Passivation of GaAs surfaces and fabrication of self-assembled In(Ga)As/GaAs quantum ring structures

This work concentrates on two topics: (i) GaAs surface passivation methods using different materials and (ii) formation of InAs islands on GaAs and transformation of InAs islands into quantum rings (QRs). All the samples are fabricated by metalorganic vapor phase epitaxy and characterized by optical spectroscopy and atomic force microscopy. InGaAs/GaAs near-surface quantum well (NSQW) structures were used in the GaAs surface passivation studies because of their sensitivity to surface states. Ultra-thin InP, GaP, GaN layers were grown in-situ on top of the NSQW structure as the passivation layer. As-P and As-N exchange on the GaAs surface were also applied for passivation. In all the passivation methods, the photoluminescence (PL) intensities and carrier lifetimes of the NSQWs were significantly increased. The enhancement factor of the PL intensity was up to two orders of magnitude. The study of time durability of the passivation after keeping the samples for months in air ambient showed that those passivation methods protect the samples against oxidation while the unpassivated samples degrade severely. The passivation effects of these materials were also studied using NSQWs fabricated on (110)-oriented GaAs substrates. The suitability of atomic layer deposited (ALD) titanium nitride layer on GaAs surface as an ex-situ passivation layer was also investigated. Although the enhancement factor of the PL intensity is smaller than that obtained by in-situ methods, smooth surface morphology and notable extension of carrier lifetime were observed in the ALD passivated samples. It is known that island formation is severely suppressed on the GaAs (110) surface. This limitation can be overcome by using a thin strain reducing layer, e.g., an InGaAs layer. Relatively uniform InAs islands with an average areal density of 109 cm-2 were obtained on GaAs (110) substrate at 400 °C using a thin InGaAs strain reducing layer. Transformation of InAs islands into rings was realized by partially capping the InAs islands and annealing under tertiarybutylarsine flow. Effects of growth conditions on ring evolution were studied by varying the thickness of the partial capping layer, annealing time and annealing temperature. It was concluded that the temperature dependence of the diffusion anisotropy of the indium atoms plays an important role in the ring evolution. The annealing process of the partially capped islands affects significantly the ring shape and the optical properties of the QR structure