Atypical phase-change alloy Ga2Te3: atomic structure, incipient nanotectonic nuclei, multilevel writing

Emerging brain-inspired computing, including artificial optical synapses, photonic tensor cores, neuromorphic networks, etc., needs phase-change materials (PCMs) of the next generation with lower energy consumption and wider temperature range for reliable long-term operation. Gallium tellurides with higher melting and crystallization temperatures appear to be promising candidates enable to achieve the necessary requirements. Using high energy X-ray diffraction and Raman spectroscopy supported by first-principles simulations, we show that vitreous g-Ga2Te3 films essentially have a tetrahedral local structure and the sp3 hybridization, similar to those in the stable fcc Ga2Te3 polymorph and in opposite to a vast majority of typical PCMs. Nevertheless, optical pump-probe laser experiments revealed high-contrast, fast and reversible multilevel SET-RESET transitions raising a question of the phase change mechanism. A recently observed nanotectonic compression in bulk glassy Ga-Te alloys seems to be responsible for the PCM performance. Incipient nanotectonic nuclei, reminiscent of monoclinic high-pressure HP-Te II and rhombohedral HP-Ga2Te3, are present as minorities (2-4%) in g-Ga2Te3 but suggested to grow dramatically with increasing temperature while interacting with appropriate laser pulses. This leads to a co-crystallization of HP-polymorphs amplified by high internal local pressure reaching 4-8 GPa. The metallic HP-forms provide an increasing optical and electrical contrast, favorable for reliable PCM operations, and higher energy efficiency.