Interdiffusion Mechanisms in GaAs/AlGaAs Quantum Well Heterostructures Induced by SiO 2 Capping and Annealing
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fusion of group III species.Although SiO 2 encapsulation-induced intermixing was investigated by several groups and quickly led to the fabrication of novel built-in optoelectronic devices such as low-loss optical waveguides, modulators, and quantum well laser diodes [2,4], the basic understanding of the specific diffusion mechanism involved has suffered from a lack of reproducibility and discrepancies due to the wide variety of experimental conditions used (differences in oxide deposition techniques, annealing conditions, epitaxial layer composition, etc.). The main objective of this paper is to investigate the basic interdiffusion mechanisms of the SiO 2 capping technique. EXPERIMENT Undoped GaAs/AIGaAs QW heterostructures of different types were grown by molecular beam epitaxy on (001) semi-insulating GaAs substrates for this study. In order to prevent possible reduction of the Si0 2 by Al and resulting in-diffusion of silicon and/or oxygen inside the epitaxial layers as reported by various authors [3], all of our structures are capped with a final 5 nm-thick GaAs layer. The SiO 2 and Si 3N4 layers used in this study were deposited by rapid thermal chemical vapor deposition (RTCVD). In previous studies [5], we have shown that Si0 2 RTCVD films give the highest quality interdiffusion compared to other deposition techniques. Conventional thermal anneals were carried out at either 850'C or 900'C in a As overpressure atmosphere. Samples were characterized before and after Si0 2 deposition, as well as after heat treatment. For patterning purposes, high resolution electron beam lithography and subsequent metal lift-off were used in association with reactive ion etching. The second RTCVD step was then performed in the case of Si0 2 /Si 3N4 arrays, and thermal annealing was carried out afterwards. A great care was taken in choosing effective characterization techniques. Our results were confronted through the use of the following complementary techniques: low temperature optical spectroscopy, photoluminescence (PL), photoluminescence excitation spectroscopy (PLE) and linear polarization anisotropy analysis, secondary ion mass spectroscopy (SIMS) and finally, cross-sectional transmission electron microscopy (XTEM). RESULTS AND DISCUSSION Intermixing under uniform SiO 2 capping layers To appreciate better the kinetics of SiO 2 capping-induced intermixing, we studied more specifically the influence of anneal duration on a multiple quantum well (MQW) heterostructure coated with a 480 nm-thick layer of RTCVD SiO 2 .The structure comprises a 0.45 lim-thick GaAs (5 nm)/ Al0 .33 Gao.67As (10 nm) MQW. Several samples were annealed at a fixed temperature of 850'C but for different durations varying from a few seconds (using either rapid thermal annealing or a pushpull procedure in a conventional furnace) up to 4 hours. Secondary ion mass spectroscopy (SIMS) was used to monitor the evolution of the Ga distribution inside the oxide layer as well as the changes in the Al depth profile within the heterostructure. Ga and Al profiles ar
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