Yu-Zung Chiou

Ultraviolet photodetectors with ZnO nanowires prepared on ZnO:Ga/glass templates

Vertically well-aligned ZnO nanowire ultraviolet (UV) photodetectors were fabricated by spin-on-glass technology on ZnO:Ga/glass templates. With 2V applied bias, it was found that dark current density of the fabricated device was only 2.0×10−7A∕cm2. It was also found that UV-to-visible rejection ratio and quantum efficiency of the fabricated ZnO nanowire photodetectors were more than 1000 and 12.6%, respectively.

GaN and InGaN Metal-Semiconductor-Metal Photodetectors with Different Schottky Contact Metals

The characterizations of n-type doped GaN, p-type doped GaN and n-type doped In0.2Ga0.8N Schottky metal-semiconductor-metal (MSM) photodetectors were reported. The epilayers were grown on sapphire by metalorganic chemical vapor deposition (MOVCD). Schottky contacts were fabricated using Au, Ti, Ni and Pt metals. The dark and illuminated current–voltage characteristics of GaN and InGaN MSM photodetectors with different Schottky metals were studied. The n-GaN MSM photodetectors with Au Schottky contacts showed better responsivity than those with other metals and they were also better than Au/p-GaN and Ti/n-In0.2Ga0.8N MSMs. The effects of the pitch width between the interdigitate fingers and the thickness of Schottky metals on the characteristics of photocurrents were also studied.

High detectivity InGaN-GaN multiquantum well p-n junction photodiodes

InGaN-GaN multiquantum well (MQW) p-n junction photodiodes with semi-transparent Ni-Au electrodes were fabricated and characterized. It was found that the fabricated InGaN-GaN p-n junction photodiodes exhibit a 20-V breakdown voltage and a photocurrent to dark current contrast ratio of /spl sim/10/sup 5/ when a 0.4-V reverse bias was applied. The peak responsivity at 380 nm was 1.28 and 1.76 A/W with a 0.1- and 3-V applied reverse bias, respectively. Furthermore, an internal gain was found from our InGaN-GaN MQW p-n junction photodiodes possibly due to the long-lifetime of GaN based materials. Also, it was found that the low frequency noise of our photodiodes was dominated by the 1/f type noise. For a given bandwidth of 500 Hz, the corresponding noise equivalent power and normalized detectivity D/sup */ were found to be 6.34/spl times/10/sup -13/ W and 4.45/spl times/10/sup 11/ cm/spl middot/Hz/sup 0.5/ W/sup -1/, respectively.

ZnO Nanowire-Based Oxygen Gas Sensor

We report growth of vertically well-aligned ZnO nanowires on ZnO:Ga/glass templates and the fabrication of resistive ZnO nanowire-based oxygen gas sensor. It was found that the ZnO nanowires are grown preferred oriented in the (002) direction with a small X-ray diffraction full-width-half-maximum. From high resolution transmission electron microscopy, scanning electron microscopy and micro-Raman measurements, it was found that the ZnO nanowires prepared in this study are single crystalline with good crystal quality. It was also found that measured sample resistance increased logarithmically as the oxygen gas pressure in the chamber was increased. Such a relationship suggests that the device is potentially useful for resistive oxygen gas sensing at room temperature.

Photo-CVD SiO/sub 2/ layers on AlGaN and AlGaN-GaN MOSHFET

High-quality SiO/sub 2/ was successfully deposited onto AlGaN by photochemical vapor deposition (photo-CVD) using a D/sub 2/ lamp as the excitation source. The resulting interface state density was only 1.1 /spl times/ 10/sup 11/ cm/sup -2/eV/sup -1/, and the oxide leakage current was dominated by Poole-Frenkel emission. Compared with AlGaN-GaN metal-semiconductor HFET (MESHFETs) with similar structure, the gate leakage current is reduced by more than four orders of magnitude by using the photo-CVD oxide layer as gate oxide in AlGaN-GaN metal-oxide-semiconductor heterojunction field-effect transistors (MOSHFETs). With a 2-/spl mu/m gate, the saturated I/sub ds/, maximum g/sub m/ and gate voltage swing (GVS) of the fabricated nitride-based MOSHFET were 572 mA/mm, 68 mS/mm, and 8 V, respectively.

GaN MSM UV photodetectors with titanium tungsten transparent electrodes

GaN metal-semiconductor-metal (MSM) ultraviolet photodetectors with titanium tungsten (TiW) transparent electrodes were fabricated and characterized. It was found that the 10-nm-thick TiW film deposited with a 300-W RF power can still provide a reasonably high transmittance of 75.1% at 300 nm, a low resistivity of 1.7/spl times/10/sup -3/ /spl Omega//spl middot/cm and an effective Schottky barrier height of 0.773 eV on u-GaN. We also achieved a peak responsivity of 0.192 A/W and a quantum efficiency of 66.4% from the GaN ultraviolet MSM photodetector with TiW electrodes. With a 3-V applied bias, it was found that minimum noise equivalent power and maximum D/sup */ of our detector were 1.987/spl times/10/sup -10/ W and 6.365/spl times/10/sup 9/ cmHz/sup 0.5/W/sup -1/, respectively.

Electrical and Optical Characteristics of UV Photodetector With Interlaced ZnO Nanowires

A zinc oxide (ZnO) nanowire (NW) photodetector was fabricated with a simple method by bridging the gap of interdigitated gallium-doped ZnO pattern deposited on a silicon oxide (SiO₂) thin film with interlaced ZnO NWs. With an incident wavelength of 375 nm, it was found that measured responsivity was 0.055 A/W for the interlaced ZnO NWs photodetector with a 1 V applied bias. The transient time constants measured during the turn-on and turn- off states were τ_ON = 12.72 ms and τ_OFF = 447.66 ms , respectively. Furthermore, the low-frequency noise spectra obtained from the ultraviolet photodetector were purely due to the 1/<i>f</i> noise. Besides, the noise equivalent power and normalized detectivity (<i>D</i>^*) of the ZnO NW photodetector were 2.32 ×10^-9 W and 7.43 ×10⁹ cm·Hz^0.5·W^-1, respectively.

High detectivity GaN metal–semiconductor–metal UV photodetectors with transparent tungsten electrodes

GaN metal–semiconductor–metal (MSM) ultraviolet photodetectors with transparent tungsten (W) electrodes were fabricated and characterized. It was found that the 10 nm thick W film deposited with a 250 W RF power could provide a reasonably high transmittance of 68.3% at 360 nm, a low resistivity of 1.5 × 10−3 Ω cm and an effective Schottky barrier height of 0.777 eV on u-GaN. We also achieved a peak responsivity of 0.15 A W−1 and a quantum efficiency of 51.8% at 360 nm from the GaN MSM UV photodetector with W electrodes. With a 2 V applied bias, it was found that the minimum noise equivalent power (NEP) and the maximum D* of our detector were 1.745 × 10−10 W and 7.245 × 109 cm Hz0.5 W−1, respectively.

Gallium nitride metal-semiconductor-metal photodetectors prepared on silicon substrates

Gallium nitride (GaN) ultraviolet metal-semiconductor-metal photodetectors (PDs) grown on Si substrates were demonstrated. The dark current of PDs fabricated on Si substrates was substantially smaller in magnitude compared to identical devices prepared on sapphire substrates. With an incident wavelength of 359nm, the maximum responsivities of the n−‐GaN MSM photodetectors with TiW and Ni∕Au contact electrodes were 0.187 and 0.0792A∕W, corresponding to quantum efficiencies of 64.7% and 27.4%, respectively. For a given bandwidth of 1kHz and a given bias of 5V, the corresponding noise equivalent powers of our n−‐GaN MSM photodetectors with TiW and Ni∕Au electrodes were 1.525×10−12 and 5.119×10−12W, respectively. Consequently, the values of detectivity (D*) determined for devices with TiW and Ni∕Au electrodes were then calculated to be 1.313×1012 and 3.914×1011cmHz0.5W−1, respectively.

Transparent TiN Electrodes in GaN Metal-Semiconductor-Metal Ultraviolet Photodetectors

GaN-based ultraviolet (UV) photodetectors were fabricated with transparent TiN electrodes. It was found that the transmittance was higher than 80% for a 50-nm-thick TiN layer. It was also found that we can significantly reduce the dark current of the photodetectors by inserting a thin Ba0.25Sr0.75TiO3 (BST) interlayer between the TiN electrode and the n-GaN. With a 3-nm-thick BST interlayer, we can realize a TiN/BST/GaN photodetector with a photocurrent-to-dark current contrast as high as 2.5×104.