PCM – Phase-Change Memory

Phase-change memory exploits a fundamentally different physical property than the technologies covered so far: certain materials can be deliberately and reversibly switched between a disordered (amorphous) and an ordered (polycrystalline) state, whose atomic structures differ in electrical resistance by several orders of magnitude. In the amorphous state, the atoms are arranged randomly, as in a frozen liquid, which strongly impedes charge transport through scattering off the disordered structure – electrical resistance is high, and this state encodes a “0”. In the crystalline state, the atoms form a regular lattice that facilitates charge transport considerably – resistance is low, and this state encodes a “1”. The resistance ratio between the two states typically spans several orders of magnitude, enabling a very robust and error-tolerant readout method.

By far the most commonly used material is an alloy of germanium, antimony, and tellurium in roughly the ratio Ge₂Sb₂Te₅, called GST for short. GST belongs to the chalcogenide material class (compounds with group-16 elements such as sulfur, selenium, or tellurium) and has the property, crucial for memory applications, that the phase transition between amorphous and crystalline can be carried out very quickly (in the nanosecond range) and reversibly, very many times, without the material's chemical composition changing in the process – unlike, for example, the electrochemical processes in batteries, where material fatigue through chemical transformation is a central cause of aging. GST's crystallization temperature is around 150 to 200 °C, well below its melting point, allowing controlled, thermally induced switching without the cell unintentionally changing state at normal operating temperatures.