The study of GaN materials for device applications

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Date
2007
Authors
Yam, Fong Kwong
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Abstract
In this project, works are focusing on the investigation of the material quality grown by different techniques, metal contacts on GaN materials as well as the study of the fundamental properties of the porous GaN materials and the fabrication of devices based on porous GaN materials. Detailed characterizations for the investigation on the GaN films quality grown by two different techniques, i.e. low pressure metal-organic chemical deposition (LPMOCVD) and plasma assisted MOCVD (PA-MOCVD) have been carried out. Apart from buffer layer was observed to be able to influence the physical properties of GaN films in LP-MOCVD, it is also found that in PA-MOCVD, hydrogenation during growth was capable of producing epitaxial GaN films at reduced temperatures. Various metal contacts on GaN materials have been investigated in this project. Pt was found to have excellent electrical properties and thermal stability at elevated temperatures among the metal contacts on n-GaN. A new Ni/Ag bi-layer ohmic contact on p-GaN has been explored; the specific contact resistivities (SCRs) of this bi-layer scheme were observed to be sensitive to the change of annealing temperatures and durations. Other than that, the study of Schottky contacts based on four different metallization schemes i.e. Ti, Ag, Ti/Ag, and Ag/Ti were also performed on p-GaN, and heat treatment was found to be able to improve the electrical properties of Schottky contacts generally. Since porous GaN is a new form of material, the properties are scarcely found in the literature. In this project, various characterization tools have been used to investigate the structural, morphological and optical properties of porous GaN generated by ultra-violet assisted electrochemical etching and Pt assisted electroless etching methods. Generally, the studies showed that the physical characteristics of the GaN were found to be influenced significantly by the porous layer. Overall the porous samples produced by these two techniques were found to have many similarities, however, different characteristics were also observed in certain areas. Other than that, study also showed that chemical treatment induced porous GaN layer was able to enhance the electrical properties of Pt Schottky contacts on GaN in which the Schottky barrier height (SBH) and leakage current were improved significantly. Four devices have been fabricated from porous GaN. Performance of the gas sensor was found to be enhanced substantially by using porous GaN layer. Photodetector fabricated from porous GaN layer also showed promise in which low dark current and higher photo-current to dark current ratio were observed. On the other hand, for light emitting Schottky diodes fabricated from as-grown p-GaN, the change of emission colors from yellow, green to violet was observed when the potential between the electrodes was increased gradually, however, there was only blue emission observed when electrical bias was applied on the porous GaN sample. In addition, the use of porous GaN layer in p-n homojunction LED structure shows no improvement on the device performance, since relatively high turn on voltage was produced for the porous sample as compared to as-grown sample. All these initial studies reveal that porous GaN layer has the potential for the substantial improvement of the performance of sensing devices. However, the potential of porous GaN layer has not been fully proven and explored when it is applied in the light emitting devices.
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PhD
Keywords
Science physic , GaN materials , Device applications
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