Effects of substrate temperature on the properties of heavily Al-doped ZnO films by simultaneous r.f. and d.c. magnetron sputtering

Transparent conducting ZnO films were heavily doped with Al (ZnO/Al) by simultaneous r.f. magnetron sputtering of ZnO and d.c. magnetron sputtering of Al. TEM image showed the coexistence of an amorphous Al complex and polycrystalline ZnO. A strong (002) preferential orientation in the ZnO/Al film was caused by the presence of ZnO. According to XRD and XPS analysis, most Al atoms were not in the ZnO crystalline and most oxygen atoms were attributed to the formation of Na zeolite structure. The Na zeolite could result in the neutral impurity scattering and cause low mobility. Electrical conductivity was primarily due to the contribution of Zn, Al interstitial atoms and oxygen vacancies. Little contribution was due to Al donors (AlZn). The resistivity decreased rapidly with the increase of substrate temperature to a minimum of 6.12×10−2 Ω cm at 150 °C, and then slightly increased. The conductivity of films was closely related to the crystallinity, which was dependent on the substrate temperature. The excellent transmission in the visible region decreased substantially at short wavelength near the ultraviolet range for films prepared at various substrate temperatures.