Browsing by Author "Lee, Chien-Ju"

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Distributed Bragg Reflectors as Broadband and Large-Area Platforms for Light-Coupling Enhancement in 2D Transition-Metal Dichalcogenides
(2018-05-16)
Chen, Yen-Chun
;
Yeh, Han
;
Lee, Chien-Ju
;
Chang, Wen-Hao
;
電子物理學系
Two-dimensional (2D) semiconductors, particularly the direct-gap monolayer transition metal dichalcogenides (TMDs), are currently being developed for various atomically thin optoelectronic devices. However, practical applications are hindered by their low quantum efficiencies in light emissions and absorptions. While photonic cavities and metallic plasmonic structures can significantly enhance the light-matter interactions in TMDs, the narrow spectral resonance and the local hot spots considerably limit the applications when broadband and large area are required. Here, we demonstrate that a properly designed distributed Bragg reflector (DBR) can be an ideal platform for light-coupling enhancement in 2D TMDs. The main idea is based on engineering the amplitude and phase of optical reflection from the DBR to produce optimal substrate-induced interference. We show that the TMDs photoluminescence, Raman, and second harmonic generation signals of monolayer WSe2 can be enhanced by a factor of 26, 34, and 58, respectively. The proposed DBR substrates pave the way for developing a range of 2D optoelectronic devices for broadband and large-area applications.
Epitaxial Growth of Optically Thick, Single Crystalline Silver Films for Plasmonics
(2019-01-23)
Cheng, Fei
;
Lee, Chien-Ju
;
Choi, Junho
;
Wang, Chun-Yuan
;
Zhan, Qiang
Single crystalline Ag films on dielectric substrates have received tremendous attention recently due to their technological potentials as low loss plasmonic materials. Two different growth approaches have been used to produce single crystalline Ag films previously. One approach is based on repetitive cycles of a two-step process (low temperature deposition followed by RT annealing) using molecular beam epitaxy (MBE), which is extremely time-consuming due to the need for repeat growth cycles. Another approach is based on rapid e-beam deposition which is capable of growing thick single crystalline Ag films (>300 nm) but lacks the precision in thickness control of thin epitaxial films. Here, we report a universal approach to grow atomically smooth epitaxial Ag films by eliminating the repetitive cycles used in the previous two-step MBE method while maintaining the precise thickness control from a few monolayers to the optically thick regime, thus overcoming the limitations of the two aforementioned methods. In addition, we develop an in situ growth of aluminum oxide as the capping layer to protect the epitaxial Ag films. The quality of the epitaxial Ag films was evaluated using a variety of techniques, and the superior optical performance of the films is demonstrated by measuring the propagation length of surface plasmon polaritons (similar to 80 mu m at 632 nm) as well as their capability to support a plasmonic nanolaser in infrared incorporating an InGaAsP quantum well as the gain media.
Ledge-directed epitaxy of continuously self-aligned single-crystalline nanoribbons of transition metal dichalcogenides
(1970-01-01)
Aljarb, Areej
;
Fu, Jui-Han
;
Hsu, Chih-Chan
;
Chuu, Chih-Piao
;
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.
Low-Threshold Plasmonic Lasers on a Single-Crystalline Epitaxial Silver Platform at Telecom Wavelength
(2017-06-01)
Lee, Chien-Ju
;
Yeh, Han
;
Cheng, Fei
;
Su, Ping-Hsiang
;
Her, Tsing-Hua
We report on the first demonstration of metal insulator semiconductor-type plasmonic lasers at the telecom wavelength (similar to 1.3 mu m) using top-down fabricated semiconductor waveguides on single-crystalline metallic platforms formed using epitaxially grown Ag films. The critical role of the Ag film thickness in sustaining. plasmonic lasing at the telecom wavelength is investigated systematically. Low-threshold (0.2 MW/cm(2)) and continuous-wave operation of plasmonic lasing at cryogenic temperatures can be achieved on a 150 nm Ag platform with minimum radiation leakage into the substrate. Plasmonic lasing occurs preferentially through higher-order surface-plasmon-polariton modes, which exhibit a higher mode confinement factor, lower propagation loss, and better field gain coupling. We observed plasmonic lasing up to 200 K under pulsed excitations. The plasmonic lasers on large-area epitaxial Ag films open up a scalable platform for on-chip integrations of plasmonics and optoelectronics at the telecom wavelength.
Plasmonic whispering-gallery modes in a semiconductor-insulator-metal hybrid structure
(2015)
Lee, Chien-Ju
;
Yeh, Han
;
Chen, Yen-Chun
;
Wang, Chun-Yuan
;
Gwo, Shangir
We demonstrate a plasmonic cavity based on a hybrid semiconductor-insulator-metal structure, which can support plasmonic whispering-gallery modes (WGMs) in the insulator layer. The hybrid structure consists of a semiconductor microdisk with embedded gain medium on top of a single-crystalline gold flake with a nanometer-thick dielectric gap in between. By optimizing the dielectric gap thickness, it is possible to form coupled modes consisting of both photonic WGM in the microdisk and plasmonic WGM in the dielectric gap, by which a high field-gain overlap and an efficient cavity feedback can be achieved for investigating strong light-matter interactions and laser applications.
Purely in-plane ferroelectricity in monolayer SnS at room temperature
(2020-05-15)
Higashitarumizu, Naoki
;
Kawamoto, Hayami
;
Lee, Chien-Ju
;
Lin, Bo-Han
;
Chu, Fu-Hsien
2D van der Waals ferroelectrics have emerged as an attractive building block with immense potential to provide multifunctionality in nanoelectronics. Although several accomplishments have been reported in ferroelectric switching for out-of-plane ferroelectrics down to the monolayer, a purely in-plane ferroelectric has not been experimentally validated at the monolayer thickness. Herein, an in-plane ferroelectricity is demonstrated for micrometer-size monolayer SnS at room temperature. SnS has been commonly regarded to exhibit the odd-even effect, where the centrosymmetry breaks only in the odd-number layers to exhibit ferroelectricity. Remarkably, however, a robust room temperature ferroelectricity exists in SnS below a critical thickness of 15 layers with both an odd and even number of layers, suggesting the possibility of controlling the stacking sequence of multilayer SnS beyond the limit of ferroelectricity in the monolayer. This work will pave the way for nanoscale ferroelectric applications based on SnS as a platform for in-plane ferroelectrics. Out-of-plane ferroelectricity is usually unstable in the two dimensional limit due to the presence of the depolarization field. Here, the authors successfully circumvent this issue by growing mu m-sized SnS monolayers that exhibit in-plane ferroelectricity that is stable at room temperature.
Ultralow Schottky Barriers in Hexagonal Boron Nitride-Encapsulated Monolayer WSe2 Tunnel Field-Effect Transistors
(2020-04-22)
Pande, Gaurav
;
Siao, Jyun-Yan
;
Chen, Wei-Liang
;
Lee, Chien-Ju
;
Sankar, Raman
To explore the potential of field-effect transistors (FETs) based on monolayers (MLs) of the two-dimensional semiconducting channel (SC) for spintronics, the two most important issues are to ensure the formation of variable low-resistive tunnel ferromagnetic contacts (FCs) and to preserve intrinsic properties of the SC during fabrication. Large Schottky barriers lead to the formation of high resistive contacts, and methods adopted to control the barriers often alter the intrinsic properties of the SC. This work aims at addressing both issues in fully encapsulated ML WSe2 FETs using bilayer hexagonal boron nitride (h-BN) as a tunnel barrier at the FC/SC interface. We investigate the electrical transport in ML WSe2 FETs with the current-in-plane geometry that yields hole mobilities of similar to 38.3 cm(2) V-1 s(-1) at 240 K and on/off ratios of the order of 10(7), limited by the contact regions. We have achieved an ultralow effective Schottky barrier (similar to 5.34 meV) with an encapsulated tunneling device as opposed to a nonencapsulated device in which the barrier heights are considerably higher. These observations provide an insight into the electrical behavior of the FC/h-BN/SC/h-BN heterostructures, and such control over the barrier heights opens up the possibilities for WSe2-based spintronic devices.