Intermediate Phase and Giant Photocontraction in Obliquely Deposited GexSe1-x Thin Films
Abstract (Summary)Obliquely deposited amorphous GexSe100-x thin-films at several compositions in the 15% < x < 33.3% range, and at several obliqueness angles in the 0 < ? < 80° range were evaporated on Si and glass substrates. Here ? 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 SEM confirm the columnar structure of obliquely (? = 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 photo-contraction (PC) of oblique films examined. Compositional trend of PC exhibit a bell shaped curve with a rather large effect ( 25%) centered in the 20 % < x < 25% range, the Intermediate Phase (IP) with the PC decreasing as 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 ?m range and a lateral width in the 50 nm range. We observe a blue shift (up to 0.38 eV) in the optical bandgap of oblique films (? = 80°) in relation to normally deposited (? = 0°) ones, a result we identify with carrier confinement in nano-filaments (< 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 nano-filaments (~Ge25Se75) that demix from a Ge-rich (~Ge40Se60) phase that fills the inter-columnar space. Loss of PC in such films is traced to the growth of the Ge-rich phase, which is stressed and photo-inactive. 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 nano-filaments. 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 photo-melting 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 near bandgap radiation.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication:01/01/2008