Optical Investigations on Donor Doped CdTe
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OPTICAL INVESTIGATIONS ON DONOR DOPED CdTe W. Stadler,
B.K.
Meyer,
D.M.
Hofmann*,
D. Sinerius+ and K.W.
Benz+
Phyikdepartment E16, Technichal University of Munich, James Franck Str., 8046 Garching. F.R.G. + University of Freiburg, Kristallographisches Institut, Hebelstr. 25, 7800 Freiburg, F.R.G. Solid State Department,
University of Lund,
Sweden
ABSTRACT The recombination luminescence involving the A-center, a Cd vacancy paired with a nearest neighbour donor, was investigated in CdTe doped with group VII and III elements. Depending on the type of donor doping, distinct differences in the A-center acceptor binding energies and electron phonon couplings are resolved. Optically detected magnetic resonance shows that the A-center behaves as a shallow effective masstype acceptor consiscent with its small binding energy. I.
INTRODUCTION
CdTe, a member of the II-VI compound semiconductors which can be made n- and p-type conductive, is widely used as a substrate material for epitaxial growth of ternary II-VI semiconductors, e.g. HgCdTe. Therefore/ the control of the electrical and optical properties in CdTe has become essential. Whereas there is increased information about the presence of the dominant extrinsic impurities Cu, Ag and P, the understanding of the native defect structures present in the material are still at the 2 3 beginnint.1' ' The limited efficiency of n-type doping resulting very often in high resistive crystals is thought to be restricted by the presence of the A-center acceptor (a Cadmium vacancy - donor pair, VcdDT,). It is also believed that the A-center, a single acceptor, plays a dominant role in the self-compensation mechanism of Il-VI semiconductors. We investigated Rridgman-growrn CdTe crystals doped with group VII Halogen donors. They show distinct differences in the A-center luminescence properties. In the first part of our paper we present the luminescence investigations. identification of the luminescence centers based on optically detected magnetic resonance is presented in the second part. II. EXPERIMENTAL DETAILS 11.1 Crystal preparation As a first step, polycrystalline CdTe was produced using Cd and Te 0 of 6N purity. The scoichiomecric charge, appr. 2 0g, was introduced into uncoated quartz-ampoules of 300mm length and 30mm diameter. After evacuation to 10-6 mbar and sealing off, synthesis was carried out in a conventional resistance furnace at II00°C for 1 hour.3 The dopantconcentrations added ranged from 1017-1021 donor-atoms/cm and could be added, for example, in form of Cadmium halides such as CdF2 to the synthesized material into the growth ampoule. The CdTe crystals were grown by the vertical Bridgman-method in a resistance furnace. Appr. 150g of doped CdTe was filled into a quartz-ampoules of 180mm length and 15mm inner diameter. The charge was premelted in the growth ampoule under e hydrogen pressure of 800mbar to remove traces of oxides. After cooling to 6 room temperature and evacuation to 10" mbar, the ampoule was sealed off. Vertical Bridgman-growth was carried o
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