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Top 20 Most Read Articles
November 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges J. Vac. Sci. Technol. A 29, 050801 (2011); http://dx.doi.org/10.1116/1.3609974 (26 pages) Online Publication Date: 18 August 2011
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Plasma-assisted atomic layer deposition (ALD) is an energy-enhanced method for the synthesis of ultra-thin films with Å-level resolution in which a plasma is employed during one step of the cyclic deposition process. The use of plasma species as reactants allows for more freedom in processing conditions and for a wider range of material properties compared with the conventional thermally-driven ALD method. Due to the continuous miniaturization in the microelectronics industry and the increasing relevance of ultra-thin films in many other applications, the deposition method has rapidly gained popularity in recent years, as is apparent from the increased number of articles published on the topic and plasma-assisted ALD reactors installed. To address the main differences between plasma-assisted ALD and thermal ALD, some basic aspects related to processing plasmas are presented in this review article. The plasma species and their role in the surface chemistry are addressed and different equipment configurations, including radical-enhanced ALD, direct plasma ALD, and remote plasma ALD, are described. The benefits and challenges provided by the use of a plasma step are presented and it is shown that the use of a plasma leads to a wider choice in material properties, substrate temperature, choice of precursors, and processing conditions, but that the processing can also be compromised by reduced film conformality and plasma damage. Finally, several reported emerging applications of plasma-assisted ALD are reviewed. It is expected that the merits offered by plasma-assisted ALD will further increase the interest of equipment manufacturers for developing industrial-scale deposition configurations such that the method will find its use in several manufacturing applications.
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Photovoltaic manufacturing: Present status, future prospects, and research needs J. Vac. Sci. Technol. A 29, 030801 (2011); http://dx.doi.org/10.1116/1.3569757 (16 pages) Online Publication Date: 29 March 2011
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In May 2010 the United States National Science Foundation sponsored a two-day workshop to review the state-of-the-art and research challenges in photovoltaic (PV) manufacturing. This article summarizes the major conclusions and outcomes from this workshop, which was focused on identifying the science that needs to be done to help accelerate PV manufacturing. A significant portion of the article focuses on assessing the current status of and future opportunities in the major PV manufacturing technologies. These are solar cells based on crystalline silicon (c-Si), thin films of cadmium telluride (CdTe), thin films of copper indium gallium diselenide, and thin films of hydrogenated amorphous and nanocrystalline silicon. Current trends indicate that the cost per watt of c-Si and CdTe solar cells are being reduced to levels beyond the constraints commonly associated with these technologies. With a focus on TW/yr production capacity, the issue of material availability is discussed along with the emerging technologies of dye-sensitized solar cells and organic photovoltaics that are potentially less constrained by elemental abundance. Lastly, recommendations are made for research investment, with an emphasis on those areas that are expected to have cross-cutting impact.
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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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Compositional study of vacuum annealed Al doped ZnO thin films obtained by RF magnetron sputtering J. Vac. Sci. Technol. A 29, 051514 (2011); http://dx.doi.org/10.1116/1.3624787 (9 pages) Online Publication Date: 18 August 2011
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Aluminum doped zinc oxide (AZO) thin films were obtained by RF magnetron sputtering. The effects of deposition parameters such as power, gas flow conditions, and substrate heating have been studied. Deposited and annealed films were characterized for composition as well as microstructure using x ray photoelectron spectroscopy and x ray diffraction. Films produced were polycrystalline in nature. Surface imaging and roughness studies were carried out using SEM and AFM, respectively. Columnar grain growth was predominantly observed. Optical and electrical properties were evaluated for transparent conducting oxide applications. Processing conditions were optimized to obtain highly transparent AZO films with a low resistivity value of 6.67 × 10−4 Ω cm.
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Imaging and phase identification of Cu2ZnSnS4 thin films using confocal Raman spectroscopy J. Vac. Sci. Technol. A 29, 051203 (2011); http://dx.doi.org/10.1116/1.3625249 (11 pages) Online Publication Date: 26 August 2011
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Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) is a potential candidate for next generation thin film solar cells because it contains abundant and nontoxic elements and exhibits high light absorption. Thin films of CZTS are typically synthesized by sulfidizing a stack of zinc, copper, and tin films. In addition to CZTS, a variety of binary and ternary metal sulfides can form and distinguishing among phases with similar crystal structure can be difficult. Herein, the authors show that confocal Raman spectroscopy and imaging can distinguish between CZTS and the other binary and ternary sulfides. Specifically, Raman spectroscopy was used to detect and distinguish between CZTS (338 cm−1), Cu2SnS3 (298 cm−1), and Cu4SnS4 (318 cm−1) phases through their characteristic scattering peaks. Confocal Raman spectroscopy was then used to image the distribution of coexisting phases and is demonstrated to be a useful tool for examining the heterogeneity of CZTS films. The authors show that, during sulfidation of a zinc/copper/tin film stack, ternary sulfides of copper and tin, such as Cu2SnS3 form first and are then converted to CZTS. The reason for formation of Cu2SnS3 as an intermediary to CZTS is the strong tendency of copper and tin to form intermetallic alloys upon evaporation. These alloys sulfidize and form copper tin sulfides first, and then eventually convert to CZTS in the presence of zinc. As a consequence, films sulfidized for 8 h at 400 °C contain both CZTS and Cu2SnS3, whereas films sulfidized at 500 °C contain nearly phase-pure CZTS. In addition, using Cu Kα radiation, the authors identify three CZTS X-ray diffraction peaks at 37.1° [(202)], 38° [(211)], and 44.9° [(105) and (213)], which are absent in ZnS and very weak in Cu2SnS3.
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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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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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Formation and transfer of GaAsN nanostructure layers J. Vac. Sci. Technol. A 29, 060601 (2011); http://dx.doi.org/10.1116/1.3630120 (6 pages) Online Publication Date: 27 September 2011
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The authors report the simultaneous formation and transfer of GaAsN nanostructure layers to alternative substrates, a process termed “ion-cut synthesis.” Ion-cut synthesis is induced by nitrogen ion implantation into GaAs (GaAs:N), followed by spin-on-glass (SOG) mediated wafer bonding and high temperature rapid thermal annealing (RTA). Due to the low ion-matrix diffusivity of GaAs:N, RTA induces the formation of both nanostructures and gas bubbles. The gas bubble pressure induces the formation and propagation of cracks, resulting in transfer of the nanostructured layer. The authors discuss the critical role of the physical properties and the thicknesses of the substrates and the SOG layer to the achievement of ion-cut synthesis.
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Atomic layer deposition for nanostructured Li-ion batteries J. Vac. Sci. Technol. A 30, 010801 (2012); http://dx.doi.org/10.1116/1.3660699 (10 pages) Online Publication Date: 21 November 2011
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Nanostructuring is targeted as a solution to achieve the improvements required for implementing Li-ion batteries in a wide range of applications. These applications range in size from electrical vehicles down to microsystems. Atomic layer deposition (ALD) could be an enabling technology for nanostructured Li-ion batteries as it is capable of depositing ultrathin films (1–100 nm) in complex structures with precise growth control. The potential of ALD is reviewed for three battery concepts that can be distinguished, i.e., particle-based electrodes, 3D-structured electrodes, and 3D all-solid-state microbatteries. It is discussed that a large range of materials can be deposited by ALD and recent demonstrations of battery improvements by ALD are used to exemplify its large potential.
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Spatially resolved study of primary electron transport in magnetic cusps J. Vac. Sci. Technol. A 30, 011301 (2012); http://dx.doi.org/10.1116/1.3656742 (9 pages) Online Publication Date: 3 November 2011
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Spatially resolved primary electron current density profiles were measured using a planar Langmuir probe in the region above a magnetic cusp in a small ion thruster discharge chamber. The probe current maps obtained were used to study the electron collection mechanics in the cusp region in the limit of zero gas flow and no plasma production, and they allowed for the visualization of primary electron transport through the cusp. Attenuation coefficients and loss widths were calculated as a function of probe distance above the anode at various operating conditions. Finally, the collection mechanics between two magnetic cusps were studied and compared. It was found that primary electron collection was dominated by the upstream magnet ring.
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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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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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J. Vac. Sci. Technol. A 29, 061301 (2011); http://dx.doi.org/10.1116/1.3645612 (9 pages) Online Publication Date: 26 October 2011
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The influence of the magnetic field strength (B) on the deposition rate and plasma properties for Ta, Cr, Ti, Al, Cu materials using the modulated pulsed power (MPP) magnetron sputtering technique in a closed field unbalanced magnetron sputtering system was investigated. The MPP deposition rates were compared to those obtained from the films deposited by direct current magnetron sputtering (DCMS) under similar experimental conditions. The time averaged ion energy and mass distributions of positive ions in the MPP plasmas at different magnetic field strengths were compared, using a Hiden electrostatic quadrupole plasma mass spectrometer. The effects of the repetition frequency and pulse length on the MPP deposition rate were investigated. For a given target power, the MPP deposition rate increased when the repetition frequency was increased. It also increased as the pulse length was increased at a constant repetition frequency and target power. The MPP deposition rate is strongly material dependent. The MPP deposition rate increased as B decreased for a given target power. For a B of 550 G, the RMPP/RDCMS ratio for Cu was in a range of 0.81–1.02, for Al it was 0.84–1.01, for Cr it was 0.64–1.01, for Ti it was 0.52–0.89, and for Ta it was 0.47–0.84. For a B of 350 G, the RMPP/RDCMS ratio for Cu was increased to 1.03–1.07, for Al it was 0.94–1.04, for Cr it was 0.8–1.03, for Ti it was 0.79–0.94, and for Ta it was 0.72–0.88. However, a decrease in the ionization of metal and gas species was observed as B was decreased, which affected the microstructure and mechanical properties of the deposited Cr films.
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Microstructural evolution during film growth J. Vac. Sci. Technol. A 21, S117 (2003); http://dx.doi.org/10.1116/1.1601610 (12 pages) Online Publication Date: 2 September 2003
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Atomic-scale control and manipulation of the microstructure of polycrystalline thin films during kinetically limited low-temperature deposition, crucial for a broad range of industrial applications, has been a leading goal of materials science during the past decades. Here, we review the present understanding of film growth processes—nucleation, coalescence, competitive grain growth, and recrystallization—and their role in microstructural evolution as a function of deposition variables including temperature, the presence of reactive species, and the use of low-energy ion irradiation during growth. © 2003 American Vacuum Society. |
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Conformality of remote plasma-enhanced atomic layer deposition processes: An experimental study J. Vac. Sci. Technol. A 30, 01A115 (2012); http://dx.doi.org/10.1116/1.3659699 (5 pages) Online Publication Date: 11 November 2011
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In total, five metal oxide and one metal plasma-enhanced atomic layer deposition (PEALD) processes were studied with respect to the conformality of the coatings. The study reveals that also high aspect ratio structures (up to 60:1) can be coated conformally with remote PEALD. Oxides could relatively easily be deposited into demanding 3D structures with rather short cycle times but not the silver metal. The key factor in achieving excellent conformality seems to be that sufficient radical density is required to overcome the loss of radicals by recombination. In the case of metals where hydrogen plasma is applied the recombination of hydrogen radicals causes major difficulties in obtaining perfect conformality.
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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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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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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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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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J. Vac. Sci. Technol. A 19, 963 (2001); http://dx.doi.org/10.1116/1.1368836 (8 pages)
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ZnO:Al/(ZaO) films were deposited on quartz substrates by dc magnetron reactive sputtering from a Zn target mixed with Al. The effect of oxygen flow rate, target to substrate distance, substrate temperature, and Al doping content on the structural, electrical and optical properties of ZAO were investigated. It was observed that the (002) peak position of all films shifts to lower angle comparable to that of bulk ZnO due to the residual stress change with deposition parameters. X-ray photoemission spectroscopy was introduced to analyze the chemical state of Al on the film surface and the results show Al enrichment. The dependences of electrical properties such as resistivity, carrier concentration and Hall mobility on substrate temperature, and Al doping content were measured. The visible transmittance of above 80% and infrared reflectance of above 80% were obtained. The minimum resistivity is 4.23×10−4 Ω cm with the carrier concentration of 9.21×1020 cm−3 and Hall mobility of 16.0 cm2 v1 s−1. The optical band gap was observed to increase with increasing carrier concentration. The probable mechanisms are discussed. © 2001 American Vacuum Society. |
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