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1 Sep 2010

Volume 108, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 108, 051101 (2010); http://dx.doi.org/10.1063/1.3474652 (20 pages)

Banqiu Wu, Ajay Kumar, and Sharma Pamarthy
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High aspect ratio silicon etch: A review

Banqiu Wu, Ajay Kumar, and Sharma Pamarthy

J. Appl. Phys. 108, 051101 (2010); http://dx.doi.org/10.1063/1.3474652 (20 pages) | Cited 16 times

Online Publication Date: 9 September 2010

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High aspect ratio (HAR) silicon etch is reviewed, including commonly used terms, history, main applications, different technological methods, critical challenges, and main theories of the technologies. Chronologically, HAR silicon etch has been conducted using wet etch in solution, reactive ion etch (RIE) in low density plasma, single-step etch at cryogenic conditions in inductively coupled plasma (ICP) combined with RIE, time-multiplexed deep silicon etch in ICP-RIE configuration reactor, and single-step etch in high density plasma at room or near room temperature. Key specifications are HAR, high etch rate, good trench sidewall profile with smooth surface, low aspect ratio dependent etch, and low etch loading effects. Till now, time-multiplexed etch process is a popular industrial practice but the intrinsic scalloped profile of a time-multiplexed etch process, resulting from alternating between passivation and etch, poses a challenge. Previously, HAR silicon etch was an application associated primarily with microelectromechanical systems. In recent years, through-silicon-via (TSV) etch applications for three-dimensional integrated circuit stacking technology has spurred research and development of this enabling technology. This potential large scale application requires HAR etch with high and stable throughput, controllable profile and surface properties, and low costs.
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81.65.Cf Surface cleaning, etching, patterning
81.65.Rv Passivation
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back to top Lasers, Optics, and Optoelectronics

Photodeterioration of the silicon nanocrystal emission

D. Koshel, D. Barba, F. Martin, and G. G. Ross

J. Appl. Phys. 108, 053101 (2010); http://dx.doi.org/10.1063/1.3475791 (4 pages) | Cited 3 times

Online Publication Date: 2 September 2010

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Photoluminescence (PL) of silicon nanocrystals (Si-nc) is investigated as a function of exposure time for several laser intensities. Both a strong decrease in luminescence and an energy shift are observed during the illumination period. Decay analysis of the PL emission intensity indicates two timescales: a short timescale (<0.5 min), associated with reversible sample heating effects and a longer timescale (>5 min), attributed to an irreversible sample damage caused by the laser excitation. Both sample heating and damage shift the PL emission toward the red. These longer timescale variations can significantly influence the precision of both PL and optical gain measurements performed at average pumping intensities as low as ∼ 1 W cm−2. The complex evolution of the Si-nc spectra after a long recovery period indicates that the permanent degradation of the PL signal results from the activation of several sample damaging mechanisms, whose possible origins are discussed.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Ap Elemental semiconductors
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Rx Nanocrystalline materials
61.82.Fk Semiconductors
81.40.Gh Other heat and thermomechanical treatments

Terahertz emission from InAs induced by photo-Dember effect: Hydrodynamic analysis and Monte Carlo simulations

Antanas Reklaitis

J. Appl. Phys. 108, 053102 (2010); http://dx.doi.org/10.1063/1.3467526 (9 pages) | Cited 1 time

Online Publication Date: 2 September 2010

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Terahertz emission from InAs excited by the femtosecond optical pulse is investigated. The hydrodynamic model is developed for the simplified analysis of the terahertz emission induced by the photo-Dember effect. The hydrodynamic model predicts that the amplitude of the transient photocurrent is proportional to the difference between the squared velocities of photoexcited electrons and holes. The predictions of the hydrodynamic model are compared to the results of the rigorous ensemble Monte Carlo simulations. The dependences of the intensity of terahertz emission on the optical fluence, on the photon energy, and on the optical pulse duration are studied. At low intensities of the optical pulse, the emitted terahertz energy is proportional to the power ranging between 3/2 and 2 of the optical pulse intensity. The emitted terahertz energy saturates at high intensities of the optical pulse. The spectral dependence of the terahertz emission from InAs is investigated. It is found that the intensity of terahertz emission profoundly depends on the optical pulse duration.
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72.40.+w Photoconduction and photovoltaic effects
79.60.-i Photoemission and photoelectron spectra
78.47.J- Ultrafast spectroscopy (<1 psec)
78.56.-a Photoconduction and photovoltaic effects
81.05.Ea III-V semiconductors

Introducing dipole-like resonance into magnetic resonance to realize simultaneous drop in transmission and reflection at terahertz frequency

ChengGang Hu, Xiong Li, Qin Feng, Xu’Nan Chen, and XianGang Luo

J. Appl. Phys. 108, 053103 (2010); http://dx.doi.org/10.1063/1.3467528 (4 pages)

Online Publication Date: 2 September 2010

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A metamaterial (MM) consisting of double layer closed rings (DCRs) and a split-wire shows the different manipulations of the transmission T(ω) and reflection R(ω) to obtain simultaneous minimums. Parametric studies on the MM and a further LC circuit model are proposed, showing that the phenomenon is dominated by the interaction of the additional dipole-like resonance supported by the split-wire and the primary magnetic resonance supported by DCRs. Simultaneous sharp drops in T(ω) and R(ω) are observed in our MM at about 2.45 THz. The potential applications for near-perfect absorption and dual-directional absorption are discussed.
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76.70.-r Magnetic double resonances and cross effects
81.05.Xj Metamaterials for chiral, bianisotropic and other complex media
42.70.-a Optical materials

Light scattering from human corneal grafts: Bulk and surface contribution

Gaël Latour, Gaëlle Georges, Laure Siozade Lamoine, Carole Deumié, John Conrath, and Louis Hoffart

J. Appl. Phys. 108, 053104 (2010); http://dx.doi.org/10.1063/1.3452334 (9 pages) | Cited 1 time

Online Publication Date: 2 September 2010

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The cornea is the only transparent tissue in the body. The transparency is the main characteristic of the corneal tissue, and depends not only on the transmission coefficient but also on the losses by scattering and absorption. The scattering properties of the cornea tissues become one of the most important parameters in the case of the corneal graft. These scattering properties are studied in this paper in the reflected half area, similar to the diagnosis configuration. We quantify the influence of the cornea thickness and of the epithelial layer on scattering level. The technique of ellipsometry on scattered field is also used to analyze the polarization properties in order to determine the origin of scattering (surface and/or bulk).
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42.66.Ct Anatomy and optics of eye

Growth and photoresponse study of PdO nanoflakes reactive-sputter deposited on SiO2

Chien-Jung Huang, Fu-Ming Pan, Hsiu-Ying Chen, and Li-Chang

J. Appl. Phys. 108, 053105 (2010); http://dx.doi.org/10.1063/1.3476567 (6 pages) | Cited 1 time

Online Publication Date: 3 September 2010

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This study prepares PdO nanostructures on a SiO2 substrate by reactive-sputter deposition and examines photoresponse characteristics of the thin films. PdO thin films deposited at 25 °C is composed of bowed nanoflakes standing on the SiO2 substrate, which have a single-crystalline structure after thermal anneal at 400 °C. The 400 °C-annealed nanoflake thin film has a band gap energy in the red-light range ( ∼ 2.06 eV), and exhibits a very sensitive photoresponse upon the UV (365 nm) illumination. The high photoresponse sensitivity of the 400 °C-annealed nanoflake thin film is ascribed to a lower density of recombination centers and traps due to an excellent crystallinity and a high carrier extraction efficiency due to a low electrical resistivity. A slight decrease in the photocurrent density during the initial stage of the UV illumination is attributed to adsorption of O2− anions on the 400 °C-annealed nanoflakes.
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81.16.-c Methods of micro- and nanofabrication and processing
68.55.ag Semiconductors
61.72.Cc Kinetics of defect formation and annealing
81.15.Cd Deposition by sputtering
73.61.Le Other inorganic semiconductors
68.43.Mn Adsorption kinetics

Transparency pump intensity and differential gain in resonantly pumped W optical pumping injection cavity lasers

L. J. Olafsen and T. C. McAlpine

J. Appl. Phys. 108, 053106 (2010); http://dx.doi.org/10.1063/1.3475504 (6 pages) | Cited 1 time

Online Publication Date: 7 September 2010

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We report the results of a cavity length characterization of a W optical pumping injection cavity (OPIC) laser with a room temperature emission wavelength of 3.2 μm using pump wavelength tuning to achieve resonant optical pumping. Devices with cavity lengths ranging from 312 to 2030 μm are characterized to determine geometry-independent figures of merit. The measurements yield transparency pump intensities Itr that range from 230 W/cm2 at 100 K to 7.8 kW/cm2 at 300 K, as gain per unit pump intensity (differential gain) decreases from 0.17 cm/W at 100 K to 0.011 cm/W at 300 K. The characteristic temperature of the transparency pump intensity is 55.6 K, while the gain per unit pump intensity demonstrates an exponential decay with a characteristic temperature of 67.9 K. Compared to other optically pumped type-II W lasers and electrically injected interband cascade lasers, there is less reduction in the differential gain with increasing temperature, consistent with the high temperature operation of these OPIC devices.
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42.55.-f Lasers
42.65.-k Nonlinear optics

Formation of metal nanoparticles of various sizes in plasma plumes produced by Ti:sapphire laser pulses

U. Chakravarty, P. A. Naik, C. Mukherjee, S. R. Kumbhare, and P. D. Gupta

J. Appl. Phys. 108, 053107 (2010); http://dx.doi.org/10.1063/1.3475512 (5 pages) | Cited 2 times

Online Publication Date: 7 September 2010

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In this paper, an experimental study on generation of nanoparticle various sizes using Ti:sapphire laser pulses, is reported. Nanoparticle formation in plasma plumes of metals like silver and copper, expanding in vacuum, has been studied using stretched pulses of 300 ps duration [subnanoseconds (sub-ns)] from a Ti:sapphire laser. It has been compared with the nanoparticle formation (of the same materials) when compressed pulses of 45 fs duration were used under similar focusing conditions. Nanoparticle formation is observed at intensities as high as 2×1016 W/cm2. The structural analysis of the nanoparticle deposition on a silicon substrate showed that, using 45 fs pulses, smaller nanoparticles of average size ∼ 20 nm were generated, whereas on using the sub-ns pulses, larger particles were produced. Also, the visible light transmission and reflection from the nanoparticle film of Ag on glass substrate showed surface plasmon resonance (SPR). The SPR curves of the films of nanoparticles deposited by femtosecond pulses were always broader and reflection/transmission was always smaller when compared with the films formed using the sub-ns pulses, indicating smaller size particle formation by ultrashort pulses. Thus, it has been demonstrated that variation in the laser pulse duration of laser offers a simple tool for varying the size of the nanoparticles generated in plasma plumes.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.77.-j Plasma applications

Design of Kerr-effect sensitive microcavity in nonlinear photonic crystal slabs for all-optical switching

Fei Qin, Ye Liu, Zi-Ming Meng, and Zhi-Yuan Li

J. Appl. Phys. 108, 053108 (2010); http://dx.doi.org/10.1063/1.3468220 (7 pages) | Cited 3 times

Online Publication Date: 13 September 2010

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We design a Kerr-effect sensitive microcavity in hybrid semiconductor nonlinear photonic crystal (PhC) slabs for application in all-optical switching. Our new concept cavity is made from infiltrating the air hole array and coating the surface of usual semiconductor PhC slabs with polystyrene, and let the polystyrene instead of the semiconductor occupy the center of the cavity. Optimization of the cavity design by modulating the structure parameter yields a quality factor Q = 1600 and shift magnitude δf ≈ 8.4 nm while pumping the cavity with a light intensity of 80 GW/cm2. This cavity configuration can help to realize very fast response speed and low pump intensity in all-optical switching devices, reduce the demand for rigorous precision during the high-Q PhC cavity fabrication, and allow for easy integration with other integrated optical components.
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42.70.Qs Photonic bandgap materials
42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.15.Eq Optical system design
42.70.Jk Polymers and organics
42.79.Wc Optical coatings
back to top Plasmas and Electrical Discharges

Attenuation of single-tone ultrasound by an atmospheric glow discharge plasma barrier

Vadim P. Stepaniuk, Tindaro Ioppolo, M. Volkan Ötügen, and Valery A. Sheverev

J. Appl. Phys. 108, 053301 (2010); http://dx.doi.org/10.1063/1.3475510 (6 pages)

Online Publication Date: 3 September 2010

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Propagation of 143 kHz ultrasound through an atmospheric pressure glow discharge in air was studied experimentally. The plasma was a continuous dc discharge formed by a multipin electrode system. Distributions of the gas temperature were also obtained in and around the plasma using laser-induced Rayleigh scattering technique. Results show significant attenuation of the ultrasound by the glow discharge plasma barrier (up to −24 dB). The results indicate that sound attenuation does not depend on the thickness of the plasma and attenuation is caused primarily by reflection of the sound waves from the plasma due to the sharp gas temperatures gradients that form at the plasma boundary. These gradients can be as high as 80 K/mm.
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52.80.Hc Glow; corona
52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
52.40.Hf Plasma-material interactions; boundary layer effects
52.70.-m Plasma diagnostic techniques and instrumentation

Disentangling fluxes of energy and matter in plasma-surface interactions: Effect of process parameters

M. Wolter, I. Levchenko, H. Kersten, S. Kumar, and K. Ostrikov

J. Appl. Phys. 108, 053302 (2010); http://dx.doi.org/10.1063/1.3475728 (5 pages) | Cited 1 time

Online Publication Date: 9 September 2010

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The possibility to discriminate between the relative importance of the fluxes of energy and matter in plasma-surface interaction is demonstrated by the energy flux measurements in low-temperature plasmas ignited by the radio frequency discharge (power and pressure ranges 50–250 W and 8–11.5 Pa) in Ar, Ar+H2, and Ar+H2+CH4 gas mixtures typically used in nanoscale synthesis and processing of silicon- and carbon-based nanostructures. It is shown that by varying the gas composition and pressure, the discharge power, and the surface bias one can effectively control the surface temperature and the matter supply rates. The experimental findings are explained in terms of the plasma-specific reactions in the plasma bulk and on the surface.
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52.40.Hf Plasma-material interactions; boundary layer effects
52.80.Pi High-frequency and RF discharges
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
52.77.Dq Plasma-based ion implantation and deposition
52.25.-b Plasma properties

Effect of hydrogen on catalyst nanoparticles in carbon nanotube growth

Michael J. Behr, E. Ashley Gaulding, K. Andre Mkhoyan, and Eray S. Aydil

J. Appl. Phys. 108, 053303 (2010); http://dx.doi.org/10.1063/1.3467971 (8 pages) | Cited 7 times

Online Publication Date: 13 September 2010

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The structures of carbon nanotubes grown from catalytic nanoparticles via plasma-enhanced chemical vapor deposition in CH4/H2 mixtures show a strong dependence on the H2-to-CH4 ratio in the feed gas. A suite of characterization techniques, including optical emission, infrared, and Raman spectroscopies combined with convergent-beam and selected-area electron diffraction, and high-resolution (scanning) transmission electron microscopy imaging were used to systematically investigate the interrelation among plasma gas phase composition, catalysts morphology, catalyst structure, and carbon nanotube structure. Hydrogen plays a critical role in determining the final carbon nanotube structure through its effect on the catalyst crystal structure and morphology. At low H2-to-CH4 ratios ( ∼ 1), iron catalyst nanoparticles are converted to Fe3C and well-graphitized nanotubes grow from elongated Fe3C crystals. High (>5) H2-to-CH4 ratios in the feed gas result in high hydrogen concentrations in the plasma and strongly reducing conditions, which prevents conversion of Fe to Fe3C. In the latter case, poorly-graphitized nanofibers grow from ductile bcc iron nanocrystals that are easily deformed into tapered nanocrystals that yield nanotubes with thick walls.
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81.16.Hc Catalytic methods
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.07.De Nanotubes
78.67.Ch Nanotubes
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Observations of the long distance exploding wire restrike mechanism

Rowan Sinton, Ryan van Herel, Wade Enright, and Pat Bodger

J. Appl. Phys. 108, 053304 (2010); http://dx.doi.org/10.1063/1.3481385 (5 pages) | Cited 2 times

Online Publication Date: 15 September 2010

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An exploding wire restrike mechanism is applied to create plasma paths up to 9 m in length. The mechanism uses enameled copper wires in a 5 to 10 kV/m region of average electric field (AEF). This relatively low AEF restrike mechanism appears to be linked to the formation of plasma beads along the wire’s length. Voltage traces, measurement of relative emitted light intensity and photographs are presented at AEFs below, inside and above the identified restrike region.
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52.80.Qj Explosions; exploding wires
52.50.Lp Plasma production and heating by shock waves and compression
52.70.La X-ray and γ-ray measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Growth and photoluminescence properties of TlInGaAsN/TlGaAsN triple quantum wells

Kang Min Kim, Shuichi Emura, Daivasigamani Krishnamurthy, Shigehiko Hasegawa, and Hajime Asahi

J. Appl. Phys. 108, 053501 (2010); http://dx.doi.org/10.1063/1.3481029 (6 pages) | Cited 2 times

Online Publication Date: 1 September 2010

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TlInGaAsN/TlGaAsN system was proposed to fabricate the temperature-insensitive both lasing wavelength and threshold current density laser diodes, which are important in the wavelength division multiplexing optical fiber communication system. The effects of indium and nitrogen concentrations in the quantum wells (QWs) and barriers on the properties of molecular-beam epitaxy grown TlInGaAsN/TlGaAsN triple QWs (TQWs) were investigated. The TQW samples having higher N concentration in the QWs have the highest Tl incorporation without deterioration of the crystalline quality. The temperature dependence of the photoluminescence (PL) peak energy was found to be the least for the highest Tl containing TQW sample. The incorporation of Tl causes the reduction in the coupling constant of the electron–phonon interaction, leading to the reduced temperature dependence of the PL peak shift. Thermal activation energies, which are deduced from the Arrhenius plot of PL intensity, are attributed to the delocalization of the excitons at low temperatures and to the escape of carriers from the QWs to the barriers at high temperatures.
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78.66.Fd III-V semiconductors
68.65.Fg Quantum wells

Temperature dependence of energies and broadening parameters of near band edge interband excitonic transitions in wurtzite ZnCdMgSe

D. O. Dumcenco, Y. S. Huang, H. P. Hsu, K. K. Tiong, F. Firszt, K. Strzałkowski, S. Łęgowski, and H. Męczyńska

J. Appl. Phys. 108, 053502 (2010); http://dx.doi.org/10.1063/1.3481378 (7 pages)

Online Publication Date: 2 September 2010

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An optical characterization of two wurtzite ZnCdMgSe crystalline alloys grown by the modified high-pressure Bridgman method was carried out by temperature-dependent photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 10–300 K and photoreflectance (PR) measurements between 300–400 K. Low temperature PL spectra of the investigated samples consisted of the excitonic line, the “edge-emission” due to radiative recombination of shallow donor-acceptor pairs and a broad band related to recombination through deep level defects. Three excitonic features, A, B, and C, in the vicinity of band edge were observed in the CER and PR spectra. The peak positions of band edge excitonic features in the PL spectra were shifted slightly toward lower energies as compared to the lowest corresponding transition energies of A exciton determined from CER and PR data. The increase in the CER-PL shift with the increasing of Mg content in the investigated crystals was explained by the rising of compositional disorder causing the smearing of the band edge energies. In addition, the parameters that describe the temperature dependence of the transition energies and broadening parameters of the band edge excitonic transitions were evaluated and discussed.
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81.10.Fq Growth from melts; zone melting and refining
78.55.Et II-VI semiconductors
71.35.-y Excitons and related phenomena
71.55.Gs II-VI semiconductors
78.20.Jq Electro-optical effects

Loss compensation in external cloaks using a thin layer of gain media

Jin Wang, Hui Yuan Dong, Wei Xiang Jiang, and Tie Jun Cui

J. Appl. Phys. 108, 053503 (2010); http://dx.doi.org/10.1063/1.3481422 (6 pages)

Online Publication Date: 2 September 2010

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Transformation optical devices based on the concept of complementary medium have attracted much attention due to their novel properties. A recent paper [ H. Chen and C. T. Chan, Opt. Lett. 34, 2649 (2009) ] showed a class of complementary medium in bipolar coordinates, which can render the object invisible at a distance. The prescription requires lossless materials composed of negative index medium (NIM), and a certain loss will cut off the cloaking effect of the device. In this paper, we suggest compensation to the lossy NIM by introducing gain media into such a device. For the realization of the whole cloaking device, we present an optimized compensation method to upgrade the function by adding a very thin layer of gain media in the surrounding medium. Numerical simulations demonstrate a dramatic improvement in the cloaking performance when the NIM is lossy.
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42.79.-e Optical elements, devices, and systems
42.25.-p Wave optics
42.70.-a Optical materials

Influence of disorder on localization and density of states in amorphous carbon nitride thin films systems rich in π-bonded carbon atoms

F. Alibart, M. Lejeune, O. Durand Drouhin, K. Zellama, and M. Benlahsen

J. Appl. Phys. 108, 053504 (2010); http://dx.doi.org/10.1063/1.3462442 (9 pages) | Cited 2 times

Online Publication Date: 2 September 2010

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We discuss in this paper the evolution of both the density of states (DOS) located between the band-tail states and the DOS around the Fermi level N(EF) in amorphous carbon nitride films (a-CNx) as a function of the total nitrogen partial pressure ratio in the Ar/N2 plasma mixture. The films were deposited by three different deposition techniques and their microstructure was characterized using a combination of infrared and Raman spectroscopy and optical transmission experiments, completed with electrical conductivity measurements, as a function of temperature. The observed changes in the optoelectronic properties are attributed to the modification in the atomic bonding structures, which were induced by N incorporation, accompanied by an increase in the sp2 carbon bonding configurations and their relative disorder. The electrical conductivity variation was interpreted in terms of local effects on the nature and energy distribution of π and π∗ states.
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71.23.Cq Amorphous semiconductors, metallic glasses, glasses
71.55.Jv Disordered structures; amorphous and glassy solids
61.43.-j Disordered solids
81.15.Cd Deposition by sputtering
78.35.+c Brillouin and Rayleigh scattering; other light scattering
68.55.-a Thin film structure and morphology

Application of the cBΩ model for the calculation of oxygen self-diffusion coefficients in minerals

Baohua Zhang, Xiaoping Wu, Junshan Xu, and Rulong Zhou

J. Appl. Phys. 108, 053505 (2010); http://dx.doi.org/10.1063/1.3476283 (6 pages) | Cited 3 times

Online Publication Date: 2 September 2010

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The cBΩ model, which suggests the defect Gibbs energy is proportional to the isothermal bulk modulus and the mean volume per atom, is first introduced to predict self-diffusion coefficients of oxygen in various silicate and oxide minerals in terms of available elastic data. We develop a new approach to determine constant c in the cBΩ model on the basis of the observed compensation effect between the activation energies and pre-exponential factors, which is critical to the diffusivity prediction. Under anhydrous conditions, the validity of this model is tested by the experimentally determined oxygen self-diffusion coefficients. Our results show that the absolute oxygen diffusion rates derived from the cBΩ model are in agreement with experimental data in a variety of rock-forming minerals including olivine, MgSiO3 perovskite, spinel, and zircon.
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66.30.H- Self-diffusion and ionic conduction in nonmetals
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli

First-principles study of native defects in TlBr: Carrier trapping, compensation, and polarization phemomenon

Mao-Hua Du

J. Appl. Phys. 108, 053506 (2010); http://dx.doi.org/10.1063/1.3476564 (4 pages) | Cited 2 times

Online Publication Date: 2 September 2010

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First-principles calculations are carried out to study the native defect properties in TlBr. Three important results emerge: (1) the native defects are benign in terms of electron trapping because the low-energy defects do not induce electron traps; (2) the dominant defects in nearly stoichiometric TlBr are Schottky defects that pin the Fermi level near the midgap, leading to high resistivity; and (3) the calculated low diffusion barriers for several native defects show that ionic conductivity can occur at room temperature. The important impacts of these material properties on the room-temperature radiation detection using TlBr are discussed.
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71.55.Ht Other nonmetals
61.72.J- Point defects and defect clusters
66.30.Dn Theory of diffusion and ionic conduction in solids

Role of phonons in the optical properties of magnetron sputtered ZnO studied by resonance Raman and photoluminescence

Sandeep Sohal, Yahya Alivov, Zhaoyang Fan, and Mark Holtz

J. Appl. Phys. 108, 053507 (2010); http://dx.doi.org/10.1063/1.3475649 (5 pages)

Online Publication Date: 3 September 2010

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The temperature dependence of phonons in ZnO has been studied using resonance Raman and photoluminescence (PL) emission measurements. Excitation with wavelength 363.8 nm (photon energy 3.409 eV) is used to establish incoming resonance near room temperature. Broad PL emission is seen at room temperature with peak position at 3.25 eV. This coincides with the overtone of the longitudinal optic (LO) band. Up to six LO phonon orders are observed. Temperature dependence of the LO phonon energy is described by a two-phonon decay mechanism with energies 100 and 496 cm−1. The temperature dependence of the PL shift is interpreted based on electron-phonon interactions. A two-phonon description is sufficient to describe the temperature shift in the band gap through occupation at average acoustic and optic phonon energies 125 and 500 cm−1, respectively. LO phonon sidebands (PSBs) are also observed at low temperature (23 to 100 K). The temperature shift in the PSB energies is interpreted based on the band gap shift combined with established theory for the PSBs.
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78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors
71.20.Nr Semiconductor compounds
63.20.kd Phonon-electron interactions
78.30.Fs III-V and II-VI semiconductors
78.66.Hf II-VI semiconductors

Simulations of luminescent solar concentrators: Effects of polarization and fluorophore alignment

S. McDowall, B. L. Johnson, and D. L. Patrick

J. Appl. Phys. 108, 053508 (2010); http://dx.doi.org/10.1063/1.3467801 (8 pages) | Cited 3 times

Online Publication Date: 3 September 2010

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We model the effects of dye molecule alignment on the collection efficiency of luminescent solar concentrators (LSCs). A Monte Carlo model for photon transport in LSC’s is derived and utilized, which incorporates the effects of fluorescent-dye-molecular alignment and the subsequent control over absorption, emission, and propagation properties. We focus on the effects of molecular alignment statistics on photon absorption and subsequent emission, including polarization and propagation direction imparted by dipole direction, to model device light-capture efficiency, defined as the ratio of the amount of light reaching particular slab edges to that incident on a face. We find that modest control of alignment, coupled with reasonable and attainable emission-absorption dipole angles, can produce very large collection efficiencies for a range of device parameters. We note that efficiencies for small values of dye molecule Stoke’s shift may be made as large as those for homogeneous (unaligned) systems with large Stoke’s shift.
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42.79.Ek Solar collectors and concentrators
78.55.-m Photoluminescence, properties and materials

Luminescence and creation of electron centers in UV-irradiated YAlO3 single crystals

L. Grigorjeva, A. Krasnikov, V. V. Laguta, M. Nikl, and S. Zazubovich

J. Appl. Phys. 108, 053509 (2010); http://dx.doi.org/10.1063/1.3459881 (10 pages) | Cited 3 times

Online Publication Date: 7 September 2010

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Luminescence and defect creation processes were studied by the photoluminescence, thermally stimulated luminescence, and electron paramagnetic resonance methods in the UV-irradiated single crystals of undoped YAlO3, containing small amounts of Ce, Mo, and Ti ions as accidental impurities. The luminescence of the electron antisite YAl2+-type centers of different structures was found around 2.45 eV and studied at 4.2–500 K. The luminescence of the Ti3+-related centers (2.03 and 1.73 eV) and Ti4+ centers (2.78 eV) was observed as well. Dependences of the number of the YAl2+-type and Ti3+-related centers on the UV irradiation energy, temperature, and duration, as well as on various crystal heat-treatment procedures were examined. As a result of the photostimulated electron transfer from the O2− ligand ions to Mo4+ and Ti4+ ions, the paramagnetic hole O−-type centers and electron Ti3+ and Mo3+ centers are created. The antisite YAl2+-type centers are created due to the photostimulated release of electrons mainly from the Mo3+ centers to the conduction band and their subsequent trapping at the YAl3+ ions located near an oxygen vacancy or a defect at the neighboring Y3+ site.
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78.55.Hx Other solid inorganic materials
76.30.Mi Color centers and other defects
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
81.40.Gh Other heat and thermomechanical treatments

Microstructural characteristics and phonon structures in luminescence from surface oxidized Ge nanocrystals embedded in HfO2 matrix

S. Das, R. K. Singha, S. Gangopadhyay, A. Dhar, and S. K. Ray

J. Appl. Phys. 108, 053510 (2010); http://dx.doi.org/10.1063/1.3475717 (7 pages) | Cited 1 time

Online Publication Date: 7 September 2010

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Ge nanocrystals embedded in HfO2 matrices were prepared by rf magnetron sputtering technique. Transmission electron micrographs reveal the formation of spherical shape Ge nanocrystals of 4–6 nm diameters for 800 °C and 6–9 nm for 900 °C annealed samples. X-ray photoelectron spectroscopy confirms the formation of surface oxidized Ge nanocrystals. Embedded Ge nanocrystals show strong photoluminescence peaks in visible and ultraviolet region even at room temperature. Spectral analysis suggests that emission in 1.58 and 3.18 eV bands originate from TΣ(TΠ)→S0, and TΠ′→S0 optical transitions in GeO color centers, respectively, and those in the range 2.0–3.0 eV are related to Ge/O defects at the interface of the oxidized nanocrystals. Temperature dependent photoluminescence study has revealed additional fine structures with lowering of temperature, the origin of which is attributed to the strong coupling of electronic excitations with local vibration of germanium oxides at the surface.
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63.22.Kn Clusters and nanocrystals
78.40.Fy Semiconductors
78.55.Ap Elemental semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.65.Mq Oxidation
81.05.Cy Elemental semiconductors

Limits for n-type doping in In2O3 and SnO2: A theoretical approach by first-principles calculations using hybrid-functional methodology

Péter Ágoston, Christoph Körber, Andreas Klein, Martti J. Puska, Risto M. Nieminen, and Karsten Albe

J. Appl. Phys. 108, 053511 (2010); http://dx.doi.org/10.1063/1.3467780 (6 pages) | Cited 3 times

Online Publication Date: 8 September 2010

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The intrinsic n-type doping limits of tin oxide (SnO2) and indium oxide (In2O3) are predicted on the basis of formation energies calculated by the density-functional theory using the hybrid-functional methodology. The results show that SnO2 allows for a higher n-type doping level than In2O3. While n-type doping is intrinsically limited by compensating acceptor defects in In2O3, the experimentally measured lower conductivities in SnO2-related materials are not a result of intrinsic limits. Our results suggest that by using appropriate dopants in SnO2 higher conductivities similar to In2O3 should be attainable.
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61.72.up Other materials
72.80.Jc Other crystalline inorganic semiconductors
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

A comparative evaluation of elasticity in pentaerythritol tetranitrate using Brillouin scattering and resonant ultrasound spectroscopy

Lewis L. Stevens, Daniel E. Hooks, and Albert Migliori

J. Appl. Phys. 108, 053512 (2010); http://dx.doi.org/10.1063/1.3467523 (4 pages) | Cited 2 times

Online Publication Date: 9 September 2010

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Elastic tensors for organic molecular crystals vary significantly among different measurements. To understand better the origin of these differences, Brillouin scattering and resonant ultrasound spectroscopy measurements were made on the same specimen for single crystal pentaerythritol tetranitrate. The results differ significantly despite mitigation of sample-dependent contributions to errors. The frequency dependence and vibrational modes probed for both measurements are discussed in relation to the observed tensor variance.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.35.+c Brillouin and Rayleigh scattering; other light scattering
43.58.-e Acoustical measurements and instrumentation
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