Preparation and electrical transport properties of TiO thin films

Abstract

Titanium monoxide has attracted great attention due to its unique superconducting characteristics and potential applications in microelectronics. In this work, a series of TiO thin films are prepared at room temperature by using the radio frequency magnetron sputtering method through changing the oxygen partial pressures. The crystal structures, valences of the elements, and electrical transport properties of the films are investigated systematically. X-ray diffraction results indicate that the films are epitaxially grown on MgO single crystal substrates along the [001] direction. After the surface of the TiO film is treated by ion etching, it is found that the Ti element mainly exists in a divalent form. For all films, the temperature coefficients of resistance are negative above superconducting transition temperature <i>T</i>_c. The films prepared under high oxygen partial pressures reveal insulator characteristics, and the temperature dependence of resistivity obeys the Mott-type variable-range-hopping law in low temperature regime. The films prepared under low oxygen partial pressures exhibit metallic properties, and enter into a superconducting state at low temperature. The superconducting transition temperature <i>T</i>_c in our film can be as high as 3.05 K. The carrier concentrations of the films are in the vicinity of ~2.0×10²² cm^–3, which is comparable to those for the typical metals. Interestingly, it is found that the main species of the charge carriers in the films transforms from electrons to holes with the oxygen partial pressure decreasing. The crossover of the species of the charge carriers could related to the changing of energy-band structure with the oxygen content in TiO film.