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15 Jun 2011

Volume 109, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 109, 121301 (2011); http://dx.doi.org/10.1063/1.3581173 (33 pages)

Leonard J. Brillson and Yicheng Lu
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ZnO Schottky barriers and Ohmic contacts

Leonard J. Brillson and Yicheng Lu

J. Appl. Phys. 109, 121301 (2011); http://dx.doi.org/10.1063/1.3581173 (33 pages) | Cited 40 times

Online Publication Date: 23 June 2011

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ZnO has emerged as a promising candidate for optoelectronic and microelectronic applications, whose development requires greater understanding and control of their electronic contacts. The rapid pace of ZnO research over the past decade has yielded considerable new information on the nature of ZnO interfaces with metals. Work on ZnO contacts over the past decade has now been carried out on high quality material, nearly free from complicating factors such as impurities, morphological and native point defects. Based on the high quality bulk and thin film crystals now available, ZnO exhibits a range of systematic interface electronic structure that can be understood at the atomic scale. Here we provide a comprehensive review of Schottky barrier and ohmic contacts including work extending over the past half century. For Schottky barriers, these results span the nature of ZnO surface charge transfer, the roles of surface cleaning, crystal quality, chemical interactions, and defect formation. For ohmic contacts, these studies encompass the nature of metal-specific interactions, the role of annealing, multilayered contacts, alloyed contacts, metallization schemes for state-of-the-art contacts, and their application to n-type versus p-type ZnO. Both ZnO Schottky barriers and ohmic contacts show a wide range of phenomena and electronic behavior, which can all be directly tied to chemical and structural changes on an atomic scale.
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73.40.Ns Metal-nonmetal contacts
73.30.+y Surface double layers, Schottky barriers, and work functions
81.65.Cf Surface cleaning, etching, patterning
61.72.Cc Kinetics of defect formation and annealing
85.40.Ls Metallization, contacts, interconnects; device isolation
73.25.+i Surface conductivity and carrier phenomena
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back to top Lasers, Optics, and Optoelectronics

Bright up-conversion green photoluminescence in Ho3+-Yb3+ co-doped Bi4Ti3O12 ferroelectric thin films

Gangjin Ding, Feng Gao, Guangheng Wu, and Dinghua Bao

J. Appl. Phys. 109, 123101 (2011); http://dx.doi.org/10.1063/1.3596597 (4 pages) | Cited 10 times

Online Publication Date: 16 June 2011

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Ho3+-Yb3+ co-doped bismuth titanate ferroelectric thin films were prepared by a chemical solution deposition method on fused silica substrates and their up-conversion luminescence characteristics excited by a 980 nm diode laser were investigated. The two emission bands centered at 546 and 656 nm in the emission spectra can be assigned to 5F4 + 5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+ ions, respectively. A bright green emission was observed even when the laser pumping power was relatively low. The dependence of the emission intensity on the pumping power indicated that the up-conversion emission in the thin films was a two-photon process. The up-conversion emission mechanism is discussed in detail. This study suggests that Ho3+ and Yb3+ co-doped Bi4Ti3O12 thin films can be applied to the fabrication of new multifunctional photoluminescence ferroelectric thin-film devices.
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78.66.Nk Insulators
77.84.Cg PZT ceramics and other titanates
77.55.fe BaTiO3-based films
77.80.-e Ferroelectricity and antiferroelectricity
78.55.Hx Other solid inorganic materials

Dynamics of laser-induced phase switching in GeTe films

W. Gawelda, J. Siegel, C. N. Afonso, V. Plausinaitiene, A. Abrutis, and C. Wiemer

J. Appl. Phys. 109, 123102 (2011); http://dx.doi.org/10.1063/1.3596562 (7 pages) | Cited 5 times

Online Publication Date: 17 June 2011

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Phase switching in GeTe thin films (grown using a modified metal organic chemical vapor deposition system) upon pulsed femtosecond and nanosecond laser irradiation has been studied. Two in situ methods, i.e., optical microscopy and real-time reflectivity measurements, have been used in order to compare the optical response before and after phase change and to follow the phase change dynamics with a time resolution close to 400 ps. The results show that cycling is possible under irradiation with both fs and ns pulses using single pulses for amorphization and multiple pulses for crystallization. The use of ns pulses favors the crystalline-to-amorphous phase transformation, with a characteristic transformation time of ∼15 ns. The presence of the liquid phase was identified and temporally resolved, featuring a well-defined transient reflectivity state, in between those of the crystalline and amorphous phases. We have also studied the role of material configuration in the phase change dynamics and the mechanisms involved in the re-crystallization process.
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68.55.-a Thin film structure and morphology
73.61.Jc Amorphous semiconductors; glasses
78.66.Jg Amorphous semiconductors; glasses
82.33.Ya Chemistry of MOCVD and other vapor deposition methods
64.70.dg Crystallization of specific substances
78.47.J- Ultrafast spectroscopy (<1 psec)

Voltage-controlled active mid-infrared plasmonic devices

K. Anglin, T. Ribaudo, D. C. Adams, X. Qian, W. D. Goodhue, S. Dooley, E. A. Shaner, and D. Wasserman

J. Appl. Phys. 109, 123103 (2011); http://dx.doi.org/10.1063/1.3600230 (5 pages) | Cited 3 times

Online Publication Date: 20 June 2011

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We demonstrate active voltage-controlled spectral tuning of mid-infrared plasmonic structures. Extraordinary optical transmission gratings were fabricated on n-doped GaAs epilayers with a HfO2 gate dielectric between the grating and the doped semiconductor. The permittivity of the GaAs was tuned by depleting charge carriers below the top grating gate upon the application of a reverse bias to the gate. Devices were characterized both electrically and optically, and resonant transmission peak spectral and transmitted intensity shifts were achieved. Possible applications for, as well as the limitations of, the demonstrated technology are discussed.
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42.79.Dj Gratings
42.82.Cr Fabrication techniques; lithography, pattern transfer

Elimination of the coherent effect in the optical Kerr measurement of bismuth glass using supercontinuum

Junyi Tong (佟俊仪), Wenjiang Tan (谭文疆), Jinhai Si (司金海), Yi Yang (杨毅), Wenhui Yi (易文辉), Feng Chen (陈烽), and Xun Hou (侯洵)

J. Appl. Phys. 109, 123104 (2011); http://dx.doi.org/10.1063/1.3597787 (4 pages) | Cited 1 time

Online Publication Date: 20 June 2011

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A strong coherent effect was observed in the monochromatic optical Kerr effect (OKE) experiment of a Bi2O3-B2O3-SiO2 oxide glass (BI glass). To eliminate the influence of the coherent effect, we proposed a simple femtosecond two-color OKE measurement method with wavelength tunability using a supercontinuum. In the two-color OKE measurement the third order nonlinear susceptibility of the BI glass was measured to be 1.64 × 10−13 electrostatic units (esu), in which the influence of the coherent effect was eliminated. The result is smaller than that obtained in a previous monochromatic OKE experiment. In addition, we discuss the discrepancies between the result obtained in this paper and those reported in other studies.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Ce Glasses, quartz
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.65.An Optical susceptibility, hyperpolarizability

Dielectric properties of aluminum silver alloy thin films in optical frequency range

Guang Yang (杨广), Jingbo Sun (孙竞博), and Ji Zhou (周济)

J. Appl. Phys. 109, 123105 (2011); http://dx.doi.org/10.1063/1.3592971 (6 pages) | Cited 4 times

Online Publication Date: 21 June 2011

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The dielectric properties of direct current (dc) magnetron sputtering aluminum silver alloy films in optical frequency have been quantitatively studied by variable angle spectroscopic ellipsometry. The structure and surface topography of the alloy films were characterized using scanning probe microscopy and x-ray diffraction. The Drude–Lorentz model was used to simulate the dielectric function of Al-Ag alloy films. Meanwhile, the effective medium theory has been utilized for the treatment of surface roughness. We found that the interband transition around 1.5 eV can be shifted through a variable annealing temperature and a changeable silver percentage of Al-Ag alloys.
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77.55.-g Dielectric thin films
77.22.Gm Dielectric loss and relaxation
77.22.Ch Permittivity (dielectric function)
81.15.Cd Deposition by sputtering
81.40.Gh Other heat and thermomechanical treatments
68.55.aj Insulators

Early plume and shock wave dynamics in atmospheric-pressure ultraviolet-laser ablation of different matrix-assisted laser ablation matrices

Thomas A. Schmitz, Joachim Koch, Detlef Günther, and Renato Zenobi

J. Appl. Phys. 109, 123106 (2011); http://dx.doi.org/10.1063/1.3592651 (15 pages) | Cited 4 times

Online Publication Date: 22 June 2011

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Pulsed laser ablation of molecular solids is important for identification and quantification in (bio-)organic mass spectrometry, for example using matrix-assisted laser desorption/ionization (MALDI). Recently, there has been a major shift to using MALDI and related laser ablation/post-ionization methods at atmospheric pressure. However, the underlying laser ablation processes, in particular early plume formation and expansion, are still poorly understood. Here, we present a study of the early ablation processes on the ns-time scale in atmospheric pressure UV-laser ablation of anthracene as well as of different common MALDI matrices such as 2,5-dihydroxybenzoic acid (2,5-DHB), α-cyano-4-hydroxycinnamic acid and sinapinic acid. Material release as well as the formation and expansion of hemi-spherical shock waves were studied by shadowgraphy with high temporal resolution (∼5 ns). The applicability of the classical Taylor-Sedov model for expansion of strong shock waves (“point-blast model”), as well as the drag force model, were evaluated to mathematically describe the observed shock wave propagation. The time- and energy-dependent expansion of the shock waves could be described using a Taylor-Sedov scaling law of the form R ∝ tq, when a q-exponent of ∼0.5 instead of the theoretical value of q = 0.4 was found, indicating a faster expansion than expected. The deviations from the ideal value of q were attributed to the non-negligible influence of ambient pressure, a weak versus strong shock regime, and additional acceleration processes present in laser ablation that surpass the limit of the point-blast model. The onset of shock wave formation at a fluence of ∼15–30 mJ/cm2 for the compounds investigated coincides with the onset of bulk material release, whereas, pure desorption below this fluence threshold did not lead to features visible in shadowgraphy.
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79.20.Eb Laser ablation
62.50.Ef Shock wave effects in solids and liquids
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
61.82.Pv Polymers, organic compounds
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Direct band Ge and Ge/InGaAs quantum wells in GaAs

V. Ya. Aleshkin and A. A. Dubinov

J. Appl. Phys. 109, 123107 (2011); http://dx.doi.org/10.1063/1.3594753 (5 pages) | Cited 1 time

Online Publication Date: 23 June 2011

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The possibility of the creation of direct band Ge and Ge/InGaAs quantum wells in GaAs is shown for small Ge quantum well thickness. Such quantum wells can emit efficiently the radiation in the 1.3–1.5 μm wavelength range and can be used in laser diodes.
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81.05.Ea III-V semiconductors
81.07.St Quantum wells
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
42.55.Px Semiconductor lasers; laser diodes
78.67.De Quantum wells
81.05.Cy Elemental semiconductors

Femtosecond laser crystallization of amorphous Ge

Omer Salihoglu, Ulas¸ Kürüm, H. Gul Yaglioglu, Ayhan Elmali, and Atilla Aydinli

J. Appl. Phys. 109, 123108 (2011); http://dx.doi.org/10.1063/1.3601356 (6 pages) | Cited 1 time

Online Publication Date: 23 June 2011

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Ultrafast crystallization of amorphous germanium (a-Ge) in ambient has been studied. Plasma enhanced chemical vapor deposition grown a-Ge was irradiated with single femtosecond laser pulses of various durations with a range of fluences from below melting to above ablation threshold. Extensive use of Raman scattering has been employed to determine post solidification features aided by scanning electron microscopy and atomic force microscopy measurements. Linewidth of the Ge optic phonon at 300 cm−1 as a function of laser fluence provides a signature for the crystallization of a-Ge. Various crystallization regimes including nanostructures in the form of nanospheres have been identified.
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64.70.dg Crystallization of specific substances
64.70.dj Melting of specific substances
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.77.Dq Plasma-based ion implantation and deposition
81.30.Fb Solidification
78.30.Am Elemental semiconductors and insulators

Scattering analysis of plasmonic nanorod antennas: A novel numerically efficient computational scheme utilizing macro basis functions

Arash Rashidi, Hossein Mosallaei, and Raj Mittra

J. Appl. Phys. 109, 123109 (2011); http://dx.doi.org/10.1063/1.3597819 (11 pages) | Cited 1 time

Online Publication Date: 23 June 2011

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In this paper we introduce a versatile and numerically efficient computational technique to model the problem of scattering from plasmonic nanorod antennas. The key to achieving the numerical efficiency is to utilize macro basis functions (MBFs) that taking into account the physics of the problem to reduce the size of matrix equation we need to solve. Closed form formulations are presented for computing the fields by the transverse and longitudinal MBFs that enable us to generate the required matrix elements rapidly, while ensuring that the matrix is well-conditioned. We show that the transverse and longitudinal components of polarization current and all of the components of the scattered fields can be computed very accurately by employing only a few MBFs, i.e., by solving a relatively small-size matrix equation. The accuracy of our modeling technique has been successfully demonstrated by comparing the simulation results with those derived by using the finite difference time domain (FDTD) technique, which is considerably more time-consuming than the present approach. Interesting physical phenomena such as surface plasmon modes for polarization currents and resonance behaviors of plasmonic nanorods are illustrated.
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78.67.Qa Nanorods
73.22.Lp Collective excitations

Enhanced output power using MgZnO/ZnO/MgZnO double heterostructure in ZnO homojunction light emitting diode

Sheng Chu, Jianze Zhao, Zheng Zuo, Jieying Kong, Lin Li, and Jianlin Liu

J. Appl. Phys. 109, 123110 (2011); http://dx.doi.org/10.1063/1.3598136 (4 pages) | Cited 8 times

Online Publication Date: 23 June 2011

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A diode with Sb-doped p-type ZnO, MgZnO/ZnO/MgZnO double heterostructure, and undoped n-type ZnO layers was grown on c-plane sapphire substrate by plasma-assisted molecular-beam epitaxy. Hall effect measurement showed that the top p-type Sb-doped ZnO layer has a hole concentration of 1 × 1017cm−3. Mesa geometry light emitting diodes were fabricated with Au/Ni and Au/Ti Ohmic contacts on top of the p-type and n-type layers, respectively. Ultraviolet emission was achieved, which yielded an output power of 457 nW at 140 mA. The enhancement of the output power is attributed to carrier confinement in the good-quality intrinsic layer of the double heterostructure. The spatial distribution of light emission was characterized.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes
85.40.Ry Impurity doping, diffusion and ion implantation technology
85.40.Sz Deposition technology

A tunable terahertz filter and its switching properties in terahertz region based on a defect mode of a metallic photonic crystal

Yong Sung Kim, Shawn-Yu Lin, Hsin-Ying Wu, and Ru-Pin Pan

J. Appl. Phys. 109, 123111 (2011); http://dx.doi.org/10.1063/1.3603009 (4 pages)

Online Publication Date: 28 June 2011

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We theoretically investigate and discuss an electrically tunable terahertz filter design and its optical switching properties based on the defect mode of a woodpile metallic photonic crystal (MPC). The model filter design is based on a dual use of an MPC as a resonator and as electrodes with a liquid crystal used as a defect layer. The static and the dynamic responses of a realistic liquid crystal are obtained using the Oseen–Frank elastic continuum theory, and the corresponding transmission spectra are calculated using an analytic modal expansion method combined with a transfer-matrix method. The tuning range of f = 1.430∼1.577 THz and the order of milliseconds switching property are observed in our design.
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84.30.Vn Filters
42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.79.Kr Display devices, liquid-crystal devices

Time-resolved photoluminescence of lithographically defined quantum dots fabricated by electron beam lithography and wet chemical etching

V. B. Verma, Martin J. Stevens, K. L. Silverman, N. L. Dias, A. Garg, J. J. Coleman, and R. P. Mirin

J. Appl. Phys. 109, 123112 (2011); http://dx.doi.org/10.1063/1.3599889 (10 pages) | Cited 1 time

Online Publication Date: 29 June 2011

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We measure the time-resolved photoluminescence characteristics of a novel type of lithographically patterned quantum dot fabricated by electron beam lithography, wet chemical etching, and overgrowth of the barrier layers by metalorganic chemical vapor deposition.We find that the quantum dot (QD) photoluminescence exhibits a bi-exponential decay that we explain in terms of the fast capture of carriers by defect states followed by a slower radiative relaxation process. We also perform a systematic investigation of the rise time and decay time as a function of the QD density, size, and temperature. These measurements indicate that the carrier capture process in this type of QD is limited by carrier drift within the GaAs barrier material.
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78.67.Hc Quantum dots
81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.65.Cf Surface cleaning, etching, patterning
78.55.Cr III-V semiconductors
back to top Plasmas and Electrical Discharges

Optical diagnostic of bipolar electrical discharges in HCl, KCl, and KOH solutions

C. Miron, M. A. Bratescu, N. Saito, and O. Takai

J. Appl. Phys. 109, 123301 (2011); http://dx.doi.org/10.1063/1.3597790 (8 pages) | Cited 1 time

Online Publication Date: 22 June 2011

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In this work, the characteristics of the plasma generated in HCl, KCl, and KOH solutions were analyzed using electrical and optical diagnostic techniques. Electrical discharges were initiated between two tungsten electrodes. Current and voltage characteristics have shown the features of a spark discharge for all of the solutions used in the experiment. The reactive species identified in the optical emission spectra depended on the type of solution used to generate the plasma. The time evolution of the reactive species depended on the nature of the solution and on the polarity of the applied pulse. The absorption spectra of the OH radical (X2Π → A2Σ+) were acquired when the voltage pulses were applied to the electrodes, with the intensity being lower in the regions between the pulses. The OH radical density was highest for the HCl solution plasma (2 × 1017 cm−3) when positive voltage pulses were applied to the electrodes.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
52.70.Ds Electric and magnetic measurements
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.80.Mg Arcs; sparks; lightning; atmospheric electricity
52.50.Dg Plasma sources

Cold atmospheric pressure plasma jets as sources of singlet delta oxygen for biomedical applications

J. S. Sousa, K. Niemi, L. J. Cox, Q. Th. Algwari, T. Gans, and D. O’Connell

J. Appl. Phys. 109, 123302 (2011); http://dx.doi.org/10.1063/1.3601347 (8 pages) | Cited 10 times

Online Publication Date: 23 June 2011

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Absolute densities of singlet delta oxygen (SDO) molecules were measured using infrared optical emission spectroscopy in the flowing effluents of two different atmospheric-pressure plasma jets (APPJs): a capacitively coupled radio-frequency-driven jet (rf-APPJ) and a lower frequency kilohertz-driven dielectric barrier discharge jet. The plasma jets were operated in helium, with small admixtures of molecular oxygen (O2 < 2%). High absolute SDO densities of up to 6.2 × 1015 cm−3 were measured at approximately 10 cm downstream. The rf-APPJ seems to be much more efficient in producing SDO. The influence of different parameters, such as gas flows and mixtures and power coupled to the plasmas, on the production of SDO by the two APPJs has been investigated. Despite the considerable differences between the two plasma jets (excitation frequency, electric field direction, inter-electrode distance, plasma propagation), similar dependencies on the oxygen admixture and on the dissipated power were found in both APPJs. However, opposite trends were observed for the gas flow dependence. The results presented in this paper show that the control of the external operating conditions of each APPJ enables the tailoring of the SDO composition of both plasma effluents. This provides scope to tune the plasma jets for desired applications, e.g., in biomedicine.
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52.50.Dg Plasma sources
52.80.Pi High-frequency and RF discharges
52.75.-d Plasma devices
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)

Electron-impact excitation cross sections into Ne(2p53p) levels for plasma applications

R. O. Jung, Garrett A. Piech, M. L. Keeler, John B. Boffard, L. W. Anderson, and Chun C. Lin

J. Appl. Phys. 109, 123303 (2011); http://dx.doi.org/10.1063/1.3597826 (8 pages)

Online Publication Date: 23 June 2011

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One requirement for plasma spectroscopy analysis and modeling of neon discharges is a set of electron-impact excitation cross sections covering excitation from both the 2p6 ground state and from the four 2p53s levels. We present experimental measurements for excitation cross sections into four J = 1 levels of the 2p53p configuration from the J = 0 and J = 2 2p53s metastable levels. A complete set of cross sections into all ten levels of the 2p53p configuration (2px in Paschen’s notation) from the ground state, the two metastable levels and the two resonance levels of the 2p53s configuration (1sy in Paschen’s notation) are compiled in convenient form. The resonance cross sections are obtained from an empirical scaling relationship between the measured metastable excitation cross sections and the corresponding optical oscillator strengths.
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34.80.Dp Atomic excitation and ionization
32.70.Cs Oscillator strengths, lifetimes, transition moments

Synthesis of higher diamondoids by pulsed laser ablation plasmas in supercritical CO2

Sho Nakahara, Sven Stauss, Toru Kato, Takehiko Sasaki, and Kazuo Terashima

J. Appl. Phys. 109, 123304 (2011); http://dx.doi.org/10.1063/1.3599887 (8 pages) | Cited 4 times

Online Publication Date: 24 June 2011

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Pulsed laser ablation (wavelength 532 nm; fluence 18 J/cm2; pulse width 7 ns; repetition rate 10 Hz) of highly oriented pyrolytic graphite was conducted in adamantane-dissolved supercritical CO2 with and without cyclohexane as a cosolvent. Micro-Raman spectroscopy of the products revealed the presence of hydrocarbons possessing sp3-hybridized carbons similar to diamond structures. The synthesis of diamantane and other possible diamondoids consisting of up to 12 cages was confirmed by gas chromatography–mass spectrometry. Furthermore, gas chromatography–mass spectrometry measurements of samples before and after pyrolysis treatment indicate the synthesis of the most compact decamantane, namely, superadamantane. It is thought that oxidant species originating from CO2 during pulsed laser ablation might lead to the selective dissociation of C-H bonds, enabling the synthesis of low H/C ratio molecules. Therefore, laser ablation in supercritical CO2 is proposed as a practical method for synthesizing diamondoids.
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52.77.Fv High-pressure, high-current plasmas (plasma spray, arc welding, etc.)
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.38.Mf Laser ablation
52.70.Nc Particle measurements

Real-time monitoring of nucleation-growth cycle of carbon nanoparticles in acetylene plasmas

Morten Hundt, Patrick Sadler, Igor Levchenko, Matthias Wolter, Holger Kersten, and Kostya (Ken) Ostrikov

J. Appl. Phys. 109, 123305 (2011); http://dx.doi.org/10.1063/1.3599893 (7 pages) | Cited 7 times

Online Publication Date: 28 June 2011

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Quantum cascade laser absorption spectroscopy was used to measure the absolute concentration of acetylene in situ during the nanoparticle growth in Ar + C2H2 RF plasmas. It is demonstrated that the nanoparticle growth exhibits a periodical behavior, with the growth cycle period strongly dependent on the initial acetylene concentration in the chamber. Being 300 s at 7.5% of acetylene in the gas mixture, the growth cycle period decreases with the acetylene concentration increasing; the growth eventually disappears when the acetylene concentration exceeds 32%. During the nanoparticle growth, the acetylene concentration is small and does not exceed 4.2% at radio frequency (RF) power of 4 W, and 0.5% at RF power of 20 W. An injection of a single acetylene pulse into the discharge also results in the nanoparticle nucleation and growth. The absorption spectroscopy technique was found to be very effective for the time-resolved measurement of the hydrocarbon content in nanoparticle-generating plasmas.
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81.05.U- Carbon/carbon-based materials
78.40.Ha Other nonmetallic inorganics
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.07.Bc Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing

Simulation of particle velocity in a laser-produced tin plasma extreme ultraviolet source

Majid Masnavi, Mitsuo Nakajima, Kazuhiko Horioka, Homaira Parchamy Araghy, and Akira Endo

J. Appl. Phys. 109, 123306 (2011); http://dx.doi.org/10.1063/1.3601346 (7 pages) | Cited 2 times

Online Publication Date: 29 June 2011

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In connection with fast heating in a laser produced plasma (LPP) extreme ultraviolet (EUV) source, the superheating behavior of bulk tin (Sn) at high heating rates is investigated. A constant temperature and pressure molecular dynamics simulation using modified Lennard-Jones and Coulomb potentials suitable for studying the liquid structure of Sn is employed in order to derive the caloric curves of the solid and liquid phases. The results have shown transient effects on the phase transitions. Superheating is observed during the melting and vaporizing processes. The velocity distribution of Sn particles against typical laser fluence in a LPP EUV light source has been numerically investigated using a simplified method including a one-dimensional, two-temperature, molecular dynamics, and steady-state ionization model. In the framework of our model, it was found that ejected Sn particles have a maximum velocity on the order of 10 to 40 km/s in plasma created using a nanosecond pre-pulse neodymium-doped yttrium aluminum garnet (Nd:YAG, 1.06 μm) laser in EUV lithography experiments.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.25.Kn Thermodynamics of plasmas
52.65.Yy Molecular dynamics methods
52.25.Jm Ionization of plasmas
52.50.Dg Plasma sources
42.72.Bj Visible and ultraviolet sources
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Symmetry-adapted non-equilibrium molecular dynamics of chiral carbon nanotubes under tensile loading

Amin Aghaei and Kaushik Dayal

J. Appl. Phys. 109, 123501 (2011); http://dx.doi.org/10.1063/1.3596827 (8 pages) | Cited 4 times

Online Publication Date: 16 June 2011

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We report on non-equilibrium molecular dynamics calculations of chiral single-wall carbon nanotubes using the framework of Objective Structures. This enables us to adapt molecular dynamics to the symmetry of chiral nanotubes and efficiently simulate these systems with small unit cells. We outline the method and the adaptation of a conventional thermostat and barostat to this setting. We then apply the method in order to examine the behavior of nanotubes with various chiralities subject to a constant extensional strain rate. We examine the effects of temperature, strain rate, and pre-compression/pre-tension. We find a range of failure mechanisms, including the formation of Stone-Wales defects, the opening of voids, and the motion of atoms out of the cross-section.
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81.40.Lm Deformation, plasticity, and creep
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
62.20.F- Deformation and plasticity
62.20.M- Structural failure of materials
62.25.-g Mechanical properties of nanoscale systems
61.43.Bn Structural modeling: serial-addition models, computer simulation

Minority-carrier diffusion length, minority-carrier lifetime, and photoresponsivity of β-FeSi2 layers grown by molecular-beam epitaxy

Keiichi Akutsu, Hideki Kawakami, Mitsushi Suzuno, Takashi Yaguchi, Karolin Jiptner, Jun Chen, Takashi Sekiguchi, Teruhisa Ootsuka, and Takashi Suemasu

J. Appl. Phys. 109, 123502 (2011); http://dx.doi.org/10.1063/1.3596565 (6 pages) | Cited 4 times

Online Publication Date: 17 June 2011

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We have epitaxially grown undoped β-FeSi2 films on Si(111) substrates via atomic-hydrogen-assisted molecular-beam epitaxy. β-FeSi2 films grown without atomic hydrogen exhibited p-type conduction with a hole density of over 1019 cm−3 at room temperature (RT). In contrast, those prepared with atomic hydrogen showed n-type conduction and had a residual electron density that was more than two orders of magnitude lower than the hole density of films grown without atomic hydrogen (of the order of 1016 cm−3 at RT). The minority-carrier diffusion length was estimated to be approximately 16 μm using an electron-beam-induced current technique; this value is twice as large as that for β-FeSi2 prepared without atomic hydrogen. This result could be well explained in terms of the minority-carrier lifetimes measured by a microwave photoconductance decay technique. The 1/e decay time using a 904 nm laser pulse was approximately 17 μs, which is much longer than that for β-FeSi2 prepared without atomic hydrogen (3 μs). The photoresponsivity reached 13 mA/W at 1.31 μm, which is the highest value ever reported for β-FeSi2 films.
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81.05.Hd Other semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.50.Mx High-frequency effects; plasma effects
73.50.Pz Photoconduction and photovoltaic effects
73.61.Le Other inorganic semiconductors

Size effect on thermal desorption of CO from Pt nanostructures on graphite

A. Julukian, T. Fadnes, S. Raaen, and M. Balci

J. Appl. Phys. 109, 123503 (2011); http://dx.doi.org/10.1063/1.3596572 (5 pages)

Online Publication Date: 17 June 2011

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Self-assembled Pt nanostructures, which are formed by evaporation and subsequent diffusion limited aggregation of Pt on graphite, have been studied by photoemission and scanning electron microscopy. Adsorption of CO has been studied by temperature programmed desorption. Charge induced Pt 4f core level shifts in the order of 1 eV that depend on the effective dimensions of the nanostructures have been observed, and effective dimensions of the Pt structures have been defined based on the Pt 4f core level shifts. The effective dimensions of the Pt structures have been correlated with changes in the thermal desorption of adsorbed CO. It is observed that smaller effective dimensions in the few nanometer range result in lower desorption temperatures of up to 50 K. The possible role of hot electrons in the adsorption process has been discussed.
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68.43.Vx Thermal desorption
79.60.Dp Adsorbed layers and thin films
81.16.Dn Self-assembly
66.30.-h Diffusion in solids
68.43.Mn Adsorption kinetics
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Quantifying the defect-dominated size effect of fracture strain in single crystalline ZnO nanowires

Mo-Rigen He, Pan Xiao, Jiong Zhao, Sheng Dai, Fujiu Ke, and Jing Zhu

J. Appl. Phys. 109, 123504 (2011); http://dx.doi.org/10.1063/1.3594655 (5 pages) | Cited 3 times

Online Publication Date: 20 June 2011

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The diameter (D) dependence of fracture strains in [0001]-oriented single crystalline ZnO nanowires (NWs) with D ranging from 18 to 114 nm is experimentally revealed via in situ uniaxial tension and is well understood based on an analytical model developed by combining molecular dynamics simulations with fracture mechanics theories. We show that the scattered fracture strains are dominated by the effective quantities of atomic vacancies, and their lower bound follows a power-form scaling law, resembling the Griffith-type behavior of single critical defects with diameter-dependent sizes, when D is larger than a critical DC. In addition, theoretical strength is expected in NWs with D < DC. Our studies provide a simple, but basic, understanding for the size effect of strengths in single crystalline NWs.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.25.Mn Fracture/brittleness
62.20.mm Fracture
68.65.La Quantum wires (patterned in quantum wells)
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

The influence of a Si cap on self-organized SiGe islands and the underlying wetting layer

M. Brehm, M. Grydlik, H. Groiss, F. Hackl, F. Schäffler, T. Fromherz, and G. Bauer

J. Appl. Phys. 109, 123505 (2011); http://dx.doi.org/10.1063/1.3594693 (9 pages) | Cited 7 times

Online Publication Date: 20 June 2011

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For the prototypical SiGe/Si(001) Stranski-Krastanow (SK) growth system, the influence of intermixing caused by the deposition of a Si cap layer at temperatures Tcap between 300°C and 700°C is studied both for the SiGe wetting layer (WL) and the SiGe islands. Systematic growth experiments were carried out with an ultrahigh resolution of down to 0.005 monolayers (ML) of deposited Ge. The properties of the samples were investigated via photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy. We studied in detail the influence of Tcap in the three main coverage regions of SiGe SK growth, which are (i) the WL build-up regime, (ii) the island nucleation regime, where most of the Ge is supplied via material transfer from the WL, and (iii) the saturation regime, where the WL thickness remains initially stable. At Tcap = 300°C, we found that both the WL and the island are essentially preserved in composition and shape, whereas at 500°C the WL becomes heavily alloyed during capping, and at 700°C the islands also become alloyed. At Tcap = 500°C we found enhanced WL intermixing in the presence of dome-shaped islands, whereas at Tcap = 700°C the WL properties become dominated by the dissolution of pyramid-shaped islands upon capping. At Ge coverages above ≈6 ML, we found an unexpected thickening of the WL, almost independently of Tcap. This finding suggests that the density and the volume of the dome-shaped islands have an upper limit, beyond which excess Ge from the external source again becomes incorporated into the WL. Finally, we compared PL spectra with AFM-based evaluations of the integral island volumes in order to determine in a straightforward manner the average composition of the SiGe islands.
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68.08.Bc Wetting
78.55.Cr III-V semiconductors
78.67.Hc Quantum dots
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Study of laser beam propagation in microholes and the effect on femtosecond laser micromachining

Sha Tao, Benxin Wu, and Shuting Lei

J. Appl. Phys. 109, 123506 (2011); http://dx.doi.org/10.1063/1.3595671 (6 pages)

Online Publication Date: 20 June 2011

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In femtosecond (fs) laser micromachining, such as microhole drilling, the hole created by previous laser pulses may act like a waveguide, whose sidewall may significantly affect the beam profile of subsequent laser pulses propagating into the hole. This effect is very important for both the fundamental study and the practical applications of fs laser micromachining, but has not been well studied in literature. The effect is investigated in this paper by numerically solving the transient Maxwell’s wave equation. The study reveals how microholes affect laser intensity profile for different laser and hole parameters, and the implied effects on fs laser micromachining. The study has provided a good fundamental physical explanation for the elliptical hole shape obtained in fs laser micromachining with linearly polarized laser beams.
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42.62.Cf Industrial applications
81.16.-c Methods of micro- and nanofabrication and processing
81.20.Wk Machining, milling
42.25.Bs Wave propagation, transmission and absorption
42.60.Fc Modulation, tuning, and mode locking
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