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Jul 2013

Volume 31, Issue 4 (partial)

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Nonvolatile memory characteristics of thin-film transistors using hybrid gate stack composed of solution-processed indium-zinc-silicon oxide active channel and organic ferroelectric gate insulator

Jun Yong Bak and Sung Min Yoon

J. Vac. Sci. Technol. B 31, 040601 (2013); http://dx.doi.org/10.1116/1.4809996 (8 pages)

Online Publication Date: 10 June 2013

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Organic-ferroelectric oxide memory thin-film transistors (OfeOx-MTFTs) were fabricated using a solution-processed indium-zinc-silicon oxide (IZSiO) active channel and a poly(vinylidene fluoride-trifluoroethylene) ferroelectric gate insulator and characterized to improve device properties such as field-effect mobility (μsat), program speed, and retention time by controlling the IZSiO channel composition. The compositions of IZSiO semiconducting layers were adjusted with different Si amounts of 0, 2, 5, and 10 mol. %. The incorporation of Si in IZSiO channel layer modulated the carrier concentration and reduced defect densities within the channel; among the fabricated OfeOx-MTFT devices, those with IZSiO of 2 mol. % Si content exhibited the best overall performance with μsat, subthreshold swing, memory window, and ratio of on/off programmed currents measured to be 23.3 cm2 V−1 s−1, 772 mV/decade, 11.9 V, and 5.7 × 105, respectively. Incorporating a suitable amount of Si optimized the compromise between the carrier concentration and defect densities within the channel, improving the OfeOx-MTFT program speed and program endurance as well as its data retention properties.
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85.50.Gk Non-volatile ferroelectric memories
85.30.Tv Field effect devices
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Performance improvement of metal-Al2O3-HfO2-oxide-silicon memory devices with band-engineered Hf-aluminate/SiO2 tunnel barriers

Jinho Oh, Heedo Na, Kyumin Lee, Hyunchul Sohn, and Min-Young Heo

J. Vac. Sci. Technol. B 31, 041201 (2013); http://dx.doi.org/10.1116/1.4807842 (5 pages)

Online Publication Date: 30 May 2013

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Physical properties of Hf-aluminate (HA) films with various compositions, deposited by atomic layer deposition, were investigated in terms of microstructure, band-gap, and band-offset with respect to Si. Charge trap flash (CTF) memory devices based on HA/SiO2 stacks as tunnel barriers were also fabricated and characterized. Modulation of HA film composition produced controlled changes of the film's band-gap and band-offset. Additionally, the tunneling efficiency of the HA/SiO2 tunnel barrier stacks was observed to be higher than that for a single SiO2 tunnel barrier, in particular at high voltage bias. The band-engineered CTF memory devices with HA/SiO2 tunnel barriers showed improved program/erase speed compared with those with single SiO2 tunnel barrier. The Al-rich HA/SiO2 tunnel barriers showed a longer charge retention time with superior endurance characteristics; in contrast, the Hf-rich HA/SiO2 tunnel barrier showed degraded charge retention because of current leakage through crystallized regions in the film.
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84.30.Sk Pulse and digital circuits
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Effect of the Al0.3Ga0.7As interlayer thickness upon the quality of GaAs on a Ge substrate grown by metal-organic chemical vapor deposition

Senlin Li, Qingqing Chen, Jin Zhang, Huiquan Chen, Wei Xu, Hui Xiong, Zhihao Wu, Yanyan Fang, Changqing Chen, and Yu Tian

J. Vac. Sci. Technol. B 31, 041202 (2013); http://dx.doi.org/10.1116/1.4809514 (4 pages)

Online Publication Date: 4 June 2013

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GaAs epilayers on Ge substrates are grown with a thin Al0.3Ga0.7As interlayer via metal-organic chemical vapor deposition with the goal of investigating the effect of the Al0.3Ga0.7As interlayer thickness upon the GaAs epilayer. The results show that as the Al0.3Ga0.7As interlayer thickness increases from 0 to 30 nm, both the crystal quality and surface morphology of the GaAs epilayer follow a trend of melioration and then deterioration. All of the Al0.3Ga0.7As interlayers investigated are seen to effectively block the diffusion of Ge atoms to the GaAs epilayers, and high crystalline quality GaAs epilayers with a smooth surface are obtained by growing a 15–23 nm-thick Al0.3Ga0.7As interlayer.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.ag Semiconductors
66.30.Ny Chemical interdiffusion; diffusion barriers
68.35.bg Semiconductors
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High precision dynamic alignment and gap control for optical near-field nanolithography

Xiaolei Wen, Luis M. Traverso, Pornsak Srisungsitthisunti, Xianfan Xu, and Euclid E. Moon

J. Vac. Sci. Technol. B 31, 041601 (2013); http://dx.doi.org/10.1116/1.4809519 (5 pages)

Online Publication Date: 6 June 2013

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The authors demonstrate the use of interferometric-spatial-phase-imaging (ISPI) to control a gap distance of the order of nanometers for parallel optical near-field nanolithography. In optical near-field nanolithography, the distance between the optical mask and the substrate needs to be controlled within tens of nanometers or less. The ISPI technique creates interference fringes from checkerboard gratings fabricated on the optical mask, which are used to determine the gap distance between the mask and the substrate surfaces. The sensitive of this gapping technique can reach 0.15 nm. With the use of ISPI and a dynamic feedback control system, the authors can precisely align the mask and the substrate and keep variation of the gap distance below 6 nm to realize parallel nanolithography.
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81.16.Nd Micro- and nanolithography
42.82.Cr Fabrication techniques; lithography, pattern transfer
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Effect of lateral tip stiffness on atomic-resolution force field spectroscopy

Berkin Uluutku and Mehmet Z. Baykara

J. Vac. Sci. Technol. B 31, 041801 (2013); http://dx.doi.org/10.1116/1.4807376 (4 pages)

Online Publication Date: 20 May 2013

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Atomic force microscopy is being increasingly used to measure atomic-resolution force fields on sample surfaces, making correct interpretation of resulting data critically important. In addition to asymmetry, elastic deformations undergone by the microscope tip are thought to affect measurements. In this study, simple analytical potentials and a model tip apex were used to theoretically analyze how lateral tip stiffness affects force spectroscopy on the surface of NaCl(001). The results suggest that lateral deformations experienced by the tip lead to certain distortions in measured force spectra, the degree of which depends on lateral tip stiffness.
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07.79.Lh Atomic force microscopes
68.35.Gy Mechanical properties; surface strains
68.37.Ps Atomic force microscopy (AFM)
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.20.D- Elasticity
62.20.F- Deformation and plasticity

Atom probe tomography of AlInN/GaN HEMT structures

Nabil Dawahre, Gang Shen, Steven N. Renfrow, Seongsin M. Kim, and Patrick Kung

J. Vac. Sci. Technol. B 31, 041802 (2013); http://dx.doi.org/10.1116/1.4807321 (6 pages)

Online Publication Date: 22 May 2013

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The authors report a correlated study of the atom probe tomography (APT) of lattice matched AlInN/GaN and strained AlGaN/GaN high electron mobility transistor structures, before and after exposure to 60Co irradiation. The AlInN/GaN exhibited a decrease in carrier density while the AlGaN/GaN was found to be more radiation tolerant. Analysis of the APT data revealed that the buried interface near the channel exhibited a much larger increase in isosurface roughness at the AlInN/GaN than at the AlGaN/GaN interfaces. This is believed to contribute to the scattering of carriers out of the channel, and the resulting deterioration of the charge transport characteristics. A proxigram analysis showed that the observed increased roughness at the AlInN/GaN heterointerface was not concurrent with a significant change in its diffuseness.
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85.30.Tv Field effect devices

Growth, doping, and characterization of ZnO nanowire arrays

Gang Shen, Nabil Dawahre, Joseph Waters, Seongsin M. Kim, and Patrick Kung

J. Vac. Sci. Technol. B 31, 041803 (2013); http://dx.doi.org/10.1116/1.4807849 (6 pages)

Online Publication Date: 30 May 2013

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Zinc oxide (ZnO) nanowire (NW) arrays were grown by chemical vapor deposition using the carbothermal reduction of ZnO powder at different pressures from 0.13 to 1.0 atm on basal plane sapphire substrates. The ZnO NWs were oriented in their [0001] direction. Lower growth pressures led to generally longer and smaller diameter wires. A model relating the length and diameter of the NWs was used to interpret the growth mechanism of these ZnO NWs as a function of pressure as the combination of adatom diffusion along the NW sidewalls and direct impingement growth on the NW tip. Al-doped ZnO NWs were synthesized by introducing Al power into the source material, resulting in an Al mole fraction up to 1.8 at. % in the NWs and a concurrent reduction in NW resistivity. Raman spectroscopy revealed slight lattice distortion to the ZnO crystal lattice, while room temperature photoluminescence showed an increase in the near band edge emission concurrently with a reduction in the green emission. The near band edge emission was also blue shifted in a manner consistent with the Burstein–Moss effect in degenerated semiconductor materials.
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81.07.Gf Nanowires
78.30.Fs III-V and II-VI semiconductors
78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors
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)
78.67.Uh Nanowires

Boron nitride growth on metal foil using solid sources

Satoru Suzuki, Roger Molto Pallares, Carlo M. Orofeo, and Hiroki Hibino

J. Vac. Sci. Technol. B 31, 041804 (2013); http://dx.doi.org/10.1116/1.4810965 (9 pages)

Online Publication Date: 14 June 2013

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Large-area, atomically thin hexagonal boron nitride (h-BN) thin films were grown simply by annealing in a vacuum from solid sources deposited on Ni or Co foils. Either a sputter-deposited amorphous boron nitride film or a spin-coated borane ammonia film can be used as the B and N source. The h-BN films were formed on the back surface of the metal foil following diffusion of B and N atoms through the foil of ∼20 μm-thick, although N is almost completely insoluble in these metals. The atomically thin h-BN film largely prevented the foil from oxidizing. The h-BN film formation was found to be restricted by the provision of N atoms. The authors propose that the mass transport of N atoms in the foil and on the back surface of the foil is dominated by grain boundary diffusion and surface migration.
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81.05.Gc Amorphous semiconductors
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
68.55.ag Semiconductors
61.43.Dq Amorphous semiconductors, metals, and alloys
61.72.Mm Grain and twin boundaries
81.05.Ea III-V semiconductors
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Micromirrors connected in series for low voltage operation in vacuum

Seibou Kotani, Hoang Manh Chu, Takashi Sasaki, and Kazuhiro Hane

J. Vac. Sci. Technol. B 31, 042001 (2013); http://dx.doi.org/10.1116/1.4807847 (6 pages)

Online Publication Date: 29 May 2013

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Micromirrors connected in series were fabricated and studied for decreasing operation voltage and investigating an energy loss of resonant mirror. Three identical mirrors (center-mirror and side-mirrors) were connected by torsion bars. The torsion bars of center-mirror were not fixed to substrate; the other sides of the torsion bars of side-mirrors were fixed to the substrate. The motions of the mirrors connected in series were studied. Using the third-mode resonance of series-mirror, the motion of center-mirror was identical to that of a single mirror in the first-mode resonance. The energy loss of mirror was studied by comparing the quality-factors of the series-mirror in the third-mode resonance and the single mirror in the first-mode resonance in vacuum. The quality-factor of the series-mirror was higher than that of the single mirror. The operation voltage of the series-mirror was decreased under the resonant condition in vacuum. The improvements in quality-factor and operation voltage of the series-mirror in the third-mode resonance were explained by the reduced anchor loss due to the virtual supporting of the center-mirror.
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42.82.-m Integrated optics
42.82.Cr Fabrication techniques; lithography, pattern transfer
06.60.Vz Workshop procedures (welding, machining, lubrication, bearings, etc.)
42.79.Bh Lenses, prisms and mirrors
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Structural and magnetic properties of NiAs-type FeSe and related alloy layers

Guibin Song, Hiroaki Matsui, Hisazumi Akai, and Hitoshi Tabata

J. Vac. Sci. Technol. B 31, 042801 (2013); http://dx.doi.org/10.1116/1.4807653 (6 pages)

Online Publication Date: 23 May 2013

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A single phase of β-FeSe was obtained under Se-rich conditions (β-FeSey: y ≧ 1.15) using pulsed laser deposition. The excess Se content in the layers resulted in a decrease of the unit cell volume owing to the nonstoichiometric composition. However, β-FeSey layers did not show any remarkable change of magnetic properties with the exception of decreased saturation magnetization. On the other hand, doping with Cr ions caused lattice distortions along the c-axis direction following a change of the crystallographic superstructure from 4c to 3c types, as confirmed by x-ray diffraction and temperature-dependent magnetization. As a consequence, high coercivity and high anisotropy of in-plane magnetism at 300 K were observed in Cr-rich β-Fe1−xCrxSe1.28 (x = 0.166) layers. Finally, the authors confirmed a correlation between electron transport and magnetism in β-Fe1−xCrxSe1.28 layers from the results of temperature-dependent resistivity and Hall measurements.
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68.55.at Other materials
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.30.Gw Magnetic anisotropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
61.72.up Other materials
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Evolution of Physics and Chemistry of Surfaces and Interfaces: A Perspective of the Last 40 Years

David K. Ferry

J. Vac. Sci. Technol. B 31, 048501 (2013); http://dx.doi.org/10.1116/1.4806761 (6 pages)

Online Publication Date: 17 May 2013

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The Physics and Chemistry of Surfaces and Interfaces conference has maintained a focus on the interfacial and surface properties of materials since its initiation in 1974. The conference continues to be a major force in this field, bringing together scientists from a variety of disciplines to focus upon the science of interfaces and surfaces. Here, a historical view of the development of the conference and a discussion of some of the themes that have been focal points for many years are presented.
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01.30.Ww Editorials
01.65.+g History of science
73.20.At Surface states, band structure, electron density of states
68.35.Ct Interface structure and roughness
68.35.bg Semiconductors
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Atomic-scale movement induced in nanoridges by scanning tunneling microscopy on epitaxial graphene grown on 4H-SiC(0001)

Peng Xu, Steven D. Barber, J. Kevin Schoelz, Matthew L. Ackerman, Dejun Qi, Paul M. Thibado, Virginia D. Wheeler, Luke O. Nyakiti, Rachael L. Myers-Ward, Charles R. Eddy, Jr., and D. Kurt Gaskill

J. Vac. Sci. Technol. B 31, 04D101 (2013); http://dx.doi.org/10.1116/1.4803137 (5 pages)

Online Publication Date: 1 May 2013

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Nanoscale ridges in epitaxial multilayer graphene grown on the silicon face of 4° off-cut 4H-SiC (0001) were found using scanning tunneling microscopy (STM). These nanoridges are only 0.1 nm high and 25–50 nm wide, making them much smaller than previously reported ridges. Atomic-resolution STM was performed near and on top of the nanoridges using a dual scanning technique in which forward and reverse images are simultaneously recorded. An apparent 100% enlarged graphene lattice constant is observed along the leading edge of the image for both directions. Horizontal movement of the graphene, due to both an electrostatic attraction to the STM tip and weak bonding to the substrate, is thought to contribute to the results.
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68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.55.-a Thin film structure and morphology
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
61.48.Gh Structure of graphene
61.46.-w Structure of nanoscale materials

Tuning the electronic states of individual Co acceptors in GaAs

Anne L. Benjamin, Donghun Lee, and Jay A. Gupta

J. Vac. Sci. Technol. B 31, 04D102 (2013); http://dx.doi.org/10.1116/1.4803841 (4 pages)

Online Publication Date: 7 May 2013

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Scanning tunneling microscope studies of individual impurities in semiconductors explore challenges associated with future nanoscale electronics and can provide insight into how new materials properties such as ferromagnetic ordering arise from impurity interactions. Atomic manipulation and tunneling spectroscopy were employed to characterize and control the acceptor states of Co atoms substituted for Ga in the GaAs(110) surface. Three states were observed whose appearance in tunneling spectra was sensitive to the tip position within the acceptor complex. The energy of these states did not follow bending of the host bands due to the tip-induced electric field, but did respond to the Coulomb potential of nearby charged defects, such as As vacancies. By applying voltage pulses with the scanning tunneling microscope tip, the vacancies could be positioned on the surface, thus enabling tunable control over the Co acceptor states.
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73.20.Hb Impurity and defect levels; energy states of adsorbed species
61.72.S- Impurities in crystals
61.72.Yx Interaction between different crystal defects; gettering effect
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Role of bias voltage and tunneling current in the perpendicular displacements of freestanding graphene via scanning tunneling microscopy

Peng Xu, Steven D. Barber, Matthew L. Ackerman, James Kevin Schoelz, and Paul M. Thibado

J. Vac. Sci. Technol. B 31, 04D103 (2013); http://dx.doi.org/10.1116/1.4804401 (5 pages)

Online Publication Date: 10 May 2013

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Systematic displacement measurements of freestanding graphene as a function of applied bias voltage and tunneling current setpoint using scanning tunneling microscopy (STM) are presented. When the bias voltage is increased, the graphene approaches the STM tip, while, on the other hand, when the tunneling current is increased the graphene contracts from the STM tip. To understand the role of the bias voltage, the authors quantitatively model the attractive force between the tip and the sample using electrostatics. For the tunneling current, they qualitatively model the contraction of the graphene using entropic concepts. These complementary results enhance the understanding of each other and highlight peculiarities of the system.
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73.40.Gk Tunneling
72.80.Vp Electronic transport in graphene
81.05.ue Graphene

Ballistic phonon thermal conductance in graphene nanoribbons

Hiroki Tomita and Jun Nakamura

J. Vac. Sci. Technol. B 31, 04D104 (2013); http://dx.doi.org/10.1116/1.4804617 (7 pages)

Online Publication Date: 14 May 2013

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Phonon dispersions for graphene nanoribbons (GNRs) have been derived from the first-principles calculations, and ballistic phonon thermal conductances have been evaluated using the Landauer theory. The phonon thermal conductance per unit width for GNR is larger than that for graphene and increases with decreasing ribbon width. The normalized thermal conductance with a unit of thermal quantum for the zigzag GNR is higher than that for the single-walled carbon nanotube that has a circumferential length corresponding to the width of GNR.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
63.22.Rc Phonons in graphene
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
71.20.Tx Fullerenes and related materials; intercalation compounds
71.15.-m Methods of electronic structure calculations
73.23.Ad Ballistic transport

Integrating MBE materials with graphene to induce novel spin-based phenomena

Adrian G. Swartz, Kathleen M. McCreary, Wei Han, Jared J. I. Wong, Patrick M. Odenthal, Hua Wen, Jen-Ru Chen, Roland K. Kawakami, Yufeng Hao, Rodney S. Ruoff, and Jaroslav Fabian

J. Vac. Sci. Technol. B 31, 04D105 (2013); http://dx.doi.org/10.1116/1.4803843 (8 pages)

Online Publication Date: 15 May 2013

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Magnetism in graphene is an emerging field that has received much theoretical attention. In particular, there have been exciting predictions for induced magnetism through proximity to a ferromagnetic insulator as well as through localized dopants and defects. Here, the authors discuss their experimental work using molecular beam epitaxy to modify the surface of graphene and induce novel spin-dependent phenomena. First, they investigate the epitaxial growth of the ferromagnetic insulator EuO on graphene and discuss possible scenarios for realizing exchange splitting and exchange fields by ferromagnetic insulators. Second, they investigate the properties of magnetic moments in graphene originating from localized pz-orbital defects (i.e., adsorbed hydrogen atoms). The behavior of these magnetic moments is studied using nonlocal spin transport to directly probe the spin-degree of freedom of the defect-induced states. They also report the presence of enhanced electron g-factors caused by the exchange fields present in the system. Importantly, the exchange field is found to be highly gate dependent, with decreasing g-factors with increasing carrier densities.
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75.76.+j Spin transport effects
72.25.-b Spin polarized transport
71.70.Gm Exchange interactions
75.30.Et Exchange and superexchange interactions
75.30.Cr Saturation moments and magnetic susceptibilities
71.70.-d Level splitting and interactions

Epitaxial growth and surface studies of the Half Heusler compound NiTiSn (001)

Jason K. Kawasaki, Thomas Neulinger, Rainer Timm, Martin Hjort, Alexei A. Zakharov, Anders Mikkelsen, Brian D. Schultz, and Chris J. Palmstrøm

J. Vac. Sci. Technol. B 31, 04D106 (2013); http://dx.doi.org/10.1116/1.4807715 (7 pages)

Online Publication Date: 30 May 2013

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The Half Heuslers are currently an attractive family of compounds for high temperature thermoelectrics research, and recently, there has been renewed interest since some of these compounds are proposed to be topological insulators. NiTiSn belongs to the family of 18 valence electron Half Heuslers that are predicted to be semiconducting, despite being composed entirely of metallic elements. The growth of the Half Heusler compound NiTiSn by molecular beam epitaxy is demonstrated. The NiTiSn films are epitaxial and single crystalline as observed by reflection high-energy electron diffraction and x-ray diffraction. Temperature dependent transport measurements suggest the films may be semiconducting, but with a high background carrier density indicative of a high density of electrically active defect states. Methods of protecting the sample surface for synchrotron-based photoemission measurements are explored. These methods may be applied to the study of surface electronic structure in unconventional materials.
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73.20.At Surface states, band structure, electron density of states
73.20.Hb Impurity and defect levels; energy states of adsorbed species
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
68.55.aj Insulators

Combined in-situ photoemission spectroscopy and density functional theory of the Sr Zintl template for oxide heteroepitaxy on Si(001)

Hosung Seo, Miri Choi, Agham B. Posadas, Richard C. Hatch, and Alexander A. Demkov

J. Vac. Sci. Technol. B 31, 04D107 (2013); http://dx.doi.org/10.1116/1.4807716 (8 pages)

Online Publication Date: 30 May 2013

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Half-monolayer Sr on Si(001) is a Zintl template necessary for epitaxial growth of SrTiO3 on Si(001). The authors investigate the reconstruction in the atomic and electronic structure of Si(001) induced by sub-monolayer Sr deposition using in-situ x-ray/ultraviolet photoemission spectroscopy and density functional theory. Sub-monolayer Sr is deposited on Si(001) using molecular beam epitaxy and the structural evolution of the surface is monitored using reflection high-energy electron diffraction. Experimentally, the authors find that the ionization energy of Si(001) decreases as a function of Sr coverage from 4.82 eV for pure Si(001) to 3.97 eV for half-monolayer Sr on Si(001) due to charge transfer from Sr to Si. They calculate the ionization energy for sub-monolayer Sr on Si(001) by considering several atomistic models and find good agreement with experiment. The authors clearly establish the Zintl character of the template by measuring the surface core level shifts of Si(001) and half-monolayer Sr/Si(001). They analyze and interpret the results of these measurements using first-principles final state theory.
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79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.bg Semiconductors
68.55.aj Insulators
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
71.15.-m Methods of electronic structure calculations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
73.20.At Surface states, band structure, electron density of states

Thermoelectric properties of a chain of coupled quantum dots embedded in a nanowire

David M.-T. Kuo and Yia-Chung Chang

J. Vac. Sci. Technol. B 31, 04D108 (2013); http://dx.doi.org/10.1116/1.4808336 (10 pages)

Online Publication Date: 3 June 2013

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The thermoelectric properties of a chain of coupled quantum dots (CCQDs) connected to metallic electrodes are theoretically investigated in the Coulomb blockade regime. An extended Hubbard model is employed to simulate the CCQD system consisted of finite number of quantum dots (QDs). The charge and heat currents are calculated in the framework of Keldysh Green's function technique. The authors obtained a closed-form Landauer expression for the transmission coefficient of the CCQD system with arbitrary number of QDs by using the method beyond mean-field theory. The electrical conductance (Ge), Seebeck coefficient (S), thermal conductance, and figure of merit (ZT) are numerically calculated and analyzed in the linear response regime. In the Coulomb blockade regime thermal conductance is dominated by phonons, the optimization of ZT is determined by the power factor (PF = S2Ge). The authors find that the optimization of ZT value favors the conditions of QD energy levels above the Fermi level of electrodes and QDs with small energy level fluctuations. The optimal ZT values are seriously suppressed by the inelastic scattering of electrons.
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72.20.Pa Thermoelectric and thermomagnetic effects
73.20.At Surface states, band structure, electron density of states
73.21.Hb Quantum wires
73.21.La Quantum dots
73.23.Hk Coulomb blockade; single-electron tunneling

CdZnS quantum dots formed by the Langmuir–Blodgett technique

Alexander G. Milekhin, Nikolay A. Yeryukov, Larisa L. Sveshnikova, Tatyana A. Duda, Dmitry Yu. Protasov, Anton K. Gutakovskii, Stepan A. Batsanov, Nikolay V. Surovtsev, Sergey V. Adichtchev, Cameliu Himcinschi, Volodymir Dzhagan, Francisc Haidu, and Dietrich R. T. Zahn

J. Vac. Sci. Technol. B 31, 04D109 (2013); http://dx.doi.org/10.1116/1.4810782 (7 pages)

Online Publication Date: 11 June 2013

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CdZnS quantum dots (QDs) with systematically varied Zn content (from 0 to 100%) are formed in an organic matrix using the Langmuir–Blodgett technique. Annealing of the QD structures leads to a removal of the organic matrix and an increase in the Zn content for free-standing CdZnS QDs. After annealing, the size of QDs as determined from UV–vis absorption experiments is in good agreement with electron microscopy measurements. Analysis of UV–vis absorption and Raman scattering data demonstrates strong changes in the content of the CdZnS QDs upon annealing. A model of the process of QD formation is developed using the precipitation model and is found to adequately describe the experimental results.
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81.05.Dz II-VI semiconductors
61.72.Cc Kinetics of defect formation and annealing
78.30.Fs III-V and II-VI semiconductors
78.40.Fy Semiconductors
78.67.Hc Quantum dots
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