We present a study of the influence of the growth parameters on the electrical and optical properties of Ge_xSi_yO_z films obtained by RF reactive sputtering of a Ge_0.85Si_0.15 target. Films are characterised measuring the composition, the optical gap and the energy activation of the electrical conductivity. We analyse the effects of the sputter gas composition, the RF power applied to the target and the bias applied to the substrate on the films properties. Compositional analysis of the films shows that O is bonded to both Si and Ge atoms, forming a homogeneous amorphous semiconductor. The optical gap and the energy activation of the electrical conductivity vary smoothly with the total oxygen content of the films. The ion bombardment induced by the application of a bias voltage to the substrate causes a modification of the band tail structure reducing the electrical resistivity of the samples. Under well-controlled experimental conditions, Ge_xSi_yO_z layers with electrical resistivities in the range of 10⁴ Ω·cm and thermal coefficients as high as 5 %K⁻¹ can be obtained.