STRUCTURAL AND OPTICAL PROPERTIES OF Al-DOPED ZnO THIN FILMS DEPOSITED BY SPRAY PYROLYSIS TECHNIQUE

Abstract

ZnO is a wide band gap (—P3.37 eV) semiconductor with a large exciton binding energy of 60 meV. It has been extensively investigated for various applications such as transparent conducting films for solar cells, surface acoustic wave (SAW) devices and opto-electronic devices. Its band structure and optical properties are very similar to those of GaN, which is known to be a good material for the fabrication of optical devices such as light-emitting diodes (LED) or laser diodes (LD). It is widely accepted that ZnO is a promising material for realizing an ultraviolet laser at room temperature. In the present work thin films of zinc oxide (ZnO) have been prepared by the spray pyrolysis technique on glass substrates using substrate temperatures (Ts); 300,400, and 500°C. The influences of the substrate temperature on the structural and optical properties of the films were investigated. X-ray diffraction results have revealed hexagonal wurtzite structure with (002) preferred orientation and increase I crystallinity with increasing substrate temperature (TS). Atomic force microscopy (AFM) measurements revealed that the surface morphology of the undoped ZnO thin films changes continuously with increase in T. The grain size and RMS average roughness increase with increase in T. UV—Vis-NIR absorption spectroscopy as the grown films showed that the band gap increases from 3.283 to 3.293 eV when substrate temperature (Ts) increases from 300 to 500°C. Photoluminescence (PL) measurements at room temperature as thin films of undoped ZnO revealed a sharp violet and one weak emission. It has been found that PL intensity increases with increasing substrate temperature (TS). The 412nm violet peak shifts from 412 to 407nm on increasing the substrate temperature from 300 to 500°C, whereas no shift in blue peak was observed with _substrate temperature. The 412nm violet luminescence is ascribed to radiative defects related to the interface traps existing at grain boundaries. With the increase of TS, the stress in the films changed from compressive to tensile, which is believed to be the came from the observed 412nm violet emission peak shifts from 412-407 nm. Thin films of Al-doped ZnO have also been prepared by the spray pyrolysis technique on glass substrates at Ts — 400°C. The influence of the Al-doping on the structural, morphological, and optical properties of the ZnO thin films has been investigated. X-ray diffraction analysis indicates that the crystallites of both undoped iv ZnO and ZnO: Al thin films are preferentially oriented along the c-axis, [002] direction of the hexagonal compact structure of type. Atomic force microscopy (AFM) measurements show that the grain size decreases with increasing Al-doping level. Energy Dispersive X-ray (EDAX) results confirm the presence of all elements in the correct ratio in all the thin films. UV Vis-NIR absorption spectroscopy reveals that the band gap increases from 3.131 to 3.301 eV with increasing doping concentration of Al. PL studies on Al-doped ZnO thin films shows a drastic decrease in the intensity of the violet emission. The violet emission at 407 nm is probably due to the radiative defects related to the interface traps existing at the grain boundaries and the emission is due to the transition between this level and the valence band. When Al is doped in the film, this occupy in the zinc lattice sites, reducing the probability of forming oxygen at zinc sites. Hence the density of oxygen antisites would be less in the doped film. This might be the reason for the decrease in intensity of the violet emission due to doping. v