Structural, electrical, and optical properties of hydrogen-doped ZnO films

Hydrogen doped ZnO thin films were deposited by radio frequency magnetron sputtering from a ceramic target on $c$-plane sapphire and fused silica using H${}_{2}$ and O${}_{2}$ as reactive gases. Structural analysis revealed that all films are polycrystalline with the $c$ axis oriented perpendicularly to the substrate surface. The lateral grain size was strongly affected by the oxygen content of the sputtering gas and decreased dramatically above a critical content of 4.5 $%$. We were able to adjust the carrier density of the films by the deposition parameters to any value between 10${}^{14}$ and 2 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{20}$ cm${}^{\ensuremath{-}3}$. Using temperature-dependent Hall-effect measurements we identified thermionic emission over Coulomb-barriers created by surface trap states at the grain boundaries and tunneling effects to dominate the carrier transport. Preparing and thoroughly characterizing the films is a prerequisite for our investigation of the dependence of the optical band gap energy on the carrier density. We use results from experiment as well as first-principles calculations (including Burstein-Moss shift, band gap renormalization, and excitonic effects) in order to understand the mechanisms that determine how free electrons influence the energy position of the optical absorption onset.