Flash DSC Measurements of Amorphous Crystalline Phase Transitions of Se1-xTex Alloys
In recent decades, chalcogenide glasses have been the subject of intense interest because of their unique physical properties such as high infrared transparency, strong light sensitivity, and high refractive index. The glasses are used for optical fibers, lenses, sensors and phase change storage systems, as well as for data storage media (RW-CD / DVD / Blu-Ray). The applications are based on reversible amorphous crystalline phase changes. Phase transitions like this exhibited by Se1-xTex alloys were investigated using ultrafast differential scanning calorimetry (Flash DSC).
Introduction
Currently, Flash EEPROMs are used to store information, for example in smart phones, laptops and USB sticks. A disadvantage of Flash EEPROMs is their limited lifetime due to the maximum number of about 106 writing cycles.
Many more writing cycles can be achieved with phase change materials (PCMs). This makes use of the properties of the different phases (e.g. amorphous and crystalline).
In this respect, chalconide glasses are an interesting class of materials in which the classical glass-forming elements silicon and oxygen are replaced by germanium and arsenic, and by sulfur, selenium and tellurium in the heavier chalconides. In chalcogenide PCMs, the change from the amorphous to the crystalline phase (Figure 1) can occur reversibly up to 1011 times [1].
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Figure 1. A phase change memory cell. |
The different phases are switched by pulses of a defined length and intensity. If high energy pulses are used, the sample is strongly heated and the crystals melt.
In these glasses, the critical cooling rate (at which no crystallization occurs) is so low that no crystals form on rapid cooling after the pulse. The material remains amorphous.
Crystalline phases occur when “low energy pulses" and longer pulse widths are used. The material heats up to the crystallization temperature and the amorphous phase changes to a crystalline phase. As indicated in Figure 1, the phase transition takes place in a spatially restricted region in order to achieve a sufficiently large data density. The phase transitions are very fast and can be simulated using ultrafast DSC.
A SeTe alloy was chosen as a model substance to study the phase transition and to investigate the following aspects:
(i) reversible phase switching through melt-quenching to an amorphous phase and recrystallization on heating,
(ii) precise control of the thermal history of the material,
(iii) the repetition of measurements using the same sample.
The aim of the research work was to gain an insight into the phase transition kinetics of this material. The measurements were performed using a METTLER TOLEDO Flash DSC 1. Detailed information on our work can be found in [2] and [3].
