Transparent and conducting intrinsic ZnO thin films prepared at high growth-rate with c-axis orientation and pyramidal surface texture

Abstract

The growth of ZnO thin films has been optimized by adjusting the intrinsic ion vacancies, by controlling the RF power applied to the plasma in magnetron sputtering. Preferred c-axis oriented intrinsic ZnO films with largest grain size and a hexagonal wurtzite structure, exhibiting high room temperature conductivity, σ∼1.37S/cm, high transparency, ∼80–90% within 450–800nm and ∼90–96% within 800–1900nm, low reflectance (<5% in the visible range) was obtained at a very high deposition rate ∼214nm/min, at 300°C, by maintaining higher concentration of Zn interstitials or singly ionized oxygen vacancy, corresponding to an optimized RF power of 200W. Films have lowest internal stress, smallest dissipation factor defined as ɛ2/ɛ1, and the specific pyramidal surface texture creates enough surface roughness that helps to improve the light scattering from the surface and makes it suitable for efficient use in thin-film silicon solar cells. With increasing RF power beyond 200W, the Zn–O bond length reduces promptly and the internal stress increases monotonically approaching toward a virtual saturation. The preferred crystallographic alignment shifts from (002) to (103), i.e., from c to a-axis orientation, as the surface energy of ZnO crystal changes due to the increase in the Zn-to-ZnO ion ratio in the plasma caused by the plausible de-oxygenation of ZnO at elevated RF powers. Oxygen deficient ZnO films having the flower-like surface texture prepared with a very high deposition rate ∼554nm/min at 500W could indeed make the material suitable for gas and chemical sensing applications.