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Top 20 Most Read Articles
September 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Double oxidation scheme for tunnel junction fabrication J. Vac. Sci. Technol. B 26, 28 (2008); http://dx.doi.org/10.1116/1.2817629 (4 pages) Online Publication Date: 2 January 2008
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The authors report a method to achieve Al–AlOx–Al tunnel junctions with high specific resistance in a controlled manner using a double oxidation technique. The technique is based on the standard method for oxidation repeated on an additional Al layer. The tunnel junctions were characterized with standard methods, such as comparison of room temperature resistance with liquid helium resistance and the authors found them to be of comparable quality to junctions fabricated with standard single oxidation. Fitting with the Simmons model suggests that both the barrier width and barrier height are consistent with those obtained in a single oxidation step. The junction specific capacitance was determined at low temperature to be 68 fF/μm2. These junctions, employed in low temperature measurements and applications, demonstrate expected and stable conductance characteristics. The double oxidation method is straightforward to implement in a basic setup for tunnel junction fabrication.
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Gas-assisted focused electron beam and ion beam processing and fabrication J. Vac. Sci. Technol. B 26, 1197 (2008); http://dx.doi.org/10.1116/1.2955728 (80 pages) Online Publication Date: 11 August 2008
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Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.
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Other one-dimensional systems and thermal properties J. Vac. Sci. Technol. B 26, 1613 (2008); http://dx.doi.org/10.1116/1.2885203 (6 pages) Online Publication Date: 13 August 2008
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Since the ICON 2007 conference was heavily focused on nanowires, one session was devoted to providing a broader picture of one-dimensional systems and how thermal properties are enhanced in these low-dimensional systems. This article is directed toward giving an overview of these topics, summarizing some of the most relevant issues, and providing a vision for the future directions of these topics.
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Nanoimprint lithography: An old story in modern times? A review J. Vac. Sci. Technol. B 26, 458 (2008); http://dx.doi.org/10.1116/1.2890972 (23 pages) Online Publication Date: 27 March 2008
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Nanoimprint lithography (NIL) is a high throughput, high-resolution parallel patterning method in which a surface pattern of a stamp is replicated into a material by mechanical contact and three dimensional material displacement. This can be done by shaping a liquid followed by a curing process for hardening, by variation of the thermomechanical properties of a film by heating and cooling, or by any other kind of shaping process using the difference in hardness of a mold and a moldable material. The local thickness contrast of the resulting thin molded film can be used as a means to pattern an underlying substrate on wafer level by standard pattern transfer methods, but also directly in applications where a bulk modified functional layer is needed. Therefore it is mainly aimed toward fields in which electron beam and high-end photolithography are costly and do not provide sufficient resolution at reasonable throughput. The aim of this review is to play between two poles: the need to establish standard processes and tools for research and industry, and the issues that make NIL a scientific endeavor. It is not the author’s intention to duplicate the content of the reviews already published, but to look on the NIL process as a whole. The author will also address some issues, which are not covered by the other reviews, e.g., the origin of NIL and the misconceptions, which sometimes dominate the debate about problems of NIL, and guide the reader to issues, which are often forgotten or overlooked.
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J. Vac. Sci. Technol. B 10, 1237 (1992); http://dx.doi.org/10.1116/1.585897 (30 pages)
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The status of research on both wurtzite and zinc‐blende GaN, AlN, and InN and their alloys is reviewed including exciting recent results. Attention is paid to the crystal growth techniques, structural, optical, and electrical properties of GaN, AlN, InN, and their alloys. The various theoretical results for each material are summarized. We also describe the performance of several device structures which have been demonstrated in these materials. Near‐term goals and critical areas in need of further research in the III–V nitride material system are identified. |
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Nanofabrication by scanning probe microscope lithography: A review J. Vac. Sci. Technol. B 23, 877 (2005); http://dx.doi.org/10.1116/1.1926293 (18 pages) Online Publication Date: 19 May 2005
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In addition to its well-known capabilities in imaging and spectroscopy, scanning probe microscopy (SPM) has recently shown great potentials for patterning of material structures in nanoscales. It has drawn the attention of not only the scientific community, but also the industry. This article examines various applications of SPM in modification, deposition, removal, and manipulation of materials for nanoscale fabrication. The SPM-based nanofabrication involves two basic technologies: scanning tunneling microscopy and atomic force microscopy. Major techniques related to these two technologies are evaluated with emphasis on their abilities, efficiencies, and reliabilities to make nanostructures. The principle and specific approach underlying each technique are presented; the differences and uniqueness among these techniques are subsequently discussed. Finally, concluding remarks are provided where the strength and weakness of the techniques studied are summarized and the scopes for technology improvement and future research are recommended.
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Band offsets of wide-band-gap oxides and implications for future electronic devices J. Vac. Sci. Technol. B 18, 1785 (2000); http://dx.doi.org/10.1116/1.591472 (7 pages)
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Wide-band-gap oxides such as SrTiO3 are shown to be critical tests of theories of Schottky barrier heights based on metal-induced gap states and charge neutrality levels. This theory is reviewed and used to calculate the Schottky barrier heights and band offsets for many important high dielectric constant oxides on Pt and Si. Good agreement with experiment is found for barrier heights. The band offsets for electrons on Si are found to be small for many key oxides such as SrTiO3 and Ta2O5 which limit their utility as gate oxides in future silicon field effect transistors. The calculations are extended to screen other proposed oxides such as BaZrO3. ZrO2, HfO2, La2O3, Y2O3, HfSiO4, and ZrSiO4. Predictions are also given for barrier heights of the ferroelectric oxides Pb1−xZrxTiO3 and SrBi2Ta2O9 which are used in nonvolatile memories. © 2000 American Vacuum Society. |
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Large areas of periodic nanoholes perforated in multistacked films produced by lift-off J. Vac. Sci. Technol. B 26, 1745 (2008); http://dx.doi.org/10.1116/1.2982240 (3 pages) Online Publication Date: 17 September 2008
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The authors report a low-cost and high-throughput method—nanosphere photolithography, for generating periodic subwavelength holes in metals/dielectrics. By combining the self-assembled and focus properties of micro-/nanospheres, the authors utilized the sphere arrays as lenses to produce large areas of nanopillars with a strong undercut in negative photoresist. Using lift-off with the nanopillars of photoresist, the authors demonstrate a large area of uniform nanoholes of as small as 50 nm in diameter at the bottom of ∼ 160 nm thick metal. The authors also show that the nanohole arrays can be generated in multistacked layers of different materials and these nanoholes can be processed with different sidewall shapes. The technique promises to be an alternative nanopatterning method that is simple, economical, fast, and flexible.
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Surface roughness generated by plasma etching processes of silicon J. Vac. Sci. Technol. B 26, 1281 (2008); http://dx.doi.org/10.1116/1.2932091 (8 pages) Online Publication Date: 12 August 2008
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The authors used atomic force microscopy to analyze the roughness generated on c-Si (100) surfaces when etched in high-density plasmas over a wide range of conditions (pressure, rf power) using SF6, CF4, Cl2, and HBr chemistries. The authors demonstrate unambiguously that high-density plasmas do not generate roughness during silicon etching; but on the contrary, they tend to smooth the existing surface roughness if already present. This is evidenced by analyzing the time evolution of the shape of self-organized silicon nanopillars (patterned on the Si wafer by using diblock copolymers as an etch mask). The 20-nm-high, 20-nm-wide pillars separated by 10 nm are rapidly smoothed by exposure to Cl2 and SF6 plasmas, thus restoring a flat silicon surface. In high-density plasmas, the local etch rate is generally limited by the availability of reactive radicals. In these conditions, the smoothing mechanism is due to the fact that the hills of a rough surface receive a higher flux of etchant radicals than the valleys. Finally, the authors show that the roughening of silicon surfaces in F-based plasma, often reported in the literature, is only due to the micromasking of silicon by AlFx particles originating from the sputtering of the (Al2O3) reactor walls. A high percentage of Al is indeed detected on the surface after etching in F-based plasmas. However, when the chamber walls are intentionally coated by a carbon layer prior to the silicon etching process, the F-based plasmas behave like the other etching chemistries investigated: they rapidly smooth any existing roughness.
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Stable room temperature deposited amorphous InGaZnO4 thin film transistors J. Vac. Sci. Technol. B 26, 959 (2008); http://dx.doi.org/10.1116/1.2917075 (4 pages) Online Publication Date: 2 May 2008
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Enhancement-mode amorphous indium gallium zinc oxide (α-IGZO) channel thin film transistors (TFTs) with a 6 μm gate length and a 100 μm gate width were fabricated on glass substrates by rf magnetron sputtering near room temperature. The resistivities of the α-IGZO films were controlled from 10−1 to 103 Ω cm by varying the deposition power of 75–300 W. The n-type carrier concentration in the channel was 6.5×1017 cm−3. The gate oxide was 90-nm-thick SiNx, deposited by plasma enhanced chemical vapor deposition at 70 °C. The bottom-gate TFTs had saturation mobility of ∼ 17 cm2 V−1 s−1 and the drain current on-to-off ratio of ∼ >105, a subthreshold gate-voltage swing of ∼ 0.5 V decade−1, and a threshold voltage of 2.1 V. In the TFT with a gate length of 6 μm and a gate width of 100 μm, the relative change of saturation mobility and threshold voltage was less than ±1.5% after 500 h aging time at room temperature. This demonstrates that α-IGZO films are promising semiconductor materials for long-term-stable transparent TFT applications.
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Nickel and nickel silicide Schottky barrier contacts to n-type silicon nanowires J. Vac. Sci. Technol. B 26, 1592 (2008); http://dx.doi.org/10.1116/1.2939256 (5 pages) Online Publication Date: 13 August 2008
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Schottky contacts to n-type silicon nanowires were fabricated using Ni or nickel silicide contacts in a wraparound or end contact geometry, respectively. Series resistance in the test structures was reduced by heavily doping the opposite end of the silicon nanowire, facilitating Ohmic contact formation and reducing the resistance of the nanowire itself. The effective Schottky barrier height is reported as a function of nanowire doping, ambient, and applied back gate bias, highlighting some of the important variables affecting current transport in Schottky contacts to semiconductor nanowires. For the silicide contact to the most lightly doped silicon nanowire, measurements in N2 showed that the effective barrier height without a back gate bias was 0.69 eV, and the ideality factor was 1.1.
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Tunable double dots and Kondo enhanced Andreev transport in InAs nanowires J. Vac. Sci. Technol. B 26, 1609 (2008); http://dx.doi.org/10.1116/1.2839634 (4 pages) Online Publication Date: 13 August 2008
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The effect of Kondo correlations on the subgap structure of a quantum dot contacted by superconducting leads is investigated experimentally in indium-arsenide nanowires. When the zero-bias Kondo effect is suppressed by the superconducting pairing of the electrons a profound enhancement of the first-order Andreev reflection is observed. Devices with local gate control allow individual tuning of multiple quantum dots along the wire, a double dot with tunable interdot coupling is demonstrated. |
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Recent advances in processing of ZnO J. Vac. Sci. Technol. B 22, 932 (2004); http://dx.doi.org/10.1116/1.1714985 (17 pages) Online Publication Date: 26 April 2004
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A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures. © 2004 American Vacuum Society. |
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J. Vac. Sci. Technol. B 26, 1700 (2008); http://dx.doi.org/10.1116/1.2970144 (5 pages) Online Publication Date: 5 September 2008
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Wire-type cold cathode was prepared by direct growth of carbon nanotube (CNT) on stainless-steel wire using thermal chemical vapor deposition. By varying the growth temperature, gas flow direction, and reactant gas mixture, improved field-emission uniformity and stability were achieved. The results show that the CNT cathodes grown by H2/C2H2 mixture exhibit better current stability than those grown by Ar/C2H2 mixture. Fully sealed fluorescent lamps have been fabricated using wire-type CNT cold cathode. Luminescent efficiencies of 37 and 21 lm/W were obtained for the fully sealed lamps with green and white phosphor screens.
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Quantum ring formation and antimony segregation in GaSb/GaAs nanostructures J. Vac. Sci. Technol. B 26, 1492 (2008); http://dx.doi.org/10.1116/1.2952451 (12 pages) Online Publication Date: 13 August 2008
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GaSb quantum rings in GaAs were studied by cross-sectional scanning tunneling microscopy. The quantum rings have an outer shape of a truncated pyramid with typical lateral extensions between 10 and 30 nm and heights between 1 and 3 nm, depending on the molecular beam epitaxy growth conditions. A clear central opening of varying diameter and more or less conical shape, filled with GaAs, is characteristic for the GaSb rings. The self-organized formation of quantum rings during the growth and subsequent fast overgrowth of GaSb quantum dots is attributed to a combination of large strain with strong Sb segregation. The latter is enabled by extensive group-V atomic exchange reactions at the GaSb/GaAs interfaces, which are quantitatively evaluated from the atomically resolved microscopy data.
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Optical and photocatalytic properties of oblique angle deposited TiO2 nanorod array J. Vac. Sci. Technol. B 26, 1350 (2008); http://dx.doi.org/10.1116/1.2949111 (9 pages) Online Publication Date: 12 August 2008
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Using an oblique angle deposition technique, an array of well-aligned and tilted TiO2 nanorods has been fabricated by electron-beam evaporation method. The as-deposited TiO2 nanorod array is amorphous and porous with fibrous fine structures on the surface of individual nanorods. After annealing, the amorphous TiO2 crystallizes gradually into an anatase phase with annealing temperature and duration, but no visible change occurs in the porous morphology and surface fine structures even when annealed at 500 °C for 5 h. The annealing under O2 flow causes the optical band gap of TiO2 to increase from 3.32 to 3.39 eV, due to the decrease in oxygen defect concentration. The photocatalytic activity of the annealed TiO2 sample is doubled compared with that of the as-deposited sample. The enhanced specific surface area by the porosity and fine structures of obliquely deposited TiO2 nanorod array is very promising for the applications such as chemical sensor, photovoltaic solar cell, and photocatalyst material for environmental protection.
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J. Vac. Sci. Technol. B 26, 1628 (2008); http://dx.doi.org/10.1116/1.2908442 (4 pages) Online Publication Date: 13 August 2008
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One-dimensional (1D) and two-dimensional (2D) metal-oxide-semiconductor field-effect transistors are compared using an approach based on the top-of-the-barrier ballistic transport model. The results for model devices show that 1D and 2D transistors behave quite similarly if the electrostatics is assumed to be perfect. Distinctive features of 1D transport are difficult to observe at room temperature. The effects of band structure on I-V and C-V characteristics of Si and InAs nanowire transistors are also examined using the sp3d5s* tight-binding model. It is found that band structure effects in 1D transistors are most distinctively reflected in the drain current versus gate bias or transconductance versus gate bias for low drain bias at low temperatures. Some effects may also be observed in nanowire C-V characteristics.
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Mechanical properties of suspended graphene sheets J. Vac. Sci. Technol. B 25, 2558 (2007); http://dx.doi.org/10.1116/1.2789446 (4 pages) Online Publication Date: 11 December 2007
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Using an atomic force microscope, we measured effective spring constants of stacks of graphene sheets (less than 5) suspended over photolithographically defined trenches in silicon dioxide. Measurements were made on layered graphene sheets of thicknesses between 2 and 8 nm, with measured spring constants scaling as expected with the dimensions of the suspended section, ranging from 1 to 5 N/m. When our data are fitted to a model for doubly clamped beams under tension, we extract a Young’s modulus of 0.5 TPa, compared to 1 TPa for bulk graphite along the basal plane, and tensions on the order of 10−7 N.
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Magnetocapacitance effect in InMnAs/InAs p-n heterojunctions J. Vac. Sci. Technol. B 26, 1526 (2008); http://dx.doi.org/10.1116/1.2929861 (4 pages) Online Publication Date: 13 August 2008
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The magnetocapacitance characteristics of an epitaxial p-n heterojunction between magnetic InMnAs and InAs are investigated. A large positive magnetocapacitance is observed at room temperature, which increases with reverse bias. For high reverse bias, the magnetocapacitance is linearly dependent on magnetic field. From capacitance-voltage measurements, the junction built-in voltage was determined and was observed to increase with magnetic field. The magnetocapacitance measurements support a model for a magnetic semiconductor heterojunction where spin-split polarized valence and conduction bands form due to the giant Zeeman effect.
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J. Vac. Sci. Technol. B 26, 1569 (2008); http://dx.doi.org/10.1116/1.2952456 (10 pages) Online Publication Date: 13 August 2008
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Admittance behavior of high-k GaAs metal-insulator-semiconductor (MIS) capacitors having an ultrathin SiNx/Si interface control double layer has been investigated in detail. The measured admittance showed characteristic features that are difficult to explain by the standard Si metal-oxide-semiconductor theory. They include (1) vertical and horizontal types of frequency dispersion in C-V curves, (2) presence of offset conductance in G/ω-f plot, and (3) discrepancy between the surface potential from the high-frequency capacitance and the corresponding relaxation frequency of interface states. All of these features are tentatively explained in a unified manner by a new distributed pinning spot (DPS) model where the MIS interface consists of DPSs in addition to pinning-free regions. When the separation of pinning spots is small, the sample shows vertical type of frequency dispersion with almost bias-independent high-frequency capacitance corresponding to pinning near midgap. When pinning spots are widely separated, the C-V curves show horizontal type of frequency dispersion, each curve showing large capacitance variation with bias. This is due to flatband voltage shifts caused by effective interface state charge at the pinning spots. The pinning spot also gives rise to conductance offset. The discrepancy related to the relaxation frequency of interface states is explained by appearance of saddle points in the potential due to interaction between pinning spots and pinning-free region.
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