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Alloys of sulfur, selenium and tellurium, often referred to as chalcogenide semiconductors, offer a highly versatile, compositionally-controllable material platform for a variety of passive and active photonic applications. They are optically nonlinear, photoconductive materials with wide transmission windows that present various high- and low-index dielectric, low-epsilon and plasmonic properties across ultra-violet, visible and infrared frequencies, in addition to an, non-volatile, electrically/optically induced switching capability between phase states with markedly different electromagnetic properties. This roadmap collection presents an in-depth account of the critical role that chalcogenide semiconductors play within various traditional and emerging photonic technology platforms. The potential of this field going forward is demonstrated by presenting context and outlook on selected socio-economically important research streams utilizing chalcogenide semiconductors. To this end, this roadmap encompasses selected topics that range from systematic design of material properties and switching kinetics to device-level nanostructuring and integration within various photonic system architectures.

By confining phase transitions to the nanoscale interface between two different crystals, interfacial phase change memory heterostructures represent the state of the art for energy efficient data storage. We present the effect of strain engineering on the electrical switching performance of the –GeTe superlattice van der Waals devices. Multiple Ge atoms switching through a two-dimensional Te layer reduces the activation barrier for further atoms to switch; an effect that can be enhanced by biaxial strain. The out-of-plane phonon mode of the GeTe crystal remains active in the superlattice heterostructures. The large in-plane biaxial strain imposed by the layers on the GeTe layers substantially improves the switching speed, reset energy, and cyclability of the superlattice memory devices. Moreover, carefully controlling residual stress in the layers of –GeTe interfacial phase change memories provides a new degree of freedom to design the properties of functional superlattice structures for memory and photonics applications.

We have measured the temperature dependence of refractive index, thermal conductivity (K) and specific heat capacity (C) for Ag6.0In4.5Sb60.8Te28.7 (AIST). For measurement of refractive index, a spectroscopic ellipsometry was employed over a wavelength range from 300 to 1700 nm whilst the temperature was increased from room temperature to 450 °C. For measurement of K, we used a novel instrument called "a nanosecond thermoreflectance measurement system" developed by Baba et al. Using this system, it is possible to easily obtain the K of thin film samples which range in thickness from sub-micron down to 100 nm. We measured the Ks up to 400 °C; a clear temperature dependence was observed whereby Ks increased with increasing temperature. For measurement of C, we used a differential scanning calorimeter. The C of AIST also increase before the melting point (530 °C); after melting the C rapidly decreased with increasing temperature.

A rotating disc technique has been applied to measure the crystallization rate dependence of Sb8Te2 on bismuth. The laser amorphised state shows an order of magnitude increase in crystallization rate upon doping with 8 at. % bismuth. This is consistent with static measurements on as-deposited films. Ab-initio methods were used to relax a long period, Sb2Te3, stacking model. The X-ray diffraction pattern was calculated from the relaxed structure and found to be consistent with experimental data; the lattice parameters of the relaxed structures were a=4.22 A and c=32.80 A in the case of the undoped film, a=4.22–4.23 A and c=32.78–33.00 A for a Bi concentration of 5.5 at. %. The calculated bulk modulus was found to increase upon doping with Bi; suggesting an increase in the crystalline viscosity. The increased crystallization rate is therefore explained in terms of either a decrease in viscosity in the amorphous phase or a reduction in the surface tension of crystalline nuclei upon doping with Bi.

Activation energy is one of the basic parameters used to estimate the physical and chemical features of optical and electrical phase-change (PC) films. However, its origin has not been discussed well because of insufficient understanding of the amorphous structures. In this paper, we reveal the origin of the activation energy using a GeSbTe-superlattice model and ab-initio local density approximation (LDA) calculations. The simulated energy required for transition from amorphous to crystal formation in a 9-atomic system was 2.34 eV; This is in good agreement with experimentally reported values.