Laser-induced metastable phase in crystalline phase-change films by confocal Raman spectrometer.

Understanding crystallization process in phase-change materials is very important for data storage application. Especially, accurately controlling the metastable phase transition as well as characterizing its structure evolution is still under investigation. In this study, phase transformations have occurs from amorphous to crystalline phases when the phase-change films were irradiated continuously by the 785 nm laser irradiation. By adjusting the laser power, the different metastable phases in conventional Ge2Sb2Te5, Sb2Te3, ZnSb, ZnSb-Al2O3 and ZnSb-ZnO were obtained and distinguished by their different Raman vibration modes. The effect of laser power on the phase-change threshold of these films was studied systematically. Large structural differences induced by laser irradiation were revealed based on the changes in Raman profiles. Our study may offer a new insight into an accurate control of distinct metastable state to realize optical multilevel memory.

Suppression for an intermediate phase in ZnSb films by NiO-doping.

The structural evolution and phase-change kinetics of NiO-doped ZnSb films are investigated. NiO-doped ZnSb films exhibit a single-step crystallization process, which is different from that of undoped ZnSb. NiO-doped ZnSb can directly crystallize into a stable ZnSb phase at temperatures greater than 320 °C with suppression of a metastable ZnSb phase. These characteristics enlarge the amorphous/crystalline resistance ratio by approximately five orders of magnitude. Moreover, NiO doping of ZnSb films increases crystallization temperature from 260 to 275 °C, improves data retention temperature from 201.7 to 217.3 °C and increases crystalline activation energy from 5.64 to 6.34 eV. The improvement of the thermal parameters in the nanocomposite can be attributed to stable ZnSb grain growth refinement owing to the dispersion of NiO particles in the sample matrix. This provides additional nucleation sites and produces more ZnSb/NiO interfaces, which can initiate the nucleation and accelerate crystallization. The kinetic exponent n decreases from 1.12 to 0.44, which confirms the ultrafast one-dimensional growth and heterogeneous phase transition of the NiO-doped ZnSb films. The improved thermal stability, larger resistance ratio and direct transition to a stable phase with ultrafast one-dimensional crystal growth indicate the good potential of these materials in phase-change memory applications.

Controllable crystal growth and fast reversible crystallization-to-amorphization in Sb2Te-TiO2 films.

The structure evolution and crystallization processes of Sb2Te-TiO2 films have been investigated. The Sb2Te-rich nanocrystals, surrounded by TiO2 amorphous phases, are observed in the annealed Sb2Te-TiO2 composite films. The segregated domains exhibit obvious chalcogenide/TiOx interfaces, which elevate crystallization temperature, impede the grain growth and increase crystalline resistance. Compared with that in conventional Ge2Sb2Te5 film, the shorter time for onset crystallization (25 ns) and amorphization (100 ns) has been achieved in as-deposited (Sb2Te)94.7(TiO2)5.3 film under 60 mW laser irradiation. The corresponding recrystallization and re-amorphization can also be realized in the film. From Johnson-Mehl-Avrami (JMA) analysis, it is further found that the one-dimensional grain growth with controlled interface is dominant for the film during the fast phase-change process. Therefore, (Sb2Te)94.7(TiO2)5.3 film with improved crystallization mechanism is promising for high-stable and fast-speed memory applications.