Chin-Hsiang Chen

GaN metal-semiconductor-metal ultraviolet photodetectors with transparent indium-tin-oxide Schottky contacts

Indium-tin-oxide (ITO) layers were deposited onto n-GaN films and/or glass substrates by electron-beam evaporation. With proper annealing, we found that we could improve the optical properties of the ITO layers and achieve a maximum transmittance of 98% at 360 nm. GaN-based metal-semiconductor-metal (MSM) photodetectors with ITO transparent contacts were also fabricated. A maximum 0.12-A photocurrent with a photocurrent to dark current contrast higher than five orders of magnitude during ultraviolet irradiation were obtained for a photodetector annealed at 600/spl deg/C. We also found that the maximum photo responsivity at 345 nm is 7.2 and 0.9 A/W when the detector is biased at 5 and 0.5 V, 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.

Photoluminescence characteristics of Mg- and Si-doped GaN thin films grown by MOCVD technique

Abstract We have studied the optical, structural and surface morphology of doped and undoped GaN thin films. The p- and n-type thin films have been successfully prepared by low-pressure MOCVD technique by doping with Mg and Si, respectively. The different carrier concentrations were obtained in the GaN thin films by varying dopant concentrations. Photoluminescence (PL) studies were carried to find the defect levels in the doped and undoped GaN thin films at low temperature. In the undoped GaN thin films, a low intensity and broad yellow band peak was observed. The donor–acceptor pair (DAP) emission and its phonon replicas were observed in both the Si or Mg lightly doped GaN thin films. The dominance of the blue and the yellow emissions increased in the PL spectra, as the carrier concentration was increased. The XRD and SEM analyses were employed to study the structural and surface morphology of the films, respectively. Both the doped and the undoped films exhibited hexagonal structure and polycrystalline nature. Mg-doped GaN thin films showed columnar structure whereas Si-doped films exhibited spherical shape grains.

AlGaN/GaN MSM photodetectors with photo-CVD annealed Ni/Au semi-transparent contacts

AlGaN/GaN metal–semiconductor–metal (MSM) ultraviolet (UV) photodetectors with photo-chemical vapour deposition (photo-CVD) annealed Ni/Au semi-transparent contacts were fabricated. It was found that the transmittances of Ni/Au films increased while the photodetector dark currents became significantly lower after annealing. With a 5 V applied bias, it was found that the photocurrent to dark current contrast ratio and the maximum responsivity were 2.3 × 104 and 0.166 A W−1 for photodetector photo-CVD annealed at 550 °C.

GaN Metal-Semiconductor-Metal Ultraviolet Photodetectors with Ir ∕ Pt Contact Electrodes

ZnO epitaxial films were grown on sapphire (0001) substrates by using rf plasma-assisted molecular beam epitaxy. Metal-semiconductor-metal (MSM) photodetectors with Iridium (Ir) electrodes were then fabricated. It was found that Schottky barrier heights at the non-annealed and 5008C-annealed Ir/ZnO interfaces were around 0.65 and 0.78 eV, respect- ively. With an incident wavelength of 370 nm and 1 V applied bias, it was found that the maximum responsivities for the Ir/ZnO/Ir MSM photodetectors with and without thermal anneal- ing were 0.18 and 0.13 A/W, respectively. From transient response measurement, it was found that time constant t of the fabricated photodetectors was 22 ms. For a given bandwidth of 100 Hz and 1 V applied bias, we found that noise equivalent power and corresponding detectivity Dwere 6 � 10 213 W and 1.18 � 10 12 cm Hz 0.5 /W, respectively. In recent years, a lot of research has been focused on semiconductor-based ultraviolet (UV) photodiodes. Photodiodes operating in the short-wavelength UV region are vitally important for various commercial and military applications. Specifically, visible-blind UV photodiodes are being used in space communications, ozone layer moni- toring and flame detection. Currently, light detection in the UV spectral range still relies on Si-based optical photo- diodes. Even though these devices are quite sensitive to visible and infrared radiations, nevertheless, the responsiv- ity in the UV region is considered relatively low because of the fact that the room temperature bandgap energy of Si is only 1.2 eV. With the optoelectronic devices start being fabricated on direct wide bandgap materials, being able to fabricate high-performance solid-state photodiodes

InGaN/GaN MQW p–n junction photodetectors

InGaN/GaN multiquantum well (MQW) p–n junction photodetectors with semi-transparent Ni/Au electrodes were fabricated and characterized. It was found that the fabricated InGaN/GaN MQW p–n junction photodetectors exhibit a 20 V breakdown voltage and a 3.5 V forward 20 mA turn on voltage. It was also found that the photocurrent to dark current contrast ratio is higher than 105 when a 0.4 V reverse bias was applied to the InGaN/GaN MQW p–n junction photodetectors. Furthermore, it was found that the maximum responsivity was 1.28 and 1.76 A/W with a 0.1 and 3 V applied reverse bias, respectively.

Nitride-Based MIS-Like Photodiodes With Semiinsulating Mg-Doped GaN Cap Layers

Nitride-based metal-insulator-semiconductor (MIS)-like photodiodes (PDs) with in situ grown 30-nm-thick unactivated semiinsulating Mg-doped GaN cap layers were fabricated. The authors found that the reverse leakage current of the aforementioned PD was comparably much smaller than that of conventional PD without the semiinsulating layer due to the facts that inserting a semiinsulating layer would result in a thicker and higher potential barrier, and also less amounts of interface states introduced. To sum up, it was determined that the benefits of incorporating a semiinsulating Mg-doped cap layer into the PD would encompass a larger photocurrent-to-dark-current contrast ratio and larger ultraviolet-to-visible rejection ratio

High-Indium-Content InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes

High-indium-content InGaN/GaN multi-quantum-well (MQW) light-emitting diode (LED) structures were epitaxially grown by metalorganic vapor phase epitaxy (MOVPE). With 70% indium in the InGaN well layers, it was found that the photoluminescence (PL) full-width at half maximum (FWHM) is stronger than that in the case of low-indium-content InGaN/GaN MQW LED structures. It was also found that the peak position of electroluminescence (EL) fabricated In0.7Ga0.3N/GaN LED depends strongly on injection current. As injection current increased from 1 mA to 150 mA, it was found that the output color of the In0.7Ga0.3N/GaN LED changed from orange to yellow, to yellowish green, and finally to yellowish white.

InGaN/GaN Multi-Quantum Well Metal-Insulator Semiconductor Photodetectors with Photo-CVD SiO2 Layers

InGaN/GaN multiple-quantum well (MQW) structure was epitaxial growth by metal-organic chemical vapor deposition (MOCVD). Their MIS photodiodes with SiO2 interlayer were fabricated successfully using photochemical vapor deposition. The normal undoped-GaN metal-semiconductor-metal (MSM) potodiodes were also prepared to compare with them. It was found that the minimum dark current of InGaN/GaN MQW photodiodes was 4.2×10-13 A with 88 nm-thick SiO2 layer under 5 V reverse bias voltage. Furthermore, it was found that we can significantly reduce the dark current of this photodiodes by inserting a thin SiO2 interlayer in between metal electrode and the underneath InGaN/GaN MQW. With a 53 nm-thick SiO2 interlayer, it was also found that we could achieve a high 1.53×103 photo current to dark current contrast.

AlGaN Metal?Semiconductor?Metal Photodetectors with Low-Temperature AlN Cap Layer and Recessed Electrodes

Al0:16Ga0:84N 320 nm near-solar-blind ultraviolet (UV) metal–semiconductor–metal photodetectors (MSM-PDs) with a low-temperature AlN (LTAlN) layer and inductively coupled plasma (ICP) recessed electrodes were successfully fabricated. Compared with the conventional MSM-PDs, it was found that the measured photocurrent was much larger for the MSM-PD with the LT-AlN layer and ICP recessed electrodes. The responsivity of the MSM-PD with the LT-AlN layer and ICP recessed electrodes was also found to be larger, which could be attributed to the ICPetching-induced photoconductive gain. # 2010 The Japan Society of Applied Physics