PAC Studies of Defects in AgCI and II-VI Compounds
- PDF / 378,006 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 42 Downloads / 191 Views
PAC Studies of Defects in AgCI and II-VI Compounds
J.C. Austin, M.L. Swanson, W.C. Hughes, S.S. Choi Department of Physics and Astronomy, University of North Carolina, Chapel Hill, N.C. 27599-3255 ABSTRACT The Perturbed Angular Correlation (PAC) method has been used to study In vacancy complexes in AgCl and II - VI compounds. Although these materials are quite different in structure and properties, they are related by photoconductive properties which make them interesting intrinsically and for application to photography (silver halides) and to the design of photodetectors and photovoltaic cells (II - VI compounds). We have obtained the first PAC results for silver halides and ternary il-VI compounds. For AgCI, measurement at 77 K clearly reveals two different 11 lIn - vacancy complexes characterized by quadrupole interaction frequencies v0 1 = 21.5 MHz and vq2 = 43 MHz, both having axially symmetric electric field gradient (EFA) tensors with principle axes along directions. Temperature dependence of the PAC signal allows a determination of the activation energy and attempt frequency for vacancy motion by measurement of the damping arising from dynamic effects. For 4% Zn - doped bulk CdTe samples, vacuum annealed in the range 4001C to 550'C, PAC shows that a large fraction of the 11In probe atoms occupy non-cubic lattice sites having an EFG characterized by vQ = 60 MHz, Ti= 0.2. The concentration of this complex was decreased by annealing between 400'C and 600*C in a Cd overpressure, suggesting that the observed EFG is caused by a 11 In - cadmium vacancy complex. Similar measurements of Cd0 .8 Mn0 .2 Te showed an identical EFG which, however, appeared at a lower annealing temperature. INTRODUCTION Perturbed angular correlation (PAC) is a nuclear technique which utilizes the hyperfine interactions between radioactive probe nuclei and their environment to study defects on an atomic scale. The technique is particularly well suited for studying defects in crystals with cubic lattice structures, since a probe nucleus occupying a perfect cubicsymmetric lattice site experiences no quadrupole interaction. The probe nucleus is therefore sensitive to perturbations caused by imperfections in the lattice, such as point defects [1]. The r -3dependence of the quadrupole interaction means that the technique is very local--it is most sensitive to defects which lie in the immediate vicinity of the probe atom, primarily those lying within a few angstroms, e.g., vacancies or impurities trapped by the probe atom. An important example is the formation of I11In - donor atom pairs in silicon [2] Our current research efforts are aimed primarily at the study of silver halides and IVI semiconductors. Although these materials are of quite different structure and properties, they are of considerable technological importance due, primarily, to their respective photoelectric properties. The silver halides are of great importance to photography, and the influence of defects is crucial to the photographic process. Of particular interest is that m
Data Loading...
