Self-assembled growth of high-quality quantum dots (QDs) is important for fabricating novel and effective electronic and photonic devices [1]. Typically, QDs are obtained via the Stranski-Krastanow (SK) growth mode, which reduces strain arising from lattice mismatch in underlying layers. A defining feature of SK growth is the formation of a wetting layer (WL) that precedes dot growth. The WL mediates the electronic interaction between the barrier states and the localized QD states influencing the optical properties of the QDs. Charge transfer between QDs via higher energy states can, for example, prove advantageous for laser operation [2]. SK growth is not possible in lattice-matched materials due to the absence of strain, droplet epitaxy (DE) is a promising technique, which enables self-assembly of strain-free QDs [3]. For the fabrication of GaAs/AlGaAs QDs, for instance, the size and density of the GaAs QDs is dependent on that of the Ga droplets, which is a function of growth temperature and Ga flux, and on the subsequent crystallisation process with As flux. However, QDs are formed without a WL, unless special growth conditions are used [4]. Besides potential advantages in terms of carrier transfer, inclusion of a GaAs WL is expected to improve the As adatom surface diffusion resulting in the growth of more uniform dots. Here, we report GaAs/AlGaAs QDs with highly uniform size and excellent optical properties. Figure 1: AFM image of (a) 3 MLs Ga droplets and (b) GaAs quantum dots

Figure 1(a) shows Ga droplets with an average diameter of 69±4 nm formed by the deposition of 3 monolayers (MLs) Ga on a GaAs substrate at 320ºC. GaAs/AlGaAs QDs with mean diameter of 99±10 nm were subsequently formed after supplying low As flux at 400ºC [Fig. 1(b)].

                      Figure2: 4K PL spectrum of GaAs QDs

The optical properties of GaAs QDs grown under similar conditions, but embedded in Al_0.3Ga_0.7As, were studied using photoluminescence (PL), shown in Fig.2. At 728 nm, the QD PL is at higher energy than the bulk GaAs emission due to confinement effects, but has lower intensity. The FWHM of the QD emission is 38 meV, which is typical for SK QDs, and indicates that although the initial size of Ga droplets is uniform, the final shape and size of the QDs on the crystallisation process during the supply of As flux. To improve the size uniformity of the QDs and the FWHM and intensity of their PL, new structure was grown where a GaAs WL was inserted immediately prior to the QD growth, and both Ga droplet deposition and crystallisation with As flux were at 320ºC. The GaAs/Al_0.3Ga_0.7As structure was also embedded in a GaAs/AlAs superlattice (SL).

Figure3: AFM image of surface of GaAs QDs from the superlattice sample

Fig 3 shows that the surface of GaAs QDs for this sample have high uniformity (99±3 nm) and areal density of 10⁸ cm^-2 .

Figure 4: (a) TEM image of GaAs/AlGaAs QDs embedded in GaAs-AlAs SL and (b) 4K and 300K PL spectra

Fig 4(a) shows a transmission electron microscopy (TEM) image of a GaAs/AlGaAs QD embedded in GaAs/AlAs SL structure. A continuous WL under the QDs is clearly observed. Low-temperature and room temperature PL spectra from the same sample are shown in Fig. 4(b). The very narrow PL emission (FWHM of 14.9 meV) observed from the QDs at 4K (centred at 689 nm) indicates high optical quality and excellent dot uniformity. Despite the low areal number density of 10⁸ cm^-2 the QD peak in Fig. 3 is now more intense than the GaAs peak, which we attribute this to efficient transfer of carriers from the SL to the WL and from the WL to the dots. At room temperature, the PL shows a series of sub-peaks, which we attribute to different quantum confined dot states.

1. D Bimberg, M. Grundmann., and N.N Ledentsov, Quantum Dots Heterostructures1999, New York: Wiley.

2. C. Cornet, et al., InAs/InP quantum dots (QD): from fundamental understanding to coupled QD 1.55 µm laser applications. physica status solidi (c), 2007. 4(2): p. 458-461.

3. M. Jo, T. Mano, and K. Sakoda, Two-Step Formation of Gallium Droplets with High Controllability of Size and Density. Crystal Growth & Design, 2011. 11(10): p. 4647-4651.

4. S. Sanguinetti, et al., Modified droplet epitaxy GaAs/AlGaAs quantum dots grown on a variable thickness wetting layer. Journal of Crystal Growth, 2003. 253(1–4): p. 71-76.

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1. Photoreflectance in the mid-infrared : a study of the bandgaps and spin-orbit splittings of ternary and pentenary InAsSb-based alloys
2. Comparison of the strain distribution obtained from multi scale and conventional approaches