Implantation-induced defects in Hg_0.78Cd_0.22Te studied using slow positrons

Slow-positron-beam studies on aluminium-implanted mercury cadmium telluride are presented. Single crystals were implanted with 320 keV Al ions up to 3*10¹² fluence at room temperature and 1*10¹⁴ ions cm^-2 fluence at 100 K and 300 K. We discuss the effect of the native oxide layer on the positron spectra and show that the oxide-crystal interface acts as a strong positron trap. By using both the core (W) and the valence (S) annihilation fractions we can separate oxide-related positron effects at the surface from the damage in the crystal. Implantation introduces small vacancy clusters. On the basis of the relative Doppler parameters of the defects created (S_d/S_b=1.05, W_d/W_b=0 80), they are most probably divacancies. The divacancy profile is found to extend from the surface to a depth comparable to the mean Al implantation depth. At room temperature divacancy creation reaches saturation at 3*10 ¹² ions cm^-2 fluence with an estimated divacancy concentration of 4*10¹⁶ cm^-3. After implantation at low temperature (100 K) and annealing at 360 K the divacancy creation exceeds 10¹⁸ cm^-3.