RF Power Effect on the Properties of Sputtered ZnO Films for Channel Layer Applications in Thin-Film Transistors

ZnO films were processed by radiofrequency (RF) magnetron sputtering under argon gas environment at room temperature, varying the RF power (90 W, 100 W, 150 W, and 200 W), on p-Si/SiO2 substrates. Structural, morphological, and electrical characteristics of the ZnO films were determined using several experimental techniques, and they showed a clear relationship with the RF power. All the ZnO films exhibited a hexagonal wurtzite polycrystalline structure with (002) preferred orientation. Atomic force microscopy (AFM) revealed the formation of grains or clusters as a result of the accumulation of nanoparticles, and the grain size increased with increasing power. An ascending trend of the root-mean-square surface roughness of the films with increasing power was also observed. ZnO film thickness and refractive index were determined by spectroscopy ellipsometry. In agreement with AFM results, the observed increase of refractive index from 2.15 to 2.44 was the result of improved film compactness on increasing the deposition power. The electrical resistivity ranged from 3.5 × 103 Ω-cm for ZnO film deposited at 200 W to 5 × 107 Ω-cm for that deposited at 100 W. The sputtered ZnO films were employed as the active channel layer in thin-film transistors, and the impact of the deposition power on device performance was studied. As the power was increased, the field-effect mobility increased from ~0.1 cm2/V s to 4.2 cm2/V s, the threshold voltage decreased from 33.5 V to 10.7 V, and the Ion/Ioff ratio decreased from 106 to 102.