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23 Jan 2012

Volume 100, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 041101 (2012); http://dx.doi.org/10.1063/1.3675885 (4 pages)

Seung Ho Choi and Young L. Kim
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Cholesteric liquid crystal-carbon nanotube hybrid architectures for gas detection

Chin-Kai Chang, Shih-Wen Chiu, Hui-Lung Kuo, and Kea-Tiong Tang

Appl. Phys. Lett. 100, 043501 (2012); http://dx.doi.org/10.1063/1.3679680 (3 pages) | Cited 5 times

Online Publication Date: 23 January 2012

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The ability of a hybrid material that is based on cholesteric liquid crystal and carbon nanotube to detect acetone vapor is investigated. We find that the phase transition in this cholesteric liquid crystal-carbon nanotube hybrid will enable carbon nanotube to form conducting networks under the higher vapor concentration. This cholesteric liquid crystal-carbon nanotube hybrid exhibits an obvious change in reflected color and electrical resistance in the early and later stages of gas diffusion, respectively. This hybrid architecture has potential application as a gas sensor with a high dynamic range.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
73.61.Wp Fullerenes and related materials

Time-resolved photocurrents in quantum well/dot infrared photodetectors with different optical coupling structures

Zhi-Hui Chen, S. Hellström, Zhong-Yuan Yu, Min Qiu, and Y. Fu

Appl. Phys. Lett. 100, 043502 (2012); http://dx.doi.org/10.1063/1.3678637 (5 pages)

Online Publication Date: 23 January 2012

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Temporal developments of photocurrents excited by an infrared radiation pulse in quantum well/dot infrared photodetectors with different optical coupling structures have been theoretically studied. It is shown that the light diffraction in a conventional reflective grating structure is a near-field effect containing severe crosstalk from neighboring pixels. A concave reflector not only eliminates the crosstalk but also strongly diffracts and focuses the incident electric field into deep active layers, which significantly increases the photocurrents in the photodetectors.
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72.40.+w Photoconduction and photovoltaic effects
78.67.De Quantum wells
78.67.Hc Quantum dots
85.60.Gz Photodetectors (including infrared and CCD detectors)

Effect of channel widths on negative shift of threshold voltage, including stress-induced hump phenomenon in InGaZnO thin-film transistors under high-gate and drain bias stress

Sung-Hwan Choi and Min-Koo Han

Appl. Phys. Lett. 100, 043503 (2012); http://dx.doi.org/10.1063/1.3679109 (3 pages) | Cited 4 times

Online Publication Date: 23 January 2012

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We investigated the degradation of indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs) for various channel widths under high-gate and drain bias stress. The threshold voltage of IGZO TFT with wide-channel width (W > 100 μm) was significantly shifted. This included stress-induced hump-effect in a negative direction after the stress, whereas IGZO TFT with narrow-channel width (W < 100 μm) shifted in a positive direction. This phenomenon may be attributed to the hole trapping into the back-interface region. In order to enhance the reliability of IGZO TFTs, we developed and verified that the multiple-channel device showed better bias-temperature stability (ΔVTH: −0.1 V), whereas the single-channel device exhibited a −0.4 VΔVTH shift.
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85.30.Tv Field effect devices

Vertical waveguides integrated with silicon photodetectors: Towards high efficiency and low cross-talk image sensors

Turgut Tut, Yaping Dan, Peter Duane, Young Yu, Munib Wober, and Kenneth B. Crozier

Appl. Phys. Lett. 100, 043504 (2012); http://dx.doi.org/10.1063/1.3678019 (3 pages)

Online Publication Date: 24 January 2012

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We describe the experimental realization of vertical silicon nitride waveguides integrated with silicon photodetectors. The waveguides are embedded in a silicon dioxide layer. Scanning photocurrent microscopy is performed on a device containing a waveguide, and on a device containing the silicon dioxide layer, but without the waveguide. The results confirm the waveguide’s ability to guide light onto the photodetector with high efficiency. We anticipate that the use of these structures in image sensors, with one waveguide per pixel, would greatly improve efficiency and significantly reduce inter-pixel crosstalk.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
42.79.Gn Optical waveguides and couplers

Ceasing of voltage switching amongst graphitic shells in multiwalled carbon nanotubes: A route toward stability

Neha Kulshrestha, Abhishek Misra, Reeti Bajpai, Soumyendu Roy, and D. S. Misra

Appl. Phys. Lett. 100, 043505 (2012); http://dx.doi.org/10.1063/1.3679397 (4 pages)

Online Publication Date: 24 January 2012

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Weakly interacting graphitic shells of different resistivities within multiwalled carbon nanotubes (MWNTs) cause instability in current and thus limit their reliability for electronic device applications. We here demonstrate voltage switching amongst graphitic shells of MWNTs by applying current sweeps with observed switching time in the range of 100–400 ms. We further demonstrate ceasing of this switching behaviour by local metal deposition on the MWNTs. After metal deposition, the graphitic shells behave like resistive wires connected altogether. This concept of metal deposition benefits in the higher conductivity and stable currents for MWNTs and proves their strong candidature as interconnecting wires.
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81.05.U- Carbon/carbon-based materials
72.60.+g Mixed conductivity and conductivity transitions
73.63.Fg Nanotubes
81.07.De Nanotubes

Large anisotropic remnant magnetization tunability in (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 multiferroic epitaxial heterostructures

Yuanjun Yang, Meng Meng Yang, Z. L. Luo, Haoliang Huang, Haibo Wang, J. Bao, Chuansheng Hu, Guoqiang Pan, Yiping Yao, Yukuai Liu, X. G. Li, Sen Zhang, Y. G. Zhao, and C. Gao

Appl. Phys. Lett. 100, 043506 (2012); http://dx.doi.org/10.1063/1.3676044 (3 pages) | Cited 3 times

Online Publication Date: 24 January 2012

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A large anisotropic remnant magnetization tunability was observed in multiferroic (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 (LSMO/PMN-0.3PT) epitaxial heterostructures. The remnant magnetization along [100] direction was suppressed by an electric field applied to the substrate while the remnant magnetization along [01math] was enhanced. The tunabilities of the remnant magnetization along the [100] and [01math] directions are about −17.9% and +157% under electric field of +7.27 kV/cm, respectively. This large anisotropic remnant magnetization tunability may find potential applications in the electrically written and magnetically read memories.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.85.+t Magnetoelectric effects, multiferroics
75.30.Gw Magnetic anisotropy

A Monte Carlo study of the low resistance state retention of HfOx based resistive switching memory

Shimeng Yu, Yang Yin Chen, Ximeng Guan, H.-S. Philip Wong, and Jorge A. Kittl

Appl. Phys. Lett. 100, 043507 (2012); http://dx.doi.org/10.1063/1.3679610 (4 pages) | Cited 5 times

Online Publication Date: 24 January 2012

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A Monte Carlo simulator is developed to investigate the low resistance state (LRS) retention of HfOx based resistive switching memory. The simulator tracks the evolution of oxygen ions and oxygen vacancies by choosing the occurrence of the generation/recombination/migration processes based on their corresponding probability at each simulation step. The current through the resulting conductive filament (CF) configuration is then calculated by a trap-assisted-tunneling solver. The simulation results are corroborated with experimental data. The LRS retention is found to be CF size dependent, and its variability is suggested to be intrinsic due to the stochastic nature of the CF dissolution. The tail bits of the failure time distribution become a limiting factor of the device reliability in a large memory array.
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84.30.Sk Pulse and digital circuits

Single charge sensing and transport in double quantum dots fabricated from commercially grown Si/SiGe heterostructures

C. Payette, K. Wang, P. J. Koppinen, Y. Dovzhenko, J. C. Sturm, and J. R. Petta

Appl. Phys. Lett. 100, 043508 (2012); http://dx.doi.org/10.1063/1.3678043 (3 pages) | Cited 3 times

Online Publication Date: 25 January 2012

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We perform quantum Hall measurements on three types of commercially available modulation-doped Si/SiGe heterostructures to determine their suitability for depletion gate defined quantum dot devices. By adjusting the growth parameters, we are able to achieve electron gases with charge densities 1–3 × 1011/cm2 and mobilities in excess of 100 000 cm2/Vs. Double quantum dot devices fabricated on these heterostructures show clear evidence of single charge transitions as measured in dc transport and charge sensing and exhibit electron temperatures of 100 mK in the single quantum dot regime.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.43.-f Quantum Hall effects
72.20.Fr Low-field transport and mobility; piezoresistance
73.21.La Quantum dots

The influence of Al composition on point defect incorporation in AlGaN

T. A. Henry, A. Armstrong, A. A. Allerman, and M. H. Crawford

Appl. Phys. Lett. 100, 043509 (2012); http://dx.doi.org/10.1063/1.3679681 (4 pages) | Cited 1 time

Online Publication Date: 26 January 2012

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The effects of point defects in AlxGa1−xN (x = 0-0.33) films with similar threading dislocation densities were investigated. The epitaxial layers were grown under conditions applicable to laser-diode quantum wells, barriers and waveguide layers. The evolution of deep level defect energy and density were quantitatively tracked versus x using deep level optical spectroscopy. Three defect levels were observed, whose defect density increased with x. The energy level of a defect suspected to be related to the group-III vacancy appeared to track the vacuum level, and a near-valence band defect level deepened with respect to the valence band maximum.
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71.55.Eq III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.jd Vacancies
68.55.ag Semiconductors
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
71.20.Nr Semiconductor compounds

Spin transport in memristive devices

Hyuk-Jae Jang, Oleg A. Kirillov, Oana D. Jurchescu, and Curt A. Richter

Appl. Phys. Lett. 100, 043510 (2012); http://dx.doi.org/10.1063/1.3679114 (4 pages) | Cited 2 times

Online Publication Date: 26 January 2012

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We report on electron spin transport through electrochemically precipitated copper filaments formed in TaOx memristive devices consisting of Co/TaOx/Cu/Py with crossbar-type electrode geometry. The devices show memristive behavior having a typical OFF/ON resistance ratio of 105. Magnetoresistance measurements performed by sweeping an external magnetic field clearly indicate spin transport through an electrochemically formed copper nano-filament as long as 16 nm in the memristive ON-state at 77 K. Spin transport vanishes in the OFF-state. These data are strong evidence that the fundamental switching mechanism in these metal-oxide devices is the formation of continuous metallic conduction paths.
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84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)
85.35.-p Nanoelectronic devices
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
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Electric resonance-rectifier circuit for renewable energy conversion

C. Boström, B. Ekergård, and M. Leijon

Appl. Phys. Lett. 100, 043511 (2012); http://dx.doi.org/10.1063/1.3680097 (3 pages) | Cited 1 time

Online Publication Date: 27 January 2012

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Variable speed generators are used more frequently for converting the energy from renewable energy sources to electric energy. The power production form a variable speed generator is dependent on the electrical damping of the generator. In this paper, a resonance circuit connected to a direct driven linear generator used for wave energy utilization is investigated. At resonance, the electrical damping in the generator increases which results in an increased power production. The results show that resonance can be achieved with the suggested circuit.
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84.30.Jc Power electronics; power supply circuits

Improved photoresponse of InAs/GaAs quantum dot infrared photodetectors by using GaAs1−xSbx strain reducing layer

Chia-Tze Huang, Yu-Cheng Chen, and Si-Chen Lee

Appl. Phys. Lett. 100, 043512 (2012); http://dx.doi.org/10.1063/1.3679132 (4 pages) | Cited 2 times

Online Publication Date: 27 January 2012

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The effect of a GaAs1−xSbx strain reducing layer on the performance of InAs/GaAs quantum-dot infrared photodetectors was investigated. The results suggest that increasing Sb composition x from 0 to 0.2 leads to an enhanced peak response and a pronounced narrowing of the band width of the spectral response from 3.3 to 1.5 μm. For a photodetector with GaAs0.8Sb0.2 strain reducing layer, the best responsivity obtained is 533 mA/W, which is 380 times higher than that without strain reducing layer. In addition, the operating temperature increases from 50 to 90 K when increasing Sb composition from 0 to 0.2.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.30.-z Semiconductor devices

Numerical analysis on the effects of bandgap energy and polarization of electron blocking layer in near-ultraviolet light-emitting diodes

Yen-Kuang Kuo (郭艷光), Yu-Han Chen (陳鈺涵), Jih-Yuan Chang (張誌原), and Miao-Chan Tsai (蔡妙嬋)

Appl. Phys. Lett. 100, 043513 (2012); http://dx.doi.org/10.1063/1.3679180 (3 pages) | Cited 1 time

Online Publication Date: 27 January 2012

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The influences of bandgap energy and polarization of the electron blocking layer (EBL) in near-ultraviolet light-emitting diodes (NUV LEDs) are systematically investigated. Design curves for the output power of NUV LEDs as a function of bandgap energy and polarization of EBL are provided. The simulation results show that, when the bandgap of the EBL increases, the polarization and polarization-induced charge increase accordingly. Both mechanisms have opposite effects for the EBL in confining electrons. The NUV LEDs with an EBL of large bandgap or small polarization have improved performance due to the enhanced efficiency of electron confining and hole injection.
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85.60.Jb Light-emitting devices
02.60.-x Numerical approximation and analysis
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