| Summary: | 碩士 === 國立清華大學 === 物理系 === 103 === Topological insulators (TI) were predicted and proven to have Dirac cone-like metallic helical surface state with strong spin-orbital coupling, making the TI a very promising spintronics material. However, the high density of intrinsic defects in TI materials has caused high doping levels, and made the Fermi level (EF) to locate in the bulk conduction band. Nevertheless, the electrical field effect may be utilized to tune the EF to move toward the conduction band. We proposed to fabricate the back gate structure by growing TI films on high- oxide layers deposited on conducting substrates. The advantage of such a structure is to leave the top surface entirely open for subsequent fabrications of FM/TI, SC/TI structures intended for various studies. Additionally, employing high- oxide thin films as the dielectrics can effectively reduce the operating voltage.
A series of high quality Bi2Se3 thin films with thickness 4QL~150QL were grown on various substrate to study the transport property. Weak anti-localization effect was observed in low temperature transport measurement indicating the presence of the time-reversal symmetry protected surface state. The derived α~0.4 to 0.7, which shows there is 1 to 2 independent 2D channels in TI, consistent with existence of the top and bottom surface states. l_ϕ is derived ranging from 100nm to 300nm, about the same order of typical sizes of the triangular domains of Bi2Se3 thin films. For the sample thickness above 20nm, l_ϕ is nearly a constant value, implying the WAL effect is not influenced by the bulk, and should be largely contributed by the surface state. For Bi2Se3 film grown on YIG, topological surface channel was observed by the much smaller α, l_ϕ and surface state contribution, suppressed by magnetic order from YIG. Moreover, a series of Hall effect measurements for Bi2Se3 films with different thickness grown on various substrate were conducted. We found that n3D decreases with increasing the film thickness regardless of the substrate, which means better film quality can be obtained with increasing the film thickness. And our film quality is superior to S. Oh’s for film thickness above 20QL. For the films grown on MoS2, surprisingly, n3D is the smallest, about only half of the sample grown on sapphire for 14QL sample. And it is found that n3D does not decrease as the lattice mismatch of the substrate becomes smaller. The effect of employing an Se capping layer to prevent film degradation in ambient condition was also studied.
High quality Bi2Se3 thin films (6QL~10QL) were grown on high- oxide including Al2O3 and Al2O3/Y2O3 multi-layer to fabricate the back gate structure. The film is highly crystalline along c-axis confirmed by RHEED, AFM, TEM and XRD. Large field effect was observed. We successfully tuned the EF within the band gap making the surface state become the dominant state responsible for carriers. Furthermore, to develop the best capability of tuning EF, various tests were performed, including cutting down TI film thickness, doping Te to Bi2Se3 to reduce intrinsic defects, and inserting Y2O3 layer to Al2O3 to enhance the effective dielectric constant. We are able to tune EF very close to Dirac point in ternary compound Bi2Te2Se thin films.
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