Insight into the role of oxygen in the phase-change material GeTe

Abstract

Oxygen is widely used to tune the performance of chalcogenide phase-change materials in the usage of phase-Change random access memory (PCRAM) which is considered as the most promising next-generation non-volatile memory. However, the microscopic role of oxygen in the write-erase process, i.e., the reversible phase transition between crystalline and amorphous state of phase-change materials is not clear yet. Using oxygen doped GeTe as an example, this work unravels the role of oxygen at the atomic scale by means of ab initio total energy calculations and ab initio molecular dynamics simulations. Our main finding is that after the amorphization and the subsequent re-crystallization process simulated by ab initio molecular dynamics, oxygen will drag one Ge atom out of its lattice site and both atoms stay in the interstitial region near the Te vacancy that was originally occupied by the oxygen, forming a "dumbbell-like" defect (O-V Te -Ge), which is in sharp contrast to the results of ab initio total energy calculations at 0 K showing that the oxygen prefers to substitute Te in crystalline GeTe. This specific defect configuration is found to be responsible for the slower crystallization speed and hence the improved data retention of oxygen doped GeTe as reported in recent experimental work.Moreover, we find that the oxygen will increase the effective mass of the carrier and thus increases the resistivity of GeTe. Our results unravel the microscopic mechanism of the oxygen-doping optimization of phase-change material GeTe, and the present reported mechanism can be applied to other oxygen doped ternary chalcogenide phase-change materials.

Photo-Seebeck study of amorphous germanium–tellurium-oxide films

“…Firstly, we observe an increase in E g from that reported for a-GeTe (* 0.75 eV) [14] to * 0.88 eV in these a-GeTe-O systems. This is in agreement with ab initio molecular dynamics simulations [8] that show substitution of Te by O results in an increase in the bandgap of * 0.06 eV, whilst the formation of 'dumbbell-like' O-V Te -Ge defects lead to an increase of * 0.08 eV. We note that such simulations typically underestimate the bandgap compared to experimental measurements hence a larger increase as observed within would not be unexpected.…”

“…As the wavelength is reduced further, S light (k) continues to increase linearly but at a greater rate up until * 970 nm (1.28 eV). The change in gradient of S light (k) at 0.88 eV is slightly above the reported optical bandgap of a-GeTe of * 0.75 eV [8] to 0.85 eV [11]. This is in line with a widening of the bandgap upon oxidation, which is well-known to occur when chalcogenide glasses are oxygen doped.…”

“…It is also apparent that the presence of oxygen in these a-GeTe films reduces the value of E f -E v from 0.31 eV in a-GeTe [14] to 0.16 eV in the studied a-GeTe-O system. This change is also reflected in the ab initio simulations [8] in the case of introducing O-V Te -Ge defects where E f -E v is significantly reduced. As such this then rules out the presence of a donor state centred at 0.25 eV above the valence band as found in a-GeTe.…”

“…Despite the significant research attention this and other applications have attracted, a full understanding the electronic and thermoelectric properties of these materials remains a challenge due to their non-crystalline structure. Recently the role of oxidation in a-GeTe has been studied, demonstrating its role in the detrimental modification of the crystallization dynamics and chemistry of thin films [6][7][8][9][10].…”

Origin of the concentration-dependent effects of N on the stability and electrical resistivity in polycrystalline Ge1Sb2Te4

“…Doping is a common but effective strategy to modify the properties of a material. Various dopants, including C, 10–12 N, 9,13,14 O, 15–17 and W, 18,19 were investigated to tailor the properties of GST alloys. Nitrogen is one of the most studied dopants among all due to its significant effect on enhancing the electrical resistivity 20 and thermal stability of the crystalline phase, 21 refining the grain size, 22 and improving the cyclability.…”