Wide range optical and electrical contrast modulation by laser-induced phase transitions in GeTe thin films

Abstract

The paper presents the results of a computational thermo-kinetic model of laser heating of GeTe thin films and their subsequent thermoconductive and radiative cooling. A comparison of this model and experimental results of observing of reversible phase transitions in thin films under impact of nanosecond pulsed laser radiation with «top hat» beam intensity distribution was performed. It was shown that the subsurface crystallization of amorphous GeTe thin film obtained on SiO2 substrates is started at threshold laser radiation energy density E = 7.5 mJ/cm2. Then crystallization process propagates up to substrate primarily in the rhombohedral α-GeTe phase in the range of E = 18.6–32 mJ/cm2 with the subsequent transition to the cubic β-GeTe phase in the range of E = 32–47.6 mJ/cm2 reaching the necessary temperatures for corresponding transitions. The reverse crystalline-amorphous transition is started at the laser energy density 62 mJ/cm2, when the film temperature reaches the melting temperature and is observed up to energy density 93 mJ/cm2 without ablation damage. It was shown that the values of energy densities, required for α-/β-phase transitions and reamorphization for the films obtained on native oxidized p-Si (100) substrates in comparison with ones required for the same transitions in films on SiO2 substrates are more on about 20–30%, because of significant higher thermal conductivity of p-Si (100) substrate. The high optical contrast in the spectral range from 200 nm to 22000 nm and high difference in electrical conductivity from σam = 0.66 Ω−1 cm−1 to σcr = 26.8 Ω−1 cm−1 between amorphous and cubic crystalline states was demonstrated. The observed experimental results are in good agreement with computational thermo-kinetic model.