Emerging Glassy and Phase-Change Chalcogenide Thin Films with Unconventional Bonding Mechanism for Adaptive Photonic Devices

Chalcogenide materials have attracted much attention over the years due to their wide range of applications. Among them, some compounds such as Ge-Sb-Te based alloys exhibit a unique portfolio of properties, which has led to their wide use for non-volatile memory applications such as optical data storage or more recently resistive phase-change memory [1,2]. In addition to a high IR transparency window and high optical nonlinearities [3], some chalcogenide glasses such as Se-based compounds exhibit an unusual conductivity behavior under high electric field, called ovonic threshold switching (OTS) effect [4]. Recently, we showed that some As-free GeSb_wS_xSe_yTe_z chalcogenide compounds exhibit outstanding reconfigurable linear and nonlinear properties when deposited in thin films. All films were obtained by industrial magnetron co-sputtering deposition.<br/>First, the linear and non-linear optical properties of innovative GeSe_1-xTe_x thin films in the amorphous phase as-deposited as well as after annealing crystallization are studied. These alloys belong to the GeSe-GeTe pseudo-binary line located between the covalent GeSe compound and the "metavalent" GeTe phase-change material (PCM). They are considered very promising candidates for high temperature non-volatile resistive memories [5], emerging all-optical neuromorphic circuits and IR photonic applications. These new PCM compounds offer promising properties for optical switch applications requiring high refractive index contrast upon crystallization with limited optical losses, especially at 1.55 µm. In addition, the amorphous films exhibit good thermal stability up to crystallization occurring above 300 °C [5], as well as a high nonlinear refractive index offering promising opportunities for nonlinear NIR-MIR on-chip devices. Second, exceptional <i>n</i>₂ values are obtained for all as-deposited GeSb_wS_xSe_yTe_z films in the NIR-MIR range. These values are 2 to 3 orders of magnitude higher than those found in SiN_x thin films, which are considered today as the reference material for on-chip nonlinear NIR photonic applications. A dramatic change in the Kerr index as a function of the composition of the GeSb_wS_xSe_yTe_z chalcogenide alloys is also observed and related to changes in the amorphous structure. The origin of the very high local electronic polarizability, related to highly polarizable structural motifs in the amorphous lattice, can be linked to the enhanced optical nonlinearities of the material [6]. These highly polarizable patterns are reminiscent of "metavalent" bonding, the unique bonding mechanism that causes the huge optical and electrical contrast between the amorphous and crystalline states of phase change materials.<br/>In summary, some innovative As-free GeSb_wS_xSe_yTe_z chalcogenide compounds exhibit outstanding linear and nonlinear optical properties that are behind their promising potential for reconfigurable and on-chip NIR-MIR applications. This results from the formation of particular structural patterns leading to a specific bonding mechanism that gives rise to their huge polarizability. These results pave the way to control and enhance the unique optical properties of chalcogenides in thin films.<br/><br/>1. P. Noé <i>et al.</i>, <i>Semicond. </i><i>Sci. Technol.</i> <b>33</b>, 013002, (2018).<br/>2. P. Noé and F. Hippert in <i>Phase Change Memory: Device Physics, Reliability and Applications</i>, A. Redaelli, Éd. Cham: Springer International Publishing, 2018, p. 125-179.<br/>3. J.-B. Dory <i>et al.</i>, <i>Scientific Reports</i> <b>10</b>, 11894, (2020).<br/>4. P. Noé <i>et al.</i>, <i>Science Advances</i> <b>6</b>, eaay2830 (2020).<br/>5. M. Tomelleri <i>et al.</i>,<i> Physica Status Solidi (RRL) – Rapid Research Letters</i> <b>15</b>, 2000451 (2021).<br/>6. J.-B. Dory <i>et al.</i>, “Microscopic origin of the uncommon optical nonlinearities of chalcogenide glasses in thin films toward on-chip highly nonlinear photonic devices”, submitted (2023).