The Fabrication of InGaAs ring-like nanostructures by droplet molecular beam epitaxy

Abstract

InGaAs ring-shaped nanostructures, or quantum rings (QRs), have been fabricated by droplet epitaxy using solid-source molecular beam epitaxy (MBE). The droplet forming conditions have been varied by changing the growth parameters including substrate temperature (Ts~120-300oC) during In0.5Ga0.5 deposition, In0.5Ga0.5 deposited amount (2-5ML), and In-mole-fraction (x~0.3-0.7) of InGa droplets. The morphology of the QRs was characterized by atomic force microscopy (AFM). The effects of each growth parameters on the InGaAs QRs are investigated. Increasing Ts results in the InGaAs QRs of a larger size but lower density due to 2-dimensional expansion and merging of InGa droplets. Furthermore, increasing In0.5Ga0.5 amount deposited results in larger QRs at low Ts. However, the QR density oscillates with increasing In0.5Ga0.5 amount due to merging of small droplets into a full-layer. At higher Ts, increasing In0.5Ga0.5 amount results in the QRs of a higher height, higher density but smaller diameter due to the accumulating compressive strain inside larger QRs and the partial relaxation. Moreover, varying In-mole-fraction (x) of InxGa1-x droplets lead to a variation of crystallized-QR size and density, i.e.; high density tiny-size QRs from high-Ga-content droplets and low density large-size QRs from high-In-content droplets. For photoluminescence (PL) measurement, another set of samples were grown under the droplet forming conditions of 2-5 ML In0.5Ga0.5 deposition at 210oC, with an additional 100-nm GaAs capping layer grown by migration-enhanced epitaxy and conventional method. The optical properties of the InGaAs QRs were analyzed by PL spectra of the respective samples at 20-100 K. The PL intensity is relatively low due to low density of the QRs (~108 cm-2). The PL measuring parameters, including excitation intensity, measuring temperature and polarization have been varied. When increase the excitation intensity, the PL intensities increase without shifting, indicating the ground-state energy of the InGaAs QR systems. With increasing the measuring temperature, the PL intensities decrease without thermal broadening. It is also observed that the spectra of 3 ML sample are not shifted. However, the spectra of 4 ML sample are red-shifted, implying the existence of the strain field. Finally, the polarized PL spectra correspond to the elongation of the QRs, confirming the anisotropy of the QRs.