Quality of Bulk CdTe Substrates and its Relation to Intrinsic Defects
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QUALITY OF BULK CdTe SUBSTRATES AND ITS RELATION TO INTRINSIC DEFECTS B. K. MEYER, D. M. HOFMANN, W. STADLER, P. EMANUELSSON*, P. OMLING*, E. WEIGEL**, G. MULLER-VOGT**, F. WIENECKE'** AND M. SCHENK*** Physikdepartment E16, Technical University Munich, D-8046 Garching, Germany. *Department of Solid State Physics, University of Lund, Box 118, S-221 00 Lund, Sweden **KristaUl- und Materiallabor, University Karlsruhe, D-7500 Karlsruhe, Germany ***Humboldt-University, D-1000 Berlin, Germany ABSTRACT Both the cadmium vacancy (Vcd) and the tellurium vacancy (VTo) in CdTe are identified by means of electron paramagnetic resonance (EPR). The VCd is a double acceptor and the EPR spectrum is observed in its single negative charge state. The symmetry is found to be trigonal, which can be explained in a model in which the hole occupies a dangling bond t2 orbital and the orbital degeneracy is removed by a static Jahn-Teller distortion. The hyperfine interaction shows that the hole is localised on one of the four Te neighbours. The EPR spectrum of VTe+ reveals cubic (unperturbed) symmetry and the hyperfine structure shows that the unpaired electron is equally spread over the four Cd neighbours. Photo-EPR measurements locate the 0/+ state of VTe at Ev + 0.2 eV and the 2-/- acceptor level of VCd to be situated less than 0.47 eV above the valence band.
INTRODUCTION CdTe is an important semiconductor to be used as a substrate material for CdxHgl-xTe infrared detectors. Like all compound semiconductors, its electrical and optical properties are strongly influenced by the stoichiometric conditions during growth. A great deal of research has therefore been performed on defects related to vacancies in CdTe. For instance have A-centres, which consist of a metal vacancy and a nearby donor, been investigated using electron paramagnetic resonance (EPR) [1] as well as optically detected magnetic resonance (ODMR) [2,3]. In this paper we summarise the previously reported EPR identifications of the isolated Te [4] and Cd [5] vacancies in CdTe. From the results of photo-EPR experiments, the level
positions in the energy gap is discussed. EXPERIMENTAL RESULTS AND ANALYSIS The EPR measurements were performed using a Bruker ESP 300 spectrometer equipped with a helium flow cryostat (Air Products) capable of controlling the temperature over the range from that of liquid helium to room temperature, The sample temperature was held at approximately 25 K. The sample could be illuminated in situ with monochromatic light from a halogen lamp attached to a monochromator.
The tellurium vacancy The starting material used was CdTe crystals doped with group III and group VII elements. After annealing for one hour in vacuum or in argon gas at temperatures between 380 'C and 780 'C, the EPR spectrum in Fig. 1 (top) appeared. It is characterised by an isotropic g-value of Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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g=2.000±0.O01 and isotropic ligand hyperfine interaction with four Cd ions. The spectrum is assigned to the VTC cent
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