AVS 53rd International Symposium
    Electronic Materials and Processing Tuesday Sessions
       Session EM-TuM

Paper EM-TuM2
Surface and Optical Properties of Zn-face versus O-face ZnO

Tuesday, November 14, 2006, 8:20 am, Room 2003

Session: Zinc Oxide
Presenter: A.A. Baski, Virginia Commonwealth University
Authors: S. Chevtchenko, Virginia Commonwealth University
J.C. Moore, Virginia Commonwealth University
U. Ozgur, Virginia Commonwealth University
X. Gu, Virginia Commonwealth University
A.A. Baski, Virginia Commonwealth University
H. Morkoc, Virginia Commonwealth University
Correspondent: Click to Email
We have compared the surface and optical properties of Zn-face (0001) and O-face (0001bar) ZnO samples. The samples used in this study were diced from the same bulk crystal prepared by Cermet Inc.@footnote 1@ Each sample had a thickness of ~0.4 mm and carrier concentration of 4.5x10@sup 16@ cm@sup -3@ at room temperature, corresponding to a Fermi level 0.11 eV below the conduction band. For optical characterization, steady-state photoluminescence (PL) using a He-Cd laser was measured at 15 K and 300 K (RT). Low-temperature PL for both surfaces showed the expected main transitions (free exciton, donor bound exciton, donor acceptor pair), with the most intense donor bound exciton transition at 3.359 eV having a FWHM of 1.7 meV, indicating a high quality sample. We did not observe any significant difference in the low-temperature PL spectrum for samples with different polarity. However, at RT the O-face sample demonstrated nearly double the near-band-edge emission intensity as that seen for the Zn-face. Using scanning Kelvin probe microscopy, we have also measured surface contact potentials of 0.39 V (Zn-face) and 0.50 V (O-face), which correspond to an upward band bending of 0.20 eV (Zn-face) and 0.09 eV (O-face), assuming an electron affinity of 4.4 eV. This relatively small difference in band bending for the polar ZnO surfaces indicates that spontaneous polarization is not a dominant contributor. Rather, we attribute band bending to surface states associated with dangling bonds. Also, the lower band bending of the O-face surface is consistent with the higher band-edge emission observed for this surface in room temperature PL studies. Finally, conductive atomic force microscopy studies show enhanced reverse-bias current conduction for the O- versus Zn-face samples. This effect may be due to different interactions of the two surfaces with ambient, in particular with water and hydrogen. @FootnoteText@ @footnote 1@ B. Nemeth and J.E. Nause; http://www.cermetinc.com.