Browsing by Author "Ku, Shin-An"

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Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators
(2015-09-15)
Tu, Chien-Ming
;
Yeh, Tien-Tien
;
Tzeng, Wen-Yen
;
Chen, Yi-Ru
;
Chen, Hsueh-Ju
Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located similar to 1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.
Nanoindentation of GaSe thin films
(2012-07-17)
Jian, Sheng-Rui
;
Ku, Shin-An
;
Luo, Chih-Wei
;
Juang, Jenh-Yih
;
電子物理學系
The structural and nanomechanical properties of GaSe thin films were investigated by means of X-ray diffraction (XRD) and nanoindentation techniques. The GaSe thin films were deposited on Si(111) substrates by pulsed laser deposition. XRD patterns reveal only the pure (000 l)-oriented reflections originating from the hexagonal GaSe phase and no trace of any impurity or additional phases. Nanoindentation results exhibit discontinuities (so-called multiple 'pop-in' events) in the loading segments of the load-displacement curves, and the continuous stiffness measurements indicate that the hardness and Young's modulus of the hexagonal GaSe films are 1.8 +/- 0.2 and 65.8 +/- 5.6 GPa, respectively.
Nanomechanical properties of GaSe thin films deposited on Si(111) substrates by pulsed laser deposition
(2012-11-25)
Jian, Sheng-Rui
;
Juang, Jenh-Yih
;
Luo, Chih-Wei
;
Ku, Shin-An
;
Wu, Kaung-Hsiung
The correlations between the crystalline structure and mechanical properties of GaSe thin films were investigated by means of X-ray diffraction (XRD) and nanoindentation techniques. The GaSe thin films were deposited on Si(111) substrates deposited at various deposition temperatures using pulsed laser deposition (PLD). The XRD results indicate that all the GaSe thin films are pure hexagonal phase with highly (000l)-oriented characteristics. Nanoindentation results revealed apparent discontinuities (so-called multiple "pop-in" events) in the load-displacement curve, while no discontinuity was observed in the unloading segment of the load-displacement curve. The hardness and Young's modulus of GaSe thin films determined by the continuous stiffness measurements (CSM) method indicated that both mechanical parameters increased with the increasing deposition temperature with the hardness and the Young's modulus being increased from 1.2 +/- 0.1 to 1.8 +/- 0.1 GPa and from 39.6 +/- 1.2 to 68.9 +/- 2.7 GPa, respectively, as the deposition temperature was raised from 400 to 475 degrees C. These results suggest that the increased grain size might have played a prominent role in determining the mechanical properties of the PLD-derived GaSe thin films. (C) 2012 Elsevier B. V. All rights reserved.