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15 Dec 2009

Volume 106, Issue 12, Articles (12xxxx)

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Electronic structure and optical properties of ZnSiO3 and Zn2SiO4

S. Zh. Karazhanov, P. Ravindran, H. Fjellvåg, and B. G. Svensson

J. Appl. Phys. 106, 123701 (2009); http://dx.doi.org/10.1063/1.3268445 (7 pages) | Cited 3 times

Online Publication Date: 16 December 2009

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The electronic structure and optical properties of orthorhombic, monoclinic, and rhombohedral (corundum type) modifications of ZnSiO3, and of rhombohedral, tetragonal, and cubic (spinel type) modifications of Zn2SiO4 have been studied using ab initio density functional theory calculations. The calculated fundamental band gaps for the different polymorphs and compounds are in the range 2.22–4.18 eV. The lowest conduction band is well dispersive similar to that found for transparent conducting oxides such as ZnO. This band is mainly contributed by Zn 4s electrons. The carrier effective masses were calculated and compared with those for ZnO. The topmost valence band is much less dispersive and contributed by O 2p and Zn 3d electrons. From the analysis of charge density, charges residing in each site, and electron localization function, it is found that ionic bonding is mainly ruling in these compounds. The calculated optical dielectric tensors show that the optical properties of ZnSiO3 and Zn2SiO4 are almost isotropic in the visible part of the solar spectra and depend negligibly on the crystal structure. Within the 0–4 eV photon energy range, the calculated magnitude of the absorption coefficient, reflectivity, refractive index, and extinction coefficient are smaller than 103 cm−1, 0.15, 2.2, and 0.3, respectively, for all the ZnSiO3 and Zn2SiO4 phases considered in this work. This suggests that zinc silicates can be used as antireflection coatings.
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71.20.Ps Other inorganic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
61.66.Fn Inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor

An alternative treatment of heat flow for charge transport in semiconductor devices

Matt Grupen

J. Appl. Phys. 106, 123702 (2009); http://dx.doi.org/10.1063/1.3270404 (7 pages) | Cited 3 times

Online Publication Date: 18 December 2009

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A unique thermodynamic model of Fermi gases suitable for semiconductor device simulation is presented. Like other models, such as drift diffusion and hydrodynamics, it employs moments of the Boltzmann transport equation derived using the Fermi–Dirac distribution function. However, unlike other approaches, it replaces the concept of an electron thermal conductivity with the heat capacity of an ideal Fermi gas to determine heat flow. The model is used to simulate a field-effect transistor and show that the external current-voltage characteristics are strong functions of the state space available to the heated Fermi distribution.
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85.30.Tv Field effect devices
47.27.te Turbulent convective heat transfer
05.60.-k Transport processes
03.75.Ss Degenerate Fermi gases

Terahertz-field-induced tunneling current with nonlinear effects in a double quantum well coupled to a continuum

Marcelo Z. Maialle, Marcos H. Degani, Justino R. Madureira, and Paulo F. Farinas

J. Appl. Phys. 106, 123703 (2009); http://dx.doi.org/10.1063/1.3270263 (5 pages) | Cited 1 time

Online Publication Date: 22 December 2009

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We have theoretically investigated the tunneling current induced by a terahertz (THz) field applied to an asymmetric double quantum well. The excitation couples an initially localized state to a nearby continuum of extended states. We have shown that the calculated current has similar features as those present in the optical spectra, such as interference effects due to the interaction between the continuum and the localized states, in addition to many-photon transition effects. The induced current is calculated as a function of the intensity of the THz field. A second THz field is used to yield nonlinear processes, useful to control the interference effects. We believe that part of the issues studied here can be useful for the integration of novel switching mechanisms based on optics (THz) and electronic current.
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73.63.Hs Quantum wells
73.21.Fg Quantum wells
73.40.Gk Tunneling
78.70.Gq Microwave and radio-frequency interactions
78.56.-a Photoconduction and photovoltaic effects
78.67.De Quantum wells

The influence of ammonia on the electrical properties of detonation nanodiamond

Mose Bevilacqua, Aysha Chaudhary, and Richard B. Jackman

J. Appl. Phys. 106, 123704 (2009); http://dx.doi.org/10.1063/1.3272912 (7 pages) | Cited 5 times

Online Publication Date: 28 December 2009

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Detonation nanodiamonds (DNDs) are an interesting class of materials for sensing applications, but little is currently understood about their electrical properties. Here, aggregated DNDs are explored with impedance spectroscopy and are found to offer near-to-ideal dielectric characteristics, which is intriguing given their nanostructure. When exposed to ammonia, two highly conductive pathways emerge through the material; these appear to be associated with grain boundary and grain interior processes, the latter potentially due to surface transfer doping. This process is reversible given modest temperature increases suggesting DNDs may offer a solid state electrical platform for ammonia sensing applications.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
73.22.-f Electronic structure of nanoscale materials and related systems
73.61.Cw Elemental semiconductors

Switching mechanism transition induced by annealing treatment in nonvolatile Cu/ZnO/Cu/ZnO/Pt resistive memory: From carrier trapping/detrapping to electrochemical metallization

Y. C. Yang, F. Pan, F. Zeng, and M. Liu

J. Appl. Phys. 106, 123705 (2009); http://dx.doi.org/10.1063/1.3273329 (5 pages) | Cited 11 times

Online Publication Date: 29 December 2009

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ZnO/Cu/ZnO trilayer films sandwiched between Cu and Pt electrodes were prepared for nonvolatile resistive memory applications. These structures show resistance switching under electrical bias both before and after a rapid thermal annealing (RTA) treatment, while it is found that the resistive switching effects in the two cases exhibit distinct characteristics. Compared with the as-fabricated device, the memory cell after RTA demonstrates remarkable device parameter improvements including lower threshold voltages, lower write current, and higher Roff/Ron ratio. A high-voltage forming process is avoided in the annealed device as well. Furthermore, the RTA treatment has triggered a switching mechanism transition from a carrier trapping/detrapping type to an electrochemical-redox-reaction-controlled conductive filament formation/rupture process, as indicated by different features in current-voltage characteristics. Both scanning electron microscopy observations and Auger electron spectroscopy depth profiles reveal that the Cu charge trapping layer in ZnO/Cu/ZnO disperses uniformly into the storage medium after RTA, while x-ray diffraction and x-ray photoelectron spectroscopy analyses demonstrate that the Cu atoms have lost electrons to become Cu2+ ions after dispersion. The above experimental facts indicate that the altered status of Cu in the ZnO/Cu/ZnO trilayer films during RTA treatment should be responsible for the switching mechanism transition. This study is envisioned to open the door for understanding the interrelation between different mechanisms that currently exist in the field of resistive memories.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.72.Cc Kinetics of defect formation and annealing
85.40.Ls Metallization, contacts, interconnects; device isolation
79.20.Fv Electron impact: Auger emission
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

High performance nanocomposite thin film transistors with bilayer carbon nanotube-polythiophene active channel by ink-jet printing

Gen-Wen Hsieh, Flora M. Li, Paul Beecher, Arokia Nathan, Yiliang Wu, Beng S. Ong, and William I. Milne

J. Appl. Phys. 106, 123706 (2009); http://dx.doi.org/10.1063/1.3273377 (7 pages) | Cited 9 times

Online Publication Date: 30 December 2009

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Nanocomposite thin film transistors (TFTs) based on nonpercolating networks of single-walled carbon nanotubes (CNTs) and polythiophene semiconductor [poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT-12)] thin film hosts are demonstrated by ink-jet printing. A systematic study on the effect of CNT loading on the transistor performance and channel morphology is conducted. With an appropriate loading of CNTs into the active channel, ink-jet printed composite transistors show an effective hole mobility of 0.23 cm2 V−1 s−1, which is an enhancement of more than a factor of 7 over ink-jet printed pristine PQT-12 TFTs. In addition, these devices display reasonable on/off current ratio of 105–106, low off currents of the order of 10 pA, and a sharp subthreshold slope (<0.8 V dec−1). The work presented here furthers our understanding of the interaction between polythiophene polymers and nonpercolating CNTs, where the CNT density in the bilayer structure substantially influences the morphology and transistor performance of polythiophene. Therefore, optimized loading of ink-jet printed CNTs is crucial to achieve device performance enhancement. High performance ink-jet printed nanocomposite TFTs can present a promising alternative to organic TFTs in printed electronic applications, including displays, sensors, radio-frequency identification (RFID) tags, and disposable electronics.
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85.30.Tv Field effect devices

Charge trapping and detrapping in polymeric materials

George Chen and Zhiqiang Xu

J. Appl. Phys. 106, 123707 (2009); http://dx.doi.org/10.1063/1.3273491 (5 pages) | Cited 1 time

Online Publication Date: 30 December 2009

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Space charge formation in polymeric materials can cause some serious concern for design engineers as the electric field may severely be distorted, leading to part of the material being overstressed. At the worst, this may result in material degradation and possibly premature failure. It is therefore important to understand charge generation, trapping, and detrapping processes in the material. In the present paper, the characteristics of charge trapping and detrapping in low density polyethylene under dc electric field have been investigated using the pulsed electroacoustic technique. It has been found that the charge decay shows very different characteristics for the sample with different periods of electric field application. To explain the results a simple trapping and detrapping model based on two trapping levels has been proposed. Qualitative analysis revealed the similar features to those observed experimentally.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
77.22.Jp Dielectric breakdown and space-charge effects
72.50.+b Acoustoelectric effects
72.80.Le Polymers; organic compounds (including organic semiconductors)
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