Development of gallium nitride-based ultraviolet detectors

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Development of gallium nitride-based ultraviolet detectors


Author: Lui, Hsian-fei Hardy
Title: Development of gallium nitride-based ultraviolet detectors
Degree: Ph.D.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Gallium nitride -- Electric properties.
Ultraviolet detectors.
Department: Dept. of Electronic and Information Engineering
Pages: xix, 179 leaves : ill. (some col.) ; 30 cm.
Language: English
Abstract: Visible-blind ultraviolet (UV) photodetectors have found many important applications nowadays, such as the ultra-high density storage optical reading head and the measurement of UV radiation (UV index) from the solar UV spectrum on the earth's surface. Gallium Nitride (GaN), having band-gap energy of 3.4 eV which corresponds to a band-edge wavelength of 364 nm, is a material of choice for the development of short wavelength optoelectronic devices. Strong atomic bonding, high chemical stability and high saturation velocity as well as a strong breakdown field are important properties that make it a superior candidate over silicon for optoelectronic applications. The role of the double buffer layer in a GaN film and its effects on the device's radiation hardness has been being addressed in the first part of this study. Two types of GaN Schottky barrier photodetectors were fabricated for the radiation hardness studies. Type I devices were fabricated on MBE-grown GaN films using a double-buffer-layer structure, whereas the type II control devices were fabricated on conventional buffer-layer structures only. The radiation hardness of the devices was studied by the accelerated optical stressing experiment. The results show that the two types of devices exhibited high-quality and comparable Schottky characteristics prior to the application of stress, in terms of their current-voltage (I-V) characteristics, capacitance-voltage (C-V) properties, low-frequency noise and responsivities. However, the device properties were found to behave quite differently upon the application of high-power UV radiation stress. All type II devices under stress exhibited catastrophic breakdown at some point during the optical-stress process and none of type I devices under test demonstrated any substantial breakdown for the entire stressing period of up to 90 hours. The increase of reverse current, decrease of capacitance and reduced UV-to-visible response ratios suggest that either significant generation of traps or the activation of metastable states within the bandgap occur, enable electrons occupying these states to be excited by sub-bandgap photons. The results from low-frequency noise measurement provided further evidence of the trap origin of the degradation phenomena. The experimental results clearly indicate that the use of the double buffer layer results in improved crystallinity and is responsible for the substantial improvements in the radiation hardness of the type I devices. The second part of this study was the fabrication and characterization of indium-tin-oxide (ITO)/GaN Schottky photodetectors. The optimal growth conditions for the ITO was systematically studied. The I-V characteristics show that the reverse leakage currents of the devices are strongly dependent on the deposition conditions of the ITO. The experimental results also indicate that interfacial traps play an important role on the optoelectronic properties of the device. Due to the fact that the ITO is deposited in an oxidizing ambient, high surface states density would exist at the ITO/GaN interface leading to a degradation in the device performance. A unique device structure was investigated in which a thin Ni layer was deposited between the ITO and GaN film for the passivation of the GaN surface. The optoelctronic properties of this novel device are found to be substantially improved.

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