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Method of doping and purification of II-VI crystals

№3 The department of photoelectric phenomena
M.K. Sheinkman, I.V. Markevich, N.O. Korsunska, L.Yu. Khomenkova, L.V. Borkovska

The method can be used in fabrication of photodetectors and light emitting devices. The method is based on the drift of impurities in electric field. For this purpose, direct electric field is applied to the electrodes that are put on crystal ends. Under electric field, negatively charged impurity ions drift to the anode and positively charged ones do to the cathode, which results in purification of crystal bulk. To dope the crystal, required impurity is introduced in one of the electrodes and then drawn in the crystal under electric field of suitable direction. As a rule, doping of II-VI compounds is carried out by diffusion from the surface or by ion implantation. The general shortcoming of these techniques is large density of unsteady intrinsic defects that are created due to high temperature of the doping or due to treatment by high-energy ions. The advantage of the proposed method is considerable acceleration of doping and purification processes due to application of electric field, which allows essential lowering of process temperature and avoidance of intrinsic defect creation. The method allows also the determination of diffusion parameters of impurities in different crystallographic directions. The method was verified on highly resistive CdS crystals that were doped with copper and silver as well as were purified from these impurities. It was found that copper diffusion perpendicular to crystal c-axis was essentially faster than that parallel to the c-axis, while for silver diffusion the reverse relation took place.

Doping and extraction of impurities were carried out under electric field Ed = 50=100 V/cm at temperature T_d = 300-400 °C and took several minutes Δt_d.

а)	b)
c)	d)

The doping of CdS crystal with copper (a,b) and extraction of silver (c,d) perpendicular (a,c) and parallel (b, d) to c-axis. a,b: T_d=620 K, E_d=70 В/см, Δt_d=10 min (a) и Δt_d=40 min (b); c, d: T_d=670 K, E_d=300 В/см, Δt_d=90 min (c) и Δt_d=25 min (d). The doped part of crystal  is dark.

The novelty of proposed method is the use of impurity drift for II-VI crystals purification as well as the choice of sutable crystallographic direction of electric field application to accelerate doping and purification processes.