ARTICLE Simulation of the structure of GeAs 4 Te 7 chalcogenide materials during memory switching 1 M. Popescu, F. Sava, A. Velea, A. Lo ˝ rinczi, and I.D. Simandan Abstract: The complex chalcogenides with excellent memory switching properties are mainly situated close to the border of glass formation domain. The simulation of the structural changes occurring during the memory switching process of a ternary chalcogenide composition has been carried out. The transition of a high resistivity GeAs 4 Te 7 amorphous cluster with 120 atoms to a low resistivity crystalline cluster was analyzed. The coordination of atoms changes from that corre- sponding to 8-N coordination rule (two for tellurium, three for arsenic, and four for germanium) in the amorphous phase to six (the same for all atoms) in metastable crystalline phase. Because of spatial constraints exercised by the amorphous matrix, the amorphous cluster cannot expand. In these circumstances Te atoms seem to be over-coordinated (up to sixfold-coordinated). During the switching process, the atoms are moving on distances up to 4.0 Å. The average displace- ment is of 2.36 Å. PACS Nos.: 47.20.Hw, 77.84.Bw, 78.55.Qr, 77.80.Fm, 83.10.Tv. Résumé : Les complexes chalcogénures avec d’excellentes propriétés de commutation de résistance a ` mémoire se situent maintenant près de la frontière de la formation de verre. Nous faisons ici une simulation des changements structuraux qui se produisent lors de la commutation de mémoire d’un composé chalcogénure tertiaire. Nous analysons la transition entre l’amas de 120 atomes de haute résistance de a-GeAs 4 Te 7 , vers l’amas cristallin de basse résistance. La coordinence des atomes change, de celle correspondant a ` un indice de coordination 8-N (deux pour le Te, trois pour l’As et quatre pour le Ge) dans la phase amorphe, a ` six (la même pour tous les atomes) dans la phase cristalline métastable. À cause des contraintes spatiales exercées par la matrice amorphe, l’amas amorphe ne peut pas s’étendre. Dans ces circonstances, l’indice de coordination des atomes de Te est très élevé (jusqu’a ` six fois). Pendant le processus de commutation, les atomes se déplacent sur des distances allant jusqu’a ` 4.0 Å. Le déplacement moyen est de 2.36 Å. [Traduit par la Rédaction] 1. Introduction The complex chalcogenides with memory switching properties (phase change materials) are the core of the nonvolatile solid- state memory devices or phase change random access memory. The knowledge of the atomic structure of phase change mate- rials, both in metastable amorphous or crystalline states, is still challenging [1, 2] and will allow us a comprehensive under- standing of the physical properties of phase change materials. Structural modelling together with experimental techniques of investigation (X-ray diffraction, extended X-ray absorption fine structure, Raman and infrared spectroscopy, Mössbauer emis- sion, and absorption spectroscopy, X-ray difference anomalous scattering, etc.) can provide important information on the structural changes occurring during of the memory switching process [3–7]. In this paper we show that the ternary chalcogenide composi- tions that exhibit excellent memory switching are related to the border of glass formation domain (GFD). Also, a computer simula- tion of the transition from the nonconducting phase (amorphous) to the conducting one (crystalline) and back, on the particular case of GeAs 4 Te 7 (GeTe-(As 2 Te 3 ) 2 or Ge 8.34 As 33.33 Te 58.33 ) composition situ- ated close to the border of glass formation domain, has been made. A comparison between the experimental X-ray structure factors S(Q) and the corresponding radial distribution functions g(r) of a-Ge 14.3 As 28.6 Te 57.1 , the most recent and the nearest experimentally investigated composition (after our knowledge) and the simulated composition (a-Ge 8.34 As 33.33 Te 58.33 ), was done. 2. GFD The GFD is a compositional domain in the binary or ternary diagrams, which shows the experimentally found compositions that can form glasses. In Fig. 1 is represented by green dotted line the GFD for the Ge–As–Te system [8–10] and by filled circles some memory switching compositions as reported in the papers [11–14]. Most of the memory switching compositions are very close to the margin of the GFD, while others are outside or inside of GFDs but not very far from this border (Fig. 1). A detailed analysis on several ternary chalcogenide systems showing that memory switching chalcogenides compositions are situated mainly in the vicinity of the border of GFD is given in ref. 15. The compositions situated close to the GFD borderline contain very probably, a mixture of nanocrystalline and nanoamorphous particles. These amorphous clusters with such very small dimen- sions can change relatively easily the amorphous structure towards a crystalline one, and back, thus ensuring a rapid switching from insulating (amorphous) structure to the conducting (crystallized) phase [16]. 3. Structural simulation method To model the amorphous structure (the initial network) of GeAs 4 Te 7 , we need to know the coordination of each type of at- Received 15 October 2013. Accepted 7 February 2014. M. Popescu, F. Sava, A. Velea, A. Lo ˝ rinczi, and I.D. Simandan. National Institute of Materials Physics, Atomistilor 105bis, RO-077125, P.O. Box MG.7, Magurele-Ilfov, Romania. Corresponding author: F. Sava (e-mail: fsava@infim.ro). 1 This paper was presented at the 25th International Conference on Amorphous and Nanocrystalline Semiconductors (ICANS25). 675 Can. J. Phys. 92: 675–680 (2014) dx.doi.org/10.1139/cjp-2013-0561 Published at www.nrcresearchpress.com/cjp on 10 February 2014. Can. J. Phys. Downloaded from www.nrcresearchpress.com by 217.156.104.10 on 07/14/14 For personal use only.