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Gamma-Ray Irradiation Effect on Ferroelectric Devices with Hafnium Aluminum Oxides
(2019-12-01)
Liu, Chien
;
Tung, Yi-Chun
;
Wu, Tian-Li
;
Cheng, Chun-Hu
;
Tseng, Chih-Yang
Herein, the gamma-ray irradiation effect on ferroelectric HfAlO(x)capacitors for biomedical and space applications is investigated. The experimental results focus on the observation of ferroelectric polarization characteristics in support of different defect mechanisms induced by annealing, endurance cycling, and gamma-ray irradiation. It is found that the HfAlO(x)with low Al doping of 6.5% become more vulnerable to radiation exposure, but a remarkable improvement in 9%-Al-doped HfAlO(x)due to less oxygen vacancies is demonstrated. In addition, the results also confirm that the fatigue property of ferroelectric HfAlO(x)capacitor is determined by the electric-stress-induced defect during endurance cycling, not mainly contributed by irradiation damage, which shows the potential for memory applications in rigorous irradiation environment.
Substrate-induced strain in 2D layered GaSe materials grown by molecular beam epitaxy
(2020-07-31)
Liu, Cheng-Wei
;
Dai, Jin-Ji
;
Wu, Ssu-Kuan
;
Nhu-Quynh Diep
;
Sa-Hoang Huynh
Two-dimensional (2D) layered GaSe films were grown on GaAs (001), GaN/Sapphire, and Mica substrates by molecular beam epitaxy (MBE). The in situ reflective high-energy electron diffraction monitoring reveals randomly in-plane orientations of nucleated GaSe layers grown on hexagonal GaN/Sapphire and Mica substrates, whereas single-orientation GaSe domain is predominant in the GaSe/GaAs (001) sample. Strong red-shifts in the frequency of in-plane E2g2 vibration modes and bound exciton emissions observed from Raman scattering and photoluminescence spectra in all samples are attributed to the unintentionally biaxial in-plane tensile strains, induced by the dissimilarity of symmetrical surface structure between the 2D-GaSe layers and the substrates during the epitaxial growth. The results in this study provide an important understanding of the MBE-growth process of 2D-GaSe on 2D/3D hybrid-heterostructures and pave the way in strain engineering and optical manipulation of 2D layered GaSe materials for novel optoelectronic integrated technologies.
Circular Polarization Dynamics during Magnetic Polaron Formation in Type-II Magnetic Quantum Dots
(2020-06-11)
Barman, Biplob
;
Pientka, James M.
;
Murphy, Joseph R.
;
Cartwright, Alexander N.
;
Chou, Wu-Ching
We used time-resolved photoluminescence spectroscopy to study the circular polarization dynamics of magnetic polarons in type-II (Zn,Mn)Te/ZnSe quantum dots in the presence of an external magnetic field. We investigated the time evolution of the peak energy of the sigma(+) and sigma(-) circularly polarized photoluminescence components and of the circular polarization of the emitted light. We also observed that the value of circular polarization, at long delay times, increases with magnetic field. We found that this system exhibits unexpected characteristics, such as different time scales for the formation of the magnetic polaron, on the one hand, and the evolution of photoluminescence circular polarization, on the other hand. These results are discussed within the framework of a theoretical model developed to describe the dependence of magnetic susceptibility as a function of temperature.
High performance negative capacitance field-effect transistor featuring low off-state current, high on/off current ratio, and steep sub-60 mV dec(-1) swing
(2020-04-01)
Liu, Chien
;
Chen, Hsuan-Han
;
Tung, Yi-Chun
;
Wang, Wei-Chun
;
Huang, Zhong-Ying
In this work, we demonstrated that the 5-nm-thick HfAlOx negative capacitance transistor (NCFET) with optimized Al doping can achieve a minimum 33 mV dec(-1) subthreshold swing (SS), an ultralow l(off) of 7.44 fA mu m(-1), and a high l(on)//l(off) ratio of 1.9 x 10(8). The NC switching with sub-60 mV dec(-1) SS can be implemented from V-DS - 0.2 V to 0.8 V due to an ultralow off-state leakage current. The experimental results reveal that well-controlled Al doping in HfAlOx not only reduces off-state leakage of the transistor, but also improves the ferroelectric NC effect to achieve a sub-60 mV dec-1 switching under a favorably low sub-1 voltage. This scaled HfAlOx NCFET with optimized Al doping and fluorine defect passivation shows the great potential for the application of low power logic devices. (C) 2020 The Japan Society of Applied Physics
Electronic structures associated with enhanced photocatalytic activity in nanogap-engineered g-C3N4/Ag@SiO2 hybrid nanostructures
(2020-06-01)
Huang, Yu-Cheng
;
Arul, K. Thanigai
;
Chen, Chi-Liang
;
Chen, Jeng-Lung
;
Chen, Jie
Graphitic carbon nitride (g-C3N4) has attracted considerable attention with regard to its use in photocatalytic solar hydrogen production by the splitting of water. High charge carrier recombination critically limits the photocatalytic activity of g-C3N4. Plasmonic metal nanoparticles that can generate localized surface plasmon resonance (LSPR) have been suggested to enhance the harvesting of visible light and to improve water splitting efficiency. However, direct contact between metal nanoparticles and g-C3N4 reduces the hydrogen generation efficiency owing to energy loss by Forster resonance energy transfer (FRET), which competes with plasmon resonance energy transfer (PRET). Decorating g-C3N4 with Ag@SiO2 core-shell plasmonic nanoparticles increases its photocatalytic ability. Tuning the size of the SiO2 nanogap can optimize the photocatalytic performance of g-C3N4/Ag@SiO2, which involves a trade-off between PRET and FRET. X-ray absorption spectroscopy (XAS) is utilized to investigate the electronic structure of g-C3N4 and its modulation with Ag@SiO2. In situ XAS reveals the dynamics of the charge carriers under solar illumination. Analytic results suggest charge redistribution, shifting of the conduction band, modification of the unoccupied states, and consequent improvement in photocatalytic activity by solar illumination. This work sheds light on the effect of LSPR on this photocatalyst with reference to its electronic structure.
Visible-Transparent Luminescent Solar Concentrators Based on Carbon Nanodots in the Siloxane Matrix with Ultrahigh Quantum Yields and Optical Transparency at High-Loading Contents
(2020-01-16)
Talite, Maria Jessabel
;
Huang, Hsiu-Ying
;
Cai, Kun-Bin
;
Co, King Chester Capinig
;
Santoso, Phoa Adelina Cynthia
Visible-transparent luminescent solar concentrators (VT-LSCs) can be integrated with solar cells for designing solar glasses. Recently, rare-earth complexes, semiconductor nanocrystals, and carbon nanodots (CNDs) have been applied in developing VT-LSCs. However, several challenges still existed, such as quantum yields (QYs) at high-loading contents, scattering/reabsorption losses, and stability. Here, highly luminescent and visible-transparent composites based on organosilane-functionalized CNDs (Si-CNDs) cross-linked in the siloxane matrix were prepared. The composites with a high-loading content (similar to 10 wt %) possess ultrahigh QYs of similar to 94% due to surface passivation, cross-linking-enhanced emission, and negligible inter-CND energy transfer. Moreover, they still appear exceptionally transparent and, thus, are suitable for VT-LSCs. Eco-friendly VT-LSCs without colored tinting were fabricated, yielding high internal and external quantum efficiencies of similar to 66% and similar to 3.9%. Our demonstration would pave a bright way for the utilization of eco-friendly VT-LSCs in solar glasses.
Screw-Dislocation-Driven Growth Mode in Two Dimensional GaSe on GaAs(001) Substrates Grown by Molecular Beam Epitaxy
(2019-11-28)
Nhu Quynh Diep
;
Liu, Cheng-Wei
;
Wu, Ssu-Kuan
;
Chou, Wu-Ching
;
Sa Hoang Huynh
Regardless of the dissimilarity in the crystal symmetry, the two-dimensional GaSe materials grown on GaAs(001) substrates by molecular beam epitaxy reveal a screw-dislocation-driven growth mechanism. The spiral-pyramidal structure of GaSe multi-layers was typically observed with the majority in epsilon-phase. Comprehensive investigations on temperature-dependent photoluminescence, Raman scattering, and X-ray diffraction indicated that the structure has been suffered an amount of strain, resulted from the screw-dislocation-driven growth mechanism as well as the stacking disorders between monolayer at the boundaries of the GaSe nanoflakes. In addition, Raman spectra under various wavelength laser excitations explored that the common epsilon-phase of 2D GaSe grown directly on GaAs can be transformed into the beta-phase by introducing a Se-pretreatment period at the initial growth process. This work provides an understanding of molecular beam epitaxy growth of 2D materials on three-dimensional substrates and paves the way to realize future electronic and optoelectronic heterogeneous integrated technology as well as second harmonic generation applications.
Optical Control of Hole Wavefunction in Type-II Magnetic Quantum Dot Structures
(2019-10-24)
Zhang, Peiyao
;
Norden, Tenzin
;
Pientka, James M.
;
Oszwaldowski, Rafal
;
Najafi, Arman
Unlike the extensively studied, spatially direct, quantum dots (QDs) with type-I band alignment where both electrons and holes are confined in the QD, in ZnTe QDs embedded in a (Zn,Mn)Se matrix only the holes are confined in the QDs. This spatially indirect type-II system provides unexplored opportunities to control the magnetic interactions between the hole spins in the nonmagnetic QDs and the spins of the magnetic ions in the matrix. Photoluminescence (PL) was excited either with photons of energy 3.06 eV [above band gap of the (Zn,Mn)Se matrix] or with photons of energy 2.54 eV [below the (Zn,Mn)Se band gap]. In the presence of an external magnetic field, the saturation red shift of the PL peak under 2.54 eV excitation exhibits an up to three-fold increase compared to the shift observed with 3.06 eV excitation. This increase is attributed to multiple hole occupancy of the QDs and the resulting increased penetration of the hole wavefunction tail further into the (Zn,Mn)Se matrix. The proposed model is supported by calculations which include the hole-hole Coulomb interactions as well as the hole-Mn spin exchange interactions.
Stabilizing Ferroelectric Domain Switching of Hafnium Aluminum Oxide Using Metal Nitride Electrode Engineering
(2019-09-17)
Liu, Chien
;
Tung, Yi-Chun
;
Tseng, Chih-Yang
;
Wang, Wei-Chun
;
Chen, Hsuan-Han
In this work, we comprehensively investigated the metal-electrode stress engineering on HfAlO metal-ferroelectric-metal capacitor. According to experimental results, we confirm that high-nitrogen tantalum-nitride electrode with good chemical stability not only mitigates the anti-ferroelectric property and gate leakage current, but also improves the wake-up effect and endurance cycling characteristics, which shows the great potential for future applications of low-power ferroelectric memories. (c) 2019 The Electrochemical Society.
Interface Engineering of Ferroelectric Negative Capacitance FET for Hysteresis-Free Switch and Reliability Improvement
(2018-01-01)
Fan, Chia-Chi
;
Tu, Chun-Yuan
;
Lin, Ming-Huei
;
Chang, Chun-Yen
;
Cheng, Chun-Hu
In this work, we successfully achieve a hysteresis-free negative capacitance field effect transistors (NCFETs) by exploiting a defect passivation scheme. This research work simultaneously provides a new insight into the gate-oxide stress reliability of NCFET. The fluorine-passivated HfAlOx NCFET shows the excellent transistor characteristics including a steep subthreshold swing of sub-30-mV/dec, a negligible hysteresis-free switch of similar to 10mV and a large on/off current reatio (I-on/I-off) of >10(7). Most importantly, fluorine passivation for NC HfAlOx effectively suppress the generation of shallow traps during electrical stress test. Besides, it is favorable to maintain NC operation and SILC immunity by in-situ fluorine passivation, which has been verified by transient pulse I-V measurement.

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