Structural and emission properties of InGaAs/GaAs ...

Structural and emission properties of InGaAs/GaAs quantum dots emitting at 1.3 μm Elias Goldmann, Matthias Paul, Florian F. Krause, Knut Müller, Jan Kettler, Thorsten Mehrtens, Andreas Rosenauer, Michael Jetter, Peter Michler, and Frank Jahnke Citation: Applied Physics Letters 105, 152102 (2014); doi: 10.1063/1.4898186 View online: http://dx.doi.org/10.1063/1.4898186 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mechanism for improvements of optical properties of 1.3- μ m InAs ∕ GaAs quantum dots by a combined InAlAs – InGaAs cap layer J. Appl. Phys. 98, 083516 (2005); 10.1063/1.2113408 1.3 to 1.5 μm light emission from InGaAs/GaAs quantum wells Appl. Phys. Lett. 85, 875 (2004); 10.1063/1.1759066 Electrically injected InGaAs/GaAs quantum-dot microcavity light-emitting diode operating at 1.3 μm and grown by metalorganic chemical vapor deposition Appl. Phys. Lett. 84, 4155 (2004); 10.1063/1.1755411 Structural study of InGaAs/GaAs quantum dots grown by metalorganic chemical vapor deposition for optoelectronic applications at 1.3 μm J. Appl. Phys. 89, 4341 (2001); 10.1063/1.1351861 Volmer–Weber and Stranski–Krastanov InAs-(Al,Ga)As quantum dots emitting at 1.3 μm J. Appl. Phys. 88, 6272 (2000); 10.1063/1.1321795

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APPLIED PHYSICS LETTERS 105, 152102 (2014)

Structural and emission properties of InGaAs/GaAs quantum dots emitting at 1.3 lm €ller,3 Jan Kettler,2 Elias Goldmann,1,a) Matthias Paul,2 Florian F. Krause,3 Knut Mu 3 3 2 Thorsten Mehrtens, Andreas Rosenauer, Michael Jetter, Peter Michler,2 and Frank Jahnke1 1

Institute of Theoretical Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany Institut f€ ur Halbleiteroptik und Funktionelle Grenzfl€ achen, Universit€ at Stuttgart, Allmandring 3, 70569 Stuttgart, Germany 3 Institute of Solid State Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany 2

(Received 11 July 2014; accepted 1 October 2014; published online 14 October 2014) A combined experimental and theoretical study of InGaAs/GaAs quantum dots (QDs) emitting at 1.3 lm under the influence of a strain-reducing InGaAs quantum well is presented. We demonstrate a red shift of 20–40 nm observed in photoluminescence spectra due to the quantum well. The InGaAs/GaAs QDs grown by metal organic vapor phase epitaxy show a bimodal height distribution (1 nm and 5 nm) and indium concentrations up to 90%. The emission properties are explained with combined tight-binding and configuration-interaction calculations of the emission wavelengths in conjunction with high-resolution scanning transmission electron microscopy investigations of QD geometry and indium concentrations in the QDs, which directly enter the calculations. QD geomeC 2014 tries and concentration gradients representative for the ensemble are identified. V AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4898186] In the past decades, semiconductor quantum dots (QDs) received continuous attention due to a broad range of applications, which include lasers, solar cells, storage media, single-photon emitters, or entanglement devices.1 Since the introduction of the concepts of quantum networks and quantum cryptography on the basis of glass fiber technology, the need for sources of single and entangled photons at the telecom low-absorption wavelength windows became immediate. The InGaAs/GaAs material system is especially interesting for emission at the telecom O-band (1.31 lm)2 since it offers good growth control and, by using the growth technique of metal-organic vapor-phase epitaxy (MOVPE), cost efficiency. Typically, Stranski-Krastanow-grown InGaAs QDs on GaAs substrate exhibit emission wavelengths around 1.0 lm due to low indium incorporation into the QDs, far away from the desired low-absorption telecommunication wavelengths of 1.31 lm and 1.55 lm, respectively. Different approaches to shift the emission of InGaAs QDs into these wavelengths were proposed, one of which being the combination of the QDs with a so-called strain-reducing layer (SRL):3,4 an additional InGaAs quantum well embedding the QDs with a typical indium concentration of 10%–40% is included before consecutive overgrowth with the buffer material. This technique is said to relieve compressive strain inside the QD, reduce interdiffusion of gallium and indium atoms in QDs and surrounding material, and result in larger QD sizes. Each of these effects leads to a stronger carrier confinement and higher ground state emission wavelengths.5–8 Our study allows to quantify the relative importance of these effects. In this letter, we present optical as well as structural investigations of MOVPE-grown InGaAs/GaAs QDs in conjunction with a microscopic theory. The results from a)

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photoluminescence (PL) experiments, high-resolution scanning transmission electron microscopy (HRSTEM), atomic force microscopy (AFM), and calculations of the emission energies using the tight-binding (TB) model provide a comprehensive understanding of the developed structures. The samples A, B, and C were fabricated by MOVPE in a horizontal flow reactor with a pressure of 100 mbars. We first deposited a 200 nm buffer layer of GaAs on the exactly oriented (100) GaAs substrates to ensure a good surface quality for the QDs. After this, the InGaAs QDs formed at a temperature of 530 C. The concentration of indium and gallium in the gas phase was nominally equal. Due to different incorporation coefficients, the effective indium content in the QDs has to be determined by HRSTEM. The reduction in lattice mismatch is shown by a thicker wetting layer confirmed by HRSTEM and a higher critical layer thickness compared to the deposition of InAs QDs. Sample A has been directly overgrown with GaAs, while for samples B and C after the formation of the QDs an additional 4 nm layer of In0.1Ga0.9As was deposited prior to the GaAs overgrowth. In contrast to sample B, for sample C, we adjusted the growth temperature and the amount of deposited material to achieve a low QD area density. An uncapped sample was investigated by AFM to estimate the area density and size distribution of the QDs. The measurement on a large ensemble of QDs provides a histogram of the QD height (Fig. 1). The first maximum at a height of 1 nm belongs to small QDs that are strongly coupled to the wetting layer (WL) and give rise to weakly confined exciton states. The remaining large QDs cover a range from 3.5 nm to 10.5 nm showing a broad maximum at 5 nm. In addition, there are also islands with a height exceeding several tens of nanometers. These are not considered in the histogram since they are not optically active. It is expected that the overgrowth process will modify the absolute size of the QDs. However, the height distribution clearly translates into the PL measurements

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FIG. 1. Measured bimodal height distribution of QDs showing QDs with heights of 1 and 5 nm, respectively. The inset shows the corresponding AFM picture of the sample before overgrowth.

(see discussion below). In order to investigate the influence of the SRL on the structural and compositional properties of the QDs, we investigated capped samples by high-angle annular dark field (HAADF) HRSTEM. These measurements give precise information on the size and the indium concentration of the QDs. This technique allows investigations of samples that also provide PL data which is not the case for uncapped samples. For the HRSTEM, we prepared lamellas of the samples by focused ion-beam for [110]-beam direction and milled them with low-energy Argon ions afterwards resulting in a

FIG. 2. HRSTEM investigation of an exemplary QD from sample A: (a) HRSTEM micrograph, (b) resulting two-dimensionally projected indium concentration map, (c) line scan of the indium composition in growth direction (indicated by white arrow) through the sole wetting layer, and (d) through the QD. Additionally shown in blue are the corresponding effective concentration profiles of the TB supercell used in the theoretical calculations assuming 70 nm specimen thickness.

FIG. 3. HRSTEM investigation of an exemplary QD from sample B: (a) HRSTEM micrograph, (b) resulting two-dimensionally projected indium concentration map, (c) line scan in growth direction (indicated by white arrow) of the indium composition next to the QD through the SRL, and (d) through the QD. Again, the blue lines indicate the corresponding effective concentration profiles of the TB supercell.

specimen thickness of around 80 nm. The HRSTEM images were acquired at an acceleration voltage of 300 kV using an HAADF-detector covering scattering angles from 36 to 250 mrad. Two exemplary micrographs are displayed in Figs. 2(a) and 3(a). The normalized intensity around each atomic column was then compared with frozen-lattice multislice simulations9,10 taking into account both biaxial strain and static atomic displacements from different covalent radii of gallium and indium as described in Ref. 11. This results in twodimensional projections of the indium concentration shown in maps in Figs. 2(b) and 3(b). Line scans through the structures in growth direction are provided in Figs. 2(c), 2(d), 3(c), and 3(d). Since the lamella thickness is higher than the QD size, we correct the projected indium values for the QDs by assuming a rotationally symmetric lens shape to extract the true indium concentration in the QDs. From these HRSTEM results, we identify the following features of the samples. Sample A (without SRL) exhibits a large WL of around 8 monolayers having QDs on top with heights of 2–4 nm and diameters between 5 and 20 nm showing approximate lens-shapes. From the concentration maps of sample A, we find a mean indium concentration of around 30% in the WL. The actual indium content within the QD has an approximately linear gradient12,13 between the WL indium content at the QD base up to the very high value of 90% at the QD tip, in good agreement with the results of Ref. 14. The analogous HRSTEM investigations of sample B (with SRL) exhibit QDs several nanometers larger in diameter as well as higher indium concentrations within the WL, as seen in Fig. 3. However, the


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FIG. 4. Low temperature PL spectra of samples A, B, and C: (a) spectra of samples A and B at an excitation power of 30 mW/cm2 exhibiting a red shift due to the SRL and emission energies calculated by empirical tightbinding theory, (b) l-PL spectrum (10 W/cm2) of a single emission line at the low energetic ensemble edge of sample A, and (c) power dependent PL spectra of low density sample C (P ¼ 3 mW/cm2) with an energetic distance of 60 meV between p- and s-shell.

corrected values for the indium concentrations inside the QDs are found to be equal to sample A within the statistical limits of this analysis. From Fig. 3(c), we can also estimate the indium concentration in the SRL by evaluation of the line scan to have an approximative negative linear concentration gradient between 20% and 10% indium, followed by a typical segregation profile into the GaAs buffer above. In essence, we find that the SRL causes in average a larger QD size but the indium concentration is not affected. The influence on the optical properties is examined by PL measurements of the samples at liquid helium temperature. We excited the QDs with a diode laser at 532 nm and analyzed the emission with a spectrometer and an InGaAs photo diode array. The bimodal QD height distribution clearly translates into the spectra (Fig. 4(a)). Due to the inhomogeneous size distribution, the PL from the QDs covers a wide spectral region from around 1050 nm to beyond 1310 nm, reaching the first telecom low absorption window. The spectral shift to longer wavelengths caused by the In0.1Ga0.9As SRL amounts to 20 nm for the small QDs and 40 nm for the larger QDs. In addition, there is almost no decrease of the PL intensity for the sample with SRL. Since the indium content in the QDs is similar for both samples, this shift is attributed to a change of the QD size as found in HRSTEM measurements and of the strain around the QDs. In Ref. 15, the same explanation is given for a red shift of 26 nm

for MBE-grown QDs utilizing an In0.2Ga0.8As SRL where no difference in indium content in the QDs is observed. In additional studies of MBE-grown samples, shifts of 26 nm and 40 nm were observed for InGaAs SRLs with x ¼ 0.1 and 0.2, respectively.16,17 Studies of MOVPE-grown QDs reported large shifts of 150 nm and beyond8,18 as well as small shifts of a few tens of nanometers.19 While the large shifts were observed for samples with InAs deposition for the formation of a QD layer, the smaller shifts occurred for an InGaAs QD layer embedded in InGaAs barriers. These growth conditions differ from ours in a way that makes a direct comparison rather difficult. Although the area density of QDs is high for both samples A and B, single emission lines can be found at the low energetic ensemble edge as shown for sample A in Fig. 4(b). These single emission lines point out the zerodimensional character of the QDs. Fig. 4(c) shows the spectrum of low density sample C with SRL. We find a much smaller area density and also a more homogeneous ensemble of QDs. Here, the small QDs are not present anymore. We only observe emission from QDs with a similar size to the large QDs of samples A and B as a consequence of the adjusted growth parameters. For higher excitation powers, a second peak with an energetic distance of 60 meV appears which is assigned to emission from the QDs’ p-shells. From our HRSTEM and PL results, we assign the observed red shift of several tens of nanometers only to the


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FIG. 5. Concentration profiles for WL, QD, and SRL in the TB supercell as extracted from the HRSTEM analysis. The actual indium concentration in the QD/SRL region is the sum of the concentrations of the WL and the QD/ SRL.

change of QD size and the strainfield around the QDs, since the indium concentration is not influenced. We can confirm the observations from PL and HRSTEM measurements by applying an atomistic empirical TB model which allows us to study single parameters separately. The HRSTEM results represent selected cuts through the sample, thus allowing to analyze individual QDs and providing complementary information of the QD structure as input for our model. Individual QDs are modeled on a lattice of atomic sites within a supercell containing about 2 107 atoms and employing periodic boundary conditions for a sufficiently large cell size. QD geometry, WL thickness, and indium concentration profiles are used according to the HRSTEM results. We start from a GaAs periodic lattice and substitute individual gallium atoms by indium ones in a way that a random distribution of indium atoms with the target concentration profile within the chosen QD geometry is achieved. Calculations of the single-particle energies and wave functions of individual QDs with and without SRL are carried out utilizing the nearest neighbour sp3 s empirical TB model of Ref. 20 featuring spin-orbit coupling and inclusion of strain arising from latticemismatch on an atomistic level. We employ the valence force field model using Keating’s potential21,22 in order to minimize the global strain energy. Having diagonalized the TB Hamiltonian with parameters of Ref. 7, we calculate the many-particle states of the quantum dot carriers by using the configuration interaction (CI) approach.23 We find excellent agreement of calculated emission wavelengths to the ensemble-PL maxima, as indicated by vertical solid and dotted lines in Fig. 4(a), by assuming lensshapes and a QD diameter and height of 11.3 nm and 3 nm

for the first PL peak and a QD diameter and height of 15.8 nm and 7 nm, respectively, for the second PL peak while assuming a linear indium concentration gradient between 0.45 and 0.9 inside the QD (Fig. 5), as suggested by the HRSTEM results. The red shift with SRL exhibited in the PL spectra is very well reproduced for both small and large QDs. We apply the linear slope of the indium concentration inside the SRL as found by the HRSTEM measurements but assume a slightly smaller amplitude between x ¼ 0.15 at WL top down to x ¼ 0.05 at SRL height. This small difference can be attributed to SRL indium concentration fluctuations on the sample. From our TB calculations, we find the energetic distance between ground and first excited states to have the large value of 99 meV (72 meV) for the QDs with 3 nm (7 nm) height. For the small QDs, this splitting changes only negligibly to 96 meV with application of the SRL, while for the large QDs the splitting changes to 63 meV. These are in excellent agreement with splittings of 60 meV found from power dependent PL measurements of sample C with SRL and a strongly reduced QD area density (Fig. 4(c)). Since the QDs, which are modeled according to the HRSTEM measurements, reproduce the PL experiments very well, we can qualitatively estimate the change in strain around the QDs from the displacements maps (Fig. 6) of the TB supercells of large QDs. The shown QDs have the same size to exclusively investigate the influence on the strain environment. As can be seen in the maps, the atomic displacements are reduced for the supercell with SRL, i.e., material around the QDs is less strained with the SRL. In conclusion, we presented a comprehensive analysis in terms of PL, HRSTEM, and TB studies of MOVPE-grown

FIG. 6. Atomic displacement maps of QDs in TB supercells with (right) and without (left) SRL. The color coded displacement of the atoms is larger for the case without SRL.


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InGaAs QDs. We applied a strain-reducing quantum well layer to reach QD emission in the telecom low-absorption window at 1.31 lm. The QD samples A and B exhibit a bimodal height distribution, responsible for the two bright peaks in the PL spectrum. From HRSTEM measurements, we find an increase of the QD size, while the high indium content in the QDs as well as the PL intensity is not affected by the SRL overgrowth. Tight-binding simulations of the QDs based on the HRSTEM data are in very good agreement with the results from PL measurements, i.e., emission wavelengths, splittings between ground and excited states, and observed red shifts. In addition, these simulations provide information on the reduction of strain around the QDs when applying an SRL. We assign the red shift of the PL emission to both the change in QD size and in the strain environment. The authors gratefully acknowledge financial support from the BMBF QK_QuaHL-Rep project and from the Deutsche Forschungsgemeinschaft. 1

P. Michler, Single Semiconductor Quantum Dots (Springer, 2009). M. B. Ward, T. Farrow, P. See, Z. L. Yuan, O. Z. Karimov, A. J. Bennett, A. J. Shields, P. Atkinson, K. Cooper, and D. A. Ritchie, Appl. Phys. Lett. 90, 063512 (2007). 3 K. Nishi, H. Saito, S. Sugou, and J.-S. Lee, Appl. Phys. Lett. 74, 1111 (1999). 4 V. Ustinov, N. Maleev, A. Zhukov, A. Kovsh, A. Y. Egorov, A. Lunev, B. Volovik, I. Krestnikov, Y. G. Musikhin, N. Bert et al., Appl. Phys. Lett. 74, 2815 (1999). 5 H. Liu, M. Hopkinson, C. Harrison, M. Steer, R. Frith, I. Sellers, D. Mowbray, and M. Skolnick, J. Appl. Phys. 93, 2931 (2003). 2

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J. Tatebayashi, M. Nishioka, and Y. Arakawa, Appl. Phys. Lett. 78, 3469 (2001). E. Goldmann, S. Barthel, M. Florian, K. Schuh, and F. Jahnke, Appl. Phys. Lett. 103, 242102 (2013). 8 A. Hospodkova, E. Hulicius, J. Pangrac, J. Oswald, J. Vyskocˇil, K. Kuldova, T. Simec ˇ ek, P. Hazdra, and O. Caha, J. Cryst. Growth 312, 1383 (2010). 9 A. Rosenauer, T. Mehrtens, K. M€ uller, K. Gries, M. Schowalter, P. V. Satyam, S. Bley, C. Tessarek, D. Hommel, K. Sebald et al., Ultramicroscopy 111, 1316 (2011). 10 T. Mehrtens, K. M€ uller, M. Schowalter, D. Hu, D. M. Schaadt, and A. Rosenauer, Ultramicroscopy 131, 1 (2013). 11 A. Rosenauer, K. Gries, K. M€ uller, A. Pretorius, M. Schowalter, A. Avramescu, K. Engl, and S. Lutgen, Ultramicroscopy 109, 1171 (2009). 12 A. Rosenauer, U. Fischer, D. Gerthsen, and A. F€ orster, Appl. Phys. Lett. 71, 3868 (1997). 13 A. Rosenauer, W. Oberst, D. Litvinov, D. Gerthsen, A. F€ orster, and R. Schmidt, Phys. Rev. B 61, 8276 (2000). 14 P. Wang, A. Bleloch, M. Falke, P. Goodhew, J. Ng, and M. Missous, Appl. Phys. Lett. 89, 072111 (2006). 15 D. Litvinov, H. Blank, R. Schneider, D. Gerthsen, T. Vallaitis, J. Leuthold, T. Passow, A. Grau, H. Kalt, C. Klingshirn et al., J. Appl. Phys. 103, 083532 (2008). 16 O. Nasr, M. H. Alouane, H. Maaref, F. Hassen, L. Sfaxi, and B. Ilahi, J. Lumin. 148, 243 (2014). 17 L. Seravalli, C. Bocchi, G. Trevisi, and P. Frigeri, J. Appl. Phys. 108, 114313 (2010). 18 J. Bloch, J. Shah, W. Hobson, J. Lopata, and S. Chu, Appl. Phys. Lett. 75, 2199 (1999). 19 A. Passaseo, R. Rinaldi, M. Longo, S. Antonaci, A. Convertino, R. Cingolani, A. Taurino, and M. Catalano, J. Appl. Phys. 89, 4341 (2001). 20 P. Vogl, H. P. Hjalmarson, and J. D. Dow, J. Phys. Chem. Solids 44, 365 (1983). 21 A. Carmele and A. Knorr, Phys. Rev. B 84, 075328 (2011). 22 M. Schowalter, K. M€ uller, and A. Rosenauer, Acta Crystallogr., Sect. A: Found. Crystallogr. 68, 68 (2012). 23 N. Baer, P. Gartner, and F. Jahnke, Eur. Phys. J. B 42, 231 (2004). 7
