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ABSTRACT: Attenuation of a surface acoustic wave is used as a highly sensitive and noninvasive probe of persistent photoconductivity effects in ZnCdSe/ZnSe quantum wells. These effects are observed over long time-scales exceeding several minutes at low temperatures. By varying the optical excitation energy and power and temperature we show that these effects arise from carriers photogenerated by interband excitation which are trapped in random potential fluctuations in the quantum wells related to compositional fluctuations. Effects related to defect levels in the band gap can be excluded and a transition of the conduction mechanism with temperature from a hopping to a percolation regime is observed. The transition temperature observed for our quantum well material is strongly reduced compared to bulk crystals. This indicates a superior structural quality giving rise to only weak potential fluctuation of <sup>></sup><sub>~</sub>3 meV .
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: Temperature dependence and recombination behavior of trapped charge carriers in ZnCdSe/ZnSe multiple quantum wells are investigated employing surface acoustic waves. These weakly perturb the carrier system, but remain highly sensitive even at small conductivities. Using this non-invasive probe we are able to detect persistent photoconductivity minutes after optical excitation. Measurement of exciting photon energies, the temperature dependence and ability to quench the conductivity with energies lower than the bandgap, support the notion of spatial separation of electrons and holes in the wells, due to random local potential fluctuations possibly induced by compositional fluctuations.
06/2009;
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ABSTRACT: Purpose – The purpose of this paper is to present a novel manufacturing process that aims to pattern metal tracks onto polyimide at atmospheric pressure and ambient environment. The process can be scaled up for industrial applications. Design/methodology/approach – From a thorough literature survey, different approaches were carried out for processing polyimide. Following a design of experiments for the processing and various characterisation techniques, a micro-coil was manufactured as a test demonstrator. Findings – The characteristics of some main formaldehyde-based electroless copper baths were compared. The quality of the sidewalls was characterised and the performance of the process was assessed. Originality/value – This paper demonstrates a high-value manufacturing technique that is mass manufacturable, low cost and suitable for use on 3D surfaces. Criteria required for the development of a direct-writing process have been described. The issues surrounding electroless plating on polyimide have been explained.
Circuit World 05/2009; 35(2):3-17. · 0.44 Impact Factor
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ABSTRACT: By making use of epitaxial lift-off, ZnCdSe quantum wells are transferred onto a LiNbO3 substrate in order to employ its enhanced piezoelectric properties. The photoluminescence emission of this hybrid structure is characterized and the influence of a surface acoustic wave on the free exciton and bound exciton emission is investigated. Finally, two counterpropagating surface acoustic waves are launched leading to a decrease in the acoustic wave mediated exciton dissociation.
Applied Physics Letters 05/2009; 94(19):193505-193505-3. · 3.84 Impact Factor
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Microelectronics Journal. 01/2009; 40:530-532.
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ABSTRACT: Copper micro-patterns have been fabricated on polyimide substrates without the use of evaporation techniques or photoresist materials. A novel easy light-directed metal patterning method has been achieved by using a photoreactive polymer reducing agent, methoxy poly(ethylene glycol) (MPEG) as a thin film coating. The interaction of UV light and MPEG in ethanol film enables the photoreduction of mobile silver ions previously incorporated within the surface-modified polyimide substrates. The silver nanoparticle patterns thus formed serve as a active catalytic seed layer for subsequent electroless copper plating. Narrow copper tracks with low resistivity close to that of bulk copper have been achieved.
Electronics System-Integration Technology Conference, 2008. ESTC 2008. 2nd; 10/2008
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ABSTRACT: Copper micro-patterns have been fabricated on polyimide substrates by ultra-violet (UV) lithography without the use of evaporation techniques or photoresist materials. Using a photoreactive polymer-reducing agent, methoxy poly(ethylene glycol) (MPEG) as a thin film coating, a novel light-directed metal-patterning method in air atmosphere was realised. The interaction of UV light and MPEG in ethanol film enabled the photoreduction of mobile silver ions available within the surface-modified polyimide substrates. The silver nanoparticle patterns thus formed served as active catalytic seed layers for subsequent electroless copper plating. Narrow copper tracks with low resistivity close to that of bulk copper were achieved. MPEG is a non-toxic, low cost and commercially available polymer which can be easily spin-coated and washed off after the patterning process. These attractive properties of MPEG together with its photoreactivity provide great potential for developments of UV direct-metallisation methods.
Micro & Nano Letters 10/2008; · 0.94 Impact Factor
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ABSTRACT: Conductive micro-patterned copper tracks were fabricated by UV direct-writing of a nanoparticle silver seed layer followed by selective electroless copper deposition. Silver ions were first incorporated into a hydrolyzed polyimide surface layer by wet chemical treatment. A photoreactive polymer coating, methoxy poly(ethylene glycol) (MPEG) was coated on top of the substrate prior to UV irradiation. Electrons released through the interaction between the MPEG molecules and UV photons allowed the reduction of the silver ions across the MPEG/doped polyimide interface. The resultant silver seed layer has a cluster morphology which is suitable for the initiation of electroless plating. Initial results showed that the deposited copper tracks were in good agreement with the track width on the photomask and laser direct-writing can also fabricate smaller line width metal tracks with good accuracy. The facile fabrication presented here can be carried out in air, at atmospheric pressure, and on contoured surfaces.
Design, Test, Integration and Packaging of MEMS/MOEMS, 2008. MEMS/MOEMS 2008. Symposium on; 05/2008
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ABSTRACT: Recent improvements in UV Raman spectrometer capability have resulted from the availability of much improved dielectric filters, and we can now observe directly both the longitudinal and the transverse optic phonon modes of the wide-bandgap zincblende semiconductor MgS. We are, thus, able to confirm the earlier measured and calculated phonon frequencies with improved accuracy. The strong sequence of overtone modes that we observe is interpreted in terms of the resonance behavior of the Raman scattering process at this excitation energy. The spectra also show features which, by comparison to ab-initio calculations of the phonon dispersion and density of states, can be identified as arising from multiple Raman scattering processes involving longitudinal acoustic phonons near the Brillouin zone boundary.
Journal- Korean Physical Society 01/2008; 53(5):2779-2781. · 0.45 Impact Factor
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ABSTRACT: We report the observation of strong exciton-photon coupling in a ZnSe based microcavity fabricated using epitaxial liftoff. Molecular beam epitaxial grown ZnSe/Zn$_{0.9}$Cd$_{0.1}$Se quantum wells with a one wavelength optical length at the exciton emission were transferred to a SiO$_2$/Ta$_2$O$_5$ mirror with a reflectance of 96% to form finesse matched microcavities. Analysis of our angle resolved transmission spectra reveals key features of the strong coupling regime: anticrossing with a normal mode splitting of $23.6 meV$ at $20 K$; composite evolution of the lower and upper polaritons; and narrowing of the lower polariton linewidth near resonance. The heavy hole exciton oscillator strength per quantum well is also deduced to be $1.78 \times 10^{13} cm^{-2}$. Comment: 3 pages, 3 figures
03/2007;
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13th International Conference on II-VI Compounds13th International Conference on II-VI Compounds, Jeju City, SOUTH KOREA; 01/2007
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ABSTRACT: We report the observation of strong exciton-photon coupling in a
ZnSe-based microcavity fabricated using epitaxial liftoff. Molecular
beam epitaxial grown ZnSe/Zn0.9Cd0.1Se quantum wells with a one
wavelength optical length at the exciton emission were transferred to
a SiO2/Ta2O5 mirror with a reflectance of 96
Semiconductor Science and Technology. 01/2007; 22:1189--1192.
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ABSTRACT: We have determined the direct and exchange electron–electron and electron–hole Coulomb energies in CdSe/ZnSe quantum dots. The experiments are based on single dot photoluminescence at low temperature. By controlling the charging with a vertical transistor structure and by applying a symmetry-breaking magnetic field, we show how we can determine all the Coulomb energies. The direct Coulomb energies are responsible for large, ∼20 meV, red-shifts of the emission on charging. The exchange Coulomb energies lead to a very pronounced fine structure splitting, up to 2.6 meV, for the neutral exciton. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 03/2006; 243(4):782 - 786. · 1.32 Impact Factor
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ABSTRACT: The effect of increasing annealing times on the emission energies and dot densities of ZnSe/CdSe quantum dots has been studied. Ensemble PL carried out at 77 K demonstrates the energy shift of the main peak attributed to fractional monolayer islands, which is due to annealing. µPL conducted at 4 K was then used to study the increased spectral separation of the emission energies from SK dots. Finally, ex-situ AFM measurements of two samples, showed a reduction in the Stranski-Krastanow dot density of approximately one order of magnitude due to annealing. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2006; 3(4):908 - 911.
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ABSTRACT: Optoelectronic devices are important for laser modulators and Q-switches as well as in optical communication and network systems where all optical switching is required. The development of II–VI semiconductor devices fabricated by molecular beam epitaxy and based on the quantum confined Stark effect in p–i–n structures where the i-region is a set of quantum wells is reviewed. In particular, the characterisation of ZnSe/ZnCdSe/ZnSe devices is discussed.
physica status solidi (b) 02/2006; 187(2):347 - 353. · 1.32 Impact Factor
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ABSTRACT: The growth of ZnO by epitaxial techniques is important for the development of new optoelectronic devices. In order to establish control over the growth, it is important to be able to measure dopant profiles through the layers to supplement Hall effect data. A standard method for many semiconductors is by electrochemical capacitance–voltage (C–V) profiling. In this letter, C–V profiles from metal organic chemical vapor deposition grown ZnO thin films are presented. It is shown that carrier density profiles can be obtained routinely and reproducibly by using 0.1 M ZnCl2. © 2004 American Institute of Physics.
Applied Physics Letters 04/2004; 84(16):3043-3045. · 3.84 Impact Factor
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ABSTRACT: An experimental study of the optical Stark effect in ZnSe-based quantum wells using pump-probe spectroscopy is presented. The Stark shift at both the heavy-hole and the light-hole resonances is measured as a function of pump detuning for samples with different heavy-hole–light-hole separations. The coupling between the heavy hole and the light hole has to be taken into account to explain the observed dependence on the pump wavelength detuning and polarization of pump and probe pulses. For a range of samples the Stark shift behavior is characterized by the ratio of the detuning to the heavy-hole to light-hole separation; higher order Coulomb correlations play a prominent role when the ratio is less than 2.
Phys. Rev. B. 02/2004; 69(8).
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ABSTRACT: We compare and contrast CdSe quantum dots grown on ZnSe and MgS. We find significant differences in the optical and structural properties of the two systems. Through an annealing process we have been able to separate the PL energy of the two peaks inherent in the photoluminescence of ZnSe/CdSe quantum dots. Temperature dependent photoluminescence was performed from 250 K to 77 K to explore the thermally induced carrier transfer in the two systems. We interpret the striking differences between the PL from CdSe quantum dots grown on MgS and those grown on ZnSe to the intermixing and alloying at the ZnSe, CdSe interface. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 02/2004; 1(4):755 - 758.
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Physica Status Solidi B-Basic Research. 01/2004; 241(3):471-474.
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ABSTRACT: In this paper we review the recent progress in the growth and spectroscopy of CdSe quantum dots. In particular, atomic layer epitaxy (ALE) has been used to grow ZnSe/CdSe and ZnSe/CdSe:Mn magnetic quantum dots. For samples grown without a ZnSe capping layer, dot densities of the order of 10<sup>9</sup> cm<sup>-2</sup> were measured by atomic force microscopy (AFM). In the capped samples, the ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV and in the high Mn concentration samples a spectrally broad emission at 2.15 eV from the internal Mn<sup>2+</sup> transition. Single dot spectroscopy was carried out by confocal microscopy and the PL linewidth was measured as a function of Mn concentration. Also, CdSe/MgS quantum dots have been grown successfully by molecular beam epitaxy using a thermally activated reorganization process that occurs during growth interruption. Unlike the ZnSe/CdSe dots the PL measurements show emission from both QDs and the wetting layer, with emission energies ranging between (2.3 and 3.8 eV). AFM topography and μm-PL measurements also show evidence of quantum dot structures and power dependent PL measurements carried out on the dots give a value of 30 meV for the bi-exciton binding energy at 77 K.
Optoelectronic and Microelectronic Materials and Devices, 2002 Conference on; 01/2003
