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14 Feb 2013

Volume 113, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 113, 064301 (2013); http://dx.doi.org/10.1063/1.4789897 (11 pages)

Y. G. Marinov, G. B. Hadjichristov, A. G. Petrov, S. Marino, C. Versace, and N. Scaramuzza
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back to top Electronic Structure and Transport

Improved conductivity of Sb-doped SnO2 thin films

A. A. Alsac, A. Yildiz, T. Serin, and N. Serin

J. Appl. Phys. 113, 063701 (2013); http://dx.doi.org/10.1063/1.4790879 (4 pages)

Online Publication Date: 8 February 2013

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Sb-doped SnO2 thin films at different thickness have been grown by sol-gel dip-coating method. All of the films exhibit degenerate semiconductor behavior and high free carrier concentrations. In the films, electrical transport can be explained reasonably well by assuming the electron-electron interactions (EEIs) contribution to the measured electrical conductivity. Our experimental observations are consistent with the theoretical description of the EEI. The effect of films thickness on the EEI contribution is also discussed. When the thickness of film reaches to 1550 nm, the agreement between the EEI theory and experimental data becomes unsatisfactory.
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81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.55.ag Semiconductors
73.61.Le Other inorganic semiconductors

Negative magnetoresistance and anomalous Hall effect in GeMnTe-SnMnTe spin-glass-like system

L. Kilanski, R. Szymczak, W. Dobrowolski, A. Podgórni, A. Avdonin, V. E. Slynko, and E. I. Slynko

J. Appl. Phys. 113, 063702 (2013); http://dx.doi.org/10.1063/1.4790321 (7 pages)

Online Publication Date: 11 February 2013

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Magnetotransport properties of spin-glass-like Ge1−x−ySnxMnyTe mixed crystals with chemical composition changing in the range of 0.083 ≤ x ≤ 0.142 and 0.012 ≤ y ≤ 0.119 are presented. The observed negative magnetoresistance we attribute to two mechanisms, i.e., weak localization occurring at low fields and spin disorder scattering giving contribution mainly at higher magnetic fields. A pronounced hysteretic anomalous Hall effect (AHE) was observed. The estimated AHE coefficient shows a small temperature dependence and is dependent on Mn-content, with changes in the range of 10−7<RS<10−6m3/C. The scaling law analysis has proven that the AHE in this system is due to the extrinsic mechanisms, mainly due to the skew scattering accompanied with the side jump processes.
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72.20.My Galvanomagnetic and other magnetotransport effects
73.43.Qt Magnetoresistance
75.47.Pq Other materials
75.50.Pp Magnetic semiconductors

Roles of core-shell and δ-ray kinetics in layered BN α-voltaic efficiency

Corey Melnick, Massoud Kaviany, and Moo-Hwan Kim

J. Appl. Phys. 113, 063703 (2013); http://dx.doi.org/10.1063/1.4790506 (8 pages)

Online Publication Date: 11 February 2013

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α-voltaics harvest electron-hole pairs created as energetic α particles collide with and ionize electrons in a semiconductor, creating δ-rays. After ionization, charged pair production continues through δ-ray impact ionization events and the Auger relaxation of core-shell holes created through K-shell ionization events. Secondary ionization events are quantified using the TPP-2M model, the fraction of K-shell ionization events is determined using the energy-loss Coulomb-repulsion perturbed-stationary-state relativistic theory, and the relaxation of the resulting holes is treated with a fully ab initio approach using multiple Fermi golden rule calculations for ranges of carrier concentrations and temperatures. The limiting rate is 15 ns−1 for small carrier concentrations and high temperatures, as compared to the radiative core-shell relaxation rate estimated here at 20 ns−1, indicating that Auger modes contribute significantly. Moreover, the K-shell ionization events are shown to dominate for low energy α particles and vanish for high energy ones. Thus, the efficiency loss due to energy dissipation in the fuel layer is mitigated, which is demonstrated by the analysis of a layered fuel-voltaic device with an efficiency from 20% to 14% for fuel layers between 5 and 10 μm thick. The design of a α-voltaic integrated with a thermoelectric generator is suggested for improved efficiency and the system-level mitigation of radiation damage and geometric inefficiency.
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79.20.Fv Electron impact: Auger emission
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
71.15.-m Methods of electronic structure calculations

Effective electron mean free path in TiN(001)

J. S. Chawla, X. Y. Zhang, and D. Gall

J. Appl. Phys. 113, 063704 (2013); http://dx.doi.org/10.1063/1.4790136 (6 pages)

Online Publication Date: 11 February 2013

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The measured resistivity ρ of smooth stoichiometric epitaxial TiN(001) is 13 and 3.0 μΩ cm at 298 and 77 K for bulk layers but is 8 and 25 times higher when the layer thickness d is reduced to 2 nm. The increase in ρ with decreasing d is attributed to diffuse electron-surface scattering and is well described by the classical Fuchs-Sondheimer (F-S) model. This is unexpected because the F-S model is based on the nearly free electron model, while TiN exhibits a highly non-spherical Fermi surface and three bands crossing the Fermi-level. The measured room temperature effective electron mean free path λ for bulk scattering is 45 ± 4 nm, which is an order of magnitude larger than the free-electron prediction. This deviation is attributed to ∼93% of charge transport in TiN being due to two slightly filled bands which represent only 4% of conduction electrons. The F-S model is applicable to TiN because these two bands are nearly parabolic and nearly degenerate, yielding a single value for λ, which is estimated based on the published band structure to be 49 nm, in excellent agreement with the experimental 45 nm. These results demonstrate that the F-S model is applicable to metals with non-spherical Fermi-surfaces as long as the charge transport is dominated by a single (or multiple degenerate) band(s).
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.61.Ng Insulators
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.20.Ps Other inorganic compounds
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)

Effect of chiral property on hot phonon distribution and energy loss rate due to surface polar phonons in a bilayer graphene

V. S. Katti and S. S. Kubakaddi

J. Appl. Phys. 113, 063705 (2013); http://dx.doi.org/10.1063/1.4790309 (5 pages)

Online Publication Date: 11 February 2013

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Effect of chiral property on hot phonon distribution and hot electron energy loss rate due to surface polar phonons is investigated in a supported bilayer graphene. Hot phonon distribution Nq(ω0) with chiral property shows a new feature of appearance of a cusp. Interestingly, minimum of the cusp is shifting toward larger q and its width is increasing as electron concentration ns increases. The maximum value of Nq(ω0) is found to shift toward lower q with the increasing ns. This unusual behavior is not found in conventional two-dimensional electron gas (2DEG). The effect of chiral property on energy loss rate is found to be less significant. Hot phonon effect on energy loss rate is studied as a function of electron temperature and electron concentration for different substrates. It is found to reduce the energy loss rate significantly and the results are compared with those in conventional 2DEG. We believe that the magnitude of hot phonon effect depends both on dielectric constant of the substrate and surface polar phonon energy.
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63.22.Rc Phonons in graphene
72.80.Vp Electronic transport in graphene
72.20.Ht High-field and nonlinear effects
77.22.Ch Permittivity (dielectric function)
79.20.Uv Electron energy loss spectroscopy

Ultrafast switching in wetting properties of TiO2/YSZ/Si(001) epitaxial heterostructures induced by laser irradiation

M. R. Bayati, S. Joshi, R. Molaei, R. J. Narayan, and J. Narayan

J. Appl. Phys. 113, 063706 (2013); http://dx.doi.org/10.1063/1.4790327 (7 pages) | Cited 1 time

Online Publication Date: 11 February 2013

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We have demonstrated dark hydrophilicity of single crystalline rutile TiO2(100) thin films, in which rapid switching from a hydrophobic to a hydrophilic surface was achieved using nanosecond excimer laser irradiation. The TiO2/YSZ/Si(001) single crystalline heterostructures were grown by pulsed laser deposition and were subsequently irradiated by a single pulse of a KrF excimer laser at several energies. The wettability of water on the surfaces of the samples was evaluated. The samples were hydrophobic prior to laser annealing and turned hydrophilic after laser annealing. Superhydrophilic surfaces were obtained at higher laser energy densities (e.g., 0.32 J.cm−2). The stoichiometries of the surface regions of the samples before and after laser annealing were examined using XPS. The results revealed the formation of oxygen vacancies on the surface, which are surmised to be responsible for the observed superhydrophilic behavior. According to the AFM images, surface smoothening was greater in films that were annealed at higher laser energy densities. The samples exhibited hydrophobic behavior after being placed in ambient atmosphere. The origin of laser induced wetting behavior was qualitatively understood to stem from an increase of point defects near the surface, which lowered the film/water interfacial energy. This type of rapid hydrophobic/hydrophilic switching may be used to facilitate fabrication of electronic and photonic devices with novel properties.
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81.15.Fg Pulsed laser ablation deposition
65.40.gp Surface energy
68.55.at Other materials
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.35.Md Surface thermodynamics, surface energies
79.20.Ds Laser-beam impact phenomena

Tunneling spectroscopy of chiral states in ultra-thin topological insulators

Gen Yin, Darshana Wickramaratne, and Roger K. Lake

J. Appl. Phys. 113, 063707 (2013); http://dx.doi.org/10.1063/1.4790804 (8 pages)

Online Publication Date: 12 February 2013

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The temperature, Fermi-level, and bias dependencies of the inter-surface tunneling current in thin-film topological insulators show unique, identifying signatures of the surface states and their opposite chiralities. The opposite chiralities of the surface states limit the tunneling to the band edges of the gapped Dirac cones. As a result, the tunneling conductance is sensitive to the temperature, the Fermi level, and the surface-surface potential difference. The temperature dependence of the tunneling conductance changes sign as the Fermi level scans through the Dirac point. The tunneling transmission is a minimum when the opposing surface Dirac cones are perfectly aligned in energy. This minimum state of the tunneling channel can result in negative differential resistance (NDR) in the presence of a built-in Rashba-like splitting. The unique thermal response of the tunneling conductance and the existence of NDR suggest a tunneling spectroscopy experiment to demonstrate the opposite chiralities of the opposing surface states.
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73.61.Ng Insulators
73.20.At Surface states, band structure, electron density of states
73.25.+i Surface conductivity and carrier phenomena
73.40.Gk Tunneling
73.50.Fq High-field and nonlinear effects

Improved chemical and electrical stability of gold silicon contacts via epitaxial electrodeposition

Azadeh Akhtari-Zavareh, Wenjie Li, Fouad Maroun, Philippe Allongue, and Karen L. Kavanagh

J. Appl. Phys. 113, 063708 (2013); http://dx.doi.org/10.1063/1.4792000 (10 pages)

Online Publication Date: 12 February 2013

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The ballistic transport and junction stability of epitaxial Au, grown on Si (111) and (001) substrates via electrodeposition, have been investigated as a function of Au thickness (6.7–14.1 nm). Scanning tunneling microscopy (STM) with ballistic emission electron microscopy (BEEM) showed hot electron transmission for diodes with Au thicknesses greater than 7 nm, whereas for thinner samples surface or interfacial oxidation meant that macroscopic current-voltage characteristics gave high barrier heights (0.8 eV) but no detectable BEEM transport at room temperature. Diodes that remained electrically stable for several months after exposure to air also show an atomically abrupt and epitaxial Au/Si interface via cross-sectional transmission electron microscopy. Degradation in rectifying diode properties through surface or interfacial reactions is indicated by poorer STM images and decreasing or no BEEM transmission, correlated with the formation of a continuous interfacial layer.
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73.40.Ns Metal-nonmetal contacts
81.15.Pq Electrodeposition, electroplating
81.65.Mq Oxidation
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
73.23.Ad Ballistic transport

Critique of charge collection efficiencies calculated through small perturbation measurements of dye sensitized solar cells

Varun Sivaram, James Kirkpatrick, and Henry Snaith

J. Appl. Phys. 113, 063709 (2013); http://dx.doi.org/10.1063/1.4789966 (6 pages)

Online Publication Date: 13 February 2013

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Through experiment and simulation, we critically examine the conventional method of extracting the charge collection efficiency, ηc, of dye-sensitized solar cells (DSSCs). We demonstrate that the collection efficiency extracted by measuring transient decay rates at short circuit deviates considerably from the true value as ηc decreases. This discrepancy arises from two sources of error: first, transient voltage decay rates are especially sensitive to perturbation size near short circuit, and second, the decay rates are influenced by transient equilibration between trapped and free charge during the measurement. Only if the collection efficiency is high (>90%) do the true and measured values coincide well. This finding finally elucidates the disparity between steady-state and transient measurements of the collection efficiency; the former is more accurate for devices with a low ηc, and the latter for devices with a high ηc. For the first time, we present a fully nondimensional model of the DSSC, allowing us to fit device parameters without extensive material knowledge. The resulting simulations and fitting of solid-state dye sensitized solar cells additionally enable us to quantify the inaccuracy of small perturbation measurements of the collection efficiency when ηc is below 90%.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells

Charge transport in lightly reduced graphene oxide: A transport energy perspective

R. S. Kajen, N. Chandrasekhar, K. L. Pey, C. Vijila, M. Jaiswal, S. Saravanan, Andrew M. H. Ng, C. P. Wong, and K. P. Loh

J. Appl. Phys. 113, 063710 (2013); http://dx.doi.org/10.1063/1.4792042 (7 pages)

Online Publication Date: 14 February 2013

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Significant variation in the charge transport behaviour in graphene oxide (GO) ranging from Schottky to Poole-Frenkel and to space charge limited transport exists. These have been extensively reported in the literature. However, the validity of such conventional charge transport models meant for delocalized carriers, to study charge transport through localised states in GO, a disordered semiconductor is open to question. In this work, we use the concept of transport energy (TE) to model charge transport in lightly reduced GO (RGO) and demonstrate that the TE calculations match well with temperature dependent experimental I-V data on RGO. We report on a temperature dependent TE ranging from a few 10 meV to 0.1 eV in slightly reduced GO. Last, we point out that, despite the success of several delocalised charge transport models in estimating barrier heights that resemble the TE level, they remain largely accidental and lack the insight in which the TE concept provides in understanding charge transport in RGO.
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72.80.Vp Electronic transport in graphene
72.20.Ht High-field and nonlinear effects
73.30.+y Surface double layers, Schottky barriers, and work functions
73.61.Wp Fullerenes and related materials

Low-bias electron transport properties of germanium telluride ultrathin films

Jie Liu and M. P. Anantram

J. Appl. Phys. 113, 063711 (2013); http://dx.doi.org/10.1063/1.4790801 (6 pages)

Online Publication Date: 14 February 2013

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The nanometer-scale size-dependent electronic transport properties of crystalline (c-) and amorphous (a-) germanium telluride (GeTe) ultrathin films sandwiched by titanium nitride (TiN) electrodes are investigated using ab initio molecular dynamics, density functional theory, and Green's function calculations. We find that a-GeTe ultrathin films scaled down to about 38 Å (12 atomic layers) still show a band gap and the electrical conductance is mainly due to electron transport via intra-gap states. If the ultrathin films are further scaled, the a-GeTe band gap disappears due to overlap of the two metal induced gap states (MIGS) regions near the TiN electrodes, leading to sharp increase of a-GeTe conductance and significant decrease of c-GeTe/a-GeTe conductance ratio. The c-GeTe/a-GeTe conductance ratio drops below one order of magnitude if the ultrathin films are scaled below about 33 Å, making it difficult to reliably perform read operations in thin film based phase change memory devices. This overlap of the MIGS regions sets up the ultimate scaling limit of phase change memory technology. Our results suggest that the ultimate scaling limit can be pushed to even smaller size, by using phase change material with larger amorphous phase band gap than a-GeTe.
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68.55.-a Thin film structure and morphology
71.15.-m Methods of electronic structure calculations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
73.50.-h Electronic transport phenomena in thin films
61.43.Bn Structural modeling: serial-addition models, computer simulation
61.43.Er Other amorphous solids

Effect of hydrogen incorporation on the negative bias illumination stress instability in amorphous In-Ga-Zn-O thin-film-transistors

Hyeon-Kyun Noh, Ji-Sang Park, and K. J. Chang

J. Appl. Phys. 113, 063712 (2013); http://dx.doi.org/10.1063/1.4792229 (6 pages)

Online Publication Date: 14 February 2013

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In amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors, negative shifts of the threshold voltage commonly occur under negative bias illumination stress (NBIS), and its origin is attributed to hole traps such as O-vacancy (VO) defects. We perform density functional calculations to investigate the effect of hydrogenation on the NBIS instability. We find that hydrogen passivates the electrical activity of VO in form of HO, in which H occupies the vacancy site. The activation energy for dissociating HO into VO and an interstitial H (Hi) is about 1.27 eV, much higher than the migration barrier of about 0.51 eV for Hi diffusion. Kinetic Monte Carlo simulations show that HO defects are quite stable upon post thermal annealing up to 200 °C. Thus, we propose that H incorporation into a-IGZO not only effectively reduces the density of VO defects but also mitigates the NBIS instability in devices fabricated at low temperatures.
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85.30.Tv Field effect devices
61.72.jd Vacancies
61.72.jj Interstitials
81.40.Gh Other heat and thermomechanical treatments
61.72.Cc Kinetics of defect formation and annealing
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
02.50.Ng Distribution theory and Monte Carlo studies
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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