Substrate and Doping Effects on the Growth Aspects of Zinc Oxide Thin Films Developed on a GaN Substrate by the Sputtering Technique

A one-micron-thick pure zinc oxide (ZnO) and nitrogen-doped zinc oxide (N-ZnO) film were fabricated on p-type, pristine (non-porous), and porous gallium nitride (GaN) substrates using a radio frequency (RF) sputtering technique at room temperature. The doping medium was nitrogen gas, which has a flow rate that ranges from 0 to 10 sccm (0 sccm refers to pure ZnO). The photoelectrochemical etching process, using ultraviolet light, was employed to etch the wafer surface and create a porous GaN substrate. ZnO films were developed on GaN with ZnO powder as the target material under vacuum conditions. This research aimed to investigate how variations in substrate and doping influenced the structural, optical, and electrical characteristics of the resulting thin films. The SEM images indicated that the pores developed on the etched GaN surface had a spherical shape. The A1 (LO) phonon peak at 750.2 cm−1 was observed in the Raman spectrum of the etched porous GaN. The X-ray diffraction (XRD) analysis confirmed that the films grown on GaN possessed a hexagonal wurtzite structure and the observed peak shift of (101) in all N-ZnO films suggested interstitial nitrogen doping. For the N-ZnO films, the UV-visible cut-off wavelength shifted towards the blue region. The root mean square (RMS) roughness of the N-ZnO films, measured using atomic force microscopy (AFM), was found to decrease with an increasing N-doping concentration. The 10 sccm sample exhibited the lowest roughness value of 1.1 nm, whereas the pure ZnO film showed the highest roughness of 3.4 nm. The N-ZnO thin films were found to exhibit p-type conductivity, as computed by Hall measurements using the van der Pauw method, and the higher value of carrier concentration obtained for the nitrogen gas flow rate of 8 sccm was 5.29 × 1021 cm−3.