Localised states and their capture characteristics in amorphous phase-change materials

Martin Rütten,Martin Salinga,Daniel Krebs,Abu Sebastian,Andreas Geilen

doi:10.1038/s41598-019-43035-7

Martin Rütten, Martin Salinga + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-43035-7

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Journal: Scientific Reports	Publication Date: Apr 29, 2019
Citations: 6	License type: open-access

Affiliation: RWTH Aachen University, IBM Research - Zurich

Abstract

As phase-change materials are poised to play a key role in next-generation computing systems, improving the current understanding of electrical transport in their amorphous phase can further strengthen their technological competitiveness. Even though the interaction of charge carriers with disorder-induced localised states largely affect the field-dependent conductivity, a clear link between electrical transport and specific features of the electronic density of states (DOS) could not be established yet due to a lack of knowledge of the capture characteristics of trap states. Here, we address this knowledge gap and employ modulated photocurrent spectroscopy (MPC) to investigate localised states in the frequently studied amorphous phase of Ge2Sb2Te5. Additionally, we present results on the DOS in the bandgap of amorphous AgIn-doped Sb2Te, which has not been subject to high-resolution DOS spectroscopy before. We find experimental evidence for clearly non-constant capture coefficients among a continuous spectrum of localised states in both studied materials. According to this observation especially in AgIn-doped Sb2Te, where no pronounced defect can be detected as main channel for carrier emission, we point out the necessity of modifying the current Poole-Frenkel-based transport modelling.

Highlights

Driven by the explosive growth of data and the speed-gap between the processing unit and conventional storage systems, memory industry introduced the storage class memory (SCM) as generation, non volatile memory class, satisfying key requirements such as speed, reliability and efficiency at a minimum cost-per-bit ratio[1,2]
Even though a revision of the current modulated photocurrent spectroscopy (MPC) analysis seems necessary before quantitative density of electronic states (DOS) characteristics can be extracted from our MPC results, we can already report two significant findings regarding the DOS of amorphous phase-change materials (PCM)
In the scenario of germanium-free Ag4In3Sb67Te26 apparently lacking such a singular defect as main channel for charge carrier emission, a realistic electrical transport model would need to account for trap- and release processes of charge carriers with a continuous spectrum of localised states

Summary

Introduction

Driven by the explosive growth of data and the speed-gap between the processing unit and conventional storage systems, memory industry introduced the storage class memory (SCM) as generation, non volatile memory class, satisfying key requirements such as speed, reliability and efficiency at a minimum cost-per-bit ratio[1,2]. PF-based transport modelling so far only includes trap states attributed to a single energy level, which is commonly attributed to a structural defect at a specific energetic distance to the band edge[16,22,23,24]. This approach appears plausible at least for PCM containing germanium such as amorphous GeTe, where underor over-coordinated Ge-atoms have been associated with electronic defect states in the bandgap[25,26,27]. In p-type conductivity materials such as amorphous PCM30, the capture coefficient for holes cp is of predominant importance for electrical transport

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