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Influence of In composition on the photoluminescence emission of In(Ga)As quantum dot bilayers
ARTICLE IN PRESS
Physica E 26 (2005) 124–128 www.elsevier.com/locate/physe
Influence of In composition on the photoluminescence emission of In(Ga)As quantum dot bilayers M.A. Migliorato, M.J. Steer, W.M. Soong, C.M. Tey, H-Y. Liu, S.L. Liew, P. Navaretti, D.J. Norris, A.G. Cullis, M. Hopkinson Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, UK Available online 18 November 2004
Abstract We discuss a novel approach to the optimisation of quantum dot bilayer structures grown by molecular beam epitaxy. Use of a kinetic segregation model has shown that a reduction of the In composition for the upper layer of a bilayer structure can be used to compensate for the excess In that exists on the surface prior to growth. Three samples have been grown with upper dot In compositions varying from 90% to 100% and have been investigated by means of optical spectroscopy and electron microscopy. r 2004 Elsevier B.V. All rights reserved. PACS: 73.63.Kv; 81.07.Ta Keywords: Quantum dots; Spectroscopy; Molecular beam epitaxy
It has been observed that vertically stacked quantum dot structures often exhibit an increase in the average size of the islands with increasing number of quantum dot layers deposited [1]. This strongly hampers the possibility of growing closely stacked quantum dots (QDs) that exhibit very narrow line widths (a bimodal photoluminescence emission is often observed), which is of paramount Corresponding author. Tel.: +44-114-2225836; fax: +44114-2754589. E-mail address: [email protected] (M.A. Migliorato).
importance for the realisation of QD lasers and detectors. The understanding of the In segregation-induced Stranski–Krastanow transition [2] suggests that, for growth conditions in which strain effects are minimised, an increase in size is primarily due to an excess of In caused by elemental segregation. Similar considerations have lead in the past to attempting to optimise the growth conditions by either removing the excess In by rapid thermal annealing (In flush) [3], or by modification of the total coverage used during the island deposition [4]. However both techniques present limitations
1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.08.037
ARTICLE IN PRESS M.A. Migliorato et al. / Physica E 26 (2005) 124–128
that cannot be easily overcome, such as the lack of size control for the initial QD layers in one case, and the fact that the ideal value of the total coverage is only calibrated to a specific spacer thickness in the other case. Therefore, we propose the use of an alternative method based on the use of the kinetic segregation model [5,6] in order to quantify the elemental segregation in the growing layers and therefore predict the optimum fraction of In that needs to be deposited in the second layer to obtain more uniform emissions from a bilayer structure of closely stacked InAs/GaAs QDs. The In composition profile is calculated after depositing 2.5 ML of InAs at 500 1C on the first layer with a 5 nm thick GaAs barrier, as shown in Fig. 1a. It is evident that the growth conditions for the second layer are different from the growth conditions of the first layer, due to the presence of a monolayer of enriched In on the surface of the GaAs separation barrier. If a second layer of InAs
125
QDs is deposited under the same growth conditions (Fig. 1b) then the second layer will incorporate an excess of around 0.05 fraction of indium, resulting in larger islands compared to the first layer. Uniformity in the deposited In can be reached by decreasing the In fraction in the impinging flux: Fig. 1c shows that the best predicted growth conditions are obtained when depositing an InxGa1xAs alloy with x ¼ 0:94; while lower fractions of In ðx ¼ 0:9Þ will result in the nucleation of smaller islands (Fig. 1d). At this stage the influence of the strain field, due to the seed layer, on the atom mobility is neglected. To test the predictions of the kinetic segregation model, three bilayer samples with varying In compositions in the upper QD layer were grown under identical conditions by molecular beam epitaxy on undoped GaAs (1 0 0) substrates. A buffer layer of 200 nm of GaAs was grown at 590 1C, the substrate temperature was reduced to 500 1C and the bilayer structure with a 5 nm GaAs
Fig. 1. (a)–(d) Calculated effect of In segregation on the initial growth conditions for the second layer of quantum dots in bilayer structures.
ARTICLE IN PRESS M.A. Migliorato et al. / Physica E 26 (2005) 124–128
126
spacer layer deposited, the second QD layer was capped with 15 nm of GaAs, a further 85 nm of GaAs was then deposited at 590 1C. For the three samples under investigation the first QD layer consisted of 2.5 ML of InAs grown at 0.1 ML/s, the composition of the second layer was varied so that the In composition in samples A, B and C were 100%, 94% and 90%, respectively, the thickness of 2.5 ML remained the same. The low-temperature (10 K) and low power of excitation (2 mW) photoluminescence for the sets of samples (Fig. 2a) shows a definite blueshift of the emission with decreasing In content of the top layer from 100%: it is however evident that the low-energy side of the emissions of samples B and C (94% and 90%) is roughly the same. This is consistent with the interpretation that the lowenergy side of the photoluminescence (PL) spectra is in general associated with the ground state emission of the top layer (larger islands) [7] in the samples where both layers are grown with a nominal 100% InAs, with the typical high-energy
I
(a)
II
III
IV
V
PL intensity(a.u.)
(b)
2mW Sample A Sample B Sample C 150mW Sample A In = 100 %
(c)
150mW Sample B In = 94 %
(d)
150mW Sample C In = 90 %
(e)
20mW Sample C In = 90 %
1.15
tail due to a low-intensity emission from the seed layer (smaller islands). For samples B and C where the In content of the top layer is lowered, the island size on the same layer is reduced and the combined PL of the two layers is blueshifted. This situation is evident in the transmission electron microscopy (TEM) micrographs of Fig. 3: larger (a), equally sized (b) and smaller (c) islands are produced on the top layer for decreasing In content on the top layer during deposition. The line widths (FWHM) of the emissions are measured at 27, 45, 113 meV for samples A, B and C, respectively. Sample C also presents a bimodal distribution of the collected luminescence, with peaks roughly separated by 70 meV. Such a bimodal emission is usually an indication of the presence of at least two distinct sets of QDs between which carrier transfer is not as efficient [8]. Instead for the other samples, carrier transfer is usually efficient enough to allow relaxation towards the lowest energy levels before recombination. The intensity of the emission from higher energy levels instead increases significantly only
1.20
1.25
1.30
1.35
1.40
1.45
Photon Energy (eV) Fig. 2. (a)–(e) Low-temperature PL for QD bilayers grown with a different In content in the second layer compared to the seed layer.
Fig. 3. Typical micrographs: larger (a), equally sized (b) and smaller (c) islands are produced on the top layer for decreasing In content on the top layer during deposition.
ARTICLE IN PRESS M.A. Migliorato et al. / Physica E 26 (2005) 124–128
when the intensity of lower energy states emission is partially saturated. This becomes evident if the excitation power is increased: the power-dependent PL spectra at 10 K was recorded between 2 and 150 mW using a diode-pumped solid-state laser (532 nm) for the three samples. To determine the peak positions the spectra were fitted with Gaussian distributions consistently for different excitation powers. For an excitation power of 150 mW the sample with nominally identical layers (sample A) shows a convoluted PL spectrum formed by a number of competing emissions which are well fitted by 5 gaussian distributions with peak positions at 1.232, 1.259, 1.289, 1.319 and 1.349 eV. Lowering the composition of the second layer to 94% results in the disappearance of the lowest energy peak (1.232 eV), which can therefore be assigned to the ground state emission of the top layer. Furthermore, the highest energy peak (1.349 eV) is also suppressed, suggesting that it is due to an excited state transition of the top layer. The lowest energy peak of sample B (1.257 eV) is very close to the second peak of sample A (1.259 eV), so that we can assign this to the ground state emission of the seed layer. Reducing the composition of the top layer to 90% results in a broadening of the emission towards higher energies: the emission is well fitted by two gaussian peaks, the energies of which (1.249 and 1.312 eV) are very close to the peak energies of sample B (1.255 and 1.318 eV). The low-energy peak is at a slightly lower energy (the difference is 8 meV) than the low-energy peak of sample B, which is probably due to a slight difference in the strain field, and therefore these peaks can still be associated with the same energy transition. A further emission at higher energy (1.355 eV) is also observed which can tentatively be assigned to emission from the top layer, although in that energy range coupling with the wetting layer states is possible [9]. The top layer of sample C is formed by very small islands (Fig. 3c), which are too small to be even considered QDs. In fact the second wetting layer is in such close proximity to the seed layer that the strain field propagating from the lower island is felt very strongly, producing a
127
structure that can be better described as a strainmodulated quantum well. It is also worth noting that the described behaviour of all three samples is very different from that of a single layer of QDs: measurements on a control sample show that the ground state PL emission is found at a substantially lower energy (1.195 eV) than that of the corresponding bilayer, with a further emission (first excited state) appearing at higher power of excitation (150 mW) at 1.251 eV. As the ground to first excited state separation is in the order of 56 meV, it is unlikely that features separated by substantially less than the former energy difference originate from the same QD layer. This helps in assigning all the fitted peaks in sample A: the I, the III and V peak are due to energy transitions in the top layer, while the II and IV originate in the seed layer. Furthermore the II and IV peaks are also found in sample B and C, which leaves unassigned only the II peak in sample B. However we can rule out that the former is due to a direct energy transition in the seed layer, as the energy separation between the three peaks is far too small (30 meV). Therefore, considering also that former peak is not clearly evident in sample C, it is likely that this emission originates from a transition in the top layer. However whether it is due to a ground state or an excited state emission we cannot verify on PL evidence alone. The theoretical model predicts that for 94% In in the top layer the islands will nucleate under more similar growth conditions, with the ideal value being probably slightly higher (1–2%). Therefore, having neglected the strain fields due to the seed layer, under these growth conditions, produces only a small correction to the ideal In fraction. The consequences of obtaining almost identical QDs on both layers is that the optical emission would behave effectively as that of a single layer of islands, if electronic coupling and interdot tunneling are negligible, or at least show a smaller line width due to a reduced presence of recombination channels, if the electronic effects are important. Our experiments show that the latter is indeed the case: for an excitation power of 150 mW the
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line width drops from 119 to 91 meV and most of all the number of peaks is reduced from 5 to 3. Furthermore, the line width of the fitted peaks of sample B is larger than the one of the corresponding peak of sample A, which would be expected if the emissions are due to QDs in both layers emitting at very similar energies resulting in an increased homogeneous broadening. Note that instead the line width at an excitation power of 2 mW is increased from 27 to 32 meV with decreasing In content from 100% to 94%. This happens because at low power of excitation non-resonant tunneling can result in the emission from the top layer (larger islands) to be more intense than that of the seed layer, and hence a reduced line width at low power of excitation cannot be used as evidence of improved coupling between QDs. However, based on this experimental evidence we cannot conclude whether we have achieved ground state emission from both seed and top layer at exactly the same energy.
In summary, we tested the validity of the kinetic segregation model in predicting the ideal In content for the optimisation of the optical emission of InAs QD bilayer. Electron microscopy and optical spectroscopy studies indicate that a reduction of around 6% results in more uniformly sized islands on the two layers and a significant reduction in the number of different optical transitions.
References [1] M.A. Migliorato, L.R. Wilson, D.J. Mowbray, M.S. Skolnick, M. Al-Khafaji, A.G. Cullis, M. Hopkinson, J. Appl. Phys. 90 (2001) 6374. [2] A.G. Cullis, D.J. Norris, T. Walther, M.A. Migliorato, M. Hopkinson, Phys. Rev. B 66 (2002) 081305(R). [3] S. Fafard, et al., Appl. Phys. Lett. 76 (2000) 2268. [4] P.B. Joyce, et al., Phys. Rev. B 66 (2002) 075316. [5] S. Fukatsu, et al., Appl. Phys. Lett. 59 (1991) 2240. [6] O. Dehaese, et al., Appl. Phys. Lett. 66 (1995) 52. [7] G.G. Tarasov, et al., J. Appl. Phys. 88 (2000) 7162. [8] Yu.I. Mazur, et al., Appl. Phys. Lett. 81 (2002) 3469. [9] S. Sauvage, et al., J. Appl. Phys. 84 (1998) 4356.
Physica E 26 (2005) 124–128 www.elsevier.com/locate/physe
Influence of In composition on the photoluminescence emission of In(Ga)As quantum dot bilayers M.A. Migliorato, M.J. Steer, W.M. Soong, C.M. Tey, H-Y. Liu, S.L. Liew, P. Navaretti, D.J. Norris, A.G. Cullis, M. Hopkinson Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, UK Available online 18 November 2004
Abstract We discuss a novel approach to the optimisation of quantum dot bilayer structures grown by molecular beam epitaxy. Use of a kinetic segregation model has shown that a reduction of the In composition for the upper layer of a bilayer structure can be used to compensate for the excess In that exists on the surface prior to growth. Three samples have been grown with upper dot In compositions varying from 90% to 100% and have been investigated by means of optical spectroscopy and electron microscopy. r 2004 Elsevier B.V. All rights reserved. PACS: 73.63.Kv; 81.07.Ta Keywords: Quantum dots; Spectroscopy; Molecular beam epitaxy
It has been observed that vertically stacked quantum dot structures often exhibit an increase in the average size of the islands with increasing number of quantum dot layers deposited [1]. This strongly hampers the possibility of growing closely stacked quantum dots (QDs) that exhibit very narrow line widths (a bimodal photoluminescence emission is often observed), which is of paramount Corresponding author. Tel.: +44-114-2225836; fax: +44114-2754589. E-mail address: [email protected] (M.A. Migliorato).
importance for the realisation of QD lasers and detectors. The understanding of the In segregation-induced Stranski–Krastanow transition [2] suggests that, for growth conditions in which strain effects are minimised, an increase in size is primarily due to an excess of In caused by elemental segregation. Similar considerations have lead in the past to attempting to optimise the growth conditions by either removing the excess In by rapid thermal annealing (In flush) [3], or by modification of the total coverage used during the island deposition [4]. However both techniques present limitations
1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.08.037
ARTICLE IN PRESS M.A. Migliorato et al. / Physica E 26 (2005) 124–128
that cannot be easily overcome, such as the lack of size control for the initial QD layers in one case, and the fact that the ideal value of the total coverage is only calibrated to a specific spacer thickness in the other case. Therefore, we propose the use of an alternative method based on the use of the kinetic segregation model [5,6] in order to quantify the elemental segregation in the growing layers and therefore predict the optimum fraction of In that needs to be deposited in the second layer to obtain more uniform emissions from a bilayer structure of closely stacked InAs/GaAs QDs. The In composition profile is calculated after depositing 2.5 ML of InAs at 500 1C on the first layer with a 5 nm thick GaAs barrier, as shown in Fig. 1a. It is evident that the growth conditions for the second layer are different from the growth conditions of the first layer, due to the presence of a monolayer of enriched In on the surface of the GaAs separation barrier. If a second layer of InAs
125
QDs is deposited under the same growth conditions (Fig. 1b) then the second layer will incorporate an excess of around 0.05 fraction of indium, resulting in larger islands compared to the first layer. Uniformity in the deposited In can be reached by decreasing the In fraction in the impinging flux: Fig. 1c shows that the best predicted growth conditions are obtained when depositing an InxGa1xAs alloy with x ¼ 0:94; while lower fractions of In ðx ¼ 0:9Þ will result in the nucleation of smaller islands (Fig. 1d). At this stage the influence of the strain field, due to the seed layer, on the atom mobility is neglected. To test the predictions of the kinetic segregation model, three bilayer samples with varying In compositions in the upper QD layer were grown under identical conditions by molecular beam epitaxy on undoped GaAs (1 0 0) substrates. A buffer layer of 200 nm of GaAs was grown at 590 1C, the substrate temperature was reduced to 500 1C and the bilayer structure with a 5 nm GaAs
Fig. 1. (a)–(d) Calculated effect of In segregation on the initial growth conditions for the second layer of quantum dots in bilayer structures.
ARTICLE IN PRESS M.A. Migliorato et al. / Physica E 26 (2005) 124–128
126
spacer layer deposited, the second QD layer was capped with 15 nm of GaAs, a further 85 nm of GaAs was then deposited at 590 1C. For the three samples under investigation the first QD layer consisted of 2.5 ML of InAs grown at 0.1 ML/s, the composition of the second layer was varied so that the In composition in samples A, B and C were 100%, 94% and 90%, respectively, the thickness of 2.5 ML remained the same. The low-temperature (10 K) and low power of excitation (2 mW) photoluminescence for the sets of samples (Fig. 2a) shows a definite blueshift of the emission with decreasing In content of the top layer from 100%: it is however evident that the low-energy side of the emissions of samples B and C (94% and 90%) is roughly the same. This is consistent with the interpretation that the lowenergy side of the photoluminescence (PL) spectra is in general associated with the ground state emission of the top layer (larger islands) [7] in the samples where both layers are grown with a nominal 100% InAs, with the typical high-energy
I
(a)
II
III
IV
V
PL intensity(a.u.)
(b)
2mW Sample A Sample B Sample C 150mW Sample A In = 100 %
(c)
150mW Sample B In = 94 %
(d)
150mW Sample C In = 90 %
(e)
20mW Sample C In = 90 %
1.15
tail due to a low-intensity emission from the seed layer (smaller islands). For samples B and C where the In content of the top layer is lowered, the island size on the same layer is reduced and the combined PL of the two layers is blueshifted. This situation is evident in the transmission electron microscopy (TEM) micrographs of Fig. 3: larger (a), equally sized (b) and smaller (c) islands are produced on the top layer for decreasing In content on the top layer during deposition. The line widths (FWHM) of the emissions are measured at 27, 45, 113 meV for samples A, B and C, respectively. Sample C also presents a bimodal distribution of the collected luminescence, with peaks roughly separated by 70 meV. Such a bimodal emission is usually an indication of the presence of at least two distinct sets of QDs between which carrier transfer is not as efficient [8]. Instead for the other samples, carrier transfer is usually efficient enough to allow relaxation towards the lowest energy levels before recombination. The intensity of the emission from higher energy levels instead increases significantly only
1.20
1.25
1.30
1.35
1.40
1.45
Photon Energy (eV) Fig. 2. (a)–(e) Low-temperature PL for QD bilayers grown with a different In content in the second layer compared to the seed layer.
Fig. 3. Typical micrographs: larger (a), equally sized (b) and smaller (c) islands are produced on the top layer for decreasing In content on the top layer during deposition.
ARTICLE IN PRESS M.A. Migliorato et al. / Physica E 26 (2005) 124–128
when the intensity of lower energy states emission is partially saturated. This becomes evident if the excitation power is increased: the power-dependent PL spectra at 10 K was recorded between 2 and 150 mW using a diode-pumped solid-state laser (532 nm) for the three samples. To determine the peak positions the spectra were fitted with Gaussian distributions consistently for different excitation powers. For an excitation power of 150 mW the sample with nominally identical layers (sample A) shows a convoluted PL spectrum formed by a number of competing emissions which are well fitted by 5 gaussian distributions with peak positions at 1.232, 1.259, 1.289, 1.319 and 1.349 eV. Lowering the composition of the second layer to 94% results in the disappearance of the lowest energy peak (1.232 eV), which can therefore be assigned to the ground state emission of the top layer. Furthermore, the highest energy peak (1.349 eV) is also suppressed, suggesting that it is due to an excited state transition of the top layer. The lowest energy peak of sample B (1.257 eV) is very close to the second peak of sample A (1.259 eV), so that we can assign this to the ground state emission of the seed layer. Reducing the composition of the top layer to 90% results in a broadening of the emission towards higher energies: the emission is well fitted by two gaussian peaks, the energies of which (1.249 and 1.312 eV) are very close to the peak energies of sample B (1.255 and 1.318 eV). The low-energy peak is at a slightly lower energy (the difference is 8 meV) than the low-energy peak of sample B, which is probably due to a slight difference in the strain field, and therefore these peaks can still be associated with the same energy transition. A further emission at higher energy (1.355 eV) is also observed which can tentatively be assigned to emission from the top layer, although in that energy range coupling with the wetting layer states is possible [9]. The top layer of sample C is formed by very small islands (Fig. 3c), which are too small to be even considered QDs. In fact the second wetting layer is in such close proximity to the seed layer that the strain field propagating from the lower island is felt very strongly, producing a
127
structure that can be better described as a strainmodulated quantum well. It is also worth noting that the described behaviour of all three samples is very different from that of a single layer of QDs: measurements on a control sample show that the ground state PL emission is found at a substantially lower energy (1.195 eV) than that of the corresponding bilayer, with a further emission (first excited state) appearing at higher power of excitation (150 mW) at 1.251 eV. As the ground to first excited state separation is in the order of 56 meV, it is unlikely that features separated by substantially less than the former energy difference originate from the same QD layer. This helps in assigning all the fitted peaks in sample A: the I, the III and V peak are due to energy transitions in the top layer, while the II and IV originate in the seed layer. Furthermore the II and IV peaks are also found in sample B and C, which leaves unassigned only the II peak in sample B. However we can rule out that the former is due to a direct energy transition in the seed layer, as the energy separation between the three peaks is far too small (30 meV). Therefore, considering also that former peak is not clearly evident in sample C, it is likely that this emission originates from a transition in the top layer. However whether it is due to a ground state or an excited state emission we cannot verify on PL evidence alone. The theoretical model predicts that for 94% In in the top layer the islands will nucleate under more similar growth conditions, with the ideal value being probably slightly higher (1–2%). Therefore, having neglected the strain fields due to the seed layer, under these growth conditions, produces only a small correction to the ideal In fraction. The consequences of obtaining almost identical QDs on both layers is that the optical emission would behave effectively as that of a single layer of islands, if electronic coupling and interdot tunneling are negligible, or at least show a smaller line width due to a reduced presence of recombination channels, if the electronic effects are important. Our experiments show that the latter is indeed the case: for an excitation power of 150 mW the
ARTICLE IN PRESS 128
M.A. Migliorato et al. / Physica E 26 (2005) 124–128
line width drops from 119 to 91 meV and most of all the number of peaks is reduced from 5 to 3. Furthermore, the line width of the fitted peaks of sample B is larger than the one of the corresponding peak of sample A, which would be expected if the emissions are due to QDs in both layers emitting at very similar energies resulting in an increased homogeneous broadening. Note that instead the line width at an excitation power of 2 mW is increased from 27 to 32 meV with decreasing In content from 100% to 94%. This happens because at low power of excitation non-resonant tunneling can result in the emission from the top layer (larger islands) to be more intense than that of the seed layer, and hence a reduced line width at low power of excitation cannot be used as evidence of improved coupling between QDs. However, based on this experimental evidence we cannot conclude whether we have achieved ground state emission from both seed and top layer at exactly the same energy.
In summary, we tested the validity of the kinetic segregation model in predicting the ideal In content for the optimisation of the optical emission of InAs QD bilayer. Electron microscopy and optical spectroscopy studies indicate that a reduction of around 6% results in more uniformly sized islands on the two layers and a significant reduction in the number of different optical transitions.
References [1] M.A. Migliorato, L.R. Wilson, D.J. Mowbray, M.S. Skolnick, M. Al-Khafaji, A.G. Cullis, M. Hopkinson, J. Appl. Phys. 90 (2001) 6374. [2] A.G. Cullis, D.J. Norris, T. Walther, M.A. Migliorato, M. Hopkinson, Phys. Rev. B 66 (2002) 081305(R). [3] S. Fafard, et al., Appl. Phys. Lett. 76 (2000) 2268. [4] P.B. Joyce, et al., Phys. Rev. B 66 (2002) 075316. [5] S. Fukatsu, et al., Appl. Phys. Lett. 59 (1991) 2240. [6] O. Dehaese, et al., Appl. Phys. Lett. 66 (1995) 52. [7] G.G. Tarasov, et al., J. Appl. Phys. 88 (2000) 7162. [8] Yu.I. Mazur, et al., Appl. Phys. Lett. 81 (2002) 3469. [9] S. Sauvage, et al., J. Appl. Phys. 84 (1998) 4356.