Browsing by Author "Liang, Chi-Te"

Now showing 1 - 13 of 13

A delta-doped quantum well system with additional modulation doping
(2011)
Luo, Dong-Sheng
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Lin, Li-Hung
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Su, Yi-Chun
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Wang, Yi-Ting
;
Peng, Zai Fong
A delta-doped quantum well with additional modulation doping may have potential applications. Utilizing such a hybrid system, it is possible to experimentally realize an extremely high two-dimensional electron gas (2DEG) density without suffering inter-electronic-subband scattering. In this article, the authors report on transport measurements on a delta-doped quantum well system with extra modulation doping. We have observed a 0-10 direct insulator-quantum Hall (I-QH) transition where the numbers 0 and 10 correspond to the insulator and Landau level filling factor nu = 10 QH state, respectively. In situ titled-magnetic field measurements reveal that the observed direct I-QH transition depends on the magnetic component perpendicular to the quantum well, and the electron system within this structure is 2D in nature. Furthermore, transport measurements on the 2DEG of this study show that carrier density, resistance and mobility are approximately temperature (T)-independent over a wide range of T. Such results could be an advantage for applications in T-insensitive devices.
Atomic-scale epitaxial aluminum film on GaAs substrate
(2017-07-01)
Fan, Yen-Ting
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Lo, Ming-Cheng
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Wu, Chu-Chun
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Chen, Peng-Yu
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Wu, Jenq-Shinn
Atomic-scale metal films exhibit intriguing size-dependent film stability, electrical conductivity, superconductivity, and chemical reactivity. With advancing methods for preparing ultra-thin and atomically smooth metal films, clear evidences of the quantum size effect have been experimentally collected in the past two decades. However, with the problems of small-area fabrication, film oxidation in air, and highly-sensitive interfaces between the metal, substrate, and capping layer, the uses of the quantized metallic films for further ex-situ investigations and applications have been seriously limited. To this end, we develop a large-area fabrication method for continuous atomic-scale aluminum film. The self-limited oxidation of aluminum protects and quantizes the metallic film and enables ex-situ characterizations and device processing in air. Structure analysis and electrical measurements on the prepared films imply the quantum size effect in the atomic-scale aluminum film. Our work opens the way for further physics studies and device applications using the quantized electronic states in metals. (C) 2017 Author(s).
Berezinskii-Kosterlitz-Thouless transition in an Al superconducting nanofilm grown on GaAs by molecular beam epitaxy
(2020-05-15)
Su, Guan-Ming
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Wu, Bi-Yi
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Fan, Yen-Ting
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Kumar, Ankit
;
Chang, Chau-Shing
We have performed extensive transport experiments on a 4 nm thick aluminum (Al) superconducting film grown on a GaAs substrate by molecular beam epitaxy (MBE). Nonlinear current-voltage (I-V) measurements on such a MBE-grown superconducting nanofilm show that V similar to I-3, which is evidence for the Berezinskii-Kosterlitz-Thouless (BKT) transition, both in the low-voltage (T-BKT 1.97 K) and high-voltage regions (T-BKT 2.17 K). In order to further study the two regions where the I-V curves are BKT-like, our experimental data are fitted to the temperature-induced vortices/antivortices unbinding model as well as the dynamical scaling theory. It is found that the transition temperature obtained in the high-voltage region is the correct T-BKT as confirmed by fitting the data to the aforementioned models. Our experimental results unequivocally show that I-V measurements alone may not allow one to determine T-BKT for superconducting transition. Therefore, one should try to fit one's results to the temperature-induced vortices/antivortices unbinding model and the dynamical scaling theory to accurately determine T-BKT in a two-dimensional superconductor.
Characterization of Single-Crystalline Aluminum Thin Film on (100) GaAs Substrate
(2013-04-01)
Lin, Shi-Wei
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Wu, Jau-Yang
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Lin, Sheng-Di
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Lo, Ming-Cheng
;
Lin, Ming-Huei
We have studied the structure and physical properties of an aluminum thin film grown on a (100) GaAs substrate. The X-ray diffraction (XRD) data shows that the Al film grown in situ by molecular beam epitaxy (MBE) is single crystalline. Compared with the polycrystalline film ex situ evaporated using an electron-gun (E-gun), the MBE-grown Al film has a high optical reflectivity in the visible and ultraviolet (UV) regime. In addition, the MBE-grown film has a 2-order-lower residue resistance, a 1-order-higher temperature coefficient of resistance, and a 2-order-larger magnetoresistance (MR) than the polycrystalline film. Owing to the long mean free time, the bulk-like electron-to-hole transition of Hall resistivity is observed for the first time in a nanoscale metal thin film. Our results suggest that MBE-grown Al thin films have great potential applications in metal-based nanoelectronics and nanophotonics. (C) 2013 The Japan Society of Applied Physics
Efficient reduction of graphene oxide catalyzed by copper
(2012)
Lin, Yu-Chuan
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Liu, Keng-Ku
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Wu, Chih-Yu
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Chu, Chih-Wei
;
Wang, Jacob Tse-Wei
We report that copper thin films deposited on top of graphene oxide (GO) serve as an effective catalyst to reduce GO sheets in a diluted hydrogen environment at high temperature. The reduced GO (rGO) sheets exhibit higher effective field-effect hole mobility, up to 80 cm(2) V-1 s(-1), and lower sheet resistance (13 k Omega square(-1)) compared with those reduced by reported methods such as hydrazine and thermal annealing. Raman and XPS characterizations are addressed to study the reduction mechanism on graphene oxide underneath copper thin films. The level of reduction in rGO sheets is examined by Raman spectroscopy and it is well correlated with hole mobility values. The conductivity enhancement is attributed to the growth of the graphitic domain size. This method is not only suitable for reduction of single GO sheets but also applicable to lower the sheet resistance of Langmuir-Blodgett assembled GO films.
Electron heating and current scaling in a two-dimensional electron system in close proximity to nanoscale scatterers
(2014-05-01)
Wang, Yi-Ting
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Yang, Chun-Kai
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Lin, Sheng-Di
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Liang, Chi-Te
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電子工程學系及電子研究所
We have performed magneto-transport measurements on a two-dimensional electron system (2DES) which is in close proximity to nanoscaled scatterers. Weak localization, as evidenced by negative magnetoresistance, is observed. In this work, we use the extracted phase coherence rate as a thermometer to measure the electron\'s effective temperature T (e) in our 2DES when a high driving current I flows through the device. We find that T (e) ae I (similar to 0.52), consistent with 1/tau (ep) similar to T (2) in two dimensions, where 1/tau (ep) is the electron-phonon scattering rate. However, the phase coherence rate 1/tau (phi) similar to T, with a very small offset, is consistent with zero-temperature dephasing. Most importantly, our experimental results are in agreement with the fact that at low temperatures, the dominant phase-breaking mechanism is electron-electron scattering, not electron-phonon scattering. Therefore we are able to investigate both electron-electron scattering and electron-phonon scattering which are, in most cases, difficult to study independently in the linear region. Our data show that the electron heating effect is a very powerful tool for studying semiconductor devices.
Magnetotransport in an aluminum thin film on a GaAs substrate grown by molecular beam epitaxy
(2011)
Lo, Shun-Tsung
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Chuang, Chiashain
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Lin, Sheng-Di
;
Chen, Kuang Yao
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Liang, Chi-Te
Magnetotransport measurements are performed on an aluminum thin film grown on a GaAs substrate. A crossover from electron- to hole-dominant transport can be inferred from both longitudinal resistivity and Hall resistivity with increasing the perpendicular magnetic field B. Also, phenomena of localization effects can be seen at low B. By analyzing the zero-field resistivity as a function of temperature T, we show the importance of surface scattering in such a nanoscale film.
On the direct insulator-quantum Hall transition in two-dimensional electron systems in the vicinity of nanoscaled scatterers
(2011)
Liang, Chi-Te
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Lin, Li-Hung
;
Chen, Kuang Yao
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Lo, Shun-Tsung
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Wang, Yi-Ting
A direct insulator-quantum Hall (I-QH) transition corresponds to a crossover/transition from the insulating regime to a high Landau level filling factor nu > 2 QH state. Such a transition has been attracting a great deal of both experimental and theoretical interests. In this study, we present three different two-dimensional electron systems (2DESs) which are in the vicinity of nanoscaled scatterers. All these three devices exhibit a direct I-QH transition, and the transport properties under different nanaoscaled scatterers are discussed.
Pure electron-electron dephasing in percolative aluminum ultrathin film grown by molecular beam epitaxy
(2015-02-18)
Lin, Shih-Wei
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Wu, Yue-Han
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Chang, Li
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Liang, Chi-Te
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Lin, Sheng-Di
We have successfully grown ultrathin continuous aluminum film by molecular beam epitaxy. This percolative aluminum film is single crystalline and strain free as characterized by transmission electron microscopy and atomic force microscopy. The weak anti-localization effect is observed in the temperature range of 1.4 to 10 K with this sample, and it reveals that, for the first time, the dephasing is purely caused by electron-electron inelastic scattering in aluminum.
Topological Transition in a 3nm Thick Al Film Grown by Molecular Beam Epitaxy
(2019-12-27)
Kumar, Ankit
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Su, Guan-Ming
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Chang, Chau-Shing
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Yeh, Ching-Chen
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Wu, Bi-Yi
We have performed detailed transport measurements on a 3nm thick (as-grown) Al film on GaAs prepared by molecular beam epitaxy (MBE). Such an epitaxial film grown on a GaAs substrate shows the Berezinskii-Kosterlitz-Thouless (BKT) transition, a topological transition in two dimensions. Our experimental data shows that the MBE-grown Al nanofilm is an ideal system for probing interesting physical phenomena such as the BKT transition and superconductivity. The increased superconductor transition temperature (similar to 2.4K) compared to that of bulk Al (1.2K), together with the ultrathin film quality, may be advantageous for future superconductor-based quantum devices and quantum information technology.
Tunable insulator-quantum Hall transition in a weakly interacting two-dimensional electron system
(2013-07-03)
Lo, Shun-Tsung
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Wang, Yi-Ting
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Lin, Sheng-Di
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Strasser, Gottfried
;
Bird, Jonathan P.
We have performed low-temperature measurements on a gated two-dimensional electron system in which electron-electron (e-e) interactions are insignificant. At low magnetic fields, disorder-driven movement of the crossing of longitudinal and Hall resistivities (rho (xx) and rho (xy)) can be observed. Interestingly, by applying different gate voltages, we demonstrate that such a crossing at rho (xx) similar to rho (xy) can occur at a magnetic field higher, lower, or equal to the temperature-independent point in rho (xx) which corresponds to the direct insulator-quantum Hall transition. We explicitly show that rho (xx) similar to rho (xy) occurs at the inverse of the classical Drude mobility 1/mu (D) rather than the crossing field corresponding to the insulator-quantum Hall transition. Moreover, we show that the background magnetoresistance can affect the transport properties of our device significantly. Thus, we suggest that great care must be taken when calculating the renormalized mobility caused by e-e interactions.
Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures
(2014-01-23)
Zhang, Wenjing
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Chuu, Chih-Piao
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Huang, Jing-Kai
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Chen, Chang-Hsiao
;
Tsai, Meng-Lin
Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light-sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108. Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.
Wafer-scale MoS2 thin layers prepared by MoO3 sulfurization
(2012)
Lin, Yu-Chuan
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Zhang, Wenjing
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Huang, Jing-Kai
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Liu, Keng-Ku
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Lee, Yi-Hsien
Atomically thin molybdenum disulfide (MoS2) layers have attracted great interest due to their direct-gap property and potential applications in optoelectronics and energy harvesting. Meanwhile, they are extremely bendable, promising for applications in flexible electronics. However, the synthetic approach to obtain large-area MoS2 atomic thin layers is still lacking. Here we report that wafer-scale MoS2 thin layers can be obtained using MoO3 thin films as a starting material followed by a two-step thermal process, reduction of MoO3 at 500 degrees C in hydrogen and sulfurization at 1000 degrees C in the presence of sulfur. Spectroscopic, optical and electrical characterizations reveal that these films are polycrystalline and with semiconductor properties. The obtained MoS2 films are uniform in thickness and easily transferable to arbitrary substrates, which make such films suitable for flexible electronics or optoelectronics.