Preparation And Properties Of P-type Transparent Conducting Oxides CuAlO₂Thin Films

Considered the theory of “the chemical modulation of the valence band”, a p-typetransparent conducting oxides CuAlO₂thin film was designed by Kawazoe etc bypulsed laser deposition for the first time. The discovery of p-type TCO opened up a newmethod for a p-type transparent conducting materials and made it possible to prepare thetransparent p-n junction and transparent transistor. However, it is difficult to get theexcellent CuAlO₂films whose optical and electrical properties are both good. Todaymany coating methods have been used to get CuAlO₂films, but there is a big differencein performance among these methods. This paper wants to synthesize p-type CuAlO₂films through pulsed laser deposition （PLD）. And we investigate the annealing effect onthe structural, optical, and electrical properties of CuAlO₂films, especially focus on theimprovement on the structure and properties of the films by the high oxygen pressureannealing, and on this basis, we study the impact of annealing oxygen pressure andannealing temperature on the capability of the films.In this paper, used Al2O3and Cu2O powers as raw materials, the high-qualityCuAlO₂ceramics targets is synthesized by solid-state reaction. XRD, SEM are used tocharacterize crystallization microstructure. After that, Cu-Al-O thin films are depositedon different substrates by PLD.It is found that the crystallization of CuAlO₂films is improved by annealing in airambience at1100°C for1h. At the same tine, the electrical properties of the films havealso been greatly improved. The average room temperature conductivity is measured tobe1.7S·m-1. However, compared with the films as-deposited, the film annealed in air at1100°C exhibits a weak transmittance. At the wavelength of500nm, the transmittanceof CuAlO₂film annealed in air ambience decreases to48.93%from68.27%（as-deposited）. And at high temperature （such as1100°C） annealing, the reproducibilityis rather poor, and low temperature annealing in the air can not reach the crystallizationtemperature of the CuAlO₂thin films, so the films are not conductive.Cu-Al-O thin films deposited on Al2O3substrates at different oxygen partial（2.8Pa、11Pa、51Pa）by PLD are annealed in high oxygen pressure furnace at800°C, as a result,some diffraction peaks of CuAlO₂can be observed, which is due to low crystallizationtemperature in the high-pressure conditions. At the long wavelength, the transmittanceof CuAlO₂film annealed in high oxygen pressure furnace at800°C increasessignificantly. But at the short wavelength, the transmittance is reduced. At the visiblewavelength of350nm⁷70nm, the average transmittance of CuAlO₂film annealedchange little. After annealing, the thicknesses of these thin films grown at different oxygen partial:2.8Pa、11Pa、51Pa, are221nm、253nm and107nm respectively, and theaverage room temperature conductivity of these films are2.5S·m-1、4.3S·m-1、0.12S·m-1,respectively.An as-deposited Cu-Al-O film on quartz in vacuum conditions is sectioned intosmall samples. Then they are annealed in high oxygen pressure furnace at800°C anddifferent oxygen pressures:1.7MPa,4.2MPa, and4.5MPa. With increase in the oxygenpressure, more and more oxygen atoms are intercalated within the Cu-Al-O thin films. Itis found that an increase in the room temperature conductivity is observed with theincrease in annealing oxygen pressure unto1.7MPa （the maximum σRT=1.24S·m-1）, butthe conductivity reduce when we continue increasing oxygen pressure. This because theconductivity is proportional to the product of carrier concentration and carrier mobility.Excess oxygen can increase the hole concentration but at the same time it will alsointroduce more defects to reduce carrier mobility thereby reducing the conductivity ofthe film.An as-deposited Cu-Al-O film on quartz in vacuum conditions is sectioned intosmall samples. Then they are annealed in high oxygen pressure furnace for1.7MPa atdifferent temperatures:600°C,700°C, and800°C. We found that an increase in theroom temperature conductivity is observed with the increase in annealing temperatureupto700°C （the maximum σRT=2.28S·m-1）, but the conductivity reduce when wecontinue increasing annealing temperature. However, the transmittance of CuAlO₂filmannealed at700°C is very weak. So considering the optical and electrical propertiestogether, we think that annealing temperature800°C and oxygen pressure1.7MPa is thebest annealing conditions.The p-CuAlO₂films were directly grown on low resistivity n-type Si substrate, thenwe get the p-CuAlO₂/n-Si heterojunction. Ag/Si/Ag and Ag/CuAlO₂/Ag display aobvious linear relationship, So we can ignore the influence of the Schottky barrier. Atthe room temperature, the I-V curves of the heterojunction show a good rectifyingcharacteristic, and the turn-on voltage is about0.87eV.