Combinatorial characterization of transparent conductive properties of Ga-doped ZnO films cosputtered from electron cyclotron resonance and rf magnetron plasma sources

The simultaneous sputtering of ZnO and Ga2O3 by electron cyclotron resonance and rf magnetron plasma sources produced Ga-doped ZnO (GZO) films with continuously varying Ga concentration over the substrate surface. Combinatorial evaluation of electrical and optical properties of GZO film grown on silica glass substrate without heater annealing enabled identification of minimum resistivity (0.5 mΩ cm) at a Ga2O3 content of 5.5 wt % with an optical transmittance of 90% in the visible wavelength. The monotonically decreasing mobility that was associated with increasing carrier concentration as Ga2O3 content was increased indicated that conduction was governed by ionized impurity scattering. Above the critical Ga2O3 content (6 wt %), carrier concentration decreased since excess Ga atoms that were incorporated beyond the solubility limit at Zn sites hindered large crystalline domains from forming. The ZnO (002) x-ray diffraction peak was suppressed and peaks assigned to Ga2O3 were observed at high Ga2O3 content. The optimum Ga2O3 content shifted to 3.5 wt % at a deposition temperature of 200 °C and 2.5 wt % at 300 °C, and the minimum resistivity was further decreased to 0.28 mΩ cm at 200 °C. However, the resistivities at these elevated temperatures were incredibly high both at the lower and higher side of the optimum Ga2O3 content.