| AVS 55th International Symposium & Exhibition | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM-TuM |
| Session: | ZnO Materials and Devices |
| Presenter: | Y. Dong, The Ohio State University |
| Authors: | Y. Dong, The Ohio State University L.J. Brillson, The Ohio State University Z.Q. Fang, Wright State University D.C. Look, Wright State University D.R. Doutt, The Ohio State University M.J. Hetzer, The Ohio State University H.L. Mosbacker, The Ohio State University |
| Correspondent: | Click to Email |
Fabricating high quality ZnO contacts remains a challenge and there is little known about the comparison between two ZnO surface polarities (O or Zn-surface) on (i) surface morphologies, (ii) surface defect concentrations and energy levels, (iii) surface reactivities with various metal contacts, and (iv) the Schottky barrier heights and their metal correlations. We used depth-resolved cathodoluminescence spectroscopy (DRCLS), current-voltage and capacitance-voltage measurements, atomic force microscopy (AFM) and deep level transient spectroscopy (DLTS) to probe the possible different behavior of metal contacts to (0001) Zn- and (000-1) O- polar surfaces of high-quality vapor-phase grown ZnO. ZnO (0001) surfaces exhibited higher quality with smaller surface roughness, higher near band edge (NBE) emission and lower surface and near-surface defect emission. Remote O_2/He plasma (ROP) can effectively decrease the 2.5 eV near-surface DRCLS defect emission by removing surface adsorbates and subsurface native defects. Au and Pd diodes in-situ deposited by e-beam evaporation on the ROP treated surfaces can form Schottky barrier diodes (SBDs), while forming good Ohmic contacts on as-received surfaces. The transport properties of the SBDs are not only dependent on metal but also very sensitive to the surface polarities. Generally, gold diodes exhibit better rectifying properties than Pd, while Pd SBDs on the Zn-face have the largest reverse current. This was correlated to the surface morphologies, the DRCLS defect emissions, the CV carrier profiles and the surface and bulk traps revealed by DLTS. The effective donor concentrations for Pd and Au SBDs on the Zn-face were decreased by a factor of ~1.5 than on the O-face in the near surface region, which was accompanied by the higher 2.5eV defect/NBE emission ratio in DRCLS and an additional trap in DLTS for the O-face. The large leakage current for Pd SBDs is due to the sharp increase of carrier concentration at the upper interface region (< 80 nm) and tunneling, especially for Pd SBDs on the Zn face. In addition, for Pd/ZnO(0001) diodes, DLTS identified a new surface trap possibly related to hydrogen at ~0.50 eV below the conduction band and localized within the outer 80-100 nm. Our findings demonstrate the importance of polar effects on forming surface and near-surface defects that control the transport properties.