Harnessing Structural Transitions in SnTe-GeTe (SGT) Alloys for Photonics

The chalcogenide semiconductor alloy system of SnTe-GeTe (SGT) offers a combination of properties that could be attractive for applications in phase change materials, thermoelectrics, and infrared optoelectronics [1,2]. Since SnTe is always Sn-deficient and GeTe is always Ge-deficient, both semiconductors are degenerately p-typed doped above 10²⁰cm^-3 [3,4]. Such a high concentration of holes leads to high free carrier reflection in the mid-infrared wavelengths, making them promising candidates for plasmonic mid-infrared materials [5]. SnTe crystalizes in a rocksalt crystal structure at 300K and undergoes a cubic to a distorted cubic (rhombohedral) structural transition at cryogenic temperatures (T_c&lt; 90K). GeTe, with a rhombohedral structure at room temperature, undergoes a structural transition to cubic through distortion along the [111] direction at T_c &gt; 700K. Because of this distortion, the rhombohedral and cubic phases are predicted to have a large difference in bandgap and reflectivity [6]. Synthesizing high-quality SGT thin films would open routes to engineer the cubic-rhombohedral ferroelectric transitions closer to room temperature, making SGT more practical for moderate-temperature thermoelectrics and infrared optoelectronics. Since the dielectric constants of materials depend on the crystal structure, the presence of a temperature-controlled structural transition in SGT offers an avenue to modulate optical properties. We aim to harness these unique properties towards integrated tunable epitaxial mirrors with applications in imaging and sensing.<br/><br/>We leverage the complete solid solution of SnTe-GeTe at high temperatures to synthesize SGT films. We deposit first a seed layer of SnTe and then a GeTe film on a Ge(001) substrate via sputtering. We intermix layers using rapid thermal annealing (RTA) at 500°C for 1 minute. To prevent materials evaporation at elevated temperatures, we use a ~40 nm SiO₂ capping layer. X-ray diffraction (XRD) measurements show that GeTe and SnTe are completely intermixed after RTA, and films are (001) out-of-plane single oriented. The SGT alloy composition is varied by changing the thickness ratio of SnTe and GeTe.<br/><br/>We show the composition-dependent structure of Sn_1-xGe_xTe thin films at room temperature is in good agreement with the bulk phase diagram. On the Sn-rich side, Sn_1-xGe_xTe adopts the rocksalt cubic structure. At higher Ge content (x&gt;0.35), SGT transitions to the rhombohedral structure. As the Ge composition of SGT films increases, we see a trend in maximum reflectance which suggests that alloy composition affects the dielectric constants. We plan to also discuss further insights into the impact of the ferroelectric distortions on the dielectric constants of our SGT films. Overall, our work lays the foundation for harnessing the cubic-rhombohedral structural transition in SGT thin films for active mid-infrared optoelectronic devices.<br/><br/>References:<br/>[1] D. Kumaar et al. … M. Yarema. <i>ACS Nano</i>, <b>17</b>(7), (2023).<br/>[2] A. Banik et al. … K. Biswas, <i>Energy Environ. </i><i>Sci.</i>, <b>12</b>, (2019).<br/>[3] H. R. Riedl et al. … R. B. Schoolar. <i>Phys. Rev.</i>, <b>162</b>(3), (1967).<br/>[4] R. Tsu et al. … L. Esaki. Phys. Rev., <b>172</b>(3), (1968).<br/>[5] T. Nguyen et al. …K. Mukherjee. <i>ACS Appl. Electron. Mater.</i>, <b>6</b>, (2024).<br/>[6] D. J. Singh. <i>J. Appl. Phys.</i>, <b>113</b>, (2013).