High Electric Fields in II-VI Materials.
Conventional electrophotographic processes operate with blocking contacts. This limits the available gain at low fields: one electron discharged per absorbed photon, at the most, during the photoconductive step of the process. A straightforward way to increase the gain is to increase the applied field until carriers multiply by impact ionization during their transit through the material. This requires high electric fields of order 10(6) V/cm, and in such high fields there is inevitably an increase in the dark current. In this competition between dark current and multiplication the irreducible source of increased dark current is Zener tunneling of carriers from valence band to conduction band. Examination of the form of current-voltage relations for tunneling and impact ionization shows that specimen dimensions have an important effect in shifting the balance in favor of multiplication. The multiplication properties have recently been studied for three II-VI materials with high band-gaps: ZnSe, CdS, and ZnO. High fields are produced with electrolyte blocking contacts to n type conducting crystals. Multiplication is seen in fields around 10(6) V/cm in ZnSe and ZnO, but not in CdS where the ultimate breakdown is by tunneling. Since the band gaps and low field mobilities are almost the same for ZnSe and CdS, this difference indicates that rather subtle features can be decisive in determining whether or not impact ionization takes place. Measurements have been made of charging and discharging of the free surface on ZnO single crystals. High fields can be achieved under these conditions, and there is tentative evidence that electron multiplication can contribute to the discharge.
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