THICKNESS DEPENDENCE OF STRUCTURE IN THIN-FILMS OF LOW-TEMPERATURE SILVER SELENIDE

The growth and structure of thin films of low-temperature (LT) silver selenide (Ag2Se) were studied by HREM and electron diffraction. Samples were produced by the reaction of vacuum-deposited epitaxial silver films (1-30 nm thick) on NaCl substrates with equivalent amounts of selenium vapour. The structure of thin Ag2Se crystals is initially found to be monoclinic, pseudo-tetragonal, while it turns to the known orthorhombic LT phase at higher thicknesses (greater-than-or-equal-to 20 nm). It is noteworthy that the difference of phases observed is only an effect of crystal thickness. Comparisons were made between such thin crystals with those grown on continuous 30 nm thick single-crystalline silver layers. A reversible phase transition between the orthorhombic LT and cubic high-temperature (HT) phases could be induced by changing the electron beam intensity during TEM investigation (75 kV). The following orientation relationships between the body-centred cubic (C, a = 0.498 nm) HT phase, the monoclinic, pseudo-tetragonal (M, a = b = 0.706 nm, c = 0.498 nm, beta = 90-degrees) and orthorhombic (O, a = 0.433, b = 0.706, c = 0.776 nm) LT phases were determined: [100]C parallel-to [110]M and [110]M, [110]C parallel-to [111BAR]M and [100]M; [201BAR]M parallel-to [100]O, [010]M parallel-to [010]O and [101]M parallel-to [001]O; [222BAR]C parallel-to[100]O, [110]C parallel-to [010]O and [112BAR]C parallel-to [001]O. No triclinic or monoclinic (different from the determined pseudo-tetragonal) phases could be observed, although numerous references (listed in this paper) about such phases exist.