Optical and electrical properties of H2 plasma-treated ZnO films prepared by atomic layer deposition using supercycles

Abstract

High mobility H-doped ZnO (ZnO:H) thin films are prepared on thermal oxide coated silicon wafers by a novel supercycle approach of thermal atomic layer deposition (ALD) technique. The influence of H2 plasma treatment and substrate composition on optical and electrical properties of ZnO films are studied using spectroscopic ellipsometry (SE) and direct electrical measurements. This work significantly expands the measured range of ZnO:H optical properties into the far infrared to obtain the complex dielectric function (ε = ε1 + iε2) spectra from 0.4 meV to 5.89 eV compared to what has been previously investigated by using SE. A blue shift in the optical band gap energy is observed with decreasing cycle ratio (n) or more frequent H2 plasma treatments during deposition. The amplitude of phonon absorption is increased with decreasing n. Free carrier transport characteristics are determined by applying the Drude model in modeling ε, and the result indicates lower n improves material conductivity as confirmed by direct electrical measurements. Regarding the effect of substrate composition, the samples prepared using the same n but on bare thermal oxide, a 5 nm ZnO:H film, or a 5 nm intrinsic ZnO film each have differences in ε and free carrier transport characteristics resulting from substrate composition, demonstrating an additional method of tailoring material properties. Effective carrier mass (m*) for the ZnO films are determined by combining results from Hall effect measurements with UV to THz spectra in ε. Thus, variation of n and substrate composition are demonstrated as an effective mean of controlling the resultant opto-electronic film properties.