AVS 51st International Symposium
    Electronic Materials and Processing Tuesday Sessions
       Session EM-TuM

Paper EM-TuM2
Near-Surface Defects and Schottky Barrier Formation at Au/ZnO(000-1) Interfaces

Tuesday, November 16, 2004, 8:40 am, Room 304B

Session: Contacts and Metallization
Presenter: H.L. Mosbacker, The Ohio State University
Authors: H.L. Mosbacker, The Ohio State University
Y.M. Strzhemechny, The Ohio State University
P.E. Smith, The Ohio State University
B.D. White, The Ohio State University
D.C. Look, Wright State University
L.J. Brillson, The Ohio State University
Correspondent: Click to Email
ZnO is rapidly emerging as a promising optoelectronic material, particularly for short wavelength light emitters. Key to such devices are an understanding and control of the metal-ZnO Schottky contact, yet clean interfaces and the role of extrinsic ZnO interface states in UHV barrier formation are relatively unexplored. We have used a combination of nanoscale depth-resolved electron-excited luminescence (NDREEL) spectroscopy, Auger electron spectroscopy (AES), atomic force microscopy (AFM), low energy electron diffraction (LEED), and current-voltage (I-V) measurements to correlate changes in Au/ZnO contacts with near-interface states and surface chemical structure. Eagle-Picher ZnO single crystals grown by chemical vapor transport and cleaned with organic solvents exhibit only minor contamination and a hexagonal LEED pattern. AFM revealed micro-pits known to render the metal/ZnO contacts ohmic. Subsequent exposure to a remote O/He plasma in a UHV-linked chamber resulted in a significant increase of rms surface roughness but an improved LEED pattern. AES surface stoichiometry improved after O/He plasma treatment with the O/Zn ratio increasing from ~ 0.55 to ~ 0.76. I-V of the in situ-evaporated Au/ZnO revealed dramatic change with plasma exposure: whereas the pre-plasma surface exhibited Ohmic behavior, the contact on the plasma-treated ZnO yielded a ~ 0.4 eV Schottky barrier with ~2 ideality factor. O plasma also significantly reduced 2.5 eV (i.e., green) NDREEL emission due to deep level (DL) traps by ~40% relative to the near band edge (NBE) emission both on and off the (semitransparent) Au contacts. In pre-plasma ZnO, the DL/NBE ratio was 40% higher at the free surface relative to that at ~ 90 nm deep. O plasma eliminated this near-surface increase. Further reduction occurred under the Au within < 20 nm of the interface. Our results are consistent with decrease in defect-assisted tunneling with plasma treatment vs. changes in Fermi level pinning.