Determination of the Surface Structure of Cadmium TELLURIDE(110) by Low Energy Positron Diffraction.
Abstract
The (110) surface of zincblende CdTe has been studied by Low Energy Positron Diffraction (LEPD). Recently, LEPD has proved to be a very accurate and sensitive tool for the study of semiconductor surfaces. For these materials, the agreement between the calculated and experimental LEPD IV profiles is 'perfect' with xray Rfactors as low as 0.07, compared to 0.16 that is typically obtained by Low Energy Electron Diffraction (LEED). LEED studies of a wide variety of zincblende (110) surfaces has led to a universal picture for their relaxation viz., these surfaces relax via an approximately bond length conserved rotation of the top layer bond by a constant angle omega _1 = 29 +/ 3^ circ. These experimental results have been borne out by tightbinding total energy minimisation calculations. Previous LEPD studies of GaAs and InP (110) surfaces have established that InP has a slightly lower omega than GaAs. These results are in accordance with theoretical predictions of Tsai et al. viz., that omega _1 decreases with increasing ionicity. However, the LEPD studies were inconclusive because GaAs and InP differ very little in their ionicity. This motivated the present study of LEPD from CdTe(110) which has a much higher ionicity and for which the theory of Tsai et al. predicts that omega_1 would be ~12^circ lower than measured by LEED. Low temperature positron diffraction experiments were carried out on the CdTe(110) surface using ^{22}Na and a solid Ar moderator. A detailed multiple scattering dynamical analysis of the measured intensities was carried out. Results of these calculations showed that omega_1=30.9 +/ 2.8^circ for CdTe, with an Rfactor of 0.098. Thus, our LEPD results support the nearly bond length conserved rotation picture suggested by LEED. As suggested by LEED studies of CuCl, secondary effects of ionicity are observed in the large (5%) contraction of the top CdTe bond. The dependence of structural parameters on ionicity may be complex as suggested by the lower omega_1 obtained for InP by LEPD and needs to be investigated further.
 Publication:

Ph.D. Thesis
 Pub Date:
 1996
 Bibcode:
 1996PhDT........12D
 Keywords:

 Physics: Condensed Matter