Fragile-to-Strong Crossover in Supercooled Liquid Ag-In-Sb-Te Studied by Ultrafast Calorimetry

Phase-change random-access memory relies on the reversible crystalline-glassy phase change in chalcogenide thin films. In this application, the speed of crystallization is critical for device performance: there is a need to combine ultrafast crystallization for switching at high temperature with high resistance to crystallization for non-volatile data retention near to room temperature. In phase-change media such as nucleation-dominated Ge2Sb2Te5, these conflicting requirements are met through the highly “fragile” nature of the temperature dependence of the viscosity of the supercooled liquid. The present study explores, using ultrafast-heating calorimetry, the equivalent temperature dependence for the growth-dominated medium Ag-In-Sb-Te. The crystallization shows (unexpectedly) Arrhenius temperature dependence over a wide intermediate temperature range. Here it is shown that this is evidence for a fragile-to-strong crossover on cooling the liquid. Such a crossover has many consequences for the interpretation and control of phase-change kinetics in chalcogenide media, helping to understand the distinction between nucleation- and growth-dominated crystallization, and offering a route to designing improved device performance.J.O., D.W.H. and A.L.G. acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC, UK), D.W.H. in part through the EPSRC Centre for Innovative Manufacturing in Photonics. J.O. and A.L.G. acknowledge support from the World Premier International Research Center Initiative (WPI), MEXT, Japan. C. A. Angell, L. Battezzati, G. Dalla Fontana and M. Salinga are thanked for helpful discussions.This is the author accepted manuscript. The final version is available from Wiley at http://onlinelibrary.wiley.com/doi/10.1002/adfm.201501607/abstract

n-type chalcogenides by ion implantation.

Carrier-type reversal to enable the formation of semiconductor p-n junctions is a prerequisite for many electronic applications. Chalcogenide glasses are p-type semiconductors and their applications have been limited by the extraordinary difficulty in obtaining n-type conductivity. The ability to form chalcogenide glass p-n junctions could improve the performance of phase-change memory and thermoelectric devices and allow the direct electronic control of nonlinear optical devices. Previously, carrier-type reversal has been restricted to the GeCh (Ch=S, Se, Te) family of glasses, with very high Bi or Pb 'doping' concentrations (~5-11 at.%), incorporated during high-temperature glass melting. Here we report the first n-type doping of chalcogenide glasses by ion implantation of Bi into GeTe and GaLaSO amorphous films, demonstrating rectification and photocurrent in a Bi-implanted GaLaSO device. The electrical doping effect of Bi is observed at a 100 times lower concentration than for Bi melt-doped GeCh glasses.This work was supported by the UK EPSRC grants EP/I018417/1, EP/I019065/1 and EP/I018050/1.This is the author accepted manuscript. The final version is available from NPG via http://dx.doi.org/10.1038/ncomms634

Spectroscopy of titanium-doped gallium lanthanum sulfide glass

Titanium-doped gallium lanthanum sulfide (Ti:GLS) and gallium lanthanum oxysulfide (Ti:GLSO) glasses have an absorption band at similar to 500-600 nm that cannot be fully resolved because of its proximity to the band edge of the glass. At concentrations >0.5% a shoulder at 980 nm is observed in Ti:GLS but not in Ti :GLSO. The emission spectra of Ti:GLS and T :GLSO both peak at 900 nm with lifetimes of 67 and 97)us, respectively. We propose that the absorption at similar to 600 nm is due to the T-2(2g) -> E-2(g) transition of octahedral Ti3+ and the 980 nm shoulder is due to Ti3+-Ti4+ pairs. (C) 2008 Optical Society of America

Concentration dependence of the fluorescence decay profile in transition metal doped chalcogenide glass

In this paper we present the fluorescence decay profiles of vanadium and titanium doped gallium lanthanum sulphide (GLS) glass at various doping concentrations between 0.01 and 1% (molar). We demonstrate that below a critical doping concentration the fluorescence decay profile can be fitted with the stretched exponential function: exp[-(t/τ)β, where τ is the fluorescence lifetime and β is the stretch factor. At low concentrations the lifetime for vanadium and titanium doped GLS was 30 μs and 67 μs respectively. We validate the use of the stretched exponential model and discuss the possible microscopic phenomenon it arises from. We also demonstrate that above a critical doping concentration of around 0.1% (molar) the fluorescence decay profile can be fitted with the double exponential function: a*exp-(t/τ )+ b*exp-(t/τ ), where τ and τ are characteristic fast and slow components of the fluorescence decay profile, for vanadium the fast and slow components are 5 us and 30 μs respectively and for titanium they are 15 μs and 67 μs respectively. We also show that the fluorescence lifetime of vanadium and titanium at low concentrations in the oxide rich host gallium lanthanum oxy-sulphide (GLSO) is 43 μs and 97 μs respectively, which is longer than that in GLS. From this we deduce that vanadium and titanium fluorescing ions preferentially substitute into high efficiency oxide sites until at a critical concentration they become saturated and low efficiency sulphide sites start to be filled

Inverted deposition and high-velocity spinning to develop buried planar chalcogenide glass waveguides for highly nonlinear integrated optics

We report on buried planar waveguides in a highly nonlinear infrared transmitting chalcogenide glass, fabricated using a combination of inverted deposition of the molten glass phase and high-velocity spinning. Films of gallium lanthanum sulphide (Ga:La:S) glass were deposited onto an expansion coefficient matched Ga:La:S cladding substrate. These amorphous films, with an optimized composition designed to be resistant against crystallization, were observed to have an excellent interface quality and uniformity. The designed planar chip had a buried core, 6 μm thick in the vertical direction, in single-mode operation at 1.064 μm and a measured propagation loss of <0.2 dB cm-1. Through this technique waveguides from Ga:La:S glass, a highly versatile optical semiconductor material, can potentially be used in nonlinear applications as well as provide passive and active integrated optic functionality into the infrared beyond 5 μm. © 2005 American Institute of Physics

Social interactions and the contemporaneous determinants of individuals’ weight

Obesity and overweight are central issues in the public health debate in most developed countries. In this debate, some of the socio-economic determinants of obesity and overweight are still relatively unexplored. This paper presents an empirical examination of the possible influence of social interactions on contemporaneous obesity and (over)underweight. A joint estimation model for obesity and self-image is applied to a sample for Spain taken from the European Union household panel for 1998. The results suggest that obesity might be in part a social phenomenon connected to individuals' social life.

Temperature dependent lifetime of Dy : 1.3 μm emission in Ge-As-S glass containing very small amount of Ga and CsBr

The lifetime of 1.3 μm emission from Dy -doped Ge-As-S glass containing very small amount of Ga and CsBr reveals interesting temperature dependence; with the addition of Ga/CsBr up to ∼0.2 mol% the lifetime decreases with increasing temperature. When the concentration of Ga/CsBr reaches ∼0.3 mol%, however, the lifetime remains relatively constant regardless of temperature change. For further addition up to ∼0.5 mol%, the lifetime increases as temperature rises. This uncommon behavior is explained through our understanding of the different temperature dependences of multiphonon relaxation and spontaneous emission rates associated with the thermally coupled Dy : ( H , F ) manifolds in our modified chalcogenide glass. © 2012 Elsevier B.V. All rights reserved

Determination of the oxidation state and coordination of a vanadium doped chalcogenide glass

Vanadium doped chalcogenide glass has potential as an active gain medium, particularly at telecommunications wavelengths. This dopant has three spin allowed absorption transitions at 1100, 737 and 578 nm, and a spin forbidden absorption transition at 1000 nm. X-ray photoelectron spectroscopy indicated the presence of vanadium in a range of oxidation states from V+ to V5+. Excitation of each absorption band resulted in the same characteristic emission spectrum and lifetime, indicating that only one oxidation state is optically active. Arguments based on Tanabe–Sugano analysis indicated that the configuration of the optically active vanadium ion was octahedral V2+. The calculated crystal field parameters (Dq/B, B and C/B) were 1.85, 485.1 and 4.55, respectively