Structural Evolution of Millisecond Laser-Induced Metastable Crystalline GeTe

Discovering new inorganic materials using solid-state synthesis in an accelerated fashion is difficult due their sluggish diffusion coefficients and long diffusion distances. Furthermore, high temperatures used in these reactions generally produce thermodynamically stable products, which provides limited control on the reaction and prevents access to functional metastable phases. Herein, we report the use of a millisecond laser annealing technique to regulate the crystallographic phases of germanium telluride films of varying thicknesses. After laser heating, we combine temperature-dependent synchrotron grazing incidence measurements and transmission electron microscopy to study the structural evolution of the post-laser-heated GeTe. On average, we observe that millisecond laser heating induced the transformation of amorphous GeTe samples up to a ∼40% to 60% mixture of cubic β-GeTe (Fm3̅m) and rhombohedral α-GeTe (R3m) for GeTe films (thicknesses between 100 nm and 2 μm) deposited on thermally conducting substrates (such as Si), as opposed to phase-pure α-GeTe, which is obtained on low thermal conductivity substrates (quartz). Further, a room-temperature thermoelectric power factor of 6.10 μV cm^-1 K^-2 was measured for a laser-heated film on quartz. These findings suggest that conformal interfaces on substrates with high thermal conductivity facilitate accelerated rates of heat extraction at the sample-substrate interface to achieve phase control. We believe our strategy opens new avenues for the development of materials that are stabilized far from their equilibrium conditions.