Effects of gallium doping on properties of a-plane ZnO films on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy

Abstract

The authors report on the structural, optical, and electrical properties of Ga-doped a-plane (112¯0) ZnO films grown by plasma-assisted molecular beam epitaxy. Ga doping level was controlled by changing the Ga cell temperatures from 350 to 470 °C with an interval of 30 °C. With up to Ga cell temperatures of 440 °C, single crystalline Ga-doped a-plane ZnO films were grown; however, the sample with a Ga cell temperature of 470 °C showed polycrystalline features. The typical striated surface morphology normally observed from undoped ZnO films disappeared with Ga doping. ZnO films doped with Ga cell temperatures up to 440 °C did not show a significant change in full width at half maximum (FWHM) values of (112¯0) x-ray rocking curves by doping. The smallest FWHM values were 0.433° (ϕ=90°) and 0.522° (ϕ=0°) for the sample with a Ga cell temperature of 350 °C. The polycrystalline ZnO film with excessive Ga doping at the Ga cell temperature of 470 °C showed significantly increased FWHM values. Hall measurements at room temperature (RT) revealed that electron concentration began to be saturated at the Ga cell temperature of 440 °C and electron mobility was drastically reduced at the Ga cell temperature of 470 °C. The carrier concentration of Ga-doped ZnO films were controlled from 7.2×1018 to 3.6×1020 cm−3. Anisotropic electrical properties (carrier concentration and Hall mobility) were observed in measurements by the van der Pauw method depending on the direction (c- or m-direction) for the undoped sample but not observed for the doped samples. RT photoluminescence (PL) spectra from the Ga-doped single crystalline ZnO films showed dominant near band edge (NBE) emissions with negligibly deep level emission. The NBE intensity in PL spectra increases with Ga doping.