Effect of hydrogen on ZnO films and Au/ZnO Schottky contacts

The structural, optical and electrical properties of ZnO films for different amounts of incorporated hydrogen (H), as well as the electrical characteristics of Au Schottky contacts based on these ZnO layers have been investigated. The films were deposited with the dc-magnetron sputtering technique, varying the H flow rate in the Ar/H sputtering gas. We found a significant improvement of the crystallinity (as obtained from x-ray diffraction spectra), Hall mobility and resistivity as the H concentration per vol. [H-2] (during deposition) increases from 0% to 33.3%, which is followed by degradation for further [H-2] increase. A high dependence of the carrier mobility on the grain size is also noted. The Schottky diodes were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements at room temperature. In correlation with the basic film properties, we obtained the best results for the Schottky diodes with [H-2] = 33.3%, in terms of higher rectification ratio, lower ideality factor (eta) and series resistance (R-s). Both the electron concentration n and the ionized donors' concentration N-D (obtained from C-V curves) increase constantly with [H-2] increase, and that seems to be consistent with our suggestion that H acts as a donor in ZnO.