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15 Feb 2012

Volume 111, Issue 4, Articles (04xxxx)

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J. Appl. Phys. 111, 043501 (2012); http://dx.doi.org/10.1063/1.3680881 (8 pages)

Gregory J. McGraw and Stephen R. Forrest
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back to top Device Physics

Energy harvesting in the super-harmonic frequency region of a twin-well oscillator

R. Masana and M. F. Daqaq

J. Appl. Phys. 111, 044501 (2012); http://dx.doi.org/10.1063/1.3684579 (11 pages) | Cited 2 times

Online Publication Date: 17 February 2012

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Nonlinear dynamical systems exhibit super-harmonic resonances that can activate large-amplitude motions at fraction integers of the fundamental frequency of the system. Such resonances offer a unique and untapped opportunity for harnessing vibratory energy from excitation sources with low-frequency components. To that end, this paper exploits the super-harmonic frequency bands of a nonlinear twin-well (bi-stable) oscillator for harvesting energy from low-frequency excitations. Theoretical and experimental studies are performed on an axially loaded clamped-clamped piezoelectric beam harvester with bi-stable potential characteristics. Voltage- and power-frequency bifurcation maps are generated near the super-harmonic resonance of order two. It is shown that, for certain base acceleration levels, the harvester can exhibit responses that are favorable for energy harvesting. These include a unique branch of large-orbit periodic inter-well oscillations, coexisting branches of large-orbit solutions, and a bandwidth of frequencies where a unique chaotic attractor exists. In these frequency regions, the harvester can produce power levels at half its fundamental frequency that are comparable to those obtained near the fundamental frequency.
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84.30.Ng Oscillators, pulse generators, and function generators
84.60.-h Direct energy conversion and storage
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
82.40.Bj Oscillations, chaos, and bifurcations

Understanding the operation of quantum dot intermediate band solar cells

A. Luque, P. G. Linares, E. Antolín, I. Ramiro, C. D. Farmer, E. Hernández, I. Tobías, C. R. Stanley, and A. Martí

J. Appl. Phys. 111, 044502 (2012); http://dx.doi.org/10.1063/1.3684968 (12 pages) | Cited 5 times

Online Publication Date: 17 February 2012

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In this paper, a model for intermediate band solar cells is built based on the generally understood physical concepts ruling semiconductor device operation, with special emphasis on the behavior at low temperature. The model is compared to JL-VOC measurements at concentrations up to about 1000 suns and at temperatures down to 20 K, as well as measurements of the radiative recombination obtained from electroluminescence. The agreement is reasonable. It is found that the main reason for the reduction of open circuit voltage is an operational reduction of the bandgap, but this effect disappears at high concentrations or at low temperatures.
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88.40.J- Types of solar cells
88.40.hj Efficiency and performance of solar cells

Current density-voltage analyses and interface characterization in Ag/DNA/p-InP structures

Ö. Güllü, O. Pakma, and A. Türüt

J. Appl. Phys. 111, 044503 (2012); http://dx.doi.org/10.1063/1.3684989 (6 pages)

Online Publication Date: 17 February 2012

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The current density-voltage (J-V) characteristics of Ag/DNA/p-InP metal-insulator-semiconductor (MIS) structures have been investigated in room temperature. We have observed that the Ag/DNA/p-InP structure shows an excellent rectifying behavior and that this structure increases the barrier height (φb0). The main electrical parameters of these structures, such as ideality factor (n), barrier height, and average series resistance values were found to be 1.087, 0.726 eV, and 66.92 Ω. This value of n was attributed to the presence of an interfacial insulator layer at the Ag/p-InP interface and the density of interface states (Nss) localized at the InP/DNA interface. The values of Nss localized at the InP/DNA interface were found at 0.675-Ev in the 1.38 × 1012 eV−1 cm−2.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
87.15.-v Biomolecules: structure and physical properties
73.40.Ei Rectification

Characterization of thin-film GaAs diodes grown on germanium-on-insulator on Si substrate

Z. Xu, S. F. Yoon, Y. C. Yeo, C. K. Chia, Y. B. Cheng, and G. K. Dalapati

J. Appl. Phys. 111, 044504 (2012); http://dx.doi.org/10.1063/1.3686182 (4 pages) | Cited 1 time

Online Publication Date: 17 February 2012

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In this study, we report the characterization of thin-film GaAs grown on germanium-on-insulator (GeOI) on Si substrate. A GaAs/GeOI diode with a 600 nm buffer layer showed a rectification of 1.0 × 107 at ±2 V and had an electrical performance similar to that of the reference sample grown on GaAs substrate. We demonstrate two thin diodes (<350 nm in thickness) that still showed high forward densities and rectification properties. The electrical performances of the diodes degraded as the diode active regions were grown closer to the GaAs/Ge interface due to the increase of defects propagating into the active regions. The experimental results were fitted with the thermionic emission equation and the Frenkel-Poole model.
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85.30.Kk Junction diodes
73.61.Ey III-V semiconductors

Analysis and experimental demonstration of an active acoustic metamaterial cell

Wael Akl and Amr Baz

J. Appl. Phys. 111, 044505 (2012); http://dx.doi.org/10.1063/1.3686210 (9 pages) | Cited 1 time

Online Publication Date: 17 February 2012

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Active acoustic metamaterials (AAMM) have been developed to overcome the limited frequency bandwidth characteristics of passive acoustic metamaterials. The AAMM rely in their operation on using piezoelectric active ingredients in a fluid-solid composite structure forming the basic building block of a larger metamaterial periodic arrangement. A prototype of AAMM composite cell is manufactured and active control strategies are implemented on the piezoelectric elements to vary its stiffness in order to control the effective dynamic density of the cell. Acoustic characterization of the developed AAMM cell is carried out by measuring its acoustic impedance and transmission loss and comparing the results with the predictions of a finite element model. The obtained experimental measurements and the predictions of the finite element model are in very good agreement for the considered frequency range. The transfer functions between the reference microphone in the impedance tube and the piezoelectric elements demonstrate the coupling nature inside the cell rendering it to a system of single acoustic properties acting as a single degree of freedom system. The proposed AAMM can be useful in manufacturing the next generation of acoustic cloaks and metamaterials with controllable directivity and dispersion characteristics.
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42.70.-a Optical materials
81.05.Xj Metamaterials for chiral, bianisotropic and other complex media
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
02.70.Dh Finite-element and Galerkin methods
42.30.Lr Modulation and optical transfer functions
43.38.Kb Microphones and their calibration

Coupled microstrip line microwave phase shifter using ferroelectric thin film varactors

Su Sheng and C. K. Ong

J. Appl. Phys. 111, 044506 (2012); http://dx.doi.org/10.1063/1.3686615 (5 pages) | Cited 2 times

Online Publication Date: 22 February 2012

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This article presents a coupled microstrip line microwave phase shifter using ferroelectric thin film varactors with parallel plate electrodes integrated on high-resistivity silicon substrate. This design is proposed as a component in low-cost beam scanning phased array antennas. The phase shifter structure consists of coupled microstrip line periodically loaded with eight BaxSr1-xTiO3 (BST) varactors and two planar Marchand baluns. The planar Marchand baluns work as the transformers of odd mode excitation and a transmission stop circuit for even mode excitation as well as the impedance matching networks. A differential phase shift of 45° was obtained with a low dc bias of 20 V at frequency of 9 GHz, which corresponds to a figure of merit of ∼23°/dB. These results represent significant progress toward compact size, low loss, and ferroelectric thin film-based phase shifters at room temperature.
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84.40.Ba Antennas: theory, components and accessories
84.40.Az Waveguides, transmission lines, striplines

Bidirectional operation of vertical organic triodes

Axel Fischer, Philipp Siebeneicher, Hans Kleemann, Karl Leo, and Björn Lüssem

J. Appl. Phys. 111, 044507 (2012); http://dx.doi.org/10.1063/1.3686744 (8 pages) | Cited 3 times

Online Publication Date: 24 February 2012

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Due to their effective short channel length of only a few hundred nanometers, vertical organic triodes (VOTs) have a high potential to overcome problems of low current densities and switching speed in current organic field effect transistors (OFETs). Furthermore, VOTs are easy to build because no sub-structuring of the base contact is necessary. Nevertheless, these devices are poorly investigated. In literature, two different working mechanisms are suggested: hot carrier transport through the metallic base or transport of charge carriers through a permeable base electrode. As a strong asymmetry is expected for function principle based on hot carriers, we are able to distinguish between both mechanisms by examining the bidirectional transmission properties of the VOT consisting of electron transporting materials. We show that high transmission values (>95%) are possible for both directions, suggesting a base contact with openings forming a grid electrode.
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84.47.+w Vacuum tubes
72.30.+q High-frequency effects; plasma effects

B segregation to grain boundaries and diffusion in polycrystalline Si with flash annealing

S. Jin, K. S. Jones, M. E. Law, and R. Camillo-Castillo

J. Appl. Phys. 111, 044508 (2012); http://dx.doi.org/10.1063/1.3688246 (5 pages)

Online Publication Date: 24 February 2012

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Three-dimensional atom probe tomography was used to characterize the segregation of B dopant atoms to grain boundaries in polycrystalline Si after flash-assisted rapid thermal annealing. Tomographic reconstructions allowed direct measurement of segregation coefficients, which were found to be greater at lower flash temperatures with thermal budgets that limit grain growth. Hall measurements confirmed the deactivation of B at the grain boundaries, while secondary ion mass spectrometry was used to measure B diffusion in the film. Experimental parameters were then simulated in a diffusion model which accurately predicts the diffusion of B in polycrystalline Si at flash temperatures of 1150 °C, 1250 °C, and 1350 °C, as well as with conventional rapid thermal annealing.
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66.30.-h Diffusion in solids
61.72.Cc Kinetics of defect formation and annealing
61.72.Mm Grain and twin boundaries
72.20.My Galvanomagnetic and other magnetotransport effects
72.80.Cw Elemental semiconductors

Investigation of the physics of sensing in organic field effect transistor based sensors

Davianne Duarte and Ananth Dodabalapur

J. Appl. Phys. 111, 044509 (2012); http://dx.doi.org/10.1063/1.3686686 (7 pages) | Cited 1 time

Online Publication Date: 28 February 2012

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In this study, we examine the physics governing the operation of chemical sensors based on field effect transistors, which use organic semiconductors as an active sensing medium. Short channel length devices can operate in the injection-limited regime and the sensing response in this regime is totally unlike the response in larger scale devices. In large geometry sensors, charge carrier trapping plays an important role in the sensor response. We describe in detail the various factors that influence charge trapping effects. Oriented dipoles from polar analytes can also influence sensor behavior and manifest as a current increase upon exposure to the analyte.
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85.30.Tv Field effect devices
82.80.-d Chemical analysis and related physical methods of analysis

Exciton transport in organic semiconductors: Förster resonance energy transfer compared with a simple random walk

K. Feron, X. Zhou, W. J. Belcher, and P. C. Dastoor

J. Appl. Phys. 111, 044510 (2012); http://dx.doi.org/10.1063/1.3687373 (7 pages) | Cited 3 times

Online Publication Date: 29 February 2012

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Förster resonance energy transfer theory (FRET) and a simple random walk (RW) are both implemented in a dynamic Monte Carlo simulation with the aim of determining the exciton diffusion length from photoluminescence (PL) measurements. The calculated diffusion lengths obtained from both models are shown to be the same. As such, given that the computational time of a random walk is typically 2–3 orders of magnitude smaller than the FRET approach, this work shows that the RW methodology can be a preferable model for the determination of diffusion lengths. We also show that the RW approach may also be implemented in Monte Carlo simulations that describe organic solar cells. Despite the fact that (compared with FRET) RW does not account for non-nearest neighbor hopping or energy relaxation, we show that the resulting overestimation of the simulated current will not exceed 2% for typical OPV parameters. In addition, by taking advantage of the gain in speed we are able to investigate the impact of the exciton diffusion length on the optimal interface distance and show that materials with longer exciton diffusion lengths are less sensitive to variations in the morphology of the active layer of an organic solar cell.
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88.40.jr Organic photovoltaics
81.05.Fb Organic semiconductors
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
78.55.Kz Solid organic materials
72.20.Ee Mobility edges; hopping transport

Characterization of the E(0.31) defect introduced in bulk n-Ge by H or He plasma exposure

C. Nyamhere, A. Venter, F. D. Auret, S. M. M. Coelho, and D. M. Murape

J. Appl. Phys. 111, 044511 (2012); http://dx.doi.org/10.1063/1.3687426 (5 pages)

Online Publication Date: 29 February 2012

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Bulk antimony (Sb) doped germanium (n-Ge) samples with doping concentrations ranging between 7.0 × 1014 cm−3 and 2.5 × 1015 cm−3 were exposed to a dc-hydrogen or helium plasma. Hydrogen exposure resulted in the introduction of a single prominent defect level at EC −0.31 eV. Exposing similar samples to He plasmas introduced the same electron trap. The trap concentration increased linearly with dopant concentration suggesting that Sb may be a component of this plasma-induced trap. Thermal annealing kinetics studies suggested that this defect anneals out by diffusion.
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71.55.Cn Elemental semiconductors
61.72.uf Ge and Si

The coexistence of two-dimensional electron and hole gases in GaN-based heterostructures

N. Al Mustafa, R. Granzner, V. M. Polyakov, J. Racko, M. Mikolášek, J. Breza, and F. Schwierz

J. Appl. Phys. 111, 044512 (2012); http://dx.doi.org/10.1063/1.3688219 (6 pages) | Cited 1 time

Online Publication Date: 29 February 2012

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The formation of two-dimensional carrier gases in gated GaN/AlGaN/GaN heterostructures is investigated theoretically. It is shown that under certain conditions a two-dimensional hole gas at the upper GaN/AlGaN interface can be formed in addition to the two-dimensional electron gas at the lower AlGaN/GaN interface. For the calculations, a Schrödinger-Poisson solver and a simple analytical model developed in the present work are used. Conditions for the formation of a two-dimensional hole gas are elaborated. It is shown that once a two-dimensional hole gas is created, it shields the coexisting two-dimensional electron gas which will result in a diminishing effect of the gate voltage on the two-dimensional electron gas.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
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