Studies of Ge-Sb-Te Phase Change Materials At and Above Melting Temperatures and Set to Reset Transition of Memory Devic
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1251-H03-02
Studies of Ge-Sb-Te Phase Change Materials at and Above Melting Temperatures and Set to Reset Transition of Memory Devices Semyon D. Savransky, Guy Wicker The TRIZ Experts; 6015 PepperTree Court, Newark, CA 94560, USA [email protected] ABSTRACT The results of calorimetric and electrical studies of bulk Ge2Sb2Te5 and GeSb2Te4 alloys around melting temperature Tm are presented together with characteristics of phase-change memory devices from such alloys. The endothermic melting region is wider in Ge2Sb2Te5 than GeSb2Te4. Electrical resistivities of the alloys in this region have semiconductor characteristics. The width of the melting region correlates with breadth of set to reset transition in devices. This empirical correlation is probably important for alloy selection for multi-level memory cells. INTRODUCTION The simplest phase-change memory (PCM) device consists of two electrodes and a phase-change alloy (PCA) whose electrical properties (resistance or threshold voltage) are significantly different between the crystalline and glassy states. Reversible transformations of PCA between the crystalline and glassy states occur due to Joule heating from electrical signals applied between the PCM electrodes. Relatively long pulses with relatively low programming currents are often used in order to crystallize glassy PCA into the so-called set state which has low resistivity. Short pulses with a sharp trailing edge and high programming current are often used in order to bring the PCA to temperatures above the melting temperature Tm and then to vitrify the PCA into the so-called reset state which has high resistivity [1]. Due to the huge difference in resistivities of the alloys in these states, multi-level cells (MLC) can be realized in PCM that store more than 1 bit of information in a single device [2]. In this paper we address the problem of selection of PCA for MLC applications of PCM. In order to shed some light on this problem we study properties of PCA near Tm and cells behavior during the set to reset transition. Programming of phase change memory usually occurs above the melting temperature Tm but information about phase change materials at such high temperatures is very limited. We present results of calorimetric and electric studies of Ge2Sb2Te5 and GeSb2Te4 around the melting point. EXPERIMENT We perform 3 types of experiments: calorimetric and electrical measurements of bulk PCA from Ge-Sb-Te family around melting temperature as well as measurements of devices based on films from such PCA. Differential scanning calorimetry (DSC) was used to study the melting of PCA. Bulk samples were prepared by sealing the material in an inert atmosphere in quartz ampoules, then melting and slowly cooling them in a rocking furnace. This produces material in the crystalline state. Differential scanning calorimetery measurements were
performed using an Omnitherm DSC1500 at Oak Ridge National Laboratory showing the endothermic behavior of the materials around the melting point [3]. In order to study c
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