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24 Dec 2001

Volume 79, Issue 26, pp. 4271-4458

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Excitation mechanisms in dye-doped organic light-emitting devices

F. Pschenitzka and J. C. Sturm

Appl. Phys. Lett. 79, 4354 (2001); http://dx.doi.org/10.1063/1.1427746 (3 pages) | Cited 25 times

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In an organic light-emitting device with a polymeric matrix concurrently doped with two different dyes, the photoluminescence (PL) and electroluminescence (EL) spectra are observed to be very different, with both dyes emitting in PL and only one in EL at room temperature. A simple model based on charge trapping and thermal excitation is introduced to explain this observation. The EL spectral change of a device at 77 K is consistent with this model. In addition, using the same model, the strong dependence of the EL efficiency (but not PL) on the concentration of a single dye in an organic film can be understood. The materials used in this experiment are the polymer poly(9-vinylcarbazole) combined with electron transport molecules, and the dyes coumarin 47, coumarin 6, and Nile red. © 2001 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.55.Kz Solid organic materials
42.70.Jk Polymers and organics

Boron carbide/n-silicon carbide heterojunction diodes

S. Adenwalla, P. Welsch, A. Harken, J. I. Brand, A. Sezer, and B. W. Robertson

Appl. Phys. Lett. 79, 4357 (2001); http://dx.doi.org/10.1063/1.1426257 (3 pages) | Cited 21 times

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The fabrication, initial structural characterization, and diode measurements are reported for a boron carbide/silicon carbide heterojunction diode. Current–voltage curves are obtained for operation at temperatures from 24 to 351 °C. Plasma-enhanced chemical-vapor deposition (PECVD) -deposited undoped boron carbide material is highly crystalline and consists of a variety of polytypes of boron carbide (BC) with crystal sizes as large as 110 nm. Crystal phases are similar to those for PECVD BC on Si but only partially match known boron and boron-rich BC phases. © 2001 American Institute of Physics.
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85.30.Kk Junction diodes
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
85.30.De Semiconductor-device characterization, design, and modeling
68.35.Ct Interface structure and roughness

Stacking fault band structure in 4H–SiC and its impact on electronic devices

M. S. Miao, Sukit Limpijumnong, and Walter R. L. Lambrecht

Appl. Phys. Lett. 79, 4360 (2001); http://dx.doi.org/10.1063/1.1427749 (3 pages) | Cited 62 times

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First principles calculations of the stacking fault (SF) in 4H–SiC indicate the occurrence of an interface band in the gap with maximum depth of 0.2–0.3 eV below the conduction band minimum at the math point. The energy of formation of SFs in 3C–, 4H–, and 6H–SiC on the other hand is found to be of order a few meV/pair. Thus, there is a thermodynamic driving force promoting growth of SF area in an n-type sample. Radiationless recombination of electrons trapped at the SF with holes is proposed to provide sufficient energy to overcome the partial dislocation motion barriers towards formation of additional SF area in a device under forward bias. © 2001 American Institute of Physics.
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61.72.Nn Stacking faults and other planar or extended defects
71.20.Nr Semiconductor compounds
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Time domain measurement of spin-dependent recombination

Christoph Boehme and Klaus Lips

Appl. Phys. Lett. 79, 4363 (2001); http://dx.doi.org/10.1063/1.1428623 (3 pages) | Cited 10 times

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A defect characterization method is presented, the time domain measurement of spin-dependent recombination (TSR). Recombination between paramagnetic states is changed rapidly by electron spin resonant excitation through strong nanosecond microwave pulses. After the pulse, a slow relaxation of the recombination rate towards its steady state takes place. By measuring the current transient after the resonant pulse, information about dissociation and recombination probabilities of spin pairs is directly obtained for a distinct recombination path. Dangling bond recombination in microcrystalline silicon was used as model process for the demonstration of TSR. © 2001 American Institute of Physics.
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76.30.-v Electron paramagnetic resonance and relaxation
71.55.-i Impurity and defect levels

Ab initio study of quantum confined unpassivated ultrathin Si films

B. K. Agrawal and S. Agrawal

Appl. Phys. Lett. 79, 4366 (2001); http://dx.doi.org/10.1063/1.1427420 (3 pages)

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An ab initio study of the electronic structure of the one-dimensional quantum-confined crystalline ultrathin Si (100) films both with the unreconstructed p(1×1) and the reconstructed c(2×2) film surfaces in a large thickness range of 2.7–29.7 Å has been made after employing a self-consistent full potential linear muffin tin orbital method along with the density functional theory in local approximation. A relatively large unit cell of 28 layers has been employed. In the case of the unreconstructed p(1×1) surface, the dangling bond states fill the fundamental energy gap completely and these states are weakly localized, i.e., the wave functions of these states die out in the inner region of the film slowly. After the c(2×2) surface reconstruction, the bonding and the antibonding dangling bond states are seen to lie near the valence band maximum and the conduction band minimum in the fundamental energy gap, respectively, and are again weakly localized. There remain dangling bond states within the gap after reconstruction. The separation between the two surface Si atoms in a dimer shows quite an appreciable change of about 36–40%. The band gaps of the various films remain practically unchanged after surface reconstruction; the changes lie well within 6%. The present results give convincing theoretical evidence for the occurrence of the weakly localized dangling bond states and the absence of too much localized states in quantum-confined systems, which may be responsible for the occurrence of luminescence observed in a comparatively low energy region in the quantum-confined nanostructures, e.g., in the infrared region in porous silicon. The presently predicted exponential rise of the band gap with the decrease in the quantum-confined size in the ultrathin film region is in sharp contrast to an approximate linear rise in the band gap observed in the spectroellipsometric measurements. © 2001 American Institute of Physics.
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71.20.Mq Elemental semiconductors
73.20.At Surface states, band structure, electron density of states
73.20.Fz Weak or Anderson localization
68.35.B- Structure of clean surfaces (and surface reconstruction)
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Synthesis and characterization of luminescent ZrO2:Mn, Cl powders

M. García-Hipólito, C. Falcony, M. A. Aguilar-Frutis, and J. Azorín- Nieto

Appl. Phys. Lett. 79, 4369 (2001); http://dx.doi.org/10.1063/1.1428110 (3 pages) | Cited 9 times

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ZrO2:Mn, Cl luminescent powders have been synthesized at temperatures ranging from 250 to 500 °C. X-ray diffraction measurements indicate changes in the crystallinity of the material as a function of the processing temperature. The photoluminescence spectra show bands associated with transitions from the 4T1 level to the 6A1 levels of the Mn2+ ions. An impurity concentration quenching is observed with the Mn contents. In addition, the presence of Cl in the synthesized material has been found to have an important role on the photoluminescence intensity emission. Preliminary results on cathodoluminescent emission are also reported. © 2001 American Institute of Physics.
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78.55.Hx Other solid inorganic materials
78.60.Hk Cathodoluminescence, ionoluminescence
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation

Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices

A. Y. Polyakov, N. B. Smirnov, A. V. Govorkov, A. V. Osinsky, P. E. Norris, S. J. Pearton, J. Van Hove, A. Wowchak, and P. Chow

Appl. Phys. Lett. 79, 4372 (2001); http://dx.doi.org/10.1063/1.1429754 (3 pages) | Cited 10 times

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Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices are compared to those of similarly doped p-GaN films. It is shown that modulation doping increases the sheet hole concentration by several times. In p-AlGaN/GaN superlattices grown on GaN underlayers, this increase is accompanied by a significant increase in hole mobility which results in a remarkable decrease in sheet resistivity of the structure compared to p-GaN films and this decrease in sheet resistivity should hold up to temperatures exceeding 350 °C. For superlattices prepared on AlGaN underlayers, the mobility decreases compared to p-GaN. In such superlattices, one also observes a strong redshift and a strong broadening of the band edge luminescence peak coming most probably from increased mosaicity and strain which would also explain the observed deterioration of mobility. The magnitude of the redshift in the position of the band edge luminescence band slightly increased upon application of reverse bias which is interpreted as a manifestation of quantum-confined Stark effect. © 2001 American Institute of Physics.
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73.63.-b Electronic transport in nanoscale materials and structures
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
73.21.Cd Superlattices
68.65.Cd Superlattices
61.72.uj III-V and II-VI semiconductors
78.20.Jq Electro-optical effects
78.60.Hk Cathodoluminescence, ionoluminescence

Localization of carriers and polarization effects in quaternary AlInGaN multiple quantum wells

E. Kuokstis, J. Zhang, M.-Y. Ryu, J. W. Yang, G. Simin, M. Asif Khan, R. Gaska, and M. S. Shur

Appl. Phys. Lett. 79, 4375 (2001); http://dx.doi.org/10.1063/1.1429753 (3 pages) | Cited 10 times

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We report on observing a long-wavelength band in low-temperature photoluminescence (PL) spectrum of quaternary Al0.22In0.02Ga0.76N/Al0.38In0.01Ga0.61N multiple quantum wells (MQWs), which were grown over sapphire substrates by a pulsed atomic-layer epitaxy technique. By comparing the excitation-power density and temperature dependence of the PL spectra of MQWs and bulk quaternary AlInGaN layers, we show this emission band to arise from the carrier and/or exciton localization at the quantum well interface disorders. PL data for other radiative transitions in MQWs indicate that excitation-dependent spectra position is determined by screening of the built-in electric field. © 2001 American Institute of Physics.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
73.21.Fg Quantum wells
81.07.St Quantum wells
71.35.Gg Exciton-mediated interactions
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