Browsing by Author "Chen, J. F."

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Admittance spectroscopy of the electric properties of 1,4-bis[N-(1-naphthyl)-N '-phenylamino]-4,4 ' diamine doped with tungsten oxide
(2006)
Hsieh, M. T.
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Chang, C. C.
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Hu, C. A.
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Huang, Y. T.
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Yang, S. L.
We report an efficient p-type doping of NPB layer using WO(3) as a p-dopant. We find that increasing the WO(3) volume percentage can reduce the activation energy of the NPB layer and increase the hole concentration in NPB layer. As a result, the hole-injection can be improved via tunneling through a narrow depletion region due to the increasing band bending which corresponds to the difference of Fermi level position between the ITO and the NPB layer.
Analysis of strain relaxation in GaAs/InGaAs/GaAs structures by spectroscopy of relaxation-induced states
(2007-02-01)
Chen, J. F.
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Chiang, C. H.
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Hsieh, P. C.
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Wang, J. S.
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電子物理學系
Strain relaxation in GaAs/In0.2Ga0.8As/GaAs structures is investigated by analyzing relaxation-induced traps. Strain relaxation is shown to cause carrier depletion by the induction of a 0.53 eV trap in the top GaAs layer, a 0.13 eV trap in the InGaAs layer, and a 0.33 eV trap in the neighboring lower GaAs layer. The 0.53 eV trap which exhibits a logarithmic function of transient capacitance is attributed to threading dislocations. The 0.33 eV trap exhibits an exponential transient capacitance, suggesting a GaAs point defect as its origin. Given its activation energy, it is assigned to the EL6 in GaAs, commonly considered to be As-i-V-Ga complexes. This trap and the 0.13 eV trap are regarded as the same, since their energy difference is comparable to the optically determined conduction-band offset. The spatial location of this trap correlates with that of misfit dislocations. Accordingly, the production of this trap is determined from the mechanism of strain relaxation. A likely mode of strain relaxation is deduced from the locations of these traps. (c) 2007 American Institute of Physics.
Bimodel onset strain relaxation in InAs quantum dots with an InGaAs capping layer
(2010-08-30)
Chen, J. F.
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Chen, Ross C. C.
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Chiang, C. H.
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Chen, Y. F.
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Wu, Y. H.
Capping InAs quantum dots (QDs) with an InGaAs layer allows strain relaxation to induce a low-energy electron state below a set of fine dot family states, which is consistent with photoluminescence (PL) spectra. The evolution of InAs thickness suggests a bimodal onset relaxation, i.e., a fine dot family that is strain-relieved by indium outdiffusion from the QDs, as suggested by transmission electron microscopy, and a low-energy dot family that is strain relaxed by the generation of lattice misfits. The indium outdiffusion can explain an abnormal PL blueshift in 70 meV in the fine dot family at onset of strain relaxation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483757]
Compensation effect and differential capacitance analysis of electronic energy band structure in relaxed InAs quantum dots
(2010-09-15)
Chen, J. F.
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Chen, Ross C. C.
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Chiang, C. H.
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Hsieh, M. C.
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Chang, Y. C.
The use of a differential capacitance technique for analyzing the effect of strain relaxation on the electronic energy band structure in relaxed InAs self-assembled quantum dots (QDs) is presented. Strain relaxation is shown to induce a deep defect state and compensate the ionized impurity in the bottom GaAs layer, leading to a double depletion width and a long emission time. An expression of capacitance at different frequency and voltage is derived for analyzing the experimental data. It has been shown that the relationship between the low-frequency and high-frequency capacitances can be well explained by a Schottky depletion model with a compensated concentration in the bottom GaAs layer. A simple expression is presented to account for the modulation of the free electrons in the top GaAs layer. This capacitance analysis shows a long low-energy tail for the electron ground state, suggesting not very uniform strain relaxation. The results of this study illustrate a carrier compensation effect of the defect state on the electronic energy band structure near the QDs. (C) 2010 American Institute of Physics. [doi:10.1063/1.3467938]
Deep-level emissions in GaAsN/GaAs structures grown by metal organic chemical vapor deposition
(2007-06-15)
Chen, J. F.
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Ke, C. T.
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Hsieh, P. C.
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Chiang, C. H.
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Lee, W. I.
This work presents the deep-level photoluminescence of coherently strained GaAsN/GaAs quantum-well (QW) structures with various GaAsN thicknesses and N contents. A broad deep-level emission at similar to 1.1 eV is observed, whose wavelength is redshifted as the GaAsN thickness increases. Based on its energy separation from the QW emission, this emission is attributed to a transition between the QW electron ground state and a deep level at similar to 0.2 eV above the GaAsN valence-band (VB) edge. This level is shown to be tied to the GaAs band edge. A transition between this level and the GaAs conduction band allows the GaAsN-GaAs band alignment to be evaluated. A type II band lineup is obtained with VB offsets of 0.03 and 0.002 eV for N=0.6% and 1.8%, respectively. The decreased VB offset suggests a transition from type II to type I with increasing N content. Thermal annealing effectively removes this level and improves the QW emission. The concentration of this level is not clearly correlated with N content, suggesting that this level is induced by a low-temperature growth of the GaAsN layer to suppress the composition fluctuation. Given its energy, this level is tentatively assigned to V-Ga. (c) 2007 American Institute of Physics.
Demonstration of slow light via population oscillation in quantum dot VCSEL
(2007)
Xuan, R.
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Chi, J. Y.
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Peng, P. C.
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Yang, H. P.
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Chao, C. L.
Experimental data for slow light delay in a quantum dot VCSEL are analyzed with a Fabry-Perot analysis. The analysis can fit the observed wavelength dependence of the delay. It can also explain that the maximum delay occur at a wavelength detuned from the resonant frequency.
Effect of antimony incorporation on the density, shape, and luminescence of InAs quantum dots
(2008-07-15)
Chen, J. F.
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Chiang, C. H.
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Wu, Y. H.
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Chang, L.
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Chi, J. Y.
This work investigates the surfactant effect on exposed and buried InAs quantum dots (QDs) by incorporating Sb into the QD layers with various Sb beam equivalent pressures (BEPs). Secondary ion mass spectroscopy shows the presence of Sb in the exposed and buried QD layers with the Sb intensity in the exposed layer substantially exceeding that in the buried layer. Incorporating Sb can reduce the density of the exposed QDs by more than two orders of magnitude. However, a high Sb BEP yields a surface morphology with a regular periodic structure of ellipsoid terraces. A good room-temperature photoluminescence (PL) at similar to 1600 nm from the exposed QDs is observed, suggesting that the Sb incorporation probably improves the emission efficiency by reducing the surface recombination velocity at the surface of the exposed QDs. Increasing Sb BEP causes a blueshift of the emission from the exposed QDs due to a reduction in the dot height as suggested by atomic force microscopy. Increasing Sb BEP can also blueshift the similar to 1300 nm emission from the buried QDs by decreasing the dot height. However, a high Sb BEP yields a quantum well-like PL feature formed by the clustering of the buried QDs into an undulated planar layer. These results indicate a marked Sb surfactant effect that can be used to control the density, shape, and luminescence of the exposed and buried QDs. (C) 2008 American Institute of Physics.
Electron emission properties of relaxation-induced traps in InAs/GaAs quantum dots and the effect of electronic band structure
(2007-08-15)
Chen, J. F.
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Wang, J. S.
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電子物理學系
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Department of Electrophysics
The electron-emission properties of relaxation-induced traps in InAs/GaAs quantum dots (QDs) are studied in detail using capacitance-voltage (C-V) profiling and bias-dependent deep-level transient spectroscopy. Strain relaxation is shown to induce a threading-dislocation-related trap in the top GaAs layer and a misfit-dislocation-related trap near the QD. The threading trap decreases its electron-emission energy from 0.63 to 0.36 eV from sample surface toward the QD, whereas the misfit trap gradually increases its electron-emission energy from 0.28 to 0.42 eV from near the QD toward the GaAs bottom layer, indicating that both traps near the QD have lower electron-emission energies. Hence, the emission-energy change is attributed to the related traps across the QD interface where a band offset exists. The C-V profiling at 300 K shows extended carrier depletion near the QD. As temperature is increased, an electron-emission peak emerges at the QD followed by a prominent peak, suggesting that the trap responsible for the prominent peak lies in energy below the QD electron ground state. From a simulation, this trap is identified to be the misfit trap located at the QD and at the observed emission energy below the GaAs conduction band. Based on the energy location of this trap, we deduce a possible mode of strain relaxation. (C) 2007 American Institute of Physics.
Electron emissions in InAs quantum dots containing a nitrogen incorporation induced defect state: the influence of thermal annealing
(2008-12-10)
Chen, J. F.
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Yu, C. C.
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Yang, C. H.
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電子物理學系
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Department of Electrophysics
With the incorporation of nitrogen (N) into InAs quantum dots (QDs), the carrier distribution near the QD displays electron emissions from a localized N-induced defect state at 0.34 eV and a weak emission at 0.15 eV from the QD. This defect state causes drastic carrier depletion in the neighboring GaAs bottom layer near the QD, which can effectively suppress tunneling emission for the QD excited states. As a result, electrons escape from the QD ground state through thermal emission to near the GaAs conduction band, rather than through thermal emission to the QD first excited state and a subsequent tunneling to the GaAs conduction band, as observed in InAs QDs without N incorporation. Thermal annealing can weaken the defect emission and enhance the QD emission, suggesting a removal of the defect state and a recovery of carriers in the QD. Increasing annealing temperature can significantly decrease the emission time and energy of the QD emission, which is explained by a weakening of tunneling suppression due to the removal of the defect state.
Evidence for the electron trap state associated with N-rich clusters in InGaAsN/GaAs quantum wells
(2007-06-13)
Chen, J. F.
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Hsieh, P. C.
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Hsiao, R. S.
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Wang, J. S.
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Chi, J. Y.
An electron trap state has been found to appear in InGaAsN/GaAs quantum wells ( QWs) as a result of composition fluctuation. Evidence of composition fluctuation is shown by photoluminescence spectra which display an additional low-energy emission due to local regions of N-rich clusters. Enhancing composition fluctuation leads to undulated InGaAsN/GaAs interface with dot-like islands. The capacitance-voltage profiling shows an additional peak following the QW electron ground state peak, suggesting the presence of an electron trap state below the QW electron ground state. The emission properties of this trap state are similar to those of quantum dots immersed in a well. Enhancing composition fluctuation can cause an energy downward shift and spectral broadening of this electron trap state, suggesting an increase in island size and fluctuations of the size and composition. Thus, it is deduced that the electron trap state originated from the N-rich clusters in the well behaving like quantum dots. Upon modulation, the electrons are thermally activated from this trap state to the QW electron ground state and subsequently emitted to the GaAs bottom electrode.
Exploration of water jet generated by Q-switched laser induced water breakdown with different depths beneath a flat free surface
(2013-01-14)
Chen, Ross C. C.
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Yu, Y. T.
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Su, K. W.
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Chen, J. F.
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Chen, Y. F.
The dynamics of a water jet on a flat free surface are investigated using a nanosecond pulsed laser for creating an oscillating bubble with different depths beneath the free surface. A thin jet is shown to deform a crater surface resulted from surface depression and cause a circular ring-shaped crater on the connection surface between the crater of surface depression and the thin jet. The collapse of this circular ring-shaped crater is proposed to the crown-like formation around a thick jet. The evolution of the bubble depth suggests a classification of four distinctive ranges of the bubble depths: non-crown formation when the parameter of bubble depth over the maximum bubble radius gamma <= 0.5, unstable crown formation when 0.5 <= gamma <= 0.6, crown-like structure with a complete crown wall when 0.6 <= gamma <= 1.1, and non-crown formation when 1.1 <= gamma. Furthermore, the orientation of the crown wall gradually turns counterclockwise to vertical direction with increasing gamma from 0.5 to 1.1, implying a high correlation between the orientation of the crown wall and the depth of the bubble. This correlation is explained and discussed by the directional change of the jet eruption from the collapse of circular ring-shaped crater. (C)2013 Optical Society of America
How do InAs quantum dots relax when the InAs growth thickness exceeds the dislocation-induced critical thickness?
(2012-01-01)
Chen, J. F.
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Lin, Y. C.
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Chiang, C. H.
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Chen, Ross C. C.
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Chen, Y. F.
A simple critical thickness for generating lattice misfits is insufficient to describe the onset strain relaxation in InAs quantum dots (QDs). A predominant dot family is shown to relieve its strain by In/Ga interdiffusion, rather than by lattice misfits, at the onset of strain relaxation. This argument is based on photoluminescence spectra, which show the emergence of a fine blueshifted transition at the onset of strain relaxation, along with a low-energy transition from a dot family degraded by lattice misfits. From the analysis of the temperature-dependent blueshift and energy separation between the ground and excited-state transitions, the blueshift is attributed to In/Ga interdiffusion. Transmission electron microscopy suggests a relaxation-induced indium migration from the interdiffused dot family to the dislocated dot family. Post-growth thermal annealing can further relieve strain by inducing more In/Ga interdiffusion in the interdiffused dot family and more dislocations in the dislocated dot family. This study explains the co-existence of strong carrier confinement in the QDs and enormous misfit-related traps in the capacitance-voltage spectra, and an elongated QD electron-emission time. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3675519]
Improving the photoluminescence properties of self-assembled InAs surface quantum dots by incorporation of antimony
(2011-08-15)
Chiang, C. H.
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Wu, Y. H.
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Hsieh, M. C.
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Yang, C. H.
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Wang, J. F.
This study investigates the effects of surfactant and segregation from InAs surface quantum dots (SQDs) by incorporating antimony (Sb) into the QD layers. The Sb surfactant effect extends planar growth and suppresses dot formation. Incorporating Sb can reduce the density of SQDs by more than two orders of magnitude. Photoluminescence (PL) reveals enhancement in the optical properties of InAs SQDs as the Sb beam equivalent pressure (BEP) increases. This improvement is caused by the segregation of Sb on the surface of SQDs, which reduces non-radiative recombination and suppresses carrier loss. The dark line at the SQDs surface in the transmission electron microscopic image suggests that the incorporated Sb probably segregates close to the surface of the SQDs. These results indicate a marked Sb segregation effect that can be exploited to improve the surface-sensitive properties of SQDs for biological sensing. Crown Copyright (C) 2011 Published by Elsevier B. V. All rights reserved.
Influence of thermal annealing on the electron emission of InAs quantum dots containing a misfit defect state
(2009-03-15)
Chen, J. F.
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Yang, C. H.
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Hsu, R. M.
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Wang, U. S.
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電子物理學系
We have investigated the effect of postgrowth thermal annealing on the electron emission from InAs quantum dots (QDs) containing a misfit-related defect state induced by strain relaxation. Additional carrier depletion in the GaAs bottom layer near the QD, caused by the defect state, can effectively suppress electron tunneling from the QD, leading to the observation of a thermal emission from the QD electron ground state to the GaAs conduction band with a large emission energy of 213 meV, in contrast to defect-free nonrelaxed QDs in which an emission of 58 meV from the QD electron ground state to first excited state is observed. The emission energy is reduced to 193 meV and to 164 meV after annealing at 650 and 700 degrees C for 1 min, respectively. This emission energy reduction is correlated with the photoluminescence blueshift which is attributed to the interdiffusion of atoms across the QD interface. The electron emission from the QD first excited and ground states is found to be a thermal emission at high temperatures and a tunneling emission at low temperatures. The tunneling energy barrier is found to be comparable to the thermal emission energy, supporting a thermal emission to the GaAs conduction band. This study illustrates a significant effect of a defect state on the electron-emission process in the QDs, suggesting the possibility of modifying the electron emission time of the QDs by purposely introducing a deep defect state.
Nonpolar a-plane GaN grown on r-plane sapphire using multilayer AlN buffer by metalorganic chemical vapor deposition
(2011-01-15)
Chiang, C. H.
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Chen, K. M.
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Wu, Y. H.
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Yeh, Y. S.
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Lee, W. I.
Mirror-like and pit-free non-polar a-plane (11-20) GaN films are grown on r-plane (1-102) sapphire substrates using metalorganic chemical vapor deposition (MOCVD) with multilayer high-low-high temperature AlN buffer layers. The buffer layer structure and film quality are essential to the growth of a flat, crack-free and pit-free a-plane GaN film. The multilayer AlN buffer structure includes a thin low-temperature- deposited AlN (LT-AlN) layer inserted into the high-temperature-deposited AlN (HT-AlN) layer. The results demonstrate that the multilayer AlN buffer structure can improve the surface morphology of the upper a-plane GaN film. The grown multilayer AlN buffer structure reduced the tensile stress on the AlN buffer layers and increased the compressive stress on the a-plane GaN film. The multilayer AlN buffer structure markedly improves the surface morphology of the a-plane GaN film, as revealed by scanning electron microscopy. The effects of various growth V/III ratios was investigated to obtain a-plane GaN films with better surface morphology. The mean roughness of the surface was 1.02 nm, as revealed by atomic force microscopy. Accordingly, the multilayer AlN buffer structure improves the surface morphology and facilitates the complete coalescence of the a-plane GaN layer. (C) 2010 Elsevier B.V. All rights reserved.
Photoluminescence studies of InAs/GaAs quantum dots covered by InGaAs layers
(2010-01-15)
Shu, G. W.
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Wang, J. S.
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Shen, J. L.
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Hsiao, R. S.
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Chen, J. F.
Photoluminescence (PL), PL excitation (PLE), and time-resolved PL were used to study effects of InGaAs layers on the optical properties of InAs/GaAs quantum dots (QDs). A rich fine structure in the excited states of confined excitons (up to n = 4 quantum states) was observed, providing useful information to study the quantum states in the InAs/GaAs QDs. A significant redshift of the PL peak energy for the QDs covered by InGaAs layers was observed, attributing to the decrease of the QD strain and the lowing of the quantum confinement. (C) 2009 Elsevier B.V. All rights reserved.
Relaxation-induced lattice misfits and their effects on the emission properties of InAs quantum dots
(2007-09-05)
Chen, J. F.
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Wang, Y. Z.
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Chiang, C. H.
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Hsiao, R. S.
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Wu, Y. H.
Strain relaxation in InAs/InGaAs quantum dots (QDs) is shown to introduce misfits in the QD and neighboring GaAs bottom layer. A capacitance -voltage profiling shows an electron accumulation peak at the QD with a long emission time, followed by additional carrier depletion caused by the misfits in the GaAs bottom layer. The emission-time increase is explained by the suppression of tunneling for the QD excited states due to the additional carrier depletion. As a result, electrons are thermally activated from the QD states to the GaAs conduction band, consistent with observed emission energies of 0.160 and 0.068 eV which are comparable to the confinement energies of the QD electron ground and first-excited states, respectively, relative to the GaAs conduction band. This is in contrast to non-relaxed samples in which emission energy of 60 meV is observed, corresponding to the emission from the QD ground state to the first-excited state.
Spectral shape and broadening of emission from AlGaInP light-emitting diodes
(2009-10-01)
Chen, N. C.
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Lien, W. C.
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Yang, Y. K.
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Shen, C.
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Wang, Y. S.
This work presents a model for describing the shape of the spontaneous emission spectrum from a quantum-well structure. A function is introduced to specify the probability distribution for the effective band gap. Based on this model, the coexisting carrier thermal broadening and effective band gap broadening in the spontaneous emission spectrum can be separated from each other. Applying this model to the spectra of AlGaInP light-emitting diodes reveals that the probability distribution functions are almost Gaussian. Therefore, the emission spectra can be described by an analytical expression with fitted parameters. Possible reasons for this band gap broadening are discussed. The determination of the junction temperatures from the emission spectra and possible deviations of the results thus determined are also elaborated. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3243319]
Strain relaxation and induced defects in InAsSb self-assembled quantum dots
(2006-06-05)
Chen, J. F.
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Hsiao, R. S.
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Huang, W. D.
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Wu, Y. H.
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Chang, L.
The onset of strain relaxation and induced defects in InAs0.94Sb0.06 quantum dots are investigated. We show that the relaxation causes partial carrier depletion in the dots and drastic carrier depletion in the top GaAs layer due to the introduction of two defect traps At 0.35 and 0.64 eV. This result is consistent with transmission electron microscopy data which show misfit dislocations on the edges of the dot upper boundary and threading dislocations in the top GaAs layer. The bottom GaAs layer is dislocation-free, and thus the strain relaxation may initially occur on the edges of the dots. (c) 2006 American Institute of Physics.
Strain relaxation in InAs self-assembled quantum dots induced by a high N incorporation
(2008-11-15)
Chen, J. F.
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Yang, C. H.
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Wu, Y. H.
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Chang, L.
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Chi, J. Y.
The effect of a high N incorporation in self-assembled InAs quantum dots (QDs) is investigated by analyzing the electronic and structural properties around QD region. Capacitance-voltage profiling and admittance spectroscopy shows that N incorporation into the InAs QD layer leads to drastic carrier depletion in the QD layer and neighboring GaAs layers due to the formation of a deep defect state at 0.34-0.41 eV. The signature of this defect state is similar to those defects observed in strain relaxed QDs or InGaAs/GaAs quantum wells when the InAs deposition thickness exceeds a critical thickness. Accordingly, the N incorporation might result in strain relaxation either by increasing localized strain or by inducing composition inhomogeneities, which provide nucleation sources for strain relaxation. The argument of strain relaxation is supported by transmission electron microscopy that reveals lattice misfits at the QD layer and neighboring GaAs layers.