Xian Gao

Investigation of Localized States in GaAsSb Epilayers Grown by Molecular Beam Epitaxy

We report the carrier dynamics in GaAsSb ternary alloy grown by molecular beam epitaxy through comprehensive spectroscopic characterization over a wide temperature range. A detailed analysis of the experimental data reveals a complex carrier relaxation process involving both localized and delocalized states. At low temperature, the localized degree shows linear relationship with the increase of Sb component. The existence of localized states is also confirmed by the temperature dependence of peak position and band width of the emission. At temperature higher than 60 K, emissions related to localized states are quenched while the band to band transition dominates the whole spectrum. This study indicates that the localized states are related to the Sb component in the GaAsSb alloy, while it leads to the poor crystal quality of the material, and the application of GaAsSb alloy would be limited by this deterioration.

The Surface and Optical Properties of Passivated GaSb with Different Passivating Agents

In this paper, (NH4)2S and Na2S were used as passivating agent for the sulphuration treatment of GaSb. Although the oxide layer can be removed by both (NH4)2S and Na2S, the etching rate of Na2S was faster than (NH4)2S, which caused the high RMS. After passivation treatment, PL instensity of GaSb increased, however, if the passivation time was too long, PL instensity would decrease. At low temperature PL spectrum, the emission located at 777meV and 795meV which can be associated with the transition from the conduction band to the native acceptor level VGaGaSb and the other peak corresponds to bound-edge-related transitions can be observed.

Investigation of localized and delocalized excitons in ZnO/ZnS core-shell heterostructured nanowires

Abstract The localized states in ZnO nanowires (NWs) through the growth of ZnS shell have been introduced in this paper. Morphology and optical properties of the ZnO/ZnS core-shell heterostructured NWs after different rapid thermal annealing (RTA) treatments are investigated. Transmission electron microscopy measurements show the gradual disappearing of the jagged boundary between ZnO and ZnS with the increase of RTA temperature, while a decrease of interfacial composition fluctuation and a formation of ZnOS phase can be found after a RTA treatment of 300°C. Temperature-dependent photoluminescence exhibits the features of “S-shape” peak positions and a “valley shape” for the emission width, implying the existence of localized excitons in the core-shell NWs. Moreover, it is noted that the RTA treatments can lower the localized degree which is confirmed by optical measurement. The results indicate that the optical behavior of excitons in ZnO/ZnS core-shell heterostructured NWs can be manipulated by appropriate thermal treatments, which is very important for their practical device applications.

Effect of rapid thermal annealing on the optical properties of GaAsSb alloys

GaAsSb ternary alloys are fundamental components of advanced electronic and optoelectronic devices in the future. The presence of localized states could greatly affect the optical properties in GaAsSb alloy, which depend on the fluctuation of alloy composition. In order to optimize the optical properties, GaAsSb alloys were treated by rapid thermal annealing (RTA) at different temperatures, and the optical behaviors of the annealed samples were investigated in detail. During RTA, a significant reduction of the localized states was observed by photoluminescence (PL) spectral analysis. Furthermore, the RTA process also altered the distribution of the components of the GaAsSb alloy, which caused a slight red-shift of the maximum PL peak at 150 K. The relationship between the localized states and the temperature of the RTA process was also investigated. The process involving the conversion of localized carriers to free carriers was proposed. Under the suitable RTA conditions, the Sb component was homogenized and the depth of carrier localization was decreased.

Surface passivation of GaAs using atomic layer deposition grown MgO

In this paper, different MgO films were deposited on a GaAs substrate by atom layer deposition (ALD), which can passivate the surface state of GaAs. From XPS results, the band structure of MgO/GaAs can be indexed to type I. Based on band alignment, the excitons could not diffuse and created nonradiative centers, which were all limited and then recombined at the MgO/GaAs interface, thus the emission intensity of the 100 nm MgO film coated sample was about 3 times higher than the uncoated sample.

Band alignment at a MgO/GaSb heterointerface using x-ray photoelectron spectroscopy measurements

The valence band offset (ΔE V) of a MgO/GaSb heterostructure was determined using x-ray photoelectron spectroscopy measurements. A ΔE V value of 2.84 ± 0.10 eV was calculated by using Ga 3d3/2 and Mg 2p1/2 binding energies as references. Taking the empirical band gaps of 7.83 eV and 0.73 eV for MgO and GaSb thin films into consideration, respectively, we obtained the type-I band alignment of a MgO/GaSb heterostructure with a conduction band offset (ΔE c) of 4.26 ± 0.10 eV, suggesting a nested interface band alignment.

The surface passivation on the optical and surface properties of InP

We propose a novel surface passivation of InP, to obtain the strong luminescence property and completely remove the surface state. InP is passivated by (NH4)2S solution, then treated by rapid thermal annealing (RTA) at different temperatures. Compared with unannealing sample, the PL intensity is increased to 1.7 times. We adopt atomic layer deposition (ALD) of Al2O3 avoiding oxidizing gradually.

Periodic nanostructures and optical properties of inp induced by femtosecond laser pulses

In this letter, InP periodic nanostructures were synthesized by linearly polarised femtosecond laser. Perfect morphologies of InP nanostructures can be obtained under an appropriate laser power. Further, to restrain surface sates of InP, the Al2O3 layers were deposited on InP nanostructures by atomic layer deposition (ALD). The optical properties of InP nanostructures were investigated by Raman and Photoluminescence (PL). And the transverse optical (TO) peak and novel luminescence emission can be observed due to the formation of the the periodic nanostructures. Copyright 2013 American Scientific Publishers. All rights reserved.