Thermal Admittance Studies of Electron-Irradiated CdGeAs 2
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of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469-0178 of Dayton Physics Department, 300 College Park, Dayton, OH 45469-2314 'AFRLJSNHX, Hanscom AFB, MA 01731-3010 b University
ABSTRACT Brudnyi, et al.', and Zwieback, et al.2 , have shown that introducing damage by irradiation with MeV electrons can alter the electrical and optical properties of undoped ptype CdGeAs 2 (CGA) crystals. Brudnyi's studies of the electrical transport properties of isochronally annealed samples demonstrated type conversion and identified at least two new centers, one a stable donor. Zwieback used multi-MeV electrons to introduce compensating donors, thereby, significantly improving the optical transparency of CGA crystals. However, at the present little is known about these centers. Therefore, we have studied these centers by observing the properties of electron-irradiated specimens using Thermal Admittance Spectroscopy (TAS) and correlated the results of these measurements with capacitancevoltage measurements and Hall effect measurements. Measurements before an after irradiation are compared. The as-grown native acceptor concentrations in our samples varied from a low in the mid 1016 cm-3 to nearly 1019 cm"3 . Significant changes in the electrically active states in the band gap were seen as a result of a single irradiation with 2 MeV electrons to a total dose of 5 x 10' 5cm-2. The samples appear to respond more strongly than Brudnyi's samples. The thermal activation energies have been determined using TAS and they will be reported.
INTRODUCTION CdGeAs 2 has shown great potential for applications as an optical parametric oscillator an frequency converter.3 Optimization of the optical properties is of great importance. It has been shown that optical absorption in this material is limited by a high concentration of lattice defects. 4 However, the concentration of these defects and, hence, the optical absorption properties can be altered by the introduction of damage into the crystal and the subsequent annealing of the damage. One of the best tools for the introduction of damage is high-energy electrons. Electrons produce point defects, hence a one-to-one correspondence between the number of defects and the total electron fluence can be assumed. The difficulty with ternary compounds such as CGA is that the defects produced by the irradiation will not all be the same. Cd, Ge, and P vacancies will all be produced. This multiplicity of defects makes identifying the source of the electrical levels induced in the material difficult. Nevertheless, we have used Thermal Admittance Spectroscopy to measure the activation energy of the
403 Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
shallow acceptor in CGA before and after irradiation with electrons in order to investigate the shallow levels in the bandgap. The technique of Thermal Admittance Spectroscopy has been described in detail in previous papers, and the reader is referred to those papers for a discussion of the experimental technique. 5 - In the
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