Hard transparent conducting hex-element complex oxide films by reactive sputtering

Hard transparent conducting oxide films of hex-element Al_xCoCrCuFeNi were deposited by reactive direct current (dc) magnetron sputtering using homogeneous alloy targets. The composition–property relation was investigated by changing the aluminum molar ratio, x value, from 0.5 to 2. The films comprise only a cubic spinel phase without other accompanying crystalline oxide phases and exhibit a high hardness up to 22.2 GPa. A small, negative deviation from Vegard’s law was observed for the spinel phase, which indicated changes in cation distribution. The optical transmittance in both the visible and infrared region is increased with aluminum content, however, together with a loss of film conductivity. The Hall measurements reveal a p-type conducting behavior for the Al_0.5CoCrCuFeNi oxide film with a conductivity of 40.1 Ω⁻¹cm⁻¹, a carrier density of 5.81 × 10¹⁸ cm⁻³, and a mobility as high as 43.2 cm²V⁻¹s⁻¹. Moreover, Hall measurements show metallic conduction behavior for the Al_0.5CoCrCuFeNi oxide film and thermal activated semiconducting properties for the Al₁CoCrCuFeNi and Al₂CoCrCuFeNi oxide films. Combine the crystal field theory and the x-ray photoelectron spectroscopy (XPS) measurements, the decrease of film conductivity is explained by the decreases of available carriers and mobility due to the fact that increasing aluminum content reduces the number of conducting cations at octahedral sites and increases the activation energy for electrical conduction. XPS analyses also show lots of excess oxygen originated from anion-rich growth condition in the films deposited at high oxygen partial pressure that produce p-type carriers lowering the electrical resistivity. The amount of excess oxygen decreases with increasing Al content and also contributes to the variation of conductivity with x value.