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Top 20 Most Read Articles
March 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Comparison of the sputter rates of oxide films relative to the sputter rate of SiO2 J. Vac. Sci. Technol. A 28, 1060 (2010); http://dx.doi.org/10.1116/1.3456123 (13 pages) Online Publication Date: 2 September 2010
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There is a growing interest in knowing the sputter rates for a wide variety of oxides because of their increasing technological importance in many different applications. To support the needs of users of the Environmental Molecular Sciences Laboratory, a national scientific user facility, as well as our research programs, the authors made a series of measurements of the sputter rates from oxide films that have been grown by oxygen plasma-assisted molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, electrochemical oxidation, or sputter deposition. The sputter rates for these oxide films were determined in comparison with those from thermally grown SiO2, a common reference material for sputter rate determination. The film thicknesses and densities for most of these oxide films were measured using x-ray reflectivity. These oxide films were mounted in an x-ray photoelectron or Auger electron spectrometer for sputter rate measurements using argon ion sputtering. Although the primary objective of this work was to determine relative sputter rates at a fixed angle, the measurements also examined (i) the angle dependence of the relative sputter rates, (ii) the energy dependence of the relative sputter rates, and (iii) the extent of ion beam induced reduction for some oxides. Oxide films examined include SiO2, Al2O3, CeO2, Cr2O3, Fe2O3, HfO2, In–Sn oxide, Ta2O5, TiO2 (anatase, rutile, and amorphous), and ZnO. The authors found that the sputter rates for the oxides can vary up to a factor of 2 (usually lower) from that observed for SiO2. The ratios of sputter rates relative to those of SiO2 appear to be relatively independent of ion beam energy in the range of 1–4 kV and for incident angles <50°. As expected, the extent of ion beam induced reduction of the oxides varies with the sputter angle.
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Dry etching of polydimethylsiloxane for microfluidic systems J. Vac. Sci. Technol. A 20, 975 (2002); http://dx.doi.org/10.1116/1.1460896 (8 pages) Online Publication Date: 7 May 2002
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A fluorine-based reactive ion etch (RIE) process has been developed to anisotropically dry etch the silicone elastomer polydimethylsiloxane (PDMS). This technique complements the standard molding procedure that makes use of forms made of thick SU-8 photoresist to produce features in the PDMS. Total gas pressure and the ratio of O2 to CF4 were varied to optimize etch rate. The RIE recipe developed in this study uses a 1:3 mixture of O2 to CF4 gas resulting in a highly directional and stable etch rate of approximately 20 μm per hour. Selective dry etching can be performed through a photolithographically patterned metal etch mask providing greater precision and alignment with preexisting molded features. The dry etch process is presented in this article along with a brief comparison to recently reported wet etch approaches. © 2002 American Vacuum Society. |
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Status and potential of atmospheric plasma processing of materials J. Vac. Sci. Technol. A 29, 020801 (2011); http://dx.doi.org/10.1116/1.3559547 (17 pages) Online Publication Date: 4 March 2011
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This paper is a review of the current status and potential of atmospheric plasma technology for materials processing. The main focus is the recent developments in the area of dielectric barrier discharges with emphasis in the functionalization of polymers, deposition of organic and inorganic coatings, and plasma processing of biomaterials. A brief overview of both the equipment being used and the physicochemical reactions occurring in the gas phase is also presented. Atmospheric plasma technology offers major industrial, economic, and environmental advantages over other conventional processing methods. At the same time there is also tremendous potential for future research and applications involving both the industrial and academic world.
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Critical review: Plasma-surface reactions and the spinning wall method J. Vac. Sci. Technol. A 29, 010801 (2011); http://dx.doi.org/10.1116/1.3517478 (25 pages) Online Publication Date: 3 January 2011
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This article reviews methods for studying reactions of atoms and small molecules on substrates and chamber walls that are immersed in a plasma, a relatively unexplored, yet very important area of plasma science and technology. Emphasis is placed on the “spinning wall” technique. With this method, a cylindrical section of the wall of the plasma reactor is rotated, and the surface is periodically exposed to the plasma and then to a differentially pumped mass spectrometer, to an Auger electron spectrometer, and, optionally, to a beam of additional reactants or surface coatings. Reactants impinging on the surface can stick and react over time scales that are comparable to the substrate rotation period, which can be varied from ∼ 0.5 to 40 ms. Langmuir–Hinshelwood reaction probabilities can be derived from a measurement of the absolute desorption product yields as a function of the substrate rotation frequency. Auger electron spectroscopy allows the plasma-immersed surface to be monitored during plasma operation. This measurement is critical, since wall “conditioning” in the plasma changes the reaction probabilities. Mass spectrometer cracking patterns are used to identify simple desorption products such as Cl2, O2, ClO, and ClO2. Desorption products also produce a measurable pressure rise in the second differentially pumped chamber that can be used to obtain absolute desorption yields. The surface can also be coated with films that can be deposited by sputtering a target in the plasma or by evaporating material from a Knudsen cell in the differentially pumped wall chamber. Here, the authors review this new spinning wall technique in detail, describing both experimental issues and data analysis methods and interpretations. The authors have used the spinning wall method to study the recombination of Cl and O on plasma-conditioned anodized aluminum and stainless steel surfaces. In oxygen or chlorine plasmas, these surfaces become coated with a layer containing Si, Al, and O, due to slow erosion of the reactor materials, in addition to Cl in chlorine plasmas. Similar, low recombination probabilities were found for Cl and O on anodized Al versus stainless steel surfaces, consistent with the similar chemical composition of the layer that forms on these surfaces after long exposure to the plasma. In chlorine plasmas, weakly adsorbed Cl2 was found to inhibit Cl recombination, hence the Cl recombination probability decreases with increasing Cl2-to-Cl number density ratios in the plasma. In mixed Cl2/O2 plasmas, Cl and O recombine to form Cl2 and O2 with probabilities that are similar to those in pure chlorine or oxygen plasmas, but in addition, ClO and ClO2 form on the surface and desorb from the wall. These and other results, including the catalytic enhancement of O recombination by monolayer amounts of Cu, are reviewed.
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Defects in m-plane ZnO epitaxial films grown on (112) LaAlO3 substrate J. Vac. Sci. Technol. A 29, 031001 (2011); http://dx.doi.org/10.1116/1.3539046 (5 pages) Online Publication Date: 31 January 2011
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The crystallographic orientations of m-plane ZnO on (112) LaAlO3 (LAO) substrate are [
 20]ZnO∥[11]LAO and [0001]ZnO∥[10]LAO. The defects in m-plane ZnO have been systematically investigated using cross section and plan-view transmission electron microscopy (TEM). High-resolution TEM observations in cross section show misfit dislocations and basal stacking faults (BSFs) at the ZnO/LAO interface. In the films, threading dislocations (TDs) with 1/3〈11 0〉 Burgers vectors are distributed on the basal plane, and BSFs have 1/6〈203〉 displacement vector. The densities of dislocations and BSFs are estimated to be 5.1×1010 cm−2 and 4.3×105 cm−1, respectively. In addition to TDs and BSFs, plan-view TEM examination also reveals that stacking mismatch boundaries mainly lie along the m-planes and they connect with planar defect segments along the r-planes. |
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Nanosphere lithography: A materials general fabrication process for periodic particle array surfaces J. Vac. Sci. Technol. A 13, 1553 (1995); http://dx.doi.org/10.1116/1.579726 (6 pages)
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In this article nanosphere lithography (NSL) is demonstrated to be a materials general fabrication process for the production of periodic particle array (PPA) surfaces having nanometer scale features. A variety of PPA surfaces have been prepared using identical single‐layer (SL) and double‐layer (DL) NSL masks made by self‐assembly of polymer nanospheres with diameter, D=264 nm, and varying both the substrate material S and the particle material M. In the examples shown here, S was an insulator, semiconductor, or metal and M was a metal, inorganic ionic insulator, or an organic π‐electron semiconductor. PPA structural characterization and determination of nanoparticle metrics was accomplished with atomic force microscopy. This is the first demonstration of nanometer scale PPA surfaces formed from molecular materials. © 1995 American Vacuum Society |
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High rate deep Si etching for through-silicon via applications J. Vac. Sci. Technol. A 29, 021009 (2011); http://dx.doi.org/10.1116/1.3543635 (6 pages) Online Publication Date: 28 January 2011
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High rate deep Si etching for through-silicon via (TSV) applications is reported. The requirements for the Si etch process is discussed from the viewpoint of TSV size and productivity, and the effective processes are described. For “small” TSV a few microns in diameter and up to 10 μm deep, profile control is the most important requirement, For “large” TSV with diameters of more than 50 μm and depths up to 100 μm and more, an ultrahigh Si etch rate is indispensable. The “medium” TSV with diameters and depths several tens of microns requires both high etch rate and profile control. Capacitively coupled plasma MERIE at high pressure is shown to be effective, by using HBr gas chemistry for small TSV, and by using SF6 gas chemistry and high rf frequency for large and medium TSV where an extremely high etch rate can be obtained.
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J. Vac. Sci. Technol. A 25, 1317 (2007); http://dx.doi.org/10.1116/1.2764082 (19 pages) Online Publication Date: 30 July 2007
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Physical vapor deposition under conditions of obliquely incident flux and limited adatom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies. This is the basis for glancing angle deposition (GLAD), a technique for fabricating porous thin films with engineered structures. The origin of the columnar structure characteristic of GLAD films is discussed in terms of nucleation processes and structure zone models. As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical applications and sensor device architectures are possible. Because the GLAD process provides precise nanoscale control over the film structure, characteristics such as the mechanical, magnetic, and optical properties of the deposited film may be engineered for various applications. Depositing onto prepatterned substrates forces the columns to adopt a planar ordering, an important requirement for photonic crystal applications.
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Plasma deposition of optical films and coatings: A review J. Vac. Sci. Technol. A 18, 2619 (2000); http://dx.doi.org/10.1116/1.1314395 (27 pages)
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Plasma enhanced chemical vapor deposition (PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin film systems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasma systems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others. © 2000 American Vacuum Society. |
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The microstructure of sputter‐deposited coatings J. Vac. Sci. Technol. A 4, 3059 (1986); http://dx.doi.org/10.1116/1.573628 (7 pages)
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Microstructure is a critical consideration when polycrystalline or amorphous thin films are used for applications such as microcircuit metallization layers and diffusion barriers. The trend in device fabrication toward lower processing temperatures means that such coatings must often be deposited at substrate temperatures T that are low relative to the coating material melting point Tm. The structure of vapor deposited coatings grown under these conditions consists typically of a columnar growth structure, defined by voided open boundaries, which is superimposed on a microstructure which may be polycrystalline (defined by metallurgical grain boundaries) or amorphous. The voided growth structure is clearly undesirable for most applications. Its occurrence is a fundamental consequence of atomic shadowing acting in concert with the low adatom mobilities that characterize low T/Tm deposition, and its formation can be enhanced by the surface irregularities which are common to microcircuit fabrication. This paper reviews some of the recent developments in understanding the fundamental aspects of the relationship between the deposition conditions and the microstructure of sputter‐deposited thin films, with particular emphasis on the origin of the growth structure and its suppression through energetic particle bombardment. |
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Work function of fluorine doped tin oxide J. Vac. Sci. Technol. A 29, 011019 (2011); http://dx.doi.org/10.1116/1.3525641 (4 pages) Online Publication Date: 5 January 2011
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Fluorine doped tin oxide (FTO) is a commonly used transparent conducting oxide in optoelectronic device applications. The work function of FTO is commonly cited as 4.4 eV, which is incommensurate with recent device performance results. Using x-ray photoelectron spectroscopy, the authors measured the work function of commercial FTO to be 5.0±0.1 eV. UV ozone treatment was found to increase the work function by ∼ 0.1 eV due to surface band bending. The origins of the much lower work function previously reported are also discussed and are found to be a result of carbon contamination and UV induced work function lowering.
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Atomic layer deposition of Ru from CpRu(CO)2Et using O2 gas and O2 plasma J. Vac. Sci. Technol. A 29, 021016 (2011); http://dx.doi.org/10.1116/1.3554691 (7 pages) Online Publication Date: 16 February 2011
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The metalorganic precursor cyclopentadienylethyl(dicarbonyl)ruthenium (CpRu(CO)2Et) was used to develop an atomic layer deposition (ALD) process for ruthenium. O2 gas and O2 plasma were employed as reactants. For both processes, thermal and plasma-assisted ALD, a relatively high growth-per-cycle of ∼ 1 Å was obtained. The Ru films were dense and polycrystalline, regardless of the reactant, yielding a resistivity of ∼ 16 μΩ cm. The O2 plasma not only enhanced the Ru nucleation on the TiN substrates but also led to an increased roughness compared to thermal ALD.
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Work function determination of zinc oxide films J. Vac. Sci. Technol. A 15, 428 (1997); http://dx.doi.org/10.1116/1.580502 (3 pages)
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Zinc oxide-silicon heterojunctions were fabricated using both n- and p-type silicon. The zinc oxide films were deposited by the magnetron sputtering process at various substrate temperatures to form these devices. The electrical properties of these devices were measured and the work function of the zinc oxide was evaluated from these properties. © 1997 American Vacuum Society. |
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Pulsed electron-beam technology for surface modification of metallic materials J. Vac. Sci. Technol. A 16, 2480 (1998); http://dx.doi.org/10.1116/1.581369 (9 pages)
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This article concerns the foundations of a new technology for surface modification of metallic materials based on the use of original sources of low-energy, high-current electron beams. The sources contain an electron gun with an explosive-emission cathode and a plasma anode, placed in a guide magnetic field. The acceleration gap and the transportation channel are prefilled with plasma with the use of spark plasma sources or a low-pressure reflected discharge. The electron-beam sources produce electron beams with the parameters as follows: electron energy 10–40 keV; pulse duration 0.5–5 μs; energy density 0.5–40 J/cm2, and beam cross-section area 10–50 cm2. They are simple and reliable in operation. Investigations performed with a variety of constructional and tool materials (steels, aluminum and titanium alloys, hard alloys) have shown that the most pronounced changes of the structure-phase state occur in the near-surface layers quenched from the liquid state, where the crystallization front velocity reaches its maximum. In these layers partial or complete dissolving of second phases and formation of oversaturated solid solutions and ordered nanosized structures may take place. This makes it possible to improve substantially the electrochemical and strength properties of the surface layers. It has been established that the deformation processes occurring in the near-surface layers have the result that the thickness of the modified layer with improved strength properties is significantly greater than that of the heat-affected zone. Some examples of the use of low-energy, high-current electron beams for improving the performance of materials and articles are given. © 1998 American Vacuum Society. |
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Ga-doped ZnO grown by pulsed laser deposition in H2: The roles of Ga and H J. Vac. Sci. Technol. A 29, 03A102 (2011); http://dx.doi.org/10.1116/1.3523296 (4 pages) Online Publication Date: 11 January 2011
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Highly conductive thin films of ZnO doped with Ga were grown by pulsed laser deposition with 10 mTorr of H2 in the growth chamber. Compared with a more conventional method of producing conductive films of ZnO, i.e., growth in O2 followed by annealing in forming gas (5% H2 in Ar), the H2 method requires no postgrowth anneal and also produces higher carrier concentrations and lower resistivities with better depth uniformity. As an example, a 65-nm-thick sample had a room-temperature mobility of 32 cm2/V s, a concentration of 6.8×1020 cm−3, and a resistivity of 2.9×10−4 Ω cm. From a scattering model, the donor and acceptor concentrations were calculated as 8.9×1020 and 2.1×1020 cm−3, respectively, as compared to the Ga and H concentrations of 11×1020 and 1×1020 cm−3. The authors conclude that growth in H2 produces higher Ga-donor concentrations but that H-donors themselves do not play a significant role.
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Deposition of ultrathin AlN films for high frequency electroacoustic devices J. Vac. Sci. Technol. A 29, 021014 (2011); http://dx.doi.org/10.1116/1.3554718 (7 pages) Online Publication Date: 14 February 2011
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The authors investigate the microstructure, crystal orientation, and residual stress of reactively sputtered aluminum nitride (AlN) films having thicknesses as low as 200 down to 25 nm. A two-step deposition process by the dual cathode ac (40 kHz) powered S-gun magnetron enabling better conditions for AlN nucleation on the surface of the molybdenum (Mo) bottom electrode was developed to enhance crystallinity of ultrathin AlN films. Using the two-step process, the residual in-plane stress as well as the stress gradient through the film thickness can be effectively controlled. X-ray rocking curve measurements have shown that ultrathin films grown on Mo using this technology are highly c-axis oriented with full widths at half maximum of 1.8° and 3.1° for 200- and 25-nm-thick films, respectively, which are equal to or even better than the results previously reported for relatively thick AlN films. High-resolution transmission electron microscopy and fast Fourier transform analyses have confirmed strong grain orientation in 25–100-nm-thick films. A fine columnar texture and a continuous lattice microstructure within a single grain from the interface with the Mo substrate through to the AlN surface have been elicited even in the 25-nm-thick film.
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Metal–organic interface and charge injection in organic electronic devices J. Vac. Sci. Technol. A 21, 521 (2003); http://dx.doi.org/10.1116/1.1559919 (11 pages) Online Publication Date: 18 March 2003
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Charge injection at the interface between metallic electrodes and organic semiconductors plays a crucial role in the performance of organic (opto-)electronic devices. This article discusses the current understanding of the formation of the metal–organic contact and the parameters which control the injection current. Organic semiconductors differ significantly from their inorganic counterparts, primarily because they are amorphous van der Waals solids. As a result the electronic states are highly localized, and charge transport is by site-to-site hopping. Organics can also form clean interfaces with many metals, free of interface states in the gap. Nevertheless, there is generally found to be a significant vacuum level offset, the origins of which are not yet fully understood. Organic semiconductors are frequently free of donor and acceptor dopants, and as a result the depletion depth is larger than the organic layer thickness. Thus the Fermi level in the organic and the charge injection barriers depend most directly on the interface offset. The charge injection process is described as thermally assisted tunneling from the delocalized states of the metal into the localized states of the semiconductor, whose energy includes contributions from the mean barrier height, the image potential, the energetic disorder, and the applied electric field. There is no completely satisfactory analytic theory for the field and temperature dependence of the injection current, which, for well characterized interfaces, exhibits behavior relating to both thermionic emission and field-induced tunneling. © 2003 American Vacuum Society. |
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On the electrochemical etching of tips for scanning tunneling microscopy J. Vac. Sci. Technol. A 8, 3570 (1990); http://dx.doi.org/10.1116/1.576509 (6 pages)
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The sharpness of tips used in scanning tunneling microscopy (STM) is one factor which affects the resolution of the STM image. In this paper, we report on a direct‐current (dc) drop‐off electrochemical etching procedure used to sharpen tips for STM. The shape of the tip is dependent on the meniscus which surrounds the wire at the air–electrolyte interface. The sharpness of the tip is related to the tensile strength of the wire and how quickly the electrochemical reaction can be stopped once the wire breaks. We have found that the cutoff time of the etch circuit has a significant effect on the radius of curvature and cone angle of the etched tip; i.e., the faster the cutoff time, the sharper the tip. We have constructed an etching circuit with a minimum cut‐off time of 500 ns which uses two fast metal–oxide semiconductor field effect transistors (MOSFET) and a high‐speed comparator. The radius of curvature of the tips can be varied from approximately 20 to greater than 300 nm by increasing the cutoff time of the circuit. |
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Growth modes of InN (000-1) on GaN buffer layers on sapphire J. Vac. Sci. Technol. A 23, 304 (2005); http://dx.doi.org/10.1116/1.1864056 (6 pages) Online Publication Date: 22 February 2005
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In this work, using atomic force microscopy and scanning tunneling microscopy, we study the surface morphologies of epitaxial InN films grown by plasma-assisted molecular beam epitaxy with intervening GaN buffer layers on sapphire substrates. On smooth GaN buffer layers, nucleation and evolution of three-dimensional InN islands at various coverages and growth temperatures are investigated. The shapes of the InN islands are observed to be predominantly mesalike with large flat (000-1) tops, which suggests a possible role of indium as a surfactant. Rough GaN buffer layers composed of dense small GaN islands are found to significantly improve uniform InN wetting of the substrates, on which atomically smooth InN films are obtained that show the characteristics of step-flow growth. Scanning tunneling microscopy imaging reveals the defect-mediated surface morphology of smooth InN films, including surface terminations of screw dislocations and a high density of shallow surface pits with depths less than 0.3 nm. The mechanisms of the three-dimensional island size and shape evolution and formation of defects on smooth surfaces are considered.
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J. Vac. Sci. Technol. A 29, 021010 (2011); http://dx.doi.org/10.1116/1.3551604 (6 pages) Online Publication Date: 31 January 2011
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Piezoelectric aluminum nitride films were deposited onto 3 in. [0001] sapphire substrates by reactive magnetron sputtering to explore the possibility of making highly (002)-textured AlN films to be used in surface acoustic wave (SAW) devices for high temperature applications. The synthesized films, typically 1 μm thick, exhibited a columnar microstructure and a high c-axis texture. The relationship between the microstructures and process conditions was examined by x-ray diffraction (XRD), transmission electron microscopy, and atomic force microscopy analyses. The authors found that highly (002)-textured AlN films with a full width at half maximum of the rocking curve of less than 0.3° can be achieved under high nitrogen concentration and moderate growth temperature, i.e., 250 °C. The phi-scan XRD reveals the high in-plane texture of deposited AlN films. The SAW devices, based on the optimized AlN films on sapphire substrate, were characterized before and after an air annealing process at 800 °C for 90 min. The frequency response, recorded after the annealing process, confirmed that the thin films were still strong in a high temperature environment and that they had retained their piezoelectric properties.
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