Study on growth and in-situ processing of InAs self-organized quantum dots for long wavelength applications

Abstract

The aim of this research work can be divided into 2 parts. First is to understand the overgrowth process of InAs/GaAs self-assembled quantum dots (QDs) to extend the emission wavelength to long wavelength region. Second is to combine strain modulation in GaAs cap layer with AsBr[superscript3] in situ etching to create the new structures such as nanoholes and QD molecules. The properties of the investigated structure were measured by ex-situ atomic force microscopy (AFM) and photoluminescence (PL). The investigated QD array is large, low-density QDs with density of about 3-6x10[superscript 9]cm[superscript 2]. The effects of strain and thickness of an In[subscript x]Ga[subscript1-x]As (x=0-0.2) cap layer grown at low temperature are systematically studied. The dot height drastically reduces and the dot shape transforms into an elongated ridge-valley structure at the early stage of GaAs overgrowth, while the dots tend to preserve their shape during InGaAs capping. The effects of elastic energy and surface energy included in the surface chemical potential can qualitatively explain the observed surface evolution. The emission wavelength from the QDs covered with InGaAs layer can be extended beyond 1.3 micrometre. The strain modulation in GaAs cap layer was combined with AsBr[subscript 3] in situ etching process to create the nanoholes. The etching process can be divided into 2 regimes depending on the nominal etching depth and the thickness of the GaAs cap layer. In the first regime, nanoholes are formed ascribing to a strain selectivity of the etchant. Further supply of the etching gas causes the hole diameter to increase, while the depth stays approximately constant since the etching gas preferentially remove the atoms at the hole edge due to the less binding energy. InAs deposited on the filled-hole layer forms into groups of closely spaced self-assembled InAs QDs—termed lateral QD molecules. Deposition of InAs onto the nanoholes causes a preferential formation of the InAs QD molecules around the holes. The number of QDs per QD molecule ranges from 2 to 6, depending on the InAs growth conditions. By decreasing the substrate temperature, the number of QDs per QD molecule increases, but the statistical distribution is wider due to a reduced In atom diffusion length. From experimental results, we propose the simple model for QDs molecule formation.