2013 Fiscal Year Research-status Report
MBE Growth of Topological Insulators
| Project/Area Number |
25400328
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Research Institution | Osaka University |
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Principal Investigator |
TASKIN Alexey 大阪大学, 産業科学研究所, 助教 (20523533)
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| Project Period (FY) |
2013-04-01 – 2016-03-31
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| Keywords | topological / MBE |
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| Research Abstract |
The focus of this project is on the growth of high-quality thin films of Topological Insulators (TIs) using Molecular Beam Epitaxy (MBE). Two main directions of the research this year were: 1) finding the growth conditions that allow to suppress the bulk conductivity in Bi2Se3/Bi2Te3 family; 2) finding/optimizing conditions for growth of the newly discovered Topological Crystalline Insulator (TCI) SnTe.
1) The successful approach to solve the problem of high bulk conductivity in Bi2(SeTe)3 is based on co-evaporation of Bi, Sb, and Te to grow (Bi1-xSbx)2Te3 compound which in the range of 0.75<x<0.85 can be grown as a bulk-insulating TI. Using these films, we were able to fabricate very efficient top-gate devices, in which surface carriers can be tuned from n- to p-type through the Dirac point, giving evidence of the truly insulating bulk with decoupled top and bottom surfaces.
2) For epitaxial growth of SnTe, the choice of the substrate was a crucial first step. We used rhombohedral Bi2Te3 buffer layers of very high quality (thanks to optimized growth conditions from our previous studies of TIs). The building block of Bi2Te3 is a quintuple layer terminated with a hexagonal Te plane, which naturally accommodates the Sn layer of the SnTe in the (111) plane. This allowed us to grow high-quality SnTe films, which are actually suitable for probing the topological surface states (TSS): We observed low-frequency SdH oscillations which give evidence for two-dimensional (2D) Dirac fermions residing on the free SnTe (111) surface. This is the first transport observation of TSS in TCI.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
Both goals for this year, to make highly insulating thin films of "ordinary" Z2 TI and to grow a new TCI, have been successfully achieved.
1) Several approaches have been tried to solve the problem of high bulk conductivity in Bi2Se3/Bi2Te3 films, including (i) Bismuth-Chalcogen stoichiometry optimization, (ii) growth of Bi2(Te1-xSex)3 series with specific emphasis on Bi2Te2Se and Bi2TeSe2 compositions (which are very insulating in a bulk-crystal form), (iii) doping during the growth with impurities such as cupper and antimony. The antimony doping turned out to be successful.
2) One of the best SnTe epitaxial films have been grown, in which we observed surface Dirac fermions for the first time in TCI. These thin-film samples create a good platform for fabricating novel devices in order to experimentally explore the physics of both Z2 TIs and TCIs.
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| Strategy for Future Research Activity |
MBE growth technique has two major advantages in the field of TI research: First, it allows to grow perfect crystals in the form of thin films with a naturally increased surface-to-bulk ratio and, second, thin films are ideally suited for micro- and nano-device fabrication indispensable for many proposed experiments based on TIs. Both of these advantages would help in the next year research. Based on what was achieved this year, we are going to elaborate on (Bi1-xSbx)2Te3 insulating films to make them a 2D TI with the ultimate domination of low dimensional conductance channels. In films with thickness below the critical one, for which the hybridization gap is expected to open, we should be able to assess the 1D edge channel using top/bottom gates for placing the Fermi level into the gap. Similar approach can be used for SnTe TCI, which has very different TSS. One of the main issues that should be solved next year for SnTe films is the growth and optimization of a new buffer layer. The reason is that Bi2Te3, which is a heavily doped n-type material and could make a p-n junction with a heavily doped p-type SnTe, in fact, forms a heterojunction with an exotic broken-gap lineup of the conduction and valence bands at the interface with SnTe (as we learned studying I-V characteristic across the SnTe/Bi2Te3 interface) and, thus, cannot be electrically isolated from SnTe layer. A new buffer layer should be insulating and can be used as a bottom gate. Other TI materials will be also explored.
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| Expenditure Plans for the Next FY Research Funding |
本年度の予算の残額は、MBE法トポロジカル絶縁体薄膜作製を最適化するための基板を購入することに充当する。
研究開始当初からのトポロジカル絶縁体薄膜成長は幸いにも既存のサファイア基板を使って成功させることができた。しかしながら、来年度はMBE用自動シャッターコントローラーシステムを導入予定であり、それに合わせて新たな基板をさらに試みることが研究推進上非常に有効であると考えられるため、予算の一部を次年度に繰り越すこととした。
利用計画は以下の物品の購入である。
1)BaF2,CdS, (CdZn)Teなどの基板 2)Si(111), Si(100), GaAs(111), GaAs(100)面を持つ基板
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