Reduction of Defect Density in Structures With InAs-GaAs Quantum

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Different parts of this study were supported by RFBR (Grant • 99-02-17356), NanOp. CC, N.N.L. acknowledges DAAD Professorship and equipment donation ...
Mat. Res. Soc. Symp. Proc. Vol. 672 © 2001 Materials Research Society

Reduction of Defect Density in Structures With InAs-GaAs Quantum Dots Grown at Low Temperature for 1.55 µm Range. 1

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P.Werner , U.Gösele , N.N.Ledentsov , D. Bimberg , N.D.Zakharov , 3 3 3 3 3 N.A.Cherkashin , N.A.Bert , B.V.Volovik , V.M.Ustinov , N.A.Maleev , A.E.Zhukov3, A.F.Tsatsul’nikov3 1 Max-Planck Institute of Microstructure Physics, Halle/Saale, GERMANY; 2 Technical University of Berlin, GERMANY 3 A.F.Ioffe Physical-Technical Institute of the RAS, St.Petersburg, RUSSIA ABSTRACT Transmission electron microscopy (TEM), and photoluminescence (PL) have been used to evaluate defects and the efficiency of defect-reduction techniques in structures with InAs quantum dots (QDs) for the 1.55 µm range grown at low substrate temperature (LT) using molecular beam epitaxy (MBE). We show that capping of the QDs with thin GaAs layer accompanied by growth interruption at 600oC (flash) allows to eliminate large islands, containing dislocations, while the smaller islands containing local defects (e.g. dislocation dipoles) still remain. If the flash procedure is accompanied with further depositing of thin AlAs cap layer, and followed by high temperature (~700oC) annealing (HTA), an almost complete elimination of defects is observed. The structures emit in the range of 1.55 µm due to lateral agglomerates of LTQDs. Simultaneously bright luminescence due to isolated QDs and GaAs matrix are detected at high excitation densities. INTRODUCTION Among the other important properties of InAs-GaAs quantum dots (QDs) there is a possibility to cover the 1.3 µm spectral range [1] which is considerably beyond the range available for pseudomorphic InGaAs-GaAs quantum well (QW) laser structures [2]. The further shift towards the 1.55µm range is desirable due to potential telecom applications, particularly in GaAs-based vertical-cavity surface-emitting lasers. Previously, we found that lateral agglomerates of InAs QDs formed at low substrate temperatures may be used for 1.5-1.7 µm emitting structures [3,4]. On the other hand, long wavelength QW and QD structures usually contain high concentration of dislocations and local defects making their device applications questionable. In the present work we apply two recently developed defect-reduction techniques [5, 6] to improve the structure of QDs and realize bright luminescence in the LTQD structures. EXPERIMENT The structures were grown by conventional solid source MBE on (001) GaAs substrate. The effective InAs thickness and the substrate temperature during InAs deposition were kept constant in all the growth runs: 4 ML and 325-350 0C respectively. O8.5.1

QDs were embedded into GaAs matrix and separated from the surface and the substrate sides by Al0.3Ga0.7As/GaAs shortperiod superlattices to avoid carrier leakage and nonradiative recombination at the surface or in the substrate. Four types of structures were grown. Sample 1: The InAs QDs were overgrown by 15-nm thick GaAs layer at the same substrate temperature 325-3500C. The GaAs growth was then followed at higher (6000C) temperatures. Sample 2: The InAs QDs were overgrown by 4 nm thin GaAs layer followed by growth interruption at 6000C. The thickness of GaAs overgrown layer was selected in such a way to cover fully small QDs leaving the large dislocated islands partially uncovered and resulting in its evaporation at 600oC short time annealing(so called “flash” procedure [5]). Sample 3: It is characterized by the deposition of 2-nm thick AlAs layer after 12 nm GaAs overgrowth at low temperature and subsequent high-temperature annealing (HTA) at 7000C. Sample 4: This type of structure represents a combination of flash and HTA procedures. The crystalline quality of the structures was investigated by TEM. The following electron microscopes were used: Philips EM420, Philips CM20T and JEOL JEM-4010 EX operating at 100kV, 200kV and 400kV, respectively. Commercial software package

Figure 1. PL spectra of all types of structures:(1) no flash, no HTA; (2) with flash; (3) with HTA; (4) with flash and HTA. Note the considerable increase in the integrated PL intensity in the structure with flash and HTA procedures. The intensity of right side peak in PL spectra 4 is

DigitalMicrograph 2.5 was used for digital processing of HRTEM images. PL was exited by the Ar+ laser and detected by cooled Ge photodiode. RESULTS The PL spectra from all kinds of structures demonstrate the existence of PL peaks at 1.2 eV (single QDs), 1.51 eV (GaAs matrix) and longwavelength (0.8 eV) emission (probably QD agglomerates) shown in Fig. 1. The sample 1 without flash and HTA demonstrates the weakest integrated PL intensity. Introduction of AlAs layer after 12 nm GaAs cap followed by HTA influenced on the intensity of the 1.2 eV PL emission. The thermal flash procedure results in increase in PL intensity for the matrix and single QD peaks (sample 2). An exceptionally strong increase in the integrated PL intensity is observed for the sample with both flash and HTA procedures. O8.5.2

Plan-view TEM images of all the samples 1-4 are shown in Figs.2. For the as grown sample 1 without flash and HTA one can observe three types of islands containing defects: small size (less than 7 nm) islands having a square shape, stripe-like islands and large islands having an irregular shape. High density of large dislocated islands (Nd= 1.7 1010 cm-2) (see A in Fig.2a), of stripe-like defect islands (Nd= 0.7 1010 cm-2), is revealed. Very high density of small coherent QDs is observed (NQD§  11 cm-2). They look like a ripple on a water surface due to relatively weak contrast. Application of flash procedure results in almost complete elimination of large dislocated islands (see A in Fig.2a) and formation of small edge type dislocation loops (Ndl=5.6 1010 cm-2, see B in Fig.2c). In spite of the density of these loops is twice higher than the density of dislocated islands in sample 1, the total perimeter of the loops is at least one order of magnitude smaller than that in the sample grown without flash. The ripple TEM contrast in the flashed sample gets finer (Fig.2c), indicating to decreasing of QDs volume. This observation agrees with the blue shift of PL lines in the flashed sample.

Figure.2. [001] plan view DF (220) TEM images of all types of structures: a) asgrown (sample 1); b) with HTA(sample 2); c) with flash procedure(sample 3); d) with flash procedure and HTA(sample 4). A-large dislocated InxGa1-xAs islands, B-small InxGa1-xAs islands in sample 2 containing dislocation loops. Coherent QDs look as a “ripple” structure in samples A and B (see magnified image in upper right corner).

Dark field cross-section images of the as grown (1) and flashed (2) samples taken in chemically sensitive (020) reflections are shown in Fig.3a,c respectively. Under these imaging conditions the In rich regions display bright contrast marked by the arrows (see Fig.3a). One can see that the In distribution is more inhomogeneous in QDs in the non-flashed samples. They usually contain several InAs enriched regions (see arrows in Fig.3a). The cross section image of one of the large island in as-grown sample taken at high resolution is shown in Fig.4a. To analyse the defect structure the image has been Fourier filtered using {220} Fourier components and positions of dislocations and orientation of their Burgers vectors are determined. Most probably the dislocations O8.5.3

are 60° ones with typical for GaAs structure Burgers vectors a/2{110}. The inhomogeneities in In distribution across the QD are marked by arrows. They are positioned in a zigzag way and are definitely correlated to the dislocation structure. The dislocations form very often a dipole configurations (AB and CD in Fig.4a) and stay mainly in between of the In enriched regions. The total Burgers vector of all dislocations in this particular case is equal zero, however it is not always so. In some cases one can observe threading dislocations punched by large QDs. The height of large QDs (≈4 nm) is comparable with the thickness of GaAs sub-layer overgrown just before flashing. The structure of QDs subjected to thermal flashing is shown in Fig.4b. Flashing results in annihilation of dislocation dipoles, flattening of QDs, and more homogeneous In distribution. The diameter and height of individual QD is about 8 nm and 2 nm

Figure 4. (a) cross-section HRTEM image of the large dislocated QD in the sample 1 without flash and HTA. The regions with high In concentration are marked by arrows. Position of dislocations and extra atomic plane orientation were determined in Fourier filtered image. (b) Structure of QDs in the sample 2 with thermal flashing. (c) Structure of QDs in sample 4 with HTA and thermal flashing.

respectively. The overlapping of individual QDs after thermal flashing is a typical feature in the flashed samples (see Fig.4b). This effect is also seen at low resolution (see Fig.3c). Application of HTA procedure after the 12-nm thick GaAs overgrowth and AlAs layer deposition results in considerable decrease in the density of large dislocated islands (by 2 times) while the density of stripe-like and small size dislocated islands does not undergo significant change. This fact is in agreement with the data available from the cross-section TEM image: dislocated islands (with height higher than 8 nm) are evaporated during HTA as they were not protected by the AlAs layer. These empty places were filled by flowed AlAs (see arrows in Fig. 3,b). At the same time small dislocated islands were overgrown by GaAs and remained in structure after HTA.

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As it was mentioned earlier, the two procedures of defect elimination, thermal flashing and AlAs capping of QDs followed by HTA, applied separately, result in some improvement of the structural quality, but one can still observe relatively high density of defects. As opposite, the combination of flashing with HTA eliminates defects completely. In the case of the structure consequently subjected to flashing and HTA one can observe the dense array of fine coherent islands (QDs) and hardly any defects [Figs. 2d,3d]. This fact can clearly explain the considarable improvement in the integrated PL intensity. The long wavelength peak due to laterally-coupled QDs broadens and somewhat shifts to higher energy after the flashing-HTA procedure, but remains of sufficient intensity. DISCUSSION Dislocation network was observed in samples with QDs formed at low substrate temperatures and not subjected to flash-HTA procedures. Large (>30-50 nm) wellseparated InAs dislocated islands are also formed indicating significant adatom diffusion and InAs redistribution through the surface at low temperatures. At the same time there is a high density of small coherent islands (see ripple contrast in Fig.2a). Low temperature growth may also result in high density of point defects formed in the GaAs allowing sufficient volume interdiffusion coefficient at elevated temperatures. The supersaturation of GaAs by point defects may explain formation of small edge type dislocation loops in the sample 2 subjected to flashing at 600oC. It was shown [7] that the edge type dislocation loops of such a small size can be formed exclusively by condensation of non-equilibrium point defects. Application of HTA at 700oC results in annealing of these loops as can be in Fig.2d. Elimination of large dislocated islands during the flashing procedure indicate that this dislocations cause more significant impact on the GaAs growth front leaving the large dislocated islands uncovered during the GaAs thin cap layer growth. Then the defect reduction procedure is essentially the same as described in [5]. Indeed, taking into account that the height of the islands is comparable with the thickness of GaAs 4 nm thick layer overgrown just before flashing, it is possible that some of larger or more relaxed (dislocated) islands are not completely covered by GaAs or AlAs due to the strain-induced GaAs expulsion force. This effect can also result in evaporation of some coherent InAs islands at flashing temperature 600oC. However it should be only a small fraction of coherent InAs QDs, otherwise we would observe QD-free regions in the cross section TEM images, the fact which was never confirmed (see Fig.3a,c). PL measurements also show that the density of small coherent QDs is high. Higher PL intensity indicates smaller concentrations of the defects in the system, which trap non-equilibrium carriers. Our results confirm the conclusion that the formation of QD’s agglomerates are most probably responsible for the appearance of the low energy peak at 0.8 eV in the PL spectra [4]. We note that sufficiently thick GaAs cap makes the defect reduction procedures ineffective. If the dislocated islands are covered with GaAs, they can’t be evaporated. By combining flash and HTA procedures with minor material loss, one can achieve enhanced optical properties via elimination of nonradiative recombination centres.

CONCLUSIONS O8.5.5

This work was devoted to defect elimination in structures with InAs-GaAs QDs emitting in the 1.5-1.7 µm range, which may be obtained by MBE at low substrate temperatures (325-3500C). Structural and optical studies are found to be in a good agreement. The longwavelength PL emissions remains in the spectra of defect-free structures supporting its assignment to radiative recombination of excitons trapped in QD lateral agglomerations. The structures subjected to flashing and HTA procedures separately demonstrate moderate improvement of the PL properties. Combination of these two approaches results in almost full elimination of the defects and leads to tremendous improvement in optical properties, particularly in integrated PL intensity. The defect reduction techniques proposed are believed to be extremely important for fabricating of defect-free long-wavelength quantum well and quantum dot InGaAsGaAs, InGaAsN-GaAs, or InGaAsSb-GaAs QW and QD structures for injection lasers operating in the 1.3 - 1.6 µm range. ACKNOWLEDGMENTS Different parts of this study were supported by RFBR (Grant • 99-02-17356), NanOp CC, N.N.L. acknowledges DAAD Professorship and equipment donation from the Alexander von Humboldt Foundation. REFERENCES 1. R.P.Mirin, J.P.Ibbetson, K.Nishi, A.C.Gossard, J.E.Bowers, Appl.Phys.Lett. 67, 3795 (1995). 2. S.L.Yellen, R.G.Waters, P.K.York, K.J.Beernink, J.J.Coleman, Electron Lett. 27, 552 (1991). 3. V.M.Ustinov, A.Yu.Egorov, A.E.Zhukov, A.R.Kovsh, N.N.Ledentsov, M.V.Maximov, B.V. Volovik, A.F. Tsatsul’nikov, P.S. Kop’ev, Zh.I. Alferov, I.P. Soshnikov, N. Zakharov, P. Werner, D. Bimberg, J. of Cryst.Growth, 201/202, 1143 (1999). 4. M.V. Maximov, A.F. Tsatsul’nikov, B.V. Volovik, D.A. Bedarev, A.Yu. Egorov, A.E. Zhukov, A.R. Kovsh, N.A. Bert, V.M. Ustinov, P.S. Kop’ev, Zh.I. Alferov, N.N. Ledentsov, D. Bimberg, I.P. Soshnikov, P. Werner, Appl. Phys. Lett. 75, 16, 2347 (1999). 5. N.N. Ledentsov, M.V. Maximov, D. Bimberg, T. Maka, Torres C.M. Sotomayor, I.V. Kochnev, I.L. Krestnikov, V.M. Lantratov, N.A. Cherkashin, Yu.G. Musikhin, Z.I. Alferov, Semicon. Sci. Technol. 15, 604 (2000) 6. I.L. Krestnikov, Cherkashin N.A., D.S. Sizov, D.A. Bedarev, Kochnev I.V., Lantratov V.M., N.N. Ledentsov, Pisma v Zhurn.Techn.Phys, 27, 34 (2001). 7. N.D. Zakharov, W. Neumann, V.N. Rozhanskii, Fizika Tverdogo Tela (USSR), 16, no.9, 2775 (1974).

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