Shih-Hsun Yu

Extremely high-performance visible light photodetector in the Sb 2 SeTe 2 nanoflake

The photocurrent was performed in the Sb2SeTe2 topological insulator at a wavelength of 532 nm. It exhibits extremely high performance that the responsivity and the photoconductive gain reach 2293 AW−1 and 5344 at 1 V. This high photoresponse is orders of magnitude higher than most reported values in topological insulators and two-dimensional transitional metal dichalcogenides. This finding suggests that the Sb2SeTe2 nanoflake has great potential for future optoelectronic device applications.

Microstructural and optical properties of high-quality ZnO epitaxially grown on a LiGaO2 substrate

High-quality ZnO films have been reproducibly grown on a LiGaO2 substrate by chemical vapor deposition with optimized growth parameters. The in-plane epitaxial relationship of ZnO[100]//LiGaO2[100] is revealed by transmission electron microscopy (TEM). The surface morphology of as-grown ZnO films characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM) demonstrates a very smooth surface with a root-mean-square roughness of 0.6 nm. The full width at half maximum value (FWHM) of 0.03° for the (0002) X-ray rocking curve shows the excellent crystalline quality of as-grown ZnO films. High transparency of the ZnO films in the visible range was revealed by optical transmittance measurements. The strong near-band-edge UV emission accompanied by a negligible defect-related green band emission suggests that the as-grown ZnO films have a high optical quality. Microstructural properties and strain state of the samples were also studied by TEM, X-ray diffraction (XRD) reciprocal space maps and Raman scattering analysis. Substrate compliant deformation, confirmed by the high-resolution TEM analysis, is suggested to be beneficial to decrease the compressive strain in the epilayer and increase the quality of the ZnO. This facile growth process towards high-quality ZnO opens up a broad potential application of high-efficiency ZnO-based devices on a LiGaO2 substrate.

The linear magnetoresistance from surface state of the Sb2SeTe2 topological insulator

A non-saturating linear magnetoresistance (MR) is observed in Sb2SeTe2 topological insulator. The results show that the MR slope and the critical magnetic field of the linear MR are proportional to the carrier mobility and inverse mobility, respectively. These are consistent with the prediction of a model, which is constructed by Parish and Littlewood [Nature 426, 162 (2003)], in the weak mobility fluctuation condition. The Kohler plot of the magnetoresistance does not collapse onto a single curve that supports the multi-carriers scattering mechanisms.

Observation of surface oxidation resistant Shubnikov-de Haas oscillations in Sb2SeTe2 topological insulator

The robustness of Sb2SeTe2 topological insulators against surface oxidation has been comparatively investigated through their magneto-transport and X-ray photoelectron spectroscopic properties with samples freshly cleaved or exposed to air over various timeframes. The magnetoresistance data exhibit Shubnikov-de Haas oscillations with the same period of oscillations for all samples regardless of surface oxidation, whereas the core-level electron binding energies of the constituent elements vary. That there is no shift in Fermi levels and no smearing-out in the amplitude of oscillations suggests that the surface states of the studied topological insulators are impervious to surface oxidation.

Two-carrier transport-induced extremely large magnetoresistance in high mobility Sb2Se3

Large magnetoresistance (MR) has been widely reported in the A2B3 (A = Sb or Bi; B = Se or Te) family of topological insulators (TIs). Sb2Se3 is not a TI that was confirmed by the extracted zero Berry phase and the X-ray diffraction. An extremely large MR was observed in the Sb2Se3 crystals. This large MR increased quadratically with the magnetic field applied. The observed MR ratio was 830% at 10 K and 9 T, which was larger than that previously reported for all A2B3 family TIs. This large MR originated from two carriers with high mobility. The inversely square root of the MR ratio was proportional to the resistance.

Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal

A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications.

Shubnikov–de Haas oscillation of Bi2Te3 topological insulators with cm-scale uniformity

A topological insulating Bi2Te3 single crystal was successfully grown with good uniformity using a home-made resistance-heated floated zone furnace. The temperature-dependent resistance and Hall voltage confirm that the transport is metallic and the overall carriersareholes. The angle and temperature dependence of the quantum Shubnikov–de Haas oscillation period amplitude suggests that the transport comes from the carriers of surface states. The Berry phase, determined from Landau level diagram, also reveals that the transport carriers are Dirac fermions. In contrast with many previous publications, the transport parameters relating to the surface carriers derived from the relationship of the Lifshitz–Kosevich (LK) theory are consistent with angle resolved photoemission spectroscopy results.

Thickness-dependent conductance in Sb 2 SeTe 2 topological insulator nanosheets

The conductivity increases as thickness decreases in a series of Sb2SeTe2 topological insulator nanosheets with thickness ranging from 80 to 200 nm, where the sheet conductance is proportional to the thickness. The corresponding sheet conductance of the surface state is 8.7 e2/h which is consistent with the values extracted from the temperature dependent Shubnikov-de Haas oscillations at high magnetic fields. The extracted Fermi momentum is the same as the results from the ARPES value, and the Berry phase is π. These support that the thickness dependent sheet conductance originates from the combination of the surface state and the bulk state.

The temperature dependence of the crossover magnetic field of linear magnetoresistance in the Cu0.1Bi2Se3

A non-saturating linear magnetoresistance (MR) is observed in Cu0.1Bi2Se3 in a wide range of temperatures. The crossover magnetic field, B*, deviating from the linear MR, increases as the temperature increases. The experimental results show that the normalized B*, inverse MR slope and mobility follow the same temperature dependence that is consistent with the model constructed by Parich and Littlewood (PL model). The mechanism of the T 2 dependent B* is systematically and comprehensively discussed through existing theories, and might be due to the electron−electron scattering in a highly uniform system with a few low mobility defects.

Enhancement of carrier transport characteristic in the Sb 2 Se 2 Te topological insulators by N 2 adsorption

The carrier transport characteristics of Sb2Se2Te topological insulators were investigated, after exposure to different levels of nitrogen gas. The magnetoresistance (MR) slope for the Sb2Se2Te crystal increased by approximately 100% at 10 K after 2-days of exposure. The Shubnikov-de Haas (SdH) oscillation amplitude increased by 30% while oscillation frequencies remained the same. MR slopes and the mobilities had the same dependency on temperature over a wide temperature range. All measured data conformed to a linear correlation between MR slope and mobility, supporting our hypothesis that the MR increase and the SdH oscillation enhancement might be caused by mobility enhancement induced by adsorbed N2 molecular.