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28 Nov 2011

Volume 99, Issue 22, Articles (22xxxx)

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Appl. Phys. Lett. 99, 221902 (2011); http://dx.doi.org/10.1063/1.3663578 (3 pages)

Aparna Deshpande, Kai Felix Braun, and Saw-Wai Hla
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Influence of threading dislocation density on early degradation in AlGaN/GaN high electron mobility transistors

M. Ťapajna, S. W. Kaun, M. H. Wong, F. Gao, T. Palacios, U. K. Mishra, J. S. Speck, and M. Kuball

Appl. Phys. Lett. 99, 223501 (2011); http://dx.doi.org/10.1063/1.3663573 (3 pages) | Cited 7 times

Online Publication Date: 28 November 2011

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Early stage degradation of AlGaN/GaN high electron mobility transistors (HEMTs) with different threading dislocation densities (TDDs) submitted to off-state voltage bias stress was studied. It was found that, for the stress conditions used, HEMTs with TDD ∼1010 cm−2 show pronounced degradation in terms of maximum drain current, gate-lag, and trap generation measured by drain current trapping characteristics, a slight degradation in gate leakage was observed also for HEMTs with TDD of ∼108 cm−2, and no significant degradation for devices with TDD in the ∼107 cm−2 range. The results illustrate the importance of TDD for degradation and reliability of AlGaN/GaN HEMTs.
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85.30.Tv Field effect devices

Fully integrable magnetic field sensor based on delta-E effect

B. Gojdka, R. Jahns, K. Meurisch, H. Greve, R. Adelung, E. Quandt, R. Knöchel, and F. Faupel

Appl. Phys. Lett. 99, 223502 (2011); http://dx.doi.org/10.1063/1.3664135 (3 pages) | Cited 3 times

Online Publication Date: 28 November 2011

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A fully integrable magnetic field sensor based on magnetic microelectromechanical systems is presented. The approach yields high application potential since it is compatible with standard micromachining techniques, operates at room-temperature, and provides high bandwidth and vector field capability. The demonstrator presented in this work consists of a tipless commercial atomic force microscope cantilever which is coated with an amorphous thin film layer of (Fe90Co10)78Si12B10. Amplitude and frequency of magnetic fields are measured via the modulation of the oscillation of the microcantilever via the delta-E effect of the FeCoSiB coating.
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07.55.-w Magnetic instruments and components
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Effect of gate dielectric scaling in nanometer scale vertical thin film transistors

M. Moradi, A. A. Fomani, and A. Nathan

Appl. Phys. Lett. 99, 223503 (2011); http://dx.doi.org/10.1063/1.3664217 (3 pages) | Cited 1 time

Online Publication Date: 28 November 2011

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A short channel vertical thin film transistor (VTFT) with 30 nm SiNx gate dielectric is reported for low voltage, high-resolution active matrix applications. The device demonstrates an ON/OFF current ratio as high as 109, leakage current in the fA range, and a sub-threshold slope steeper than 0.23 V/dec exhibiting a marked improvement with scaling of the gate dielectric thickness.
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85.30.Tv Field effect devices

Investigation on interface related charge trap and loss characteristics of high-k based trapping structures by electrostatic force microscopy

Chenxin Zhu, Zhongguang Xu, Zongliang Huo, Rong Yang, Zhiwei Zheng, Yanxiang Cui, Jing Liu, Yumei Wang, Dongxia Shi, Guangyu Zhang, Fanghua Li, and Ming Liu

Appl. Phys. Lett. 99, 223504 (2011); http://dx.doi.org/10.1063/1.3664222 (3 pages) | Cited 1 time

Online Publication Date: 28 November 2011

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Charge trap and loss characteristics of high-k based trapping layer structures are investigated by electrostatic force microscopy, which proves that the interfaces provide dominate trap sites. The effects of post-deposition anneal of HfO2 and Al2O3 single layer are determined. Based on aforementioned findings, we demonstrate the HfO2/Al2O3 bi-layers trapping structure with improved performance. The lateral charge spreading properties are also evaluated by extracted diffusion coefficients to further understand the interface effect. The study may provide insights into fundamental assessment and optimization for charge trapping structures, especially for high-density NAND flash applications.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

A generator with nonlinear spring oscillator to provide vibrations of multi-frequency

Bin Yang, Jingquan Liu, Gang Tang, Jiangbo Luo, Chunsheng Yang, and Yigui Li

Appl. Phys. Lett. 99, 223505 (2011); http://dx.doi.org/10.1063/1.3664223 (3 pages) | Cited 3 times

Online Publication Date: 28 November 2011

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A piezoelectric generator with nonlinear spring oscillator is proposed to provide multiple resonant modes for operation and improve conversion efficiency. In order to scavenge the vibration energy of multiple frequencies from a certain vibration source, two types of nonlinear springs have been employed and tested. The maximum output power of 5, 17.83, and 23.39 μW for the nonlinear spring of 8.3 N/m with 1 g acceleration has been obtained under the resonant frequency of 89, 104, and 130 Hz, respectively. Its total output power of 46.22 μW is obviously larger than the one of 28.35 μW for traditional second-order spring-mass linear system.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
84.30.Ng Oscillators, pulse generators, and function generators
84.70.+p High-current and high-voltage technology: power systems; power transmission lines and cables

Role of oxygen in the OFF-state degradation of AlGaN/GaN high electron mobility transistors

Feng Gao, Bin Lu, Libing Li, Stephen Kaun, James S. Speck, Carl. V. Thompson, and Tomás Palacios

Appl. Phys. Lett. 99, 223506 (2011); http://dx.doi.org/10.1063/1.3665065 (3 pages) | Cited 3 times

Online Publication Date: 29 November 2011

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The physical degradation of AlGaN/GaN high electron mobility transistors during OFF-state stress experiments has been systematically studied. Oxide particles and stringers were found to form along the gate edge of stressed devices. When the gate electrode is removed, pits are seen to have formed underneath each particle. The observed room-temperature oxidation process is strongly dependent on the duration of the electrical stressing and the electric field. Moreover, the oxidation can be significantly reduced in vacuum (3 × 10−5 Torr), with a corresponding 30% reduction of current collapse. Finally, a degradation process with electric-field-driven oxidation of the AlGaN surface has been proposed.
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85.30.Tv Field effect devices

Effects of non-ideal energy selective contacts and experimental carrier cooling rate on the performance of an indium nitride based hot carrier solar cell

P. Aliberti, Y. Feng, S. K. Shrestha, M. A. Green, G. Conibeer, L. W. Tu, P. H. Tseng, and R. Clady

Appl. Phys. Lett. 99, 223507 (2011); http://dx.doi.org/10.1063/1.3663862 (3 pages) | Cited 3 times

Online Publication Date: 29 November 2011

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The performance of an InN based hot carrier solar cell with a bulk InN absorber has been evaluated using an innovative approach that takes into account absorber energy-momentum dispersion relations, energy conservation, Auger recombination and impact ionization mechanisms simultaneously. The non ideality of the energy selective filters has also been included in the model. In order to obtain practical achievable values of conversion efficiency, the actual thermalisation velocity of hot carriers in InN has been measured using time resolved photoluminescence. Results of the computations shown limiting efficiencies of 24% for 1000 suns and 36.2% for maximal concentration.
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88.40.hj Efficiency and performance of solar cells
79.20.Fv Electron impact: Auger emission
72.20.Ht High-field and nonlinear effects
88.40.jm Thin film III-V and II-VI based solar cells

Milliwatt-level output power in the sub-terahertz range generated by photomixing in a GaAs photoconductor

E. Peytavit, S. Lepilliet, F. Hindle, C. Coinon, T. Akalin, G. Ducournau, G. Mouret, and J.-F. Lampin

Appl. Phys. Lett. 99, 223508 (2011); http://dx.doi.org/10.1063/1.3664635 (3 pages) | Cited 1 time

Online Publication Date: 29 November 2011

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It is shown from accurate on-wafer measurement that continuous wave output powers of 1.2 mW at 50 GHz and 0.35 mW at 305 GHz can be generated by photomixing in a low temperature grown GaAs photoconductor using a metallic mirror Fabry-Pérot cavity. The output power is improved by a factor of about 100 as compared to the previous works on GaAs photomixers. A satisfactory agreement between the theory and the experiment is obtained in considering both the contribution of the holes and the electrons to the total photocurrent.
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72.40.+w Photoconduction and photovoltaic effects

Entanglement generation and quantum state transfer between two quantum dot molecules mediated by quantum bus of plasmonic circuits

Mu-Tian Cheng, Xiao-San Ma, Ya-Qin Luo, Pei-Zhen Wang, and Guang-Xing Zhao

Appl. Phys. Lett. 99, 223509 (2011); http://dx.doi.org/10.1063/1.3664637 (3 pages) | Cited 1 time

Online Publication Date: 29 November 2011

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We investigate theoretically entanglement generation between two distant quantum dot molecules (QDMs) mediated by quantum bus of metal nanoring with surface plasmon polaritons. We show that the two QDMs can be in an entangled state by adjusting the external gate voltage and the coupling strength between the QDMs and the surface plasmon polaritons. The quantum state transfer between the two QDMs is also discussed. Our scheme may find applications in on-chip quantum networks and integrated optoelectronics devices.
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73.21.La Quantum dots
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

The influence of charged InAs quantum dots on the conductance of a two-dimensional electron gas: Mobility vs. carrier concentration

B. Marquardt, A. Beckel, A. Lorke, A. D. Wieck, D. Reuter, and M. Geller

Appl. Phys. Lett. 99, 223510 (2011); http://dx.doi.org/10.1063/1.3665070 (3 pages)

Online Publication Date: 30 November 2011

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Using time-resolved transport spectroscopy, we investigate the influence of charge-tunable InAs quantum dots (QDs) on the conductance of a nearby two-dimensional electron gas (2DEG). Loading successively electrons into the self-assembled QDs decreases the carrier concentration and mobility in the 2DEG. We are able to quantify how these transport properties change for each additional charge in the s- or p-shell. It is found that mobility and carrier concentration contribute equally to the overall change in conductance.
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73.63.Kv Quantum dots
78.47.D- Time resolved spectroscopy (>1 psec)
72.20.Fr Low-field transport and mobility; piezoresistance
73.21.La Quantum dots

Direct measurement of nanowire Schottky junction depletion region

E. Koren, N. Berkovitch, O. Azriel, A. Boag, Y. Rosenwaks, E. R. Hemesath, and L. J. Lauhon

Appl. Phys. Lett. 99, 223511 (2011); http://dx.doi.org/10.1063/1.3665182 (3 pages) | Cited 2 times

Online Publication Date: 30 November 2011

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We have used Kelvin probe force microscopy to measure the surface potential of both doped and unintentionally doped (UID) Si nanowires Schottky junctions. The imaging of the Schottky junction together with 3D potential simulation and consideration of the convolution of the scanning tip enables us to determine the real surface potential. Highly doped n-type nanowires show smaller depletion regions compared to UID nanowires, and their potential profile was successfully modeled. For the UID nanowires, the measured potential profiles and, consequently, the depletion region indicate the presence of bulk deep traps with a concentration of ∼5 × 1017 cm−3.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts

Even-odd symmetry and the conversion efficiency of ideal and practical graphene transistor frequency multipliers

Kristen N. Parrish and Deji Akinwande

Appl. Phys. Lett. 99, 223512 (2011); http://dx.doi.org/10.1063/1.3664112 (3 pages) | Cited 1 time

Online Publication Date: 30 November 2011

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The conversion efficiency of field-effect transistors with even-odd symmetry is elucidated in this work. From symmetry considerations, this work reveals that even symmetry, due to electron-hole symmetry in graphene, affords efficient even-harmonic multiplication. Odd symmetry, associated with linear charge transport, affords suppression of odd-harmonic signals. For the ideal symmetric transistor multiplier, conversion efficiency with relatively large power gain is achievable, while for practical graphene transistors, the efficiency can be substantially less than unity due to non-idealities such as contact resistance, high impurity densities, and low gate capacitance. In the quantum capacitance limit of graphene transistor, near-lossless conversion efficiency is available.
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85.30.Tv Field effect devices
85.65.+h Molecular electronic devices
84.30.-r Electronic circuits

Role of mechanical stress in the resistance drift of Ge2Sb2Te5 films and phase change memories

M. Rizzi, A. Spessot, P. Fantini, and D. Ielmini

Appl. Phys. Lett. 99, 223513 (2011); http://dx.doi.org/10.1063/1.3664631 (3 pages) | Cited 4 times

Online Publication Date: 30 November 2011

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In a phase change memory (PCM), the device resistance increases slowly with time after the formation of the amorphous phase, thus affecting the stability of stored data. This work investigates the resistance drift in thin films of amorphous Ge2Sb2Te5 and in PCMs, demonstrating a common kinetic of drift in stressed/unstressed films and in the nanometer-size active volume of a PCM with different stress levels developed via stressor layers. It is concluded that stress is not the root cause of PCM drift, which is instead attributed to intrinsic structural relaxation due to the disordered, metastable nature of the amorphous chalcogenide phase.
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85.30.De Semiconductor-device characterization, design, and modeling
84.30.Sk Pulse and digital circuits

Improvement of transfer characteristic for carbon nanotube field effect transistor with poly crystalline PbZrxTi1-xO3 gate by ionic liquid

Shogo Kataoka, Takayuki Arie, and Seiji Akita

Appl. Phys. Lett. 99, 223514 (2011); http://dx.doi.org/10.1063/1.3665186 (3 pages) | Cited 1 time

Online Publication Date: 1 December 2011

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We have investigated the transfer characteristics of carbon nanotube (CNT) field effect transistors (FETs) with poly crystalline PbZrxTi1-xO3 (PZT) gate with the application of ionic liquid. The transconductance of the devices was improved more than three times by applying ionic liquid with high reproducibility. In addition, the FETs including metallic CNTs are preferable to obtain clockwise hysteresis induced by polarization reversal of the PZT gate because of the efficient electric field concentration just beneath the CNT channel. The low voltage operation around 1 V has been achieved due to the electric field concentration even for the polycrystalline PZT gate.
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85.30.Tv Field effect devices
85.75.Hh Spin polarized field effect transistors
77.22.Ej Polarization and depolarization
77.80.Dj Domain structure; hysteresis

Enhanced performance of hybrid solar cells using longer arms of quantum cadmium selenide tetrapods

Kyu-Sung Lee, Inho Kim, Sravani Gullapalli, Michael S. Wong, and Ghassan E. Jabbour

Appl. Phys. Lett. 99, 223515 (2011); http://dx.doi.org/10.1063/1.3662839 (3 pages) | Cited 1 time

Online Publication Date: 1 December 2011

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We demonstrate that enhanced device performance of hybrid solar cells based on tetrapod (TP)-shaped cadmium selenide (CdSe) nanoparticles and conjugated polymer of poly (3-hexylthiophene) (P3HT) can be obtained by using longer armed tetrapods which aids in better spatial connectivity, thus decreasing charge hopping events which lead to better charge transport. Longer tetrapods with 10 nm arm length lead to improved power conversion efficiency of 1.12% compared to 0.80% of device having 5 nm short-armed tetrapods:P3HT photoactive blends.
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88.40.J- Types of solar cells

On the “U-shaped” continuum of band edge states at the Si/SiO2 interface

J. T. Ryan, R. G. Southwick, J. P. Campbell, K. P. Cheung, C. D. Young, and J. S. Suehle

Appl. Phys. Lett. 99, 223516 (2011); http://dx.doi.org/10.1063/1.3664772 (3 pages) | Cited 1 time

Online Publication Date: 1 December 2011

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The historical and near universal acceptance that a U-shaped continuum of band edge states intrinsically exists at the Si/SiO2 boundary is re-examined. Using a recently developed interface defect spectroscopy method, we show that the U-shape continuum of band edge states does not exist in high quality Si/SiO2 interfaces. We then show that the U-shape continuum of band edge state can be readily measured in relatively poor quality devices, proving that the absence of states in the high quality sample is not due to measurement limitations. This observation calls into question firmly established beliefs about the intrinsic nature of the continuum.
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73.20.At Surface states, band structure, electron density of states
71.55.-i Impurity and defect levels
73.20.Hb Impurity and defect levels; energy states of adsorbed species

Bias application hard x-ray photoelectron spectroscopy study of forming process of Cu/HfO2/Pt resistive random access memory structure

T. Nagata, M. Haemori, Y. Yamashita, H. Yoshikawa, Y. Iwashita, K. Kobayashi, and T. Chikyow

Appl. Phys. Lett. 99, 223517 (2011); http://dx.doi.org/10.1063/1.3664781 (3 pages) | Cited 6 times

Online Publication Date: 1 December 2011

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The forming process of Cu/HfO2/Pt, which is an oxide based resistive random access memory (ReRAM), structure that exhibited resistance switching behavior at a voltage of 1.3 V was investigated by hard x-ray photoelectron spectroscopy under bias operation. A bias application to the structure reduced the Cu2O bonding state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2, providing the evidence of Cu2O reduction and Cu diffusion into the HfO2 layer. We directly observed Cu diffusion at the Cu/HfO2 interface under device operation, which is the evidence of the metal filament formation in the oxide-based ReRAM.
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84.30.Sk Pulse and digital circuits
79.60.Jv Interfaces; heterostructures; nanostructures

Electrical characteristics of asymmetrical silicon nanowire field-effect transistors

Soshi Sato, Kuniyuki Kakushima, Kenji Ohmori, Kenji Natori, Keisaku Yamada, and Hiroshi Iwai

Appl. Phys. Lett. 99, 223518 (2011); http://dx.doi.org/10.1063/1.3665261 (3 pages) | Cited 1 time

Online Publication Date: 2 December 2011

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This letter reports the electrical characteristics of nonuniform silicon nanowire nFETs with asymmetric source and drain widths. For electrostatic properties, reduced drain-induced barrier lowering (DIBL) is achieved in a device in which the source is wider than the drain. For carrier transport properties, higher values of surface-roughness-limited mobility (μSR) are obtained in the sample with the wider drain size. Our electrostatic model shows that the concentration of lines of electric force is relaxed near the wider source edge, which results in smaller DIBL. The asymmetric μSR is attributed to the channel surface morphology with (110)- and (100)-faceted surfaces.
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85.30.Tv Field effect devices
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