Shang-Chao Hung

ZnO Branched Nanowires and the p-CuO/n-ZnO Heterojunction Nanostructured Photodetector

The authors report the growth of ZnO branched nanowires on the CuO nanowires and the fabrication of p-CuO/n-ZnO heterojunction nanostructured photodetector (PD). It was found that the hydrothermally grown ZnO branched nanowires were reasonably uniform with an average length of 200 nm and an average diameter of 50 nm. Under forward bias, it was found that turn on voltage of the fabricated PD reduced from ~0.7 to ~0.2 V under ultraviolet (UV) illumination. It was also found that UV-to-visible rejection ratio of the fabricated device was larger than 100.

Nitride-Based LEDs With Phosphoric Acid Etched Undercut Sidewalls

We propose a simple defect-selective wet etching method to form oblique sidewalls for GaN-based epitaxial layers with phosphoric acid. Using the same defect-selective wet etching, we also prepared GaN-based light-emitting diodes (LEDs) with undercut sidewalls. Compared with conventional LEDs with vertical sidewalls, it was found that output intensity of the LEDs prepared by defect-selective wet etching was 30% higher.

CuO Nanowire-Based Humidity Sensor

The authors report the growth of CuO nanowires on an oxidized Cu wire and the fabrication of a CuO nanowire humidity sensor. It was found that we could transform a Cu wire into CuO/Cu2O/Cu core-shell tri-layers covered with high density CuO nanowires by thermal annealing. It was also found that steady state currents of the sensor were about 2.44, 2.32, 2.23, and 2.15 μA , respectively, when measured with 20, 40, 60, and 80% relative humidity. Furthermore, it was found that sensing property of the fabricated device was stable and reproducible.

Buckling instabilities in GaN nanotubes under uniaxial compression

We report experimental observations of shell buckling instabilities in free-standing, vertically aligned GaN nanotubes subjected to uniaxial compression. Highly uniform arrays of the GaN nanotubes standing on a GaN template were fabricated and subjected to uniaxial compression using a nanoindenter. The buckling load was found to be of the order of 150 microN for the GaN nanotubes with an outer radius of 40 nm, an inner radius of 20 nm, and heights of 500 and 300 nm. Good agreement was found between the experimental observations, the stress-strain relation equation study findings and the predictions from the cylindrical shell buckling theory.

Growth of Ga

The authors report the growth of nanowires by heating the GaN/sapphire template. It was found that density, average length, and average diameter of the nanowires all increased as we increased the grown temperature. It was also found that β-Ga₂O ₃ nanowires with good crystal quality could be achieved only at high temperatures (i.e., 1050 and 1100 °C). Solar-blind β-Ga₂O ₃ nanowire photodetector was also fabricated by depositing interdigitated contact electrodes. With an incident light wavelength of 255 nm and an applied bias of 5 V, it was found that measured responsivity of the photodetector was 3.72×10^-1 mA/W.

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The authors report the growth of needle-like high density quaternary Zn0.87Cd0.13Se0.98Te0.02nanotips on oxidized Si(100) substrate. It was found that average length and average diameter of the nanotips were 1.3 μm and 91 nm, respectively. It was also found that the as-grown ZnCdSeTe nanotips exhibit mixture of cubic zinc-blende and hexagonal wurtzite structures. Furthermore, it was found that the operation speeds of the fabricated ZnCdSeTe nanotip photodetector were fast with turn-on and turn-off time constants both less than 2 s.

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The authors demonstrate the fabrication of 1 mm × 1 mm GaN-based power flip-chip light-emitting diodes (LEDs) with an internal electrostatic discharge (ESD) protection diode on Cu sub-mount. With the internal diode, it was found that forward voltage of the LED increased from 3.22 to 3.38 V while output power decreased from 366.5 to 273.9 mW when under 350 mA current injection due to the reduced light emitting area. It was also found that we can achieve a significantly better ESD robustness by building the internal diode inside the LED chip. Furthermore, It was found that 90% of the LEDs with internal diode survived with an applied reverse ESD surge of 12000 V and 25% of the LEDs can even endure 20000 V reverse ESD stressing.

_{\bm 3}

We report the fabrication of GaN-based ultraviolet (UV) metal-semiconductor-metal photodetectors with a semi-insulating AlInN cap layer. It was found that we can achieve a smaller dark leakage current, a larger UV-to-visible rejection ratio, a smaller noise level, and a larger detectivity while maintaining the sharp transition by inserting the semi-insulating AlInN cap layer.

Nanowires and the Fabrication of Solar-Blind Photodetector

Nitride-based power flip-chip (FC) LEDs with Cu submount were proposed and prepared. With a much higher thermal conductivity, it was found that we can achieve a lower operation voltage under high-current injections and lower junction temperature from the FC LEDs with Cu submount. Compared with the power FC LEDs with Si submount, the reliability of the proposed device was also better.

A Quaternary ZnCdSeTe Nanotip Photodetector

The authors report the conversion of AlGaN epitaxial layer into (Al_xGa_1-x)₂O₃ thin film by furnace oxidation at high temperature in oxygen containing ambient. A vacuum-ultraviolet (Al_xGa_1-x)₂O₃ photodetector (PD) was also fabricated. It was found that external quantum efficiency of the fabricated PD measured at 220 nm was 30%. It was found that the fabricated PD exhibits extremely large deep-UV-to-visible rejection ratio.