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Breakthrough to Non-Vacuum Deposition of Single-Crystal, Ultra-Thin, Homogeneous Nanoparticle Layers: A Better Alternative to Chemical Bath Deposition and Atomic Layer Deposition
(2017-04-01)
Liao, Yu-Kuang
;
Liu, Yung-Tsung
;
Hsieh, Dan-Hua
;
Shen, Tien-Lin
;
Hsieh, Ming-Yang
Most thin-film techniques require a multiple vacuum process, and cannot produce high-coverage continuous thin filmswith the thickness of a fewnanometers on rough surfaces. We present a new "paradigm shift" non-vacuum process to deposit high-quality, ultra-thin, single-crystal layers of coalesced sulfide nanoparticles (NPs) with controllable thickness down to a few nanometers, based on thermal decomposition. This provides high-coverage, homogeneous thickness, and large-area deposition over a rough surface, with little material loss or liquid chemical waste, and deposition rates of 10 nm/min. This technique can potentially replace conventional thin-film deposition methods, such as atomic layer deposition (ALD) and chemical bath deposition (CBD) as used by the Cu(In, Ga) Se2 (CIGS) thin-film solar cell industry for decades. We demonstrate 32% improvement of CIGS thin-film solar cell efficiency in comparison to reference devices prepared by conventional CBD deposition method by depositing the ZnS NPs buffer layer using the new process. The new ZnS NPs layer allows reduction of an intrinsic ZnO layer, which can lead to severe shunt leakage in case of a CBD buffer layer. This leads to a 65% relative efficiency increase.
Electrically sign-reversible transverse g-factors of holes in droplet epitaxial GaAs/AlGaAs quantum dots under uniaxial stress
(2017-08-23)
Wu, Yu-Nien
;
Wu, Ming-Fan
;
Ou, Ya-Wen
;
Chou, Ying-Lin
;
Cheng, Shun-Jen
We present a theoretical investigation of anisotropic g-factor tensors of single holes confined in droplet epitaxial GaAs/AlGaAs quantum dots under electrical and mechanical controls using the gauge-invariant discretization method within the framework of four-band Luttinger-Kohn (k) over right arrow.(p) over right arrow theory. We reveal an intrinsic obstacle to realize the electrical sign reversal of the hole g-factors, being a key condition required for a full spin control in the scheme of g-tensor modulation, for the quantum dots solely with electrical bias control. Constructively, our studies show that, besides electrical gating, slightly stressing an inherently unstrained droplet epitaxial GaAs/AlGaAs quantum dot can offset the transverse hole g-factor to be nearly zero and make the electrical sign reversal of the hole g-factors feasible.
Diamagnetic Response of Exciton Complexes in Semiconductor Quantum Dots
(2008-12-31)
Tsai, Ming-Fu
;
Lin, Hsuan
;
Lin, Chia-Hsien
;
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.
Magnetic anisotropy in symmetric magnetic colloidal quantum dots doped with few Mn2+ impurities
(2009-06-01)
Cheng, Shun-Jen
;
電子物理學系
;
Department of Electrophysics
We present numerical exact diagonalization studies of the magnetism in magnetic II-VI colloidal semiconductor quantum dots charged with controlled number of electrons coupled to few magnetic-ion (Mn2+) dopants. The interplay between various relevant spin interactions and the discrete nature of Mn2+ spin distribution is shown to be essential in the magnetism of few Mn2+-doped quantum dots. An interesting revealed feature is the existence of pronounced magnetic anisotropies in symmetric magnetic quantum dots, which are related to the orbital quenchings induced by electron-Mn spin interactions. The orbital quenching effects further lead to significant deviations of the magnetizations of randomly Mn-doped dots from the standard Brillouin-function descriptions.
Spin characteristics of excitonic absorption in multiply charged quantum dots
(2007-08-01)
Cheng, Shun-Jen
;
電子物理學系
;
Department of Electrophysics
The excitonic absorption spectra of negatively charged self-assembled quantum dots as a function of resident electron number (N-e=0-5) are theoretically studied using the configuration interaction method. Spin Pauli blockade, Coulomb (exchange and correlation) interactions, and symmetry of dot shape are identified as the three main underlying mechanisms, sensitively depending on N-e, in the spin characteristic spectra. In particular, we predict that polarized absorption in triply charged dots exhibits anomalous spin-dependent features, resulting from the intrinsic correlations in charged exciton X-3.
Theory of magnetism in diluted magnetic semiconductor nanocrystals
(2008-03-01)
Cheng, Shun-Jen
;
電子物理學系
;
Department of Electrophysics
We present a theoretical investigation of magnetism in II-VI diluted magnetic semiconductor nanocrystals (NCs). The energy spectra, magnetizations, and magnetic susceptibilities of singly charged NCs with few substitutional magnetic Mn2+ ions are studied as functions of NC size, magnetic dopant distribution, and concentration by using the technique of exact diagonalization. For NCs containing long range interacting Mn ions, the quantum size effects improve the stability of ferromagnetic magnetic polarons. The carrier-mediated spin interactions between magnetic ions result in the enhancement of magnetism. By contrast, the ground states of NCs containing short ranged Mn clusters undergo a series of magnetic phase transitions from antiferromagnetism to ferromagnetism, with decreasing size of NC. An analysis based on a simplified constant interaction model, supported by the numerical calculations of local mean field theory, is presented for the magnetic NCs with many magnetic ions over a wide range of Mn concentration and NC size. Accordingly, we derive the condition for the formation of magnetic polaron and predict the observable signatures of the magnetic phases in magnetization measurements.
Nonresonant carrier transfer in single InGaAs/GaAs quantum dot molecules
(2008-06-01)
Chang, Wen-Hao
;
Lin, Hsuan
;
Wang, Sheng-Yun
;
Lin, Chia-Hsien
;
Cheng, Shun-Jen
We present a spectroscopic study of single quantum dot molecules (QDMs) formed by two closely stacked In(0.5)Ga(0.5)As layers. The exciton fine structures as well as direct and indirect excitonic species associated with QDMs were identified by power dependent and polarization resolved microphotoluminescence measurements. As the temperature was increased, a directional energy transfer between the direct and indirect excitons in single QDMs was observed. A rate-equation model was developed to explain our data. We show that a phonon-assisted nonresonant tunneling of the hole between the two adjacent dots is responsible for such directional energy transfers in QDMs.
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
;
You, Wen-Ting
;
Chou, Hsiang-Yu
;
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.
Engineered spin-state transitions of two interacting electrons in semiconductor nanowire quantum dots
(2010-06-11)
Chen, Yan-Ting
;
Cheng, Shun-Jen
;
Tang, Chi-Shung
;
電子物理學系
;
Department of Electrophysics
Spin properties of two interacting electrons in a quantum dot (QD) embedded in a nanowire with controlled aspect ratio and longitudinal magnetic fields are investigated by using a configuration-interaction (CI) method. The developed CI theory based on a three-dimensional parabolic model provides explicit formulations of the Coulomb matrix elements and allows for straightforward and efficient numerical implementation. Our studies reveal fruitful features of spin-singlet-triplet transitions of two electrons confined in a nanowire QD, as a consequence of the competing effects of geometry-controlled kinetic-energy quantization, Coulomb interaction, and spin-Zeeman energy. The developed theory is further employed to study various spin states of two quantum-confined electrons in the regime of "crossover" dimensionality, from quasi-two-dimensional (disklike) QDs to finite one-dimensional (rodlike) QDs.
Tunneling Effects on Fine-Structure Splitting in Quantum-Dot Molecules
(2010-05-21)
Ramirez, Hanz Y.
;
Cheng, Shun-Jen
;
電子物理學系
;
Department of Electrophysics
We theoretically study the effects of bias-controlled interdot tunneling in vertically coupled quantum dots on the emission properties of spin excitons in various bias-controlled tunneling regimes. As a main result, we predict substantial reduction of optical fine-structure splitting without any drop in the optical oscillator strength for the coupled dots with high tunneling rates. This special reduction diminishes the distinguishability of polarized decay paths in cascade emission processes suggesting the use of stacked quantum-dot molecules as entangled photon-pair sources.

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