2014 Fiscal Year Research-status Report
MBE Growth of Topological Insulators
| Project/Area Number |
25400328
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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 insulator / MBE |
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| Outline of Annual Research Achievements |
Two main directions of the research this year were: 1) finding the growth conditions for insulating (Bi1-xSbx)2Te3 (BST) films that allow to make devices with highly efficient electrostatic gating to control the surface transport of Dirac fermions; 2) finding/optimizing conditions for growth of the family of Topological Crystalline Insulators (TCI).
1) The first prerequisite for successful control of the surface transport is a bulk-insulating state of grown films, which has been achieved last year. The second step was a successful fabrication of a very efficient top gate using a low-temperature deposition of SiNx. The further achievement was a developing a comprehensive method for fabricating dual-gated devices that includes the growth of insulating BST films which can be transferred onto a Si/SiO2 wafer. The dual gating allows effective tuning of the chemical potentials of both the top and bottom surfaces across the Dirac point.
2) We have grown series of high-quality (Pb1-xSnx)Te and (Pb1-xSnx)Se thin films on various substrates and buffer layers, including BaF2, CdTe, and Bi2Te3. The challenge to probe surface states (SS) in TCIs stems from a very high concentration of holes in these materials. To solve this problem, we used an enhanced surface-to-bulk ratio in thin films. For series of SnTe films with decreasing thickness we were able to observe the change of carrier type, signifying the domination of surface charge in ultrathin films. Substituting Sn for Pb is another approach which potentially can reduce the bulk conductivity in this family of TCI materials.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
Both goals for this year, 1) finding the growth conditions for BST films which can be used for highly efficient electrostatic gating and 2) optimizing conditions for growth of high-quality (Pb1-xSnx)Te and (Pb1-xSnx)Se thin films, have been successfully achieved.
1) Experimental studies of novel quantum phenomena predicted for 3D topological insulators often require tuning of the Fermi level across the Dirac point. Both back gating and top gating techniques have been successfully applied to 3D TI thin films, however, with a single gate the chemical potential of only one surface can be controlled effectively. We have succeeded in developing a comprehensive method for fabricating dual-gated devices on high-quality insulating BST films. This dual gating allows effective tuning of the chemical potentials of the top and bottom surfaces across the Dirac point, which is manifested in a large peak of the sheet resistance accompanied by a sharp sign change of Ryx upon sweeping the top and bottom gate voltages.
2) High-quality (Pb1-xSnx)Te and (Pb1-xSnx)Se films have been grown on various substrates and buffer layers. The best quality films have been obtained on Bi2Te3-on-Al2O3. However, both SnTe and Bi2Te3 are topological materials, and, moreover, they form a broken gap interface which cannot electrically isolate both layers. To solve this problem, we developed the growth of (Pb1-xSnx)Te and (Pb1-xSnx)Se films on including BaF2–on-GaAs and CdTe–on-GaAs buffer layers. The main remaining problem is a relatively high bulk conductance of grown TCI films.
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| Strategy for Future Research Activity |
Next year research will use both major advantages of MBE growth technique in the field of TI research, the growth of high-quality crystals in the thin-film form with increased surface-to-bulk ratio and their compatibility with micro- and nano-device fabrication indispensable for many proposed experiments based on TIs. Based on the developed this year comprehensive method for fabricating dual-gated devices, we are going to test several exiting proposals related to the surface transport of Dirac electrons in topological insulators. In particular, the role of spin-momentum locking in spin transport and magnetoresistance in films where the concentration, type of charge carries, and, hence, their helicity, can be controlled by top and bottom gates independently. For now, there are only several groups in the world which can produce such devices.
For TCIs, we will focus on (Pb1-xSnx)Se, which has a highest mobility among our grown TCI films. To reduce bulk conductivity, we plan to optimize the doping of this material with elemental Eu by its co-evaporation during MBE growth.
We also consider the growth of new topological materials. In particular, it seems interesting to try to grow a new natural hetero-structure Pb5Bi24Se41 material as a thin film.
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| Causes of Carryover |
JPY17818 was left to buy equipment in the next year.
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| Expenditure Plan for Carryover Budget |
I plan to use JPY17818 left from the previous year to buy bakable sensor for MBE electron beam deposition chamber.
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