Growth of ZnO:Al by atomic layer deposition: Deconvoluting the contribution of hydrogen interstitials and crystallographic texture on the conductivity

Abstract

Aluminium doped ZnO (AZO) is an interesting low cost transparent conducting oxide with further use as an inorganic transport layer in multilayer solar cells. Here we present our work on atomic layer deposited (ALD) thin films where, with optimised growth conditions, we can obtain resistivities of 1 × 10−3 Ωcm even in 50–80 nm thin films grown at low temperatures (250 °C). We discuss the influence of crystallographic texture for ALD grown films by comparing plain glass, c-plane Al2O3, and a-plane Al2O3 substrates. We show that the doping mechanism in ALD grown AZO is more complex than for e.g. sputtered material as a substantial hydrogen interstitial related background doping occurs. We compare results from as grown samples with those briefly annealed at 320 °C in nitrogen. This process leads to an increased Hall mobility due to improved grain boundary passivation, but reduced carrier concentration due to partial loss of hydrogen interstitials.