- Email: [email protected]
GaAs quantum dots
Journal of Crystal Growth 370 (2013) 303–306
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Graded GaAsSb strain reducing layers covering InAs/GaAs quantum dots A. Hospodkova´ n, M. Zı´kova´, J. Pangra´c, J. Oswald, K. Kuldova´, J. Vyskocˇil, E. Hulicius ´ 10, 162 00 Prague 6, Czech Republic Institute of Physics AS CR, v.v.i., Cukrovarnicka
a r t i c l e i n f o
abstract
Available online 25 August 2012
We have grown new InAs/GaAs quantum dot (QD) structures with graded Sb concentration of GaAs(1 x)Sbx strain reducing layer (SRL). New types of GaAsSb SRLs with graded concentration of Sb are theoretically and experimentally studied. We compare properties of three different Sb concentration gradients in SRL, constant, increasing and decreasing during the growth. Both types of non-constant gradients help us to prevent transition of the InAs(QD)/GaAsSb(SRL) heterojunction from type I to type II, to increase emission wavelength and to retain high luminescence intensity of these types of QD structures. Comparison of photoluminescence of samples with different concentration gradients and similar average Sb concentration in SRLs is shown. The longest wavelength of type I ground state transition was achieved on sample with decreasing gradation of Sb content in SRL—1399 nm (0.886 eV). & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Band alignment A1. Photoluminescence A1. Strain reducing layer A1. Quantum dot A3. MOVPE B2. InAs/GaAs
1. Introduction The overgrowth of QDs by strain reducing layer (SRL) is used to red-shift the quantum dot (QD) photoluminescence (PL) compared to simple GaAs capping [1]. Indium dissolution considerably decreases during the dot capping process [2,3], thus preserving the original size and shape of QDs with high aspect ratio (height/base diameter ratio). This is especially important for MOVPE grown QD structures which are usually grown at higher temperatures. SRLs also decreases the strain inside QDs and energy barriers of covering layer which also helps to red-shift the QD PL. GaAsSb SRL in comparison to InGaAs one provides higher energy barrier for electrons. The high electron energy barriers of GaAsSb SRL together with the higher aspect ratio keeps or even sometimes increases the energy difference between the ground and first excited QD electron states, which is important for the stability of lasing wavelength. GaAsSb SRL also improves overlap of electron and hole wave functions in QD which increases considerably the PL efficiency [4]. The disadvantage of GaAsSb SRL is formation of type II heterojunction between QDs and SRL when the Sb content in GaAsSb exceeds 14%. The drawback of type II heterostructure is the separation of electrons and holes, electrons are located in QDs and holes in GaAsSb SRL. This decreases recombination efficiency, causes blue-shift of emission wavelength under higher excitation intensities and enhances probability of emission from excited states, which have better overlap of electron and hole wave
n
Corresponding author. Tel.: þ420 22 0318538; fax: þ 420 23 3343184. E-mail address: [email protected] (A. Hospodkova´).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.08.007
functions. InAs/GaAs QD structures with GaAsSb SRL emitting at 1.3 mm and above were reported to be type II up till now [4–7]. The heterojunction type transition depends not only on the composition of QDs or GaAsSb SRL as is usually assumed, but also on the other parameters of the structure. The thickness of GaAsSb SRL was also reported to influence the heterojunction type [8]. The aim of this work is to achieve the longest possible photoluminescence (PL) maximum of the QD structure with GaAsSb SRL while keeping the type I heterojunction between QDs and GaAsSb SRL. The influence of graded composition of GaAsSb SRL on the emission wavelength was studied.
2. Experimental InAs/GaAs QD structures were prepared by LP MOVPE in AIXTRON 200 on semiinsulating (100) GaAs substrates using Stranski– Krastanow growth mode. Trimethylgallium (TMGa), triethylgallium (TEGa), trimethylindium (TMIn), arsine (AsH3), tertiarybutylarsine (TBAs) and triethylantimony (TESb) were used as precursors for the structure growth. The structures were grown at a total pressure of 7 kPa, total flow rate through the reactor was 8 slpm. The growth temperature was 650 1C for the first buffer layer (TMGaþAsH3). Then the temperature was lowered to 510 1C for the growth of the rest of the structure: second buffer layer (TEGaþtBAs), InAs QDs (TMInþ tBAs), GaAsSb SRL (TEGaþtBAsþTESb) and capping layer (TEGaþtBAs). The growth rate of InAs was 0.05 ML/s. The growth interruptions for QD formation were 15 s. The details of the QD growth are described in [9]. The growth rate of GaAsSb SRL was 0.1 nm/s. The graded composition of GaAsSb SRL was controlled by TBAs flow rate.
´ et al. / Journal of Crystal Growth 370 (2013) 303–306 A. Hospodkova
304
Reflectance anisotropy spectroscopy in situ measurement at 2.65 eV using EpiRAS 200 TT (LayTec) [10] monitored the formation and development of InAs QDs, as well as the growth of GaAs and GaAsSb layers [11]. PL of QDs was excited by a semiconductor laser (670 nm) with the excitation power density from 0.15 to 10 Wcm 2. PL was detected by a Ge detector using standard lock-in technique. All PL measurements were performed at room temperature. X-ray diffraction and reflection were used for the determination of average composition and thickness of layers (CuKa1l ¼ ˚ 4-crystal monochromator, scintillation detector, beam 1.5406 A, size 0.3 8 mm2).
3. Theoretical simulations Simulation of the strain, band alignment and transition energies in QD structures was performed using the nextnano3 3D simulator [12]. For a given structure, the computation started by minimizing the total elastic energy using a conjugate gradient method. This yielded the local strain tensor, which in turn determined the band offsets and light/heavy holes splitting. ¨ Subsequently, the multi-band-Schrodinger and Poisson equations were solved using the GaAs, InAs and GaAsSb band parameters from [13].
In all simulated structures the shape of QDs was semiellipsoid with the circular base, the aspect ratio (height/base diameter ratio) was kept 0.33 and height was 6 nm. The thickness of GaAsSb SRL was 5 nm for all simulated structures. The Sb incorporation into GaAsSb SRL above QDs in our model was dependent on the angle from QD axis (see Fig. 1) to approximate more precisely the Sb incorporation in real structures studied by X-STM [3]. 5 nm thickness of GaAsSb SRL was supposed to be the same on the QD top as well as on the flat surface (see scheme in Fig. 1), while the composition above QDs was supposed to be higher (average 17%), than in region between QDs (average 10%), since we suppose, that the incorporation of Sb atoms is also driven by the strain and lattice constant of the epitaxial surface. The values of average composition used for simulations were chosen to be near the type I/type II heterojunction transition. We have taken into account the suppression of possible intermixing of materials using SRLs, as described in [14,15]. The calculations were performed for three different types of GaAsSb SRLs: with the constant (structure A), increasing (structure B) or decreasing (structure C) content of Sb in SRL in the growth direction (see Table 1). The conduction and valence band structure alignment in the QD axis calculated for structures A, B and C from Table 1 is in Fig. 2. The band bending is caused by the strain in the structure. The conduction band alignment is similar for all three types of structures, while the valence band alignments differ significantly. The calculated visualization of hole probability density as well as ground state transition energies for all three types of structure are summarized in Table 1. It can be seen that the structure C with the highest Sb concentration in QD apex vicinity is close to the type II heterojunction, since the hole wave function starts to penetrate into the SRL region. This structure has the lowest calculated transition energy.
4. Experimental results and discussion Fig. 1. Scheme of simulated InAs QD structure with GaAsSb SRL with increasing content of Sb in SRL in the growth direction (marked by scaling the gray tone). The Sb content above QD is decreased with the j angle to approximate more precisely the Sb incorporation in real structures.
The PL spectra of seven samples with type B (increasing) Sb gradation and different average Sb content are shown in Fig. 3. The Sb content was measured by X-ray diffraction for samples
Table 1 Parameters of three types of QD structures with different Sb gradient: Sb content used for simulation, band structure scheme and results of simulation and PL measurement. Structure type (Sb gradient)
Sb content in SRL
Band structure scheme
Hole probability density
Simulated transition energy
Measured transition energy
Above QD
On the flat surface
A (constant)
17%
10%
0.892 eV
0.953 eV
B (increasing)
12-22%
8-12%
0.887 eV
0.924 eV
C (decreasing)
22-12%
12-8%
0.884 eV
0.886 eV
´ et al. / Journal of Crystal Growth 370 (2013) 303–306 A. Hospodkova
305
Fig. 2. The calculated conduction and valence band structure alignment in the QD axis (in the growth direction) calculated for structures A, B and C from Table 1. Fig. 4. Normalized room temperature PL spectra of QD structures with 5 nm GaAs1 xSbx SRL with equal average Sb content (x¼ 0.1 aprox.) for all three types of composition gradient—constant, decreasing and increasing. The energy of PL maxima can be found in Table 1.
The longest wavelength of type I ground state transition was achieved on sample type C (decreasing gradation of Sb content in SRL)—1399 nm (0.886 eV), with high PL intensity and FWHM of 35 meV. The average content of Sb in SRL was 10%. The QD PL spectra of three samples with SRL type A, B and C with approximately the same average content 10% of Sb in SRL are shown in Fig. 4. The red-shift of PL maxima for structures B and C with graded composition of SRL predicted by simulations was confirmed by experimental data. It is caused by higher maximal Sb concentration in graded SRL. The shortest ground state wavelength was achieved on sample with type A SRL (without Sb content gradation) in agreement with expectation from simulated values. The ground state transition energies are summarized in Table 1. The higher achieved ground state transition energies in comparison to simulated values may be caused by differences between structure parameters used for simulation and parameters of the real grown structures (QD size or shape, Sb concentration). Fig. 3. The room temperature PL spectra of samples with different average Sb content with increasing Sb gradient (type B). Inset shows the dependence of the ground state transition energy on the average Sb content in SRL for all B type measured samples.
with 9% and 14% of Sb. The SRL composition of other samples was extrapolated according to the technological growth parameters. The highest peak intensity PL was achieved for sample with 7% average Sb content in SRL probably due to best overlap of electron and hole wave functions [4]. The transition between type I and type II heterojunction occurs in average Sb content 14–18% in agreement with theoretical expectation and published values [5–7]. The longest wavelength type I PL maximum achieved on type B structure was 1394 nm (0.8895 eV) with very low full width at half maximum (FWHM) of the ground state transition 23 meV. The average content of Sb in SRL was 14%. The PL intensity was lower in comparison to samples with lower Sb content in SRL. The PL spectra of type II structure for 18% (average) in GaAsSb SRL has approximately two orders of magnitude lower PL intensity than for type I heterostructure. The spatial separation of electrons to QDs and holes to GaAsSb SRL causes low overlap of electron and hole wave functions and decreases the PL intensity.
5. Conclusion Three types of GaAsSb SRL with constant, increasing or decreasing concentration of Sb during the SRL growth were studied. It was shown that both types of Sb concentration gradients, increasing and decreasing, help us to increase emission wavelength and to retain high luminescence intensity of QD structures with the same average content of Sb in SRL. Theoretical expectation was confirmed by experimental results. The longest wavelength of type I ground state transition was achieved on sample with decreasing gradation of Sb content in SRL—1399 nm (0.886 eV), with high PL intensity and FWHM of 35 meV. This is according to our knowledge the longest reported wavelength on GaAsSb caped QDs with type I heterojunction between InAs QDs and GaAsSb SRL.
Acknowledgment This work was supported by Czech Science Foundation Project P102/10/1201, the Research Program of Institute of Physics AV0Z
306
´ et al. / Journal of Crystal Growth 370 (2013) 303–306 A. Hospodkova
10100521, and by the Scientific Program MSMT-LNSM for Support of Large Infrastructures VVI. We would like to thank Ondˇrej Caha for X-ray measurements. References [1] P. Hazdra, J. Oswald, V. Komarnitskyy, K. Kuldova´, A. Hospodkova´, E. Hulicius, J. Pangra´c, Self-assembled InAs/GaAs quantum dots covered by different strain reducing layers exhibiting strong photo- and electroluminescence in 1.3 and 1.55 mm bands, Journal of Nanoscience and Nanotechnology 11 (2011) 6804–6809. [2] G. Costantini, A. Rastelli, C. Manzano, P. Acosta-Diaz, R. Songmuang, G. Katsaros, O.G. Schmidt, K. Kern, Interplay between thermodynamics and kinetics in the capping of InAs/GaAs (001) quantum dots, Physical Review Letters 96 (2006) 226106-1–226106-4. [3] J.M. Ulloa, R. Gargallo-Caballero, M. Bozkurt, M. del Moral, A. Guzma´n, P.M. Koenraad, A. Hierro, GaAsSb-capped InAs quantum dots: from enlarged quantum dot height to alloy fluctuations, Physical Review B 81 (2010) 165305-1–165305-7. [4] A. Hospodkova´, E. Hulicius, J. Pangra´c, J. Oswald, J. Vyskocˇil, K. Kuldova´, ˇ ˇ ek, P. Hazdra, O. Caha, InGaAs and GaAsSb strain reducing layers T. Simec covering InAs/GaAs quantum dots, Journal of Crystal Growth 312 (2010) 1383–1387. [5] H.Y. Liu, M.J. Steer, T.J. Badcock, D.J. Mowbray, M.S. Skolnick, P. Navaretti, K.M. Groom, M. Hopkinson, R.A. Hogg, Long-wavelength light emission and lasing from InAs/GaAs quantum dots covered by a GaAsSb strain-reducing layer, Applied Physics Letters 86 (2005) 143108-1–143108-3. [6] K. Akahane, N. Yamamoto, N. Ohtani, Long-wavelength light emission from InAs quantum dots covered by GaAsSb grown on GaAs substrates, Physica E 21 (2004) 295–299.
[7] C.Y. Jin, H.Y. Liu, S.Y. Zhang, Q. Jiang, S.L. Liew, M. Hopkinson, T.J. Badcock, E. Nabavi, D.J. Mowbray, Optical transitions in type-II InAs/GaAs quantum dots covered by a GaAsSb strain-reducing layer, Applied Physics Letters 91 (2007) 021102-1–021102-3. [8] A. Pancholi, S.P. Bremner, J. Boyle, V.G. Stoleru, C.B. Honsberg, Variability of heterostructure type with thickness of barriers and temperature in the InAs/ GaAsSb quantum dot system, Solar Energy Materials & Solar Cells 94 (2010) 1025–1030. [9] A. Hospodkova´, J. Pangra´c, J. Oswald, P. Hazdra, K. Kuldova´, J. Vyskocˇil, E. Hulicius, Influence of strain reducing layers on electroluminescence and photoluminescence of InAs/GaAs QD structures, Journal of Crystal Growth 315 (2011) 110–113. [10] E. Steimetz, J.-T. Zettler, F. Schienle, T. Trepk, T. Wethkamp, W. Richter, I. Sieber, In situ monitoring of InAs on GaAs quantum dot formation in MOVPE by reflectance-anisotropy-spectroscopy, Applied Surface Science 107 (1996) 203–211. [11] A. Hospodkova´, J. Vyskocˇil, J. Pangra´c, J. Oswald, E. Hulicius, K. Kuldova´, Surface processes during growth of InAs/GaAs quantum dot structures monitored by reflectance anisotropy spectroscopy, Surface Science 604 (2010) 318–321. [12] /http:/www.wsi.tu-muenchen.de/nextnano3S. [13] I. Vurgaftman, J.R. Meyer, L.R. Ram-Mohan, Band parameters for III–V compound semiconductors and their alloys, Journal of Applied Physics 89 (2001) 5815–5875. [14] J. He, F. Bao, J. Zhang, Capping effect of GaAsSb and InGaAsSb on the structural and optical properties of type II GaSb/GaAs quantum dots, Applied Physics Letters 100 (2012) 171914-1–171914-4. [15] J. He, Y. Wu, K.L. Wang, Structure and composition profile of InAs/GaAs quantum dots capped by an InGaAs and InAlAs combination layer, Nanotechnology 21 (2010) 255705-1–255705-4.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Graded GaAsSb strain reducing layers covering InAs/GaAs quantum dots A. Hospodkova´ n, M. Zı´kova´, J. Pangra´c, J. Oswald, K. Kuldova´, J. Vyskocˇil, E. Hulicius ´ 10, 162 00 Prague 6, Czech Republic Institute of Physics AS CR, v.v.i., Cukrovarnicka
a r t i c l e i n f o
abstract
Available online 25 August 2012
We have grown new InAs/GaAs quantum dot (QD) structures with graded Sb concentration of GaAs(1 x)Sbx strain reducing layer (SRL). New types of GaAsSb SRLs with graded concentration of Sb are theoretically and experimentally studied. We compare properties of three different Sb concentration gradients in SRL, constant, increasing and decreasing during the growth. Both types of non-constant gradients help us to prevent transition of the InAs(QD)/GaAsSb(SRL) heterojunction from type I to type II, to increase emission wavelength and to retain high luminescence intensity of these types of QD structures. Comparison of photoluminescence of samples with different concentration gradients and similar average Sb concentration in SRLs is shown. The longest wavelength of type I ground state transition was achieved on sample with decreasing gradation of Sb content in SRL—1399 nm (0.886 eV). & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Band alignment A1. Photoluminescence A1. Strain reducing layer A1. Quantum dot A3. MOVPE B2. InAs/GaAs
1. Introduction The overgrowth of QDs by strain reducing layer (SRL) is used to red-shift the quantum dot (QD) photoluminescence (PL) compared to simple GaAs capping [1]. Indium dissolution considerably decreases during the dot capping process [2,3], thus preserving the original size and shape of QDs with high aspect ratio (height/base diameter ratio). This is especially important for MOVPE grown QD structures which are usually grown at higher temperatures. SRLs also decreases the strain inside QDs and energy barriers of covering layer which also helps to red-shift the QD PL. GaAsSb SRL in comparison to InGaAs one provides higher energy barrier for electrons. The high electron energy barriers of GaAsSb SRL together with the higher aspect ratio keeps or even sometimes increases the energy difference between the ground and first excited QD electron states, which is important for the stability of lasing wavelength. GaAsSb SRL also improves overlap of electron and hole wave functions in QD which increases considerably the PL efficiency [4]. The disadvantage of GaAsSb SRL is formation of type II heterojunction between QDs and SRL when the Sb content in GaAsSb exceeds 14%. The drawback of type II heterostructure is the separation of electrons and holes, electrons are located in QDs and holes in GaAsSb SRL. This decreases recombination efficiency, causes blue-shift of emission wavelength under higher excitation intensities and enhances probability of emission from excited states, which have better overlap of electron and hole wave
n
Corresponding author. Tel.: þ420 22 0318538; fax: þ 420 23 3343184. E-mail address: [email protected] (A. Hospodkova´).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.08.007
functions. InAs/GaAs QD structures with GaAsSb SRL emitting at 1.3 mm and above were reported to be type II up till now [4–7]. The heterojunction type transition depends not only on the composition of QDs or GaAsSb SRL as is usually assumed, but also on the other parameters of the structure. The thickness of GaAsSb SRL was also reported to influence the heterojunction type [8]. The aim of this work is to achieve the longest possible photoluminescence (PL) maximum of the QD structure with GaAsSb SRL while keeping the type I heterojunction between QDs and GaAsSb SRL. The influence of graded composition of GaAsSb SRL on the emission wavelength was studied.
2. Experimental InAs/GaAs QD structures were prepared by LP MOVPE in AIXTRON 200 on semiinsulating (100) GaAs substrates using Stranski– Krastanow growth mode. Trimethylgallium (TMGa), triethylgallium (TEGa), trimethylindium (TMIn), arsine (AsH3), tertiarybutylarsine (TBAs) and triethylantimony (TESb) were used as precursors for the structure growth. The structures were grown at a total pressure of 7 kPa, total flow rate through the reactor was 8 slpm. The growth temperature was 650 1C for the first buffer layer (TMGaþAsH3). Then the temperature was lowered to 510 1C for the growth of the rest of the structure: second buffer layer (TEGaþtBAs), InAs QDs (TMInþ tBAs), GaAsSb SRL (TEGaþtBAsþTESb) and capping layer (TEGaþtBAs). The growth rate of InAs was 0.05 ML/s. The growth interruptions for QD formation were 15 s. The details of the QD growth are described in [9]. The growth rate of GaAsSb SRL was 0.1 nm/s. The graded composition of GaAsSb SRL was controlled by TBAs flow rate.
´ et al. / Journal of Crystal Growth 370 (2013) 303–306 A. Hospodkova
304
Reflectance anisotropy spectroscopy in situ measurement at 2.65 eV using EpiRAS 200 TT (LayTec) [10] monitored the formation and development of InAs QDs, as well as the growth of GaAs and GaAsSb layers [11]. PL of QDs was excited by a semiconductor laser (670 nm) with the excitation power density from 0.15 to 10 Wcm 2. PL was detected by a Ge detector using standard lock-in technique. All PL measurements were performed at room temperature. X-ray diffraction and reflection were used for the determination of average composition and thickness of layers (CuKa1l ¼ ˚ 4-crystal monochromator, scintillation detector, beam 1.5406 A, size 0.3 8 mm2).
3. Theoretical simulations Simulation of the strain, band alignment and transition energies in QD structures was performed using the nextnano3 3D simulator [12]. For a given structure, the computation started by minimizing the total elastic energy using a conjugate gradient method. This yielded the local strain tensor, which in turn determined the band offsets and light/heavy holes splitting. ¨ Subsequently, the multi-band-Schrodinger and Poisson equations were solved using the GaAs, InAs and GaAsSb band parameters from [13].
In all simulated structures the shape of QDs was semiellipsoid with the circular base, the aspect ratio (height/base diameter ratio) was kept 0.33 and height was 6 nm. The thickness of GaAsSb SRL was 5 nm for all simulated structures. The Sb incorporation into GaAsSb SRL above QDs in our model was dependent on the angle from QD axis (see Fig. 1) to approximate more precisely the Sb incorporation in real structures studied by X-STM [3]. 5 nm thickness of GaAsSb SRL was supposed to be the same on the QD top as well as on the flat surface (see scheme in Fig. 1), while the composition above QDs was supposed to be higher (average 17%), than in region between QDs (average 10%), since we suppose, that the incorporation of Sb atoms is also driven by the strain and lattice constant of the epitaxial surface. The values of average composition used for simulations were chosen to be near the type I/type II heterojunction transition. We have taken into account the suppression of possible intermixing of materials using SRLs, as described in [14,15]. The calculations were performed for three different types of GaAsSb SRLs: with the constant (structure A), increasing (structure B) or decreasing (structure C) content of Sb in SRL in the growth direction (see Table 1). The conduction and valence band structure alignment in the QD axis calculated for structures A, B and C from Table 1 is in Fig. 2. The band bending is caused by the strain in the structure. The conduction band alignment is similar for all three types of structures, while the valence band alignments differ significantly. The calculated visualization of hole probability density as well as ground state transition energies for all three types of structure are summarized in Table 1. It can be seen that the structure C with the highest Sb concentration in QD apex vicinity is close to the type II heterojunction, since the hole wave function starts to penetrate into the SRL region. This structure has the lowest calculated transition energy.
4. Experimental results and discussion Fig. 1. Scheme of simulated InAs QD structure with GaAsSb SRL with increasing content of Sb in SRL in the growth direction (marked by scaling the gray tone). The Sb content above QD is decreased with the j angle to approximate more precisely the Sb incorporation in real structures.
The PL spectra of seven samples with type B (increasing) Sb gradation and different average Sb content are shown in Fig. 3. The Sb content was measured by X-ray diffraction for samples
Table 1 Parameters of three types of QD structures with different Sb gradient: Sb content used for simulation, band structure scheme and results of simulation and PL measurement. Structure type (Sb gradient)
Sb content in SRL
Band structure scheme
Hole probability density
Simulated transition energy
Measured transition energy
Above QD
On the flat surface
A (constant)
17%
10%
0.892 eV
0.953 eV
B (increasing)
12-22%
8-12%
0.887 eV
0.924 eV
C (decreasing)
22-12%
12-8%
0.884 eV
0.886 eV
´ et al. / Journal of Crystal Growth 370 (2013) 303–306 A. Hospodkova
305
Fig. 2. The calculated conduction and valence band structure alignment in the QD axis (in the growth direction) calculated for structures A, B and C from Table 1. Fig. 4. Normalized room temperature PL spectra of QD structures with 5 nm GaAs1 xSbx SRL with equal average Sb content (x¼ 0.1 aprox.) for all three types of composition gradient—constant, decreasing and increasing. The energy of PL maxima can be found in Table 1.
The longest wavelength of type I ground state transition was achieved on sample type C (decreasing gradation of Sb content in SRL)—1399 nm (0.886 eV), with high PL intensity and FWHM of 35 meV. The average content of Sb in SRL was 10%. The QD PL spectra of three samples with SRL type A, B and C with approximately the same average content 10% of Sb in SRL are shown in Fig. 4. The red-shift of PL maxima for structures B and C with graded composition of SRL predicted by simulations was confirmed by experimental data. It is caused by higher maximal Sb concentration in graded SRL. The shortest ground state wavelength was achieved on sample with type A SRL (without Sb content gradation) in agreement with expectation from simulated values. The ground state transition energies are summarized in Table 1. The higher achieved ground state transition energies in comparison to simulated values may be caused by differences between structure parameters used for simulation and parameters of the real grown structures (QD size or shape, Sb concentration). Fig. 3. The room temperature PL spectra of samples with different average Sb content with increasing Sb gradient (type B). Inset shows the dependence of the ground state transition energy on the average Sb content in SRL for all B type measured samples.
with 9% and 14% of Sb. The SRL composition of other samples was extrapolated according to the technological growth parameters. The highest peak intensity PL was achieved for sample with 7% average Sb content in SRL probably due to best overlap of electron and hole wave functions [4]. The transition between type I and type II heterojunction occurs in average Sb content 14–18% in agreement with theoretical expectation and published values [5–7]. The longest wavelength type I PL maximum achieved on type B structure was 1394 nm (0.8895 eV) with very low full width at half maximum (FWHM) of the ground state transition 23 meV. The average content of Sb in SRL was 14%. The PL intensity was lower in comparison to samples with lower Sb content in SRL. The PL spectra of type II structure for 18% (average) in GaAsSb SRL has approximately two orders of magnitude lower PL intensity than for type I heterostructure. The spatial separation of electrons to QDs and holes to GaAsSb SRL causes low overlap of electron and hole wave functions and decreases the PL intensity.
5. Conclusion Three types of GaAsSb SRL with constant, increasing or decreasing concentration of Sb during the SRL growth were studied. It was shown that both types of Sb concentration gradients, increasing and decreasing, help us to increase emission wavelength and to retain high luminescence intensity of QD structures with the same average content of Sb in SRL. Theoretical expectation was confirmed by experimental results. The longest wavelength of type I ground state transition was achieved on sample with decreasing gradation of Sb content in SRL—1399 nm (0.886 eV), with high PL intensity and FWHM of 35 meV. This is according to our knowledge the longest reported wavelength on GaAsSb caped QDs with type I heterojunction between InAs QDs and GaAsSb SRL.
Acknowledgment This work was supported by Czech Science Foundation Project P102/10/1201, the Research Program of Institute of Physics AV0Z
306
´ et al. / Journal of Crystal Growth 370 (2013) 303–306 A. Hospodkova
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