Investigation of the formation of InAs QD's in a AlGaAs matrix

Transcription

1 10th Int. Symp. "Nanostructures: Physics and Technology" St Petersburg, Russia, June 17-21, IOFFE Institute NT.16p Investigation of the formation of InAs QD's in a AlGaAs matrix D. S. Sizov, N. V. Kryzhanovskaya, A. G. Gladyshev, Yu. B. Samsonenko, G. E. Cirlin, N. K. Polyakov, V. A. Egorov, A. A. Tonkih, Yu. G. Musikhin A. F. Tsatsul'nikov, N. N. Ledentsov and V. M. Ustinov Ioffe Physico-Technical Institute, St Petersburg, Russia Abstract. Optical and structural properties of self organised InGaAs quantum dots (QD), deposited in Al 0.3 Ga 0.7 As matrix, were investigated. Samples were grown by molecularbeam epitaxy (MBE). It is shown, that deposition of monolayer of InAs on Al 0.3 Ga 0.7 As surface results in formation of nanoscale QDs on 1 2 monolayer thick wetting layer (Stranski Krastanov growth mode) [1]. Large exciton localization energy of the InAs QDs in Al 0.3 Ga 0.7 As in compare with QDs in GaAs is demonstrated. This is due to increase in size of these QDs and significant bandgap offset in the case of InAs/AlGaAs system in compare with InAs/GaAs one. Introduction At recent time, progress in the development of light emitting devices and injection lasers based on self-organised In(Ga)As QDs become evident [2]. Lasers grown on GaAs substrates with QD-based active region allow to realize 1.3 µm emission. This lasers are promising for using in fiber optic communication systems. Vertical cavity surface emitting lasers with AlGaAs distributed Bragg reflector and QD based active region, were fabricated. Deep carrier localization in QDs leads to another advantage of QD-based devices temperature stability of the parameters of lasers with QD's in active region [3]. Localization energy more than 800 mev can be achieve in In(Ga)As/Al 0.3 Ga 0.7 As QD's system. In principle, up to now main investigations were concentrated on In(Ga)As QD's in GaAs matrix [4]. In this work we have investigated InAs QD's grown in Al 0.3 Ga 0.7 As matrix which exhibit photoluminescence (PL) in wide optical range ( µm). Wide bandgap Al 0.3 Ga 0.7 As allows dramatically increase carrier localization in QD. Localization energy of about 0.7 ev for InAs QDs in a Al 0.3 Ga 0.7 As matrix was achieved that exceed the value for the same QD's in GaAs matrix (about 0.46 ev). Obtained result indicates that InGaAs QDs in AlGaAs can be used for fabrication of high temperature stability injection laser [5].

2 Experiment Investigated structures were grown by molecular beam epitaxy on GaAs [100] substrates. The samples contain active region with InAs QD embedded in Al 0.3 Ga 0.7 As matrix. This active region was confined from both sides by AlAs/GaAs superlattices, to prevent leakage of the carriers to defect areas which located near buffer layer and surface. The substrate temperature during the deposition of QDs and following 50 Å Al 0.3 Ga 0.7 As layer was 485 C. The other layers of the structures were grown at 600 C. PL was excited by Ar + laser (~500 W/cm 2 ) and registered by cooled Ge photodiode. Transmission electron microcopy (TEM) measurements were performed using a Phillips EM 420 microscope in plan-view and cross-section geometry at an acceleration voltage of 100 kv. Result and discussion Cross section and plain view TEM pictures of the structures with QDs obtained by the deposition of 2.5 ML of InAs in the Al 0.3 Ga 0.7 As and GaAs matrixes are presented in Fig. 1. WL was revealed in the images that indicate Stransky Krastanov growth mode for both cases. Lateral size of the QDs grown in Al 0.3 Ga 0.7 As matrix is about to 18 nm, whereas, lateral size of QDs in GaAs matrix is about 12 nm. This effect can be connected with roughness of Al 0.3 Ga 0.7 As surface or different growth mechanism for InAs on Al 0.3 Ga 0.7 As. Fig 1. TEM pictures of structure with QDs in AlGaAs matrix in compare with structure with QDs in GaAs matrix. PL lines associated with recombination via ground state of the QDs for the samples with different InAs average coverage in Al 0.3 Ga 0.7 As matrix are presented in Fig. 2. Observed lines are inhomoheneously broadened, and they have Gauss function shape. QD peak shifts in red side with increase in InAs layer thickness up to 4.5 ML.

3 Fig 2. PL spectra of structures with QDs in AlGaAs matrix. QD peak shift with in red side increasing InAs average coverage up to 4.5 ML. Dependencies of the QD peak position on InAs average thickness for the QDs in GaAs and Al 0.3 Ga 0.7 As are shown in Fig. 3. In case of GaAs matrix, red shift of QD peak can be observed with increasing of InAs coverage up to 2.5 ML and then the shift of emission saturates [6]. In case of AlGaAs matrix, this red shift continue up to 3 ML, and saturate over these value. Energy separation between InAs ground states and matrix states can be achieved of 0.7 ev for the AlGaAs matrix in compare with 0.46 ev for GaAs matrix. This large shift is due to significant localization of the carriers in QDs, that results in smaller effect of matrix bandgap on energy levels in QDs. Beside, conservation of the shape of QDs at overgrowth of the QDs by AlGaAs, resulting in red shift of emission was demonstrated [7]. Fig 3. Dependences of the QD peak position on InAs coverage for the QDs in GaAs and AlGaAs.

4 It's known, that overgrowth of InAs QDs by In 0.15 Ga 0.85 As alloy leads to considerable redshift of the QD emission. This method allows to achieve 1.3 µm lasing in structures with InGaAs/GaAs QD [6]. We propose apply this method for QD embedded in Al 0.3 Ga 0.7 As matrix. Active region of this sample contains 2.5 ML thick InAs layer overgrown by 5 nm In 0.15 Ga 0.85 As layer. PL spectrum of this sample is presented in Fig. 4. Maximum QD peak position wavelength corresponds to 1.25 µm. Further optimization of growth parameters can increase QD emitting wavelength, and so emission at 1.3 µm can be achieved. Fig 4. PL spectrum of structure with InAs QD overgrown by In 0.15 Ga 0.85 As in AlGaAs matrix. Conclusions It was demonstrated, that InAs QDs are grown in Stransky-Krastanov growth mode in Al 0.3 Ga 0.7 As matrix. These QDs have larger lateral size in compare with QDs in GaAs matrix. This fact, as well significant bandgap offset in the InAs/AlGaAs system, allows fabricate QDs with localization energy about 0.7 ev. Possibility to achieve PL at 1.3 µm wavelength by using of InAs QDs overgrown by InGaAs layer was demonstrated. Acknowledgments This work was supported by: RFBR NanoOp Program, Intas and CRDF grant (RE1-2217). References 1. V. M. Ustinov, A. E. Zhukov, A. Yu Egorov, A. R. Kovsh, A. F. Tsatsul'nikov, M. V. Maximov, S. V. Zaitsev, N. Yu Gordeev, P. S. Kop'ev and Zh. I. Alferov, Proc. Int. Symp. on Compound Semiconductors, St Peterspurg, Russia, V. M. Ustinov and A. E. Zhukov, Semicond. Sci. Technol. 15, R41 (2000).

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