Effects of substrates on the crystalline growth and UV photosensitivity of glancing angle deposited porous ZnO nanostructures

Abstract

We prepared ZnO nanostructures on three different substrates such as quartz, sapphire (Al2O3) (001) and MgO (100) by glancing angle deposition (GLAD) technique using KrF excimer pulsed laser and investigated the effects of various substrates on the growth and optical properties. The GLAD technique promotes a vertical growth of the nanostructures and hence creates necessary porosity for penetration of light through the top of the surface. Although these nanostructures are porous, they are highly crystalline and oriented along c-axis of the wurtzite structure. Notably, despite the amorphous nature of the quartz substrate, nanostructures on quartz are preferentially oriented towards the c-axis. All three samples show intense ultraviolet (UV) emission with a highly suppressed visible emission, where the suppression depends on the growth and porous morphology of nanostructures. The variation in morphology and the porosity strongly influenced the electrical properties and UV photo-detection of these nanostructures when exposed under UV (254 nm) light. Although we used the same technique of the growth, the lowest dark current (0.14 nA) with the highest UV photosensitivity (182 %) is achieved for the highly porous and yet crystalline nanostructures grown on the sapphire substrate. Generally, it is extremely difficult to maintain crystalline structure with porosity; however, here we demonstrate a promising approach in designing visible-blind photodetectors by combining these two opposites, porosity and crystallinity, in ZnO nanostructures using the GLAD-assisted pulsed laser deposition method.