Optical Properties of Innovative GeSe_1-xTe_x Phase-Change Material Thin Films for On-Chip Active Components, Non-Linear and Brain-Like Computing Applications

In this work, the linear and nonlinear optical properties of innovative GeSe<i>_1−x</i>Te<i>_x</i> thin films in amorphous as-deposited state as well as after crystallization by annealing are studied. These alloys, obtained by industrial magnetron co-sputtering of GeSe and GeTe targets, belong to the GeSe-GeTe pseudo-binary line lying between the covalent GeSe compound and the “metavalently” bonded GeTe phase-change material (PCM). They are considered as very promising candidates for high temperature non-volatile resistive memory [1], emerging all-optical neuromorphic circuits and IR photonic applications. In fact, they exhibit fast and reversible phase transformations between amorphous and crystalline states with unprecedented large contrast of electronic and optical properties, a very high thermal stability of the amorphous phase compared to other PCMs [1] as well as a high transparency in the NIR-MIR range in both states. By modifying the Te content of the GeSe<i>_1−x</i>Te<i>_x</i> thin films, one can tailor their linear and non-linear optical properties for a wide range of innovative optical and photonic applications.<br/>To compare candidate PCMs and evaluate the impact of their optical losses on the performances of optical switch applications, a figure-of-merit was introduced as <i>FOM=Δn/Δk</i>, where <i>Δn=n_cr-n_am</i> and <i>Δk=k_cr-k_am</i>, with <i>n</i> and <i>k</i> refractive index and extinction coefficient of the amorphous (am-) and crystalline (cr-) phase, respectively. Ideally, PCMs with large <i>Δn</i> and small <i>Δk</i> are desired. In this work, the best <i>FOM</i> at 1.55 μm wavelength is found for GeSe_0.4Te_0.6, for which <i>FOM </i>= 13, almost 6 times higher than that of GST-225 reference material with a low <i>FOM</i> of 2.3 at 1.55 μm. Moreover, all the studied compositions showed very high <i>FOM</i> values compared to the most promising composition previously proposed in the literature (Ge₂Sb₂Se₄Te₁ alloy with a <i>FOM</i> value of 4.2 [2]). Besides, the high transparency window in the IR, coupled with large optical non-linearities observed in chalcogenide glasses, also offers opportunities for the development of innovative near- and mid-infrared (NIR-MIR) components such as super-continuum (SC) laser sources, optical sensors for gas spectroscopy, IR microlens arrays and all-optical integrated circuits. The non-linear refractive indices <i>n₂</i> of the am-films were obtained by using the Sheik-Bahae model. The latter is an analytical approach that estimates the <i>n₂</i> indices using the values of the linear refractive index <i>n</i> and the optical band gap energy <i>E_g⁰⁴</i>. Outstanding <i>n₂</i> values are obtained for all GeSe<i>_1-x</i>Te<i>_x</i> am-films in the NIR-MIR range. These values are two to three orders of magnitude higher than that found in SiN<i>_x</i>thin films, considered nowadays as the reference material for on-chip non-linear NIR photonic applications.<br/>To summarize, the study of the optical properties of GeSe<i>_1-x</i>Te<i>_x</i> thin films shows that these new exciting PCM compounds offer promising <i>FOM</i> (<i>Δn/Δk</i>) for purpose of optical switch applications requiring high refractive index contrast upon crystallization with limited optical losses, in particular at 1.55 µm. Besides, the amorphous films exhibit a good thermal stability up to crystallization occurring above 300 °C [1], as well as high non-linear refractive index offering promising opportunities for on chip non-linear NIR-MIR devices.<br/><br/>[1] M. Tomelleri, F. Hippert, T. Farjot, C. Licitra, N. Vaxelaire, J.-B. Dory, D. Benoit, V. Giordano, and P. Noé, Physica Status Solidi (RRL) – Rapid Research Letters <b>15</b>, 2000451 (2021).<br/>[2] Y. Zhang, J.B. Chou, J. Li, H. Li, Q. Du, A. Yadav, S. Zhou, M.Y. Shalaginov, Z. Fang, H. Zhong, C. Roberts, P. Robinson, B. Bohlin, C. Ríos, H. Lin, M. Kang, T. Gu, J. Warner, V. Liberman, K. Richardson, and J. Hu, Nature Communications <b>10</b>, 1 (2019).