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Epitaxy and Thin Film Deposition/Formation |
Epitaxial growth program in the Department is based on metal organic chemical vapour deposition (MOCVD) of III-V compound semiconductors. Using our state of the art facilities, we have been growing a variety of semiconductor structures based on GaAs/AlGaAs/InGaAs/GaAsN and InP/InGaAs/InGaAsP materials systems. Main emphasis of the research has been to understand fundamental growth related issues to fabricate novel optoelectronic devices. Other thin film deposition techniques such as plasma enhance chemical vapour deposition, e-beam evaporation, thermal evaporation are used to create dielectric and metallic layer structures. Examples of research programs in this area are given below.
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III-AsSb for long wavelength applications
Qiang (Michael) Gao, H. Hoe Tan, Chennupati Jagadish
GaAsSb and InAsSb based materials are of interests for optoelectronic applications in the 2-4um wavelength range such as in molecular spectroscopy, remote sensing, environmental/pollution monitoring, bio-hazardous material detection and other defence applications. However, one of the fundamental limitations to InAsSb QW laser is the difficulty to incorporate more Sb into the active region due to strain. To achieve longer wavelength operation up to about 50% of Sb has to be included into the InAs QW. In this project we investigate the growth mechanism of III-AsSb on GaAs, GaSb and InP susbtrates to acheive device quality materials. In addition, we also investigate the incorporation of N into this material system. Due to the huge negative bandgap bowing effect from N, the emission/absorption wavelength of these material Sb can be further extended to longer wavelength just by adding a few percent of N.
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Nanowires
Hannah Joyce, Qiang (Michael) Gao, H. Hoe Tan, Chennupati Jagadish
The growth of GaAs, InGaAs, InAs, InP and GaSb nanowires is done via the vapour-liquid-solid mechanism where gold nano-particles are used to nucleate the one-dimensional growth on the semiconductor substrates. The size and shapes of these nanowires are very dependent of growth temperature, which is typically 100-200C lower than normal MOCVD growth. By varying the deposition conditions, both the axial and radial growth can be controlled. Very long nanowires (~10 um) with diameter of few 10’s nm can be achieved with excellent crystallinity. Strong photoluminescence could also be observed from the nanowires. These nanowires are of great interests due to the possibility of using them as nano-building blocks (both active devices and interconnects) of future optoelectronics devices.
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Pulsed Laser Deposition of ZnO
Imam Rofi'i, Almamun Ashrafi, H. Hoe Tan, Chennupati Jagadish
ZnO is a future material which is very attractive for a range of optoelectronic devices due to its wide band gap of 3.4 eV and large exciton binding energy of 60 meV at room temperature. To be able to fabricate devices, a p-n junction is usually required. However for ZnO, p-type doping is still an unresolved issue. A newly acquired pulsed laser deposition system is used to investigate the p-doping issue and to deposit device quality heterostructures.
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Quantum Dots
, , H. Hoe Tan, Chennupati Jagadish
Stranski-Krastanow growth of InGaAs and InAs QDs on GaAs substrates by Metal-Organic-Chemical-Vapor-Deposition (MOCVD) is challenging and extremely sensitive to growth conditions. Great care has to be taken to growth these materials to ensure that they are pseudomorphically-strained without any defects. Furthermore, stacking these QDs into many layers is an issue due to strain accumulation of each layer and also on the smoothness of the overlaying surface prior to the deposition of the subsequent QD layer.
In the growth of InAs quantum dots on InP substrate, we can minimise the pronounced effect of As-P exchange by growing a very thin interlayer of GaAs or InGaAs prior to the deposition of QDs. Also by changing the amount of In content inof this interlayer, we can control the emission wavelength of the QDs and also their size and density.
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Selective Area Epitaxy for Device Integration
, H. Hoe Tan, Chennupati Jagadish
Selective area epitaxy is a technique where growth occurs over a pre-patterned substrate. The mask used is typically a dielectric and under certain growth conditions, no growth takes place on the mask. In the openings at vicinity of the mask, the local growth rate is then enhanced due to diffusion of the adatoms from the mask regions. Hence by controlling the dimensions of the masks and openings, the local growth rates and also the composition in the opening regions can be controlled. Using this technique for quantum dot growth, we are able to control the size, density and composition of the quantum dots. In addition, we are also able to selectively form quantum dots in one region but only the wetting layer (quantum well) in other regions. SAE proves to be a powerful technique for the integration of quantum dot-based optoelectonic devices such as a laser, modulator and waveguide.
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