Toshiyuki Kawaharamura

Carrier concentration dependence of band gap shift in n-type ZnO:Al films

Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2∕3 for ne≤4.2×1019 cm−3, which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4−8.4×1019 cm−3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widen...

Carrier concentration induced band-gap shift in Al-doped Zn1-xMgxO thin films

Transparent conducting Al-doped Zn1−xMgxO thin films were grown on glass substrates by chemical vapor deposition. The resistivity could be lowered to 10−3Ωcm with optical transmittance above 85% in visible regions. The influence of carrier concentration on band-gap shift in Zn1−xMgxO alloys was systematically studied. The shift of energy gap could be fully explained by the Fermi-level band filling and band-gap renormalization effects. As the Mg content increased, the electron effective masses in Zn1−xMgxO (x=0–0.21) alloys increased from 0.30m0 to 0.49m0. The Al-doping efficiency was reduced with the increase in alloy composition.

Growth of Crystalline Zinc Oxide Thin Films by Fine-Channel-Mist Chemical Vapor Deposition

A simple, cost-effective, and safe growth technique, that is, mist chemical vapor deposition method, has been applied to the growth of single crystalline ZnO thin films on a-plane sapphire substrates. A water solution of zinc acetic acid and compressed air were used as sources, and the growth was performed at 800 °C. Under optimized conditions, the X-ray diffraction rocking curves full-width at half maximum was 224 arcsec, rotational domains were hardly observed, and the photoluminescence spectra showed predominant near-band-edge emission without noticeable deep-level emissions. The potential of the technique for the growth of single-crystalline ZnO thin films was apparently shown, and efforts to improve source purity, which affects the optical and electrical properties of the films, as well as to further optimize the growth conditions will contribute to the fabrication of multifunctional ZnO-based thin films towards wide application fields without a heavy load to the environment.

Linear-Source Ultrasonic Spray Chemical Vapor Deposition Method for Fabrication of ZnMgO Films and Ultraviolet Photodetectors

A linear-source ultrasonic spray chemical vapor deposition method has been developed and applied to fabricate ZnMgO ternary alloy thin films on glass substrates. A water solution of zinc acetate and magnesium acetate was ultrasonically atomized to form aerosol particles of water containing the sources, and then they were supplied onto the heated substrate by a nitrogen carrier gas through a nozzle with a linear aperture to form ZnMgO films. The source concentration ratios in the water solution successfully controlled the solid composition and hence raised the band gap of ZnMgO up to 3.75 eV, keeping the optical transmission higher than 90% for the visible-light region. An UV photodetector fabricated using the ZnMgO film showed the photoresponsivity to be as high as a few A/W, suggesting that this simple and cost-effective technique is promising for fabricating ZnMgO films for various applications.

Low-temperature growth of ZnO thin films by linear source ultrasonic spray chemical vapor deposition

ZnO films were prepared on soda-glass substrates and flexible polyimide films at low temperatures by linear source ultrasonic spray chemical vapor deposition without intentionally enhancing the energy of precursor reactions. Highly transparent ZnO films were deposited on both glass and polyimide at 180 °C using zinc acetate in a mixture of methanol and water. The deposition at low temperatures is attributed to water-activated precursor reactions.

Successful Growth of Conductive Highly Crystalline Sn-Doped -Ga2O3 Thin Films by Fine-Channel Mist Chemical Vapor Deposition

Highly crystalline α-phase gallium oxide (Ga2O3) thin films were grown by fine-channel mist chemical vapor deposition on c-sapphire substrates at 400 °C at a deposition rate of more than 20 nm/min. The thin films were doped with Sn(IV) atoms, which were obtained from Sn(II) chloride by the reaction SnCl2+ H2O2+ 2HCl→SnCl4+ 2H2O. Conductive α-phase Ga2O3 thin films were successfully grown from source solutions containing less than 10 at. % Sn(IV). The source solution containing 4 at. % Sn(IV) resulted in obtaining a thin film with an n-type conductivity as high as 0.28 S cm-1, a mobility of 0.23 cm2 V-1 s-1, a carrier concentration of 7×1018 cm-3, and a full width at half maximum (FWHM) of the (0006) reflection X-ray rocking curve as low as 64 arcsec.

Physics on development of open-air atmospheric pressure thin film fabrication technique using mist droplets: Control of precursor flow

Mist CVD, which is one of the functional thin-film fabrication methods carried out under open-air atmospheric pressure, has been developed for advanced control of precursor streams in order to achieve the fabrication of uniform thin films. In order to obtain a uniform stream of flow including mist droplets, collisional mixing has been considered, and a fine channel (FC) nozzle has been developed using this technology. In the FC, mist droplets are affected by the lift forces toward the direction of the substrate, the gas temperature of the entire area reaches a constant, and owing to the Leidenfrost effect, the evaporation time of a mist droplet is remarkably long as several hundred milliseconds and the migration of a mist droplet extends to a distance of several hundred millimeters. From these characteristic phenomena in the FC, the formation of uniform thin films on the entire surface has been achieved using the FC-type mist (FCM-) CVD system.

Electrical Properties of the Thin-Film Transistor With an Indium–Gallium–Zinc Oxide Channel and an Aluminium Oxide Gate Dielectric Stack Formed by Solution-Based Atmospheric Pressure Deposition

We developed a thin-film transistor (TFT) with an amorphous-indium-gallium-zinc oxide (IGZO) channel and aluminium oxide (AlO_x) gate dielectric stack that was formed using a solution-based atmospheric pressure chemical vapor deposition. A breakdown electric field of 5.9 MV/cm and a dielectric constant of 6.8 were achieved for the AlO_x gate dielectric. The nonvacuum-processed IGZO TFT gave a field-effect mobility of 4.2 cm² · V^-1 · s^-1 and an on/off current ratio of over 10⁸. Moreover, the proposed deposition method is a powerful tool for material research to explore multicomponent oxide insulators and semiconductors.

Effect of Surface Treatment of Gate-Insulator on Uniformity of Bottom-Gate ZnO Thin Film Transistors

The effect of interface treatment of ZnO/gate insulator on the variations in both the electrical properties and the trap density of bottom-gate ZnO thin film transistors (TFTs) were investigated. Two kinds of interface treatments, as a diluted HF wet treatment and a nitrous oxide (N 2 O) plasma treatment, are individually applied to the gate-insulator surface of the ZnO TFTs. The threshold voltage uniformity of ZnO TFTs with the N 2 O plasma-treated gate insulator was drastically improved. The trap densities extracted from the ZnO TFTs revealed that the variation in the trap density in deep energy level is reduced significantly by the N 2 O plasma treatment.

Mist-CVD Grown Sn-Doped

This paper demonstrates the use of cost-effective solution-processed