Excitonic emission in heavily Ga-doped zinc oxide films grown on GaN

Abstract

There has been a great interest in development of materials technologies of transparent conducive electrodes for efficient solar cells, light-emitting diodes, radiation scintillators, displays and smart windows. Due to several advantages over other oxide and nitride materials, ZnO doped by gallium is a top contender in the considered field. Understanding the fundamental physics behind Ga doping effect on the optical and electronic properties of ZnO is thus a cornerstone of conceptualization of more efficient optoelectronic devices. Here, we use atmospheric pressure MOCVD approach for the preparation of well-aligned columnar Ga-doped ZnO films on GaN substrates. By performing complementary X-ray photoelectron spectroscopy and Raman spectroscopy studies, we evidence Ga substitutional doping leading to an upshift of the Fermi level and significant increase in the carrier density up to ~4·1020 cm−3. Temperature-dependent photoluminescence studies enabled unraveling the nature of the excitonic emission in heavily Ga-doped ZnO nanostructured films. Unlike well-known gradual thermalization of the donor bound excitons and prevailing of free excitonic emission in the high temperature regime for undoped ZnO films, we observe faster thermalization of D0X excitons and dominating FX-LO phonon replica for ZnO:Ga. A significant band gap renormalization of ~755 meV was found to be a major effect in heavily Ga-doped ZnO films. The obtained knowledge could be useful for smart design of ZnO-based transparent conductive electrodes with enhanced conductivity, improved quantum yield of emission and controlled optical transparency window.