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15 Aug 2005

Volume 98, Issue 4, Articles (04xxxx)

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Intersubband absorption of strain-compensated Si1−xGex valence-band quantum wells with 0.7 ⩽ x ⩽ 0.85

T. Fromherz, M. Meduňa, G. Bauer, A. Borak, C. V. Falub, S. Tsujino, H. Sigg, and D. Grützmacher

J. Appl. Phys. 98, 044501 (2005); http://dx.doi.org/10.1063/1.1997292 (7 pages) | Cited 11 times

Online Publication Date: 16 August 2005

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Strain-compensated, p-type SiGe quantum wells with a high Ge concentration of up to 85% have been grown on commercially available Si0.5Ge0.5 pseudosubstrates by molecular-beam epitaxy. Structural investigations by transmission electron microscopy and high-resolution x-ray reflection and diffraction showed that at a growth temperature around T = 300 °C, samples in excellent compliance with the design parameters, comparatively sharp interfaces, and negligible increase of growth-induced surface roughness can be grown. Comparison of polarization-dependent intersubband absorption measurements with simulated intersubband absorption spectra shows that for the quantum wells investigated in this work, the hole eigenstates, their in-plane dispersion, and the polarization-dependent intersubband transition matrix elements are accurately described by a strain-dependent, six-band k∙p Luttinger-Kohn Hamiltonian in which only one fitting parameter—the intersubband transition linewidth—is used.
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81.07.St Quantum wells
78.67.De Quantum wells
68.65.Fg Quantum wells
71.20.Nr Semiconductor compounds
73.21.Fg Quantum wells
71.15.-m Methods of electronic structure calculations
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Lp Transmission electron microscopy (TEM)

Single-component light-emitting electrochemical cell fabricated from cationic polyfluorene: Effect of film morphology on device performance

Ludvig Edman, Bin Liu, Martin Vehse, James Swensen, Guillermo C. Bazan, and Alan J. Heeger

J. Appl. Phys. 98, 044502 (2005); http://dx.doi.org/10.1063/1.2006222 (8 pages) | Cited 24 times

Online Publication Date: 16 August 2005

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Planar light-emitting electrochemical cells (LECs) were prepared by drop casting the conjugated polyelectrolyte poly(9,9-bis[6′-(N,N,N,-trimethylammonium)hexyl]fluorene-co-alt-1,4-phenylene) bromide (PFN+Br−) onto substrates patterned with micrometer-sized interelectrode (Au) gaps. When operated at room temperature (RT), such LECs exhibit a gap-size-dependent turn-on voltage for light emission, but when operated at 140 °C, the turn-on voltage is essentially gap-size independent and close to the band gap of PFN+Br−. This temperature dependence is consistent with operation below and above the melting temperature (Tm = 130 °C) of PFN+Br− and is consequently a signature of anion mobility. The RT morphology of PFN+Br− is dependent on the film preparation process: slow evaporation of the solvent via drop casting produces a partially crystalline film, while a fast evaporation via spin casting produces a glassy film. The glassy spin-cast film can, however, be transformed into a partially crystalline film by slow cooling (0.5 °C/min) from an elevated temperature (180 °C) to RT. The photoluminescence (PL) from a partially crystalline film is slightly blueshifted and its vibronic structure better resolved than the PL from a glassy spin-cast film.
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82.47.Jk Photoelectrochemical cells, photoelectrochromic and other hybrid electrochemical energy storage devices
82.45.Wx Polymers and organic materials in electrochemistry
82.35.Rs Polyelectrolytes
82.45.Gj Electrolytes
72.40.+w Photoconduction and photovoltaic effects
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
78.55.Kz Solid organic materials
73.61.Ph Polymers; organic compounds
78.66.Qn Polymers; organic compounds
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.55.-a Thin film structure and morphology
81.20.-n Methods of materials synthesis and materials processing
81.30.Fb Solidification
71.20.Rv Polymers and organic compounds
64.70.F- Liquid-vapor transitions
64.70.D- Solid-liquid transitions
61.41.+e Polymers, elastomers, and plastics
63.50.-x Vibrational states in disordered systems
63.20.-e Phonons in crystal lattices
78.66.Jg Amorphous semiconductors; glasses

Electronic transport imaging in a multiwire SnO2 chemical field-effect transistor device

Sergei V. Kalinin, J. Shin, S. Jesse, D. Geohegan, A. P. Baddorf, Y. Lilach, M. Moskovits, and A. Kolmakov

J. Appl. Phys. 98, 044503 (2005); http://dx.doi.org/10.1063/1.2001144 (8 pages) | Cited 30 times

Online Publication Date: 17 August 2005

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The electronic transport and the sensing performance of an individual SnO2 crossed-nanowires device in a three-terminal field-effect transistor configuration were investigated using a combination of macroscopic transport measurements and scanning surface-potential microscopy (SSPM). The structure of the device was determined using both scanning electron- and atomic force microscopy data. The SSPM images of two crossed one-dimensional nanostructures, simulating a prototypical nanowire network sensors, exhibit large dc potential drops at the crossed-wire junction and at the contacts, identifying them as the primary electroactive elements in the circuit. The gas sensitivity of this device was comparable to those of sensors formed by individual homogeneous nanostructures of similar dimensions. Under ambient conditions, the dc transport measurements were found to be strongly affected by field-induced surface charges on the nanostructure and the gate oxide. These charges result in a memory effect in transport measurements and charge dynamics which are visualized by SSPM. Finally, scanning probe microscopy is used to measure the current-voltage characteristics of individual active circuit elements, paving the way to a detailed understanding of chemical functionality at the level of an individual electroactive element in an individual nanowire.
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85.35.-p Nanoelectronic devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.30.Tv Field effect devices

Time-dependent all-optical logic gates based on two coupled waves in bacteriorhodopsin film

Guiying Chen, Chunping Zhang, Zongxia Guo, Xinyu Wang, Jianguo Tian, and Q. W. Song

J. Appl. Phys. 98, 044504 (2005); http://dx.doi.org/10.1063/1.2006225 (5 pages) | Cited 4 times

Online Publication Date: 18 August 2005

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When two coherent green beams offset from each other by a small angle are coupled in bacteriorhodopsin film, the diffraction intensity rises from zero to a maximum, and then decreases along with the writing time. Based on the change of the diffraction intensity with the writing time, we proposed and demonstrated a time-dependent all-optical exclusive not or (XNOR) operation and a time-dependent all-optical logical inverter (NOT) operation. Based on the relation between the self-diffraction intensity and the polarization states of the input beams, the time-dependent all-optical XNOR logic gate was achieved, and based on the relation between the polarization states of the diffraction beams and that of the recording beams, the time-dependent all-optical NOT logic gate was obtained.
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42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
42.79.Wc Optical coatings
42.70.-a Optical materials

dc modulation in field-effect transistors operating under microwave irradiation for quantum readout

Giorgio Ferrari, Laura Fumagalli, Marco Sampietro, Enrico Prati, and Marco Fanciulli

J. Appl. Phys. 98, 044505 (2005); http://dx.doi.org/10.1063/1.2007852 (4 pages) | Cited 7 times

Online Publication Date: 19 August 2005

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With a view to using microwaves to excite the single-spin resonance of an electron trapped in a defect at the Si/SiO2 interface of a metal-oxide-semiconductor field-effect transistor (MOSFET), we report on the experimental evidence for a stationary current in such devices operated under microwave radiation. The stationary current is examined as a function of the microwave power and of the operating voltage of the MOSFET. The transistor behavior is reproduced by a model exploiting the nonlinearity of the MOSFET channel resistance as a component of the circuit coupled with the electromagnetic field. We conclude that, in operating a MOSFET under microwaves, one has to pay attention to the generation of spurious stationary currents that may alter the likelihood to observe spin-dependent phenomena in the random telegraph signal observed in a MOSFET.
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85.30.Tv Field effect devices
84.40.-x Radiowave and microwave (including millimeter wave) technology
85.75.Hh Spin polarized field effect transistors
03.67.Lx Quantum computation architectures and implementations

High peak luminance of molecularly dye-doped organic light-emitting diodes under intense voltage pulses

B. Wei, M. Ichikawa, K. Furukawa, T. Koyama, and Y. Taniguchi

J. Appl. Phys. 98, 044506 (2005); http://dx.doi.org/10.1063/1.2009081 (5 pages) | Cited 2 times

Online Publication Date: 19 August 2005

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The performance and efficiency of molecularly doped organic light-emitting devices (OLEDs) using voltage pulses have been investigated. The maximum current density and peak luminance have been found to depend on the pulse duration and device size, which was attributed to the heat effect in device. The self-quenching of dye molecules, especially at a high current density of ∼ A/cm2, can be effectively inhibited using a low concentration of acceptor in donor system as a light-emitting layer (EML). For an OLED using a 0.4-mol % rubrene in Alq3 as an EML, a maximum peak luminance of 5.66×106 cd/m2 at 52.8 V can be sustained under the voltage pulses of duration of 5 μs. The current efficiency has been observed to decrease rapidly with the current density in device, which was believed to result from the annihilation of excitons. We have also investigated the performance of a pulsed OLED with an emitter layer of 4.0-mol % 1,4-bis[2-[4-[N,N-di(p-tolyl)amino]phenyl]vinyl]benzene in 4,4′-bis(9-carbazolyl)biphenyl, which has been known to exhibit low-amplified spontaneous emission under a pulsed nitrogen laser.
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85.60.Jb Light-emitting devices

Property investigation of C+-ion-implanted LiNbO3 planar optical waveguides

Feng Chen, Hui Hu, Xue-Lin Wang, Fei Lu, and Ke-Ming Wang

J. Appl. Phys. 98, 044507 (2005); http://dx.doi.org/10.1063/1.2011782 (5 pages) | Cited 6 times

Online Publication Date: 19 August 2005

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The LiNbO3 planar optical waveguides are fabricated by 3.0-MeV C+ ion implantation at the doses of 1.5–10.5×10(14) cm−2. Both TE and TM modes are observed by prism coupling method (m lines). The reconstructed refractive index profiles of the guiding region show that extraordinary index has an enhancement compared with the substrate of LiNbO3, while ordinary index experiences a typical “barrier” change. The modal analysis of the waveguides shows a good confinement of light in fundamental TM mode. The tendency of effective refractive index (decrease after the annealing for TE and increase after annealing for TM) suggests a moderate recovery of the lattice.
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42.79.Gn Optical waveguides and couplers
42.82.Et Waveguides, couplers, and arrays
61.72.up Other materials
42.82.Cr Fabrication techniques; lithography, pattern transfer

Synthesis for semiconductor device design

Jason Thalken, Stephan Haas, and A. F. J. Levi

J. Appl. Phys. 98, 044508 (2005); http://dx.doi.org/10.1063/1.2014942 (8 pages) | Cited 1 time

Online Publication Date: 22 August 2005

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Synthesis of semiconductor device design requires access to realistic physical models and adaptive algorithms. To demonstrate that such synthesis is feasible we design elements of a quantum-confined Stark-effect modulator. Optimization with respect to a target function is achieved using a genetic algorithm. It is then shown how automated searches of configuration space may be performed without the need to input a specific target function.
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85.30.De Semiconductor-device characterization, design, and modeling
02.60.Pn Numerical optimization

Three-dimensional simulation on current-density distribution in flip-chip solder joints under electric current stressing

T. L. Shao, S. W. Liang, T. C. Lin, and Chih Chen

J. Appl. Phys. 98, 044509 (2005); http://dx.doi.org/10.1063/1.2000667 (8 pages) | Cited 19 times

Online Publication Date: 23 August 2005

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Three-dimensional simulations on current-density distribution in solder joints under electric current stressing were carried out by finite element method. Five underbump metallization (UBM) structures were simulated, including Ti/Cr–Cu/Cu thin-film UBM, Al/Ni(V)/Cu thin-film UBM, Cu thick-film UBM, Ni thick-film UBM, and Cu/Ni thick-film UBM. The maximum current density inside the solder occurs in the vicinity of the entrance of the Al trace into the solder joint, while there is no obvious current crowding effect in the substrate side of the joint. The crowding ratio, which is defined as the maximum current density inside the solder divided by the average value in the UBM opening, is as high as 24.7 for the solder with the Ti/Cr–Cu/Cu UBM. However, it decreases to 23.4, 13.5, 8.7, and 7.2 for the rest of the UBM structures, respectively. Solder joints with thick UBMs were found to have a better ability to relieve the current crowding effect. The simulation results are in reasonable agreement with limited published data. The solder joints with higher current crowding ratios have a shorter electromigration failure time.
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85.40.Ls Metallization, contacts, interconnects; device isolation

Correlation between the performance of double-barrier quantum-well infrared photodetectors and their microstructure: On the origin of the photovoltaic effect

E. Luna, A. Trampert, A. Guzmán, and E. Calleja

J. Appl. Phys. 98, 044510 (2005); http://dx.doi.org/10.1063/1.2006990 (4 pages)

Online Publication Date: 24 August 2005

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In this work we show clear evidences that silicon segregation out of quantum wells (QWs) is the mechanism responsible for the unexpected photovoltaic (PV) effect exhibited by AlGaAs/AlAs/GaAs double-barrier quantum-well infrared photodetectors. Our results are based on the combined analysis of the detectors’ microstructure [obtained by transmission electron microscopy (TEM)] and their electro-optical characteristics (dark current and responsivity versus bias). A TEM image intensity analysis yields the result of an unintentional asymmetry between the two AlAs barriers adjacent to the QW attributed to the presence of segregated Si at the interface. Moreover, we find that the higher this compositional asymmetry, the higher the asymmetries in the electro-optical response of the detector. Additionally we show here direct evidences of how the growth-induced nonequivalence of the AlAs tunnel barriers can be ruled out as the origin of the PV effect.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Metastable defect migration under high carrier injection in hydrogenated amorphous silicon p-i-n solar cells

U. Dutta, P. Chatterjee, S. Tchakarov, M. Uszpolewicz, and P. Roca i Cabarrocas

J. Appl. Phys. 98, 044511 (2005); http://dx.doi.org/10.1063/1.2010623 (8 pages)

Online Publication Date: 25 August 2005

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The evolution of excess defects in hydrogenated amorphous silicon p-i-n solar cells, induced by a forward current in the dark, has been studied by modeling their measured dark and illuminated current-voltage and quantum efficiency characteristics at different stages of degradation. Our electrical-optical model is based on the solution of Poisson’s and continuity equations. Modeling reveals that metastable defects are mainly produced in regions where tail-to-tail recombination of injected electrons and holes is high. These regions are characterized by either high defect density or low electric field. Simulation of experimental characteristics after 1 h of current injection indicates that the spatial generation of current-induced defects is highly nonuniform, with the main defect formation occurring near the p/i interface, and to a lesser extent towards the n/i interface. Few defects are generated over the bulk intrinsic layer. Modeling of the characteristics after a longer duration of current injection indicates a broadening of the current-induced defect zone from the interfaces to the bulk intrinsic layer. After prolonged current injection, the density of excess dangling-bond defects in the bulk intrinsic layer increases significantly, while the defect density near the p/i interface actually decreases, resulting in a more uniform distribution of excess metastable defects. Evidence from modeling suggests that some metastable defects have migrated from the interfaces towards the bulk. We thus conclude that prolonged current injection not only produces excess metastable defects, but also causes these defects to migrate to regions of lower defect density.
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84.60.Jt Photoelectric conversion
71.55.Cn Elemental semiconductors
78.20.Jq Electro-optical effects
85.30.De Semiconductor-device characterization, design, and modeling
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