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17 Jan 2011

Volume 98, Issue 3, Articles (03xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 98, 031101 (2011); http://dx.doi.org/10.1063/1.3529469 (3 pages)

Sinan Balci, Askin Kocabas, Coskun Kocabas, and Atilla Aydinli
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Inelastic electron tunneling study of crystallization effects and defect energies in hafnium oxide gate dielectrics

Eun Ji Kim, Michael Shandalov, Krishna C. Saraswat, and Paul C. McIntyre

Appl. Phys. Lett. 98, 032108 (2011); http://dx.doi.org/10.1063/1.3527977 (3 pages) | Cited 1 time

Online Publication Date: 20 January 2011

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Changes in phonon spectra and point defect populations that accompany crystallization of HfO2 were investigated by inelastic tunneling across Al/HfO2/SiO2/Si. Spectral features from tetragonal- and monoclinic-HfO2 vibrational modes are observed in annealed films, while they are not detected in as-deposited samples, consistent with selected area electron diffraction analysis. In addition to features indexed as vibrational modes, peaks whose amplitude and energy vary with bias history were detected for p-type Si. We attribute these features to point defect-related states in the HfO2 band gap and find good agreement between their energies and those predicted theoretically for oxygen vacancy levels in HfO2.
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73.40.Gk Tunneling
77.55.D- High-permittivity gate dielectric films
81.40.Gh Other heat and thermomechanical treatments
71.55.Ht Other nonmetals
71.20.Ps Other inorganic compounds
61.72.jd Vacancies

Disorder induced Coulomb gaps in graphene constrictions with different aspect ratios

B. Terrés, J. Dauber, C. Volk, S. Trellenkamp, U. Wichmann, and C. Stampfer

Appl. Phys. Lett. 98, 032109 (2011); http://dx.doi.org/10.1063/1.3544580 (3 pages) | Cited 1 time

Online Publication Date: 21 January 2011

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We present electron transport measurements on lithographically defined and etched graphene nanoconstrictions with different aspect ratios including different lengths (l) and widths (w). A roughly length-independent disorder induced effective energy gap can be observed around the charge neutrality point. This energy gap scales inversely with the width even in regimes where the length of the constriction is smaller than its width (l<w). In very short constrictions, we observe both resonances due to localized states or charged islands and an elevated overall conductance level (0.1−1e2/h), which is strongly length-dependent in the gap region. This makes very short graphene constrictions interesting for highly transparent graphene tunneling barriers.
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73.22.Pr Electronic structure of graphene
73.40.Gk Tunneling
72.80.Vp Electronic transport in graphene

Nanowatt logic stochastic resonance in branched resonant tunneling diodes

F. Hartmann, A. Forchel, I. Neri, L. Gammaitoni, and L. Worschech

Appl. Phys. Lett. 98, 032110 (2011); http://dx.doi.org/10.1063/1.3548539 (3 pages) | Cited 2 times

Online Publication Date: 21 January 2011

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The authors have fabricated branched resonant tunneling diodes (RTDs). Using two branches as inputs, universal logic-gate operation was investigated as a function of noise added to the input signal. The difference in the output voltage, used as a measure for logic operation, shows a peak in the noise power characteristic associated with logic stochastic resonance. The split RTD allows morphing between universal logic-gates solely controlled by the noise level with power differences in the nanowatt range.
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85.75.Mm Spin polarized resonant tunnel junctions
84.30.Sk Pulse and digital circuits

Phonon-limited and effective low-field mobility in n- and p-type [100]-, [110]-, and [111]-oriented Si nanowire transistors

Mathieu Luisier

Appl. Phys. Lett. 98, 032111 (2011); http://dx.doi.org/10.1063/1.3540689 (3 pages) | Cited 10 times

Online Publication Date: 21 January 2011

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Ultrascaled n- and p-type Si nanowire field-effect transistors (NW FETs) with [100], [110], and [111] as channel orientations are simulated in the presence of electron-phonon scattering using an atomistic quantum transport solver based on the sp3d5s∗ tight-binding model for electrons and holes, a modified Keating model for phonons, and the nonequilibrium Green’s function formalism. The channel resistances of devices with different gate lengths and carrier concentrations are computed at room temperature and used to extract phonon-limited, ballistic, and effective low-field mobilities. It is found that a [110] channel represents the best choice for high n- and p-type NW FET performances.
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85.30.Tv Field effect devices
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
81.07.Vb Quantum wires
85.75.Hh Spin polarized field effect transistors
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Cooling electrons from 1 to 0.4 K with V-based nanorefrigerators

O. Quaranta, P. Spathis, F. Beltram, and F. Giazotto

Appl. Phys. Lett. 98, 032501 (2011); http://dx.doi.org/10.1063/1.3544058 (3 pages) | Cited 2 times

Online Publication Date: 19 January 2011

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The fabrication and operation of V-based superconducting nanorefrigerators is reported. Specifically, electrons in an Al island are cooled, thanks to hot-quasiparticle extraction provided by tunnel-coupled V electrodes. Electronic temperature reduction down to 400 mK starting from 1 K is demonstrated with an estimated cooling power of ∼ 20 pW at 1 K for a junction area of 0.3 μm2.
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85.25.-j Superconducting devices
85.35.-p Nanoelectronic devices
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High dielectric permittivity and low percolation threshold in polymer composites based on SiC-carbon nanotubes micro/nano hybrid

Jin-Kai Yuan, Wei-Long Li, Sheng-Hong Yao, You-Qin Lin, Alain Sylvestre, and Jinbo Bai

Appl. Phys. Lett. 98, 032901 (2011); http://dx.doi.org/10.1063/1.3544942 (3 pages) | Cited 2 times

Online Publication Date: 21 January 2011

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Vertically aligned carbon nanotubes (CNTs) were synthesized on SiC microplates. Resultant SiC-CNTs hybrids were further incorporated into poly(vinylidene fluoride) (PVDF) as conductive filler to prepare percolative composites. They exhibited a much low percolation threshold (1.47 vol %), but the dielectric permittivity was enhanced significantly. The largest dielectric permittivity of 8700 was observed in the SiC-CNTs/PVDF composites with 2.3 vol % CNTs at 100 Hz. A microcapacitor model was proposed to explain the obtained results based on the special geometry of the hybrid SiC-CNTs filler.
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81.07.Pr Organic-inorganic hybrid nanostructures
77.22.Ch Permittivity (dielectric function)
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Epitaxial growth and structural property of graphene on Pt(111)

M. Gao, Y. Pan, L. Huang, H. Hu, L. Z. Zhang, H. M. Guo, S. X. Du, and H.-J. Gao

Appl. Phys. Lett. 98, 033101 (2011); http://dx.doi.org/10.1063/1.3543624 (3 pages) | Cited 3 times

Online Publication Date: 18 January 2011

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We report on epitaxial growth of graphene on Pt(111) surface. It was found out that the proportion of different rotational domains varies with growth temperature and the graphene quality can be improved by adjusting both the growth temperature and ethylene exposure. Rippled and unrippled domains of high quality graphene are observed. The adhesive energy and electronic structure of two models, representing rippled and unrippled graphene, are obtained with density functional theory calculation, which shows that the interaction between graphene and Pt(111) surface is very weak and the electronic structure is nearly the same as that of a free standing graphene.
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61.48.Gh Structure of graphene
73.22.Pr Electronic structure of graphene
81.05.ue Graphene
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Highly selective spectral response with enhanced responsivity of n-ZnO/p-Si radial heterojunction nanowire photodiodes

Han-Don Um, Syed Abdul Moiz, Kwang-Tae Park, Jin-Young Jung, Sang-Won Jee, Cheol Hyoun Ahn, Dong Chan Kim, Hyung Koun Cho, Dong-Wook Kim, and Jung-Ho Lee

Appl. Phys. Lett. 98, 033102 (2011); http://dx.doi.org/10.1063/1.3543845 (3 pages) | Cited 7 times

Online Publication Date: 18 January 2011

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A radial heterojunction nanowire diode (RND) array consisting of a ZnO (shell)/Si (core) structure was fabricated using conformal coating of a n-type ZnO layer that surrounded a p-type Si nanowire. In both ultraviolet (UV) and visible ranges, the photoresponsivity of the RND was larger than that of a planar thin film diode (PD) owing to the efficient carrier collection with improved light absorption. Compared to a PD, in the forward bias, a 6 μm long RND resulted in a ∼ 2.7 times enhancement of the UV responsivity at λ = 365 nm, which could be explained based on the oxygen-related hole-trap mechanism. Under a reverse bias, UV-blind visible detection was observed while the UV response was suppressed.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.60.Dw Photodiodes; phototransistors; photoresistors

Behind the change of the photoluminescence property of metal-coated ZnO nanowire arrays

Y. J. Fang, J. Sha, Z. L. Wang, Y. T. Wan, W. W. Xia, and Y. W. Wang

Appl. Phys. Lett. 98, 033103 (2011); http://dx.doi.org/10.1063/1.3543902 (3 pages) | Cited 3 times

Online Publication Date: 18 January 2011

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The effect of metal coating on the photoluminescence (PL) properties of ZnO nanowire arrays has been investigated in detail in this letter. The Zn coating induces remarkable enhancement of the ultraviolet and green emissions of the nanowires, while the deposition of Ag leads to notable decrement of them. A model considering the type of contacts formed between metals and ZnO is proposed to interpret the change of the PL spectra. Also, this model is strongly supported by the PL variation of the nanowires after coating with other kinds of metals.
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78.55.Et II-VI semiconductors
78.40.Fy Semiconductors
73.40.Ns Metal-nonmetal contacts
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
78.67.Uh Nanowires
73.21.Hb Quantum wires

Energy and matter-efficient size-selective growth of thin quantum wires in a plasma

K. Ostrikov and H. Mehdipour

Appl. Phys. Lett. 98, 033104 (2011); http://dx.doi.org/10.1063/1.3540645 (3 pages) | Cited 1 time

Online Publication Date: 18 January 2011

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It is shown that plasmas can minimize the adverse Gibbs-Thompson effect in thin quantum wire growth. The model of Si nanowire nucleation includes the unprecedented combination of the plasma sheath, ion- and radical-induced species creation and heating effects on the surface and within an Au catalyst nanoparticle. Compared to neutral gas thermal processes, much thinner, size-selective wires can nucleate at the same temperature and pressure while much lower energy and matter budget is needed to grow same-size wires. This explains the experimental observations and may lead to energy- and matter-efficient synthesis of a broader range of one-dimensional quantum structures.
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68.65.La Quantum wires (patterned in quantum wells)
52.40.Kh Plasma sheaths
73.21.Hb Quantum wires
73.63.Nm Quantum wires
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

Air-gap heterostructures

Ch. Heyn, M. Schmidt, S. Schwaiger, A. Stemmann, S. Mendach, and W. Hansen

Appl. Phys. Lett. 98, 033105 (2011); http://dx.doi.org/10.1063/1.3544047 (3 pages)

Online Publication Date: 19 January 2011

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We demonstrate the fabrication of thin GaAs layers which quasi hover above the underlying GaAs substrate. The hovering layers have a perfect epitaxial relationship to the substrate crystal lattice and are connected to the substrate surface only by lattice matched nanopillars of low density. These air-gap heterostructures are created by combining in situ molecular beam epitaxy compatible self-assembled droplet-etching and ex situ selective wet-chemical etching.
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81.05.Ea III-V semiconductors
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.65.Cf Surface cleaning, etching, patterning

Direct image of a molecular orientation of a liquid crystal using directional polymerization of photoreactive mesogen

You-Jin Lee, Chang-Jae Yu, Young-Ki Kim, Soo In Jo, and Jae-Hoon Kim

Appl. Phys. Lett. 98, 033106 (2011); http://dx.doi.org/10.1063/1.3537959 (3 pages)

Online Publication Date: 19 January 2011

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We present a method for visualizing the geometrical distribution of a liquid crystal (LC) arrangement using the replication of the molecular orientation by a mixture of alignment layers. We found that the anisotropic diffusion of the reactive mesogen (RM) monomers, mixed with an alignment layer, in contact with the LC was governed by the LC director through application of external voltage and allowed the directional polymerization under ultraviolet light illumination. The directional polymerization of a RM transferred and visualized the LC arrangement even at optically indistinct LC distributions.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
82.35.-x Polymers: properties; reactions; polymerization
66.10.-x Diffusion and ionic conduction in liquids

Influence of hydrogen functionalization on thermal conductivity of graphene: Nonequilibrium molecular dynamics simulations

Shih-Kai Chien, Yue-Tzu Yang, and Cha’o-Kuang Chen

Appl. Phys. Lett. 98, 033107 (2011); http://dx.doi.org/10.1063/1.3543622 (3 pages) | Cited 6 times

Online Publication Date: 20 January 2011

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The influence of hydrogen coverage on the thermal conductivity of typical armchair hydrogen functionalized graphene is investigated using a nonequilibrium molecular dynamics simulation. We also study the effects of easy-rotation of unsupported sp3 bonds. We find that the system exhibits a rapid drop in thermal conductivity with hydrogen coverage, where hydrogen coverage down to as little as 2.5% of the graphene carbon atoms reduces the thermal conductivity by about 40%. The simulation results indicate that the effect is due to a reduction in the phonon mean free path.
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65.80.Ck Thermal properties of graphene
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
63.22.Rc Phonons in graphene

Growth and electrical properties of Al-catalyzed Si nanowires

Seok-Youl Choi (최석열), Wayne Y. Fung, and Wei Lu

Appl. Phys. Lett. 98, 033108 (2011); http://dx.doi.org/10.1063/1.3544933 (3 pages) | Cited 1 time

Online Publication Date: 20 January 2011

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We report the growth and electrical characterization of small-diameter, Al-catalyzed silicon nanowires. Large quantities of Si nanowires can be grown with a fast growth rate at a low temperature of 520 °C via a vapor-liquid-solid process using Al as catalyst. The use of high H2 partial pressure was found to effectively mitigate the Al catalyst oxidation problem, and higher SiH4 partial pressure was found to produce smaller diameter nanowires. Electrical measurements show that small diameter (d< ∼ 25 nm) Al-catalyzed nanowires behave as p-type semiconductors with good gate response, while larger diameter Al-catalyzed nanowires behave as degenerately Al-doped semiconductors.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
73.61.Cw Elemental semiconductors
81.07.Gf Nanowires
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Improving optical performance of low bandgap polymer solar cells by the two-mode moderate microcavity

Yongbing Long (龙拥兵)

Appl. Phys. Lett. 98, 033301 (2011); http://dx.doi.org/10.1063/1.3541960 (3 pages)

Online Publication Date: 18 January 2011

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A two-mode moderate microcavity (MM) structure has been constructed to confine large optical electric field in the low band gap polymer solar cells by inserting a titanium dioxide (TiO2) layer into the devices. Simulations demonstrate the two-mode MM can significantly improve the light absorption at two discrete wavelength ranges and yield an improvement of about 13.8% in total absorbed photons (TAPs) for the device with a 60 nm thick active layer. Combined with an optical spacer, the MM structure leads to a larger improvement of 32.8% in TAPs. The efficacy of the MM structure is revealed to be strongly dependent on the active layer thickness.
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88.40.jr Organic photovoltaics
FREE

Efficient organic light-emitting devices with platinum-complex emissive layer

Xiaohui Yang, Fang-Iy Wu, Hanna Haverinen, Jian Li, Chien-Hong Cheng, and Ghassan E. Jabbour

Appl. Phys. Lett. 98, 033302 (2011); http://dx.doi.org/10.1063/1.3541447 (3 pages) | Cited 2 times

Online Publication Date: 18 January 2011

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We report efficient organic light-emitting devices having a platinum-complex emissive layer with the peak external quantum efficiency of 17.5% and power efficiency of 45 lm W−1. Variation in the device performance with platinum-complex layer thickness can be attributed to the interplay between carrier recombination and intermolecular interactions in the layer. Efficient white devices using double platinum-complex layers show the external quantum efficiency of 10%, the Commission Internationale d’Énclairage coordinates of (0.42, 0.41), and color rendering index of 84 at 1000 cd m−2.
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85.60.Jb Light-emitting devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
FREE

Stability of polarization in organic ferroelectric metal-insulator-semiconductor structures

R. Kalbitz, P. Frübing, R. Gerhard, and D. M. Taylor

Appl. Phys. Lett. 98, 033303 (2011); http://dx.doi.org/10.1063/1.3543632 (3 pages) | Cited 3 times

Online Publication Date: 19 January 2011

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Dielectric measurements have been carried out on all-organic metal-insulator-semiconductor structures with the ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) as the gate insulator. It is shown that the polarization states remain stable after poling with accumulation and depletion voltage. However, negative charge trapped at the semiconductor-insulator interface during the depletion cycle masks the negative shift in flatband voltage expected during the sweep to accumulation voltages.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
84.32.Tt Capacitors
FREE

Mobility determination using frequency dependence of imaginary part of impedance (Im Z) for organic and polymeric thin films

Durgesh C. Tripathi, Awnish K. Tripathi, and Y. N. Mohapatra

Appl. Phys. Lett. 98, 033304 (2011); http://dx.doi.org/10.1063/1.3544935 (3 pages)

Online Publication Date: 21 January 2011

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In view of a growing need for purely electrical techniques of determining carrier mobility under actual device conditions in organic semiconductors, we demonstrate that instead of difference susceptance, the frequency dependence of imaginary part of impedance is a simpler, more convenient, and powerful method of determination of mobility. We illustrate it by using the method to determine field dependence of mobility, and hence Poole–Frenkel coefficient, for the case of small molecule m-MTDATA and polymeric MEH-PPV. We also determine the correct numerical factors required to obtain true transit times and compare that with electroluminescence transient. We also show that these measurements are robust in presence of dispersive transport avoiding interpretative difficulties in determination of mobility.
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73.61.Ph Polymers; organic compounds
81.05.Fb Organic semiconductors
72.20.Ht High-field and nonlinear effects
78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds
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Constant voltage stress induced charge trapping and detrapping characteristics of the Si3N4 uniaxial strained n-channel metal-oxide-semiconductor field-effect-transistor with fluorinated HfO2/SiON gate stack

Yung-Yu Chen, Chih-Ren Hsieh, and Fang-Yu Chiu

Appl. Phys. Lett. 98, 033501 (2011); http://dx.doi.org/10.1063/1.3541878 (3 pages)

Online Publication Date: 18 January 2011

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Charge trapping and detrapping characteristics of Si3N4 contact etch stop layer (SiN CESL) uniaxial strained n-channel metal-oxide-semiconductor field-effect-transistor (nMOSFET) with fluorinated HfO2/SiON gate stack have been investigated for the first time. Smaller threshold voltage shift can consequently obtain for the SiN CESL strained nMOSFET with fluorinated HfO2/SiON gate stack, primarily due to passivation of oxygen vacancies and dangling bonds by either nitrogen or fluorine atoms. However, the SiN CESL strained nMOSFET with fluorinated gate stack inevitably exhibits less charge detrapping ratio, which means greater part of stress-induced charges would remain in the gate stack after nitrogen or fluorine incorporation, respectively.
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85.30.Tv Field effect devices
81.65.Rv Passivation
61.72.jd Vacancies
71.55.-i Impurity and defect levels

Interface-modified random circuit breaker network model applicable to both bipolar and unipolar resistance switching

S. B. Lee, J. S. Lee, S. H. Chang, H. K. Yoo, B. S. Kang, B. Kahng, M.-J. Lee, C. J. Kim, and T. W. Noh

Appl. Phys. Lett. 98, 033502 (2011); http://dx.doi.org/10.1063/1.3543776 (3 pages) | Cited 6 times

Online Publication Date: 18 January 2011

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We observed reversible-type changes between bipolar (BRS) and unipolar resistance switching (URS) in one Pt/SrTiOx/Pt capacitor. To explain both BRS and URS in a unified scheme, we introduce the “interface-modified random circuit breaker network model,” in which the bulk medium is represented by a percolating network of circuit breakers. To consider interface effects in BRS, we introduce circuit breakers to investigate resistance states near the interface. This percolation model explains the reversible-type changes in terms of connectivity changes in the circuit breakers and provides insights into many experimental observations of BRS which are under debate by earlier theoretical models.
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84.70.+p High-current and high-voltage technology: power systems; power transmission lines and cables
84.32.Tt Capacitors

Strong Stark effect in electroluminescence from phosphorous-doped silicon-on-insulator metal-oxide-semiconductor field-effect transistors

J. Noborisaka, K. Nishiguchi, Y. Ono, H. Kageshima, and A. Fujiwara

Appl. Phys. Lett. 98, 033503 (2011); http://dx.doi.org/10.1063/1.3543849 (3 pages) | Cited 1 time

Online Publication Date: 18 January 2011

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We report a strong Stark effect in electroluminescence (EL) from thin silicon-on-insulator metal-oxide-semiconductor field-effect transistors. The EL peak shows a large Stark shift of up to 50 meV when a gate-induced electric field is applied. Such a large shift is observed because of the strong confinement of carriers by a Si/SiO2 quantum well. Furthermore, we observe a sudden decrease of EL intensity at a specific electric field. This feature is ascribed to electron dissociation from phosphorous atom to subbands in a triangular well at the Si/SiO2 interface by the applied electric field.
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85.30.Tv Field effect devices

Transition of dominant instability mechanism depending on negative gate bias under illumination in amorphous In-Ga-Zn-O thin film transistor

Himchan Oh, Sung-Min Yoon, Min Ki Ryu, Chi-Sun Hwang, Shinhyuk Yang, and Sang-Hee Ko Park

Appl. Phys. Lett. 98, 033504 (2011); http://dx.doi.org/10.1063/1.3540500 (3 pages) | Cited 8 times

Online Publication Date: 18 January 2011

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The gate bias dependence on the negative bias instability under illumination was examined. As the gate bias got more negative, dominant mechanism was changed from simple charge trapping to that accompanied by generation of subgap states. Degree of threshold voltage shift was not monotonously dependent on the magnitude of negative gate bias. It is strongly related with the corresponding instability modes for different gate bias regimes. The transition of instability mechanism depends on how much the gate bias stabilizes ionized oxygen vacancy states.
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85.30.Tv Field effect devices
81.05.Gc Amorphous semiconductors
61.72.jd Vacancies

Diagnosis of low-frequency noise sources in contact resistance of staggered organic transistors

Y. Xu, R. Gwoziecki, R. Coppard, M. Benwadih, T. Minari, K. Tsukagoshi, J. A. Chroboczek, F. Balestra, and G. Ghibaudo

Appl. Phys. Lett. 98, 033505 (2011); http://dx.doi.org/10.1063/1.3544583 (3 pages) | Cited 3 times

Online Publication Date: 20 January 2011

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A diagnostic procedure for the contact low-frequency noise sources of staggered organic transistors is presented. Like the conventional process for channel noise, the two models of carrier number fluctuations and Hooge mobility fluctuations are concerned. Both models apply to bis(triisopropylsilylethynyl) pentacene transistors at lower and higher current densities, where the contact defect density is 1000 times higher than that in the channel and a Hooge parameter around 1 is obtained, respectively. The Hooge mobility fluctuations model well accounts for the contact noise in pentacene transistors with a Hooge parameter in the range of 0.5–3.
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85.30.Tv Field effect devices

On the symmetry of efficiency-versus-carrier-concentration curves in GaInN/GaN light-emitting diodes and relation to droop-causing mechanisms

Qi Dai, Qifeng Shan, Jaehee Cho, E. Fred Schubert, Mary H. Crawford, Daniel D. Koleske, Min-Ho Kim, and Yongjo Park

Appl. Phys. Lett. 98, 033506 (2011); http://dx.doi.org/10.1063/1.3544584 (3 pages) | Cited 4 times

Online Publication Date: 20 January 2011

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The internal quantum efficiency (IQE)-versus-carrier-concentration (n) curves of GaN-based light-emitting diodes have been frequently described by the ABC model: IQE = Bn2/(An+Bn2+Cn3). We show that this model predicts IQE-versus-n curves that have even symmetry. Phase-space filling makes the B and C coefficients concentration-dependent. We also show that IQE-versus-n curves that take into account phase-space filling possess even symmetry. In contrast, experimental IQE-versus-n curves exhibit asymmetry. The asymmetry requires a fourth-power or higher-power contribution to the recombination rate and provides insight into the mathematical form of the droop-causing mechanisms.
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85.60.Jb Light-emitting devices

Low noise and wide bandwidth of NbN hot-electron bolometer mixers

Ivan Tretyakov, Sergey Ryabchun, Matvey Finkel, Anna Maslennikova, Natalia Kaurova, Anastasia Lobastova, Boris Voronov, and Gregory Gol’tsman

Appl. Phys. Lett. 98, 033507 (2011); http://dx.doi.org/10.1063/1.3544050 (3 pages) | Cited 3 times

Online Publication Date: 21 January 2011

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We report a record double sideband noise temperature of 600 K (5hν/kB) offered by a NbN hot-electron bolometer receiver at 2.5 THz. Allowing for standing wave effects, this value was found to be constant in the intermediate frequency range 1–7 GHz, which indicates that the mixer has an unprecedentedly large noise bandwidth in excess of 7 GHz. The insight into this is provided by gain bandwidth measurements performed at the superconducting transition. They show that the dependence of the bandwidth on the mixer length follows the model for an HEB mixer with diffusion and phonon cooling of the hot electrons.
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85.25.Pb Superconducting infrared, submillimeter and millimeter wave detectors
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
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