First-principles study of the properties for crystal Ge2Sb2Te5 with Ge vacancy

Abstract

Ge2Sb2Te5 (GST) is a technologically important phase-change material for data storage, where the fast reversible phase transition between crystalline and amorphous states is used for recording information. The effects of vacancies on crystal GST were investigated by ab initio calculations. Based on analysis of the vacancy formation energy, the GST structure with Ge vacancy (VGe) was found to be the most stable. Thereafter, the influence of VGe defects on crystal GST structure was deliberated by analyzing the band structure, electron density difference, total density of states (TDOS) and partial density of states (PDOS) of GST structure. The results reveal that VGe can promote the Fermi level enter into the valence band, which makes the GST material exhibit more pronounced properties of P-type semiconductors. Nevertheless, VGe shows a slight effect on the chemical bond characters. When VGe concentration maintained at 20% in the GST structure, the band gap is the widest about 0.45eV. Moreover, VGe can result in the electrons in s orbital of Ge, Sb and p, d orbitals of Te make a contribution to the valence band, while electrons in p and d orbitals of Ge, Sb are more favorable to conduction band.

Notes on Reproducibility of Ge2Sb2Te5 Properties

“…These methods produce a‐GST films with similar optical absorption‐edge spectra, whereas there may be some differences in subgap absorptions, the details of the dc‐sputtered films being inspected recently . We have also seen that dc sputtering at high voltages with slow rates (≈5 nm min −1 ) deposits more compact amorphous films, which could favor producing dense cubic films possessing fewer cation (Sb/Ge) vacancies, smaller N , and lower σ . In contrast, although with no preparation details being given, Lees’ films might be deposited at a faster rate, e.g., ≈5 nm s −1 , in a similar way to those commonly used, which may contain more vacancies and/or voids, giving rise to a smaller refractive index, more dangling bonds and N , and higher σ .…”

An Investigation into the Kinetics and Mechanism of Phase Transitions in Optical Phase Change Ternary Alloy $Ge_2Sb_2Te_5$

“…The electron behaviour and the density of states in the longer and shorter Ge − T e bonds is also speculated to play a role in the optical contrast displayed by these materials. [32] [34] [44] 3 Macroscopic Kinetics of Solidification and Phase transformation GST − 225 and other phase change alloys have been used in next generation rewritable optical media for the last few years and are being considered for use in high speed non-volatile memory. The primary method for inducing this phase change, which is used for writing and rewriting bits onto a memory cell, is a SET and RESET laser pulse of varying power and temporal width.…”