First-principles simulations of amorphous GeSe compounds for memory selectors

Abstract

Ovonic Threshold Switching (OTS) is a nonlinear current-voltage phenomenon in certain amorphous materials. Recently, OTS selectors based on chalcogenide compounds have been successfully integrated with innovative phase-change memories in 3D cross-point arrays for switching memory. The selector controls data accessibility and transmission quality within and outside the entire memory cell. a-GexSe1-x are among the most promising compounds for this application. Understanding electronic properties and the related structural features is crucial for designing and optimizing these materials. This work addresses some of the most significant challenges in generating amorphous materials using first-principles calculations. Density functional theory methods are used to gain new insights into the structural and electronic properties of the undoped and doped a-GexSe1-x. We have developed realistic density functional theory-based structural models of a-GexSe1-x without resorting to experimental information or adjusted interatomic potential. A series of a-GexSe1-x with three stoichiometries (x = 0.4, 0.5, and 0.6) and dopants (Si, As, P, S, Te) with various concentrations (1%, 3%, 5%,7%, 10%, 15%) has been generated with the melt and quench protocol. We analyze each sample's structural and electronic properties and unveil the link between each localized state's electronic and structural properties within the mobility gap. Our results indicate that the a-GexSe1-x is mainly Ge(3): Se(3) coordinated. The results show that there is Ge and Se clustering in Ge and Se-rich structures, respectively; the Ge-rich structures tend to have a larger coordination number, opposite to the Se-rich structures. Well-defined Ge-Se first-neighbor shells are found, similar to the Ge-Se bond in the GeSe crystalline structure. We have analyzed the electronic properties, including the inverse partition ratio and the density of states. We have also analyzed the correlation between their most relevant features and the microscopic structure of our models. We have found that the mobility gap is very sensitive to the composition around the stochiometric GeSe compound, and in particular, that it decreases with Ge concentration, to the point of becoming essentially metallic already for x=0.6. We show the structural motifs at the origin of the localized states in the mobility gap. Specifically, we found that the localized states are predominantly due to the Se-Se bonds and a minor contribution to Se lone pairs and tetrahedra. These bonding configurations are at the origin of conduction and valence band tail states, leading to nonlinear conduction in OTS. The a-Ge0.6Se0.4 sample is metallic. We have also generated and studied the structure of the three a-GexSe1-x compounds in the presence of dopants species (Si, As, P, S, Te) with various concentrations (1%, 3%, 5%,7%, 10%, 15%). The dopants behave similarly with their iso-valent hosting ions, with some delicate differences. We found a new kind of structural motif, the germanium chalcogenide cubane, symbolizing the presence of structurally ordered assemblies in disordered materials. The structural origin of localized states has also been explored. Bicoordinated and pentacoordinated Ge give rise to localized states in all the dopants. For the Si-doped and pnictogen-doped a-GexSe1-x, we observe the presence of defect states originating from substitutional doping. In the case of chalcogenide doping, we also observe the chalcogenide lone pairs, homopolar bonds, and the valence alternation pair, all of which yield localized states. These results are useful to understand the electronic properties of these materials and could help to provide guidelines for improving the memory-switching performance of a-GexSe1-x.

Date of Award	27 Jan 2025
Original language	English
Supervisor	Pablo Jesús Ordejón Rontome (Director) & Roberta Farris (Director)