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4 Feb 2008

Volume 92, Issue 5, Articles (05xxxx)

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Appl. Phys. Lett. 92, 053301 (2008); http://dx.doi.org/10.1063/1.2838457 (3 pages)

Michael S. Arnold, Gregory J. McGraw, Stephen R. Forrest, and Richard R. Lunt
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193 nm deep-ultraviolet solar-blind cubic boron nitride based photodetectors

A. Soltani, H. A. Barkad, M. Mattalah, B. Benbakhti, J.-C. De Jaeger, Y. M. Chong, Y. S. Zou, W. J. Zhang, S. T. Lee, A. BenMoussa, B. Giordanengo, and J.-F. Hochedez

Appl. Phys. Lett. 92, 053501 (2008); http://dx.doi.org/10.1063/1.2840178 (3 pages) | Cited 20 times

Online Publication Date: 4 February 2008

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Deep-ultraviolet (DUV) solar-blind photodetectors based on high-quality cubic boron nitride (cBN) films with a metal/semiconductor/metal configuration were fabricated. The design of interdigitated circular electrodes enables high homogeneity of electric field between pads. The DUV photodetectors present a peak responsivity at 180 nm with a very sharp cutoff wavelength at 193 nm and a visible rejection ratio (180 versus 250 nm) of more than four orders of magnitude. The characteristics of the photodetectors present extremely low dark current, high breakdown voltage, and high responsivity, suggesting that cBN films are very promising for DUV sensing.
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Carrier distribution in (0001)InGaN/GaN multiple quantum well light-emitting diodes

Aurélien David, Michael J. Grundmann, John F. Kaeding, Nathan F. Gardner, Theodoros G. Mihopoulos, and Michael R. Krames

Appl. Phys. Lett. 92, 053502 (2008); http://dx.doi.org/10.1063/1.2839305 (3 pages) | Cited 59 times

Online Publication Date: 4 February 2008

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We study the carrier distribution in multi quantum well (multi-QW) InGaN light-emitting diodes. Conventional wisdom would assume that a large number of QWs lead to a smaller carrier density per QW, enabling efficient carrier recombination at high currents. We use angle-resolved far-field measurements to determine the location of spontaneous emission in a series of multi-QW samples. They reveal that, no matter how many QWs are grown, only the QW nearest the p layer emits light under electrical pumping, which can limit the performances of high-power devices.
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85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
85.60.Jb Light-emitting devices

Impurity bound-to-unbound terahertz sensors based on beryllium and silicon δ-doped GaAs/AlAs multiple quantum wells

D. Seliuta, J. Kavaliauskas, B. Čechavičius, S. Balakauskas, G. Valušis, B. Sherliker, M. P. Halsall, P. Harrison, M. Lachab, S. P. Khanna, and E. H. Linfield

Appl. Phys. Lett. 92, 053503 (2008); http://dx.doi.org/10.1063/1.2839585 (3 pages) | Cited 9 times

Online Publication Date: 4 February 2008

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Beryllium and silicon δ-doped GaAs/AlAs multiple quantum wells (MQWs) were designed and fabricated for selective sensing of terahertz radiation. Optical characterization of the structures by photoreflectance spectroscopy has indicated a presence of built-in electric fields–from 18 up to 49 kV/cm depending on the structure design–located mainly in the cap and buffer layers, but do not penetrating into the MQWs region. Terahertz sensing under normal incidence via impurity bound-to-unbound transitions is demonstrated in photocurrent experiments within 0.6–4.2 THz in silicon-doped MQWs and 3.5–7.3 THz range in beryllium-doped MQWs at low temperatures.
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81.07.St Quantum wells
78.67.De Quantum wells
68.65.Fg Quantum wells

Electronic transport and layer engineering in multilayer graphene structures

H. M. Wang, Y. H. Wu, Z. H. Ni, and Z. X. Shen

Appl. Phys. Lett. 92, 053504 (2008); http://dx.doi.org/10.1063/1.2840713 (3 pages) | Cited 10 times

Online Publication Date: 6 February 2008

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We demonstrate a reproducible layer engineering technique for multilayer graphene through controllable oxidation via a SiO2 capping layer. The oxidation method is able to reduce the thickness of few layer graphene to a single layer, as determined by a combination of contrast and Raman spectroscopies. We have also studied the electrical transport properties of graphene sheets with different thicknesses by focusing on their minimum conductivity. The average minimum conductivity of single layer graphene was found to be 0.3×4e2/h, while that of multilayer graphene consisting of n layers is approximately 1.2×4e2/h for n = 2, 2.4×4e2/h for n = 3, and 4ne2/h for n>3. The results suggest that the substrate plays an important role in determining the transport properties of thin graphene sheets with n<3, while its influence is relatively small in thicker graphene sheets.
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68.65.Ac Multilayers
81.65.Mq Oxidation
78.30.Hv Other nonmetallic inorganics
73.63.-b Electronic transport in nanoscale materials and structures
78.66.Nk Insulators

Alignment-free process for asymmetric contact electrodes and their application in light-emitting organic field-effect transistors

Tomo Sakanoue, Masayuki Yahiro, Chihaya Adachi, Jeremy H. Burroughes, Yoshiaki Oku, Noriyuki Shimoji, Takayoshi Takahashi, and Akio Toshimitsu

Appl. Phys. Lett. 92, 053505 (2008); http://dx.doi.org/10.1063/1.2839895 (3 pages) | Cited 8 times

Online Publication Date: 6 February 2008

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We developed an alignment-free process for asymmetric contacts of Au and Al and applied it to light-emitting organic field-effect transistors. Because electrons were injected efficiently from Al contacts, the emission intensity and onset voltages for light were significantly better than those in a device with conventional Au/Cr contacts. Moreover, a device with 1 μm channel length asymmetric contacts of Au and Al showed about 50 times higher current than that of the device with conventional Au/Cr contacts. This significant improvement can be ascribed to both dual space-charge formation of holes and electrons and low carrier injection barriers.
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85.30.Tv Field effect devices
85.60.Jb Light-emitting devices

Ultraviolet bandpass Al0.17Ga0.83N/GaN heterojunction phototransitors with high optical gain and high rejection ratio

M. L. Lee, J. K. Sheu, and Yung-Ru Shu

Appl. Phys. Lett. 92, 053506 (2008); http://dx.doi.org/10.1063/1.2838307 (3 pages) | Cited 1 time

Online Publication Date: 6 February 2008

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Ultraviolet Al0.17Ga0.83N/GaN-based heterojunction phototransistors (HPTs) grown by metal-organic vapor-phase epitaxy were demonstrated. The HPTs showed a bandpass spectral responsivity ranging from 280 to 390 nm. With a bias voltage of 6 V, the responsivity at an incident of 340 nm was as high as 1500 A/W, corresponding to a quantum gain of 5.47×103. In contrast to GaN-based photoconductors, the HPTs also featured high contrast in spectral response. With a bias voltage of 3 V, the spectral response showed high rejection ratios of approximately 4×105 and 1×104 for the long-wavelength side (340/400 nm) and the short-wavelength side, respectively. The high contrast of spectral response for the long-wavelength side could be due to the long trapping time of holes blocked by the base-emitter heterojunction and the low defect-to-band response that is caused by defect-related gap states.
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85.60.Dw Photodiodes; phototransistors; photoresistors

Nanodipole photovoltaics

Diana Shvydka and V. G. Karpov

Appl. Phys. Lett. 92, 053507 (2008); http://dx.doi.org/10.1063/1.2835972 (3 pages) | Cited 5 times

Online Publication Date: 8 February 2008

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We propose a principle of photovoltaics utilizing the built-in electric field generated by aligned nanosize dipoles in a semiconductive host, either polymer or liquid, or amorphous, or polycrystalline. Such dipoles are readily found with the wurtzite CdS, CdSe, and some other nanoparticles. We show that their generated field can be uniform and strong enough, ∼ 3×104 V/cm, to separate electron-hole pairs and run significant drift currents. Our suggested structure does not rely on p-n or Schottky junctions and can be tunable in a broad range of parameters.
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72.40.+w Photoconduction and photovoltaic effects
73.63.Bd Nanocrystalline materials

Large inverse magnetoresistance in fully epitaxial Fe/Fe3O4/MgO/Co magnetic tunnel junctions

F. Greullet, E. Snoeck, C. Tiusan, M. Hehn, D. Lacour, O. Lenoble, C. Magen, and L. Calmels

Appl. Phys. Lett. 92, 053508 (2008); http://dx.doi.org/10.1063/1.2841812 (3 pages) | Cited 14 times

Online Publication Date: 8 February 2008

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Fully epitaxial Fe(001)/Fe3O4(001)/MgO(001)/Co micron-sized magnetic tunnel junctions have been elaborated on MgO(001) substrates. X-ray reflectivity and high-resolution transmission electron microscopy revealed a good quality and epitaxial growth of the stack with abrupt interfaces. The magnetotransport measurements exhibit a large negative tunneling magnetoresistance (TMR) value for magnetic tunnel junctions including an Fe3O4 layer and a MgO tunnel barrier (−8.5% at 300 K and −22% at 80 K). Moreover, the sign of the TMR changes with the applied bias. We discuss here the structural quality of the samples and the transport measurement results.
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72.20.My Galvanomagnetic and other magnetotransport effects
75.47.Pq Other materials

A constructive combination of antireflection and intermediate-reflector layers for a-Si/μc-Si thin film solar cells

Chandan Das, Andreas Lambertz, Juergen Huepkes, Wilfried Reetz, and Friedhelm Finger

Appl. Phys. Lett. 92, 053509 (2008); http://dx.doi.org/10.1063/1.2841824 (3 pages) | Cited 14 times

Online Publication Date: 8 February 2008

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This device design approach combines sputter-deposited TiO2 antireflection layer (ARL) and plasma-enhanced chemical vapor deposition-grown SiOx intermediate-reflector layer (IRL) in superstrate a-Si/μc-Si thin film solar cell. The loss of current from either the component cells with individual application of ARL and IRL has been recovered with their combined application. With both ARL and IRL in a-Si/μc-Si cell, (a) the top cell current and (b) the sum of top and bottom cell current increases. An initial efficiency of 11.8% [Voc = 1.42 V, FF = 0.74, Jsc (top) = 11.5 mA cm−2, Jsc (bottom) = 11.2 mA cm−2] is achieved from such an a-Si/μc-Si cell with a total Si layer thickness less than 2 μm.
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84.60.Jt Photoelectric conversion
42.79.Wc Optical coatings
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