Doping limits in II-VI compounds - Challenges, problems and solutions

Wide-band-gap II-VI semiconductors have a potential for a variety of applications especially in the areas of light-emitting and light-detecting devices, photovoltaic conversion (solar cells), X-ray and gamma-ray detection, etc. In all applications, a good bipolar electrical conduction, i.e. efficient doping from both n- and p-side is essential, but due to the reasons which are not yet fully understood, it is still difficult to achieve. In this paper, a number of possible doping-limiting mechanisms in II-VI's are critically analyzed, in particular: self-compensation by spontaneous formation of native defects, amphoteric behavior of several potential dopants, lattice relaxation around some doping atoms, insufficient solubility of the others, and 'softness' of the lattice of the W-VI compounds. Zn the third part of the paper, various approaches to overcome doping difficulties have been analyzed, in particular growth and doping under non-equilibrium conditions (low-temperature growth/doping techniques, particularly MBE, MOVPE, MOCVD), doping by ion implantation, co-doping with more than one dopant, non-equilibrium doping using ultra-fast techniques, etc., as well as the reinforcement of crystal lattice by alloying with some IIA-VI compounds. The results of these efforts are overviewed, including the status of maximum p- and n-doping so far achieved in each of II-VI compounds. It is concluded that a much greater range of applications of the II-VI's, in accordance with their extraordinary properties and potential in many fields, can be expected in the foreseeable future.