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2 Apr 2007

Volume 90, Issue 14, Articles (14xxxx)

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Appl. Phys. Lett. 90, 142110 (2007); http://dx.doi.org/10.1063/1.2720640 (3 pages)

M. T. Björk, O. Hayden, H. Schmid, H. Riel, and W. Riess
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Ni/Au Ohmic contacts to p-type Mg-doped CuCrO2 epitaxial layers

W. T. Lim, L. Stafford, P. W. Sadik, D. P. Norton, S. J. Pearton, Y. L. Wang, and F. Ren

Appl. Phys. Lett. 90, 142101 (2007); http://dx.doi.org/10.1063/1.2719150 (3 pages) | Cited 6 times

Online Publication Date: 2 April 2007

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Ohmic contact formation on p-type Mg-doped CuCrO2 layers grown by pulsed-laser deposition was investigated. While the current-voltage characteristics from Ti/Au contacts showed back-to-back Schottky behavior, a specific contact resistance of ∼ 1×10−4 Ω cm2 was achieved by using Ni instead of Ti. The contact resistivity was fairly independent of measurement temperature, suggesting that tunneling is the dominant transport mechanism. The contact resistance remained practically constant upon annealing in the 100–400 °C range. Above 500 °C, the morphology became rough and the contact showed rectifying behavior. This degradation resulted from both the out-diffusion of oxygen and the in-diffusion of Ni and Au in CuCrO2.
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73.40.Ns Metal-nonmetal contacts
73.40.Gk Tunneling
73.40.Ei Rectification
73.40.Cg Contact resistance, contact potential
61.72.Cc Kinetics of defect formation and annealing
66.30.J- Diffusion of impurities

Enhancement of ballistic transport in an AlGaAs/InGaAs high electron mobility transistor at low temperatures

Nambin Kim, Yongmin Kim, Soohyun Kang, Kyooho Jung, Woong Jung, Hyunsik Im, Hyungsang Kim, J.-K. Rhee, and Donghoon Shin

Appl. Phys. Lett. 90, 142102 (2007); http://dx.doi.org/10.1063/1.2719231 (3 pages)

Online Publication Date: 2 April 2007

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The authors investigated the current-voltage characteristics of a 100 nm AlGaAs/InGaAs pseudomorphic high electron mobility transistor as a function of temperature. The drain current in the linear region showed a dramatic increase when the temperature was lowered below a critical value (TB) and the drain voltage is increased. A quantitative analysis based on self-consistent Schrödinger-Poisson and simple electrostatic band potential profile calculations was performed to model the transmission coefficient. The modeled results are consistent with the measured data, confirming that the main transport mechanism switches from a classical drift-diffusion transport into a quasiballistic transport when decreasing temperature below TB and increasing drain voltage.
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85.30.Tv Field effect devices

Modulated charge separation at tetraphenyl porphyrin/Au interfaces

Y. Zidon, Yoram Shapira, and Th. Dittrich

Appl. Phys. Lett. 90, 142103 (2007); http://dx.doi.org/10.1063/1.2719629 (3 pages) | Cited 3 times

Online Publication Date: 2 April 2007

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A frequency and temperature dependent change of the sign of modulated surface photovoltage signals from metal-free tetraphenyl porphyrin (H2TPP)/Au junctions has been observed. The sign of the modulated photovoltage signal could be switched by using continuous (bias) illumination, while the sign depends on the wavelength of the bias light. The light induced modulation of the negative and positive charges in the H2TTP layer can be interpreted in terms of internal photoemission of electrons from Au into H2TTP and of hole escape after exciton dissociation at the H2TPP/Au junction.
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73.40.Ns Metal-nonmetal contacts
72.40.+w Photoconduction and photovoltaic effects
73.25.+i Surface conductivity and carrier phenomena
71.35.-y Excitons and related phenomena
73.50.Pz Photoconduction and photovoltaic effects
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Semiconducting graphene nanostrips with edge disorder

D. Gunlycke, D. A. Areshkin, and C. T. White

Appl. Phys. Lett. 90, 142104 (2007); http://dx.doi.org/10.1063/1.2718515 (3 pages) | Cited 59 times

Online Publication Date: 3 April 2007

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Results of calculations are presented which show that edge disorder can easily transform semiconducting graphene nanostrips into Anderson insulators. However, it is also shown that this problem could be overcome by adjusting the nanostrip aspect ratio to decrease the effects of the edge disorder without making the nanostrip so wide as to close the semiconducting band gap or so short as to allow tunneling through the band gap.
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71.30.+h Metal-insulator transitions and other electronic transitions
71.20.Nr Semiconductor compounds

Frequency-dependent magnetoresistance and magnetocapacitance properties of magnetic tunnel junctions with MgO tunnel barrier

P. Padhan, P. LeClair, A. Gupta, K. Tsunekawa, and D. D. Djayaprawira

Appl. Phys. Lett. 90, 142105 (2007); http://dx.doi.org/10.1063/1.2719032 (3 pages) | Cited 8 times

Online Publication Date: 4 April 2007

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The frequency-dependent impedance of magnetic tunnel junctions (MTJs) with MgO barriers was investigated. The capacitance of the MTJs switches from high to low when the relative electrode magnetizations change from parallel to antiparallel, opposite the resistance change. Additionally, for parallel magnetizations, the capacitance varies with temperature though resistance remains approximately constant. The low frequency resistance and the tunneling magnetoresistance are in agreement with dc values. The capacitance is found to be larger than the expected (geometrical) capacitance, in contrast to MTJs with Al2O3 barriers. These results are explained by screening due to charge and spin accumulation at the interfaces.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Bias current effects on the magnetoresistance of a ferromagnetic-semiconductor-ferromagnetic trilayer

S. Bala Kumar, S. G. Tan, and M. B. A. Jalil

Appl. Phys. Lett. 90, 142106 (2007); http://dx.doi.org/10.1063/1.2719156 (3 pages) | Cited 3 times

Online Publication Date: 4 April 2007

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The authors apply a self-consistent ballistic-diffusive theoretical model to study the bias current j effect on the magnetoresistance of a ferromagnet (FM)-semiconductor (SC)-FM trilayer, with SC highly doped (n++). The interfacial resistance becomes a dynamic parameter and its decrease with increasing j would be responsible for the decrease of magnetoresistance (MR) with j. The underlying physics of this model is based on a self-consistent treatment between the spin drift diffusion transport of electrons in the bulk and ballistic transmission at the interfaces. This model applies qualitatively to the more common FM-nonmagnetic-FM, metal-based current-perpendicular-to-plane spin valve, which has shown experimentally observed decline of MR with j.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
72.25.Mk Spin transport through interfaces
73.23.Ad Ballistic transport

Transition between grain boundary and intragrain scattering transport mechanisms in boron-doped zinc oxide thin films

J. Steinhauser, S. Faÿ, N. Oliveira, E. Vallat-Sauvain, and C. Ballif

Appl. Phys. Lett. 90, 142107 (2007); http://dx.doi.org/10.1063/1.2719158 (3 pages) | Cited 55 times

Online Publication Date: 4 April 2007

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A comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented. The optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements. By analyzing the results for samples with large variation of grain size and doping level, the respective influences on the transport of potential barriers at grain boundaries and intragrain scattering could be separated unambiguously. A continuous transition from grain boundary scattering to intragrain scattering is observed for doping level increasing from 3×1019 to 2×1020 cm−3.
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73.61.Ga II-VI semiconductors
78.66.Hf II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
61.72.uj III-V and II-VI semiconductors
61.72.Mm Grain and twin boundaries

On the effect of periodic Mg distribution in GaN:δ-Mg

C. Simbrunner, M. Wegscheider, M. Quast, Tian Li, A. Navarro-Quezada, H. Sitter, A. Bonanni, and R. Jakiela

Appl. Phys. Lett. 90, 142108 (2007); http://dx.doi.org/10.1063/1.2719171 (3 pages) | Cited 8 times

Online Publication Date: 4 April 2007

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p-type doping of group-III nitrides represents a well recognized crucial challenge in the realization of optical and magneto-optical devices based on wide band gap semiconductors. The insertion of Mg in a δ-doping fashion into a GaN matrix is expected to be very promising for the enhanced incorporation of acceptors. The authors present a comprehensive study of the GaN:δ-Mg material system, starting from the optimization of the metal organic chemical vapor deposition process monitored in situ via spectroscopic ellipsometry and reflectometry and giving then evidence of the periodic distribution of the Mg ions through high-resolution secondary ion mass spectroscopy measurements. Furthermore, the effect of the acceptor distribution in the layers on the optical and transport properties is highlighted.
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81.05.Ea III-V semiconductors
61.72.S- Impurities in crystals
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Influence of hole transport units on the efficiency of polymer light emitting diodes

Frédéric Laquai and Dirk Hertel

Appl. Phys. Lett. 90, 142109 (2007); http://dx.doi.org/10.1063/1.2719212 (3 pages) | Cited 19 times

Online Publication Date: 4 April 2007

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Charge transport and electroluminescence efficiency in spirobifluorene polymers containing triarylamine as hole transport unit have been studied. The hole mobility as measured by the time-of-flight technique was found to be on the order of 10−4 cm2 V−1 s−1. The field and temperature dependences of the mobility are in accord with the Gaussian disorder model. Best device efficiencies of the blue emitting materials are in the range of 2.9 cd/A. The influence of the hole transport unit on charge transport and device efficiency is analyzed. The efficiency depends critically on the amount of the hole transport unit and decreases with increasing hole mobility.
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85.60.Jb Light-emitting devices

Vertical surround-gated silicon nanowire impact ionization field-effect transistors

M. T. Björk, O. Hayden, H. Schmid, H. Riel, and W. Riess

Appl. Phys. Lett. 90, 142110 (2007); http://dx.doi.org/10.1063/1.2720640 (3 pages) | Cited 35 times

Online Publication Date: 5 April 2007

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One of the fundamental limits in the scaling of metal oxide semiconductor field-effect transistor technology is the room-temperature (RT) limit of ∼ 60 mV/decade in the inverse subthreshold slope. Here, the authors demonstrate vertical integration of a single surround-gated silicon nanowire field-effect transistor with an inverse subthreshold slope as low as 6 mV/decade at RT that spans four orders of magnitude in current. Operation of the device is based on avalanche breakdown in a partially gated vertical nanowire, epitaxially grown using the vapor-liquid-solid method. Low-power logic based on impact ionization field-effect transistors in combination with a vertical architecture is very promising for future high-performance ultrahigh-density circuits.
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85.30.Tv Field effect devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

High current density InN/AlN heterojunction field-effect transistor with a SiNx gate dielectric layer

Yu-Syuan Lin, Shun-Hau Koa, Chih-Yuan Chan, Shawn S. H. Hsu, Hong-Mao Lee, and Shangjr Gwo

Appl. Phys. Lett. 90, 142111 (2007); http://dx.doi.org/10.1063/1.2719223 (3 pages) | Cited 11 times

Online Publication Date: 5 April 2007

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InN/AlN metal-insulator-semiconductor heterojunction field-effect transistors with a gate-modulated drain current and a clear pinch-off characteristic have been demonstrated. The devices were fabricated using high-quality InN (26 nm)/AlN (100 nm) epifilms grown by plasma-assisted molecular-beam epitaxy on Si (111) substrates. The devices exhibited a current density higher than ∼ 530 mA/mm with a 5 μm gate length. The pinch-off voltage was at ∼ −7 V with an associated drain leakage current less than 10 μA/mm. The observed high current density may be attributed to the high sheet carrier density due to the large spontaneous polarization difference between InN and AlN.
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85.30.Tv Field effect devices

Organic photodetectors with electrically bistable electron acceptors and nanotubes

Satyajit Sahu, Sudip K. Batabyal, and Amlan J. Pal

Appl. Phys. Lett. 90, 142112 (2007); http://dx.doi.org/10.1063/1.2720296 (3 pages)

Online Publication Date: 5 April 2007

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The authors provide two routes to increase the photocurrent of organic photodetectors that are based on electron-donor and electron-acceptor materials. During device fabrication, they introduce carbon nanotubes, which act as channels for electron transport. The channels in effect provide an approach to address the problem of low carrier mobility in the conjugated organics. The authors also make use of the (electrically) bistable nature of the acceptor material, namely, rose bengal in a favorable way. During device characterization, they switch its conductivity to decrease device resistance and yield higher photocurrent. Both the routes enhance photocurrent in these donor-acceptor-type photodetectors.
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Valence band tunneling model for charge transfer of redox-active molecules attached to n- and p-silicon substrates

Srivardhan Gowda, Guru Mathur, and Veena Misra

Appl. Phys. Lett. 90, 142113 (2007); http://dx.doi.org/10.1063/1.2720337 (3 pages) | Cited 7 times

Online Publication Date: 5 April 2007

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In this work, monolayers of the redox-active molecules, with cationic- accessible states, were incorporated on p- and n-type silicons of varying doping concentrations. The redox voltages and kinetics were found to be strongly dependent on the silicon doping concentrations, and ambient light in case of n-Si substrate, while there was no significant impact of substrate doping concentration or ambient light in case of p-Si substrate. These results suggest the redox energy states in the molecule align within the valence band of the silicon substrate. Based on this, a model for electronic coupling and charge transfer at the molecule-semiconductor interfaces is proposed.
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71.20.Mq Elemental semiconductors
73.23.Hk Coulomb blockade; single-electron tunneling
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
85.65.+h Molecular electronic devices

Fabrication of Ta2O5/GeNx gate insulator stack for Ge metal-insulator-semiconductor structures by electron-cyclotron-resonance plasma nitridation and sputtering deposition techniques

Yohei Otani, Yasuhiro Itayama, Takuo Tanaka, Yukio Fukuda, Hiroshi Toyota, Toshiro Ono, Minoru Mitsui, and Kiyokazu Nakagawa

Appl. Phys. Lett. 90, 142114 (2007); http://dx.doi.org/10.1063/1.2720345 (3 pages) | Cited 4 times

Online Publication Date: 5 April 2007

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The authors have fabricated germanium (Ge) metal-insulator-semiconductor (MIS) structures with a 7-nm-thick tantalum pentaoxide (Ta2O5)/2-nm-thick germanium nitride (GeNx) gate insulator stack by electron-cyclotron-resonance plasma nitridation and sputtering deposition. They found that pure GeNx ultrathin layers can be formed by the direct plasma nitridation of the Ge surface without substrate heating. X-ray photoelectron spectroscopy revealed no oxidation of the GeNx layer after the Ta2O5 sputtering deposition. The fabricated MIS capacitor with a capacitance equivalent thickness of 4.3 nm showed excellent leakage current characteristics. The interface trap density obtained by the modified conductance method was 4×1011 cm−2 eV−1 at the midgap.
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84.32.Tt Capacitors
85.30.Tv Field effect devices

Nitride-based near-ultraviolet light emitting diodes with meshed p‐GaN

C. H. Kuo, H. C. Feng, C. W. Kuo, C. M. Chen, L. W. Wu, and G. C. Chi

Appl. Phys. Lett. 90, 142115 (2007); http://dx.doi.org/10.1063/1.2720347 (3 pages) | Cited 15 times

Online Publication Date: 5 April 2007

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This investigation presents nitride-based near ultraviolet light emitting diodes (LEDs) with a meshed p‐GaN layer. With 20 mA injection current, it was found that forward voltages were 3.33 and 3.39 V while output powers were 9.0 and 10.6 mW for the meshed indium-tin-oxide (ITO) LED and meshed p‐GaN LED, respectively. The larger LED output power is attributed to increased light extraction efficiency.
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85.60.Jb Light-emitting devices

Observation of a P-associated defect in HfO2 nanolayers on (100)Si by electron spin resonance

K. Clémer, A. Stesmans, and V. V. Afanas’ev

Appl. Phys. Lett. 90, 142116 (2007); http://dx.doi.org/10.1063/1.2435337 (3 pages)

Online Publication Date: 5 April 2007

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Electron spin resonance analysis has detected a P-donor related point defect in nanometer-thick HfO2 films on (100)Si after annealing in the range of 500–900 °C. Based on the principal g matrix (axial; g‖ = 1.9965; g⊥ = 1.9975) and hyperfine tensor values (A1 = 1425±10 G, A2 = 1245±10 G, and A3 = 1160±10 G) inferred from consistent K- and Q-band spectrum simulations, the center is assigned to a P2-type defect—a P substituting a Hf atom—similar to P2 in silica, where the unpaired spin is strongly localized on the P atom. The annealing impact is linked to the onset of crystallization enabling substitutional positioning of the P impurities. The centers may act as detrimental charge trapping sites.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
76.30.Lh Other ions and impurities
61.72.J- Point defects and defect clusters
64.70.K- Solid-solid transitions
81.40.Gh Other heat and thermomechanical treatments

Negative small-signal impedance of nanoscale GaN diodes in the terahertz frequency regime

V. N. Sokolov, K. W. Kim, V. A. Kochelap, and P. M. Muntiian

Appl. Phys. Lett. 90, 142117 (2007); http://dx.doi.org/10.1063/1.2720758 (3 pages) | Cited 4 times

Online Publication Date: 6 April 2007

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A physical mechanism of electrical instability in the terahertz frequency range is explored in a nanoscale two-terminal configuration of nitride-based structures under a high-field transport regime. The investigated instability utilizes the negative differential resistance effect that is known to occur in bulk group-III nitride semiconductors. A distinctive feature of the proposed mechanism for terahertz generation is that the diode steady-state I-V characteristics maintain a positive differential resistance for a stable operation, while the small-signal response provides negative impedance within the desired frequency range. An additional advantage is that the frequency window of negative small-signal impedance can be tuned electrically.
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85.30.Kk Junction diodes

On the nature of the spin-polarized hole states in a quasi-two-dimensional GaMnAs ferromagnetic layer

E. Dias Cabral, M. A. Boselli, A. T. da Cunha Lima, A. Ghazali, and I. C. da Cunha Lima

Appl. Phys. Lett. 90, 142118 (2007); http://dx.doi.org/10.1063/1.2721129 (3 pages) | Cited 7 times

Online Publication Date: 6 April 2007

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A self-consistent calculation of the density of states and the spectral density function is performed in a two-dimensional spin-polarized hole system based on a multiple-scattering approximation. Using parameters corresponding to GaMnAs thin layers, a wide range of Mn concentrations and hole densities have been explored to understand the nature, localized or extended, of the spin-polarized holes at the Fermi level for several values of the average magnetization of the Mn system. The authors show that, for a certain interval of Mn and hole densities, an increase on the magnetic order of the Mn ions come together with a change of the nature of the states at the Fermi level. This fact provides a delocalization of spin-polarized extended states antialigned to the average Mn magnetization and a higher spin polarization of the hole gas. This nonmetal-to-metal transition caused by the increase of Mn concentration once the ferromagnetic phase is reached was observed experimentally since the first samples were produced. These results are consistent with the occurrence of ferromagnetism with relatively high transition temperatures observed in some thin film samples and multilayered structures of this material.
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73.61.Ey III-V semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
72.25.Dc Spin polarized transport in semiconductors
72.25.Mk Spin transport through interfaces
75.47.Pq Other materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
71.20.Nr Semiconductor compounds
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
71.30.+h Metal-insulator transitions and other electronic transitions
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
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