Direct evidence for phase transition in thin Ge1Sb4Te7 films using in situ UV-Vis-NIR spectroscopy and Raman scattering studies

Phase-change materials (PCM) show remarkable property-contrast from amorphous to crystalline phase that forms the basis for high-speed non-volatile memory device applications. Despite understanding the local structure and physical properties of these phases, a systematic study on the phase-change behavior is essential. Here, we used in situ UV-Vis-NIR spectroscopic measurements to study a systematic evolution of optical band gap (E-g) and the local disorder described by Tauc parameter (B), for the temperatures from 90 to 480K on amorphous and cubic phases of Ge1Sb4Te7 thin films. It has been found that the E-g of amorphous phase decreases with increasing temperature from 90 to 400 K, while the disorder as exemplified by B, increases owing to thermal vibrations. At 420 K, a rapid decrease in the E-g from 0.47 to 0.33 eV and also a sharp reduction of similar to 13% in the value of B-1/2 is observed evidencing the signature of amorphous-to-cubic phase transition. Furthermore, the hexagonal phase is more disordered compared to cubic phase. The Raman results are consistent with optical measurements, which indicate that the degree of disorder reduces from amorphous to cubic phase, while hexagonal phase with an increased disorder is attributed to elongated bonds. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim