Origin of giant photocontraction in obliquely deposited amorphous Ge_xSe_1-x thin films and the intermediate phase

Abstract

Obliquely deposited amorphous Ge_xSe_100-x thin films at several compositions in the 15%<x<33.3% range and at several obliqueness angles in the 0<α<80° range at each x were evaporated on Si and glass substrates. Here α designates the angle between film normal and direction of vapor transport. Raman scattering, IR reflectance, and optical absorption measurements were undertaken to characterize the vibrational density of states and optical band gaps. Edge views of films in scanning electron microscopy (SEM) confirm the columnar structure of obliquely (α=80°) deposited films. Films, mounted in a cold stage flushed with N₂ gas, were irradiated to UV radiation from a Hg-Xe arc lamp, and photocontraction (PC) of oblique films were examined. Compositional trend of PC exhibit a bell-shaped curve with a rather large effect (0.25 μm) centered in the 20%<x<25% range, the intermediate phase (IP) with the PC decreasing at x>25%, the stressed rigid phase, and at x<20%, the flexible phase. IR reflectance confirmed absence of photo-oxidation of films under these conditions. The IP represents a range of compositions across which stress-free networks form. Columns observed in SEM reveal a high aspect ratio, with typical lengths in the 1-2-μm range and a lateral width in the 50-nm range. We observe a blueshift (up to 0.38 eV) in the optical band gap of oblique films (a=80°) in relation to normally deposited (α=0°) ones, a result we identify with carrier confinement in nanofilaments (<10 nm) that form part of columns observed in SEM. In the IP, the large PC results due to the intrinsically stress-free character of filaments, which undergo facile photomelting resulting in film densification. Ge-rich films (25%<x<33.3%) are intrinsically nanoscale phase separated and consist of nanofilaments (∼Ge₂₅Se₇₅) that demix from a Ge-rich (∼Ge₄₀Se₆₀) phase that fills the intercolumnar space. Loss of PC in such films is traced to the growth of the Ge-rich phase, which is intrinsically stressed and photoinactive. In contrast, Se-rich films are homogeneous, and loss of PC as x decreases below 20% is traced to the accumulation of network stress in the Se-rich nanofilaments. The microscopic origin of the giant PC effect in amorphous semiconducting thin films can be traced, in general, to three conditions being met: (i) growth of a columnar structure leading to porous films, (ii) formation of columns that are rigid but intrinsically stress free, and (iii) an appropriate flux of pair-producing radiation leading to photomelting of columns. These findings lead us to predict that PC will, in general, be maximized in obliquely deposited films of semiconducting networks glasses residing in their IP when irradiated with supra-band-gap radiation.