Transiente Schalt- und Relaxationseffekte in Phasenwechselnanostrukturen

  • Transient switching and relaxation effects in phase-change nanostructures

Wimmer, Martin; Wuttig, Matthias (Thesis advisor); Waser, Rainer (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2015)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2015

Abstract

Due to their special physical properties phase change materials are one of the most promisingcandidates for a future electronic data storage technology. The high potential ofthis class of material results from a huge contrast in electrical properties and a very fastphase transition between the amorphous and crystalline state of the solid. Electrical memoriesbased on phase change materials are non-volatile and provide extremly fast readand write cycles, a high storage density as well as a low energy consumption. Previousstudies on the crystallization kinetics revealed the write speed of phase change materialscan be comparable to DRAM (Dynamic Random Access Memory). Phase change memoriescan even reach storage densities higher than hard-disks or flash-memories, becauseof an high scalability and the ability to store multiple bits in one physical cell (multilevelstorage). Additionally, a phase change memory does not need to have a transistoron a single-crystalline silicon substrate. Therefore, scaling in three dimensions becomespossible, which largely increases the available volume for data storage. The reduction inenergy consumption is a beneficial side effect of scaling a phase change memory. Becauseof the large contrast in resistivity between the crystalline and the amorphous phase ofthree or more orders of magnitude and the necessary amount of joule heating in orderto perform a fast phase transition, the strong non-linearity in the IV characteristics aswell as the „threshold-switching“ phenomena are the most important properties in thesematerials. The effect of threshold-switching thereby describes an abrupt break-down inelectrical resistivity in the amorphous phase in the regime of high electrical fields.The present work focuses on the study of the threshold-switching effect as well as thetemporal evolution of the electronic properties after creating an amorphous state (theso-called resistance drift). In the course of this work it becomes undoubtfully clear thatboth effects should not be investigated individually but hand in hand, since the resistancedrift effect also causes a strong change in the threshold-switching properties. Although thethreshold-switching effect was already discovered in 1968 by Ovshinsky in semi-conductingglasses, different opinions on the physical nature of this effect still exist today. The mostimportant theoretical models are therefore presented and tested by new experimentalfindings. The aim of this work is to build an extended physical understanding of themechanisms behind the effects of threshold-switching and resistance drift.In a first part the transient effect of threshold-switching is studied at a fixed time after amorphization. Here the material dynamics before showing the threshold-switching effectbecome visible by the usage of a custom made fast electrical tester. These experimentsgive crucial hints to a purely electronic excitation mechanism for the first time. Moreoverthe experimental findings allow a definition of a minimal electrical field for the insetof threshold-switching. Besides of physical interest the presented results are relevant foran electronic phase change memory application. On the one side the readout processof a stored information should be done with voltages as high as possible to guarantee aprecise determination of the cell state (signal-to-noise-ratio). On the other side the readoutvoltage should be low enough to leave the cell state unchanged. The definition of a minimalelectrical field for threshold-switching therefore gives a clear limit between the read andwrite window.The second part of this work focuses on the resistance drift effect in various amorphous „asdeposited“ phase change materials at low electrical fields, namely: Ge2Sb2Te5, AgIn-Sb2Teund GeTe. In addition to solely measure the electrical resistance, the special design of theexperiment enables a pristine characterisation of the activation energy for conductionas well as the arrhenic pre-factor as a function of time. Models proposed in literaturefor Ge2Sb2Te5 only describe a change in activation energy as a function of time. Theexperimental findings within this work undoubtfully prove a general explanation for theresistance drift effect can only be correct if it is able to cover the temporal evolution ofthe activation energy as well as the arrhenic pre-factor.In the last part a combined study of the resistance drift and threshold-switching effect ispresented. In these experiments the electrical resistance is not characterized exclusivelybut as a function of the applied voltage up to the regime of threshold-switching. Furthermore,the drift in the current-voltage characteristics of lateral phase change nanostructuresis studied on short time scales after amorphization. An interpretation of the temporal evolutionof the current-voltage characteristics of AgIn-Sb2Te within the Poole-Model leadsto an increase in trap density. This observation strongly contradicts the results foundfor Ge2Sb2Te5 in literature and could indicate a different drift behaviour in these twomaterials. The development of a new experiment allows an in situ study of the temporalevolution of the IV characteristic and the threshold-switching parameter in the same„melt-quenched“ amorphous state for the first time. In addition, critical parameters likethe threshold-voltage, threshold-current, threshold-power and the delay time for switchingas a function of voltage are investigated and interpreted within the conflicting theoreticalapproaches. The experimental data indicate a nearly constant electrical power for the insetof threshold switching. All these studies give important hints on the threshold-switchingcondition and thereby help to unravel the physical mechanisms behind.