Thickness dependence of structure in thin films of low-temperature silver selenide

Abstract

The growth and structure of thin films of low-temperature (LT) silver selenide (Ag 2Se) 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 Ag 2Se crystals is initially found to be monoclinic, pseudo-tetragonal, while it turns to the known orthorhombic LT phase at higher thicknesses (⩾ 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, β = 90°) and orthorhombic ( O, a = 0.433, b = 0.706, c = 0.776 nm) LT phases were determined: [100] C‖[001] M and [110] M, [110] C‖[111] M and [100] M; [20 1 ] M‖[100] O, [010] M‖[010] O and [101] M‖[001] O; [ 2 22] C‖[100] O, [110] C ‖[010] O and [1 1 2] C‖[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.