Electronic Excitation Induced Solid-State Amorphization in Ge-Sb-Te Alloy
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Electronic Excitation Induced Solid-State Amorphization in Ge-Sb-Te Alloy
Xian-Bin Li,1 X.Q. Liu,2 Xin Liu,3, 4 Dong Han,1 Z. Zhang,2 X.D. Han,2 and S. B. Zhang1, 3 1
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and
Engineering, Jilin University, Changchun 130012, China 2
Institute of Microstructure and Property of Advanced Materials, Beijing University of
Technology, Beijing100022, China 3
Department of Physics, Applied Physics, & Astronomy, Rensselaer Polytechnic Institute, Troy,
New York 12180, USA 4
School of Chemistry, Dalian University of Technology, Dalian 116024, China
ABSTRACT On the basis of ab initio molecular dynamics modeling, we show that Ge-Sb-Te alloy under excitation can realize amorphization without going through a liquid phase. The electronic structure analysis further reveals that the excitation mainly involves the Ge s-like states near the valence band maximum. After the phase transition, the coordination number of Ge is reduced from six to four, while the change in the coordination number for Sb is noticeably less. INTRODUCTION Phase change materials, such as Ge-Sb-Te alloys, have been widely used in the field of optical storage and may also play an important role in the next-generation microelectronic memory devices due to their extremely fast phase-change speed and non-volatile characteristics [1]. The binary memory effect is achieved under ultrashort laser pulse or an electrical pulse by transforming between a high-reflectivity/low-resistivity crystal phase and a low-reflectivity/highresistivity amorphous phase. It is remarkable that the time scale of the phase transition is in the range of nanosecond or less. A number of questions regarding the phase change dynamics at the atomic scale remain elusive. By extended X-ray absorption fine-structure spectroscopy (EXAFS) investigation of local configuration for crystalline and laser induced amorphous samples, Kolobov et al. [2] proposed a mechanism of Ge umbrella flip from an octahedral position to a tetrahedral position. However, Kim et al. [3] carefully analyzed the electronic structure of amorphous samples based on hard X-ray photoemission spectroscopy (HX-PES) and concluded that amorphous structure of Ge2Sb2Te5 (GST) is essentially sustained by p-like bonds as in the crystalline phase, thus their experimental results are not compatible with the sp3-like bond
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formation mechanism proposed by Kolobov. On the other hand, ab initio molecular dynamics (AIMD) simulation has unique advantage of providing insights into atom-scale motion during phase transition. Recently, calculation by Sun et al. [4] showed large voids in GST that are mainly surrounded by Te atoms both in molten and amorphous GST. They concluded that vacancies and Te may play an important role in the fast phase transition. At present, it is generally accepted that amorphization of Ge-Sb-Te is a result of melting and subsequently rapid quenching. However, the effect of a laser on a solid may not only be a simple “heating” especially w
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