Browsing by Author "Chang, Wen-Hao"

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2D Layered Semiconductors beyond MoS2
(2019-01-01)
Chang, Wen-Hao
;
交大名義發表
;
National Chiao Tung University
All-Color Plasmonic Nanolasers with Ultralow Thresholds
(2013)
Lu, Yu-Jung
;
Kim, Jisun
;
Chen, Hung-Ying
;
Chang, Wen-Hao
;
Shih, Chih-Kang
Diffraction-unlimited semiconductor nanolasers are demonstrated by using single shape-controlled InGaN/GaN core-shell nanorods as laser gain media on Ag films epitaxially grown on Si substrates. The use of atomically smooth Ag films allows us to fabricate low-loss plasmonic cavities for ultralow-threshold, continuous-wave (CW) nanolaser operation above liquid nitrogen temperature. Furthermore, the tunable band-gap energy of the InxGa1-xN alloy makes it possible to realize laser emission in the full visible spectrum.
All-Color Plasmonic Nanolasers with Ultralow Thresholds: Autotuning Mechanism for Single-Mode Lasing
(2014-08-01)
Lu, Yu-Jung
;
Wang, Chun-Yuan
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Kim, Jisun
;
Chen, Hung-Ying
;
Lu, Ming-Yen
We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spasers) emitting in the full visible spectrum. These nanolasers are based on a single metal oxide semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al2O3-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresholds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of "thresholdless" lasing for the blue plasmonic nanolaser.
All-Color Plasmonic Nanolasers with Ultralow Thresholds: Autotuning Mechanism for Single-Mode Lasing
(2014-08-01)
Lu, Yu-Jung
;
Wang, Chun-Yuan
;
Kim, Jisun
;
Chen, Hung-Ying
;
Lu, Ming-Yen
We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spasers) emitting in the full visible spectrum. These nanolasers are based on a single metal oxide semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al2O3-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresholds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of "thresholdless" lasing for the blue plasmonic nanolaser.
Anticorrelation between the splitting and polarization of the exciton fine structure in single self-assembled InAs/GaAs quantum dots
(2011-02-28)
Lin, Chia-Hsien
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You, Wen-Ting
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Chou, Hsiang-Yu
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Cheng, Shun-Jen
;
Lin, Sheng-Di
We report on a systematic correlation between the fine-structure splitting and polarization anisotropy of excitons in InAs/GaAs quantum dots, but with an unexpected reversal in order of the polarization eigenaxes. Such an anticorrelation is explained by a large valence-band-mixing induced splitting due to the shape and strain anisotropies. The strength and phase of valence band mixing are also found to play an important role in the tuning of fine structure splitting using an in-plane magnetic field.
Atomic scale depletion region at one dimensional MoSe2-WSe2 heterointerface
(2018-12-10)
Chu, Yu-Hsun
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Wang, Li-Hong
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Lee, Shin-Ye
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Chen, Hou-Ju
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Yang, Po-Ya
Lateral heterojunctions based on two dimensional (2D) transition metal dichalcogenides (TMDCs) potentially realize monolayer devices exploiting 2D electronic structures and the functions introduced by the presence of 1D heterointerfaces. Electronic structures of a lateral MoSe2-WSe2 junction have been unveiled using scanning tunneling microscopy and spectroscopy. A smooth and narrow depletion region exists despite a defect-rich heterointerface deviating from the preferred zigzag orientations of the TMDC lattice. From the characteristics of the depletion region, a high carrier concentration and high internal electric fields are inferred, offering to benefit designs of lateral TMDC devices. Published by AIP Publishing.
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
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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
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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
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Hsu, Chang-Lung
Bandgap tunability at single-layer molybdenum disulphide grain boundaries
(2015-02-01)
Huang, Yu Li
;
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.
Bidirectional All-Optical Synapses Based on a 2D Bi2O2Se/Graphene Hybrid Structure for Multifunctional Optoelectronics
(1970-01-01)
Yang, Chia-Ming
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Chen, Tsung-Cheng
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Verma, Dharmendra
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Li, Lain-Jong
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Liu, Bo
Neuromorphic computing has been extensively studied to mimic the brain functions of perception, learning, and memory because it may overcome the von Neumann bottleneck. Here, with the light-induced bidirectional photoresponse of the proposed Bi2O2Se/graphene hybrid structure, its potential use in next-generation neuromorphic hardware is examined with three distinct optoelectronic applications. First, a photodetector based on a Bi2O2Se/graphene hybrid structure presents positive and negative photoresponsibility of 88 and -110 A W-1 achieved by the excitation of visible wavelength and ultraviolet wavelength light at intensities of 1.2 and 0.3 mW cm(-2), respectively. Second, this unique photoresponse contributes to the realization of all optically stimulated long-term potentiation or long-term depression to mimic synaptic short-term plasticity and long-term plasticity, which are attributed to the combined effect of photoconductivity, bolometric, and photoinduced desorption. Third, the devices are applied to perform digital logic functions, such as "AND" and "OR," using full light modulation. The proposed Bi2O2Se/graphene-based optoelectronic device represents an innovative and efficient building block for the development of future multifunctional artificial neuromorphic systems.
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.
Cathodoluminescence studies of GaAs nano-wires grown on shallow-trench-patterned Si
(2010-11-19)
Lee, Ling
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Fan, Wen-Chung
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Ku, Jui-Tai
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Chang, Wen-Hao
;
Chen, Wei-Kuo
The optical properties of GaAs nano-wires grown on shallow-trench-patterned Si(001) substrates were investigated by cathodoluminescence. The results showed that when the trench width ranges from 80 to 100 nm, the emission efficiency of GaAs can be enhanced and is stronger than that of a homogeneously grown epilayer. The suppression of non-radiative centers is attributed to the trapping of both threading dislocations and planar defects at the trench sidewalls. This approach demonstrates the feasibility of growing nano-scaled GaAs-based optoelectronic devices on Si substrates.
Cathodoluminescence studies of GaAs nano-wires grown on shallow-trench-patterned Si
(2010-11-19)
Lee, Ling
;
Fan, Wen-Chung
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Ku, Jui-Tai
;
Chang, Wen-Hao
;
Chen, Wei-Kuo
The optical properties of GaAs nano-wires grown on shallow-trench-patterned Si(001) substrates were investigated by cathodoluminescence. The results showed that when the trench width ranges from 80 to 100 nm, the emission efficiency of GaAs can be enhanced and is stronger than that of a homogeneously grown epilayer. The suppression of non-radiative centers is attributed to the trapping of both threading dislocations and planar defects at the trench sidewalls. This approach demonstrates the feasibility of growing nano-scaled GaAs-based optoelectronic devices on Si substrates.
DC Characteristics of InAlAs/InGaAsSb/InGaAs Double Heterojunction Bipolar Transistors
(2010-12-01)
Chen, Shu-Han
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Chang, Chao-Min
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Chiang, Pei-Yi
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Wang, Sheng-Yu
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Chang, Wen-Hao
DC electrical characteristics of a series of InAlAs/InGaAsSb/InGaAs double heterojunction bipolar transistors (DHBTs) that are grown on InP by molecular beam epitaxy are reported and analyzed. The InGaAsSb base of the transistors leads to a type-I base-emitter junction and a type-II base-collector junction, resulting in unique device characteristics, such as low turn-on voltage, low crossover current, and constant current gain over a wide current range. In addition, the DHBTs exhibit rather high current gains despite the use of a heavily doped thick InGaAsSb base layer. This indicates the long minority carrier lifetime of the InGaAsSb material. A high current gain over base sheet resistance ratio is, thus, realized with these novel DHBTs.
Design of multifold Ge/Si/Ge composite quantum-dot heterostructures for visible to near-infrared photodetection
(2015-05-15)
Kuo, Ming-Hao
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Lai, Wei-Ting
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Lee, Sheng-Wei
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Chen, Yen-Chun
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Chang, Chia-Wei
We demonstrate an effective approach to grow high-quality thin film (>1 mu m) ofmultifold Ge/Si/Ge composite quantum dots ( CQDs) stacked heterostructures for near infrared photodetection and optical interconnect applications. An otherwise random, self-assembly of variable-fold Ge/Si CQDs has been grown on Si through the insertion of Si spacer layers to produce micron-scale-thick, stacked Ge/Si CQD layers with desired QD morphology and composition distribution. The high crystalline quality of these multifold Ge CQD heterostructures is evidenced by low dark current density of 3.68 pA/mu m(2), superior photoresponsivity of 267 and 220 mA/W under 850 and 980 nm illumination, respectively, and very fast temporal response time of 0.24 ns measured on the Ge/Si CQD photodetectors. (C) 2015 Optical Society of America
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
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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.
Diamagnetic Response of Exciton Complexes in Semiconductor Quantum Dots
(2008-12-31)
Tsai, Ming-Fu
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Lin, Hsuan
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Lin, Chia-Hsien
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Lin, Sheng-Di
;
Wang, Sheng-Yun
We report measurements of diamagnetic shifts for different exciton complexes confined in small InAs quantum dots. The measured diamagnetic responses are sensitive to the number of carriers in the exciton complexes, with systematic differences between neutral excitons, biexcitons, and trions. Theoretical calculations suggest that such systematic differences arise from very different extents of electron and hole wave functions confined in small quantum dots. The measured magnetic response of Coulomb energies is found to vary with the cube of the wave function extent, and can be a sensitive probe to the electron-hole wave function asymmetry.
Dielectric impact on exciton binding energy and quasiparticle bandgap in monolayer WS2 and WSe2
(2019-04-01)
Hsu, Wei-Ting
;
Quan, Jiamin
;
Wang, Chun-Yuan
;
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.
Diffraction-Unlimited Plasmonic Nanolaser
(2015-01-01)
Lu, Yu-Jung
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Kim, Jisun
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Chen, Hung-Ying
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Wu, Chihhui
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Dabidian, Nima