Study On Optical And Electrical Properties And Applications Of Al_xZn_1-xO Thin Film

ZnO is a wide-band-gap semiconductor that has been widely investigated. At roomtemperature, its band gap is3.37eV which corresponds with the energy of nearultraviolet （UV） band in light spectrum. So it can be used to fabricate detectors for theradiation of this band. The exciton binding energy of ZnO is60meV, which makes itcapable to be used to fabricate laser devices of short-wavelength at room temperature.Moreover, ZnO is stable under thermal and chemical environment, and has goodradiation-resistant ability. Thus, it enables ZnO with much more applicability and longlifetime in applications. ZnO can be used to form a well-grown n-type semiconductorafter being doped into Al ions with low concentration, which is able to preparetransparent conductive oxidation （TCO） thin film. Meanwhile, after being heavilyincorporated with Al ions, ZnO could form an alloy semiconductor. And this processcould modulate the optical band gap of this semiconductor to the suitable value forsolar-blind UV detecting. Based on the different concentration of Al ions, ZnO-basedmaterials can be used in both civil and military area with special functions.This dissertation presents the realization of ZnO:Al （AZO） transparent conductivethin film with resistivity about3.8×10^-4Ω cm and transmittance about90%in the visibleregion, and Al_0.3Zn_0.7O thin film whose optical band gap is4.41eV and thecorresponding cutoff wavelength is281nm, based on the different Al concentration andoptimum preparation parameters. After specific study on the structure, characteristics,and theoretical fundament of these two types of functional film, the details are discussedin this thesis, which are as follows:Firstly, we investigated the approaches to improve the crystalline quality of AZOtransparent conductive thin film, and summarized the optimum dopant concentrationand deposition conditions. Under the optimized parameters, we obtained AZO thin filmwith efficient Al substitution and less scattering probability. So that films with highconductivity and transmittance were achieved. Specific and systematic characterizationof AZO thin film was performed, and the structure, defect state, and composition of itwere attained. AZO film with relatively high mobility （32cm²/Vs） comparing with common ZnO thin films was obtained. The non-ignorable existence of Zn vacancy （VZn）has also been verified. The antisite defect OZnwas identified to be responsible for thegreen emission band in photoluminescence （PL） spectrum, after annealing process hadbeen carried out in different ambience.Secondly, we prepared the Al_0.3Zn_0.7O thin film which was fit for the detecting ofsolar-blind UV. Fabricate the solar-blind UV detector using this film as photocathode,based on Burstein-Moss theory. Massive electrons were generated due to highAl-incorporated concentration. Owing to the narrow conduction-band minimum andinsufficient effective state density in conduction band of ZnO, it does not have enoughstates to hold a large number of electrons beneath conduction band, and excess electronswould occupy states in conduction band, which shifted the Fermi level upwards,accordingly. This condition did not only enlarge the optical band gap, but also decreasethe surface work function of film. This Al_0.3Zn_0.7O thin film was verified to be inamorphous nanoscaled polycrystalline structure, have a smooth surface, and have theability to suppress surface adsorption. The optical band gap of this film is estimated tobe4.41eV and it only absorbs illumination whose wavelength is shorter than281nm.X-ray Photoelectron spectroscopy was used to estimate the enlargement values inconduction band and valence band, respectively. Kelvin probe microscope result labeledthe surface barrier height. These results proved the capability of Al_0.3Zn_0.7O thin film asphotocathode layer for solar-blind detecting. A photodetector was fabricated based onthis Al_0.3Zn_0.7O thin film as photocathode layer after being activated by Cs, and variousmeasurements of this photodetector were carried out to verify its solar-blind detectingability. This detector only responded to the illumination of254nm, but the illuminationwith longer wavelength, which revealed the direct transition process of electrons fromvalence band. UV/dark contrast nearly3orders of magnitude has been achieved of thisdetector, under254nm UV illumination of0.5mW/cm²and80V bias. Current variationunder periodical exposal of254nm illumination indicated repeatability of this detector.Spectral response result proved that the detecting range of this detector fitted well withthe solar-blind band. Cs activation was utilized to decrease surface workfunction owingto its small work function （¹.8eV）, which would incease the emission efficiency offilm. Through the deduction and fitting of I-V curve, we found out the essence ofemission and put forward the relationship between surface barrier height with vacuum level and activation process. The results told that the decrease of surface barrier heightbetween10^-6Pa and10^-3Pa was about0.042eV, and the surface barrier height performeda0.165eV decrease after Cs activation.