P 10 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
ELECTRONIC STRUCTURE OF IRRADIATED CdO THIN FILMS
I.N. Demchenko1∗, R. Minikayev1, T. Tyliszczak2, M. Chernyshova3, K.M. Yu2, J.D. Denlinger2, D. Speaks2,4, and W. Walukiewicz2
1Institute of Physics PAS, al. Lotnikow 32/46, 02–668, Warsaw, Poland
2Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Bldg4R0230, Berkeley, CA 94720–8235, USA
3Institute of Plasma Physics and Laser Microfusion, 23 Hery Street, 01–497 Warsaw, Poland
4Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA Keywords: synchrotron radiation, X-ray absorption, X-ray emission
∗e-mail : demch@ifpan.edu.pl
There has been an increasing interest in group II-oxides, a class of semiconductors analogous to the very extensively studied and commercially success- ful group III-nitrides. In spite of thoroughly inves- tigated ZnO less attention has been paid to other group-II oxides. One of them, cadmium oxide, be- ing synthesized by different methods, has still not very well established properties [1]-[6]. Up to date there is controversy regarding the type, direct or in- direct, of the fundamental band gap of this material.
Recent studies have shown that the Fermi energy on the free CdO surface is pinned at about 1.15 eV above the conduction band edge (EC) [7]. This re- sembles the extensively studied case of InN where the Fermi energy was found to be pinned deeply in the conduction band at EC+ 0.9 eV. It is significant to note that, as in InN, the surface Fermi level pin- ning energy for semiconductors is expected to be like the bulk Fermi level stabilization energy in heavily damaged materials [8, 9]. In a striking contrast, a recent study has shown that intentional introduc- tion of native point defects using irradiation with 1 MeV He+ions led to stabilization of the bulk Fermi energy at about EC+ 0.4 eV in CdO [10] which is almost 3 times lower than the surface Fermi level pinning energy.
To better understand the properties of native de- fects in CdO, we have studied the effects of high con- centration of native point defects on CdO electronic structure. The defects were generated by high en- ergy Ne ions bombardment. Significant differences between experimental NEXAFS spectra gathered in total fluorescence yield (TFY, bulk sensitive) and total electron yield (TEY, rather surface sensitive) detection modes are observed. Such an observation confirms the fact that irradiation process drasti- cally modifies the surface layer of investigated films.
An interpretation of NEXAFS spectra at the K - edge of oxygen in “as-grown” and “irradiated ” CdO films, within the ab initio DFT formalism will be shown. Different models with point defects (oxygen
vacancy) in the host CdO matrix were considered and respective theoretical spectra were calculated.
Comparison of the experimental (“irradiated ” ver- sus “as grown”) and theoretical data allows conclud- ing that electronic levels of defects push the Fermi level into the conductive band and shift the absorp- tion threshold to a higher energy. The observed leading edge differences of TFY and TEY spectra could be likely explained basing on electron accumu- lation at the surface of n-type CdO due to the pres- ence of positively charged donor-type surface states.
Our conclusions are confirmed by optical absorption measurements.
Acknowledgments: The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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