Transparent Conductive ZnO Thin Films Co-doped with H and Al
|School||Wuhan University of Science and Technology|
|Keywords||AZO thin films Hydrogen doping Magnetron sputtering Resistivity Transmittance Energy gap (E_g)|
Al-doped ZnO （AZO） films have become alternative materials to ITO because of their low cost, non-toxicity and stability in hydrogen plasma. Magnetron sputtering is the most commonly used method to prepare AZO films because it can perform high deposition rate and obtain highly uniform and larger scale films with strong adhesion. How, the resistivity of the AZO films deposited at low substrate temperature is usually higher and their transmittance is lower, which can not reach the requirement of optical-electrical devices. Recently, it is reported that the optical-electrical properties of AZO films can be greatly improved by doping hydrogen （H） at low substrate temperature. When H2 is introduced into deposition atmosphere, however, the effects of sputtering parameter on the characteristics of AZO have not been systemically investigated at present. Thus, this thesis systemically studies the effects of introducing H2 on the transparent conductive properties of AZO films at different sputtering parameters. We hope that the effects of H can be summarized and the AZO films with excellent transparent conductive properties can be obtained.First, AZO films, having film thickness of about 501200nm, were deposited at substrate temperature of 100300℃by magnetron sputtering in Ar atmosphere. Structural, electrical, and optical properties of as-deposited AZO films have been studies as a function of film thickness and substrate temperature. With increasing film thickness or substrate temperature, the crystalline quality of the film improves and its stress relaxes, and preferred （002） orientation is found at substrate temperature above 100℃. The decrease of resistivity of the film is generally accompanied by the increase of carrier concentration and mobility with increasing film thickness or substrate temperature due to the improvement of crystallinity. The minimum resistivity of 1.12～1.59?10-3 ??cm is obtained at film thickness of about 1000 nm and substrate temperature of 200300℃. The transmission spectra measurements of AZO films indicate that average visible transmittance decreases from about 86% to 70% with increasing film thickness. The obtained maximal energy gap （Eg） of the films is found at substrate temperature of 200℃, which can be attributed to high carrier concentration and compressive stress. With increasing film thickness, the decrease of compressive stress and/or increase of crystallite size result in the decreased or unchanged tendency of Eg of the films although the carrier concentration increases.On the basis of above study, the AZO films were deposited at different substrate temperatures and H2/（H2+Ar） flow ratios by magnetron sputtering in Ar+H2 atmosphere. （002） preferred orientation is observed in all the films. As H2 flow ratio increasing, the stress in the films obviously decreases and crystallinity greatly improves for the films deposited at RT, but stress and crystallinity have no obvious change for the films deposited at 100300℃. By introducing H2, carrier concentration and mobility of the films obviously increase at substrate temperature of RT and 100℃, and they slightly increase at substrate temperature of 200℃, but they are almost unchanged at substrate temperature of 300℃. When the H2 flow ratio reaches 3%, the resistivity of the films deposited at RT and 100℃is even lower than that of the films deposited at 200 and 300℃. The minimum resistivity of 1.15?10-3 ??cm is obtained at substrate temperature of 100℃and H2 flow ratio of 6%. These results indicate that optical-electrical properties of AZO films can be evidently improved by introducing H2 and H is doped into ZnO lattice as interstitial H. It is found that the transmittance of the films can be increased by introducing H2 into deposition atmosphere. As for broadening of Eg of the films, it can be mainly attributed to carrier concentration except that the fine grain and large compressive stress can also be considered for the films deposited at RT.Finally, the effects of sputtering pressure, power and film thickness on the structural and optical-electrical properties of AZO films were investigated at substrate temperature of RT and H2 flow ratio of 5%. When sputtering power is fixed at 150W, the crystallinity of the films degrades with sputtering pressure; the minimum resistivity of 2.69×10-3Ωcm is obtained at sputtering pressure of 0.8 Pa. When the sputtering pressure is fixed at 0.8 Pa, the crystallinity of the films improves, and the carrier concentration and mobility show increased trend and thus resistivity shows decreased trend with sputtering pressure. The film deposited at 100 W shows maximum carrier concentration and relatively larger mobility, and thus it shows minimum resistivity of 1.43×10-3Ωcm. When the sputtering pressure and power are fixed at 0.8 Pa and 100 W, it is found the （002） diffraction peak becomes sharp, stress in films relaxes, and the crystalline quality improves as film thickness increasing from 80 to 1100 nm. The carrier concentration and mobility increase and resistivity decreases with film thickness, and minimum resistivity of 3.13?10-4 ??cm is obtained at film thickness of 803 nm. The transmittance of the films is mainly dependent on the film thickness, indicating by that the transmittance decreases from 90% to 77% with film thickness but it changes between 86 and 90% with sputtering power and pressure. It is found that the change tendency of Eg of films is consistent with that of the carrier concentration, i.e., Eg increases with carrier concentration due to the BM effect.The study of this thesis indicates that high performance AZO films can be obtained at low substrate temperature （even RT） by magnetron sputtering in Ar+ H2 atmosphere and adjusting sputtering power, pressure and film thickness, and thus deposited AZO films are suitable for application in solar cells and organic light emitting diodes as transparent conductive electrode layers.