Crystal growth and orientation in sputtered films of bismuth telluride

Abstract

Abstract Films in the thickness range from 300 Å to 3 μ have been prepared by sputtering from a cathode of composition Bi2Te3 in an atmosphere of pure argon. At low substrate temperatures the stoichiometric Bi2Te3 composition is retained in the sputtered deposits, but with increasing temperature, above 250°c, progressive loss of Te occurs, until at 420°c the film composition corresponds approximately to BiTe. The Te-deficient film structures are identified as metastable β-type phases which are of the same structure type as Bi2Te3. On amorphous substrates fibre textures are developed which change from a {1015} to a {0001} type as the temperature is raised. The corresponding single-crystal orientations are obtained by epitaxy on the (100) and (111) planes respectively of NaCl crystals. The texture changes are accompanied by striking changes in the crystal habit. Hexagonal platelets appear at low temperatures and very high temperatures, while well-developed octahedra are observed at intermediate temperatures. These effects are interpreted by referring the structure to a reduced pseudo-cubic unit cell which assumes a random distribution of Bi and Te atoms on a cubic close-packed lattice. In terms of this unit, the low- and high-temperature textures are identified respectively as {100} and {111} nucleation orientations. The textures occurring at intermediate temperatures are attributed to growth orientations.