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Photoluminescence measurements of InP quantum dots in pulsed magnetic fields
Physica B 256±258 (1998) 203±206
Photoluminescence measurements of InP quantum dots in pulsed magnetic ®elds R. Provoost a
a,*
, M. Hayne a, M.K. Zundel b, K. Eberl b, V.V. Moshchalkov
a
Laboratorium voor Vaste-Stoysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium b Max-Planck-Instit ut f ur Festk orperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany
Abstract We report the ®rst photoluminescence (PL) measurements on stacked layers of self-assembled InP quantum dots in pulsed magnetic ®elds up to 50 T at 4.2 K. A strong increase of the diamagnetic shift with magnetic ®eld parallel and perpendicular to the growth direction is observed by decreasing the stacked layer separation from 8 to 4 nm, indicating that in latter case the quantum dot layers are strongly coupled. This is con®rmed by the decrease in PL line width, which is determined by the height homogeneity of a single dot or by the ¯uctuation of the total height of the coupled layers. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Photoluminescence; InP; Quantum dots; Pulsed magnetic ®elds
1. Introduction There has been much interest in the growth of self-assembled quantum dots (InAs, InP,. . .) in fundamental and applied physics due to their optoelectronic properties [1]. The epitaxial growth of a few monolayers of a semiconductor on a lattice mismatched Gax In1 ÿ x P buer layer results in the nucleation of the quantum dots upon a wetting layer due to the large strain ®elds [2]. By this method, known as the Stransky±Krastanov growth mode, a very large number of quantum dots are easily grown with relatively small sizes (about 10 nm). The photoluminescence (PL) line related with the con®ned excitons in the dots is very intense. The single-dot spectroscopy reveals very sharp PL lines [3]. However, the large number
of dots probed by conventional PL experiments results in a width of about 40 meV, determined by the averaging eects due to the presence of size ¯uctuations and shape non-uniformity of the dots. On the other hand, by a repetition of layers of dots separated by a few nanometers a reduction of the line width has been obtained due to the more homogeneous size distribution and the coupling between the layers [4]. Here we report PL measurements in ®elds up to 50 T on stacked layers of self-assembled InP quantum dots. By measuring the diamagnetic shift of the PL line one can study the spatial anisotropy [5] and electronic coupling between stacked layers of quantum dots as a function of the layer separation.
2. Experimental details * Corresponding author. Tel.: +32 16 327120; fax: +32 16 327983; e-mail: [email protected].
The InP quantum dots were grown on a GaAs substrate with a Ga0:52 In0:48 P buer layer. The lat-
0921-4526/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 5 1 6 - X
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R. Provoost et al. / Physica B 256±258 (1998) 203±206
tice mismatch between Ga0:52 In0:48 P and InP is 3.7%. For sample A, only one layer of quantum dots was grown on this structure by depositing three monolayers of InP. For sample B, C, and D three layers of InP dots were grown, separated by a GaInP layer with a thickness of 8, 4, and 2 nm, respectively. 50 nm GaInP covered the third layer of InP dots. A detailed description of these samples can be found in Ref. [4]. TEM micrographs showed that for sample B, C, and D the InP quantum dots are nicely stacked above each other. The dots have typical dimensions of 3 nm in height and 16 nm diameter and with density of 5 ´ 1010 cmÿ2 [4]. The PL measurements were performed at 4.2 K in pulsed magnetic ®elds up to 50 T, aligned parallel and perpendicular to the growth direction z. The excitation was provided by a solid-state laser operating at 532 nm and transmitted to the sample through an optical ®bre. The diameter of the illuminated region was about 2 mm with a total laser power of 20 mW. The PL was collected by pick-up ®bres surrounding the excitation ®bre, dispersed by a 150 g/mm grating of a monochromator, and projected on a CCD detector. By only measuring the PL at the peak of the 20 ms magnetic ®eld pulse, a constant ®eld (1%) can be obtained during an exposure time of 1.8 ms.
3. Results and discussion In Fig. 1 we present the evolution of the PL energy as a function of magnetic ®eld applied parallel and perpendicular to the growth direction. The energy of the PL line is given by the centre of mass of the peak as determined from a numerical analysis. In some cases, a small oset is observed for the PL peak positions for the two ®eld directions. This systematic error is a product of the experimental arrangement, but it does not aect the relative movement of the PL lines with ®eld. There is no change in the shape of the PL lines with magnetic ®eld. The complexity of the factors causing the line shifts in magnetic ®elds, must include for example, the electronic coupling of the dots as well as the change from the spatial to magnetic con®nement [6], is beyond the scope of this work. Therefore, we have ®tted the PL peak positions with third order polynomials, to determine the relative shift between zero ®eld and 50 T (Table 1). We shall thus limit ourselves to a qualitative discussion of the results. For sample A with only a single layer of InP dots, we observe a very small shift of 4 meV with the ®eld perpendicular to z. This illustrates the large con®nement of the excitons in the z-direction. The shifts of the PL line for sample B are
Fig. 1. Centre of mass position of the PL energy at 4.2 K with magnetic ®elds B parallel and perpendicular to the growth direction z for the sample with a single layer of dots, and for the three samples with three layers of dots, spaced by 8, 4, and 2 nm of GaInP.
R. Provoost et al. / Physica B 256±258 (1998) 203±206
205
Table 1 Details of the InP quantum dot samples A, B, C, and D; shift of the PL line between zero ®eld and 50 T for magnetic ®elds parallel and perpendicular to the growth direction, the line width of the PL mode, and the stacking height for the samples with three layers of InP dots Sample
B // z
B^z
Line width
Stack height
A (single) B (8 nm) C (4 nm) D (2 nm)
7 10 20 17
4 5 13 8
43 40 28 24
) 20 nm 12 nm 8 nm
meV meV meV meV
meV meV meV meV
larger in both measured geometries. During the deposition of this multilayer a stacking of dots is grown due to the strain ®eld formed by the dots in the lower layer. However, the strain ®eld of small dots is not high enough to continue a stacking, so only larger dots will be formed in the upper layers, increasing the average size of the dots compared to a single layer. Such a weakening of the spatial con®nement will enhance the ®eld induced diamagnetic shift. For sample C a very pronouncing increase in shift is observed for both geometries, indicating less spatial con®nement. Since there are no large dierences in size and shape distribution of the dots of sample B and C, we conclude that there is a strong coupling between the InP dot layers for sample C, producing a large decrease in the electronic con®nement in the growth direction. The PL line of the multilayer of dots spaced by 2 nm (sample D) shows almost the same shift with ®eld parallel to z as for sample C, thus indicating that coupling exists between the layers. However, the shift of the line with ®eld perpendicular to z for sample D is signi®cantly smaller than for sample C. This can be attributed to the total height of the stacked layer which is now reduced to about 8 nm compared with 12 nm in sample C, increasing the con®nement and reducing the diamagnetic shift again. The coupling of the layers also produces a drop in the line width [4], shown in Fig. 2 and given in Table 1. This is a result of the decrease in relative size of a small (e.g. monolayer) ¯uctuation in the height of the dots. The stack of coupled dots has a height of 12 nm for sample C, and 8 nm for sample D compared with 3 nm for the individual dots. The large line width of sample B implies that the layers are too widely separated to be strongly coupled.
meV meV meV meV
The measured PL energy versus ®eld parallel to z can be ®tted for all samples with B2 up to at least 30 T (not shown here), indicating the diamagnetic shift of the electron and hole levels [7]. For higher ®elds, the shift tends to a more distinctly linear dependence [6], which can be seen in Fig. 1. The low ®eld and high ®eld behaviour of the PL line with ®eld perpendicular to z, is rather dierent for the 4 nm spaced sample (C). At low ®elds a small increase in transition energy is observed up to 5±10 T, which becomes more parabolic behaviour at higher ®elds. The increase at low ®elds is reproducible and it has also been seen in measurements with a higher resolution grating (600 g/mm). This contrasts with the constant or even decreasing PL energy observed by others in single layer (InGa)As dots in the same ®eld con®guration [5±7]. We have no detailed explanation for this behaviour as yet, but it is most likely that it is associated with the coupling of the stacked quantum dots.
Fig. 2. PL spectra of the samples recorded in 1.8 ms at zero ®eld and 4.2 K.
206
R. Provoost et al. / Physica B 256±258 (1998) 203±206
4. Conclusions
Acknowledgements
We have analysed the PL spectra of InP quantum dots in a single layer or in three layers spaced by 8, 4, and 2 nm in pulsed magnetic ®elds parallel and perpendicular to z. A comparison of the size of the shifts for each of the samples, demonstrates that the 4 nm and the 2 nm spaced samples are strongly coupled. This is con®rmed by a reduction in the line width. If the layers are separated by 8 nm of GaInP, the coupling seems to be almost negligible. In low magnetic ®elds an increase of the PL energy has been observed in the sample with 4 nm layer separation. Since this has never been seen in a sample with a single layer of dots, we conclude that this eect has its origin in the stacking and the coupling of the dots.
This research has been supported by the Fund for Scienti®c Research Flanders (FWO-Vlaanderen), the Concerted Research Action (GOA), and the Interuniversity Attraction Poles (IUAP) programs. M.H. is a fellow of the Research Council of the KU Leuven. References [1] [2] [3] [4] [5] [6] [7]
N.N. Ledentsov et al., Phys. Rev. B 54 (1996) 8743. A. Kurtenbach et al., J. Electron. Mater. 25 (1996) 395. M. Grundmann et al., Phys. Rev. Lett. 74 (1995) 4043. M.K. Zundel et al., Appl. Phys. Lett. 71 (1997) 2972. P.D. Wang et al., Phys. Rev. B 53 (1996) 16458. S.T. Stoddart et al., Appl. Surf. Sci. 123±124 (1998) 366. I.E. Itskevich et al., Appl. Phys. Lett. 70 (1997) 505.
Photoluminescence measurements of InP quantum dots in pulsed magnetic ®elds R. Provoost a
a,*
, M. Hayne a, M.K. Zundel b, K. Eberl b, V.V. Moshchalkov
a
Laboratorium voor Vaste-Stoysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium b Max-Planck-Instit ut f ur Festk orperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany
Abstract We report the ®rst photoluminescence (PL) measurements on stacked layers of self-assembled InP quantum dots in pulsed magnetic ®elds up to 50 T at 4.2 K. A strong increase of the diamagnetic shift with magnetic ®eld parallel and perpendicular to the growth direction is observed by decreasing the stacked layer separation from 8 to 4 nm, indicating that in latter case the quantum dot layers are strongly coupled. This is con®rmed by the decrease in PL line width, which is determined by the height homogeneity of a single dot or by the ¯uctuation of the total height of the coupled layers. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Photoluminescence; InP; Quantum dots; Pulsed magnetic ®elds
1. Introduction There has been much interest in the growth of self-assembled quantum dots (InAs, InP,. . .) in fundamental and applied physics due to their optoelectronic properties [1]. The epitaxial growth of a few monolayers of a semiconductor on a lattice mismatched Gax In1 ÿ x P buer layer results in the nucleation of the quantum dots upon a wetting layer due to the large strain ®elds [2]. By this method, known as the Stransky±Krastanov growth mode, a very large number of quantum dots are easily grown with relatively small sizes (about 10 nm). The photoluminescence (PL) line related with the con®ned excitons in the dots is very intense. The single-dot spectroscopy reveals very sharp PL lines [3]. However, the large number
of dots probed by conventional PL experiments results in a width of about 40 meV, determined by the averaging eects due to the presence of size ¯uctuations and shape non-uniformity of the dots. On the other hand, by a repetition of layers of dots separated by a few nanometers a reduction of the line width has been obtained due to the more homogeneous size distribution and the coupling between the layers [4]. Here we report PL measurements in ®elds up to 50 T on stacked layers of self-assembled InP quantum dots. By measuring the diamagnetic shift of the PL line one can study the spatial anisotropy [5] and electronic coupling between stacked layers of quantum dots as a function of the layer separation.
2. Experimental details * Corresponding author. Tel.: +32 16 327120; fax: +32 16 327983; e-mail: [email protected].
The InP quantum dots were grown on a GaAs substrate with a Ga0:52 In0:48 P buer layer. The lat-
0921-4526/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 5 1 6 - X
204
R. Provoost et al. / Physica B 256±258 (1998) 203±206
tice mismatch between Ga0:52 In0:48 P and InP is 3.7%. For sample A, only one layer of quantum dots was grown on this structure by depositing three monolayers of InP. For sample B, C, and D three layers of InP dots were grown, separated by a GaInP layer with a thickness of 8, 4, and 2 nm, respectively. 50 nm GaInP covered the third layer of InP dots. A detailed description of these samples can be found in Ref. [4]. TEM micrographs showed that for sample B, C, and D the InP quantum dots are nicely stacked above each other. The dots have typical dimensions of 3 nm in height and 16 nm diameter and with density of 5 ´ 1010 cmÿ2 [4]. The PL measurements were performed at 4.2 K in pulsed magnetic ®elds up to 50 T, aligned parallel and perpendicular to the growth direction z. The excitation was provided by a solid-state laser operating at 532 nm and transmitted to the sample through an optical ®bre. The diameter of the illuminated region was about 2 mm with a total laser power of 20 mW. The PL was collected by pick-up ®bres surrounding the excitation ®bre, dispersed by a 150 g/mm grating of a monochromator, and projected on a CCD detector. By only measuring the PL at the peak of the 20 ms magnetic ®eld pulse, a constant ®eld (1%) can be obtained during an exposure time of 1.8 ms.
3. Results and discussion In Fig. 1 we present the evolution of the PL energy as a function of magnetic ®eld applied parallel and perpendicular to the growth direction. The energy of the PL line is given by the centre of mass of the peak as determined from a numerical analysis. In some cases, a small oset is observed for the PL peak positions for the two ®eld directions. This systematic error is a product of the experimental arrangement, but it does not aect the relative movement of the PL lines with ®eld. There is no change in the shape of the PL lines with magnetic ®eld. The complexity of the factors causing the line shifts in magnetic ®elds, must include for example, the electronic coupling of the dots as well as the change from the spatial to magnetic con®nement [6], is beyond the scope of this work. Therefore, we have ®tted the PL peak positions with third order polynomials, to determine the relative shift between zero ®eld and 50 T (Table 1). We shall thus limit ourselves to a qualitative discussion of the results. For sample A with only a single layer of InP dots, we observe a very small shift of 4 meV with the ®eld perpendicular to z. This illustrates the large con®nement of the excitons in the z-direction. The shifts of the PL line for sample B are
Fig. 1. Centre of mass position of the PL energy at 4.2 K with magnetic ®elds B parallel and perpendicular to the growth direction z for the sample with a single layer of dots, and for the three samples with three layers of dots, spaced by 8, 4, and 2 nm of GaInP.
R. Provoost et al. / Physica B 256±258 (1998) 203±206
205
Table 1 Details of the InP quantum dot samples A, B, C, and D; shift of the PL line between zero ®eld and 50 T for magnetic ®elds parallel and perpendicular to the growth direction, the line width of the PL mode, and the stacking height for the samples with three layers of InP dots Sample
B // z
B^z
Line width
Stack height
A (single) B (8 nm) C (4 nm) D (2 nm)
7 10 20 17
4 5 13 8
43 40 28 24
) 20 nm 12 nm 8 nm
meV meV meV meV
meV meV meV meV
larger in both measured geometries. During the deposition of this multilayer a stacking of dots is grown due to the strain ®eld formed by the dots in the lower layer. However, the strain ®eld of small dots is not high enough to continue a stacking, so only larger dots will be formed in the upper layers, increasing the average size of the dots compared to a single layer. Such a weakening of the spatial con®nement will enhance the ®eld induced diamagnetic shift. For sample C a very pronouncing increase in shift is observed for both geometries, indicating less spatial con®nement. Since there are no large dierences in size and shape distribution of the dots of sample B and C, we conclude that there is a strong coupling between the InP dot layers for sample C, producing a large decrease in the electronic con®nement in the growth direction. The PL line of the multilayer of dots spaced by 2 nm (sample D) shows almost the same shift with ®eld parallel to z as for sample C, thus indicating that coupling exists between the layers. However, the shift of the line with ®eld perpendicular to z for sample D is signi®cantly smaller than for sample C. This can be attributed to the total height of the stacked layer which is now reduced to about 8 nm compared with 12 nm in sample C, increasing the con®nement and reducing the diamagnetic shift again. The coupling of the layers also produces a drop in the line width [4], shown in Fig. 2 and given in Table 1. This is a result of the decrease in relative size of a small (e.g. monolayer) ¯uctuation in the height of the dots. The stack of coupled dots has a height of 12 nm for sample C, and 8 nm for sample D compared with 3 nm for the individual dots. The large line width of sample B implies that the layers are too widely separated to be strongly coupled.
meV meV meV meV
The measured PL energy versus ®eld parallel to z can be ®tted for all samples with B2 up to at least 30 T (not shown here), indicating the diamagnetic shift of the electron and hole levels [7]. For higher ®elds, the shift tends to a more distinctly linear dependence [6], which can be seen in Fig. 1. The low ®eld and high ®eld behaviour of the PL line with ®eld perpendicular to z, is rather dierent for the 4 nm spaced sample (C). At low ®elds a small increase in transition energy is observed up to 5±10 T, which becomes more parabolic behaviour at higher ®elds. The increase at low ®elds is reproducible and it has also been seen in measurements with a higher resolution grating (600 g/mm). This contrasts with the constant or even decreasing PL energy observed by others in single layer (InGa)As dots in the same ®eld con®guration [5±7]. We have no detailed explanation for this behaviour as yet, but it is most likely that it is associated with the coupling of the stacked quantum dots.
Fig. 2. PL spectra of the samples recorded in 1.8 ms at zero ®eld and 4.2 K.
206
R. Provoost et al. / Physica B 256±258 (1998) 203±206
4. Conclusions
Acknowledgements
We have analysed the PL spectra of InP quantum dots in a single layer or in three layers spaced by 8, 4, and 2 nm in pulsed magnetic ®elds parallel and perpendicular to z. A comparison of the size of the shifts for each of the samples, demonstrates that the 4 nm and the 2 nm spaced samples are strongly coupled. This is con®rmed by a reduction in the line width. If the layers are separated by 8 nm of GaInP, the coupling seems to be almost negligible. In low magnetic ®elds an increase of the PL energy has been observed in the sample with 4 nm layer separation. Since this has never been seen in a sample with a single layer of dots, we conclude that this eect has its origin in the stacking and the coupling of the dots.
This research has been supported by the Fund for Scienti®c Research Flanders (FWO-Vlaanderen), the Concerted Research Action (GOA), and the Interuniversity Attraction Poles (IUAP) programs. M.H. is a fellow of the Research Council of the KU Leuven. References [1] [2] [3] [4] [5] [6] [7]
N.N. Ledentsov et al., Phys. Rev. B 54 (1996) 8743. A. Kurtenbach et al., J. Electron. Mater. 25 (1996) 395. M. Grundmann et al., Phys. Rev. Lett. 74 (1995) 4043. M.K. Zundel et al., Appl. Phys. Lett. 71 (1997) 2972. P.D. Wang et al., Phys. Rev. B 53 (1996) 16458. S.T. Stoddart et al., Appl. Surf. Sci. 123±124 (1998) 366. I.E. Itskevich et al., Appl. Phys. Lett. 70 (1997) 505.