Generation of metastable shallow donors induced by cooling in hexagonal II-VI semiconductors

A highly conductive layer is found to arise on a (0001) plane of CdS, CdSe and CdSSe crystals under cooling in darkness from 300 to 77 K. This effect is shown to be caused by the increase of free electron density in a thin (500-100 AA) subsurface layer. At 77 K, the free electron density in this layer reaches approximately 10¹⁸ cm^-3. Detailed investigations of the dark conductivity, photoconductivity, Hall effect and bound exciton luminescence lead to the conclusion that the increase of conductivity is due to creation of metastable donors. This effect is supposed to be stimulated by both a pyroelectric field and mechanical stresses arising under cooling.