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13 Aug 2007

Volume 91, Issue 7, Articles (07xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 071906 (2007); http://dx.doi.org/10.1063/1.2753092 (3 pages)

Cheng Zhang, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishta
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Largely variable electroluminescence efficiency with current and temperature in a blue (In, Ga)N multiple-quantum-well diode

Y. Yamane, K. Fujiwara, and J. K. Sheu

Appl. Phys. Lett. 91, 073501 (2007); http://dx.doi.org/10.1063/1.2769754 (3 pages) | Cited 1 time

Online Publication Date: 13 August 2007

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Electroluminescence (EL) efficiency of a blue In0.3Ga0.7N multiple-quantum-well diode has been investigated as a function of current between 0.001 and 20 mA at various temperatures (20–300 K). The low-temperature EL quenching previously observed below 100 K at a driving current of 20 mA does not occur at 0.001 mA and is found to be strongly dependent on the current level. Largely variable temperature-dependent EL quantum efficiency with current suggests that the injected carrier capture efficiency by radiative recombination centers plays a decisive role for determination of the EL efficiency under the forward bias condition.
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78.60.Fi Electroluminescence
85.60.Jb Light-emitting devices

Propagation and spatiotemporal summation of electrical pulses in semiconductor nerve fibers

A. Samardak, S. Taylor, A. Nogaret, G. Hollier, J. Austin, and D. A. Ritchie

Appl. Phys. Lett. 91, 073502 (2007); http://dx.doi.org/10.1063/1.2770773 (3 pages) | Cited 3 times

Online Publication Date: 13 August 2007

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The authors report the propagation and analog summation of electrical impulses in artificial nerve fibers made of submicron p-n wires. These wires model the longitudinal conductivities of K+ and Na+ ions inside and outside a nerve capillary as well as the transverse capacitance of the nerve membrane and the nonlinear conductance of its ion channels. They demonstrate the summation and annihilation of electrical impulses at room temperature which form the basis for making spike timing neural networks.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Selective epitaxial germanium on silicon-on-insulator high speed photodetectors using low-temperature ultrathin Si0.8Ge0.2 buffer

T. H. Loh, H. S. Nguyen, R. Murthy, M. B. Yu, W. Y. Loh, G. Q. Lo, N. Balasubramanian, D. L. Kwong, J. Wang, and S. J. Lee

Appl. Phys. Lett. 91, 073503 (2007); http://dx.doi.org/10.1063/1.2769750 (3 pages) | Cited 16 times

Online Publication Date: 13 August 2007

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The authors report the performance of selective epitaxial Ge (400 nm) on Si-on-insulator p-i-n mesa-type normal incidence photodiodes using ∼ 14 nm low-temperature Si0.8Ge0.2 buffer without cyclic annealing. At −1 V, very low bulk dark current densities of 1.5–2 mA/cm2 were obtained indicating good material quality, and the peripheral surface leakage current densities were 14–19.5 μA/cm. For 28 μm diameter round photodiode, the highest achieved external quantum efficiencies at −5 V were 27%, 9%, and 2.9% for 850 nm, 1.3 μm, and 1.56 μm optical wavelengths, respectively. 15×15 μm2 square photodiode has 3 dB bandwidth ≥ 15 GHz at −1 V. Good performance was achieved without high-temperature annealing, suggesting easy integration of Ge/Si photodiode unto existing complementary metal-oxide-semiconductor process.
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85.60.Dw Photodiodes; phototransistors; photoresistors

In-plane bistable nematic liquid crystal devices based on nanoimprinted surface relief

Jin Seog Gwag, Jun-ichi Fukuda, Makoto Yoneya, and Hiroshi Yokoyama

Appl. Phys. Lett. 91, 073504 (2007); http://dx.doi.org/10.1063/1.2769946 (3 pages) | Cited 35 times

Online Publication Date: 13 August 2007

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The authors present a bistable nematic device, using a fourfold symmetrical bidirection nanometer-scale surface grating fabricated by the nanoimprinting lithography. The bistability is achieved by a composite action between two orthogonal surface undulations, which tend to stabilize the nematic director along either of the two diagonal axes. The switching between the bistable states is easily driven by orthogonal in-plane electric fields. A recent model of groove-induced surface anchoring due to Fukuda et al. [Phys. Rev. Lett. 98, 187803 (2007) ] accounts for the azimuthal bistability in the present system.
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42.79.Kr Display devices, liquid-crystal devices

Phase change memory cell with an upper amorphous nitride silicon germanium heating layer

Feng Rao, Zhitang Song, Liangcai Wu, Min Zhong, Songlin Feng, and Bomy Chen

Appl. Phys. Lett. 91, 073505 (2007); http://dx.doi.org/10.1063/1.2771053 (3 pages) | Cited 13 times

Online Publication Date: 13 August 2007

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The advanced phase change memory (PCM) cell with an amorphous nitride silicon germanium (SiGexNy) upper heating layer was fabricated. Applying a proper height of reset voltage pulse to the cell, the amorphous SiGexNy heating layer was crystallized along with the melt of the Ge1Sb2Te4 layer. Then the SiGexNy heating layer preserved its crystalline state during the successive programming cycles. With this crystalline SiGexNy heating layer, the set and reset threshold voltage values were reduced, which enhanced the heat efficiency and decreased the power consumption of the PCM cell. Meanwhile, the PCM cell showed good endurance characteristics.
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85.30.-z Semiconductor devices
84.30.Sk Pulse and digital circuits

Selectable resistance-area product by dilute highly charged ion irradiation

J. M. Pomeroy, H. Grube, A. C. Perrella, and J. D. Gillaspy

Appl. Phys. Lett. 91, 073506 (2007); http://dx.doi.org/10.1063/1.2768894 (3 pages) | Cited 9 times

Online Publication Date: 13 August 2007

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Considerable effort worldwide has been invested in producing low resistance-area (RA) product magnetic tunnel junction sensors for future hard drive read heads. Here the authors present a method of producing tunnel barriers with a selectable RA value spanning orders of magnitude. A single process recipe is used with only the dose of highly charged ions (HCIs) varied. The HCIs reduce the tunnel barrier integrity, providing enhanced conduction that reduces the overall RA product. The final RA product is selected by appropriate choice of the HCI density; e.g., 100 HCIs/μm2 typically results in the RA product being reduced by a factor of 100.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
07.55.-w Magnetic instruments and components
61.82.-d Radiation effects on specific materials

Broadband all-optical ultrasound transducers

Yang Hou, Jin-Sung Kim, Shai Ashkenazi, Sheng-Wen Huang, L. Jay Guo, and Matthew O’Donnell

Appl. Phys. Lett. 91, 073507 (2007); http://dx.doi.org/10.1063/1.2771058 (3 pages) | Cited 8 times

Online Publication Date: 14 August 2007

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A broadband all-optical ultrasound transducer has been designed, fabricated, and tested for high-resolution ultrasound imaging. It consists of a two-dimensional gold nanostructure on a glass substrate, followed by a 3 μm polydimethylsiloxane layer and a 30 nm gold layer. The signal to noise ratio of a pulse-echo signal is over 10 dB in the far field of the transducer, where the center frequency is 40 MHz with −6 dB bandwidth of 57 MHz. The potential for high-frequency ultrasound arrays using this technology is demonstrated using multiple measurements from the transducer to image a 25 μm diameter wire.
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43.38.Zp Acoustooptic and photoacoustic transducers
42.79.Jq Acousto-optical devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Thickness dependence of the trap states in organic thin film of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine

Ta-Ya Chu and Ok-Keun Song

Appl. Phys. Lett. 91, 073508 (2007); http://dx.doi.org/10.1063/1.2771536 (3 pages) | Cited 8 times

Online Publication Date: 14 August 2007

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The authors have investigated the relationship between the trap states (exponential trap distribution in energy and density) and the thickness of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB). The thickness dependent hole mobility of NPB can be attributed to the trap states. The origin of deep trap states at thinner film can be attributed to both surface dipole of buckminsterfullerene and the interaction between NPB and indium tin oxide at the interface. The influence of interfacial trap states on charge drift mobility is getting weaker as the thickness increases and is negligible when the thickness of NPB is thicker than 300 nm.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.20.At Surface states, band structure, electron density of states

Simulation of twin boundary effect on characteristics of single grain-silicon thin film transistors

F. Yan, P. Migliorato, and R. Ishihara

Appl. Phys. Lett. 91, 073509 (2007); http://dx.doi.org/10.1063/1.2769951 (3 pages) | Cited 1 time

Online Publication Date: 14 August 2007

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The influence of twin boundaries on the characteristics of single grain-silicon thin film transistors has been analyzed by three-dimensional simulation. The simulations show that the orientation and the location of a twin boundary could affect the field-effect mobility and the leakage current of a device. The field-effect mobility increases with the increase of the angle between the normal direction of the twin boundary and the channel direction. A single twin boundary in contact with the drain can lead to higher leakage current because electron-hole generation is greatly enhanced by the trap states in the twin boundary.
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85.30.De Semiconductor-device characterization, design, and modeling
85.30.Tv Field effect devices

Quantum well intrasubband photodetector for far infared and terahertz radiation detection

David Z.-Y. Ting, Yia-Chung Chang, Sumith V. Bandara, and Sarath D. Gunapala

Appl. Phys. Lett. 91, 073510 (2007); http://dx.doi.org/10.1063/1.2770766 (3 pages) | Cited 7 times

Online Publication Date: 14 August 2007

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The authors present a theoretical analysis on the possibility of using the dopant-assisted intrasubband absorption mechanism in quantum wells for normal-incidence far infrared/terahertz radiation detection. The authors describe the proposed concept of the quantum well intrasubband photodetector (QWISP), which is a compact semiconductor heterostructure device compatible with existing GaAs focal-plane array technology, and present theoretical results demonstrating strong normal-incidence absorption and responsivity in the QWISP.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Nonvolatile memory devices with Cu2S and Cu-Pc bilayered films

Liang Chen, Yidong Xia, Xuefei Liang, Kuibo Yin, Jiang Yin, Zhiguo Liu, and Yong Chen

Appl. Phys. Lett. 91, 073511 (2007); http://dx.doi.org/10.1063/1.2771064 (3 pages) | Cited 21 times

Online Publication Date: 14 August 2007

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An organic bistable device with a structure Cu/Cu2S/copperphthalocyanine (Cu-Pc)/Pt was fabricated. Compared to the single layer organic device composed of Cu/Cu-Pc/Pt, the bilayer devices were observed to show distinct bistability with the resistance ratio of the off/on states up to 107 and low switch voltage (0.75–0.85 V). At least 105 switching cycles were achieved in the “write-read-erase-read” cycle voltage. The filament mechanism for the device is supported by the “metallic” behavior in their temperature dependence of the resistance in the on state. Such a memory device provides a promising structure for the nonvolatile memory.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects
84.30.Sk Pulse and digital circuits

Demonstration of critical coupling in microfiber loops wrapped around a copper rod

Xin Guo, Yuhang Li, Xiaoshun Jiang, and Limin Tong

Appl. Phys. Lett. 91, 073512 (2007); http://dx.doi.org/10.1063/1.2771526 (3 pages) | Cited 8 times

Online Publication Date: 15 August 2007

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The authors demonstrate critical coupling condition in microfiber loops wrapped around a copper rod. The critical coupling condition is achieved by tuning the coupling coefficient to balance the circulation loss. A maximum extinction of 30 dB has been obtained around the critical coupling point with a quality factor of about 4000. The resonance wavelength can be tuned by applying an electric current through the copper rod. The copper rod, usually deemed as a high-loss medium for handling light at optical frequency, serves as a low-index robust support for achieving critical coupling in the microfiber loops with acceptable loss.
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42.81.Qb Fiber waveguides, couplers, and arrays
42.81.Dp Propagation, scattering, and losses; solitons

Scaling in back-illuminated GaN avalanche photodiodes

K. Minder, J. L. Pau, R. McClintock, P. Kung, C. Bayram, M. Razeghi, and D. Silversmith

Appl. Phys. Lett. 91, 073513 (2007); http://dx.doi.org/10.1063/1.2772199 (3 pages) | Cited 10 times

Online Publication Date: 15 August 2007

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Avalanche p-i-n photodiodes of various mesa areas were fabricated on AlN templates for back illumination for enhanced performance through hole-initiated multiplication, and the effects of increased area on device performance were studied. Avalanche multiplication was observed in mesa sizes up to 14 063 μm2 under linear mode operation. Uniform gain and a linear increase of the dark current with area were demonstrated.
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85.60.Dw Photodiodes; phototransistors; photoresistors

Effect of tensile stress on the various components of the off current of n-channel metal-oxide-semiconductor transistors

Peizhen Yang, W. S. Lau, V. Ho, C. H. Loh, S. Y. Siah, and L. Chan

Appl. Phys. Lett. 91, 073514 (2007); http://dx.doi.org/10.1063/1.2769960 (3 pages) | Cited 2 times

Online Publication Date: 15 August 2007

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The authors found that tensile stress actually slightly increases the on current and the subthreshold off current but it slightly decreases the gate leakage current and the drain junction leakage current for n-channel metal-oxide-semiconductor transistors. For short transistors, the subthreshold off current dominates over the other two components of the off current such that tensile stress slightly increases both the on current and the off current. However, in the on current versus the logarithm of off current plot, tensile stress increases the on current for a constant off current such that the overall effect of tensile stress is an improvement.
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85.30.Tv Field effect devices

Effect of GaP strain compensation layers on rapid thermally annealed InGaAs/GaAs quantum dot infrared photodetectors grown by metal-organic chemical-vapor deposition

L. Fu, I. McKerracher, H. H. Tan, C. Jagadish, N. Vukmirović, and P. Harrison

Appl. Phys. Lett. 91, 073515 (2007); http://dx.doi.org/10.1063/1.2770765 (3 pages) | Cited 2 times

Online Publication Date: 15 August 2007

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The effect of GaP strain compensation layers was investigated on ten-layer InGaAs/GaAs quantum dot infrared photodetectors (QDIPs) grown by metal-organic chemical-vapor deposition. Compared with the normal QDIP structure, the insertion of GaP has led to a narrowed spectral linewidth and slightly improved detector performance. A more significant influence of GaP was observed after the structure was annealed at various temperatures. While a similar amount of wavelength tuning was obtained, the GaP QDIPs exhibited much less degradation in device characteristics with increasing annealing temperature.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.60.Gz Photodetectors (including infrared and CCD detectors)
85.40.Sz Deposition technology

Quantum dot resonant tunneling diode for telecommunication wavelength single photon detection

H. W. Li, B. E. Kardynał, P. See, A. J. Shields, P. Simmonds, H. E. Beere, and D. A. Ritchie

Appl. Phys. Lett. 91, 073516 (2007); http://dx.doi.org/10.1063/1.2768884 (3 pages) | Cited 11 times

Online Publication Date: 15 August 2007

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The authors present a quantum dot (QD) based single photon detector operating at a fiber optic telecommunication wavelength. The detector is based on an AlAs/In0.53Ga0.47As/AlAs double-barrier resonant tunneling diode containing a layer of self-assembled InAs QDs grown on an InP substrate. The device shows an internal efficiency of about 6.3% with a dark count rate of 1.58×10−6 ns−1 for 1310 nm photons.
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85.30.Kk Junction diodes
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Highly efficient and stable white light organic light-emitting devices

Mei-Fang Lin, Lei Wang, Wai-Kwok Wong, Kok-Wai Cheah, Hoi-Lam TAM, Meng-Ting Lee, Meng-Huan Ho, and Chin H. Chen

Appl. Phys. Lett. 91, 073517 (2007); http://dx.doi.org/10.1063/1.2769762 (3 pages) | Cited 7 times

Online Publication Date: 16 August 2007

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A highly efficient two-element white light organic light-emitting device with a sky blue dopant BUBD-1 and a yellow dopant TBRb was fabricated. The device achieved an electroluminescence efficiency of 17.1 cd/A and 7.9 lm/W at 20 mA/cm2. From 112 to 32 010 nits, the CIEx,y coordinates variation was Δ(x,y)<±(0.01,0.01), which is superior to common white light organic light-emitting devices (OLEDs) that tend to change color with drive conditions. Moreover, the device reached a half-decay t1/2 lifetime over 40 000 h at an initial luminance of 300 cd/m2. This result is among the best records of fluorescent white light OLEDs.
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85.60.Jb Light-emitting devices

Origin of the efficiency improvement in all-polymer solar cells upon annealing

M. M. Mandoc, W. Veurman, J. Sweelssen, M. M. Koetse, and P. W. M. Blom

Appl. Phys. Lett. 91, 073518 (2007); http://dx.doi.org/10.1063/1.2772185 (3 pages) | Cited 28 times

Online Publication Date: 16 August 2007

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The origin of the enhanced efficiency upon annealing in solar cells based on blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] and poly{9,9-dioctylfluorene-2,7-diyl-alt-1,4-bis[2-(5-thienyl)-1-cyanovinyl]-2-methoxy-5-(3,7-dimethyl-octyloxy)benzene} is investigated. Current-voltage measurements on solar cells and single-carrier diodes reveal that the electron and hole transport is not improved for thermally treated devices. From the analysis of the photocurrent, the authors show that the dissociation efficiency of the bound electron-hole pairs increases upon annealing, due to an increase of the initial electron-hole separation distance.
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84.60.Jt Photoelectric conversion
81.40.Gh Other heat and thermomechanical treatments

Highly efficient organic tandem solar cells using an improved connecting architecture

A. G. F. Janssen, T. Riedl, S. Hamwi, H.-H. Johannes, and W. Kowalsky

Appl. Phys. Lett. 91, 073519 (2007); http://dx.doi.org/10.1063/1.2772208 (3 pages) | Cited 39 times

Online Publication Date: 16 August 2007

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Tandem solar cells based on the combination of a poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester and a copper phthalocyanine:fullerene subcell are reported. By using a highly transparent, high-work function WO3 layer as part of the interconnecting system for the two subcells, the authors demonstrate stacked devices with power conversion efficiencies as high as 4.6%. The efficiency of the stacked devices is close to the sum of the efficiencies of the individual subcells.
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84.60.Jt Photoelectric conversion

Light controlled spin polarization in asymmetric n-type resonant tunneling diode

L. F. dos Santos, Y. Galvão Gobato, G. E. Marques, M. J. S. P. Brasil, M. Henini, and R. Airey

Appl. Phys. Lett. 91, 073520 (2007); http://dx.doi.org/10.1063/1.2772662 (3 pages) | Cited 4 times

Online Publication Date: 17 August 2007

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The authors have observed a strong dependence of the circular polarization degree from the quantum well emission in an asymmetric n-type GaAs/AlAs/AlGaAs resonant tunneling diode on both the laser excitation intensity and the applied bias voltage. The sign of the circular polarization can be reversed by increasing the light excitation intensity when the structure is biased with voltages slightly larger than the first electron resonance. The variation of polarization is associated with a large density of photogenerated holes accumulated in the quantum well, which is enhanced due to the asymmetry of the structure.
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85.30.Kk Junction diodes
85.75.Mm Spin polarized resonant tunnel junctions
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Organic solar cells on indium tin oxide and aluminum doped zinc oxide anodes

Kerstin Schulze, Bert Maennig, Karl Leo, Yuto Tomita, Christian May, Jürgen Hüpkes, Eduard Brier, Egon Reinold, and Peter Bäuerle

Appl. Phys. Lett. 91, 073521 (2007); http://dx.doi.org/10.1063/1.2771050 (3 pages) | Cited 19 times

Online Publication Date: 17 August 2007

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The authors compare organic solar cells using two different transparent conductive oxides as anode: indium tin oxide (ITO) and three kinds of aluminum doped zinc oxide (ZAO). These anodes with different work functions are used for small molecule photovoltaic devices based on an oligothiophene derivative as donor and fullerene C60 as acceptor molecule. It turns out that cells on ITO and ZAO have virtually identical properties. In particular, the authors demonstrate that the work function of the anode does not influence the Voc of the photovoltaic device due to the use of doped transport layers.
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84.60.Jt Photoelectric conversion
85.60.-q Optoelectronic devices
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