Origin of giant photocontraction in obliquely deposited amorphousGexSe1−xthin films and the intermediate phase

Abstract

Obliquely deposited amorphous Ge x Se 100−x thin films at several compositions in the 15% Ͻ x Ͻ 33.3% range and at several obliqueness angles in the 0 Ͻ ␣ Ͻ 80°range at each x 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 scanning electron microscopy ͑SEM͒ confirm the columnar structure of obliquely ͑␣ = 80°͒ deposited films. Films, mounted in a cold stage flushed with N 2 gas, were irradiated to UV radiation from a Hg-Xe arc lamp, and photocontraction ͑PC͒ of oblique films were examined. Compositional trend of PC exhibit a bell-shaped curve with a rather large effect ͑0.25 m͒ centered in the 20% Ͻ x Ͻ 25% range, the intermediate phase ͑IP͒ with the PC decreasing at x Ͼ 25%, the stressed rigid phase, and at x Ͻ 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 blueshift ͑up to 0.38 eV͒ in the optical band gap of oblique films ͑␣ = 80°͒ in relation to normally deposited ͑␣ =0°͒ ones, a result we identify with carrier confinement in nanofilaments ͑Ͻ10 nm͒ 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% Ͻ x Ͻ 33.3%͒ are intrinsically nanoscale phase separated and consist of nanofilaments ͑ϳGe 25 Se 75 ͒ that demix from a Ge-rich ͑ϳGe 40 Se 60 ͒ 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: ͑i͒ growth of a columnar structure leading to porous films, ͑ii͒ formation of columns that are rigid but intrinsically stress free, and ͑iii͒ 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.

Raman scattering as a probe of intermediate phases in glassy networks

“…8. These features uniquely fix the stoichiometry 57 of the Se-rich phase to close to the centroid of the reversibility window (Fig. 6).…”

Melt Homogenization and Self‐Organization in Chalcogenides‐Part I

“…The exciting radiation of 1.16 eV used in FT‐Raman is near the middle of the optical gap of Ge x Se 100‐ x glasses, and is transparent to melts profiled in the experiments. However, because of the use of a notch filter to suppress the laser line, the low frequency (<100/cm) vibrational densities of states (VDOS) are inaccessible in FT‐Raman experiments.…”

Monitoring structural phase transition of (Ge2S3)x(As2S3)1−x chalcogenide glass with Raman spectroscopy

“…Recent research of similar systems [10][11][12][13] showed that properties of glasses with Z around 2.67 can be explained with nanoscale phase separation effects. On the other hand, ternary As-Ge-S system, because of a mix of As centered pyramids and Ge centered tetrahedra, promotes chemical disorder and prevents nanoscale phase separation [13] making that systems suitable for investigation of rigidity percolation effects.…”