Structure and properties of transparent conductive ZnO films grown by pulsed laser deposition (PLD)

Zinc Oxide (ZnO) has attracted interest due to its potential applications including nonlinear optical devices, blue‐violet emission device, buffer layer for GaN‐based devices, visible‐range transparent electrodes for solar cell and flat panel displays, surface acoustic wave devices, piezoelectric and piezo‐ optic devices, gas sensors for oxygen, and integrated optical devices such as optical waveguides. Furthermore, ZnO is nontoxic and inexpensive and has comparable electrical and optical properties to commercially available Indium Tin Oxide (ITO). ZnO thin films were grown on glass substrates by the pulsed laser deposition (PLD) technique using a KrF excimer laser with wavelength of 248nm. The influence of ambient oxygen pressure, substrate temperature, laser fluence, the number of pulses and doping with silicon was investigated. The structural, optical and electrical properties of ZnO thin films were investigated using X‐ray diffraction (XRD), atomic force microscopy (AFM), scanning electronic microscopy (SEM), spectrophotometers and resistance measurement by the four‐point probe. Regarding the influence of oxygen pressure, XRD measurements indicate that the ZnO thin films deposited at the oxygen pressure of 10 mTorr (1.3 Pa) crystallize well. AFM results show that the surface roughness of ZnO film increases with an increase of oxygen pressure. The electrical resistivity showed a minimum for an oxygen partial pressure of 10 mTorr (1.3 Pa). The effect of the substrate temperatures in the range of 100 to 500 on the properties of ZnO thin films was investigated with the oxygen pressure kept at 10 mTorr. The lowest resistivity was observed for a substrate temperature of 200 C. The effect of post‐growth annealing showed that for temperatures < 300 , it is observed that the resistivity of the ZnO films decreases slightly. As the annealing temperature increases up to 500 , the resistivity of the ZnO films increases dramatically. Furthermore, The ZnO films show an increase of transmittance with annealing temperature, which probably indicates that the annealing treatment improved the crystallinity. ZnO thin films were deposited using a different number of pulses in order to investigate the thickness dependence of the structural, electrical and optical properties of the films. It was found that correlation between thickness and resistivity of ZnO thin films is negative due to the thicker films having fewer point defects such as oxygen vacancies. The transmittance of ZnO thin films is decreased due to the thickness effect. Laser energy density was varied in the range from 1.16 J/cm to 2.73 J/cm and it was observed that the resistivity of ZnO thin films declines with increasing laser fluence up to 1.51 J/cm. However, at higher fluences up to 2.27 J/cm, the resistivity dramatically increases, and the relatively low resistivity of the ZnO thin films of 3.1x10 Ω/cm is found at a laser fluence of 1.51J/cm. The optical transmittance increases with increase of laser fluence, from 1.16 J/cm to 2.27 J/cm in the range of wavelength of 350 nm to 450 nm. However, as the laser fluence increases up to 2.73 J/cm, the optical transmittance significantly declines. The influence of impurity doping was investigated by using a 2 wt% Si‐ZnO target. It was observed that the doped films had lower resistivity than pure ZnO films (from 9 x10 Ω cm to 7 x10 Ω cm). Probably due to the increase in carrier concentration with doping the Si ions entering into the ZnO lattice caused a shift in the absorption edge and the average transmittance increased to 85 % in the visible region after doping with 2 wt% silicon.