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Effect of Strains and V-Shaped Pit Structures on the Performance of GaN-Based Light-Emitting Diodes
(2020-04-01)
Chen, Shuo-Wei
;
Chang, Chia-Jui
;
Lu, Tien-Chang
;
光電工程學系
;
Department of Photonics
Strains and V-shaped pits are essential factors for determining the efficiency of GaN-based light-emitting diodes (LEDs). In this study, we systematically analyzed GaN LED structures on patterned sapphire substrates (PSSs) with two types of growth temperature employed for prestrained layers and three different thickness of n-type GaN layers by using cathodoluminescence (CL), microphotoluminescence (PL), and depth-resolved confocal Raman spectroscopy. The results indicated that V-pits formation situation can be analyzed using CL. From the emission peak intensity ratio of prestrained layers and multiple quantum wells (MQWs) in the CL spectrum, information regarding strain relaxation between prestrained layers and MQWs was determined. Furthermore, micro-PL and depth-resolved confocal Raman spectroscopy were employed to validate the results obtained from CL measurements. The growth conditions of prestrained layers played a dominant role in the determination of LED performance. The benefit of the thick layer of n-GaN was the strain reduction, which was counteracted by an increase in light absorption in thick n-type doped layers. Consequently, the most satisfactory LED performance was observed in a structure with relatively lower growth temperature of prestrained layers that exhibited larger V-pits, leading to higher strain relaxation and thinner n-type GaN layers, which prevent light absorption caused by n-type GaN layers.
Nitride-stressor and quantum-size engineering in Ge quantum-dot photoluminescence wavelength and exciton lifetime
(2020-03-01)
Kuo, Yu-Hong
;
Chiu, Shih-Hsuan
;
Tien, Che-Wei
;
Lin, Sheng-Di
;
Chang, Wen-Hao
We report local nitride-stressor engineering in combination with quantum-size tunability in Ge quantum dots (QDs) for tailoring photoluminescence (PL) wavelength and exciton lifetime. Spherical-shaped Ge QDs with tunable diameters ranging from 25-90 nm embedded within layers of thermally-grown SiO2 and chemical-vapor-deposited Si3N4 were fabricated by thermal oxidation of lithographically-patterned poly-Si0.85Ge0.15 pillars on top of buffer layers of SiO2 and Si3N4, respectively, in a self-organization approach. The effects of local stressors and quantum confinement on the strain and optical properties of Ge QDs were systematically investigated using Raman and PL measurements. We observed that when Ge QD diameter gets smaller than 60 nm, quantum confinement sets in and has a predominant influence on PL wavelength and exciton lifetime. Our self-organized Ge QD/Si3N4 system provides a generic building block for the fabrication of active photonic devices on Si3N4 platform.
Structural and optical characteristics of non-polar ZNO epitaxial films grown by radio-frequency magnetron sputtering
(2018-08-01)
Hsu, Chia-Hung
;
Lin, Bi-Hsuan
;
Liu, Wei-Rein
;
Hsieh, Wen-Fong
;
光電工程學系
Distinctive Signatures of the Spin- and Momentum-Forbidden Dark Exciton States in the Photoluminescence of Strained WSe2 Monolayers under Thermalization
(2019-04-01)
Peng, Guan-Hao
;
Lo, Ping-Yuan
;
Li, Wei-Hua
;
Huang, Yan-Chen
;
Chen, Yan-Hong
With both spin and valley degrees of freedom, the low-lying excitonic spectra of photoexcited transition-metal dichalcogenide monolayers (TMDC-MLs) are featured by rich fine structures, comprising the intravalley bright exciton states as well as various intra- and intervalley dark ones. The latter states can be classified as those of the spin- and momentum-forbidden dark excitons according to the violated optical selection rules. Because of their optical invisibility, these two types of the dark states are in principle hardly observed and even distinguished in conventional spectroscopies although their impacts on the optical and dynamical properties of TMDC-MLs have been well noticed. In this Letter, we present a theoretical and computational investigation of the exciton fine structures and the temperature-dependent photoluminescence spectra of strained tungsten diselenide monolayers (WSe2-MLs) where the intravalley spin-forbidden dark exciton lies in the lowest exciton states and other momentum-forbidden states are in the higher energies that are tunable by external stress. The numerical computations are carried out by solving the Bethe-Salpeter equation for an exciton in a WSe2-ML under the stress-control in the tight-binding scheme established from the first principle computation in the density functional theory. According to the numerical computation and supportive model analysis, we reveal the distinctive signatures of the spin- and momentum-forbidden exciton states of strained WSe2-MLs in the temperature-dependent photoluminescences and present the guiding principle to infer the relative energetic locations of the two types of dark excitons.
A Model for Neutral Defect Limited Electron Mobility in Strained-Silicon Inversion Layers
(2017-03-01)
Hsieh, Shang-Hsun
;
Chen, Ming-Jer
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
On the strained silicon metal-oxide-semiconductor field-effect transistors (MOSFETs), we show how to derive a formalism dealing with the scattering of a 2-D electron by a neutral defect. The corresponding neutral defect limited inversion-layer electron mobility, mu(n), is calculated in the momentum relaxation time approximation. The calculated results lead to a new analytical model: mu(n) = cN(n)(-1) where N-n is the neutral defect density per unit area and c is the coefficient independent of the inversion-layer density, the strain, and the temperature. The validity and applicability of the model are confirmed by citing three independent experiments on strained silicon MOSFETs undergoing different implantation sources and different annealing budgets. Importantly, this paper clarifies for the first time that strain will not change neutral defect limited mobility unless changing the neutral defect density. This reasonably explains the two experimental observations during implantation and annealing: 1) the implantation-induced strain relaxation in strained sample does not occur and 2) the neutral defect density is much higher in strained sample than in unstrained sample.
The Effect of Tensile Strain on Optical Anisotropy and Exciton of m-Plane ZnO
(2015-04-01)
Wang, H. H.
;
Tian, J. S.
;
Chen, C. Y.
;
Huang, H. H.
;
Yeh, Y. C.
The near band edge emission of the tensile-strained m-plane ZnO film grown on (112)LaAlO3 substrates shows abnormal low polarization degree (rho = 0.1). The temperature dependency of polarization degree clarifies the origins of different emission peaks. In tensile-strained m-plane ZnO, the [0001] polarized state is upper shifted and is overlapping with the [11 (2) over bar0] polarized state. This phenomenon causes the abnormal low polarization degree and reveals the effect of strain on the emission anisotropy of m-plane ZnO.
Mobility enhancement in local strain channel nMOSFETs by stacked a-Si/poly-Si gate and capping nitride
(2005-04-01)
Lu, TY
;
Chao, TS
;
交大名義發表
;
National Chiao Tung University
A local strained channel nMOSFET has been fabricated by a stress control technique utilizing a stacked a-Si/poly-Si gate and a SiN capping layer. It is found that the transconductance (G(M)) of nMOSFETs increases as the thickness of a-Si is increased. We also found that the G(M) of devices with the SiN capping layer exhibits a 17% increase compared to that of its counterparts. The stacked gate a-Si/poly-Si with the capping layer can improve the G(M) further to 29% more than the single-poly-Si gate structure without SiN capping layer.
Nondestructive Mapping of Long-Range Dislocation Strain Fields in an Epitaxial Complex Metal Oxide
(2019-03-01)
Simons, Hugh
;
Jakobsen, Anders Clemen
;
Ahl, Sonja Rosenlund
;
Poulsen, Henning Friis
;
Pantleon, Wolfgang
The misfit dislocations formed at heteroepitaxial interfaces create long-ranging strain fields in addition to the epitaxial strain. For systems with strong lattice coupling, such as ferroic oxides, this results in unpredictable and potentially debilitating functionality and device performance. In this work, we use dark-field X-ray microscopy to map the lattice distortions around misfit dislocations in an epitaxial film of bismuth ferrite (BiFeO3), a well-known multiferroic. We demonstrate the ability to precisely quantify weak, long-ranging strain fields and their associated symmetry lowering without modifying the mechanical state of the film. We isolate the screw and edge components of the individual dislocations and show how they result in weak charge heterogeneities via flexoelectric coupling. We show that even systems with small lattice mismatches and additional mechanisms of stress relief (such as mechanical twinning) may still give rise to measurable charge and strain heterogeneities that extend over mesoscopic length scales. This sets more stringent physical limitations on device size, dislocation density, and the achievable degree of lattice mismatch in epitaxial systems.
Strain Coupling of Conversion-type Fe3O4 Thin Films for Lithium Ion Batteries
(2017-06-26)
Hwang, Sooyeon
;
Meng, Qingping
;
Chen, Ping-Fan
;
Kisslinger, Kim
;
Cen, Jiajie
Lithiation/delithiation induces significant stresses and strains into the electrodes for lithium ion batteries, which can severely degrade their cycling performance. Moreover, this electrochemically induced strain can interact with the local strain existing at solid-solid interfaces. It is not clear how this interaction affects the lithiation mechanism. The effect of this coupling on the lithiation kinetics in epitaxial Fe3O4 thin film on a Nb-doped SrTiO3 substrate is investigated. In situ and ex situ transmission electron microscopy (TEM) results show that the lithiation is suppressed by the compressive interfacial strain. At the interface between the film and substrate, the existence of LixFe(3)O(4) rock-salt phase during lithiation consequently restrains the film from delamination. 2D phase-field simulation verifies the effect of strain. This work provides critical insights of understanding the solid-solid interfaces of conversion-type electrodes.
Investigation of strain-induced phase transformation in ferroelectric transistor using metal-nitride gate electrode
(2017-03-01)
Chiu, Yu-Chien
;
Cheng, Chun-Hu
;
Chang, Chun-Yen
;
Tang, Ying-Tsan
;
Chen, Min-Cheng
In this work, we report a ferroelectric memory with strainedgate engineering. The memory window of the high strain case was improved by similar to 71% at the same ferroelectric thickness. The orthorhombic phase transition (from ferroelectric to antiferroelectric transition) plays a key role in realizing negative capacitance effect at high gate electric field. Based on a reliable first principles calculation, we clarify that the gate strain accelerates the phase transformation from metastable monoclinic to orthorhombic and thus largely enhances the ferroelectric polarization without increasing dielectric thickness. This ferroelectric strain technology shows the potential for emerging device application.

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