XRD analysis of TRAM composed from [Sb 2 Te 3 /GeTe] superlattice film and its switching characteristics
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XRD analysis of TRAM composed from [Sb2Te3/GeTe] superlattice film and its switching characteristics T.Ohyanagi1, M.Kitamura1, S.Kato2, M.Araidai3, N.Takaura1 and K. Shiraishi2,3 1 Low-power Electronics Association & Projects, AIST West7A, Tsukuba, Japan 2 Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan 3 Department of Computational Science and Engineering, Nagoya University, Nagoya, Japan ABSTRACT We studied GeTe structures in topological switching random access memories (TRAMs) with a [GeTe/Sb2Te3] superlattice by using X-ray diffraction (XRD) analysis. We examined the electrical characteristics of the TRAMs deposited at different temperatures. We found that XRD spectra differed between the films deposited at 200 and 240°C and that the differences corresponded to the differences in the GeTe sequences in the films. INTRODUCTION Interfacial phase change memory (i-PCM) [1] was first introduced by Dr. Tominaga’s group at The National Institute of Advanced Industrial Science Technology (AIST) in Japan. This memory was composed of a stacked structure (superlattice structure) of Sb2Te3 and GeTe. However, the transition mechanism was slightly different from that in conventional PCM, such as that obtained with Ge2Sb2Te5. The transitions between a high resistance state (HRS) and a low resistance state (LRS) in conventional PCM occur due to changes between amorphous and crystalline phases and these changes are caused by heat. The transitions between HRS and LRS in iPCM occur due to short movements by Ge atoms at the interface between GeTe and Sb2Te3 [2]. Although these changes are not caused by heat [3], the driving force behind them is not yet known. We have frequently been asked why we used the term of “phase change” for i-PCM. The mechanism responsible for transitions in i-PCM is completely different from that in conventional PCM. We therefore needed to find another term to distinguish i-PCM from conventional PCM. The superlattice of Sb2Te3 and GeTe also recently demonstrated interesting topological insulating behaviors [4]. Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap of such systems are protected by time-reversal symmetry [5]. Moreover, topological switching means the change in resistance caused by an ordered change in atomic and electronic structures. We and Dr. Tominaga give created the acronym “TRAM” (topological switching RAM) for as the new i-PCM [6]. TRAM is a promising candidate for replacing flash memories as next-generation solidstate memories. It has been found that their reset energy is approximately one order of magnitude lower than that of conventional PCMs [1, 2].
EXPERIMENT TRAM films with a [Sb2Te3/GeTe] superlattice were prepared with a multi-cathode DC sputtering system. These films were deposited on 300 mm Φ SiO2/Si substrates and the deposition temperatures ranged from 200 to 240°C. The Sb2Te3 was 5 nm thick and the GeTe was 1 nm thick and there were 8 repetitions [7]. Moreover, the TRAM films
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