Effect of sputtering power and post annealing process on properties of gallium-doped zinc oxide thin films

Transparent conductive oxide films are materials used in many different technological works today. They can be used in planar solar cells, low emission window glasses, transparent heat reflectors, electrical curtain applications and touch screen technologies. Zinc oxide (ZnO) films, which are an alternative to thin films such as tin oxide (SnO2) and indium-tin oxide (ITO), which have been used commercially so frequently, have recently started to be used. Additive elements such as aluminum, gallium, magnesium and indium can be added to ZnO films. By the influence of additive elements, the carrier concentration of ZnO films are increased and the electrical properties of thin film are improved. In the course of the growth of thin films, there are certain parameters that influence film properties. These parameters are the sputtering power, gas pressure, target-substrate distance and substrate temperature. In order to improve the desired properties in accordance with the application area, some additional operations can be performed after the coating of the transparent conductive oxide films. Heat treatment and etching are the mainstays of these processes. In heat treatment, the thin film structure changes with temperature effect and electrical, optical and properties show improvement at appropriate temperature, time and ambient conditions. In this study, the effect of sputtering power on the electrical, optical and structural properties of ZnO:Ga thin film was investigated. After evaluating the obtained results and determining the appropriate sputtering power, the effect of the post annealing process applied to the optical, electrical and structural properties of ZnO: Ga thin films grown in this sputtering power was investigated. Experiments to investigate the effect of applied post annealing on the electrical, optical and structural properties of ZnO: Ga thin filmin were carried out under vacuum at different temperatures and durations. The most suitable sputtering power was selected for the properties obtained from ZnO:Ga thin films grown at 5 different sputtering powers. Then, the films grown at the selected sputtering power were subjected to heat treatment at 150-250-350-450oC for 2 hours for each temperature value in a vacuum environment. As a result of heat treatments applied at different temperatures, heat treatment was applied for 1 hour and 3 hours at the selected temperature (2500C) according to their properties. The most suitable sputtering power obtained after all the operations was determined as 1.2kW heat treatment temperature 250oC and heat treatment time as 2 hours. In order to confirm that the selected heat treatment gives the best result at the selected sputtering power, 5 different glazes were heat treated in a vacuum environment of 250oC for 2 hours. Many different techniques have been used to characterize the enlarged ZnO: Ga thin films. Film thicknesses are measured by surface profilometry, while initial resistivity values are measured by a 4-point measuring probe. Optical transmittance values were measured by spectrophotometer. Surface roughness was determined by atomic force microscopy (AFM). Structural properties were visualized by scanning electron microscopy. The electrical properties of the films were measured by Hall effect measuring device. The the ratio of the average optical transmittance and resistivity, referred to as "figure of merit for evaluating the efficiency in ZnO:Ga thin films. The electrical and optical properties of ZnO:Ga thin films are change with sputtering power. The highest optical transmittance value was measured as 67% in the 1.2kW sputtering power. Again, the lowest resistivity value and the lowest surface roughness were obtained at the same sputtering power. As a result of the post annealing experiments, the resistivity values of ZnO: Ga films reached 3x10-4 ohm.cm and the optical transmittance values were measured at 77%. Bandgap values increased from 3.77eV to 3.92eV. The lowest resistivity, highest transmittance, suitable surface roughness, high hall mobility and high carrier concentration values were obtained in ZnO: Ga thin film grown at 1.2kW sputtering power and annealed in vacuum at 250oC for 2 hours.