Electrical properties of some thin-film semiconductor alloys

The Müller technique of flash evaporating powdered compound semiconductors was used to form polycrystalline thin-film solid solutions of CdSe-CdS and CdSe-CdTe onto heated glass substrates. The composition of each alloy was varied from 10 to 90 percent of one compound by weight. X-ray diffractometer was used to determine their crystalline structures and lattice parameters. The results show that all CdSe-CdS alloys have a hexagonal structure regardless of their composition and that CdSe-CdTe alloys have both cubic and hexagonal structures depending on the composition of the constituent compounds. The lattice parameters were found to vary linearly with the composition for each alloy. Electrical properties of these thin-film alloys in terms of Hall mobility and carrier concentration were determined at room temperature as a function of substrate temperature which ranged from 50°C to 200°C. The results indicate that the film properties depend heavily on the preparation conditions, such as evaporation rate, vacuum pressure, and substrate temperature. The latter factor appeared to have dominated their properties to a great extent. Films prepared at higher substrate temperatures tended to have both low carrier mobility and concentration. All films, however, show an n-type conduction regardless of their composition and preparation conditions. Measurements of dark current versus temperature were also carried out in nitrogen atmosphere. From these measurements, the imperfection ionization energies of films prepared at various substrate temperatures were determined.