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15 Feb 2010

Volume 96, Issue 7, Articles (07xxxx)

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Appl. Phys. Lett. 96, 072501 (2010); http://dx.doi.org/10.1063/1.3314301 (3 pages)

T. Hesjedal and T. Phung
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Superlattice electronic devices as high-performance oscillators between 60–220 GHz

Heribert Eisele, Suraj P. Khanna, and Edmund H. Linfield

Appl. Phys. Lett. 96, 072101 (2010); http://dx.doi.org/10.1063/1.3324697 (3 pages) | Cited 3 times

Online Publication Date: 16 February 2010

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Negative differential resistance devices were fabricated from four epitaxial wafers with different GaAs/AlAs superlattices and evaluated in resonant-cap full-height waveguide cavities. These devices generated output powers in the fundamental mode between 62–108 GHz. The best RF powers were 58 mW at 66 GHz, 42 mW at 78 GHz, and 28 mW at 94 GHz. The RF power of 15 mW at 101 GHz constitutes a 30-fold improvement over previous results; the highest fundamental oscillation frequency was 108 GHz. In a second-harmonic mode, one device yielded 2.0 mW at 216 GHz, the highest second-harmonic frequency to date for a GaAs/AlAs superlattice.
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84.30.Ng Oscillators, pulse generators, and function generators
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Charge-pumping characterization of interface traps in Al2O3/In0.75Ga0.25As metal-oxide-semiconductor field-effect transistors

W. Wang, J. Deng, J. C. M. Hwang, Y. Xuan, Y. Wu, and P. D. Ye

Appl. Phys. Lett. 96, 072102 (2010); http://dx.doi.org/10.1063/1.3315870 (3 pages) | Cited 4 times

Online Publication Date: 16 February 2010

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Charge pumping was used to characterize the interface traps between Al2O3 and In0.75Ga0.25As in an n-channel inversion-mode metal-oxide-semiconductor field-effect transistor (MOSFET). By analyzing the charge pumped under gate voltage pulses of different rise and fall times, the interface trap density was extracted across the band gap of In0.75Ga0.25As. The interface trap density was found to be 4×1012 cm−2 eV−1 near the conduction band and to peak at 3×1013 cm−2 eV−1 mid-gap. The result helps explain the promising on-state performance of the Al2O3/In0.75Ga0.25As MOSFET and the need to further improve the interface so that its off-state performance can be on par with that of the Si MOSFET.
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85.30.Tv Field effect devices
73.20.At Surface states, band structure, electron density of states
71.20.Nr Semiconductor compounds

Improvement of reproducible resistance switching in polycrystalline tungsten oxide films by in situ oxygen annealing

D. S. Shang, L. Shi, J. R. Sun, B. G. Shen, F. Zhuge, R. W. Li, and Y. G. Zhao

Appl. Phys. Lett. 96, 072103 (2010); http://dx.doi.org/10.1063/1.3300637 (3 pages) | Cited 17 times

Online Publication Date: 17 February 2010

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The electric-field-induced resistance switching in polycrystalline tungsten oxide films was investigated. Compared with the as-deposited film, the film annealed in an oxygen atmosphere shows a more stable switching behavior, a higher low-to-high resistance ratio, and a better endurance and retention. Based on the x-ray photoemission spectroscopy analysis, the resistance switching was attributed to the change in the interfacial barrier potential, due to the electron trapping/detrapping in the surface states, and the switching improvement was attributed to the decrease in the surface density of states. The present work suggests a possible approach controlling the resistance switching property by surface modification.
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72.80.Ga Transition-metal compounds
73.40.Cg Contact resistance, contact potential
73.40.Sx Metal-semiconductor-metal structures
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.20.At Surface states, band structure, electron density of states

Surface photovoltage studies of nonstoichiometric rutile titanium dioxide

L. R. Sheppard, T. Dittrich, J. Nowotny, and T. Bak

Appl. Phys. Lett. 96, 072104 (2010); http://dx.doi.org/10.1063/1.3318465 (3 pages) | Cited 1 time

Online Publication Date: 17 February 2010

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The aim of the present investigation was to assess how processing at elevated temperatures impacts upon the room temperature electronic structure of rutile TiO2. Consequently, strongly reduced and oxidized rutile TiO2 pellets were processed under controlled conditions and studied using surface photovoltage spectroscopy. Under illumination with photon energies above the band gap energy, reduced and oxidized rutile, respectively, displayed positive and negative surface photovoltages. This result indicates that undoped TiO2 may be tailored to promote either photoreduction or photo-oxidation reactions. Several transitions involving surface and bulk electronic states within the band gap were observed in oxidized rutile, but in contrast, similar states could not be readily distinguished in reduced rutile. A band diagram of oxidized rutile is proposed.
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73.20.At Surface states, band structure, electron density of states
72.40.+w Photoconduction and photovoltaic effects
71.20.Ps Other inorganic compounds

A nondestructive analysis of the B diffusion in Ta–CoFeB–MgO–CoFeB–Ta magnetic tunnel junctions by hard x-ray photoemission

Xeniya Kozina, Siham Ouardi, Benjamin Balke, Gregory Stryganyuk, Gerhard H. Fecher, Claudia Felser, Shoji Ikeda, Hideo Ohno, and Eiji Ikenaga

Appl. Phys. Lett. 96, 072105 (2010); http://dx.doi.org/10.1063/1.3309702 (3 pages) | Cited 11 times

Online Publication Date: 17 February 2010

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This work reports on hard x-ray photoelectron spectroscopy (HAXPES) of CoFeB based tunnel junctions. Aim is to explain the role of the boron diffusion for the observed improvement of the tunneling magnetoresistance ratio with increasing annealing temperature. The high bulk sensitivity of HAXPES was used as a nondestructive technique to analyze CoFeB–MgO–CoFeB magnetic tunnel junctions. The investigated samples were processed at different annealing temperatures from 523 to 923 K. Hard x-ray core level spectroscopy reveals an enforced diffusion of boron from the CoFeB into the adjacent Ta layer with increasing annealing temperature. The dependence of the tunneling magnetoresistance on the annealing temperature is explained by the combined effects of an improved crystalline structure together with a change in the spin polarization at the Fermi energy caused by the removal of boron from the CoFeB layer and Ta diffusion at high annealing temperature.
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79.60.Jv Interfaces; heterostructures; nanostructures
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
68.35.Fx Diffusion; interface formation

Thermally formed conducting filaments in a single-crystalline NiO thin film

Masanori Kawai, Kimihiko Ito, Noriya Ichikawa, and Yuichi Shimakawa

Appl. Phys. Lett. 96, 072106 (2010); http://dx.doi.org/10.1063/1.3309709 (3 pages) | Cited 14 times

Online Publication Date: 17 February 2010

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Effects of thermal annealing on resistance switching properties of a single-crystalline NiO thin film grown on a Pt0.8Ir0.2 bottom electrode were investigated. Annealing the NiO thin film above 430 °C produced conducting filaments in the single-crystalline NiO and the resulting low-resistance state changed to a high-resistance state at an electrical reset bias voltage. Rearrangements of crystalline defects such as Ni vacancies play a role in forming the conducting filaments. This “thermal forming” is essentially the same as “electrical forming” but does not need large bias voltages to make the initially ON state in unipolar switching devices.
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73.61.Ng Insulators
81.40.Gh Other heat and thermomechanical treatments
61.72.jd Vacancies
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Current deep level transient spectroscopy analysis of AlInN/GaN high electron mobility transistors: Mechanism of gate leakage

W. Chikhaoui, J.-M. Bluet, M.-A. Poisson, N. Sarazin, C. Dua, and C. Bru-Chevallier

Appl. Phys. Lett. 96, 072107 (2010); http://dx.doi.org/10.1063/1.3326079 (3 pages) | Cited 14 times

Online Publication Date: 18 February 2010

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In order to assess possible mechanisms of gate reverse-bias leakage current in AlInN/GaN high electron mobility transistors (HEMTs) grown by metalorganic chemical-vapor deposition on SiC substrates, temperature-dependent current-voltage measurements combined with Fourier transform current deep level transient spectroscopy (FT-CDLTS) are performed in the temperature range of 200–400 K. In this range of temperature reverse-bias leakage current flow is found to be dominated by Poole–Frenkel emission. Based on CDLTS measurements, a model of leakage current transport via a trap state located at the AlInN/metal interface with an activation energy of 0.37 eV is suggested. The trap nature is shown to be an extended trap, most probably associated with dislocations in the AlInN barrier layer.
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85.30.Tv Field effect devices
85.30.Hi Surface barrier, boundary, and point contact devices
85.30.Kk Junction diodes
07.60.Rd Visible and ultraviolet spectrometers
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
71.55.Eq III-V semiconductors

Insight into the performance enhancement of double-gated polycrystalline silicon thin-film transistors with ultrathin channel

Zer-Ming Lin, Horng-Chih Lin, Wei-Chen Chen, and Tiao-Yuan Huang

Appl. Phys. Lett. 96, 072108 (2010); http://dx.doi.org/10.1063/1.3327336 (3 pages) | Cited 1 time

Online Publication Date: 18 February 2010

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In this letter, characteristics of independently-controllable double-gated polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with ultrathin channel are characterized and analyzed experimentally and theoretically. As compared with the single-gated mode where only one of the gates is used for driving the device, 1.3–2.1 fold increase in drive current is achieved under double-gated mode as the two gates are biased simultaneously for driving the device. A remarkable lowering of barrier height 7–12 meV in the latter case due to the coupling of the two gate biases is identified as the major origin for such performance enhancement.
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85.30.Tv Field effect devices
81.05.Cy Elemental semiconductors
79.40.+z Thermionic emission

Enhanced spin injection efficiency in a four-terminal quantum dots system

Ling Qin, Hai-Feng Lü, and Yong Guo

Appl. Phys. Lett. 96, 072109 (2010); http://dx.doi.org/10.1063/1.3298640 (3 pages) | Cited 3 times

Online Publication Date: 18 February 2010

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Within the scheme of quantum rate equations, we investigate the spin-resolved transport through a double quantum dot system with four ferromagnetic terminals. We find that the injection efficiency of spin-polarized electrons can be significantly improved. When the magnetization in one of four ferromagnetic terminals is antiparallel with the other three, the current polarization rate through one dot can be greatly enhanced, accompanied by the drastic decrease through the other one. The mechanism is the exchange interaction between electrons in the two dots, which can be a promising candidate for the improvement of the spin injection efficiency.
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73.63.Kv Quantum dots
72.25.Dc Spin polarized transport in semiconductors
71.70.Gm Exchange interactions

Coherent transport through a double donor system in silicon

J. Verduijn, G. C. Tettamanzi, G. P. Lansbergen, N. Collaert, S. Biesemans, and S. Rogge

Appl. Phys. Lett. 96, 072110 (2010); http://dx.doi.org/10.1063/1.3318271 (3 pages) | Cited 2 times

Online Publication Date: 19 February 2010

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In this letter, we describe the observation of the interference of conduction paths induced by two donors in a nanoscale silicon transistor, resulting in a Fano resonance. This demonstrates the coherent exchange of electrons between two donors. In addition, the phase difference between the two conduction paths can be tuned by means of a magnetic field, in full analogy to the Aharonov–Bohm effect. One of the crucial ingredients for donor based quantum computation is phase coherent manipulation of electrons. This has not been achieved as yet, and this work presents a stepping stone.
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85.30.Tv Field effect devices

In situ photoemission spectroscopy study on formation of HfO2 dielectrics on epitaxial graphene on SiC substrate

Q. Chen, H. Huang, W. Chen, A. T. S. Wee, Y. P. Feng, J. W. Chai, Z. Zhang, J. S. Pan, and S. J. Wang

Appl. Phys. Lett. 96, 072111 (2010); http://dx.doi.org/10.1063/1.3327834 (3 pages) | Cited 5 times

Online Publication Date: 19 February 2010

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High quality HfO2 dielectrics have been grown on epitaxial graphene on 4H-SiC substrates and have been studied by using in situ x-ray photoemission spectroscopy. The in situ thermal treatment shows that the HfO2/graphene/4H-SiC heterojunctions have good thermal stability up to 650 °C. A shift of core-level spectra from graphene layer implies that charge transfer takes place at the interface. The high thermal stability and sufficient barrier heights between HfO2 and graphene indicate that high-k dielectric grown on graphene is very promising for the development of graphene-based electronic devices.
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81.15.Cd Deposition by sputtering
68.55.A- Nucleation and growth
68.60.Dv Thermal stability; thermal effects
79.60.Jv Interfaces; heterostructures; nanostructures
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
77.55.D- High-permittivity gate dielectric films
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