Shinji Nakagomi

Sol-gel prepared β-Ga2O3 thin films for ultraviolet photodetectors

β-Ga2O3 thin films have been prepared on (0001) sapphire substrates by the sol-gel method. X-ray diffraction showed that β-Ga2O3 polycrystalline films were formed at heat-treatment temperatures above 600°C. With increasing heat-treatment temperature above 900°C, the lattice constants of the β-Ga2O3 films decreased, while the band gap increased. Planar geometry photoconductive detectors based on the sol-gel prepared β-Ga2O3 thin films have been fabricated. They showed the photoresponse only for the wavelengths shorter than 270nm, which correspond to the solar-blind region. The peak wavelength in the spectral response depended on the heat-treatment temperature in the sol-gel process.

Deep ultraviolet photodiodes based on β-Ga2O3/SiC heterojunction

A deep Ultraviolet (UV) photodiode was fabricated using a heterojunction between β-Ga2O3 with a band gap of 4.9 eV, and 6H-SiC with a band gap of 3.02 eV, and investigated its UV sensitivity. A thin β-Ga2O3 layer (200 nm) was prepared on a p-type 6H-SiC substrate through gallium evaporation in oxygen plasma. The device showed good rectifying properties. Under reverse bias, the current increased linearly with increasing deep-UV light intensity. The responsivity of the photodiode was highest to deep-UV light below a wavelength of 260 nm. The photodiodes response time to deep-UV light was in the order of milliseconds.

Solar-blind photodiodes composed of a Au Schottky contact and a β-Ga2O3 single crystal with a high resistivity cap layer

We fabricated Ga2O3 photodiodes composed of a Au Schottky contact and a β-Ga2O3 single-crystal substrate with a sol–gel prepared high resistivity cap layer. The photodiodes with the cap layer showed solar-blind photosensitivity under both forward and reverse biases in contrast to conventional Schottky photodiodes. Finally, we proposed energy band diagram of the i-n junction to determine the photodetection mechanism of our photodiodes. The photoconductive device model explained the high responsivity of over 1 A/W at forward bias. In this model, the cap layer behaves like a photoconductor, and the substrate behaves like an electrode that replenishes electrons.

Influence of carbon monoxide, water and oxygen on high temperature catalytic metal-oxide-silicon carbide structures

High temperature sensors, Schottky diodes and capacitors, based on catalytic metal-oxide-silicon carbide devices are investigated. Reducing gases like hydrogen and other hydrogen containing gases, ...

Preparation of ZnO Thin Films on Sapphire Substrates by Sol-Gel Method

ZnO films have been prepared on (0001) sapphire substrates by the sol-gel method from a 2-methoxyethanol solution of zinc acetate dihydrate stabilized by monoethanolamine. The structural properties of the films have been characterized by X-ray diffraction, compared with the films on silica glass substrates. The crystallinity was enhanced in the ZnO films on sapphire substrates by increasing heat-treatment temperature, whereas it was degraded in the films on silica glass substrates at heat-treatment temperatures above 700°C. Completely (0001)-oriented ZnO films were successfully prepared by heat-treatment at temperatures above 800°C on (0001) sapphire substrates annealed at 1000°C in air. The c-axis lattice constants of the ZnO films on sapphire substrates were larger than those of bulk ZnO and ZnO films on silica glass substrates.

Electrical characterization of carbon monoxide sensitive high temperature sensor diode based on catalytic metal gate-insulator-silicon carbide structure

Field-effect gas sensors based on catalytic metal-in- sulator-silicon carbide (MISiC) devices are investigated. For the evaluation of the barrier height, the temperature dependence of the current-voltage ( - ) and the capacitance-voltage ( - ) charac- teristics of MISiC Schottky diodes were investigated in CO and O atmospheres. Four methods were used to evaluate how a change in gas ambient influences the barrier height of the diode: a change of the intersection current at zero voltage in the forward direction of the - curve, a change of the temperature dependence in the for- ward direction and the reverse direction, respectively, of the - curve, and a change of the intersection voltage of versus plot. The four methods gave similar changes in the barrier height for the device in 8000 ppm CO and 4000 ppm O. The values of bar- rier height obtained from the - curves were here normalized by the ideality factor calculated from - measurements. The corre- lation between the barrier height change obtained from the - and the - measurements, respectively, is discussed regarding the ideality factor. It is proposed that absolute value of the bar- rier height under flat-band condition is the most important for the evaluation of the barrier height. In the mixture of CO and O , the change of barrier height obtained from the - curve had roughly the same values as that from the - curve when normal- ized by the ideality factors.

Electrical characterization and hydrogen gas sensing properties of a n-ZnO∕p-SiC Pt-gate metal semiconductor field effect transistor

A new hydrogen gas sensitive n-ZnO∕p-SiC Pt-gate metal semiconductor field effect transistor (MESFET) is reported. The observed current-voltage curves for the source to drain region indicate that this MESFET operates in enhancement mode. A change in gate potential, due to different ambient atmospheres caused a change in the width of the depletion region, hence modulating the current in the n channel (ZnO layer). The H2 gas sensing mechanism of the presented MESFET structure is discussed using energy band diagrams.

New Materials for chemical and biosensors

ABSTRACT Wide band gap materials such as SiC, AlN, GaN, ZnO, and diamond have excellent properties such as high operation temperature when used as field effect devices and a high resonating frequency of the substrate materials used in piezoelectric resonator devices. Integration of FET and resonating sensors on the same chip enables powerful miniaturized devices, which can deliver increased information about a gas mixture or complex liquid. Examples of sensor devices based on different wide band gap materials will be given.

Hydrogen sensitive negative switching behavior in metal-oxide-semiconductor devices

Abstract Hydrogen sensitive switching device with a Pd-thin SiO 2 -(n)Si-(p + )Si structure which has thyristor-like S-shape property and of which threshold voltage was sensitive to hydrogen concentration was studied by using an impedance measurement for the first time. The switching device was considered to be a series connection of MIS junction and p + n junction. Four equivalent circuit parameters of two parallel connections of a capacitance and a resistance corresponding to two junctions were estimated under several bias and hydrogen conditions. Dependence of the parameters on bias voltage and on ambient hydrogen concentration is shown. The properties of these parameters reveal the behavior of MIS and p + n junction and their mutual interaction. A hydrogen sensitive switching mechanism is discussed. The impedance method is available to clarify the operation mechanism of the switching device.

Effects of Ambient Gases on Current-Voltage Characteristics of Pt-GaN Schottky Diodes at High Temperatures.

The electrical characteristics of a Pt Schottky barrier to n-type GaN grown by metalorganic chemical vapor deposition and the effects of ambient gases on their properties have been investigated at elevated temperatures of up to 600°C. The current–voltage (I–V) characteristics of Schottky diodes remained steady at high temperatures of up to 600°C, although the rectifying ratio decreased with a rise in temperature. The I–V characteristics of Pt–GaN Schottky diodes depended on the ambient gases. Hydrogen decreases the barrier height of Pt–GaN Schottky diodes, whereas oxygen increases it. The barrier height changed significantly in the temperature range from 100 to 400°C due to the change of atmosphere from H2 to O2.