Low-temperature interface reactions in layered Au/Sb films: In situ investigation of the formation of an amorphous phase.

Abstract

Photoelectron-spectroscopy methods combined with electrical-resistance measurements were employed to study the effects of intermixing at Au/Sb interfaces at low temperatures. For the purpose of characterizing the growth processes of the intermixed phase on a ML scale, Au/Sb bilayers (layer thicknesses ${\mathit{D}}_{\mathrm{Au}}$=0.5--75 ML and ${\mathit{D}}_{\mathrm{Sb}}$=150 ML) were evaporated at 77 K and the different in situ techniques allowed a comparison to vapor-quenched amorphous ${\mathrm{Au}}_{\mathit{x}}$${\mathrm{Sb}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ alloys. For Au thicknesses between 0.5 and 0.9 ML, a change from a semiconducting to a metallic behavior of the samples has been detected, as indicated by the development of a steplike photoelectron intensity at the Fermi level. Evidence has been found that for Au coverages \ensuremath{\le} 6 ML chemical reactions at the Au/Sb interface occur, leading to the formation of a homogeneously intermixed amorphous layer with a maximum thickness of about 2.3 nm and Au concentrations as high as x\ensuremath{\approxeq}80 at. %. This latter value corresponds to the limiting Au content where amorphous alloys can be prepared at low temperature (0 at. % \ensuremath{\le}x\ensuremath{\le} 80 at. %). For nominal coverages beyond 6 ML polycrystalline Au films were formed. Consequently, Au/Sb multilayers with sufficiently small modulation lengths, which were prepared at 130 K by ion-beam sputtering, were observed to grow as a homogeneous amorphous phase over a broad range of compositions, as evidenced by in situ resistance measurements and by comparing the obtained crystallization temperatures to those of vapor-quenched amorphous alloys. Variation of the deposition temperature ${\mathit{T}}_{\mathit{s}}$ revealed that an amorphous interface layer is only formed for ${\mathit{T}}_{\mathit{s}}$\ensuremath{\le} 220 K. This is consistent with the fact that for multilayers with large modulation lengths containing unreacted polycrystalline Au and Sb layers, long-range interdiffusion is found to set in at temperatures above 230 K. This interdiffusion, however, results in the formation of polycrystalline Au-Sb alloys.