The fabrication of ultra-wide bandgap GeO2 thin films by DC magnetron sputtering: The impacts of growth temperature and post-annealing process

Abstract

Ultra-wide bandgap semiconductors (UWBSs), possessing the bandgap greater than 4 eV, have emerged as an advanced platform for optoelectronic device applications. The exploration of novel UWBSs becomes compelling research focus on the solar blind photodetectors (SBPDs). Here, we have deposited ultra-wide bandgap GeO2 films on c-plane sapphire substrates using direct-current (DC) magnetron sputtering method and investigated the effects of the growth and annealing temperatures on surface morphologies, crystal structure, oxygen vacancies, and bandgap energies of GeO2 films. The high quality GeO2 film with the hexagonal phase can be obtained at the growth temperature of 500 °C, which transforms to the mixture of hexagonal and tetragonal phases after annealing at 900 °C. By analyzing the energy loss spectra of O 1s peaks, GeO2 films exhibit the ultra-wide bandgap ranging from 5.78 to 5.90 eV, which is verified by optical transmittance measurements. Moreover, 200 nm SBPDs based on GeO2 films have been fabricated and an obvious improvement in the photoresponse current of the annealed device has been found due to the induced additional electrons by the hole capture of oxygen vacancies. Our findings on the development of ultra-wide bandgap GeO2 films pave a way towards the realization of robust and high-performance SBPDs.