Electrical switching and aluminium speciation in Al-As-Te glasses

Bulk AlxAs30Te70-x glasses in the range 5 ≤ x ≤ 20 prepared by melt quenching method has been found to exhibit memory for x < 10 and threshold switching for x ≥ 10. Upon heating in a differential scanning calorimeter (DSC), glasses with x ≤ 10 show crystallization reaction whereas glasses with x > 10 do not show crystallization reaction. Glasses with x ≤ 10 crystallized into Te and As2Te3 crystal structures when annealed at their crystallization temperature. Glasses with x ≥ 15 crystallized to As2Te3 structure only. The glasses cooled from its melting temperature (Tm) in water at room temperature (28 °C) solidified into crystalline structure for x = 5, whereas x ≥ 10 solidified into amorphous structure. Magic angle spinning nuclear magnetic resonance (MAS NMR) results show 27Al resides in [4]Al and [6]Al coordination. Raman spectra of the as-quenched, SET and RESET samples of the memory glass reveal that the SET state is crystalline and RESET state is amorphous. Raman spectra of threshold switching glass reveal no structural change even after 50 switching cycles. The progressive addition of Al promotes the transfer of lone-pair electrons of Te to Al atoms. We propose this lone-pair tailoring by Al may form a band above the valence band that decreases the density of charge carriers. As a result, a decrease in threshold current (Ith) and an increase in threshold voltage (Vth) is observed. The higher coordinated Al cross links the structural network heavily. This makes structural reorganization difficult resulting in the observed threshold switching in AlxAs30Te70-x for glasses with x > 5.