Metal insulator transition in phase change material material Ge1Sb2Te4

  Irina Pozin  
Tel Aviv University
Palevsli Aleksander

In general, the mechanisms leading to metal–insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), however usually MIT is interplay of both these mechanisms so distinction between them is difficult. Particular feature of Ge1Sb2Te4 is that the metal–insulator transition in this material is due to strong disorder

without a structural change (in hexagonal crystal structure) usually associated only with amorphous solids. 

In our study MIT is explored through transport studies via magnetoresistance measurements. For addition, this data is supported by performing structural analysis of GST films and corresponding correlation of the material structure and transition from the insulator to the metal state.

The experiments show that annealing of Ge1Sb2Te4 100 nm of width films to 1600C under argon pressure of 130 Torr leads to phase transition from amorphous to cubic phase and from cubic to hexagonal phase upon further annealing of the material at least to the 2500C.

Ge1Sb2Te4 shows a metallic resistance-vs-temperature curve after high-temperature annealing above second crystallization temperature. While higher disorder that gain this material in low-temperature annealing leads to relatively high resistivity and accompanied by negative temperature coefficient (TCR). For addition, insulating behavior is observed in cases of even higher disorder reflected by variable-range hopping mechanism that matches Mott’s law. All these and the zero TCR that was observed for the sample annealed to 2460C indicate the occurrence of the metal insulator transition.

Furthermore, the MIT is explored through magnetoresistance of Ge1Sb2Te4. The dependences of resistance on temperature and magnetic field can be consistently explained by considering the mechanisms of weak antilocalization (WAL) and disorder-enhanced electron-electron interaction.

The positive magnetoresistance observed via WAL is due to strong spin-orbit scattering, and the low-field magnetoresistance data allows us to analyze the temperature-dependent electron dephasing field and rate. We show that the transition from the diffusive to hopping regime of electronic conductivity is accompanied with the change of the sign of the magnetoresistance at small fields.