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1 Dec 1999

Volume 86, Issue 11, pp. 5927-6612

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Light-emitting devices based on a poly(p-phenylene vinylene) derivative with ion-coordinating side groups

J. Morgado, R. H. Friend, F. Cacialli, B. S. Chuah, S. C. Moratti, and A. B. Holmes

J. Appl. Phys. 86, 6392 (1999); http://dx.doi.org/10.1063/1.371702 (4 pages) | Cited 29 times

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We report the phase-separation and the optoelectronic properties of an alternating poly(p-phenylene vinylene)-based copolymer, to which poly(ethylene oxide), PEO, and or lithium triflate are added. The alternating units of this copolymer (DB-BTEM-PPV) are 2,3-dibutoxy-1,4-phenylene vinylene, and 2,5-bis(triethoxymethoxy)-1,4-phenylene vinylene, a moiety containing side groups allowing ion solvation and transport. Upon addition of the ion-transporting polymer PEO to DB-BTEM-PPV blended with lithium triflate, we have found a sizeable efficiency increase (from 0.9 to 1.5 cd/A), with a concomitant increase of the response time. We propose that this is due to solvation and complexation of lithium triflate by the PEO, which simultaneously reduces the quenching of photoluminescence (and electroluminescence) efficiency by the ionic charge, and the effectiveness of formation of highly doped, low-barrier, polymer/electrode interfaces. We discuss charge transport and injection in the copolymer and in the blend with reference to the diodes characteristics, and to the phase separation of PEO, which we investigated with atomic force microscopy. © 1999 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.55.Kz Solid organic materials
64.75.-g Phase equilibria
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials

Vertical and lateral forces between a permanent magnet and a high-temperature superconductor

John R. Hull and Ahmet Cansiz

J. Appl. Phys. 86, 6396 (1999); http://dx.doi.org/10.1063/1.371703 (9 pages) | Cited 92 times

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The vertical and horizontal forces and associated stiffnesses on a permanent magnet (PM) above a high-temperature superconductor (HTS) were measured during vertical and horizontal traverses in zero-field cooling (ZFC) and in field cooling (FC). In ZFC, the vertical stiffness was greater in the first descent than in the first ascent and second descent, and the stiffness in the second descent was between those of the first descent and the first ascent. At the FC position, the vertical stiffness was two times greater than the lateral stiffness at each height, to within 1% of the vertical stiffness value. The cross stiffness of vertical force with respect to lateral position was positive for FC, but negative for ZFC. Free-spin-down experiments of a PM levitated above a HTS were also performed. These results showed that the coefficient of friction is double valued at frequencies just below the rotor resonance, a result attributed to cross stiffness in the PM/HTS interaction. A frozen-image model was used to calculate the vertical and horizontal forces and stiffnesses, and reasonable agreement with the data occurred for vertical or horizontal movements of the PM less than several mm from the FC position. © 1999 American Institute of Physics.
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75.50.Ww Permanent magnets
75.80.+q Magnetomechanical effects, magnetostriction
85.70.Rp Magnetic levitation, propulsion and control devices

Return current in hysteretic Josephson junctions: Experimental distribution in the thermal activation regime

M. G. Castellano, G. Torrioli, F. Chiarello, C. Cosmelli, and P. Carelli

J. Appl. Phys. 86, 6405 (1999); http://dx.doi.org/10.1063/1.371704 (7 pages) | Cited 8 times

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We present an experimental study on the retrapping process of a hysteretic, high-quality Josephson junction; namely, we have measured the distribution of the values at which the junction switches back from the voltage state to the zero-voltage state, as a function of the applied magnetic field. While the opposite process (escape from the zero-voltage state) has been extensively studied in the past, both from the theoretical and the experimental point of view, little is found in the literature on the retrapping process. In terms of the tilted washboard potential, the process corresponds to the retrapping from the running state to a locked state in a potential well. The interest of the measurements is in the fact that the value of the return current can be directly related to the dissipation in the junction. While the deterministic behavior, experimentally measured through the I–V curve, appears to be in agreement with the theoretical predictions, even in minor details, the statistical behavior is strongly different from what is expected. The disagreement is found even in zero-applied magnetic field and it cannot be attributed to external noise in the system. From the experimental statistical properties, we find values for the effective dissipation much lower than those obtained from the deterministic curves, a result which could be of interest in experiments on the observation of macroscopic quantum phenomena. © 1999 American Institute of Physics.
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74.50.+r Tunneling phenomena; Josephson effects
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)

Analysis of torque in nested magnetic cylinders

T. R. Ní Mhíocháin, D. Weaire, S. M. McMurry, and J. M. D. Coey

J. Appl. Phys. 86, 6412 (1999); http://dx.doi.org/10.1063/1.371705 (13 pages) | Cited 9 times

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Rotatable nested cylindrical magnets may be used to generate variable magnetic fields. A mutual torque exists between the cylinders which is an important consideration in practical designs. We present a thorough investigation of this torque, combining experiment, computation, and theory. In the finite length case, in which the magnetization varies according to the prescription of Halbach, the torque is an end effect. It is proportional to sin α, where α is the angle defining the relative orientation of the cylinders. Practical designs use a discrete number N of permanent magnet segments to approximate the continuously varying magnetization of the ideal design. These introduce higher components sin(nα) to the angular dependence of torque, where the allowed values of n are restricted to n = (kN±1) according to the segmented magnet torque theorem. © 1999 American Institute of Physics.
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85.70.Ay Magnetic device characterization, design, and modeling
07.55.Db Generation of magnetic fields; magnets
75.50.Ww Permanent magnets
41.20.Gz Magnetostatics; magnetic shielding, magnetic induction, boundary-value problems

Strain optimization for high differential gain and low current operation in 1.55 μm InGaAs/InGaAsP quantum well lasers

O. Gilard, F. Lozes-Dupuy, G. Vassilieff, S. Bonnefont, P. Arguel, J. Barrau, and P. Le Jeune

J. Appl. Phys. 86, 6425 (1999); http://dx.doi.org/10.1063/1.371706 (6 pages) | Cited 6 times

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The effect of strain on threshold current and differential gain in 1.55 μm InGaAs/InGaAsP quantum well lasers is reviewed. A constant decrease in the threshold current with increasing stress is predicted if a conventional model of Auger recombination is used. We propose a more realistic model based on an accurate method for the derivation of the Auger recombination rate in quantum wells. Unlike the conventional theory, a realistic valence band structure along with Fermi–Dirac statistics and analytic expressions of the transition matrix element for both bound–bound and bound-unbound Auger processes are employed. Contrary to conventional modeling, computation results show an optimal compressive strain of approximately 1% that minimizes the threshold current and maximizes the differential gain. These results are in good agreement with the experimental results reported in the literature. © 1999 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
79.20.Fv Electron impact: Auger emission
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Current transport and the role of barrier inhomogeneities at the high barrier n-InP ∣ poly(pyrrole) interface

Frank E. Jones, Ben P. Wood, James A. Myers, Carrie Daniels-Hafer, and Mark C. Lonergan

J. Appl. Phys. 86, 6431 (1999); http://dx.doi.org/10.1063/1.371707 (11 pages) | Cited 55 times

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A detailed study of current transport at the Schottky-type n-InP ∣ poly(pyrrole) interface is presented. At room temperature, this interface exhibits an average quality factor of n = 1.02±0.02, a C–V barrier height of qϕbCV = 0.78±0.01 eV, and a surface recombination velocity over two orders-of-magnitude slower than at ideal n-InP metal interfaces. These latter two parameters imply an effective barrier height of 0.9 eV, which is among the highest values ever reported for an n-InP Schottky-type diode. The quality factor increases monotonically with decreasing temperature reaching a value of 1.23 at 98 K. Substantial curvature is also observed in a Richardson plot at reduced temperature. These temperature dependencies can be quantitatively modeled using thermionic emission theory in the presence of barrier inhomogeneities. Standard models, including thermionic emission with image force effects, interfacial layer models with and without surface states, and tunneling, do not adequately explain the temperature dependence of the quality factor and the curvature in the Richardson plot. © 1999 American Institute of Physics.
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73.61.Ey III-V semiconductors
73.61.Ph Polymers; organic compounds
73.40.Ns Metal-nonmetal contacts
68.35.Ct Interface structure and roughness
73.25.+i Surface conductivity and carrier phenomena
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Analytical model of transient photoresponse of quantum well infrared photodetectors

M. Ershov, S. Satou, and Y. Ikebe

J. Appl. Phys. 86, 6442 (1999); http://dx.doi.org/10.1063/1.371716 (9 pages) | Cited 8 times

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This article presents a theory of the transient photoresponse of quantum well infrared photodetectors (QWIPs). As has been shown recently [M. Ershov, Appl. Phys. Lett. 69, 3480 (1996)], the photocurrent in a QWIP in response to a step-like infrared illumination is composed of fast and slow transients. Analytical models for both fast and slow transients in the linear regime are proposed. The amplitudes and time constants of the transients are expressed in terms of the basic QWIP characteristics — capture time to the QWs, photocurrent gain, and differential conductivity of the emitter contact. Closed-form analytical expressions for the transient photocurrent in the time and frequency domains are obtained. The validity of the proposed analytical model is confirmed by comparison with numerical simulations. © 1999 American Institute of Physics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
85.30.De Semiconductor-device characterization, design, and modeling
02.30.-f Function theory, analysis

Electrical, optical, and structural properties of indium–tin–oxide thin films for organic light-emitting devices

H. Kim, C. M. Gilmore, A. Piqué, J. S. Horwitz, H. Mattoussi, H. Murata, Z. H. Kafafi, and D. B. Chrisey

J. Appl. Phys. 86, 6451 (1999); http://dx.doi.org/10.1063/1.371708 (11 pages) | Cited 214 times

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High-quality indium–tin–oxide (ITO) thin films (200–850 nm) have been grown by pulsed laser deposition (PLD) on glass substrates without a postdeposition annealing treatment. The structural, electrical, and optical properties of these films have been investigated as a function of target composition, substrate deposition temperature, background gas pressure, and film thickness. Films were deposited from various target compositions ranging from 0 to 15 wt % of SnO2 content. The optimum target composition for high conductivity was 5 wt % SnO2+95 wt % In2O3. Films were deposited at substrate temperatures ranging from room temperature to 300 °C in O2 partial pressures ranging from 1 to 100 mTorr. Films were deposited using a KrF excimer laser (248 nm, 30 ns full width at half maximum) at a fluence of 2 J/cm2. For a 150-nm-thick ITO film grown at room temperature in an oxygen pressure of 10 mTorr, the resistivity was 4×10−4 Ω cm and the average transmission in the visible range (400–700 nm) was 85%. For a 170-nm-thick ITO film deposited at 300 °C in 10 mTorr of oxygen, the resistivity was 2×10−4 Ω cm and the average transmission in the visible range was 92%. The Hall mobility and carrier density for a 150-nm-thick film deposited at 300 °C were 27 cm2/V s and 1.4×1021 cm−3, respectively. A reduction in the refractive index for ITO films can be achieved by raising the electron density in the films, which can be obtained by increasing the concentration of Sn dopants in the targets and/or increasing deposition temperature. Atomic force microscopy measurements of these ITO films indicated that their root-mean-square surface roughness (∼5 Å) was superior to that of commercially available sputter deposited ITO films (∼40 Å). The PLD ITO films were used to fabricate organic light-emitting diodes. From these structures the electroluminescence was measured and an external quantum efficiency of 1.5% was calculated. © 1999 American Institute of Physics.
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81.05.Hd Other semiconductors
81.15.Fg Pulsed laser ablation deposition
68.55.-a Thin film structure and morphology
73.61.Le Other inorganic semiconductors
78.66.Li Other semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
68.35.B- Structure of clean surfaces (and surface reconstruction)
85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.40.Fy Semiconductors

Electrical properties of thin SiON/Ta2O5 gate dielectric stacks

M. Houssa, R. Degraeve, P. W. Mertens, M. M. Heyns, J. S. Jeon, A. Halliyal, and B. Ogle

J. Appl. Phys. 86, 6462 (1999); http://dx.doi.org/10.1063/1.371709 (6 pages) | Cited 25 times

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The electrical characteristics of metal–oxide–semiconductor capacitors with SiON/Ta2O5 gate dielectric stacks with thin Ta2O5 layers (6–10 nm) are investigated. From the field and temperature dependence of the current of the gate stacks, it is shown that the main conduction mechanism at low bias is tunneling through the stack and that Poole–Frenkel conduction in the Ta2O5 layer becomes important at larger bias and temperature. From the analysis of the data in the high voltage and temperature range, taking into account the field distribution in both layers, the refractive index n of Ta2O5 and the energy level ϕB of traps involved in Poole–Frenkel conduction are found to be 2.3 and 0.85 eV, respectively. It is also shown that the gate current density of the stack is reduced by one to three orders of magnitude as compared to SiO2 layers with equivalent electrical thickness (2.5–3 nm). The temperature acceleration effect on the time-dependent dielectric breakdown is shown to be much reduced in the SiON/Ta2O5 stack as compared to SiO2 layers with equivalent electrical thickness. © 1999 American Institute of Physics.
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77.55.-g Dielectric thin films
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
84.32.Tt Capacitors
72.20.Ht High-field and nonlinear effects
73.50.Fq High-field and nonlinear effects
73.40.Gk Tunneling
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
77.22.Jp Dielectric breakdown and space-charge effects
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

A comprehensive study of AlGaAs/GaAs beryllium- and carbon-doped base heterojunction bipolar transistor structures subjected to rapid thermal processing

Hong Wang, Geok Ing Ng, Haiqun Zheng, and Penghua Zhang

J. Appl. Phys. 86, 6468 (1999); http://dx.doi.org/10.1063/1.371710 (6 pages) | Cited 3 times

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AlGaAs/GaAs single heterojunction bipolar transistor (HBT) structures with Be- and C-doped bases have been annealed at different temperatures using rapid thermal processing (RTP). Both electrical and low-temperature photoluminescence measurements were used to investigate their thermal stability. We found that the conventional AlGaAs/GaAs abrupt HBT structures could undergo significant degradation at temperatures commonly encountered in typical RTP for device fabrication. The decrease of current gain was observed in both molecular beam epitaxy-grown HBTs with a Be-doped base and metalorganic chemical vapor deposition-grown HBTs with a C-doped base after RTP at temperatures greater than 600 °C. Our studies show that high-temperature RTP could induce undesirable degradation in AlGaAs/GaAs HBTs. Different degradation mechanisms, which are similar to those for the degradation of the Be- and C-doped base HBTs under current-induced stress, are responsible for the degradation of the Be- and C-doped HBTs subjected to RTP. The degradation of Be-doped HBTs is believed to be due to the outdiffusion of Be from the highly doped base, whereas the decrease of current gain for C-doped HBTs is closely related to the unintentionally incorporated hydrogen during material growth. © 1999 American Institute of Physics.
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85.30.Pq Bipolar transistors
78.55.Cr III-V semiconductors
61.72.Cc Kinetics of defect formation and annealing
61.72.uj III-V and II-VI semiconductors
85.40.Ry Impurity doping, diffusion and ion implantation technology

Light emitting porous silicon diode based on a silicon/porous silicon heterojunction

L. Pavesi, R. Chierchia, P. Bellutti, A. Lui, F. Fuso, M. Labardi, L. Pardi, F. Sbrana, M. Allegrini, S. Trusso, C. Vasi, P. J. Ventura, L. C. Costa, M. C. Carmo, and O. Bisi

J. Appl. Phys. 86, 6474 (1999); http://dx.doi.org/10.1063/1.371711 (9 pages) | Cited 6 times

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A new structure is proposed to improve the external quantum efficiency of porous silicon (PS) light emitting diodes (LED). It is based on a heterojunction between n-type doped silicon and PS. The heterojunction is formed due to the doping selectivity of the etching process used to form PS. The improvement of the proposed LED structure with respect to usual metal/PS LED is demonstrated. This is thought to be due to a different injection mechanism for which carriers are injected directly into conduction band states. Anodic oxidation experiments show further improvements in the LED efficiency.© 1999 American Institute of Physics.
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85.60.Jb Light-emitting devices
81.65.Cf Surface cleaning, etching, patterning
81.05.Cy Elemental semiconductors
81.05.Rm Porous materials; granular materials
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
61.72.uf Ge and Si
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