Electron Traps and Deep Levels in Cadmium Selenide

Abstract

Electron traps and deep levels in cadmium selenide are investigated using the techniques of photocapacitance, infrared quenching of photocapacitance, deep level transient spectroscopy (DLTS), and optical DLTS. The Cdse crystals used are grown from the vapour phase in sealed capsules and have resistivities of the order of 0.01 Ω cm. In order to prepare successful Schottky diodes for examination with the techniques mentioned, it is necessary to increase the resistivity of the Cdse to the range 1 to 10 Ω cm. This is done by annealing the Cdse in selenium vapour at 550 °C, either for three or thirty days. Three principal acceptor levels at 0.15, 0.58, and 0.71 eV are observed in material annealed for three days. However, only one main acceptor at 0.64 eV is detected in material annealed for one month. The hole capture cross‐section of this defect is large, ≈︁ 10−14 cm2, and the indications are that the defect is the sensitising centre for photoconductivity in Cdse. Photocapacitance measurements reveal one donor level 0.11 to 0.12 eV below the conduction band. The DLTS technique also suggests that there is only one electron trap in this region of the forbidden gap, but the slope of the Arrhenius plot leads to an activation energy of 0.22 eV. It is probable that the donor and trapping level revealed by the two techniques are one and the same, and that the discrepancy between the ionisation energy of 0.11 to 0.12 eV and the activation energy of 0.22 eV can be accounted for by a thermally activated capture cross‐section of the donor (trap) defect.

Effect of oxygen on the properties of encapsulated polycrystalline CdSe films

“…[19][20][21] Yodogawa 19 ascribed the center at 0.65 eV to Se vacancies, Robinson 20 and Bube reported that the level at 1.1 eV was a defect involving native acceptor defects such as Cd vacancies while Ture 21 interpreted the level at 1.3 eV also as an acceptor level. As with the 0.1 eV level, observed in the resistivity measurements, these levels could also be attributed to impurities substituting on the lattice sites.…”