ABSTRACT
Quantum well intermixing (QWI) using the impurity free vacancy disordering (IFVD) technique is one of the simplest and most versatile ways of altering the bandgap of a quantum well (QW) structure after growth, particularly in GaAs/AlGaAs structures [1]. The technique involves depositing a Si02 cap onto the surface of the GaAs and annealing at temperatures of 900° C or higher. At these temperatures Ga has a very high diffusion coefficient in SiC>2, an effect which was has been reported as far back as 1957 [2], although the detailed mechanism for this diffusion is still unknown. Secondary ion mass
spectroscopy (SIMS) profiles [1] of quantum well samples, which have an uppermost layer of GaAs, annealed with SiC>2 caps indicate, first of all, that there is minimal Si diffusion from the SiC>2 into the GaAs layer. Furthermore, there is a significant level of Ga detected in the Si02 cap after annealing with large Ga accumulation at the Si(>2 free surface. The outdiffusion of the Ga atoms into the SiC>2 during annealing generates group III vacancies, Vm in the GaAs, which subsequently diffuse through the structure. Due to the concentration gradient between Ga in the QW and A1 in the barriers, the Ga vacancies promote the diffusion of A1 into a buried GaAs QW and Ga into AlGaAs barriers, hence changing the QW bandgap to a higher energy by partially intermixing the QW. One of the attractions of this process is that no impurities are introduced in the process, only vacancies, unlike impurity induced disordering.
