Structure, Electrical And Optical Properties Of ZnO: Al Thin Film

ZnO is a direct wide band-gap semiconductor material, which is widely used in optoelectronic devices. In this work, ZnO:Al thin films were fabricated on glass substrates by RF magnetron sputtering. The dependence of substrate temperature and O₂ flow with the structural, electrical properties and optical properties were systematically studied. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), physical property measurement system (PPMS), photoluminescence (PL) and UV-VIS scanning spectrophotometer were used to measure the crystalline quality, the chemical binding state, the electrical properties and the optical properties, respectively.It was found that all the samples deposited at different substrate temperature and O₂ flow showed strong (002) preferred orientation. Zn and Al were in the form of Zn²⁺ and Al³⁺ independent of the deposition conditions. Increasing the substrate temperature and suitable O₂ flow can decrease the room temperature resistivity. The resistivity reached a minimum at Ts = 550℃with a value of 2.662×10^-4Ωcm. The temperature dependence of resistivity from 2 to 300 K was studied. It was observed that the film grown at 550℃showed metallic conductivity and the film grown at 450℃revealed semiconductor behavior. The conduction electrons in the two films are likely to be predominantly scattered by phonons and ionized impurity, respectively. The films under different O₂ flow all showed negative magnetoresistance, which is believed to originate from localized magnetic moment.All the films showed good transmittance with an average transmission over 80 %, and the optical band gap is wider than that of pure ZnO. The lower the resistivity, the wider the band gap and this phenomenon is due to the Burstein-Moss effect. The observed green emission at 520 nm was specially studied to the samples grown under different O₂ flow. The spectral analyses suggested that the green emission may arise from the recombination of the O vacancies (V_O) and Zn interstitials (Zn_i) to Zn vacancies (V_Zn).