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15 Feb 2012

Volume 111, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

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 Lasers, Optics, and Optoelectronics

Photothermal and thermo-refractive effects in high reflectivity mirrors at room and cryogenic temperature

Alessandro Farsi, Mario Siciliani de Cumis, Francesco Marino, and Francesco Marin

J. Appl. Phys. 111, 043101 (2012); http://dx.doi.org/10.1063/1.3684626 (12 pages) | Cited 1 time

Online Publication Date: 16 February 2012

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Increasing requirements in the sensitivity of interferometric measurements is a common feature of several research fields, from gravitational wave detection to quantum optics. This motivates refined studies of high reflectivity mirrors and of noise sources that are tightly related to their structure. In this work we present an experimental characterization of photothermal and thermo-refractive effects in high reflectivity mirrors, i.e., of the variations in the position of their effective reflection plane due to weak residual power absorption. The measurements are performed by modulating the impinging power in the range 10Hz÷100kHz. The experimental results are compared with an expressly derived theoretical model in order to fully understand the phenomena and exploit them to extract useful effective thermo-mechanical parameters of the coating. The measurements are extended to cryogenic temperature, where most high sensitivity experiments are performed (or planned in future versions) and where characterizations of dielectric film coatings are still poor.
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42.79.Bh Lenses, prisms and mirrors
07.60.Ly Interferometers
42.79.Wc Optical coatings

Microstructured silicon with two impurity bands for solar cells

Huili He, Changshui Chen, and Songhao Liu

J. Appl. Phys. 111, 043102 (2012); http://dx.doi.org/10.1063/1.3684669 (7 pages)

Online Publication Date: 16 February 2012

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A low-cost, large area, random, maskless texturing scheme independent of crystal orientation is expected to have an impact on terrestrial photovoltaic technology significantly. We investigated microstructured silicon formed by irradiating the surface of a Si wafer with femtosecond laser pulses in the presence of a sulfur-bearing gas. It is widely believed that microstructured silicon is a potential material for efficient multi intermediate bands silicon solar cell. Using a four-band model, the loss of sunlight for silicon with two different chalcogenide impurity bands was analyzed, and the levels of the minimum loss of sunlight can be found. Then the effect of the position at which the intermediate bands that have minimum sunlight loss rate on the theoretical conversion efficiency of the corresponding solar cell was discussed with the Detailed Balance Theory. After that, the influence of impact ionization on the efficiency of four-band Solar Cells was analyzed in detail. Finally, problems that need to be resolved in making intermediate band solar cell based on femtosecond laser microstructured silicon are pointed out with great emphasis.
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88.40.H- Solar cells (photovoltaics)

Formation of carriers in Ti-oxide thin films by substitution reactions

Y. S. Liu, Y. H. Lin, Y. S. Wei, and C. Y. Liu

J. Appl. Phys. 111, 043103 (2012); http://dx.doi.org/10.1063/1.3685448 (6 pages)

Online Publication Date: 16 February 2012

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Conductive Ti-oxide thin films are produced using a reactive sputtering and post-annealing process. The lowest resistivity of Ti-oxide thin films (2.30 × 10–2 Ω-cm) can be achieved after annealing for 1 h at 400 °C in ambient O2. Additionally, the Hall measurement results indicate that the carrier concentration increases during the initial 1-h annealing process before decreasing during subsequent annealing. By curve fitting the Ols core-level peaks in the x ray photoelectron spectroscopy (XPS) spectrum of the annealed Ti-oxide thin films, we found that the oxygen (O) vacancy concentration monotonically increases with annealing time, which differs from the behavior of the carrier concentration regarding annealing time. This means that the O-vacancy mechanism alone cannot explain the formation of carriers in Ti-oxide thin films. By curve-fitting core-level Ti peaks in the XPS spectrum of annealed Ti-oxide thin films, a Ti3+-to-Ti4+ substitution reaction in the TiO2 phase of the Ti-oxide thin film after annealing plays the dominant role in the formation of conduction carriers. Instead of the O-vacancy mechanism, the Ti3+-to-Ti4+ substitution mechanism can explain the concentration of carriers in Ti-oxide thin films following annealing.
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73.61.Le Other inorganic semiconductors
68.55.ag Semiconductors
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
81.15.Cd Deposition by sputtering
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Energy transfer in CaYAlO4: Ce3+, Pr3+ for sensitization of quantum-cutting with the Pr3+-Yb3+ couple

A. Guille, A. Pereira, G. Breton, A. Bensalah-Ledoux, and B. Moine

J. Appl. Phys. 111, 043104 (2012); http://dx.doi.org/10.1063/1.3686614 (5 pages) | Cited 3 times

Online Publication Date: 17 February 2012

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Silicon based solar cells show a poor yield in the UV-blue range of the solar spectrum. The thermalization of the charge carriers is one major loss mechanism, which could be overcome by depositing a quantum-cutting material in front of the solar cell. This phenomenon has recently been demonstrated in materials doped with the rare-earth couple Pr3+-Yb3+, where Pr3+ is able to sensitize two Yb3+ ion for one absorbed blue photon. Unfortunately, the weak absorption cross-section of Pr3+ ion makes it not suitable for thin films applications. Therefore, it is necessary to sensitize it to obtain a significant light yield. In that respect, Ce3+ ion could be used as a sensitizer of Pr3+ because its 4f-5d transition shows a high cross-section absorption. The purpose of this article is to study energy transfers between Ce3+ and Pr3+ in CaYAlO4 with a view to evaluate the potential of Ce3+ as a sensitizer of quantum-cutting with Pr3+-Yb3+ couple. We present results obtained in the bulk material and in a thin film deposited by pulsed laser deposition and we demonstrate that, in CaYAlO4, Ce3+ ion sensitizes the 3Pj levels of Pr3+ ion, but also that this may be followed by a cross-relaxation involving the 4f levels of Ce3+ ion, which would be in competition with energy transfer toward Yb3+ ion.
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88.40.H- Solar cells (photovoltaics)
88.40.J- Types of solar cells
68.55.-a Thin film structure and morphology
81.15.Fg Pulsed laser ablation deposition
78.40.Fy Semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Electronic and optical properties of quantum wells embedded in wrinkled nanomembranes

P. Cendula, S. Kiravittaya, and O. G. Schmidt

J. Appl. Phys. 111, 043105 (2012); http://dx.doi.org/10.1063/1.3684544 (5 pages) | Cited 1 time

Online Publication Date: 21 February 2012

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The authors theoretically investigate quantum confinement and transition energies in quantum wells (QWs) asymmetrically positioned in wrinkled nanomembranes. Calculations reveal that the wrinkle profile induces both blue- and redshifts, depending on the lateral position of the QW probed. Relevant radiative transitions include the ground state of the electron (hole) and excited states of the hole (electron). Energy shifts as well as stretchability of the structure are studied as a function of wrinkle amplitude and period. Large tunable bandwidths of up to 70 nm are predicted for highly asymmetric, wrinkled QWs.
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78.67.De Quantum wells
73.63.Hs Quantum wells
81.07.St Quantum wells
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Crystallization and surface texturing of amorphous-Si induced by UV laser for photovoltaic application

Lei Hong, Xincai Wang, Rusli, Hao Wang, Hongyu Zheng, and Hongyu Yu

J. Appl. Phys. 111, 043106 (2012); http://dx.doi.org/10.1063/1.3686612 (6 pages) | Cited 2 times

Online Publication Date: 22 February 2012

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The DPSS Nd:YVO4 UV laser is used to anneal amorphous silicon (a-Si) film to achieve crystallization and nano-dome surface texturing simultaneously in a one-step annealing process. With pulse energy of 380 mJ/cm2 and repetition rate of 20 kHz, the a-Si can be crystallized by the sequential lateral solidification process, which is evidenced by both SEM characterization and Raman spectra. In addition, the nano-dome like structure is confirmed by AFM characterization, which can lead to ∼200% boost in terms of light absorption as measured by UV-Visible - Near-infrared scanning spectrophotometer. This study highlights the great potential of Nd:YVO4 UV laser for its application in thin film Si solar cell industry to improve the film quality and light trapping capability.
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81.05.Cy Elemental semiconductors
68.55.jm Texture
81.30.Fb Solidification
81.05.Gc Amorphous semiconductors
78.66.Jg Amorphous semiconductors; glasses
78.66.Db Elemental semiconductors and insulators

High density InAlAs/GaAlAs quantum dots for non-linear optics in microcavities

R. Kuszelewicz, J.-M. Benoit, S. Barbay, A. Lemaître, G. Patriarche, K. Meunier, A. Tierno, and T. Ackemann

J. Appl. Phys. 111, 043107 (2012); http://dx.doi.org/10.1063/1.3682466 (9 pages) | Cited 1 time

Online Publication Date: 22 February 2012

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Structural and optical properties of InAlAs/GaAlAs quantum dots grown by molecular beam epitaxy are studied using transmission electron microscopy and temperature- and time-resolved photoluminescence. The control of the recombination lifetime (50 ps-1.25 ns) and of the dot density (5.10–8-2.1011 cm–3) strongly suggest that these material systems can find wide applications in opto-electronic devices as focusing non-linear dispersive materials as well as fast saturable absorbers.
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68.65.Hb Quantum dots (patterned in quantum wells)
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
78.67.Hc Quantum dots
81.07.Ta Quantum dots
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Ground-state power quenching in two-state lasing quantum dot lasers

Mariangela Gioannini

J. Appl. Phys. 111, 043108 (2012); http://dx.doi.org/10.1063/1.3682574 (9 pages) | Cited 1 time

Online Publication Date: 22 February 2012

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The paper analyses theoretically the quenching of the ground state (GS) power observed in InAs/GaAs quantum dot lasers when emitting simultaneously from both ground state and excited state. The model, based on a set of rate equations for the electrons, holes, and photons, shows that the power quenching is caused by the different time scales of the electron and hole intra-level dynamic, as well as by the long transport time of the holes in the GaAs barrier. The results presented also evidence how the very different dynamics of electrons and holes have other important consequences on the laser behavior; we show for example that the electron and hole carrier densities of the states resonant with lasing modes are never clamped at the threshold value, and that the damping of relaxation oscillations is strongly influenced by the hole dynamics.
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42.55.Px Semiconductor lasers; laser diodes

Detecting and tuning anisotropic mode splitting induced by birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser

J. L. Yu, Y. H. Chen, C. Y. Jiang, X. L. Ye, and H. Y. Zhang

J. Appl. Phys. 111, 043109 (2012); http://dx.doi.org/10.1063/1.3682648 (4 pages) | Cited 1 time

Online Publication Date: 22 February 2012

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The mode splitting induced by birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser (VCSEL) has been studied by reflectance difference spectroscopy (RDS) at room temperature. The mode splitting, anisotropic broadening width, and the anisotropic integrated area of the cavity mode are determined. Uniaxial strain on the order of 10−4 is introduced to tune the mode splitting. The mode splitting can be linearly tuned by the uniaxial strain, which agrees very well with theoretical calculations using a Jones matrix approach. We demonstrate that the RDS is a powerful, nondestructive tool to study the cavity anisotropy of VCSELs.
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42.60.Fc Modulation, tuning, and mode locking
42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Improved two level model of mid-infrared quantum cascade lasers for analysis of direct intensity modulation response

J. F. Webb and M. K. Haldar

J. Appl. Phys. 111, 043110 (2012); http://dx.doi.org/10.1063/1.3684660 (5 pages)

Online Publication Date: 23 February 2012

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The direct intensity modulation response of quantum cascade lasers can be accurately calculated from a three-level model. However, the calculations are algebraically complex. An alternative two-level model gives simpler expressions and more readily provides insight into the role of parameters influencing the modulation response. The object of this paper is to improve the two-level model and analyze the direct intensity modulation response. The modified model will simplify theoretical investigation of modulation related phenomena.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Low-order harmonic generation in metal ablation plasmas in nanosecond and picosecond laser regimes

M. López-Arias, M. Oujja, M. Sanz, R. A. Ganeev, G. S. Boltaev, N. Kh. Satlikov, R. I. Tugushev, T. Usmanov, and M. Castillejo

J. Appl. Phys. 111, 043111 (2012); http://dx.doi.org/10.1063/1.3686740 (7 pages) | Cited 2 times

Online Publication Date: 23 February 2012

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Low-order harmonics, third and fifth, of IR (1064 nm) laser emission have been produced in laser ablation plasmas of the metals manganese, copper and silver. The harmonics were generated in a process triggered by laser ablation followed by frequency up-conversion of a fundamental laser beam that propagates parallel to the target surface. These studies were carried out in two temporal regimes by creating the ablation plasma using either nanosecond or picosecond pulses and then probing the plasma plume with pulses of the same duration. The spatiotemporal behavior of the generated harmonics was characterized and reveals the distinct composition and dynamics of the plasma species that act as nonlinear media, allowing the comparison of different processes that control the generation efficiency. These results serve to guide the choice of laser ablation plasmas to be used for efficient high harmonic generation of laser radiation.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.79.Nv Optical frequency converters
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
52.38.Mf Laser ablation

Fast electro-optics of a single self-assembled quantum dot in a charge-tunable device

Jonathan H. Prechtel, Paul A. Dalgarno, Robert H. Hadfield, Jamie McFarlane, Antonio Badolato, Pierre M. Petroff, and Richard J. Warburton

J. Appl. Phys. 111, 043112 (2012); http://dx.doi.org/10.1063/1.3687375 (7 pages)

Online Publication Date: 29 February 2012

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The response of a single InGaAs quantum dot, embedded in a miniaturized charge-tunable device, to an applied GHz bandwidth electrical pulse is investigated via its optical response. Quantum-dot response times of 1.0 ± 0.1 ns are characterized via several different measurement techniques, demonstrating GHz-bandwidth electrical control. Furthermore, a novel optical detection technique based on resonant electron-hole pair generation in the hybridization region is used to map fully the voltage pulse experienced by the quantum dot, showing, in this case, a simple exponential rise.
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85.30.De Semiconductor-device characterization, design, and modeling
back to top Plasmas and Electrical Discharges

Investigation of the swirl flow on anode surface in high-current vacuum arcs

Shenli Jia, Dingge Yang, Lijun Wang, and Zongqian Shi

J. Appl. Phys. 111, 043301 (2012); http://dx.doi.org/10.1063/1.3684974 (6 pages)

Online Publication Date: 17 February 2012

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multimedia

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The anode activities are critical for high-current vacuum arc characteristics, especially the interruption performance of vacuum interrupters. The serious anode melting and sputter of liquid droplets into arc column often lead to interruption failure. In our previous work, the obvious anode melting and swirl flow of melted anode metal were detected at the center of anode surface when arc current exceeded a critical value under axial magnetic field (AMF). It is found that the AMF has great influence on the anode swirl flow, no swirl flow is found on the butt plate anode without AMF, but obvious swirl flow can be found when a moderate AMF is applied. Meanwhile, the swirl flow direction reversed if the AMF direction also reversed. The electromagnetic forcej×Bin anode melting pool and the impact force of ions coming from cathode plasma jets which are inclined to the arc axis on anode surface were thought to be two main possible reasons. In order to discover the physics behind the anode swirl flow phenomenon, special experiments have been conducted in this paper. Both the theoretical analysis and experimental results indicate that it is not the electromagnetic force but the interaction between ions from cathode plasma jets and anode melting pool that leads to the observed anode swirl flow. The inclination direction of cathode plasma jets, which is consistent with the direction of magnetic field vector, determines the direction of swirl flow.
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82.45.-h Electrochemistry and electrophoresis
64.70.D- Solid-liquid transitions

Ion current density measurements in a copper vacuum arc with different refractory anode thicknesses

I. I. Beilis, Y. Koulik, and R. L. Boxman

J. Appl. Phys. 111, 043302 (2012); http://dx.doi.org/10.1063/1.3686183 (6 pages)

Online Publication Date: 22 February 2012

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The time-dependent ion current density was measured in a hot refractory anode vacuum arc (HRAVA) sustained between a consumed water-cooled cylindrical Cu cathode and non-consumed cylindrical W anodes with thickness d = 5, 10, 15, 20, or 30 mm separated by an h = 10 mm gap. Arc currents of I = 130, 150, 175, and 200 A were applied for a period of 90 s. Ion current density Ji extracted from the plasma was measured using a probe located at varying distances from the electrode axis. The active surface of the probe was oriented to be either perpendicular or parallel to the radially expanding plasma to measure the directed or random component of Ji, respectively. Ji started at arc ignition and grew slowly, passed through a peak, and reached a final steady state level. This level increased with arc current and decreased with probe distance. The time to reach the steady state decreased when d was decreased from 30 to 5 mm, for I = 200 A from 48 to 12 s and for I = 150 A from 69 to 20 s, and weakly depended on probe orientation. Ji to the perpendicular probe significantly exceeded that for the parallel orientation.
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52.70.Ds Electric and magnetic measurements
52.80.Vp Discharge in vacuum
52.80.Mg Arcs; sparks; lightning; atmospheric electricity

Phase locking of high power relativistic backward wave oscillator using priming effect

Yan Teng, Wei Song, Jun Sun, Renzhen Xiao, Zhimin Song, Ligang Zhang, Zhiqiang Zhang, Lijun Zhang, Yuchuan Zhang, Jiawei Li, and Jinyong Fang

J. Appl. Phys. 111, 043303 (2012); http://dx.doi.org/10.1063/1.3686620 (7 pages) | Cited 3 times

Online Publication Date: 23 February 2012

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The phase-locking approach using the priming effect is developed for high power relativistic backward wave oscillators (RBWO). A plasma switch is conceived to avoid the feedback effect. In experiment, multicavity RBWO of 200 MW with the 73 MHz half power bandwidth is phase-locked under the injection power ratio 0.044 for the frequency separation of 20 MHz. We found that it takes more time to reach stable phase-locking than to achieve saturation of RBWO generation. The external signal of higher power results in the longer time duration of phase locking. Besides phase-locking, the priming effect leads to longer microwave pulse duration.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
52.75.Kq Plasma switches (e.g., spark gaps)

Correlation of current drop, filling gas pressure, and ion beam emission in a low energy Mather-type plasma focus device

R. A. Behbahani and F. M. Aghamir

J. Appl. Phys. 111, 043304 (2012); http://dx.doi.org/10.1063/1.3686753 (5 pages) | Cited 1 time

Online Publication Date: 24 February 2012

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The behavior of current drop and its correlation with ion beam emission during the radial phase of a high inductance low energy Mather type plasma focus device have been studied. The study includes two ranges of filling gas pressure, namely the low range of 0.2–0.8 mbar and the high range of 0.8–1.5 mbar. Two different current simulation processes were performed to aid the interpretation of the experimental results. Within the low range of operating pressure, an acceptable match between the computed and experimental current signals was achieved when the effects of anomalous resistances were contemplated. While in the high range of pressure, the computed and experimental current traces were in line even without considering the effects of anomalous resistances. The analysis shows that by decreasing the filling gas pressure the effects of instabilities are intensified. The computed and experimental current traces, along with ion beam signals gathered from a faraday cup, show that there is a strong correlation between the intensity of ion beam and its duration with the current drop during the radial phase.
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52.59.Fn Multistage accelerated heavy-ion beams
52.75.-d Plasma devices
52.25.-b Plasma properties
52.65.-y Plasma simulation
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Real-time observation of the capacitance variation in a surface dielectric layer in radio frequency discharge

Jin-Young Bang, Kyoung Yoo, and Chin-Wook Chung

J. Appl. Phys. 111, 043305 (2012); http://dx.doi.org/10.1063/1.3688052 (5 pages)

Online Publication Date: 27 February 2012

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The capacitance of an Al2O3 layer coated on a probe was measured in real-time in plasma using the harmonic method. The measured capacitance was influenced by the applied power and the exposure time to the plasma. Upon varying the power, the capacitance quickly changed and then slowly saturated. The change in the capacitance was partly understood to be due the temperature dependence of the dielectric constant. However, the rate of the capacitance change as a function of temperature was higher than that typically observed, and the quick response to varying power was too fast to be explained by the temperature variation. These results showed that other effects besides temperature should be considered to explain this phenomenon.
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52.80.Pi High-frequency and RF discharges
73.61.Ng Insulators
77.22.Ch Permittivity (dielectric function)
77.55.-g Dielectric thin films
52.25.Fi Transport properties
52.70.Ds Electric and magnetic measurements
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Fluid dynamics and mass transport in organic vapor jet printing

Gregory J. McGraw and Stephen R. Forrest

J. Appl. Phys. 111, 043501 (2012); http://dx.doi.org/10.1063/1.3680881 (8 pages) | Cited 1 time

Online Publication Date: 16 February 2012

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Organic vapor jet printing (OVJP) is a high-resolution mask and solvent-free organic thin film deposition and patterning technique. In this work, we analyze factors that affect the material deposition rate and the dynamics of gas flow through an OVJP print head consisting of an array of micron-sized nozzles. Scaling laws governing carrier gas flow dynamics through a system of microchannels and nozzles are developed. From these laws, we predict the deposition rate and doping ratio of organic material as a function of carrier gas flow. The analysis is applied to an experimental print head, and is compared to results obtained via a more complex, direct simulation Monte Carlo model. The print head is used to grow efficient green phosphorescent organic light emitting diodes having an external quantum efficiency and luminance comparable to analogous devices grown by vacuum thermal evaporation.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.72.U- Doping and impurity implantation
68.55.aj Insulators

Determination of the composition and thickness of semi-polar and non-polar III-nitride films and quantum wells using X-ray scattering

M. E. Vickers, J. L. Hollander, C. McAleese, M. J. Kappers, M. A. Moram, and C. J. Humphreys

J. Appl. Phys. 111, 043502 (2012); http://dx.doi.org/10.1063/1.3678631 (13 pages)

Online Publication Date: 16 February 2012

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There is increasing interest in III-nitride films and multiple quantum well structures grown in non-polar or semi-polar orientations for application in light-emitting devices. We describe a method to obtain the compositions and the thicknesses of layers within III-nitride quantum well or superlattice structures grown in non-polar or semi-polar orientations, based on X-ray scattering. For each new crystallographic orientation considered, new axes were obtained and both the lengths and angles of these new axes calculated relative to the original conventional reference axes. These angles provide the coefficients of the matrix to transform the elastic constants published in the conventional setting (as used for polar c-plane oriented III-nitrides) into the appropriate new values. The new characteristic lengths and new elastic constants are then put into the general equation that relates the composition of a fully strained layer to the experimentally measured out-of-plane alloy d-spacing. Thus we have (a) determined the alloy composition from the difference between the experimentally measured alloy d-spacing and that of the substrate and (b) calculated the strained d-spacing for a given alloy composition for input to simple kinematical simulation software. In addition for quantum well structures the thickness ratio of well-to-barrier must be determined. Here we use the minima in the low angle reflectivity data. The repeat thickness and thus the thicknesses of the well and barrier layers, can be obtained from either the low or the high-angle data. We then cross-check by comparing the experimental and the simulated high-angle diffraction data. This method has been applied successfully to heteroepitaxial non-polar and semi-polar GaN/AlGaN and InGaN/GaN multiple quantum well structures and may also be used to find the composition of epilayers. The method works even in the presence of tilt between the superlattice and the GaN “template’, although in this case additional high-angle diffraction data at different settings must be collected.
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78.70.Ck X-ray scattering
81.07.St Quantum wells
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.dq Other elastic constants
68.55.ag Semiconductors
68.65.Cd Superlattices
68.65.Fg Quantum wells

Phase transformation and crystallization kinetics of a-Ge/Cu bilayer for blue-ray recording under thermal annealing and pulsed laser irradiation

Yung-Chiun Her, Wei-Ting Tu, and Ming-Hsin Tsai

J. Appl. Phys. 111, 043503 (2012); http://dx.doi.org/10.1063/1.3684966 (6 pages)

Online Publication Date: 16 February 2012

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Similar phase formation and crystallization behaviors have been observed in the a-Ge/Cu bilayer under thermal annealing and pulsed laser irradiation. The Cu3Ge phase would form prior to the crystallization of a-Ge. The crystallization temperature and activation energy for crystallization of a-Ge were reduced to 310 °C and 2.75 eV, respectively, due to the fast Ge diffusion in the already formed germanide phases. The reaction exponent m of ∼2.0 for the a-Ge/Cu bilayer corresponds to a crystallization process in which grain growth occurs with nucleation, and the nucleation rate decreases with the progress of the grain growth process. Under pulsed laser irradiation, the maximum data-transfer-rates of 44, 56, 74, and 112 Mbit/s can be achieved in the write-once blue-ray disk at the recording powers of 3, 4, 5, and 6 mW, respectively. The a-Ge/Cu bilayer also demonstrated sufficient optical contrast and adequate absorptance for low power and high speed write-once blue-ray recording.
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64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
66.30.-h Diffusion in solids
61.82.Ms Insulators
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
64.60.Q- Nucleation

Femtosecond electron diffraction: Preparation and characterization of (110)-oriented bismuth films

Gustavo Moriena, Masaki Hada, Germán Sciaini, Jiro Matsuo, and R. J. Dwayne Miller

J. Appl. Phys. 111, 043504 (2012); http://dx.doi.org/10.1063/1.3684975 (4 pages) | Cited 1 time

Online Publication Date: 16 February 2012

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Here, we present a new approach to synthesize (110)-oriented ultrathin membranes of bismuth (Bi). This rather exotic orientation was achieved by directing the growth through rationale control of lattice matching. Bi films were hetero-epitaxially grown on the (100)-surface of freshly cleaved potassium chloride crystals. The sample orientation was characterized by x-ray and electron diffraction. In addition, high quality free-standing films were obtained after dissolution of the substrate in water and controlled evaporation. Femtosecond electron diffraction (FED) was, therefore, used to monitor the coherent shear acoustic phonons in (110)-oriented free-standing Bi films produced by impulsive femtosecond optical excitation. The small de Broglie wavelength (flat Ewald sphere) of keV-electrons combined with an off-Bragg detection scheme provided a magnified view of shear atomic motions, i.e., lattice distortions in the transverse direction. All-optical pump-probe experiments are usually insensitive to shear displacements, a fact that makes FED a unique non-contact method to achieve the complete characterization of elastic properties of nanoscale materials.
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81.05.Bx Metals, semimetals, and alloys
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
68.55.A- Nucleation and growth
63.22.Dc Free films

Optimization on a three-level heat engine working with two noninteracting fermions in a one-dimensional box trap

Jianhui Wang and Jizhou He

J. Appl. Phys. 111, 043505 (2012); http://dx.doi.org/10.1063/1.3681295 (6 pages) | Cited 2 times

Online Publication Date: 16 February 2012

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We setup a three-level heat engine model that works with two noninteracting fermions in a one-dimensional box trap. Besides two quantum adiabatic processes, the quantum heat engine cycle consists of two isoenergetic processes, along which the particles are coupled to energy baths at a high constant energy EH and a low constant energy EC, respectively. Based on the assumption that the potential wall moves at a very slow speed and there exists a heat leakage between two energy baths, we derive the expressions of the power output and the efficiency, and then obtain the optimization region for the heat engine cycle. Finally, we present a brief performance analysis of a Carnot engine between a hot and a cold bath at temperatures TH and TC, respectively. We demonstrate that under the same conditions, the efficiency η = 1-(EC/EH) of the engine cycle is bounded from above the Carnot efficiency ηc = 1-(TC/TH).
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05.30.Fk Fermion systems and electron gas
05.70.-a Thermodynamics

Damage property of incompletely spalled aluminum under shock wave loading

Meilan Qi, Chao Luo, Hongliang He, Yonggang Wang, Duan Fan, and Shilin Yan

J. Appl. Phys. 111, 043506 (2012); http://dx.doi.org/10.1063/1.3681301 (6 pages) | Cited 1 time

Online Publication Date: 16 February 2012

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The nucleation, growth, and coalescence of microscopic voids are induced inside ductile metal when it is subjected to dynamic tension, and this eventually results in a catastrophic fracture of the specimen. In the present work, this failure property is studied by using ultrapure aluminum (99.999%) as a representative candidate for the ductile metal and, further, the micro-void growth behavior (or the damage evolution) before the catastrophic fracture has been investigated. Measurements of the free surface velocity profile and statistical analysis of micro-voids were used to determine the spall characteristic and damage evolution behavior of dynamic tensile fracture in ultrapure aluminum. Through comparison of the differences between the incompletely and completely spalled signals, the spall characteristics of ultrapure aluminum from incomplete to complete spall were analyzed. Moreover, an obvious critical characteristic is found on the basis of the association between the maximum damage caused within samples and the product of peak stress and tensile duration. The damage variable slowly increased in a linear manner in the initial stages but changed to nonlinear growth and rapidly approached a fracture state as the damage variable extended beyond the critical value, which is approximately 0.09. A physical explanation for this transition is discussed and implicates micro-void linkage behavior during the dynamic failure.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mm Fracture
62.50.Ef Shock wave effects in solids and liquids
64.60.qj Studies of nucleation in specific substances
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Identification of electrically active defects in thin dielectric films by spectroscopic ellipsometry

J. Price, G. Bersuker, and P. S. Lysaght

J. Appl. Phys. 111, 043507 (2012); http://dx.doi.org/10.1063/1.3684605 (6 pages)

Online Publication Date: 17 February 2012

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A study of resonant photo-absorption features in high-k dielectric film stacks using vacuum ultraviolet spectroscopic ellipsometry demonstrates that all optically observable dielectric-related defects are located in the interfacial SiO2 layer rather than in the bulk high-k film. The defects, located at 2.9 eV, 3.6 eV, 3.9 eV, and 4.75 eV within the bandgap of this bottom interface, are found to be strongly affected by processing conditions. These results are supported by both electrical and physical characterization measurements that identify a consistent trend in the evolution of charge trapping defects for samples subjected to identical processing conditions. In addition, evidence is provided correlating the optically active 2.9 eV defect to positively charged oxygen vacancies in the bottom interfacial layer that have recently been proposed as contributing to the flatband voltage roll-off phenomenon. The close connection between these results and both ab initio calculations and experimental findings substantiate the use of spectroscopic ellipsometry as a unique characterization method for identifying process-induced defects during development and fabrication of dielectric film stacks.
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77.55.D- High-permittivity gate dielectric films
61.72.jd Vacancies

Structural and optical evaluation of InGaN/GaN multi-quantum wells on template consisting of in-plane alternately arranged relaxed InGaN and GaN

Narihito Okada, Yoichi Yamada, and Kazuyuki Tadatomo

J. Appl. Phys. 111, 043508 (2012); http://dx.doi.org/10.1063/1.3684606 (5 pages)

Online Publication Date: 21 February 2012

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Templates consisting of in-plane alternately arranged relaxed InGaN and GaN were obtained by (11–22) facet growth and mass transport via metal-organic vapor phase epitaxy. InGaN/GaN multi-quantum wells (MQWs) were grown on the novel templates, and their optical and structural properties were investigated. From photoluminescence measurement, longer and shorter peak emission wavelengths were observed from the MQWs on the InGaN and GaN regions, respectively. The discrepancy of the peak emission wavelength for the MQWs on the alternately arranged relaxed InGaN and GaN was caused by the compositional-pulling effect, which contributed to the InN molar fraction and the thickness of the InGaN quantum well. The reduction of the quantum-confined Stark effect on the InGaN region of the template was confirmed by revealing the structural and optical properties of the MQWs.
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81.07.St Quantum wells
78.67.De Quantum wells
78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
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