Browsing by Author "Hsu, Wei-Ting"

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Band Alignment of 2D Transition Metal Dichalcogenide Heterojunctions
(2017-05-18)
Chiu, Ming-Hui
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Tseng, Wei-Hsuan
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Tang, Hao-Ling
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Chang, Yung-Huang
;
Chen, Chang-Hsiao
It is critically important to characterize the band alignment in semiconductor heterojunctions (HJs) because it controls the electronic and optical properties. However, the well-known Anderson's model usually fails to predict the band alignment in bulk HJ systems due to the presence of charge transfer at the interfacial bonding. Atomically thin 2D transition metal dichalcogenide materials have attracted much attention recently since the ultrathin HJs and devices can be easily built and they are promising for future electronics. The vertical HJs based on 2D materials can be constructed via van der Waals stacking regardless of the lattice mismatch between two materials. Despite the defect-free characteristics of the junction interface, experimental evidence is still lacking on whether the simple Anderson rule can predict the band alignment of HJs. Here, the validity of Anderson's model is verified for the 2D heterojunction systems and the success of Anderson's model is attributed to the absence of dangling bonds (i.e., interface dipoles) at the van der Waal interface. The results from the work set a foundation allowing the use of powerful Anderson's rule to determine the band alignments of 2D HJs, which is beneficial to future electronic, photonic, and optoelectronic devices.
Band alignment tuning of InAs quantum dots with a thin AlGaAsSb capping layer
(2013-04-29)
Liao, Yu-An
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Hsu, Wei-Ting
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Huang, Shih-Han
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Chiu, Pei-Chin
;
Chyi, Jen-Inn
We investigate the optical properties of InAs quantum dots (QDs) capped with a thin AlxGa1-xAsSb layer. As evidenced from power-dependent and time-resolved photoluminescence (PL) measurements, the GaAsSb-capped QDs with type-II band alignment can be changed to type-I by adding Al into the GaAsSb capping layer. The evolution of band alignment with the Al content in the AlGaAsSb capping layer has also been confirmed by theoretical calculations based on 8-band k . p model. The PL thermal stability and the room temperature PL efficiency are also improved by AlGaAsSb capping. We demonstrate that using the quaternary AlGaAsSb can take the advantages of GaAsSb capping layer on the InAs QDs while retaining a type-I band alignment for applications in long-wavelength light emitters. (C) 2013 AIP Publishing LLC.
Band Gap-Tunable Molybdenum Sulfide Selenide Monolayer Alloy
(2014-07-09)
Su, Sheng-Han
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Hsu, Yu-Te
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Chang, Yung-Huang
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Chiu, Ming-Hui
;
Hsu, Chang-Lung
Bandgap tunability at single-layer molybdenum disulphide grain boundaries
(2015-02-01)
Huang, Yu Li
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Chen, Yifeng
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Zhang, Wenjing
;
Quek, Su Ying
;
Chen, Chang-Hsiao
Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40 +/- 0.05 eV for single-layer, 2.10 +/- 0.05 eV for bilayer and 1.75 +/- 0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85 +/- 0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.
Carrier dynamics of type-II InAs/GaAs quantum dots covered by a thin GaAs(1-x)Sb(x) layer
(2008-07-21)
Chang, Wen-Hao
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Liao, Yu-An
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Hsu, Wei-Ting
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Lee, Ming-Chih
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Chiu, Pei-Chin
Carrier dynamics of InAs/GaAs quantum dots (QDs) covered by a thin GaAs(1-x)Sb(x) layer were investigated by time-resolved photoluminescence (PL). Both the power dependence of PL peak shift and the long decay time constants confirm the type-II band alignment at the GaAsSb-InAs interface. Different recombination paths have been clarified by temperature dependent measurements. At lower temperatures, the long-range recombination between the QD electrons and the holes trapped by localized states in the GaAsSb layer is important, resulting in a non-single-exponential decay. At higher temperatures, optical transitions are dominated by the short-range recombination with the holes confined to the band-bending region surrounding the QDs. (C) 2008 American Institute of Physics.
Determination of s-d exchange coupling in GaMnN by time-resolved Kerr rotation spectroscopy
(2014-09-29)
Hsu, Wei-Ting
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Hsieh, Ting-Yen
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Chen, Hsin-Feng
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Huang, Feng-Wen
;
Chen, Po-Cheng
Coherent electron-spin dynamics in Ga1-xMnxN has been investigated by time-resolved Kerr rotation spectroscopy. The effective electron g factor shows a linear increase with the Mn concentration due to the s-d exchange coupling between the conduction electrons and the d-shell electrons of Mn3+ impurities. The magnitude and sign of the s-d exchange constant are determined precisely to be N-0 alpha = +0.23 +/- 0.02 eV, indicative of a ferromagnetic s-d exchange coupling in GaMnN. The determined N-0 alpha is consistent with the typical value found in most diluted magnetic semiconductors and reveals that GaMnN is indeed not an exception.
Dielectric impact on exciton binding energy and quasiparticle bandgap in monolayer WS2 and WSe2
(2019-04-01)
Hsu, Wei-Ting
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Quan, Jiamin
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Wang, Chun-Yuan
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Lu, Li-Syuan
;
Campbell, Marshall
Excitons, bound electron-hole pairs in a 2D plane, dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). A large exciton binding energy on the order of 0.5 eV was theoretically predicted and experimentally determined recently. These ultrastable excitons thus open an avenue to explore the exciton physics such as Bose-Einstein condensation and superfluidity at room temperature (Kasprzak et al 2006 Nature 443 409; Plumhof et al 2014 Nat. Mater. 13 247; Fogler et al 2014 Nat. Commun. 5 4555; Jiang and John 2014 Sci. Rep. 4 7432). Recent experiments further demonstrated the concept of Coulomb engineering via dielectric environments based on either solutions or few-layer graphene. However, the conducting nature of these dielectrics can lead to quenching of optical transitions. Thus, in order to utilize 'dielectric tuning' of the exciton binding energy and quasiparticle band gaps, one must use insulating dielectrics. Here, we investigate the impact of insulating dielectric environments on the exciton binding energy of monolayer WS2 and WSe2 by exciton Rydberg spectroscopy, in which the dielectric environment is systematically varied from kappa= 1.49 to 3.82. We found that, with increasing kappa value, the exciton binding energy and quasiparticle bandgap exhibit significant reductions. Quantitatively, our result follows the prediction of nonlocally-screened Keldysh potential very well. The fitted 2D polarizability, Chi(2D), agrees rather well with previous density function theory calculations. Their close agreement validates the nonlocally-screened Keldysh model which can be used to quantitatively predict the exciton binding energy for monolayer TMDs (and possibly other 2D materials) in different dielectric environments. Such a predictive model will play an important role for the design of van der Waals heterostructures and TMD-based optoelectronic devices.
Effects of GaAsSb capping layer thickness on the optical properties of InAs quantum dots
(2011-08-15)
Hsu, Wei-Ting
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Liao, Yu-An
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Hsu, Feng-Chang
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Chiu, Pei-Chin
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Chyi, Jen-Inn
The optical properties of GaAsSb-capped InAs quantum dots (QDs) with different capping layer thickness are investigated. Both the emission energy and the recombination lifetime are found to be correlated with the capping layer thicknesses. Theoretical calculations indicate that the quantum confinement and the wave function distribution of hole states are sensitive to the GaAsSb capping layer thickness. The Sb induced change in QD size also plays a role in the optical properties of GaAsSb-capped QDs. Controlling the GaAsSb capping layer thickness is a feasible way to tailor the InAs QDs for long-wavelength applications. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624464]
Effects of thermal annealing on the emission properties of type-II InAs/GaAsSb quantum dots
(2009-02-02)
Liao, Yu-An
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Hsu, Wei-Ting
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Chiu, Pei-Chin
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Chyi, Jen-Inn
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Chang, Wen-Hao
We report the effects of thermal annealing on the emission properties of type-II InAs quantum dots (QDs) covered by a thin GaAs(1-x)Sb(x) layer. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAsSb layer. Evidences of the evolution from type-II to type-I band alignments are obtained from time-resolved and power-dependent photoluminescence measurements. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAsSb type-II QDs.
Evidence of indirect gap in monolayer WSe2
(2017-10-13)
Hsu, Wei-Ting
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Lu, Li-Syuan
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Wang, Dean
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Huang, Jing-Kai
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Li, Ming-Yang
Monolayer transition metal dichalcogenides, such as MoS2 and WSe2, have been known as direct gap semiconductors and emerged as new optically active materials for novel device applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS2 and WSe2. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe2-MoS2 and MoSe2-WSe2 lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe2 is actually an indirect gap material, as manifested in the observed photoluminescence intensity-energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe2 exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.
Fast Visible-Light Phototransistor Using CVD-Synthesized Large-Area Bilayer WSe2
(2014)
Liu, Pang-Shiuan
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Chen, Chang-Hsiao
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Hsu, Wei-Ting
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Lin, Chih-Pin
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Lin, Tzu-Ping
P-channel transition metal dichalcogenide ultrathin-body phototransistor (UTB-PT) with a response time as fast as 100 mu s has been demonstrated for the first time using the CVD-synthesized large-area bilayer WSe2. Because of its excellent compatibility with mass production, the application of WSe2 UTB-PT for high-speed proximity interactive display has been proposed.
Influences of Contact Metals on the Performances of MoS2 Devices under Strains
(2019-12-19)
Tseng, Sian-Hong
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Chen, Hung-Yi
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Hsu, Wei-Ting
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Wang, Hsiang-Chen
;
Li, Yuan-Yao
The growth of large-area MoS2 films was achieved through atmospheric pressure chemical vapor deposition via vapor management in this study. The thickness of the MoS2 film changed upon varying the amount of vapor deposited on the substrate. Gratifyingly, large-area monolayer MoS2 was obtained under controlled conditions. Studies using transmission electron microscopy and second harmonic generation confirmed that the MoS2 films were composed of grains having sizes ranging from 40 to 70 mu m. Flexible MoS2 device arrays were fabricated on a polyimide substrate; the device arrays displayed high spatial uniformity in their carrier transport properties. The contact metals affected the electrical characteristics of the MoS2 devices under strain; the sensitivity of devices featuring Schottky contacts was higher than that of those with ohmic contacts. Importantly, the device arrays exhibit sensitive and endurance performances under strain cycles of up to 10(5) times. These results suggest a means for the feasible growth of large-area single-layer MoS2 films, as well as the exploitation of flexible MoS2 device arrays in strain and human motion sensor applications.
Layered MoS2 grown on c-sapphire by pulsed laser deposition
(2015-03-01)
Ho, Yen-Teng
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Ma, Chun-Hao
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Luong, Tien-Tung
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Wei, Lin-Lung
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Yen, Tzu-Chun
Layered growth of molybdenum disulphide (MoS2) was successfully achieved by pulsed laser deposition (PLD) method on c-plane sapphire substrate. Growth of monolayer to a few monolayer MoS2, dependent on the pulsed number of excimer laser in PLD is demonstrated, indicating the promising controllability of layer growth. Among the samples with various pulse number deposition, the frequency difference (A1gE12g) in Raman analysis of the 70 pulse sample is estimated as 20.11 cm(1), suggesting a monolayer MoS2 was obtained. Two-dimensional (2D) layer growth of MoS2 is confirmed by the streaky reflection high energy electron diffraction (RHEED) patterns during growth and the cross-sectional view of transmission electron microscopy (TEM). The in-plane relationship, (0006) sapphire//(0002) MoS2 and inline image sapphire//inline imageMoS(2) is determined. The results imply that PLD is suitable for layered MoS2 growth. Additionally, the oxide states of Mo 3d core level spectra of PLD grown MoS2, analysed by X-ray photoelectron spectroscopy (XPS), can be effectively reduced by adopting a post sulfurization process. (C) 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim
Ledge-directed epitaxy of continuously self-aligned single-crystalline nanoribbons of transition metal dichalcogenides
(1970-01-01)
Aljarb, Areej
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Fu, Jui-Han
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Hsu, Chih-Chan
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Chuu, Chih-Piao
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Wan, Yi
Two-dimensional transition metal dichalcogenide nanoribbons are touted as the future extreme device downscaling for advanced logic and memory devices but remain a formidable synthetic challenge. Here, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer and single-crystalline MoS(2)nanoribbons on beta-gallium (iii) oxide (beta-Ga2O3) (100) substrates. LDE MoS(2)nanoribbons have spatial uniformity over a long range and transport characteristics on par with those seen in exfoliated benchmarks. Prototype MoS2-nanoribbon-based field-effect transistors exhibit high on/off ratios of 10(8)and an averaged room temperature electron mobility of 65 cm(2) V-1 s(-1). The MoS(2)nanoribbons can be readily transferred to arbitrary substrates while the underlying beta-Ga(2)O(3)can be reused after mechanical exfoliation. We further demonstrate LDE as a versatile epitaxy platform for the growth of p-type WSe(2)nanoribbons and lateral heterostructures made of p-WSe(2)and n-MoS(2)nanoribbons for futuristic electronics applications. Aligned arrays of single-crystalline monolayer TMD nanoribbons with high aspect ratios, as well as their lateral heterostructures, are realized, with the growth directed by the ledges on the beta-Ga(2)O(3)substrate. This approach provides an epitaxy platform for advanced electronics applications of TMD nanoribbons.
Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection
(2014-08-01)
Chang, Yung-Huang
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Zhang, Wenjing
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Zhu, Yihan
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Han, Yu
;
Pu, Jiang
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection
(2014-08-01)
Chang, Yung-Huang
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Zhang, Wenjing
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Zhu, Yihan
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Han, Yu
;
Pu, Jiang
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal alpha-In2Se3
(2018-05-01)
Xue, Fei
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Zhang, Junwei
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Hu, Weijin
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Hsu, Wei-Ting
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Han, Ali
Piezoelectric materials have been widely used for sensors, actuators, electronics, and energy conversion. Two-dimensional (2D) ultrathin semiconductors, such as monolayer h-BN and MoS2 with their atom-level geometry, are currently emerging as new and attractive members of the piezoelectric family. However, their piezoelectric polarization is commonly limited to the in-plane direction of odd-number ultrathin layers, largely restricting their application in integrated nanoelectromechanical systems. Recently, theoretical calculations have predicted the existence of out-of-plane and in-plane piezoelectricity in monolayer alpha-In2Se3. Here, we experimentally report the coexistence of out-of-plane and in-plane piezoelectricity in monolayer to bulk alpha-In2Se3, attributed to their noncentrosymmetry originating from the hexagonal stacking. Specifically, the corresponding d(33) piezoelectric coefficient of alpha-In2Se3 increases from 0.34 pm/V (monolayer) to 5.6 pm/V (bulk) without any odd even effect. In addition, we also demonstrate a type of alpha-In2Se3-based flexible piezoelectric nanogenerator as an energy-harvesting cell and electronic skin. The out-of-plane and in-plane piezoelectricity in alpha-In2Se3 flakes offers an opportunity to enable both directional and nondirectional piezoelectric devices to be applicable for self-powered systems and adaptive and strain tunable electronics/optoelectronics.
Negative circular polarization emissions from WSe2/MoSe2 commensurate heterobilayers
(2018-04-10)
Hsu, Wei-Ting
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Lu, Li-Syuan
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Wu, Po-Hsun
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Lee, Ming-Hao
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Chen, Peng-Jen
Van der Waals heterobilayers of transition metal dichalcogenides with spin-valley coupling of carriers in different layers have emerged as a new platform for exploring spin/valleytronic applications. The interlayer coupling was predicted to exhibit subtle changes with the interlayer atomic registry. Manually stacked heterobilayers, however, are incommensurate with the inevitable interlayer twist and/or lattice mismatch, where the properties associated with atomic registry are difficult to access by optical means. Here, we unveil the distinct polarization properties of valley-specific interlayer excitons using epitaxially grown, commensurate WSe2/MoSe2 heterobilayers with well-defined (AA and AB) atomic registry. We observe circularly polarized photoluminescence from interlayer excitons, but with a helicity opposite to the optical excitation. The negative circular polarization arises from the quantum interference imposed by interlayer atomic registry, giving rise to distinct polarization selection rules for interlayer excitons. Using selective excitation schemes, we demonstrate the optical addressability for interlayer excitons with different valley configurations and polarization helicities.
Optical detection of anisotropic g-factor and nuclear spin polarization in a single CdTe quantum well
(2015-03-01)
Yan, Li-Ping
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Kurosawa, Masahiro
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Hsu, Wei-Ting
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Chang, Wen-Hao
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Adachi, Satoru
Longitudinal and in-plane electron g-factors, and a nuclear spin polarization (NSP) have been evaluated precisely in a CdTe/Cd0.85Mg0.15Te single quantum well by using the time-resolved Kerr rotation and double lock-in detection techniques. Resident electron spin polarization (RESP) was formed via the negative trion formation and recombination, and RESP gave rise to NSP in an oblique magnetic field configuration. We observed the effective nuclear field of a few mT which was weak compared with that in III-V semiconductor nanostructures as expected, but the nuclear field can be converted to the maximal NSP of 12% in Faraday geometry. (C) 2015 The Japan Society of Applied Physics
Optically initialized robust valley-polarized holes in monolayer WSe2
(2015-11-01)
Hsu, Wei-Ting
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Chen, Yen-Lun
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Chen, Chiang-Hsiao
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Liu, Pang-Shiuan
;
Hou, Tuo-Hung
A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics. Although monolayer transition metal dichalcogenides with inherent spin-valley coupling offer a unique platform to develop such valleytronic devices, the anticipated long-lived valley pseudospin has not been observed yet. Here we demonstrate that robust valley-polarized holes in monolayer WSe2 can be initialized by optical pumping. Using time-resolved Kerr rotation spectroscopy, we observe a long-lived valley polarization for positive trion with a lifetime approaching 1 ns at low temperatures, which is much longer than the trion recombination lifetime (similar to 10-20 ps). The long-lived valley polarization arises from the transfer of valley pseudospin from photocarriers to resident holes in a specific valley. The optically initialized valley pseudospin of holes remains robust even at room temperature, which opens up the possibility to realize room-temperature valleytronics based on transition metal dichalcogenides.