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15 Oct 2010

Volume 108, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 108, 081101 (2010); http://dx.doi.org/10.1063/1.3493111 (18 pages)

Yiping Wang
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back to top Device Physics

Can we enhance two-dimensional electron gas from ferroelectric/GaN heterostructures?

Jihua Zhang, Chuanren Yang, Ying Liu, Min Zhang, Hongwei Chen, Wanli Zhang, and Yanrong Li

J. Appl. Phys. 108, 084501 (2010); http://dx.doi.org/10.1063/1.3494040 (6 pages)

Online Publication Date: 18 October 2010

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Ferroelectric (FE)/semiconductor heterostructures are very promising for future electronic devices. This paper examined several kinds of devices based on heterostructures made from FE and GaN semiconductor. Results showed that although two-dimensional electron gas (2DEG) density in GaN could be tuned by the polarization of FE, it was harsh to enhance the 2DEG greatly for a practical structure even in theory. We proposed that beside the device process, structure design of the device was also important to 2DEG characteristics. To keep or enhance the 2DEG density need novel materials and/or device structures. Our theory predictions may provide some references to design of new electronic devices and promote experimental studies for FE/GaN heterostructures.
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73.20.At Surface states, band structure, electron density of states
77.80.-e Ferroelectricity and antiferroelectricity
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
77.22.Ej Polarization and depolarization

Point defects analysis of zinc oxide thin films annealed at different temperatures with photoluminescence, Hall mobility, and low frequency noise

Lin Ke, Szu Cheng Lai, Jian Dong Ye, Vivian Lin Kaixin, and Soo Jin Chua

J. Appl. Phys. 108, 084502 (2010); http://dx.doi.org/10.1063/1.3494046 (6 pages) | Cited 1 time

Online Publication Date: 18 October 2010

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Zinc oxide (ZnO) thin films annealed at different temperatures were studied with photoluminescence (PL), electrical resistivity, Hall mobility, and 1/f noise spectroscopy. Relatively high electrical conductivity and carrier concentration in sample annealed at 400 °C suggested the presence of ZnO interstitials. Rapid reduction in electrical conductivity and carrier concentration upon increasing the annealing temperature suggested that ZnO interstitials could be eliminated by high temperature annealing. Presence of G-R noise in sample annealed at 400 °C indicated high level of electron trapping activities. Density of Zn vacancies acting as electron traps was estimated by Lorentzian fitting on the G-R noise. PL spectra exhibiting dominant green emission in all samples suggested the presence of Zn vacancies in high concentration. Yellow-orange emission in PL in samples annealed at 600 °C and below indicated the presence of O interstitials, while the same emission in samples annealed at higher temperatures were ascribed to Si impurities diffused from the substrate. Sharp reduction in mobility and surge in Hooge’s parameter in sample annealed at 700 °C implied high level of electron scattering due to large extrinsic Si impurities. Gradual rise in green-yellow emission and electron concentration as annealing temperature increased from 500 to 700 °C were ascribed to the gradual formation of O vacancies.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
81.40.Gh Other heat and thermomechanical treatments
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
61.72.Cc Kinetics of defect formation and annealing
61.72.jj Interstitials
61.72.jd Vacancies

Phase-change activities on gallium-doped indium oxide

S.-L. Wang and L.-H. Peng

J. Appl. Phys. 108, 084503 (2010); http://dx.doi.org/10.1063/1.3494089 (5 pages)

Online Publication Date: 19 October 2010

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This study investigated the phase-change activities on a gallium-doped indium oxide (Ga:InO) device that can be supplied with a constant heat flow via symmetric contact to a pair of rodlike heating elements. A device set/reset current of 0.8/18 μA and resistance window of 2.6×105 to 107 Ω can be found on Ga:InO with a 6.2 μm2 device area and a thickness of 40 nm. Analysis of a log-log plot revealed slopes of 1.07±0.01 and −1.12±0.03 that were found to correlate with the switching current and resistance change between the high-/low-value states of the Ga:InO device area, respectively. These observations lead to the estimated energy densities of 1.77±0.11 pJ/μm3 and 7.26±0.44 pJ/μm3 required to initiate the set and reset process in Ga:InO, respectively. Through differential scanning calorimetry analysis and acceleration tests of the Ga:InO film and the device, an activation energy of ∼ 1.275±0.005 eV was found, which corresponded to the high-/low-resistance state change. A data retention time of ten years was further estimated when the Ga:InO device is operated at 75 °C. According to the transmission electron microscopy analysis, these observations are correlated with an amorphous to cubic phase transition in In2O3, which takes place at a crystallization temperature of 252 °C, and suggest that the phase-change activities originate from the Joule heating effect.
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61.43.Dq Amorphous semiconductors, metals, and alloys
81.05.Gc Amorphous semiconductors
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.70.K- Solid-solid transitions

Analysis of thermal properties of GaInN light-emitting diodes and laser diodes

Qifeng Shan, Qi Dai, Sameer Chhajed, Jaehee Cho, and E. Fred Schubert

J. Appl. Phys. 108, 084504 (2010); http://dx.doi.org/10.1063/1.3493117 (8 pages) | Cited 4 times

Online Publication Date: 19 October 2010

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The thermal properties, including thermal time constants, of GaInN light-emitting diodes (LEDs) and laser diodes (LDs) are analyzed. The thermal properties of unpackaged LED chips are described by a single time constant, that is, the thermal time constant associated with the substrate. For unpackaged LD chips, we introduce a heat-spreading volume. The thermal properties of unpackaged LD chips are described by a single time constant, that is, the thermal time constant associated with the heat spreading volume. Furthermore, we develop a multistage RthCth thermal model for packaged LEDs. The model shows that the transient response of the junction temperature of LEDs can be described by a multiexponential function. Each time constant of this function is approximately the product of a thermal resistance, Rth, and a thermal capacitance, Cth. The transient response of the junction temperature is measured for a high-power flip-chip LED, emitting at 395 nm, by the forward-voltage method. A two stage RthCth model is used to analyze the thermal properties of the packaged LED. Two time constants, 2.72 ms and 18.8 ms are extracted from the junction temperature decay measurement and attributed to the thermal time constant of the LED GaInN/sapphire chip and LED Si submount, respectively.
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42.55.Px Semiconductor lasers; laser diodes
85.60.Jb Light-emitting devices
42.60.By Design of specific laser systems

Realizing the frequency quality factor product limit in silicon via compact phononic crystal resonators

Drew Goettler, Mehmet Su, Zayd Leseman, Yasser Soliman, Roy Olsson, and Ihab El-Kady

J. Appl. Phys. 108, 084505 (2010); http://dx.doi.org/10.1063/1.3475987 (5 pages) | Cited 3 times

Online Publication Date: 21 October 2010

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High-Q (quality factor) resonators are a versatile class of components for radio frequency micro-electromechanical systems . Phononic crystals provide a promising method of producing these resonators. In this article, we present a theoretical study of the Q factor of a cavity resonator in a two-dimensional phononic crystal comprised of tungsten rods in a silicon matrix. One can optimize the Q of a phononic crystal resonator by varying the number of inclusions or the cavity harmonic number. We conclude that using higher harmonics marginally increases Q while increasing crystal length via additional inclusions causes Q to increase by orders of magnitude. Incorporating loss into the model shows that the silicon material limit on Q is achievable using a two-dimensional phononic crystal design with a reasonable length. With five layers of inclusions on either side of the cavity, the material limit on Q is achieved, regardless of the harmonic number.
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77.65.Fs Electromechanical resonance; quartz resonators
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.10.Cm Micromechanical devices and systems
42.55.Tv Photonic crystal lasers and coherent effects

Self-consistent electrothermal Monte Carlo simulation of single InAs nanowire channel metal-insulator field-effect transistors

Toufik Sadi, Jean-Luc Thobel, and François Dessenne

J. Appl. Phys. 108, 084506 (2010); http://dx.doi.org/10.1063/1.3496658 (7 pages)

Online Publication Date: 22 October 2010

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Electron transport and self-heating effects are investigated in metal-insulator field-effect transistors with a single InAs nanowire channel, using a three-dimensional electrothermal Monte Carlo simulator based on finite-element meshing. The model, coupling an ensemble Monte Carlo simulation with the solution of the heat diffusion equation, is carefully calibrated with data from experimental work on these devices. This paper includes an electrothermal analysis of the device basic output characteristics as well the microscopic properties of transport, including current-voltage curves, heat generation and temperature distributions, and electron velocity profiles. Despite the low power dissipation, results predict significant peak temperatures, due to the high power density levels and the poor thermal management in these structures. The extent of device self-heating is shown to be strongly dependent on both device biasing configuration as well as geometry.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling

Reset dynamics and latching in niobium superconducting nanowire single-photon detectors

Anthony J. Annunziata, Orlando Quaranta, Daniel F. Santavicca, Alessandro Casaburi, Luigi Frunzio, Mikkel Ejrnaes, Michael J. Rooks, Roberto Cristiano, Sergio Pagano, Aviad Frydman, and Daniel E. Prober

J. Appl. Phys. 108, 084507 (2010); http://dx.doi.org/10.1063/1.3498809 (7 pages) | Cited 7 times

Online Publication Date: 22 October 2010

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We study the reset dynamics of niobium (Nb) superconducting nanowire single-photon detectors (SNSPDs) using experimental measurements and numerical simulations. The numerical simulations of the detection dynamics agree well with experimental measurements, using independently determined parameters in the simulations. We find that if the photon-induced hotspot cools too slowly, the device will latch into a dc resistive state. To avoid latching, the time for the hotspot to cool must be short compared to the inductive time constant that governs the resetting of the current in the device after hotspot formation. From simulations of the energy relaxation process, we find that the hotspot cooling time is determined primarily by the temperature-dependent electron-phonon inelastic time. Latching prevents reset and precludes subsequent photon detection. Fast resetting to the superconducting state is, therefore, essential, and we demonstrate experimentally how this is achieved. We compare our results to studies of reset and latching in niobium nitride SNSPDs.
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85.25.Oj Superconducting optical, X-ray, and γ-ray detectors (SIS, NIS, transition edge)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.25.Pb Superconducting infrared, submillimeter and millimeter wave detectors

Electrical annealing for flexible organic light-emitting diodes having poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) anodes

Byoungchoo Park, Chan Hyuk Park, Younchan Yim, and Jongwoon Park

J. Appl. Phys. 108, 084508 (2010); http://dx.doi.org/10.1063/1.3499286 (6 pages)

Online Publication Date: 25 October 2010

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We present a postproduction method of electrical annealing (E-annealing) to improve the performance of flexible organic light-emitting diodes (FOLEDs) having conductive polymer anodes on a polyethersulfone substrate. The polymer that was used for the anodes was dimethylsulfoxide-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate). It was found that E-annealing of the fabricated FOLEDs can reduce the turn-on voltage and enhance the brightness with reduced flowing current, thereby enhancing the device efficiency. With the E-annealing method, we have successfully demonstrated efficient solution-processed green FOLEDs, which show a peak luminescence of 6 100 cd/m2 and a maximum current efficiency of 16.4 cd/A. An ion migration model to explain the phenomena related to the improvement of FOLEDs is also proposed.
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78.55.Kz Solid organic materials
78.60.Fi Electroluminescence
42.55.Sa Microcavity and microdisk lasers
42.25.Fx Diffraction and scattering

Comparison of noise floor and sensitivity for different magnetoelectric laminates

Junqi Gao, Jaydip Das, Zengping Xing, Jiefang Li, and D. Viehland

J. Appl. Phys. 108, 084509 (2010); http://dx.doi.org/10.1063/1.3486483 (3 pages) | Cited 5 times

Online Publication Date: 25 October 2010

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We present a comparison of the magnetoelectric (ME) response and magnetic-field sensitivities of engineered laminate sensors comprised of magnetostrictive and piezoelectric phases. The ME voltage coefficients for Metglas and single crystal fibers of Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) or Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) are about 2.8 times larger than those with Metglas-Pb(Zr,Ti)O3 (PZT) ceramic ones. This results in a 1.7 times enhancement in the magnetic-field sensitivity for the structures with single crystals. Accordingly, the noise floors are about three to four times lower for composites with PMN-PT or PZN-PT fibers than those with PZT.
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85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
75.85.+t Magnetoelectric effects, multiferroics

Influence of interdot hopping and intradot many-body interaction on conductance through parallel triple-quantum-dot device: Nonequilibrium Green’s function approach

Hua-Hua Fu and Kai-Lun Yao

J. Appl. Phys. 108, 084510 (2010); http://dx.doi.org/10.1063/1.3500503 (9 pages)

Online Publication Date: 26 October 2010

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By means of the nonequilibrium Green’s function technique, influence of interdot hoppings t’s and intradot many-body interaction U on electronic transport through parallel triple-quantum-dot (tQD) device in the Coulomb blockade regime are theoretically investigated. In the symmetrical tQD device with uniform interdot hopping t, as t increases starting from zero, the conductance is enhanced due to the opening of new channels for the electronic transport, but as t increases over its critical tc, the interdot hoppings have an opposite influence, which are well consistent with those obtained by the numerical renormalization-group method. The effect of the intradot electron interaction U on the electronic transport through the tQD device is strongly relative to the strength of t. For small t, U suppresses the electronic transport and the Coulomb blockade effect on the dots is dominant. However, for relatively strong t, a small U has a contribution to enhance the electronic transport through the device. These theoretical results can be applied to explain the recent experimental findings on a parallel tQD device. In the asymmetrical parallel tQD device, the symmetry-breaking in the interdot hoppings leads to the increasing on the electronic transport owing to the appearance of more resonance levels in the conductance spectrum. On the basis of this feature, it is proposed that we can obtain some devices with desirable transport properties, such as the molecular charge rectifier, through adjusting the interdot hoppings and the intradot many-body interaction.
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85.35.Ds Quantum interference devices
85.35.Gv Single electron devices

Determination of free polaron lifetime in organic bulk heterojunction solar cells by transient time analysis

Kejia Li, Yang Shen, Nabanita Majumdar, Chong Hu, Mool C. Gupta, and Joe C. Campbell

J. Appl. Phys. 108, 084511 (2010); http://dx.doi.org/10.1063/1.3493114 (5 pages) | Cited 1 time

Online Publication Date: 26 October 2010

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A transient response technique that is widely used to measure the minority carrier lifetime in inorganic semiconductors is proposed to measure the lifetime of free polarons in a polymer:fullerene bulk heterojunction (BHJ) solar cell. A numerical model that can be used to describe the transient behavior of BHJ devices has been developed. Using the proposed method, the lifetime of free polarons in poly (3-hexylthiophene) and [6, 6]-phenyl C61-butyric acid methyl ester blend film is estimated to be in the range of 300–400 ns.
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88.40.H- Solar cells (photovoltaics)
71.38.-k Polarons and electron-phonon interactions

Model for localized buildup and thinning of metal due to electromigration

P. A. Flinn and J. C. Doan

J. Appl. Phys. 108, 084512 (2010); http://dx.doi.org/10.1063/1.3498819 (4 pages)

Online Publication Date: 27 October 2010

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We present a model to account for the highly localized buildup and thinning of metal that has been observed during electromigration testing. It assumes variations in mobility along the line, rather than complete blockage. Using reasonable estimates for the parameters needed, it is possible to obtain a quantitative fit to previously observed data.
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66.30.Qa Electromigration

TiAlNiAu contacts for ultrathin AlN/GaN high electron mobility transistor structures

Lin Zhou, C. Y. Chang, S. J. Pearton, F. Ren, Amir Dabiran, and David. J. Smith

J. Appl. Phys. 108, 084513 (2010); http://dx.doi.org/10.1063/1.3501106 (5 pages) | Cited 1 time

Online Publication Date: 28 October 2010

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The morphology of ultrathin AlN/GaN high electron mobility transistors without or with TiAlNiAu Ohmic contacts, and annealed from 750 to 950 °C, has been investigated using transmission electron microscopy and associated analytical techniques. After annealing, the contact surface roughness was degraded due to intermixing and phase separation of the metal layers. TiN contact inclusions (CIs) that had penetrated through the AlN layers into the underlying GaN layers along threading dislocations, were observed in all annealed samples. The CI density increased with increasing annealing temperature but the lowest specific contact resistivity was obtained for structures annealed at 850 °C. Annealing at 950 °C caused cracking on the contact metal surface. The AlN layers remained intact in dislocation-free areas of all samples. The relationship between annealing temperature, interfacial structure and contact resistance is also discussed.
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85.30.Tv Field effect devices

Nonideal parasitic resistance effects in bulk heterojunction organic solar cells

John R. Tumbleston, Doo-Hyun Ko, Edward T. Samulski, and Rene Lopez

J. Appl. Phys. 108, 084514 (2010); http://dx.doi.org/10.1063/1.3494100 (8 pages) | Cited 1 time

Online Publication Date: 28 October 2010

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A common assumption in both experimental measurements and device modeling of bulk heterojunction (BHJ) organic solar cells is that parasitic resistances are ideal. In other words, series resistance (Rsr) is near zero while shunt resistance (Rsh) approaches infinity. Relaxation of this assumption affects device performance differently depending on the chosen BHJ material system. Specifically, the impact of nonideal Rsr is controlled by the electric field dependence of the probability of charge transfer (CT) state dissociation (PCT). This is demonstrated by evaluating the experimental current density versus voltage response within the framework of a drift/diffusion model for two BHJ systems that strongly differ in PCT. Second, light intensity measurements of devices with nonideal Rsr and Rsh are shown to convolute the scaling of short-circuit current and open-circuit voltage with light intensity, which is a common technique to study BHJ device physics. Finally, we show the connection between the drift/diffusion and equivalent circuit model with regard to each model’s treatment of CT state dissociation. In particular, the equivalent circuit model utilizes a light intensity dependent Rsh to describe this dissociation process and predicts a photocurrent under reverse bias that exceeds the photocurrent permitted by light absorption.
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88.40.H- Solar cells (photovoltaics)
88.40.J- Types of solar cells
66.30.-h Diffusion in solids
72.40.+w Photoconduction and photovoltaic effects
79.60.-i Photoemission and photoelectron spectra
78.20.-e Optical properties of bulk materials and thin films

Acoustic confinement and waveguiding with a line-defect structure in phononic crystal slabs

Abdelkrim Khelif, Saeed Mohammadi, Ali Asghar Eftekhar, Ali Adibi, and Boujamaa Aoubiza

J. Appl. Phys. 108, 084515 (2010); http://dx.doi.org/10.1063/1.3500226 (5 pages) | Cited 2 times

Online Publication Date: 28 October 2010

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We present a new way of forming phononic crystal waveguides by coupling a series of line-defect resonators. The dispersion proprieties of these coupled resonator acoustic waveguides (CRAW) can be engineered by using their geometrical parameters. We show that single-mode guiding over a large bandwidth is possible in CRAW formed in a honeycomb-lattice phononic crystal slab of holes in zinc oxide. In addition, a finite length of CRAW structure acts as an efficient selective acoustic filter for Lamb waves.
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43.35.Pt Surface waves in solids and liquids
77.65.Dq Acoustoelectric effects and surface acoustic waves (SAW) in piezoelectrics
43.20.-f General linear acoustics

Band gap dependent thermophotovoltaic device performance using the InGaAs and InGaAsP material system

R. S. Tuley and R. J. Nicholas

J. Appl. Phys. 108, 084516 (2010); http://dx.doi.org/10.1063/1.3488903 (7 pages)

Online Publication Date: 29 October 2010

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Thermophotovoltaic cells with a range of band gaps are modeled under a variety of illumination conditions, including a range of source temperatures and a variable degree of spectral control. Thus, the balance between the requirements of high power densities and high efficiencies can be investigated. The influence of elevated cell temperatures, cell cooling, Auger recombination, and series resistances have been included, and all weight the optimum band gap thermophotovoltaic cell toward higher band gaps than the ∼ 0.5–0.6 eV conventional optimum. The cells have been modeled using fundamental physical parameters from the InGaAs and InGaAsP material system which accurately reproduce reported device performance and allow a comparison to theoretical limits.
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84.60.Jt Photoelectric conversion
85.30.De Semiconductor-device characterization, design, and modeling

Physical loss mechanisms for resonant acoustical waves in boron doped poly-SiGe deposited with hydrogen dilution

Steve Stoffels, Enrico Autizi, Rita Van Hoof, Simone Severi, Robert Puers, Ann Witvrouw, and Harrie A. C. Tilmans

J. Appl. Phys. 108, 084517 (2010); http://dx.doi.org/10.1063/1.3499319 (11 pages)

Online Publication Date: 29 October 2010

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In this paper, the physical loss mechanisms in boron doped poly-SiGe are analyzed theoretically and experimentally. The phonon losses were calculated theoretically for different germanium and doping concentrations. The theoretical analysis showed that Akhiezer damping sets a fundamental lower limit to the internal damping. Calculated limits for the f×Q due to Akhiezer damping were ∼ 1×1014 Hz for SiGe with low Ge content and ∼ 2×1013 Hz for SiGe with high Ge content. However, in the experiments it was found that an internal friction loss mechanism limits the maximum achievable f×Q in our material to 3×1012 at a frequency of 130 MHz. Experimentally the loss mechanisms were studied further by preparing SiGe layers with different Ge/H/B content. The acoustical losses were measured by fabricating a micromechanical resonator from the layers. The measurements identified a thermally activated loss mechanism. By studying the microstructure of the SiGe layers, we identified interface defects and interstitial as the most important loss mechanisms. Furthermore, the experiments show that at high frequencies (>130 MHz) the achievable f×Q-products of SiGe are close to the values, which can be achieved with single crystal materials.
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62.65.+k Acoustical properties of solids
63.20.-e Phonons in crystal lattices
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.72.jj Interstitials
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