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12 Apr 2004

Volume 84, Issue 15, pp. 2721-2955

Issue Cover Spotlight Figure

Appl. Phys. Lett. 84, 2883 (2004); http://dx.doi.org/10.1063/1.1702137 (3 pages)

Pu Xian Gao and Zhong L. Wang
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Current transport mechanisms and their effects on the performances of InP-based double heterojunction bipolar transistors with different base structures

Zhi Jin, W. Prost, S. Neumann, and F. J. Tegude

Appl. Phys. Lett. 84, 2910 (2004); http://dx.doi.org/10.1063/1.1695638 (3 pages) | Cited 7 times

Online Publication Date: 7 April 2004

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InP-based double heterojunction bipolar transistors (DHBTs) with different base structures were studied. The base structures are InGaAs with and without graded composition and GaAsSb. Both of the terminal currents of InP/GaAsSb/InP DHBT in forward and reverse modes are limited by the carrier transport across the base layer. This causes the offset voltage to be determined by the difference between the base-collector and base-emitter areas and by the normal common-base current gain. The emitter currents of both graded- and abrupt-base InP/InGaAs/InP DHBTs in the reverse mode are also limited by the carrier transport across the base layer, while their collector currents are limited by the band discontinuity of the base–emitter junction. The different current transport mechanisms of the terminal currents in the forward and reverse modes result in the larger offset voltage. © 2004 American Institute of Physics.
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85.30.Pq Bipolar transistors
73.61.Ey III-V semiconductors

Dual role of LiF as a hole-injection buffer in organic light-emitting diodes

J. M. Zhao, S. T. Zhang, X. J. Wang, Y. Q. Zhan, X. Z. Wang, G. Y. Zhong, Z. J. Wang, X. M. Ding, W. Huang, and X. Y. Hou

Appl. Phys. Lett. 84, 2913 (2004); http://dx.doi.org/10.1063/1.1695444 (3 pages) | Cited 33 times

Online Publication Date: 7 April 2004

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It is demonstrated experimentally that the effect of a LiF buffer layer inserted at the ITON,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB) interface on the hole injection is greatly dependent on the initial barrier height (IBH) existing at the interface. Only for a large IBH, will the introduction of the LiF show improvement effect. For small one, it will weaken the hole injection. These phenomena are explained in terms of tunneling model and calculations based on this model show a good agreement with the experimental results. This further confirms that the energy level realignment and the change in carrier tunneling probability are mainly responsible for the variation of current injection induced by the insulating buffers in organic light-emitting diodes. © 2004 American Institute of Physics.
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85.60.Jb Light-emitting devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Electron blocking and hole injection: The role of N,N′-Bis(naphthalen-1-y)-N,N′-bis(phenyl)benzidine in organic light-emitting devices

S. T. Zhang, Z. J. Wang, J. M. Zhao, Y. Q. Zhan, Y. Wu, Y. C. Zhou, X. M. Ding, and X. Y. Hou

Appl. Phys. Lett. 84, 2916 (2004); http://dx.doi.org/10.1063/1.1699472 (3 pages) | Cited 26 times

Online Publication Date: 7 April 2004

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The current density-luminance-voltage characteristics of organic light-emitting devices (OLEDs) with N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB) of various thicknesses as the hole transport layer have been investigated. It is found that for conventional structures of indium–tin–oxide/NPB/tris(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/LiF (0.5 nm)/Al the optimal hole injection and luminescence efficiencies appear at NPB thicknesses of 5 and 20 nm, respectively. The large difference between the two optimal thicknesses suggests that the effective block of the NPB layer against electrons from across the Alq3/NPB interface is essential for high-efficiency operation of the OLEDs. The electron blocking effect of NPB is further confirmed by the electroluminescence (EL) behavior of devices with the structure of ITO/NPB(5 nm)/Alq3:4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) (30 nm)/NPB/Alq3(60 nm)/LiF(0.5 nm)/Al. The proportion of DCM EL to the whole EL decreases with increasing NPB thickness. This suggests that the NPB layer blocks electron transport to the Alq3:DCM layer. The Förster energy transfer from the 60 nm Alq3 layer to the DCM molecules is ruled out by the EL behavior observed after quenching excitons in the Alq3 layer. The origin of the difference in the optimal N,N′-Bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) thicknesses reported by other two different groups is also discussed. © 2004 American Institute of Physics.
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85.60.Jb Light-emitting devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.Fr Low-field transport and mobility; piezoresistance
78.60.Fi Electroluminescence
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

MgO/p-GaN enhancement mode metal-oxide semiconductor field-effect transistors

Y. Irokawa, Y. Nakano, M. Ishiko, T. Kachi, J. Kim, F. Ren, B. P. Gila, A. H. Onstine, C. R. Abernathy, S. J. Pearton, C.-C. Pan, G.-T. Chen, and J.-I. Chyi

Appl. Phys. Lett. 84, 2919 (2004); http://dx.doi.org/10.1063/1.1704876 (3 pages) | Cited 40 times

Online Publication Date: 7 April 2004

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We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 1.75 MV cm−1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 μS mm−1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs. © 2004 American Institute of Physics.
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85.30.Tv Field effect devices
61.72.uj III-V and II-VI semiconductors
73.61.Ey III-V semiconductors

Dual insulated-gate field-effect transistors with cadmium sulfide active layer and a laminated polymer dielectric

J. S. Meth, S. G. Zane, and G. Nunes

Appl. Phys. Lett. 84, 2922 (2004); http://dx.doi.org/10.1063/1.1704875 (3 pages) | Cited 2 times

Online Publication Date: 7 April 2004

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We report the fabrication of dual insulated gate thin-film transistors with chemical-bath deposited cadmium sulfide active layers. The cadmium sulfide was deposited from solution onto thermally oxidized silicon wafers to form the first semiconductor–dielectric interface. The terpolymer poly(tetrafluoroethylene-co-vinylidenefluoride-co-propylene) was laminated onto the semiconductor to create the second semiconductor–dielectric interface. This device geometry allows direct comparison of the behavior of the accumulated charge at these two very different interfaces. The mobility values for these devices are in the 0.1–1 cm2/Vs range, while the on/off ratios vary from 102 to 105. The ability to laminate a dielectric to a semiconductor enables new processing routes for large area transistor arrays. © 2004 American Institute of Physics.
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85.30.Tv Field effect devices
81.05.Dz II-VI semiconductors
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

Trilayer hybrid polymer-quantum dot light-emitting diodes

Sumit Chaudhary, Mihrimah Ozkan, and Warren C. W. Chan

Appl. Phys. Lett. 84, 2925 (2004); http://dx.doi.org/10.1063/1.1699476 (3 pages) | Cited 45 times

Online Publication Date: 7 April 2004

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We report a trilayer hybrid polymer-quantum-dot light-emitting diode fabricated by sandwiching a CdSe-ZnS core–shell quantum-dot (QD) layer, a few monolayers thick, between films of polyvinylcarbazole (PVK) and an oxadiazole derivative (butyl-PBD). All the layers have been deposited by a controlled spin-coating technique. Stable aqueous dispersion of QDs has been prepared to make possible the spin coating of multiple layers without affecting the layer underneath. Our device shows 20 times the quantum efficiency (0.2%) and less than half the threshold voltage (5 V) of a single-layer device made from the PVK-QD-PBD blend. This is attributed to balanced carrier conduction, enhanced recombination, and reduced quenching of emission due to a better electro-optical device design. © 2004 American Institute of Physics.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
81.07.Ta Quantum dots
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
78.55.Kz Solid organic materials
78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds
78.67.Hc Quantum dots
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

Influence of a carrier supply layer on carrier density and drift mobility of AlGaN/GaN/SiC high-electron-mobility transistors

M. Marso, J. Bernát, P. Javorka, and P. Kordoš

Appl. Phys. Lett. 84, 2928 (2004); http://dx.doi.org/10.1063/1.1704854 (3 pages) | Cited 3 times

Online Publication Date: 7 April 2004

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Sheet carrier concentration and low-field drift mobility of intentionally undoped and modulation-doped AlGaN/GaN heterostructures on SiC substrate were evaluated by capacitance-voltage and channel conductivity measurements. Sheet carrier concentration and average mobility at 0 V gate bias correspond to standard Hall results. Sheet carrier density increases from 6.8×1012 cm−2 for the undoped sample up to 1×1013 cm−2 for the device with the highest doping concentration, while the mobility decreases from 1800 to 1620 cm2/V s. The local mobility, on the other hand, depends only on the actual sheet carrier density and is not influenced by the doping concentration of the carrier supply layer. It reaches a maximum value of 2100 cm2/V s at a carrier density of 3×1012 cm−2. © 2004 American Institute of Physics.
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85.30.Tv Field effect devices
81.05.Ea III-V semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
61.72.S- Impurities in crystals

Warm-white-light emitting diode utilizing a single-phase full-color Ba3MgSi2O8:Eu2+, Mn2+ phosphor

J. S. Kim, P. E. Jeon, J. C. Choi, H. L. Park, S. I. Mho, and G. C. Kim

Appl. Phys. Lett. 84, 2931 (2004); http://dx.doi.org/10.1063/1.1695441 (3 pages) | Cited 75 times

Online Publication Date: 7 April 2004

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The Ba3MgSi2O8:Eu2+, Mn2+ shows three emission colors: 442, 505, and 620 nm. The 442 and 505 nm emission originate from Eu2+ ions, while the 620 nm emission originates from Mn2+ ions. The excitation bands of three emission colors are positioned around 375 nm. Electron paramagnetic resonance measurement demonstrates that Eu2+ ions are occupied with three different Ba2+ sites. The red emission of Mn2+ ions has a long decay time of 750 ms due to persistent energy transfer from oxygen vacancies to Mn2+ ions, while the blue and green bands of Eu2+ ions have decay times of 0.32 and 0.64 μs, respectively. The fabricated white-light emitting diode using a 400-nm-emissive chip with a white-light emitting Ba3MgSi2O8:Eu2+, Mn2+ phosphor shows warm white light and higher color stability against input power in comparison with a commercial GaN-pumped (Y1−xGdx)3(Al1−yGay)5O12:Ce3+ phosphor. © 2004 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.55.Hx Other solid inorganic materials
76.30.Kg Rare-earth ions and impurities
61.72.J- Point defects and defect clusters

InP/InGaAs heterojunction bipolar transistors with low-resistance contact on heavily doped InP emitter layer

Moonjung Kim, Choul-Young Kim, and Young-Se Kwon

Appl. Phys. Lett. 84, 2934 (2004); http://dx.doi.org/10.1063/1.1713053 (3 pages)

Online Publication Date: 7 April 2004

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InP/InGaAs heterojunction bipolar transistors (HBTs) with low-resistance Ti/Pt/Au contact directly on heavily doped InP emitter layer have been demonstrated. A specific contact resistance of Ti/Pt/Au to n-type InP with doping concentration of 2×1019 cm−3 was investigated with the tunneling model theoretically and transmission line model (TLM) experimentally, revealing that it depends greatly on the doping level. TLM measurements exhibited a low specific contact resistance of 3.5×10−7 Ω cm2, which can be applicable to the fabrication of HBTs. InP/InGaAs HBTs with n+-InP emitter layer have been demonstrated with excellent dc characteristics, including a low offset voltage of 0.12 V, a knee voltage of 0.5 V, and a current gain of 28. These results verify that the heavily doped InP emitter layer allows a low-resistance contact. © 2004 American Institute of Physics.
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85.30.Pq Bipolar transistors
73.40.Cg Contact resistance, contact potential
61.72.S- Impurities in crystals
61.72.uj III-V and II-VI semiconductors

Current density mapping and pinhole imaging in magnetic tunnel junctions via scanning magnetic microscopy

B. D. Schrag, Xiaoyong Liu, Weifeng Shen, and Gang Xiao

Appl. Phys. Lett. 84, 2937 (2004); http://dx.doi.org/10.1063/1.1695194 (3 pages) | Cited 2 times

Online Publication Date: 7 April 2004

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We have applied a magnetoresistive microscopy technique to the imaging of current densities and pinhole formation in magnetic tunnel junction devices. In this work, we demonstrate how the magnetic field distribution at the surface of the device can be used to understand the flow of current within the junction itself. By imaging the current-induced fields before and after pinhole formation in several different junctions, we find that many junctions exhibit an unexpectedly complicated current distribution after high-voltage-induced breakdown. Further, we have seen that pinhole locations can be correlated with current inhomogeneities observed before junction breakdown. Finally, we present the results of finite-element simulations which are in good agreement with experimental results. © 2004 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
72.15.Gd Galvanomagnetic and other magnetotransport effects
77.22.Jp Dielectric breakdown and space-charge effects
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