| 研究課題/領域番号 |
18K13807
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| 研究機関 | 国立研究開発法人産業技術総合研究所 |
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研究代表者 |
Aurelie Spiesser 国立研究開発法人産業技術総合研究所, エレクトロニクス・製造領域, 研究員 (90793513)
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| 研究期間 (年度) |
2018-04-01 – 2020-03-31
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| キーワード | oxide tunnel barrier / epitaxial growth / spin transport in Si / coherent tunneling |
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| 研究実績の概要 |
As a first step, metallic Fe/SrO/MgO/Fe magnetic tunnel junctions (MTJs) were fabricated to investigate whether coherent tunneling occurs through the SrO oxide tunnel barrier. SrO was evaporated from SrO crystals using electron beam gun evaporation in an ultra-high vacuum molecular beam epitaxy system. During the growth, the reflection high energy electron diffraction (RHEED) was used to monitor the crystallinity of the grown layers. Streaky RHEED patterns were obtained after the deposition of SrO layer on MgO(001) on Fe(001) at Tg = 100C, indicating the growth of epitaxial SrO(001) on MgO(001). The sample was processed into MTJ devices for magneto-transport measurements. The tunnel magnetoresistance ratio (TMR) of Fe/SrO/MgO/Fe magnetic tunnel junctions reached ~ 90% at room temperature, suggesting that coherent tunneling occurs through SrO tunnel barrier.
In a second part, I investigated the epitaxial growth of SrO on a Si(001) substrate. 2 nm-thick SrO layers were grown on Si at substrate temperatures (Tg) ranging from 0 to 150 °C using a two-step method. This method consists in the deposition of the first 0.5 nm thick SrO layer without oxygen pressure, followed by the deposition of the rest of the SrO layer under a partial oxygen pressure. Crystalline SrO(001) was obtained only at Tg = 30C, indicating the growth of epitaxial ultrathin SrO film on Si(001). A 10 nm thick Fe layer was then deposited on the top of the crystalline SrO layer. The sample is currently under processing for the fabrication of spin transport devices.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
3: やや遅れている
理由
Since SrO is better lattice-matched to Si compared to MgO and the growth of epitaxial SrO(001) layers on Si(001) has already been reported, I investigated the use of SrO as a novel oxide tunnel contact for spin injection in Si. However, the epitaxial growth of SrO on Si(001)
remains challenging mostly due to the strong reactivity and instability of SrO. So far, only SrO with a poor crystallinity were obtained on Si, according to the RHEED observations. One possible origin for this is the use of SrO crystals with an electron beam gun instead of metallic Sr in an effusion cell with O2 gas supply.
Therefore, the fabrication of Si-based spin transport devices using Fe/SrO magnetic tunnel contacts has been delayed because further improvement of the crystalline quality was first required. Note that a large TMR ratio was obtained in metallic Fe/SrO/MgO/Fe magnetic tunnel junctions, indicating that coherent tunneling occurs through the SrO tunnel barrier. This first result suggests that SrO has the potential to be an effective tunnel barrier for spin injection into Si.
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| 今後の研究の推進方策 |
In my next experiments, I will further improve the growth conditions of SrO on Si(001) in order to obtain a fully epitaxial Fe/SrO on Si(001). In particular, I will explore the use of different types of Si(001) surface reconstructions at various growth temperature. I also plan to use a strontium-induced deoxidation process of the Si surface before the growth of SrO on Si(001).
If these experiments are not successful, I will investigate another oxide tunnel contact on Si, such as the single-crystalline spinel MgAl2O4(001), for spin injection into Si. Since coherent tunneling effect has been demonstrated in MgAl2O4(001) barrier and over 300% TMR ratios at room temperature were reported in MgAl2O4-based MTJs, this spinel oxide has a potential to be an efficient tunnel barrier for Si.
I also intend to explore the incorporation of dopants such as Sr, Ba, or Ca in the crystalline MgO(001) oxide barrier of a Fe/MgO tunnel contact on Si to tune the lattice constant of MgO to better match that of Si and eventually enhance coherent spin tunneling transport in the junction.
After obtaining fully epitaxial ferromagnet (FM)/oxide tunnel contacts on Si, I will fabricate lateral spin transport devices and carry out magneto-transport measurements to quantify the spin polarization of the FM/oxide tunnel contact on Si and compare it with the current Fe/MgO tunnel contact on Si.
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