Observation of carrier localization in cubic crystalline Ge2Sb2Te5 by field effect measurement

Hang Qian,Ling-Jun Zhou,Ming Xu,Hao Tong,Xiang-Shui Miao,Ming-Ze He,Hong-Kai Ji

doi:10.1038/s41598-017-18964-w

Abstract

The tunable disorder of vacancies upon annealing is an important character of crystalline phase-change material Ge2Sb2Te5 (GST). A variety of resistance states caused by different degrees of disorder can lead to the development of multilevel memory devices, which could bring a revolution to the memory industry by significantly increasing the storage density and inspiring the neuromorphic computing. This work focuses on the study of disorder-induced carrier localization which could result in multiple resistance levels of crystalline GST. To analyze the effect of carrier localization on multiple resistant levels, the intrinsic field effect (the change in surface conductance with an applied transverse electric field) of crystalline GST was measured, in which GST films were annealed at different temperatures. The field effect measurement is an important complement to conventional transport measurement techniques. The field effect mobility was acquired and showed temperature activation, a hallmark of carrier localization. Based on the relationship between field effect mobility and annealing temperature, we demonstrate that the annealing shifts the mobility edge towards the valence-band edge, delocalizing more carriers. The insight of carrier transport in multilevel crystalline states is of fundamental relevance for the development of multilevel phase change data storage.

Highlights

Phase change memory (PCM) is one of the most important candidates for future nonvolatile memory devices owing to its high scalability, high speed, and long endurance[1]
As has been demonstrated by Siegrist et al using the samples of Ge1Sb2Te48, this annealing effect in the electrical resistance of crystalline state takes place prior to the metal-insulator transition (MIT) transition when GST transforms into the hexagonal phase
From the step annealing process, we observed the annealing effect in the electrical resistivity of crystalline state, which is consistent with the MIT demonstrated by previous research[8,9]

Summary

Introduction

Phase change memory (PCM) is one of the most important candidates for future nonvolatile memory devices owing to its high scalability, high speed, and long endurance[1]. Due to its close relationship with carrier transport, the analysis of field effect (the intrinsic effect of a transverse electric field on surface conductance) has been well practiced in MIT mechanisms including electron correlation (Mott transition)[20,21] and disorder (Anderson localization)[22,23]. Field effect studies of semiconductors have been a very active area of interest for many years, because they can probe the localized states of carriers which control the electronic properties of materials[24,25,26]. Carrier transport of crystalline GST observed from field effect has not been reported before. This study provides observation of localized carriers in different crystalline states of GST and enriches the research of MIT in the GeSbTe compounds as an addition to conventional transport measurement techniques. Insights of carrier transport in GeSbTe compounds have implications for the development of low power consumption and multilevel memory devices

Methods

Results

Conclusion