Changes in electrical transport and density of states of phase change materials upon resistance drift

Abstract

Phase-change memory technology has become more mature in recent years. But some fundamental problems linked to the electrical transport properties in the amorphous phase of phase-change materials still need to be solved. The increase of resistance over time, called resistance drift, for example, poses a major challenge for the implementation of multilevel storage, which will eventually be necessary to remain competitive in terms of high storage densities. To link structural properties with electrical transport, a broader knowledge of (i) changes in the density of states (DoS) upon structural relaxation and (ii) the influence of defects on electrical transport is required. In this paper, we present temperaturedependent conductivity and photo-conductivity measurements on the archetype phase change material GeTe. It is shown that trap-limited band transport at high temperatures (above 165 K) and variable range hopping at low temperatures are the predominating transport mechanism. Based on measurements of the temperature dependence of the optical band gap, modulated photo-conductivity and photo-thermal deflection spectroscopy, a DoS model for GeTe was proposed. Using this DoS, the temperature dependence of conductivity and photo-conductivity has been simulated. Our work shows how changes in the DoS (band gap and defect distributions) will affect the electrical transport before and after Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.temperature-accelerated drift. The decrease in conductivity upon annealing can be explained entirely by an increase of the band gap by about 12%. However, low-temperature photo-conductivity measurements revealed that a change in the defect density may also play a role. New J. Phys. 16 (2014) 043015 D Krebs et al New J. Phys. 16 (2014) 043015 D Krebs et al New J. Phys. 16 (2014) 043015 D Krebs et al 7 New J. Phys. 16 (2014) 043015 D Krebs et al 8

High-field electrical transport in amorphous phase-change materials

“…15 In agreement with Ref. 15, we observe a decrease in activation energy for GST and GeTe below $170 K (see supplementary material 17 ). At this point, we note again that our fitted values for K Á l 0 increase strongly for temperatures below 200 K, which is in line with this decrease in activation energy.…”

“…Such an energetic position of a deep defect is also compatible with values that have been inferred from modeling the density of states based on modulated photocurrent (MPC) measurement, photothermal deflection spectroscopy (PDS), 30 and with the variable-range hopping activation energy observed by Krebs et al 15 For AIST, the drastic increase in inter-trap distance s (30 nm to 90 nm over the entire temperature range) can be modeled with E D ¼ 0.12 eV.…”

“…At low fields and lower temperatures (T 200 K), the validity of the 2CPF model is limited because of the dominant influence of variable range hopping. 15 A characteristic sign for hopping transport in GeTe is a low activation energy of conduction due to the large defect density in the vicinity of the Fermi level. 15 In agreement with Ref.…”

“…15 A characteristic sign for hopping transport in GeTe is a low activation energy of conduction due to the large defect density in the vicinity of the Fermi level. 15 In agreement with Ref. 15, we observe a decrease in activation energy for GST and GeTe below $170 K (see supplementary material 17 ).…”

Dielectric properties of amorphous phase-change materials

“…Nevertheless, as can be inferred from the ongoing discussion [17,18], the atomic arrangement and bonding in the amorphous state as well as the phase-change mechanism are still under debate. Consequently, the already mentioned amorphous-state phenomena resistance drift and threshold switching [19] are not yet fully understood, which hampers the development of PCM-based electronic devices and the realization of multilevel memories [20].…”