Kyslyuk V. Electrodiffusion of ions as a method to operate concentration of shallow donors in cadmium sulfide

II-VI compounds (where CdS belongs) are used in optoelectronics and photovoltaics as UV- and visible light photodetectors, lasers and solar cells due to appropriate values of the energy gap. However, such their application runs across two principal restrictions: i) instability of their parameters, which is attributed to defect reactions, the key reagents being mobile atoms; ii) most of attempts to grow p-type CdS, CdSe etc. have been unsuccessful because the incorporated acceptor atoms were compensated, lacked solubility in the host matrix, or formed deep levels. These restrictions are attributed to unique properties of CdS, CdSe etc. (grown from gaseous phase): i)they contain over-stoichiometric atoms of metalloid (e.g. Cd atoms in CdS); ii)these atoms are shallow donors; iii)these donors are ionized at moderate temperatures (>350K), with ionization energy being 0.03 eV; vi)the ions are rather mobile at such temperatures. The ion electrodiffusion (ion drift under electric field) has been found to dis tribute mobile ions in such a way that near anode region was depleted with them. This region is a "swept" part of the crystal. It can be cut off and utilized with the following advantages as compared with initial crystal: 1. higher stability of parameters; 2. higher content of vacancies, which makes it possible to incorporate impurities (Li, N or other more appropriate) substitutionally to form shallow acceptors. The work is devoted to theoretical and experimental research on distribution of shallow donors of interstitial cadmium in CdS under electric field. Theoretical models considered for stationary conditions (diffusion and drift fluxes of ions are equalized) at various ratios of mobile and immobile charges in the bulk demonstrate: i)at low values of external voltage (near several kT/e) the distribution of ions is linear; ii)the most effective "sweeping" takes place at the situation when, among immobile centers, positively charged centers prevail. Illuminating compensated crystals which contain immobile c enters (both acceptors and donors) liberates electrons bounded on the acceptors and, thus, switches the system into the state with immobile positive charge. The main obstacle to apply the electrodiffusion for ion distribution in compensated crystals is a reduction of conductivity at the near anode region, which gives rise to tunnel emission of holes from anode. To reduce the tunnel emission we illuminated near anode region, which allowed us to get 0.5 cm region "swept" of mobile atoms in 2 cm sample. Transient process of the electrodiffusion has been described analytically for low values of voltage. The non-stationary distribution of mobile ion concentration can be presented as two movable fronts which devide the bulk into three regions: two near electrode regions with stationary distribution and central region with initial concentration of the ions. These fronts move to meet each other. Analytical time-dependence of voltage U(t) at the constant value of current through the sample contain diffusivity of ion s Dp as parameter. Key words: interstitial cadmium, CdS, electrodiffusion, concentration profile, photosensitivity, diffusivity.