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Amorphous Ge2Sb2Te5 (GST) alloy, upon heating crystallize to a metastable NaCl structure around 150°C and then to a stable hexagonal structure at high temperatures (≥250°C). It has been generally understood that the phase change takes place between amorphous and the metastable NaCl structure and not between the amorphous and the stable hexagonal phase. In the present work, it is observed that the thermally evaporated (GST)1-xSex thin films (0 ≤ x ≤ 0.50) crystallize directly to the stable hexagonal structure for x ≥ 0.10, when annealed at temperatures ≥ 150°C. The intermediate NaCl structure has been observed only for x < 0.10. Chemically ordered network of GST is largely modified for x ≥ 0.10. Resistance, thermal stability and threshold voltage of the films are found to increase with the increase of Se. The contrast in electrical resistivity between the amorphous and crystalline phases is about 6 orders of magnitude. The increase in Se shifts the absorption edge to lower wavelength and the band gap widens from 0.63 to 1.05 eV. Higher resistance ratio, higher crystallization temperature, direct transition to the stable phase indicate that (GST)1-xSex films are better candidates for phase change memory applications.

We experimentally demonstrate the coexistence of two opposite photo-effects, viz. fast photodarkening (PD) and slow photobleaching (PB) in Ge 19 As 21 Se 60 thin films, when illuminated with a laser of wavelength 671nm, PD appears to begin instantaneously upon light illumination and saturates in tens of seconds. By comparison, PB is a slower process that starts only after PD has saturated. Although we could observe the coexistence of PD/PB even at moderate, one order of magnitude lower intensity of 0.2 W/cm 2 , the kinetics of transformation is significantly slowed down. However, both PD and PB follow stretched exponetial dependence on time. Modeling of overall change as a linear sum of two contributions suggests that the changes in As and Ge parts of glass network respond to light indepndent of each other.

In this Letter, we present the interesting results of photodarkening (PD), transition toward photostability, and a slow crossover from PD to photobleaching when composition of the chalcogenide glassy thin film changes from Ge-deficient to rich. A subsequent Raman analysis on these as-prepared and irradiated samples provide the direct evidence of photoinduced structural rearrangement, i.e., photocrystallization of Se and the removal of edge-sharing GeSe4 tetrahedra. Further, our experimental results clearly demonstrate that light-induced effects can be effectively controlled by choosing the right composition and provide valuable information on synthesizing photostable/sensitive glasses.

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