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Top 20 Most Read Articles
March 2012
The 20 articles with the most full-text downloads during the month, in descending order.
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Degradation of superhard nanocomposites by built-in impurities J. Vac. Sci. Technol. B 22, L5 (2004); http://dx.doi.org/10.1116/1.1689305 (5 pages) Online Publication Date: 24 March 2004
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Impurities such as oxygen and chlorine can strongly decrease the hardness of superhard nc-TiN/a-Si3N4 and similar nanocomposites when incorporated into the coatings during their deposition. It is shown that 1–1.5 at. % of oxygen causes a hardness decrease to about 30 GPa, as compared to 45–55 GPa for the pure material. This may explain some of the contradictory results found by other authors, particularly for coatings deposited by physical vapor deposition at relatively low nitrogen pressure, deposition temperature, and deposition rates. © 2004 American Vacuum Society. |
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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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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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Recent developments and design challenges in continuous roller micro- and nanoimprinting J. Vac. Sci. Technol. B 30, 010801 (2012); http://dx.doi.org/10.1116/1.3661355 (28 pages) Online Publication Date: 1 December 2011
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As an emerging technology for the manufacture of micro- and nano-scale patterns, continuous imprinting; otherwise known as roll-to-roll or roller imprinting, is attracting interest from researchers around the world because of its inherent advantages of low cost, high throughput, large area patterning. This technology is an evolutionary advance on the more traditional nanoimprint lithography developed in the 1990s, which is considered a batch mode, or dis-continuous patterning approach. In recent years, a number of commercial applications have been discovered which require low cost, large area patterning, particularly displays, optical coatings and films, and biological applications such as anti-fouling surfaces and micro-fluidic devices. This review covers a variety of continuous imprinting approaches, highlights challenges, and surveys progress towards high speed production of micro- and nanoscale features for these applications and others using this platform technology.
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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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Fabrication and performance of graphene nanoelectromechanical systems J. Vac. Sci. Technol. B 29, 050801 (2011); http://dx.doi.org/10.1116/1.3623419 (10 pages) Online Publication Date: 9 September 2011
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As a result of the recent progress in fabricating large-area graphene sheets, graphene-based mechanical devices have become vastly easier to manufacture and now show even greater promise for a range of applications. This article reviews the progress of resonant graphene nanoelectromechanical systems and the possible applications of this technology to signal processing, sensing, and other areas. After discussing recent advances in fabrication and measurement techniques that make graphene resonators a viable technology, the article presents what is known about the performance of graphene mechanical systems. The authors also highlight unresolved questions, such as the source of the dissipation in graphene resonators, and discuss the progress made on these issues to date. The authors conclude with a discussion of important future directions for graphene research and the applications for which graphene nanomechanical devices may be well suited.
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Characteristic comparison of AlGaN/GaN enhancement-mode HEMTs with CHF3 and CF4 surface treatment J. Vac. Sci. Technol. B 30, 021201 (2012); http://dx.doi.org/10.1116/1.3680115 (6 pages) Online Publication Date: 1 February 2012
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In this study, enhancement-mode (E-mode) AlGaN/GaN HEMTs that underwent CHF3 and CF4 plasma treatment beneath the gate metal were fabricated. These treatments were applied because, although previous studies have formed AlF3 compound layers after fluorine-based plasma treatment to suppress the polarization-induced charge density, the surface negative charges still influenced the device gate leakage current and trap density. In the device in this study, unlike in previous CF4 plasma-treated GaN E-mode devices, the hydrogen atoms of the CHF3 plasma were introduced to compensate for vacancies by donating an electron to a vacancy acceptor level, thereby reducing the number of vacancy induced traps. Based on the measured subthreshold slope (SS) and the effective interface state density (Dit) results, the SS value of a CHF3-treated HEMT was 80 mV/decade and the Dit was 1.23 × 1012 cm−2. Moreover, the CHF3-treated HEMT exhibited a current gain cut-off frequency, a maximum oscillation frequency, and an output power of 6.7, 26, and 14.8 dBm (302 mW/mm), respectively. The 1/f noise measurement results of the CHF3-treated HEMT indicated that the flicker noise-induced generation-recombination noise and gate leakage-induced generation-recombination noise were also improved. Therefore, the CHF3-treated HEMT has great potential for use in low-distortion power amplifiers and logic control circuits.
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Damage evolution in GaN under MeV heavy ion implantation J. Vac. Sci. Technol. B 27, 2342 (2009); http://dx.doi.org/10.1116/1.3244591 (5 pages) Online Publication Date: 22 October 2009
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Damage evaluation processes in patterned GaN implanted by 3 MeV Au2+ ions were investigated as a function of ion fluences and annealing temperatures. Surface swelling was observed by using atomic force microscopy and the results showed that the swelling height depends on ion fluence and annealing temperature. The authors observed four-stage implantation-induced damage evolution including point-defect formation, defect clustering, amorphization/bubble formation, and eventually, decomposition. This evolution is contributed to irradiation-induced defect production and defect migration/accumulation occurred at different levels of displacement per atom. Craterlike holes were observed on the surface of GaN implanted at the ion fluence of 2×1016 cm−2, which is evidence of N loss, and broken bubbles formed during implantation.
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Graphene functionalization and seeding for dielectric deposition and device integration J. Vac. Sci. Technol. B 30, 030801 (2012); http://dx.doi.org/10.1116/1.3693416 (21 pages) Online Publication Date: 19 March 2012
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Graphene has recently attracted wide-spread attention because of its unique transport and physical properties that are appealing for a wide range of electronic applications. Integration with scalable high-κ dielectrics is important for the realization of graphene-based top-gated electronic devices, including next generation THz applications. Atomic layer deposition (ALD), a low temperature deposition method based on two separate self-limiting surface reactions, is a preferred technique to achieve high-quality, conformal, ultrathin dielectric films with precise control of thickness and chemical composition at the atomic scale. Unfortunately, ALD of oxides on graphene is hindered by the inertness of the graphene surface. To alleviate this graphene-oxide incompatibility, several different functionalization and seeding methods have recently been developed to render the graphene more susceptible to the ALD process of high-κ dielectrics including: ozone, wet chemical and fluorine pretreatments, low-k polymer seed, e-beam metal, and oxide seed layers. The ability of each approach to enable conformal, uniform high-κ dielectrics on graphene while maintaining its inherent transport properties for low power, high-frequency device applications is discussed.
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J. Vac. Sci. Technol. B 29, 010801 (2011); http://dx.doi.org/10.1116/1.3532949 (35 pages) Online Publication Date: 14 January 2011
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Photolithographic patterning of organic materials and plasma-based transfer of photoresist patterns into other materials have been remarkably successful in enabling the production of nanometer scale devices in various industries. These processes involve exposure of highly sensitive polymeric nanostructures to energetic particle fluxes that can greatly alter surface and near-surface properties of polymers. The extension of lithographic approaches to nanoscale technology also increasingly involves organic mask patterns produced using soft lithography, block copolymer self-assembly, and extreme ultraviolet lithographic techniques. In each case, an organic film-based image is produced, which is subsequently transferred by plasma etching techniques into underlying films/substrates to produce nanoscale materials templates. The demand for nanometer scale resolution of image transfer protocols requires understanding and control of plasma/organic mask interactions to a degree that has not been achieved. For manufacturing of below 30 nm scale devices, controlling introduction of surface and line edge roughness in organic mask features has become a key challenge. In this article, the authors examine published observations and the scientific understanding that is available in the literature, on factors that control etching resistance and stability of resist templates in plasma etching environments. The survey of the available literature highlights that while overall resist composition can provide a first estimate of etching resistance in a plasma etch environment, the molecular structure for the resist polymer plays a critical role in changes of the morphology of resist patterns, i.e., introduction of surface roughness. Our own recent results are consistent with literature data that transfer of resist surface roughness into the resist sidewalls followed by roughness extension into feature sidewalls during plasma etch is a formation mechanism of rough sidewalls. The authors next summarize the results of studies on chemical and morphological changes induced in selected model polymers and advanced photoresist materials as a result of interaction with fluorocarbon/Ar plasma, and combinations of energetic ion beam/vacuum ultraviolet (UV) irradiation in an ultrahigh vacuum system, which are aimed at the fundamental origins of polymer surface roughness, and on establishing the respective roles of (a) polymer structure/chemistry and (b) plasma-process parameters on the consequences of the plasma-polymer interactions. Plasma induced resist polymer modifications include formation of a thin ( ∼ 1–3 nm) dense graphitic layer at the polymer surface due to ion bombardment and deeper-lying modifications produced by plasma-generated vacuum ultraviolet (VUV) irradiation. The relative importance of the latter depends strongly on initial polymer structure, whereas the ion bombardment induced modified layers are similar for various hydrocarbon polymers. The formation of surface roughness is found to be highly polymer structure specific. Beam studies have revealed a strong ion/UV synergistic effect where the polymer modifications introduced at various depths by ions or ultraviolet/UV photons can interact. A possible fundamental mechanism of initial plasma-induced polymer surface roughness formation has been proposed by
Bruce et al. [J. Appl. Phys. 107, 084310 (2010)]
. In their work, they measured properties of the ion-modified surface layer formed on polystyrene (PS) polymer surfaces, and by considering the properties of the undamaged PS underlayer, they were able to evaluate the stressed bilayer using elastic buckling theory. Their approach was remarkably successful in reproducing the wavelength and amplitude of measured surface roughness introduced for various ion bombardment conditions, and other variations of experimental parameters. Polymer material-dependent VUV modifications introduced to a depth of about 100 nm can either soften (scission) or stiffen (cross-linking) this region, which produce enhanced or reduced surface roughness.
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Fabrication of top-gated epitaxial graphene nanoribbon FETs using hydrogen-silsesquioxane J. Vac. Sci. Technol. B 30, 03D104 (2012); http://dx.doi.org/10.1116/1.3693593 (4 pages) Online Publication Date: 12 March 2012
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Top-gated epitaxial graphene nanoribbon (EGNR) field effect transistors (FETs) were fabricated on epitaxial graphene substrates which demonstrated the opening of a substantial bandgap. Hydrogen silsesquioxane (HSQ) was used for the patterning of 10 nm size linewidth as well as a seed layer for atomic layer deposition (ALD) of a high-k dielectric aluminum oxide (Al2O3). It is found that the resolution of the patterning is affected by the development temperature, electron beam dose, and substrate materials. The chosen gate stack of HSQ followed by Al2O3 ALD permits stable device performance and enables the demonstration of the EGNR-FET.
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Nanofabrication of doped, complex oxides J. Vac. Sci. Technol. B 30, 011804 (2012); http://dx.doi.org/10.1116/1.3669645 (6 pages) Online Publication Date: 15 December 2011
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Complex oxides have many promising attributes, including wide band gaps for high temperature semiconductors, ion conducting electrolytes in fuel cells, ferroelectricity and ferromagnetism. Bulk and thin film oxides can be readily manufactured and tested however these physically hard and chemically inert materials cannot be nanofabricated by direct application of conventional methods. In order to study these materials at the nanoscale there must first be a simple and effective means to achieve the desired structures. Here we discuss the use of pulsed laser deposition at room temperature onto electron beam lithography defined templates of poly methyl methacrylate photoresist. Following a resist liftoff in organic solvents, a heat treatment was used to crystallize the nanostructures. The morphology of these structures was studied using scanning electron microscopy and atomic force microscopy. Crystallinity and composition as determined by x ray diffraction and photo-electron spectroscopy respectively is reported for thin film analogues of the nanostructured oxide. The oxide studied in this report is Nb doped SrTiO3, which has been investigated for use as a high temperature thermoelectric material; however the approach used is not materials-dependent.
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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 30, 020601 (2012); http://dx.doi.org/10.1116/1.3678490 (4 pages) Online Publication Date: 24 January 2012
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The authors report on the formation of highly reliable Ti/Al-based ohmic contacts to N-face n-GaN for high-performance vertical light-emitting diodes by using Ti(Ga) solid solution and TiN layers. The Ti(Ga) solid solution layer is used to minimize the outdiffusion of Ga atoms from the n-GaN surface region. Unlike the Ti/Al contacts, the Ti(Ga)/Ti/Al and Ti(Ga)/TiN/Al samples exhibit ohmic behavior with contact resistivities of 3.9 – 4.8 × 10−4 Ωcm2 after annealing at 250 °C. It was further shown that unlike the Ti(Ga)/TiN/Al samples, the Ti/Al and the Ti(Ga)/Ti/Al samples are largely electrically degraded when annealed at 300 °C in an oven. Based on x-ray photoemission spectroscopy and secondary ion mass spectrometry results, ohmic formation and degradation mechanisms are briefly described and discussed.
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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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Investigation of graphene piezoresistors for use as strain gauge sensors J. Vac. Sci. Technol. B 29, 06FE01 (2011); http://dx.doi.org/10.1116/1.3660784 (5 pages) Online Publication Date: 10 November 2011
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The primary objective of this research is to fabricate a graphene-based piezoresistive strain gauge and characterize its sensitivity. The strain gauge consists of mechanically exfoliated graphene sheets and electrical electrodes located on a silicon wafer. Instead of using e-beam lithography, which is the most widely applied methods in experimental studies of graphene, a new fabrication method utilizing conventional photolithography was used to easily fabricate a new nanoelectromechanical system strain gauge. The proposed fabrication technique is easy and only requires a few types of microfabrication equipment, thereby opening up a new way to broadly spread and facilitate associated graphene research, especially for those laboratories with limited resources. To characterize the piezoresistive sensitivity of the graphene-based strain gauge, a strain-detection system built by an equivalent-stress macrocantilever was set up to generate mechanical bending strain where a calibrated commercial strain gauge was packaged to display the generated strain. Utilizing this measurement setup, the electrical properties of the graphene-based piezoresistive strain gauge were reliably investigated. A high gauge factor of ∼150 was experimentally measured with the graphene device, which promises a new strain gauge of high sensitivity.
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Study of Ti etching and selectivity mechanism in fluorocarbon plasmas for dielectric etch J. Vac. Sci. Technol. B 30, 021804 (2012); http://dx.doi.org/10.1116/1.3690643 (10 pages) Online Publication Date: 6 March 2012
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The authors studied the behavior of Ti hardmasks in CF4/Ar and C4F8/Ar discharges using conditions relevant to pattern transfer processes into organosilicate glass (OSG), a reference low-k material investigated in parallel. The authors examined various material erosion stages and determined the dependencies of etch rates (ERs) and etching selectivities (ESs) on the following plasma parameters: self-bias voltage (50–150 V), processing pressure (20–60 mTorr) and %CF4 (10–30 %) in CF4/Ar discharges, and O2 addition (0–10 %) and N2 addition (0–20 %) to C4F8/Ar discharges. Erosion behavior and ERs were characterized by real-time ellipsometric measurements and multilayer optical modeling. These measurements were complemented by x ray photoelectron spectroscopy to study the surface composition. The impact of plasma parameter changes were investigated by comparing ERs and corresponding ESs (OSG ER/Ti ER). During the erosion of Ti, the initially oxidized film surface was transformed into a TiFx layer (x ∼ 3) covered by a FC film. The FC film thickness strongly depended on the FC feed gas and was significantly thicker for the C4F8-based etch (1.5 nm) than for the CF4-based etch (0.9 nm). Ti erosion was found to be dependent on the energy deposited on the film surface by ion bombardment and to exponentially decrease with increasing FC film thicknesses. For thin FC films (< 1 nm), erosion was ion driven, i.e., “chemical sputtering”, and, for thick FC films (> 1 nm), erosion was limited by the amount of F that could diffuse through the FC layer to the Ti interface. In contrast to organic masking materials, Ti hardmasks have lower ESs for the more polymerizing C4F8-based discharges than for CF4-based discharges. This can be explained by the consumption of the limited supply of F at the OSG surface by C and H impurities, which form volatile CF4 and HF etch products. For thin FC films and low ion energy deposition by ion bombardment, ESs up to 15 have been achieved.
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Sub-10 nm imprint lithography and applications J. Vac. Sci. Technol. B 15, 2897 (1997); http://dx.doi.org/10.1116/1.589752 (8 pages)
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New developments, further details, and applications of imprint lithography are presented. Arrays of 10 nm diameter and 40 nm period holes were imprinted not only in polymethylmethacrylate (PMMA) on silicon, but also in PMMA on gold substrates. The smallest hole diameter imprinted in PMMA is 6 nm. All the PMMA patterns were transferred to a metal using a liftoff. In addition, PMMA mesa’s of a size from 45 nm to 50 μm were obtained in a single imprint. Moreover, imprint lithography was used to fabricate the silicon quantum dot, wire, and ring transistors, which showed the same behavior as those fabricated using electron (e)-beam lithography. Finally, imprint lithography was used to fabricate nanocompact disks with 10 nm features and 400 Gbits/in.2 data density—near three orders of magnitude higher than current critical dimensions (CDs). A silicon scanning probe was used to read back the data successfully. The study of wear indicates that due to the ultrasmall force in tapping mode, both the nano-CD and the scanning probe will not show noticeable wear after a large number of scans. © 1997 American Vacuum Society. |
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J. Vac. Sci. Technol. B 30, 011205 (2012); http://dx.doi.org/10.1116/1.3671020 (7 pages) Online Publication Date: 21 December 2011
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AlGaN/GaN high electron mobility transistors (HEMTs) with similar active layers structures were grown on SiC or sapphire substrates using different buffer layer structures, including GaN of different thickness (1 or 2 μm) or composite AlGaN/GaN buffers. The highest density of hole traps was observed in the buffer on sapphire, while the lowest density of hole traps was obtained in the thick (2 μm) GaN buffer on SiC. The reverse leakage currents in HEMTs were lower in the devices grown on SiC substrates and the on-off ratios improved by two orders of magnitude for thicker GaN buffers or composite AlGaN/GaN buffers compared to a standard 1 μm GaN buffer. The maximum drain-source currents and tranconductances were all larger for the devices on SiC compared to sapphire.
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J. Vac. Sci. Technol. B 16, 558 (1998); http://dx.doi.org/10.1116/1.589862 (3 pages)
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Etching results using the solution system of citric acid/H2O2 and de-ionized H2O/buffered oxide etch are shown to provide good selective wet etching of AlGaAs/GaAs structures. For AlxGa1−xAs(x<0.5) layers the selective characteristics of each Al composition strongly depend on the volume ratio of the citric acid/H2O2 solution. The turning volume ratio of the solution, at which etching starts, sensitively depends on Al composition. Additionally, the etch rate of AlyGa1−yAs (y>0.7) quickly decreases with decreasing Al composition in de-ionized H2O/buffered oxide etch solution, providing a high degree of etching selectivity. These simple selective etching processes have been applied to define AlAs/GaAs distributed Bragg reflector mesas in a vertical-cavity surface-emitting laser structure. © 1998 American Vacuum Society. |
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