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(A) study on electrical and nano-structural characteristics of In-Sb-Te ternary alloys with graphene and carbon for high density phase change memory 원문보기

  • 저자

    김현수

  • 학위수여기관

    Graduate School, Korea University

  • 학위구분

    국내박사

  • 학과

    전기전자전파공학과

  • 지도교수

    성만영

  • 발행년도

    2014

  • 총페이지

    xvii, 112장

  • 키워드

    PRAM Graphene stack Carbon doping;

  • 언어

    eng

  • 원문 URL

    http://www.riss.kr/link?id=T13541893&outLink=K  

  • 초록

    For the multi-bit storage phase change memory, new chalcogen alloys such as In-Sb-Te, Ga-Te-Sb and Si-Sb-Te have been suggested because Ge2Sb2Te5 (GST) has a limitation for the application of multibit storage. Therefore, it is very important to understand the phase change mechanism of new chalcogen materials because the feasibility of multi-bit storage closely related to the multi-level phase change mechanism. According to previous results, In3Sb1Te2 (IST) shows multi-level phase transformation phenomena from amorphous to crystalline IST through several intermediate phases. However, this multi-level phase transformation lead an abrupt volume change because of the movements of vacancy and the atomic migration. In this thesis, carbon doping In-Sb-Te materials, which is suspect to enhance the reliability rather than InSbTe ternary alloy, is investigated to reveal the effect of carbon doping in In3Sb1Te2. Bi-layer of carbon doped InTe (C-InTe) and InSb is annealed over crystallization temperature to study the movement of carbon atoms. The crystallization mechanism of carbon doped IST can be explained by the observation of the boundary reaction of C-InTe / InSb with a high resolution-TEM and AES. In3Sb1Te2 (IST) shows multi-level phase transformation from amorphous to several crystalline materials of IST, intermediate phases such as InSb, SbTe and InTe. However, its volume can change abruptly in the multi-level phase transformation, and this change can lead to vacancy movement and atomic migration, which are related to failures and reliability issues. First, the carbon doped In3Sb1Te2 (C-IST) alloys was investigated on thermal and micro structural properties to look for higher retention ability than the IST ternary alloy. Carbon atoms delay crystallization and prevent volume change during the set/reset operation. It is proved by measuring sheet resistance with increasing temperature and taking differential scanning calorimeter (DSC) analysis. The carbon concentration is 12.5 %, and the activation energy increases from 5.1 eV to 5.4 eV. To investigate its electrical characteristics, PRAM devices were fabricated using proposed chalcogen materials and structure. The device had a simple layered structure (bottom electrode/chalcogen layer/top electrode). The contact region between the bottom and phase change materials was fabricated by focused ion beam (FIB) method. The contact dimension is changed from 100nm to 500nm. Reset and Set Pulse generated with Kitheley 3402 pulse generator and current voltage (I-V) characteristics were measured with Kitheley-4200 semiconductor analyzer. The I-V results show us that the four distinguishable state and retention ability is over than 103 times. Overall the study of the microstructure, thermal stability and electrical properties, the carbon doping InSbTe materials can be summarized three points. First, the carbon doping has affected grain size because of higher crystallization activation energy. Second, the carbon atoms exist in the vacancy site of InSbTe, so the atomic migration is difficult to occur. Third, the crystal graphite also blocks InSb crystalline which mixed with InTe. The carbon atoms limited volume changes during set/reset operations. It also increased the crystallization activation energy. The activation energies are 5.138, 5.278 and 5.398 eV when carbon concentrations are 0 %, 8.4 % and 12.5 %, respectively. The activation energy increases with carbon concentration. This means that the carbon atoms are an impurity that obstructs the crystallization of IST and make more robust materials than IST and Ge2Sb2Te5, which activation energy are 5.1 eV and 3.31 eV, respectively. Carbon atoms also limited volume changes during set/reset operations. It also increased the crystallization activation energy. Carbon atoms prevent the contact between InSb and InTe phase. Therefore, carbon doped IST films are promising to improve multi-level-cell ability based on these results. Second, using the graphene as a stable interlayer for a multi-stacking structure of GST and IST is studied. The results show that the graphene layer remains as a stable barrier between GST and IST during set/reset operations. The microstructure of 1st chalcogen/graphene/2nd chalcogen layers does not show atomic inter diffusion between first chalcogen and second chalcogen materials through the graphene layer. It is observed that the graphene layer prevents atomic movement through it, even at the melting temperatures of GST and IST. Consequently, the multi-stacking structure exhibits six distinguishably separated resistance levels due to the unique thermal and electrical properties of the graphene interlayer.


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