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13 Jun 2005

Volume 86, Issue 24, Articles (24xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 86, 241913 (2005); http://dx.doi.org/10.1063/1.1946181 (3 pages)

E. Placidi, F. Arciprete, V. Sessi, M. Fanfoni, F. Patella, and A. Balzarotti
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Band engineering of carbon nanotube field-effect transistors via selected area chemical gating

Xiaolei Liu, Zhicheng Luo, Song Han, Tao Tang, Daihua Zhang, and Chongwu Zhou

Appl. Phys. Lett. 86, 243501 (2005); http://dx.doi.org/10.1063/1.1944898 (3 pages) | Cited 12 times

Online Publication Date: 6 June 2005

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This letter presents an approach to engineer the band structure of carbon nanotube field-effect transistors via selected area chemical gating. By exposing the center part, or the contacts, of nanotube devices to oxidizing or reducing gases, a good control over the threshold voltage and subthreshold swing has been achieved. Our experiments reveal that NO2 shifts the threshold voltage positively, while NH3 shifts it negatively for both center-exposed and contact-exposed devices. However, modulations to the subthreshold swing are in opposite directions for center-exposed and contact-exposed devices: NO2 lowers the subthreshold swing of the contact-exposed devices, but increases that of the center-exposed devices. In contrast, NH3 reduces the subthreshold swing of the center-exposed devices, but increases that of the contact-exposed devices.
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85.35.Kt Nanotube devices
85.30.Tv Field effect devices

Extreme bendability of single-walled carbon nanotube networks transferred from high-temperature growth substrates to plastic and their use in thin-film transistors

Seung-Hyun Hur, O Ok Park, and John. A. Rogers

Appl. Phys. Lett. 86, 243502 (2005); http://dx.doi.org/10.1063/1.1947380 (3 pages) | Cited 44 times

Online Publication Date: 7 June 2005

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In this paper we describe printing methods for transferring single-walled carbon nanotubes (SWNTs) from high-temperature growth substrates to flexible, low-cost plastic supports. Thin-film transistors (TFTs) built with networks of transferred SWNTs grown by chemical vapor deposition show good performance—mobilities and on∕off current ratios similar to those of devices fabricated on the growth substrates for a wide range of channel lengths. Bending tests on these TFTs show that their output current varies only in a narrow (±5%) range, even for bend radii that induce surface strains larger than 1%. Similar structures evaluated under sharp folding, with strains larger than 20%, show that the SWNT networks are operational even under extreme bending conditions. This level of mechanical robustness, the good electrical performance, and optical transparency make transferred SWNT networks an attractive type of electronic material for applications in macroelectronics, sensors, and other systems that require wide area coverage and unusual substrates.
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81.07.De Nanotubes
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
85.30.Tv Field effect devices

Hydrogen-selective sensing at room temperature with ZnO nanorods

H. T. Wang, B. S. Kang, F. Ren, L. C. Tien, P. W. Sadik, D. P. Norton, S. J. Pearton, and Jenshan Lin

Appl. Phys. Lett. 86, 243503 (2005); http://dx.doi.org/10.1063/1.1949707 (3 pages) | Cited 170 times

Online Publication Date: 8 June 2005

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The sensitivity for detecting hydrogen with multiple ZnO nanorods is found to be greatly enhanced by sputter-depositing clusters of Pd on the surface. The resulting structures show a change in room- temperature resistance upon exposure to hydrogen concentrations in N2 of 10–500 ppm of approximately a factor of 5 larger than without Pd. Pd-coated ZnO nanorods detected hydrogen down to <10 ppm, with relative responses of >2.6% at 10 ppm and >4.2% at 500 ppm H2 in N2 after a 10 min exposure. There was no response at room temperature to O2. Approximately 95% of the initial ZnO conductance after exposure to hydrogen was recovered within 20 s by exposing the nanorods to either air or pure O2. This rapid and easy recoverability make the Pd-coated nanorods suitable for practical applications in hydrogen-selective sensing at ppm levels at room temperature with <0.4 mW power consumption.
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81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
82.80.-d Chemical analysis and related physical methods of analysis
68.47.Jn Clusters on oxide surfaces
81.65.-b Surface treatments
73.61.Ga II-VI semiconductors

Dynamic avalanche breakdown of a p‐n junction: Deterministic triggering of a plane streamer front

Pavel Rodin and Igor Grekhov

Appl. Phys. Lett. 86, 243504 (2005); http://dx.doi.org/10.1063/1.1947915 (3 pages) | Cited 7 times

Online Publication Date: 8 June 2005

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We discuss the dynamic impact ionization breakdown of a high voltage p‐n junction which occurs when the electric field is increased above the threshold of avalanche impact ionization on a time scale smaller than the inverse thermogeneration rate. The avalanche-to-streamer transition characterized by generation of dense electron-hole plasma capable of screening the applied external electric field occurs in such regimes. We argue that the experimentally observed deterministic triggering of the plane streamer front at the electric-field strength above the threshold of avalanche impact ionization, yet below the threshold of band-to-band tunneling, is generally caused by field-enhanced ionization of deep-level centers. We suggest that the process-induced sulfur centers and native defects such as EL2, HB2, and HB5 centers initiate the front in Si and GaAs structures, respectively. In deep-level-free structures the plane streamer front is triggered by Zener band-to-band tunneling.
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72.20.Ht High-field and nonlinear effects
77.22.Jp Dielectric breakdown and space-charge effects
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
71.55.Eq III-V semiconductors
71.55.Cn Elemental semiconductors

Analysis of high-power packages for phosphor-based white-light-emitting diodes

Hong Luo, Jong Kyu Kim, E. Fred Schubert, Jaehee Cho, Cheolsoo Sone, and Yongjo Park

Appl. Phys. Lett. 86, 243505 (2005); http://dx.doi.org/10.1063/1.1949282 (3 pages) | Cited 58 times

Online Publication Date: 8 June 2005

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An optimized packaging configuration for high-power white-light-emitting diode (LED) lamps that employs a diffuse reflector cup, a large separation between the primary emitter (the LED chip) and the wavelength converter (the phosphor) and a hemispherically shaped encapsulation is presented. Ray tracing simulations for this configuration show that the phosphor efficiency can be enhanced by up to 50% over conventional packages. Dichromatic LED lamps with phosphor layers on the top of a diffuse reflector cup were fabricated and studied experimentally. The experimental enhancement of phosphor efficiency is 15.4% for blue-pumped yellow phosphor and 27% for ultraviolet-pumped blue phosphor. Those improvements are attributed to reduced absorption of the phosphorescence by the LED chip and the reduction of deterministic optical modes trapped inside the encapsulant.
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85.60.Pg Display systems
42.72.Bj Visible and ultraviolet sources

Efficient photovoltaic energy conversion in tetracene-C60 based heterojunctions

Chih-Wei Chu, Yan Shao, Vishal Shrotriya, and Yang Yang

Appl. Phys. Lett. 86, 243506 (2005); http://dx.doi.org/10.1063/1.1946184 (3 pages) | Cited 51 times

Online Publication Date: 9 June 2005

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We report organic solar cells fabricated with small-molecule organic semiconductor tetracene/C60 heterojunction as the photoactive layer. The external power conversion efficiency of the devices under AM 1.5 solar illumination at 100 mW/cm2 (1 sun) is 2.3±0.5% with relatively high open-circuit voltage (Voc = 0.58±0.06 V) compared to most of the other small-molecular donor-acceptor (D-A) heterojunction solar cells reported so far. Using atomic force microscopy and x-ray diffraction we found that tetracene thin films consist of submicron-sized grains with rough surface and well defined molecular order. Therefore, using high mobility polycrystalline tetracene thin films for D-A heterojunction devices dramatically increases area of tetracene and C60 interface for exciton diffusion to reduce the recombination.
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84.60.Jt Photoelectric conversion
68.37.Ps Atomic force microscopy (AFM)
71.35.-y Excitons and related phenomena
68.35.Ct Interface structure and roughness

Gate inversion effect in Si1−xGex/HfO2/Si metal-oxide-semiconductor devices

J. Costa e Silva, E. L. de Oliveira, J. S. de Sousa, J. A. K. Freire, V. N. Freire, and G. A. Farias

Appl. Phys. Lett. 86, 243507 (2005); http://dx.doi.org/10.1063/1.1946919 (3 pages) | Cited 2 times

Online Publication Date: 10 June 2005

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The electrical properties of polycrystalline Si1−xGex/HfO2/Si metal-oxide-semiconductor (MOS) devices are investigated theoretically by solving self-consistently the Poisson and Schrödinger equations. Our calculations demonstrate that the combination of HfO2 as dielectric film and polycrystalline gates causes charge inversion in both substrate and gate layer for ultrathin oxide layers. This effect is a consequence of the high dielectric constant of HfO2, and it is expected to occur for all high-κ-based polygated MOS devices. However, the relative doping concentration between polycrystalline gate and substrate can be used to decrease the effect of gate inversion.
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85.30.Tv Field effect devices
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
61.72.S- Impurities in crystals
77.22.Ch Permittivity (dielectric function)
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