Effect of hot-filament annealing in a hydrogen atmosphere on the electrical and structural properties of Nb-doped TiO 2 sputtered thin films

Abstract

In this work Nb-doped TiO2 thin films were deposited by d.c.-pulsed reactive magnetron sputtering at 500 °C from a composite target with weight fractions of 96% Ti and 4% Nb, using oxygen as reactive gas. In order to enhance the conductive properties, the as-deposited samples were treated in vacuum with atomic hydrogen at a substrate temperature of 500 °C. The atomic hydrogen flow was generated by a hot filament, inside a high-vacuum chemical vapour deposition reactor, at a temperature of 1750 °C. In order to optimise the hydrogen hot-wire treatments, the H2 pressure was varied between 1.3 and 67 Pa, the treatment time was monitored between 1 and 5 min and the hot-filament current was changed between 12 and 17 A. Dark conductivity was measured as a function of temperature and its value at room temperature was extrapolated and used to assess the effect of the hydrogen annealing on the charge transport properties. A two-order of magnitude increase in dark conductivity was typically observed for optimised hydrogen treatments (10 Pa), when varying the hydrogen pressure, resulting in a minimum resistivity of ~3× 103 cm at room temperature. The maximum amount of atomic H incorporation in oxygen vacancies was determined to be ~5.7 at.%. Carrier mobility and resistivity were also investigated using Hall effect measurements. Correlations between structural and electrical properties and the hydrogen treatment conditions are discussed. The purpose of these films is to provide a transparent and conductive front contact layer for a-Si based photovoltaics, with a refractive index that better matches that of single and tandem solar cell structures. This can be achieved by an appropriate incorporation of a very small amount of cationic doping (Nb5+) into the titanium dioxide lattice.