Istituto dei materiali per l'elettronica ed il magnetismo     
Trevisi G., Seravalli L., Frigeri P., Prezioso M., Rimada Herrera J. C., Gombia E., Mosca R., Nasi L., Bocchi C., Franchi S. The effects of quantum dot coverage in InAs/(In)GaAs nanostructures for long wavelength emission. In: WRA-LDSD - Workshop on Recent Advances of Low Dimensional Structures and Devices (Nottingham, UK, 7-9 April 2008).
Different approaches have been used to redshift the emission of quantum dots (QD) in the 1.3-1.6 Ám spectral window [1]: i) the decrease in the QD material energy-gap, obtained by using antimonides or nitrides for QDs or by reducing the QD strain induced by confining layers (CL) [2], ii) the decrease in QD-CL band discontinuities and iii) the increase in QD sizes. While the decrease in band discontinuities inevitably results in the decrease of activation energy for thermal quenching of luminescence [2], then reducing the light emission efficiency at room temperature (RT), the increase in QD size does not have such a drawback and can be simply obtained by increasing the QD coverage. However, according to equilibrium theories of mismatched heteroepitaxial growth [3], above the Stranski-Krastanow critical coverage stable islands are nucleated with increasing size and density until a second critical coverage is reached, beyond which the nucleation of unstable, large-sized, ripened islands coexisting with the stable ones is anticipated. The elastic energy accumulated in large-sized islands can be relaxed through the formation of defects that may spoil the photonic properties of the structures. This work is aimed at studying the effects of QD size on the properties of MBE grown InAs/(In)GaAs dots with coverages in the 1.5 - 3.0 ML range. The structures were characterised by photoluminescence (PL), atomic force microscopy (AFM), capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). At low coverages (from 1.5 up to 2.2 ML) InAs/GaAs QD structures [4] are characterised by coherent QDs and by accumulation peaks of the apparent carrier density that can be related to quantum confined carriers in QDs and wetting layers. On the other hand, for coverages exceeding 2.2 ML we observe: i) large-sized, ripened QDs, ii) threading dislocations originating from ripened QDs, iii) reduced accumulation due to trapping of carriers by acceptor-like defects in the QD region, iv) deep levels detected in the QD region that we relate to extended defects and v) the decrease by 40% of the low-temperature integrated PL intensity. In InAs/In0.15Ga0.85As QD structures for redshifted emission, by increasing the QD coverage from 2.0 to 3.0 ML, a PL emission wavelength exceeding 1.3 Ám is observed at RT but: i) large-sized islands are nucleated which, along with typical features of layers grown on partially relaxed structures, make the surface morphology peculiar [5], ii) the accumulation peaks centered at the depth of QDs disappear, iii) deep levels related to extended defects show up and iv) the low-temperature integrated PL intensity decreases by 50%. A suitable choice of growth conditions to kinetically limit the QD ripening and the combination of different approaches to redshift the emission, such as QD strain engineering [2], should be considered for the preparation of structures with optimised optical and electrical properties. The work has been partially supported by the "SANDiE" Network of Excellence of EU, contract no NMP4-CT-2004-500101. The AFM characterisation has been carried out at CIM, University of Parma. [1] N.N. Ledentsov, IEEE J. Select. Topics Quantum Electron. 8, 1015 (2002). [2] L. Seravalli, M. Minelli, P. Frigeri, S. Franchi, G. Guizzetti, M. Patrini et al, J. Appl. Phys. 101, 024313 (2007). [3] I. Daruka and A.L. Barabasi, Phys. Rev. Lett. 79, 3708 (1997). [4] P. Frigeri, L. Nasi, M. Prezioso, L. Seravalli, G. Trevisi, E. Gombia et al, J. Appl. Phys. 102, 083506 (2007). [5] K. Samonji, H. Yonezu, Y. Takagi, and N. Ohshima, J. Appl. Phys. 86, 1331 (1999).
Subject quantum dots
long wavelength emission
molecular beam epitaxy
quantum dot ripening

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