The effects of quantum dot coverage in InAs/(In)GaAs nanostructures for long wavelength emission
Introduction
In recent years, great interest has been devoted to optoelectronic devices based on quantum dot (QD) structures [1], [2] and one of the major concerns is actually related to the preparation of long wavelength (>1.3 μm) emitting QD structures grown on GaAs substrates. To this aim, different approaches have been used, among which the growth of QDs on metamorphic buffers [3], [4] proved to be useful for the achievement of room-temperature (RT) emission wavelengths exceeding 1.5 μm [3], [5]. While redshifted emissions are obtained by decreasing the band discontinuities between QDs and confining layers (CL), it has been shown that the activation energy for the thermal quenching of luminescence inevitably decreases when such band discontinuities are reduced [6]. As a consequence, QD structures engineered to emit at long wavelength may suffer from poor emission efficiency at RT. Redshifting emission wavelength by increasing the QD sizes does not have such a drawback and, since it can be simply obtained by increasing the QD coverage (θ), it is usually combined with other more sophisticated approaches in order to reach the 1.3–1.5 μm spectral window. While several experimental results report on the detrimental effects of large-sized QDs on the properties of QDs embedded in GaAs CLs [7], [8], such studies have not yet been reported for QD structures grown on InGaAs metamorphic buffers.
This work is aimed at studying the effects of θ on the properties of InAs QDs embedded in GaAs and in In0.15Ga0.85As metamorphic CLs in structures grown by molecular beam epitaxy (MBE) on (0 0 1) GaAs substrates. Capped QD structures were characterised by photoluminescence (PL), capacitance–voltage (C–V), deep-level transient spectroscopy (DLTS), transmission electron microscopy (TEM) and X-ray diffraction, while the morphological and structural properties of uncapped QD structures were studied by atomic force microscopy (AFM) and plan-view TEM.
Section snippets
Experimental
InAs/GaAs QD structures were prepared by MBE with a coverage θ in the 1.5–2.9 ML range according to Ref. [9]. InAs/In0.15Ga0.85As metamorphic QD structures with θ ranging between 2.0 and 3.0 ML were grown by MBE on GaAs semi-insulating substrates and consist of a single layer of InAs QDs grown by Atomic Layer MBE [10] embedded in a 20-nm-thick undoped In0.15Ga0.85As layer. Such a layer was grown on a 0.8 μm n-doped In0.15Ga0.85As lower CL (LCL) and it was capped by an In0.15Ga0.85As upper CL (UCL)
Results
By increasing θ beyond 1.55 ML in the InAs/GaAs QD system, we observe the nucleation of three-dimensional (3D) islands that undergo a fast increase in density, until a saturation value of ∼2×1011 islands/cm2 is reached when θ is 2.2 ML. The island heights and base diameters, given by the most frequent values of the corresponding AFM size distributions, increase with increasing θ and reach the maximum values of 2.7 and 15 nm, respectively. Noticeably, while θ increases in the density-saturation
Discussion and conclusions
The effect of QD coverage on the properties of InAs/(In)GaAs QD structures has been presented. We have shown that high QD coverages lead to the nucleation of large-sized ripened QDs that coexist with the small-sized ones, consistent with the thermodynamic theory of lattice-mismatched growth by Daruka and Barabasi [12]. Such ripened QDs undergo relaxation through the formation of V-shaped defects that we relate to: (i) the decrease in the low-temperature integrated PL intensity, (ii) the
Acknowledgements
This work has been partially supported by the “SANDiE” Network of Excellence of EU, Contract no NMP4-CT-2004-500101. J.C.R. gratefully acknowledges the support of the “ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy”. The AFM characterisation has been carried out at CIM, University of Parma.
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