Study on phase-changing molybdenum ditelluride for optoelectronic applications

Abstract

Reconfigurable photonic devices operating at visible and near infrared (NIR) regime are necessary in various applications such as memories, displays, sensors and integrated photonics. The essential parts of these devices are tunable structures or active photonic materials. Opto-mechanical devices undergo structural change and show high optical performances than that of electro-optical devices. However, even with advancements of nanotechnology, these devices suffer from bulky sizes and complex fabrication processes. Electro-optical materials are cost-effective, high-speed, integrable in a small-scaled device, but, the tunabililty is low due to small change. Germanium-antimony-tellurium (GST)-based phase-change materials show high reconfigurability due to large refractive change in broadband optical regime. However, these involve time-temperature transformation with high-power consumptions. Seeking for novel active photonic materials, molybdenum ditelluride (MoTe$_2$) can be switched from semiconductor to semi-metal and is considered to be next phase-changing platform due to its small-thickness, low-power consumptions and large optical change. However, molybdenum ditelluride is not yet fully understood in an optical manner and its phase-changing optoelectronic application is not demonstrated.We perform optical characterization of phase-changing molybdenum ditelluride with various methods. Raman spectroscopic measurement is used to fully investigate phononic modes of semiconductor (2H) and semi-metallic (1T’) phases. Temperature-resolved Raman inspection reveal 2H and 1T’ mixed phase and show phase hysteresis along various temperatures. Based on Raman inspection, we extract complex refractive indices of 2H- and 1T’-phases of MoTe$_2$. The reflectance of each phase is measured in various thicknesses. Through characterization matrix formalism, reflectance is fitted through parametrization of complex dielectric functions. The contrast of refractive index is distinct in visible regime when compared to synthesized GST material. Finally, we propose vertical metal-MoTe$_2$-metal memory cell with top electrode adopted as transparent electrode to observe electric-field induced optical change of MoTe$_2$due to phase-transition. The low-resistance state of switched MoTe$_2$show lower optical reflectance than that of high-resistance state. The complex refractive indices of electrically switched low- and high-resistance state are extracted with iterative method. The optical change is significant in visible regime, indicating MoTe$_2$as promising candidate as an active photonic material in various visible applications.