Unraveling the optical contrast in Sb2Te and AgInSbTe phase-change materials

Shehzad Ahmed,Xu-Dong Wang,Riccardo Mazzarello,Liang Sun,Wei Zhang,Yu-Xing Zhou

doi:10.1088/2515-7647/ac051b

Abstract

Chalcogenide phase-change materials (PCMs) show a significant contrast in optical reflectivity and electrical resistivity upon crystallization from the amorphous phase and are leading candidates for non-volatile photonic and electronic applications. In addition to the flagship Ge2Sb2Te5 phase-change alloy, doped Sb2Te alloys, in particular AgInSbTe used in rewritable optical discs, have been widely investigated for decades, and nevertheless the theoretical insights on the optical properties of this important family of PCMs are scarce. Here, we carry out thorough ab initio simulations to gain an atomistic understanding of the optical properties of Sb2Te and AgInSbTe. We show that the large optical contrast between the amorphous and crystalline phase stems from the change in bond type in the parent compound Sb2Te. Ag and In impurities serve mostly the purpose of stabilization of the amorphous phase, and have marginal impact on the large variation in the dielectric function upon the phase transitions.

Highlights

It was reported that hybrid functional calculations can satisfactorily reproduce the experimental optical contrast of typical phase-change materials, such as GeTe, GST and Sb2Te3.[81] we employed the projector augmented wave (PAW) method[95] in combination with both PBE and hybrid Heyd-Scuseria-Ernzerhof (HSE06)
We showed that the large imaginary part of the dielectric function in c-Sb2Te stems from highly extended bond chains of aligned p orbitals with 3 p electrons per lattice site on average, promoting electron delocalization and the formation of metavalent bonding
We assessed the impurity effects of common alloy elements, namely, Ag and In, which result in moderate weakening of absorption due to the breaking of the alignment of p orbitals in the crystalline phase

Summary

Introduction

Phase-change materials (PCMs) undergo fast and reversible transitions between crystalline and amorphous states exhibiting electrical and optical contrast, enabling a number of important applications.[1,2,3,4,5,6,7,8] The first commercial products of PCMs were rewritable optical media (CD, DVD and Blu-ray Disc), which exploited the reflectivity contrast and employed laser pulses to switch the material.[9,10,11] More recently, the large resistance difference between the amorphous and crystalline phases is being exploited in electronic devices such as the 3D Xpoint non-volatile memories,[12,13,14,15] where electrical pulses are used to induce the structural transitions Such devices offer an attractive combination of properties including excellent scalability, stability, high speed, and non-volatility.[16,17,18,19,20,21] PCMs are promising for applications in neuro-inspired computing devices, aiming to eliminate the von Neumann bottleneck of traditional computing.[22,23,24,25,26,27,28,29,30] Very recently, with the booming development of optoelectronics and photonics, the optical properties of PCMs have gained again strong interests. In addition to photonic non-volatile memories[31,32,33,34,35,36] and neuro-inspired computing,[37,38,39,40] various emerging techniques based on PCMs have been proposed and demonstrated, such as non-volatile optoelectronic displays, reconfigurable optical metamaterials, mid-infrared absorbers, thermal emitters and others.[41,42,43,44,45,46,47,48,49]

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Results

Conclusion