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.
Description
PhD
Keywords
Science physic , GaN materials , Device applications