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Title: A study of back-to-back schottky barrier ultraviolet photodetectors fabricated on molecular beam epitaxy grown gallium nitride
Authors: Chan, Ngai-hung
Degree: M.Phil.
Issue Date: 2002
Abstract: Gallium nitride (GaN) with wide and direct band gap has a wide range of applications in optoelectronics. Among them, ultraviolet (UV) photodetectors with valuable properties such as visible blindness are needed in many applications including spectral imaging, flame detection and safeguard, missile tracking and space communications. In this study, back-to-back Schottky baffler UV photodetectors, using transparent indium tin oxide (ITO) as electrodes, in the form of coplanar inter-digitated fingers on GaN substrate with SiO2 cladding layer on the top, have been fabricated and characterized. The GaN films were grown on sapphire substrates by Molecular Beam Epitaxy (MBE) using a novel intermediate temperature buffer layer (ITBL) method. Before device fabrication, the photoluminescence (PL) spectrum of the GaN films on sapphire substrates, with optimized intermediate temperature buffer layer thickness, was measured to show a dominant near-bandgap transition at 364.3 nm with unobservable yellow band emission. The photoluminescence spectrum is consistent with the abrupt cut-off of more than 2 orders of magnitude at bandgap position shown in the spectral responsivity measurement of the detector. The absence of the widely reported yellow luminescence is indicative of the superior optical quality of the MIBE grown GaN sample. The current-voltage measurement of the detector gives a dark current of 7.9x10-6 A at 1V bias, and the excess dark current was attributed to the conduction by trap-assisted tunneling in addition to the typical thermionic emission. The dark currents in metal-semiconductor-metal (MSM) devices, fabricated using different metal contacts (platinum and nickel) on GaN with different carrier concentration and surface polarity, have been compared with that using ITO. The work function of the contact materials and the surface polarity of GaN were found to have their effects on the dark current as reported. We suspected that the sputter-induced damage should be the major factor causing the excess dark current. The responsivity of the detector was found to depend on the applied bias and light chopping frequency. These show the existence of a widely reported internal gain mechanism through the trapping and re-injection of charge carriers in the depletion region. The existence of trapping centers has also been confirmed by the generation-recombination noise measurement, in which a trap level of higher density (possibly due to sputter-induced defect during electrode film deposition) with activation energy of 252+-13 meV was identified. Electron beam evaporation of the Schottky contact material without the presence of energetic particles which normally exist in the plasma during sputtering, and Ga-face gallium nitride with a low carrier concentration, are recommended to further improve the device performance.
Subjects: Hong Kong Polytechnic University -- Dissertations
Optical detectors
Gallium nitride
Pages: vii, 85 leaves : ill. ; 30 cm
Appears in Collections:Thesis

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