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15 Jul 2008

Volume 104, Issue 2, Articles (02xxxx)

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Strain-induced modulation of band structure of silicon

S. Zh. Karazhanov, A. Davletova, and A. Ulyashin

J. Appl. Phys. 104, 024501 (2008); http://dx.doi.org/10.1063/1.2940135 (8 pages) | Cited 1 time

Online Publication Date: 16 July 2008

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This work presents ab initio study of strain-induced modulation of band structure of Si. It is shown that at straining pressures >12 GPa, band structure of Si can be turned from indirect to direct. Both the bottommost conduction band and topmost valence band are located at the Γ point. The conduction band minimum at the Γ point of the strained Si is found to be much more dispersive than that at the X point of the unstressed Si. Consequently, electrical conductivity through the Γ valley is suggested to be more superior than the X point of the unstressed Si. Barrier height, which is needed to transfer electrons in the Γ point to X/L points or from Γ point to X/L to Γ point have been calculated. The results have been applied to explain peculiarities of electronic structure and light emission of Si based materials containing dislocations and voids.
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71.20.Mq Elemental semiconductors
71.15.-m Methods of electronic structure calculations
78.55.Ap Elemental semiconductors
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Tailoring gas sensing properties of carbon nanotubes

Sean Brahim, Steve Colbern, Robert Gump, and Leonid Grigorian

J. Appl. Phys. 104, 024502 (2008); http://dx.doi.org/10.1063/1.2956395 (10 pages) | Cited 6 times

Online Publication Date: 17 July 2008

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An approach to tune the chemical properties (electronic density of states, chemical potential) of carbon nanotubes (CNTs) for gas/vapor sensing is presented. The technique involves infiltrating CNTs with transition metal as opposed to decorating the outer sidewalls. NO2 was chosen as a model gas to demonstrate the variable/tunable sensing behavior of the metal-CNT hybrids. By varying the nature of the transition metal infiltrated into the CNT, we observe dramatically different responses among the hybrid sensors upon exposure to ppm levels of the target analyte. These responses are manifested as simultaneous resistive and capacitive components of the CNT impedance. While some hybrid materials demonstrated superior sensitivity (approximately two-fold increase) to detection of NO2 at room temperature compared to the pristine SWCNT sensor, other hybrid sensors showed suppressed sensitivity (approximately 20-fold decrease) to NO2 relative to the single wall CNT sensor. These differential impedimetric responses created from a library of metal-CNT hybrid materials may function as a practical approach for offering enhanced sensitivity and selectivity for gas sensing.
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82.80.-d Chemical analysis and related physical methods of analysis
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Sheet resistance determination of electrically symmetric planar four-terminal devices with extended contacts

Martin Cornils and Oliver Paul

J. Appl. Phys. 104, 024503 (2008); http://dx.doi.org/10.1063/1.2956333 (10 pages) | Cited 2 times

Online Publication Date: 18 July 2008

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This paper reports an analytic method to determine the sheet resistance Rsq of symmetric planar four-terminal devices based on resistance measurements. Using the technique of conformal mapping it is first shown that any such device is electrically equivalent to a corresponding symmetric unit disk with the same Rsq and invariant under rotations by 90°. Two independent resistances measurable on these devices are expressed analytically as a function of Rsq and of the contact opening angle α. These two resistances fully characterize the electrical properties of such planar conductive devices. A simple procedure to extract both α and Rsq from the resistance values is then presented. These findings are corroborated by the experimental characterization of four-contact devices of ten different geometries fabricated using a commercial complementary metal oxide semiconductor process. From these widely different devices, the sheet resistance of a n-well is extracted to be 1042 Ω with a relative uncertainty of only 0.45%.
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85.30.Tv Field effect devices
85.40.Ls Metallization, contacts, interconnects; device isolation
84.32.Dd Connectors, relays, and switches

Quantifying the relation between the morphology and performance of polymer solar cells using Monte Carlo simulations

Bao Lei, Yan Yao, Ankit Kumar, Yang Yang, and Vidvuds Ozolins

J. Appl. Phys. 104, 024504 (2008); http://dx.doi.org/10.1063/1.2956689 (6 pages) | Cited 13 times

Online Publication Date: 21 July 2008

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Morphology is a crucially important factor determining the efficiency of photocurrent generation in bulk heterojunction polymer solar cells. Morphology, which depends on the characteristics of the polymers as well as on the conditions of phase separation, affects the performance of solar cells by influencing the rate of exciton dissociation and the efficiency of charge carrier transport. Using Monte Carlo simulations, we investigate the effects of annealing time on the morphology of phase separation and charge transfer behavior inside the active layers of polymer solar cells. We find that a suitably defined correlation distance is an effective parameter that quantitatively characterizes different morphologies and can be used to establish a direct link with transmission electron microscopy images of real polymer solar cells. Optimal morphologies have been investigated, showing results that are consistent with experimental data.
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84.60.Jt Photoelectric conversion

Reverse-magnetic-field reciprocity in conductive samples with extended contacts

Martin Cornils and Oliver Paul

J. Appl. Phys. 104, 024505 (2008); http://dx.doi.org/10.1063/1.2951895 (7 pages) | Cited 2 times

Online Publication Date: 21 July 2008

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This paper reports on the extension of the reverse-magnetic-field reciprocity (RMFR) principle to electrically linear devices with arbitrary shape and extended contacts. The RMFR principle is a consequence of Onsager’s relation for the conductivity tensor depending on the magnetic field. It states the identity of resistance measurements on linearly conductive four-contact samples in a magnetic field B upon contact switching and magnetic field reversal. The previously available proof was restricted to samples with pointlike contacts and thus did not directly apply to real devices. Implications for two-contact and three-contact structures, for planar and symmetric devices, and the use of the structures as magnetic sensors are discussed. Experimental results obtained with complementary metal oxide semiconductor test structures of different geometries are in excellent agreement with the theoretical findings.
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73.40.Cg Contact resistance, contact potential

Analysis of carrier transport in quaterrylene thin film transistors formed by ultraslow vacuum deposition

Ryoma Hayakawa, Matthieu Petit, Toyohiro Chikyow, and Yutaka Wakayama

J. Appl. Phys. 104, 024506 (2008); http://dx.doi.org/10.1063/1.2956696 (5 pages) | Cited 4 times

Online Publication Date: 22 July 2008

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Quaterrylene field-effect transistors (FETs) with top-contact Au electrodes were formed on a SiO2 (200 nm)/p-Si (001) substrate by an ultraslow vacuum deposition technique, and their carrier transport was investigated. The quaterrylene FETs showed typical p-channel transistor behavior. The dependence of carrier mobility on grain size, film thickness, and temperature was examined to gain insight into the transport mechanism. Carrier mobility increased with grain size, showing that carrier transport was limited by grain boundaries. Temperature dependence in the range from 300 to 60 K was divided into two distinct behaviors. Above 210 K, carrier mobility showed thermally activated behavior, with energies of 100–150 meV required to overcome the potential barriers at grain boundaries. In contrast, the conduction mechanism became tunnel-transfer-like below 210 K. In the low temperature range, tunnel transfer through potential barriers at grain boundaries predominated over the thermally activated type. The change in carrier mobility was correlated with film thickness. Carrier mobility rose sharply with increasing thickness in the two-dimensional (2D) growth region, followed by saturation at 3 or 4 ML, where the growth process changed from 2D to three-dimensional mode. This result reveals that the first few layers of 2D growth work as an effective transistor channel. Enhancement in 2D growth in the vertical direction is crucial to improving carrier transport.
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85.30.Tv Field effect devices
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Modeling of InGaN/Si tandem solar cells

L. Hsu and W. Walukiewicz

J. Appl. Phys. 104, 024507 (2008); http://dx.doi.org/10.1063/1.2952031 (7 pages) | Cited 26 times

Online Publication Date: 22 July 2008

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We investigate theoretically the characteristics of monolithic InGaN/Si two-junction series-connected solar cells using the air mass 1.5 global irradiance spectrum. The addition of an InGaN junction is found to produce significant increases in the energy conversion efficiency of the solar cell over that of one-junction Si cells. Even when Si is not of high quality, such two-junction cells could achieve efficiencies high enough to be practically feasible. We also show that further, though smaller, improvements of the efficiency can be achieved by adding another junction to form an InGaN/InGaN/Si three-junction cell.
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84.60.Jt Photoelectric conversion

Tuning of ungated plasmons by a gate in the field-effect transistor with two-dimensional electron channel

V. V. Popov, A. N. Koudymov, M. Shur, and O. V. Polischuk

J. Appl. Phys. 104, 024508 (2008); http://dx.doi.org/10.1063/1.2955731 (6 pages) | Cited 13 times

Online Publication Date: 23 July 2008

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We show that voltage variations at a short gate can effectively tune higher-order ungated plasmon resonances in field-effect transistors (FETs). These higher-order ungated plasmon resonances may be excited by incoming terahertz radiation with much greater efficiency than the gated plasmon resonances. We calculate the spectra of the terahertz plasmon absorption in the frame of a rigorous electromagnetic approach, which allows us to estimate the radiation resistance. Based on the calculation results, we explain the behavior of different plasmon resonances in terms of the alternating-current FET equivalent circuit. The results may help design high performance plasmonic devices operating in the terahertz frequency range.
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85.30.Tv Field effect devices
84.30.Bv Circuit theory
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

Experimental study of a hybrid single flux quantum digital superconducting quantum interference device magnetometer

Torsten Reich, Pascal Febvre, Thomas Ortlepp, F. Hermann Uhlmann, Juergen Kunert, Ronny Stolz, and Hans-Georg Meyer

J. Appl. Phys. 104, 024509 (2008); http://dx.doi.org/10.1063/1.2958327 (9 pages) | Cited 6 times

Online Publication Date: 24 July 2008

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Digital superconducting quantum interference devices (SQUIDs) have been proposed to extend the dynamic range of analog SQUIDs whenever magnetic fields varying in a wide range need to be measured. We developed a digital SQUID concept, based on the single flux quantum technique, using the reliable 1 kA/cm2 niobium trilayer technology of the FLUXONICS Foundry. We present an extensive experimental analysis to evaluate the digital SQUID magnetometer operation in shielded and unshielded conditions. Bit error rate measurements have shown reliable operation of the digital superconducting circuitry with operation margins of ±11%, confirmed by stable operation in unshielded magnetometer mode for more than 10 h. The best intrinsic magnetic field resolution, corresponding to one magnetic single flux quantum (Φ0), was found to be about 4.2 nT. Peak-to-peak amplitudes of 14 800 and 2810 flux quanta Φ0 could be measured, respectively, for inductively coupled current and magnetic field inputs. A flux noise level of 220 mΦ0/math at 0.1 Hz has been measured, corresponding to a dynamic range of 76 dB (e.g., above 12 bits). Furthermore, preliminary experiments have shown that the integration of a digital SQUID with an analog dc SQUID in a hybrid magnetometer is a solution to reach simultaneously a high dynamic range and an increased field sensitivity. Such a device is of utmost importance to detect magnetic fields with high dynamic range to monitor earthquakes in the presence of high amplitude external magnetic perturbations.
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85.25.Dq Superconducting quantum interference devices (SQUIDs)
07.55.Ge Magnetometers for magnetic field measurements

Poly(3-hexylthiophene) thin-film transistors with variable polymer dielectrics for transfer-printed flexible electronics

D. R. Hines, A. Southard, and M. S. Fuhrer

J. Appl. Phys. 104, 024510 (2008); http://dx.doi.org/10.1063/1.2959821 (5 pages) | Cited 8 times

Online Publication Date: 24 July 2008

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The fabrication of high quality organic thin-film transistors onto flexible, plastic substrates has been extended to include the polymeric semiconductor material poly(3-hexlythiophene). The transfer printing method is used to easily assemble these devices onto either polyethylene terephthalate (PET) or polycarbonate (PC) substrates. A PC dielectric layer is used in conjunction with the PC substrate while both poly(methyl methacrylate) and polystyrene dielectric layers are used in conjunction with the PET substrate. In all cases the mobility of the transfer-printed devices, 0.019–0.041 cm2/V s, is significantly higher than that of the unprinted reference devices (SiO2 dielectric layer on a Si substrate), 0.007 cm2/V s. The width-normalized contact resistance is also lower for the transfer-printed devices, 0.18 MΩ cm, as compared to that for the reference devices, 0.56 MΩ cm. For the devices reported, the threshold voltage becomes more positive as the polar component of the surface energy of the polymer dielectric material increases. These results illustrate the simplicity, power, and versatility of the transfer printing method for the fabrication of high quality flexible electronics.
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85.30.Tv Field effect devices

Correlation of photoluminescent quantum efficiency and device characteristics for the soluble electrophosphorescent light emitter with interfacial layers

Nam Su Kang, Byeong-Kwon Ju, Ji Whan Kim, Jang-Joo Kim, Jae-Woong Yu, and Byung Doo Chin

J. Appl. Phys. 104, 024511 (2008); http://dx.doi.org/10.1063/1.2959817 (8 pages) | Cited 2 times

Online Publication Date: 25 July 2008

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We have investigated the effects of interfacial layers on the properties of soluble phosphorescent organic light emitting devices. Two kinds of polyfluorene-based interfacial layer materials have been studied; both were spin coated on top of PEDOT:PSS to form the insoluble layers by thermal annealing. The molecular-doped, phosphorescent light emitting layer comprising a polymeric host, small molecular host, and guest molecule was fabricated onto the thin interfacial layer. The photoluminescence quantum yield (PLQY) of these layers was measured with an integrating sphere. We have calculated the PLQY values of the single phosphorescent light emitting layer and various organic multilayers incorporating the interfacial layers, showing that a reduction in PLQY due to the interfacial quenching is more significant in the thicker interfacial layer structures. In spite of the decrease in PLQY induced by the triplet energy mismatch, polyfluorene-based interfacial layers improved the charge injection from PEDOT:PSS to the emitting layer, which results in the enhanced brightness and current. The triplet quenching by the interfacial layer could explain the reduction in luminous efficiency of the devices compared to the reference. This was also investigated by studying the charge carrier trapping, change in the spectral characteristics induced by the shift in the emission zone, and the analysis on the carrier balance of devices.
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85.60.Jb Light-emitting devices
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.55.Kz Solid organic materials
78.60.Fi Electroluminescence
61.72.Cc Kinetics of defect formation and annealing

Ion beam synthesis and charge storage behavior of Au nanocrystals in thin SiO2 layers

V. Beyer, F. Eichhorn, J. von Borany, A. Mücklich, and T. Müller

J. Appl. Phys. 104, 024512 (2008); http://dx.doi.org/10.1063/1.2957076 (7 pages) | Cited 1 time

Online Publication Date: 25 July 2008

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Au nanocrystals (NCs) were synthesized in a thin SiO2 layer by ion implantation and annealing in a tight distribution close to the Si/SiO2 interface. Between the NCs and the Si substrate a thin tunneling oxide forms self-organized during annealing totally depleted from Au NCs. Memory behavior is demonstrated by electron charging and discharging on metal–oxide–semiconductor capacitors. Lenticular liquid Au:Si droplets nucleate at the Si/SiO2 interface from silicon regions supersaturated by Au close to the oxide. Au NCs embedded in SiO2 above these droplets are stabilized during annealing due to a modified detailed balance of Au atom detachment and attachment. Capacitance-voltage and spreading resistance measurements reveal the impact of the Au contamination in the Si substrate. Structure and distribution of Au droplets and NCs are characterized by x-ray diffraction and transmission electron microscopy.
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81.07.Bc Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing
61.72.up Other materials
61.72.Cc Kinetics of defect formation and annealing
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)

The role of creep in the time-dependent resistance of Ohmic gold contacts in radio frequency microelectromechanical system devices

O. Rezvanian, C. Brown, M. A. Zikry, A. I. Kingon, J. Krim, D. L. Irving, and D. W. Brenner

J. Appl. Phys. 104, 024513 (2008); http://dx.doi.org/10.1063/1.2953072 (5 pages) | Cited 8 times

Online Publication Date: 28 July 2008

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It is shown that measured and calculated time-dependent electrical resistances of closed gold Ohmic switches in radio frequency microelectromechanical system (rf-MEMS) devices are well described by a power law that can be derived from a single asperity creep model. The analysis reveals that the exponent and prefactor in the power law arise, respectively, from the coefficient relating creep rate to applied stress and the initial surface roughness. The analysis also shows that resistance plateaus are not, in fact, limiting resistances but rather result from the small coefficient in the power law. The model predicts that it will take a longer time for the contact resistance to attain a power law relation with each successive closing of the switch due to asperity blunting. Analysis of the first few seconds of the measured resistance for three successive openings and closings of one of the MEMS devices supports this prediction. This work thus provides guidance toward the rational design of Ohmic contacts with enhanced reliabilities by better defining variables that can be controlled through material selection, interface processing, and switch operation.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
84.32.Dd Connectors, relays, and switches
73.40.Cg Contact resistance, contact potential

Analysis of resonant detection of terahertz radiation in high-electron mobility transistor with a nanostring/carbon nanotube as the mechanically floating gate

V. G. Leiman, M. Ryzhii, A. Satou, N. Ryabova, V. Ryzhii, T. Otsuji, and M. S. Shur

J. Appl. Phys. 104, 024514 (2008); http://dx.doi.org/10.1063/1.2957589 (7 pages) | Cited 4 times

Online Publication Date: 28 July 2008

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We develop a device model for a resonant detector of electromagnetic radiation with a frequency in the terahertz (THz) range modulated by megahertz (MHz) or gigahertz (GHz) signals based on a micromachined high-electron mobility transistor (HEMT) with a metallized nanostring (NS) or metallic carbon nanotube (CNT) as mechanically the floating gate and analyze the detector operation. The device model describes both the NS/CNT mechanical motion and plasma effects in the HEMT two-dimensional electron channel. Using this model, we calculate the output gate alternating current and the detector responsivity as functions of the carrier (in the THz range) and modulation frequencies, which are in the THz and MHz (or GHz range), respectively. It is shown that the THz detector responsivity exhibits sharp and high maxima under the conditions of both mechanical and plasma resonances.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.30.Tv Field effect devices
85.35.Kt Nanotube devices

Hydrogen sensing properties of a Pt-oxide-GaN Schottky diode

Yan-Ying Tsai, Kun-Wei Lin, Huey-Ing Chen, I-Ping Liu, Ching-Wen Hung, Tzu-Pin Chen, Tsung-Han Tsai, Li-Yang Chen, Kuei-Yi Chu, and Wen-Chau Liu

J. Appl. Phys. 104, 024515 (2008); http://dx.doi.org/10.1063/1.2959841 (6 pages) | Cited 3 times

Online Publication Date: 29 July 2008

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The interesting hydrogen sensing properties of a Pt-oxide-GaN metal-oxide-semiconductor-type Schottky diode are comprehensively studied and demonstrated. In the hydrogen-containing environment, the shift in current-voltage curves and decrease in turn-on voltage are found to be caused by the lowering of Schottky barrier height. Also, the corresponding series resistance is decreased from 191.8 (in air) to 155.3 Ω (for a 9970 ppm H2/air gas) at 30 °C. As the carrier gas is replaced by a nitrogen gas, a significant variation of 0.32 V and 19.56 Ω in the turn-on voltage Von and series resistance Rs values, respectively, is obtained at 30 °C, even at an extremely low hydrogen concentration of 4.3 ppm H2/N2. Since the oxygen atoms will be dissolved on the Pt metal surface and react with hydrogen atoms by the formation of hydroxyl and water, the number of adsorbed hydrogen atoms on the Pt surface is reduced. Moreover, the shorter response time constant and the larger initial rate of current density variation are found even at room temperature.
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82.80.-d Chemical analysis and related physical methods of analysis
85.30.Kk Junction diodes
85.30.Tv Field effect devices

The evolution of Ga and As core levels in the formation of Fe/GaAs (001): A high resolution soft x-ray photoelectron spectroscopic study

Jamie D. W. Thompson, James R. Neal, Tiehan H. Shen, Simon A. Morton, James G. Tobin, G. Dan Waddill, Jim A. D. Matthew, Denis Greig, and Mark Hopkinson

J. Appl. Phys. 104, 024516 (2008); http://dx.doi.org/10.1063/1.2942395 (12 pages) | Cited 2 times

Online Publication Date: 29 July 2008

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A high resolution soft x-ray photoelectron spectroscopic study of Ga and As 3d core levels has been conducted for Fe/GaAs (001) as a function of Fe thickness. This work has provided unambiguous evidence of substrate disrupting chemical reactions induced by the Fe overlayer—a quantitative analysis of the acquired spectra indicates significantly differing behavior of Ga and As during Fe growth, and our observations have been compared with existing theoretical models. Our results demonstrate that the outdiffusing Ga and As remain largely confined to the interface region, forming a thin intermixed layer. Whereas at low coverages Fe has little influence on the underlying GaAs substrate, the onset of substrate disruption when the Fe thickness reaches 3.5 Å results in major changes in the energy distribution curves (EDCs) of both As and Ga 3d cores. Our quantitative analysis suggests the presence of two additional As environments of metallic character: one bound to the interfacial region and another which, as confirmed by in situ oxidation experiments, surface segregates and persists over a wide range of overlayer thickness. Analysis of the corresponding Ga 3d EDCs found not two, but three additional environments—also metallic in nature. Two of the three are interface resident whereas the third undergoes outdiffusion at low Fe coverages. Based on the variations of the integrated intensities of each component, we present a schematic of the proposed chemical makeup of the Fe/GaAs (001) system.
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73.20.At Surface states, band structure, electron density of states
79.60.Jv Interfaces; heterostructures; nanostructures
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.35.Fx Diffusion; interface formation
68.35.Dv Composition, segregation; defects and impurities

Surface potential and magnetic properties of La0.7Sr0.3MnO3 periodic arrays fabricated by direct electron beam writing

Ming-Chung Wu, Yi-Jen Wu, Yu-Ching Huang, Chih-Min Chuang, Kuo-Chung Cheng, Ching-Fuh Lin, Yang-Fang Chen, and Wei-Fang Su

J. Appl. Phys. 104, 024517 (2008); http://dx.doi.org/10.1063/1.2957489 (6 pages) | Cited 1 time

Online Publication Date: 29 July 2008

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It is demonstrated that magnetic periodic arrays can be easily fabricated from direct writing water developable La0.7Sr0.3MnO3 electron beam resist. Two unique features of our approach are (1) the patterned La0.7Sr0.3MnO3 resist film can be developed using nontoxic and environmentally friendly pure water and (2) either positive or negative patterns can be fabricated depending on the dosage of electron beam. The mechanism of the dual function characteristic of the resist was studied using Kelvin probe microscope. The surface potential of patterned La0.7Sr0.3MnO3 resist increases with increasing electron beam dosage due to the changes of resist composition. The formations of periodic magnetic arrays were confirmed by the studies of scanning electron microscope and magnetic force microscope. The magnetization of La0.7Sr0.3MnO3 can be enhanced by postsintering the sample at 900 °C after electron beam irradiation. We have therefore provided a one-step, simple, and convenient alternative technique for the fabrication of nanoscale magnetic patterns, which form the building blocks for the study of physical properties in periodic magnetic arrays.
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81.07.Bc Nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.46.Hk Nanocrystals
61.80.Fe Electron and positron radiation effects
61.82.Rx Nanocrystalline materials
81.16.Nd Micro- and nanolithography

Influence of nanocrystal size on the transport properties of Si nanocrystals

Xin Zhou, Kouichi Usami, M. A. Rafiq, Yoshishige Tsuchiya, Hiroshi Mizuta, and Shunri Oda

J. Appl. Phys. 104, 024518 (2008); http://dx.doi.org/10.1063/1.2952036 (4 pages) | Cited 10 times

Online Publication Date: 30 July 2008

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In this study, we have investigated the carrier transport mechanism across silicon nanocrystals with the Al/p-Si/Si nanocrystals/Al structure. Sizes of silicon nanocrystals were controlled at diameters of ∼ 6, ∼ 8, and ∼ 11 nm. It is shown that the conductivity σ of silicon nanocrystals, both as-grown and annealed, exhibits σ∝exp[−(T0/T)]1/2 behavior under low electrical fields and over a wide temperature range. The phenomenon of material constant T0 increasing with the decrease of nanocrystal size has been observed. Considering nanocrystal size effect, experimental results can be explained by the hopping-percolation model. The influence of nanocrystal size on transport properties has been discussed. Based on this model, changes in T0 after annealing treatment are attributed to an increase in effective decay length.
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73.61.Cw Elemental semiconductors
72.20.Ee Mobility edges; hopping transport
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.72.Cc Kinetics of defect formation and annealing

Polarization sensitive detection of 100 GHz radiation by high mobility field-effect transistors

M. Sakowicz, J. Łusakowski, K. Karpierz, M. Grynberg, W. Knap, and W. Gwarek

J. Appl. Phys. 104, 024519 (2008); http://dx.doi.org/10.1063/1.2957065 (5 pages) | Cited 7 times

Online Publication Date: 30 July 2008

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Detection of 100 GHz electromagnetic radiation by a GaAs/AlGaAs high electron mobility field-effect transistor was investigated at 300 K as a function of the angle α between the direction of linear polarization of the radiation and the symmetry axis of the transistor. The angular dependence of the detected signal was found to be A0 cos2(α−α0)+C with A0, α0, and C dependent on the electrical polarization of the transistor gate. This dependence is interpreted as due to excitation of two crossed phase-shifted oscillators. A response of the transistor chip (including bonding wires and the substrate) to 100 GHz radiation was numerically simulated. Results of calculations confirmed experimentally observed dependencies and showed that the two oscillators result from an interplay of 100 GHz currents defined by the transistor impedance together with bonding wires and substrate related modes.
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29.40.-n Radiation detectors
85.30.Tv Field effect devices
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