizing regions rapidly increases, resulting in a larger density of spikes and a stronger integrated intensity of line B relative to its higher-energy partner, since one now has a larger density of the 0D exciton centers to be "lled before one begins to "ll states in the extended 2D ZnCdSe island regions. In conclusion, we have performed both macroPL and micro-PL studies on a series of samples consisting of CdSe layers deposited on ZnSe as a function of nominal layer thickness. We observe strongly con"ned zero-dimensional excitons for coverages as low as 1.1 ML, i.e., far below the critical thickness for CdSe on ZnSe. We also observe strong red shifts of the PL signal from both QDs and extended 2D CdZnSe layers as the CdSe deposition increases. These "ndings suggest that in the CdSe/ZnSe system the change from 2D to 3D morphology occurs gradually (starting with as little as 1 ML CdSe deposition), and therefore is more complex than the spontaneous and abrupt change implied by the Stranski}Krastanow growth mode.
We acknowledge the support by the NSF (DMR-9705443, DMR-9705064, ECS-9412772), and ARO (DAAG55-97-1-0378). We also appreciate conversations with J.C. Kim.
References [1] R. Notzel, J. Temmyo, T. Tamamura, Nature (London) 369 (1994) 131. [2] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petro!, Appl. Phys. Lett. 63 (1993) 3203. [3] R. Heitz, T.R. Ramachandran, A. Kalburge, Q. Xie, I. Mukhametzhanov, P. Chen, A. Madhukar, Phys. Rev. Lett. 78 (1997) 4071. [4] J.L. Merz, S. Lee, J.K. Furdyna, J. Crystal Growth 184/185 (1998) 228. [5] S.H. Xin, P.D. Wang, Aie Yin, C.S. Kim, M. Dobrowolska, J.K. Merz, J.K. Furdyna, Appl. Phys. Lett. 69 (1996) 3884. [6] K. Leonardi, H. Heinke, K. Ohkawa, D. Hommel, H. Selke, F. Gindele, U. Woggon, Appl. Phys. Lett. 71 (1997) 1510. [7] T. Kummell, R. Weigand, G. Bacher, A. Forchel, K. Leonardi, D. Hommel, H. Selke, Appl. Phys. Lett. 73 (1998) 3105. [8] F. Flack, N. Samarth, U. Nikitin, P.A. Crowell, J. Shi, J. Levy, D.D. Awschalom, Phys. Rev. B 54 (1996) R17 312. [9] H. Kirmse, R. Schneider, M. Rabe, W. Neumann, F. Henneberger, Appl. Phys. Lett. 72 (1998) 1329. [10] J.C. Kim, H. Rho, L.M. Smith, H.E. Jackson, S. Lee, M. Dobrowolska, J.K. Furdyna, Appl. Phys. Lett. 75 (1999) 214. [11] G. Karczewski, S. Mackowski, M. Kutrowski, T. Wojtowicz, J. Kossut, Appl. Phys. Lett. 74 (1999) 3011. [12] Y. Wu, K. Arai, T. Yao, Phys. Rev. B 53 (1996) R10-485. [13] D.I. Lubyshev, P.P. Gonz'alez-Borrero, E. Marega Jr., E. Petitprez, N. La Scala Jr., P. Basmaji, Appl. Phys. Lett 68 (1996) 205. [14] H. Lee, W. Yang, P.C. Sercel, Phys. Rev. B 55 (1997) 9757. [15] A. Polimeni, A. Patane, M. Henini, L. Eaves, P.C. Main, Phys. Rev. B 59 (1999) 5064. [16] M. Strassburg, V. Kutzer, V.W. Pohl, A. Ho!mann, I. Broser, N.N. Ledenstsov, B. Bimberg, A. Rosenauer, V. Fisher, D. Gerthsen, I.L. Krestnikov, M.V. Maximov, P.S. KopH ev, Zh.I. Alferov, Appl. Phys. Lett. 72 (1998) 942. [17] J.E. Guyer, P.W. Voorhees, Phys. Rev. Lett. 74 (1995) 4031.
Optical observation of quantum-dot formation in sub-critical CdSe layers grown on ZnSe C.S. Kim , M. Kim , S. Lee , J.K. Furdyna , M. Dobrowolska *, H. Rho, L.M. Smith, Howard E. Jackson Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA Department of Physics, University of Cincinnati, Cincinnati, OH 45221-0011, USA
Abstract The evolution of CdSe quantum dot (QD) formation on ZnSe was investigated on a series of samples grown by MBE, with nominal CdSe coverages ranging from 1.1 to 2.6 monolayers (ML). Micro-PL data suggest strongly con"ned zero-dimensional excitons even for coverages as low as 1.1 ML. PL and micro-PL data show emission from the QDs as well as from an accompanying 2 D layer; the temperature dependence of these emissions is notably di!erent. Strong red-shifts of both emissions as the CdSe coverage increases are observed. 2000 Elsevier Science B.V. All rights reserved. PACS: 78.66.Hf; 78.66.!w; 78.55.Et; 68.65.#g; 81.15.Hi Keywords: II}VI semiconductors; Self-assembling; Quantum dots; Photoluminescence; MBE
Intense interest in the surface morphology of overlayers deposited on lattice-mismatched surfaces has been sparked by the interest in mechanisms leading to the formation of quantum dots (QDs). The epitaxial system most widely studied in this context is that of InAs overlayers on (0 0 1) GaAs. The growth of InAs QDs on GaAs substrates is usually described in terms of the Stranski}Krastanow (S}K) mechanism, where coherent islands form after deposition of 1 to 2 monolayers (ML) of InAs, referred to as the wetting layer [1}3]. The situation in II}VI materials is less clear. Here the * Corresponding author. Tel.: #1-219-631-6962; fax: #1219-631-5952. E-mail address: [email protected] (M. Dobrowolska).
most extensively explored materials combination is that of CdSe on (0 0 1) ZnSe [4}10]. In this paper we will present optical data which strongly suggest that in the case of CdSe grown on ZnSe, 3-D islands (QDs) begin to form well before the deposited layer reaches its critical thickness, indicating that the S}K mechanism is inadequate to describe the 2-D to 3-D transition that occurs in this system } atleast under conditions used for growing our CdSe/ZnSe specimens, described below. The samples were grown by molecular-beam epitaxy (MBE). A ZnSe bu!er was "rst grown at 3003C on a (1 0 0) GaAs substrate to a thickness of approximately 2 lm. For the growth of CdSe layers, the substrate temperature was raised to
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 2 2 1 - 9
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C.S. Kim et al. / Journal of Crystal Growth 214/215 (2000) 761}764
3503C. For the purposes of the present study we have grown four specimens, with nominal thicknesses of the CdSe layers of 1.1, 1.5, 1.9 and 2.6 ML. We observed the RHEED streaks transforming into spots only for the sample with 2.6 ML CdSe deposition. After deposition, the CdSe layers were capped by 50 nm of ZnSe. In our early work we performed AFM studies on both bare ZnSe and uncapped CdSe dots on ZnSe [4]. While AFM pictures taken on ZnSe sample show very smooth surface, the pictures taken on CdSe clearly show very well-developed dots. However, this study also revealed a ripening of the dots in room temperature. This rather quick ripening prevented us from using AFM method for systematic study of QD formation as a function of CdSe deposition. Therefore, in this paper we use the optical method to study the QD formation process. Systematic photoluminescence (PL) and microPL studies were carried out on the series of specimens described above. The micro-PL experiments were performed at 6 K using a Dilor spectrometer with a liquid-nitrogen-cooled CCD camera and a microscope. We recorded the spectrally dispersed luminescence from 1.7 lm-diameter regions of each sample. The samples were excited at 438 nm by a doubled mode-locked Ti-sapphire laser. Fig. 1 shows micro-PL spectra observed on four samples with di!erent CdSe coverages. One can see a clear progression of the broad PL spectrum towards lower energies as the CdSe coverage increases. Superimposed on the broad luminescence,
Fig. 1. Micro-photoluminescence spectra taken at 6 K on a series of CdSe/ZnSe QD samples with di!erent CdSe coverages (solid curves), together with best-"t two Gaussian simulations (dotted curves).
we observe many sharp lines, with a typical width of 200 leV. The most striking feature in Fig. 1 is that the sharp lines are seen on all of the measured samples, including the one with 1.1 ML CdSe coverage; the presence of these sharp spikes is more noticeable as coverage increases. The observation of the sharp peaks superimposed on the broad luminescence indicates the presence of 0}D excitons con"ned to quantum-dot regions. Another important feature of the data shown in Fig. 1 is the asymmetry of the broad PL lines, most obviously seen for samples with 1.5 and 1.9 ML coverages. Since the spectrum observed on the 1.5 ML sample suggests the presence of two lines, we model the broad luminescence as a superposition of two Gaussian lines. Dotted curves in Fig. 1 show two-line "ts to the data which provide a good description of the PL lineshape observed on samples with 1.1, 1.5, and 1.9 ML coverages. The luminescence observed on the sample with 2.6 ML coverage can be modeled satisfactorily with a single line. Three additional features emerge from the data displayed in Fig. 1. First, the intensity ratio of the lower-energy line (line B) to the higher-energy line (line A) increases with increasing coverage. Secondly, the sharp spikes superimposed on the broad emission are much more pronounced on the lower-energy side of each PL spectrum. Thirdly, we see in Fig. 1 that the PL energies of both lines A and B shift towards lower energies, and the energy separations between them systematically increase, as the thickness of the CdSe layer increases. In order to gain insight into the nature of both lines making up the broad spectrum, we performed a temperature study of the macro-PL. We separated the contributions from lines A and B by modeling all spectra as a superposition of two Gaussian lines, and followed the temperature dependence of the PL energy, the integrated intensity, and the FWHM of both lines. Fig. 2 shows the energy position of lines A and B as a function of temperature for samples with 1.1 and 1.9 ML deposition. The open symbols show the temperature behavior of line A, while full symbols represent line B. To facilitate comparison between the two lines, we approximated all curves by straight lines in the "gure.
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763
Fig. 2. Temperature dependence of the energy position of lines B (full symbols) and A (open symbols).
Fig. 3. Temperature dependence of full-width at half-maximum of line B (open circles) and of line A (full circles) observed on the CdSe/ZnSe sample with 1.9 ML CdSe deposition.
For a given coverage, the red shift of the PL energy of line B always increases faster than that of line A. For instance the slopes of the straightline "ts shown in Fig. 2 are as follows: for 1.1 ML coverage, the slopes are !1.6;10\ and !2.8;10\; for 1.9 ML, !2.4;10\ and !2.7;10\ for lines A and B, respectively. We "nd that the di!erence between the slopes of lines A and B increase with CdSe coverage. Note, for example, that for a coverage of 1.9 ML, the rate of energy shift with increasing temperature for line B is an order of magnitude larger than for line A. This di!erence in the temperature dependence of the PL emissions has been attributed to 0D and 2D con"ned excitons in both II}VI [11,12] and III}VI [13] systems, with the 0D or QD emission (line B) having a much more rapid temperature dependence of the energy shift than the 2D or QW (or wetting layer) emission (line A). In Fig. 3 we display the temperature dependence of the FWHM for the QD (line B) and QW (line A) for the 1.9 ML CdSe sample. The FWHM of the QD line shows signi"cant narrowing as the temperature increases. This narrowing as temperature increases has been observed in other QD systems and explained by a redistribution of carriers to the lower energy QD states. [11,13}15]. The QW line (line A), on the other hand, shows a behavior typical of the emission of excitons con"ned in a quantum well, where the FWHM increases monotonically with temperature. Thus, the temper-
ature dependence of the peak emission energies and the line width data are consistent with line B being associated with QDs or zero-dimensionally-con"ned excitons, and with line A being associated with more extended regions that are 2D in character (see also below). Since we observe line B in the PL spectrum for the sample with only a 1.1 ML coverage, we conclude that the precursors of the dots form at an early stage of CdSe deposition. This indicates that the onset of quantum dot formation in the case of CdSe layers grown on ZnSe is not abrupt as found in the InAs/GaAs case, but occurs more gradually. Based on our optical "ndings lead as to speculate on the following possible scenario for the early evolution of 2D and 0D growth. As we deposit CdSe, extended ZnCdSe 2-D islands form, with some graded composition containing local maxima of Cd concentration (and possibly accompanied by thickness #uctuations) [16]. The local maxima of Cd concentration provide localization for 0D excitons and give rise to the spikes associated with the lower-energy emission band (line B in Fig. 1). During CdSe deposition both the extended 2D layer and the localizing #uctuations evolve simultaneously; and as the amount of Cd delivered increases, both regions become richer in Cd, so that the two PL lines (A and B) shift to lower energies. Furthermore, because of strain energy considerations, the Cd-rich regions also preferentially accumulate more material, that may in turn lead to the
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development of island-like thickness #uctuations (which also relieves the signi"cant strain in the layer). This morphological picture is consistent with compositional nonuniformities driven by strain that stabilize "lm growth of lattice-mismatched alloys, as discussed by Guyer and Voorhees [17]. The same kind of model can also describe the relative intensity variation of lines A and B with increasing CdSe deposition. At the very early stage of dot formation (i.e., at coverages slightly exceeding 1 ML) the density of the dots themselves is very low, which gives rise to a very weak line B. Moreover, the microscopic regions which give rise to the spikes seen in line B can be easily saturated because of the characteristically low densities of states of the zero-dimensional objects, so that the states in the extended 2D ZnCdSe islands (which lie at a higher energy) can also be partially "lled. As the coverage increases from 1.5 to 1.9 ML, the density of the localizing regions rapidly increases, resulting in a larger density of spikes and a stronger integrated intensity of line B relative to its higher-energy partner, since one now has a larger density of the 0D exciton centers to be "lled before one begins to "ll states in the extended 2D ZnCdSe island regions. In conclusion, we have performed both macroPL and micro-PL studies on a series of samples consisting of CdSe layers deposited on ZnSe as a function of nominal layer thickness. We observe strongly con"ned zero-dimensional excitons for coverages as low as 1.1 ML, i.e., far below the critical thickness for CdSe on ZnSe. We also observe strong red shifts of the PL signal from both QDs and extended 2D CdZnSe layers as the CdSe deposition increases. These "ndings suggest that in the CdSe/ZnSe system the change from 2D to 3D morphology occurs gradually (starting with as little as 1 ML CdSe deposition), and therefore is more complex than the spontaneous and abrupt change implied by the Stranski}Krastanow growth mode.
We acknowledge the support by the NSF (DMR-9705443, DMR-9705064, ECS-9412772), and ARO (DAAG55-97-1-0378). We also appreciate conversations with J.C. Kim.
References [1] R. Notzel, J. Temmyo, T. Tamamura, Nature (London) 369 (1994) 131. [2] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petro!, Appl. Phys. Lett. 63 (1993) 3203. [3] R. Heitz, T.R. Ramachandran, A. Kalburge, Q. Xie, I. Mukhametzhanov, P. Chen, A. Madhukar, Phys. Rev. Lett. 78 (1997) 4071. [4] J.L. Merz, S. Lee, J.K. Furdyna, J. Crystal Growth 184/185 (1998) 228. [5] S.H. Xin, P.D. Wang, Aie Yin, C.S. Kim, M. Dobrowolska, J.K. Merz, J.K. Furdyna, Appl. Phys. Lett. 69 (1996) 3884. [6] K. Leonardi, H. Heinke, K. Ohkawa, D. Hommel, H. Selke, F. Gindele, U. Woggon, Appl. Phys. Lett. 71 (1997) 1510. [7] T. Kummell, R. Weigand, G. Bacher, A. Forchel, K. Leonardi, D. Hommel, H. Selke, Appl. Phys. Lett. 73 (1998) 3105. [8] F. Flack, N. Samarth, U. Nikitin, P.A. Crowell, J. Shi, J. Levy, D.D. Awschalom, Phys. Rev. B 54 (1996) R17 312. [9] H. Kirmse, R. Schneider, M. Rabe, W. Neumann, F. Henneberger, Appl. Phys. Lett. 72 (1998) 1329. [10] J.C. Kim, H. Rho, L.M. Smith, H.E. Jackson, S. Lee, M. Dobrowolska, J.K. Furdyna, Appl. Phys. Lett. 75 (1999) 214. [11] G. Karczewski, S. Mackowski, M. Kutrowski, T. Wojtowicz, J. Kossut, Appl. Phys. Lett. 74 (1999) 3011. [12] Y. Wu, K. Arai, T. Yao, Phys. Rev. B 53 (1996) R10-485. [13] D.I. Lubyshev, P.P. Gonz'alez-Borrero, E. Marega Jr., E. Petitprez, N. La Scala Jr., P. Basmaji, Appl. Phys. Lett 68 (1996) 205. [14] H. Lee, W. Yang, P.C. Sercel, Phys. Rev. B 55 (1997) 9757. [15] A. Polimeni, A. Patane, M. Henini, L. Eaves, P.C. Main, Phys. Rev. B 59 (1999) 5064. [16] M. Strassburg, V. Kutzer, V.W. Pohl, A. Ho!mann, I. Broser, N.N. Ledenstsov, B. Bimberg, A. Rosenauer, V. Fisher, D. Gerthsen, I.L. Krestnikov, M.V. Maximov, P.S. KopH ev, Zh.I. Alferov, Appl. Phys. Lett. 72 (1998) 942. [17] J.E. Guyer, P.W. Voorhees, Phys. Rev. Lett. 74 (1995) 4031.