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Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films

Issue 3 • Date May 2011

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Displaying Results 1 - 25 of 44
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  • Effects of vacuum ultraviolet radiation on deposited and ultraviolet-cured low-k porous organosilicate glass

    Page(s): 030602 - 030602-4
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    The authors compare the effects of vacuum ultraviolet (VUV) irradiation on pristine and UV-cured low-k porous organosilicate glass (SiCOH). The authors find that during VUV irradiation, more trapped charges are generated in UV-cured SiCOH as compared to pristine SiCOH. VUV is also used as a tool to investigate effects of UV curing. From comparison of VUV spectroscopy and photoinjection current of the two samples, the authors find that UV curing reduces the number of defect states in SiCOH. The authors also find that UV-cured SiCOH has higher photoconductivity and intrinsic conductivity from VUV spectroscopy and trapped-charge decay rate, respectively. View full abstract»

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  • Photovoltaic manufacturing: Present status, future prospects, and research needs

    Page(s): 030801 - 030801-16
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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. View full abstract»

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  • Defects in m-plane ZnO epitaxial films grown on (112) LaAlO3 substrate

    Page(s): 031001 - 031001-5
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    The crystallographic orientations of m-plane ZnO on (112) LaAlO3 (LAO) substrate are [1210]ZnO||[111]LAO and [0001]ZnO||[110]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<1120> Burgers vectors are distributed on the basal plane, and BSFs have 1/6<2023> 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. View full abstract»

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  • Effect of ion bombarding energies on photocatalytic TiO2 films growing in a pulsed dual magnetron discharge

    Page(s): 031301 - 031301-7
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    Photocatalytic crystalline TiO2 films were deposited by a pulsed dc dual magnetron system. The depositions were performed using two unbalanced magnetrons with planar titanium targets of 50 mm diameter in Ar+O2 gas mixtures at a total pressure of 0.9 Pa with oxygen partial pressures ranging from 0.2 to 0.9 Pa. The maximum substrate surface temperature was 160 °C. Both magnetrons operated in the same asymmetric bipolar mode at the repetition frequencies of 100 and 350 kHz with a fixed 50% duty cycle and the average target power densities of 52–74 Wcm-2 in the negative voltage phase of the pulses, but the magnetron operations were shifted by a half of the period. Time-averaged energy-resolved mass spectroscopy was performed at a substrate position located 100 mm from the targets. The measured structure of the ion energy distributions was correlated with the distinct pulse phases of the magnetron discharges. A decrease in the energy delivered by fast ions (E≥10 eV) to the unit volume of the growing films, together with possible effects of plasma-chemical processes, during the depositions at the oxygen partial pressures of 0.5–0.75 Pa and the repetition frequency of 350 kHz resulted in a strong predominance of the highly photoactive crystalline anatase phase in the TiO2 films. View full abstract»

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  • Analysis of organic pollutant degradation in pulsed plasma by coherent anti-Stokes Raman spectroscopy

    Page(s): 031302 - 031302-7
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    The degradation of p-benzoquinone (p-BQ) in water was investigated by the coherent anti-Stokes Raman spectroscopy (CARS) method, in which the change of the anti-Stokes signal intensity corresponding to the vibrational transitions of the molecule is monitored during and after solution plasma processing (SPP). In the beginning of SPP treatment, the CARS signal intensity of the ring vibrational molecular transitions at 1233 and 1660 cm-1 increases under the influence of the electric field of the plasma, depending on the delay time between the plasma pulse and the laser firing pulse. At the same time, the plasma contributes to the degradation of p-BQ molecules by generating hydrogen and hydroxyl radicals, which decompose p-BQ into different carboxylic acids. After SPP, the CARS signal intensity of the vibrational bands of p-BQ ceased and the degradation of p-BQ was confirmed by UV-visible absorption spectroscopy and liquid chromatography analysis. View full abstract»

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  • Reaction mechanisms of thermal atomic oxygen interaction with organosilicate low k dielectric materials from ab initio molecular dynamics simulations

    Page(s): 031303 - 031303-7
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    The interactions of the oxidizing plasma with the low k dielectric materials and the associated damage mechanisms are of great technological interest for processing current and next generation low k materials. Density functional theory based ab initio molecular dynamics simulations have been performed to evaluate the reaction mechanisms of thermal atomic oxygen [in triplet (3P) or singlet (1D) state] with the organosilicate low k materials represented by model systems. The threshold kinetic energies of attacking atomic oxygen and the reaction pathway were found to be highly incident angle dependent. Carbon abstraction through methyl radical formation can happen at energy barriers as low as 0.1 eV when O radical attack occurs along the axes inclined to the Si–C bond. The simulation results agree well with recent experiments and support diffusion-controlled etching rate dependence, and dielectric constant increases due to oxygen plasma etching. View full abstract»

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  • Modification of transparent conductive ZnO and Ga-doped ZnO films by irradiation with electron cyclotron resonance argon plasma

    Page(s): 031304 - 031304-8
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    By irradiating undoped ZnO and Ga-doped ZnO (GZO) films with electron cyclotron resonance Ar plasma, their electrical and optical properties have been modified. Both the carrier concentration and the Hall mobility increased with continuous irradiation in conductive ZnO films with a defect-rich crystalline lattice. Improved crystallinity was verified by intensifying the ZnO(002) x-ray diffraction peak and increasing the optical transmission level in the visible wavelength. This observation suggests network rearrangement through Zn atoms at interstitials or grain boundaries being oxidized or accommodated into the crystalline network. For insulating ZnO films that contain a sufficient number of oxygen atoms, the changes were better scaled with sheet resistance rather than resistivity. The interference fringes redshifted without lowering the transmittance level. Here, Ar ion impact will create oxygen vacancy donors in the near-surface region or desorb interstitial oxygen atoms that act as donor killers. In contrast, GZO films with resistivities in the 10-4 Ω cm range revealed little changes because there were very few imperfections in the crystalline lattice. The reduced amount of resistivity corresponded to the creation of oxygen vacancies in more resistive GZO films in the 10-3 Ω cm range. View full abstract»

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  • Resonance hairpin and Langmuir probe-assisted laser photodetachment measurements of the negative ion density in a pulsed dc magnetron discharge

    Page(s): 031305 - 031305-7
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    The time-resolved negative oxygen ion density n- close to the center line in a reactive pulsed dc magnetron discharge (10 kHz and 50% duty cycle) has been determined for the first time using a combination of laser photodetachment and resonance hairpin probing. The discharge was operated at a power of 50 W in 70% argon and 30% oxygen gas mixtures at 1.3 Pa pressure. The results show that the O- density remains pretty constant during the driven phase of the discharge at values typically below 5×1014 m-3; however, in the off-time, the O- density grows reaching values several times those in the on-time. This leads to the negative ion fraction (or degree of electronegativity) α=n-/ne being higher in the off phase (maximum value α∼1) than in the on phase (α=0.05–0.3). The authors also see higher values of α at positions close to the magnetic null than in the more magnetized region of the plasma. This fractional increase in negative ion density during the off-phase is attributed to the enhanced dissociative electron attachment of highly excited oxygen molecules in the cooling plasma. The results show that close to the magnetic null the photodetached electron density decays quickly after the laser pulse, followed by a slow decay over a few microseconds governed by the negative ion temperature. However, in the magnetized regions of the plasma, this decay is more gradual. This is attributed to the different cross-field transport rates for electrons in these two regions. The resonance hairpin probe measurements of the photoelectron densities are compared directly to photoelectron currents obtained using a conventional Langmuir probe. There is good agr- - eement in the general trends, particularly in the off-time. View full abstract»

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  • Electrodeposition of cobalt nanowires on H-terminated conductive Si(111) surfaces using coblock polymer templating

    Page(s): 031401 - 031401-5
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    The authors have investigated the formation of block copolymer nanocavities on H-terminated conducting Si(111) surfaces as templates for the electrochemical growth of perpendicular metallic nanowire arrays. Poly(styrene)-block-poly(methyl methacrylate) block copolymers (PS-b-PMMA) of appropriate block length and PS to PMMA ratio were used to create a self-assembled array of perpendicular nanocavities in which the PS majority phase is continuous and surrounds cylinders of the minority PMMA phase. Here, we report that H-terminated conducting Si(111) surfaces are also capable of inducing a perpendicular orientation in block copolymers, which—in all likelihood—is a direct result of the H-termination (i.e., removal of the oxide layer). Atomic force microscopy reveals that an acetic acid wash of the annealed block copolymer causes the minority PMMA component to be rearranged, giving rise to cavities that are perpendicular to the conducting Si substrate. Subsequently, scanning electron microscopy reveals that electrodeposition into the nanocavities can be achieved, producing an array of metallic nanopillars, 20 nm in diameter. View full abstract»

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  • Low work function CsI coatings for enhanced field emission properties

    Page(s): 031402 - 031402-6
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    Thin films of cesium iodide (CsI) were deposited by pulsed laser deposition and by thermal evaporation onto Si substrates and were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Kelvin probe measurements. The thermally evaporated films were found to be stoichiometric whereas the pulsed laser deposited films showed the presence of a Cs/CsI mixture. The latter is supported by UPS measurements whose Fermi edge indicates the presence of a metallic component (elemental Cs). The presence of a Cs/CsI mixture is also supported by the Kelvin probe work function values found to be in the range of 2.6–2.8 eV, a value in excess of the 2.1 eV reported for elemental Cs. This paper addresses the physical mechanisms responsible for the presence of the elemental Cs in the films and its ramification to their field emission properties. View full abstract»

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  • Preparation of clean Bi2Te3 and Sb2Te3 thin films to determine alignment at valence band maxima

    Page(s): 031403 - 031403-5
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    The thermoelectric application of Bi2Te3 and Sb2Te3 thin film structures relies on the relative alignment of the valence band maxima for good electrical conduction. In order to determine the valence band maxima of the bulk films, the authors propose a simple repeatable treatment of a chemical etching in dilute hydrochloric acid solution and a subsequent annealing at ∼150 °C under ultrahigh vacuum to prepare clean surfaces of Bi2Te3 and Sb2Te3 thin films. High-resolution photoemission spectroscopy using synchrotron radiation is used to investigate the chemical states of epitaxial Bi2Te3 and Sb2Te3 thin films grown on GaAs by low-temperature metal-organic chemical vapor deposition. The valence band and core-level photoemission spectra indicate that the surface contaminations and oxides were removed. After chemical etching in acid solution, elemental Te was observed on the surface; a follow-up anneal in ultrahigh vacuum creates a stoichiometric oxide-free surface. View full abstract»

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  • Processing-structure-property relationships in electron beam physical vapor deposited yttria stabilized zirconia coatings

    Page(s): 031501 - 031501-8
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    The physical and mechanical properties of yttria stabilized zirconia (YSZ) coatings deposited by the electron beam physical vapor deposition technique have been investigated by varying the key process variables such as vapor incidence angle and sample rotation speed. The tetragonal zirconia coatings formed under varying process conditions employed were found to have widely different surface and cross-sectional morphologies. The porosity, phase composition, planar orientation, hardness, adhesion, and surface residual stresses in the coated specimens were comprehensively evaluated to develop a correlation with the process variables. Under transverse scratch test conditions, the YSZ coatings exhibited two different crack formation modes, depending on the magnitude of residual stress. The influence of processing conditions on the coating deposition rate, column orientation angle, and adhesion strength has been established. Key relationships between porosity, hardness, and adhesion are also presented. View full abstract»

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  • Y thin films grown by pulsed laser ablation

    Page(s): 031502 - 031502-5
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    The effects of laser fluence on the growth characteristics and surface morphology of yttrium films grown by pulsed laser deposition are investigated. The presence of droplets in the deposited films, which is the main drawback of pulsed laser deposition technique, was studied at different laser fluences. The morphology and the structure of the grown films were studied by scanning-electron microscopy and x-ray diffraction, respectively. Careful scanning-electron microscope investigations obtained by tilting the samples show that the droplets arrive to the substrate in the molten phase. The ablation rate measured at five different laser fluences (0.9–7.6 J/cm2) shows a nonlinear trend correlated with the presence of the plasma-shielding effect. The present interest in the deposition of yttrium thin films by laser ablation is due to the well-known photoemission characteristics of this metal. Depositing good-quality thin films with high adhension and low droplet density will improve the performance of photocathodes by reducing both the dark current and the thermal emittance of the photoemitted current. View full abstract»

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  • Improving the thermal stability of nickel monosilicide thin films by combining annealing with the use of an interlayer and a capping layer

    Page(s): 031503 - 031503-5
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    The authors investigated the effects of preannealing a 2-nm-thick Pd interlayer and a 20-nm-thick TiN capping layer on the electrical and thermal stability of nickel silicides as a function of the annealing temperature. The preannealed samples (prepoly-Si) produce lower sheet resistances compared to the samples without preannealing. For the preannealed samples, NiSi remains stable up to 600 °C. Transmission electron microscopy results show that the preannealed samples have a higher resistance against layer inversion. The addition of a Pd interlayer at the Ni film/prepoly-Si interface increases the formation temperature of NiSi2 to 900 °C. The use of the capping layer on the Pd-interlayered prepoly-Si samples improves the electrical and morphological stabilities of NiSi. The possible mechanisms for the preannealing and interlayer-induced improvement of the thermal stabilities of the Ni-silicide samples are discussed in terms of grain growth and simple thermodynamic relations. View full abstract»

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  • Direct comparison of the x-ray emission and absorption of cerium oxide

    Page(s): 031504 - 031504-3
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    X-ray emission spectroscopy and x-ray absorption spectroscopy have been used to investigate the photon emission and absorption associated with the Ce 3d5/2 and Ce 3d3/2 core levels in Ce oxide. A comparison of the two processes and their spectra will be made. View full abstract»

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  • Transparent conductive Al-doped ZnO thin films grown at room temperature

    Page(s): 031505 - 031505-6
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    Aluminum-doped ZnO (ZnO:Al, AZO) thin films were prepared on glass substrates by dc reactive magnetron sputtering from a Zn–Al alloy target at room temperature. The effects of the Ar-to-O2 partial pressure ratios on the structural, electrical, and optical properties of AZO films were studied in detail. AZO films grown using 100:4 to 100:8 Ar-to-O2 ratio result in acceptable quality films with c-axis orientated crystals, uniform grains, 10-3 Ω cm resistivity, greater than 1020 cm-3 electron concentration, and high transmittance, 90%, in the visible region. The lowest resistivity of 4.11×10-3 Ω cm was obtained under the Ar-to-O2 partial pressure ratio of 100:4. A relatively strong UV emission at ∼3.26 eV was observed in the room-temperature photoluminescence spectrum. X-ray photoelectron spectroscopy analysis confirmed that Al was introduced into ZnO and substitutes for Zn and doped the film n-type. View full abstract»

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  • Structure and properties of nanocrystalline ZrNxOy thin films: Effect of the oxygen content and film thickness

    Page(s): 031506 - 031506-7
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    The main objective of this study was to investigate the structure and properties of ZrNxOy thin films associated with oxygen content and film thickness. ZrNxOy thin films were deposited using hollow cathode discharge ion plating on Si (100) substrate. The thickness of ZrNxOy films increased with increasing oxygen flow rate, ranging from 143 to 894 nm. Phase separation from ZrNxOy to ZrN and monoclinic ZrO2 (m-ZrO2) was observed by x-ray diffraction (XRD). The electrical and mechanical properties were influenced by the film thickness and the amount of separated phase, m-ZrO2. ZrNxOy thin films with smaller thickness or deposited at higher O2 flow rate were found to have higher electrical resistivity. Hardness of the ZrNxOy thin films increased with increasing thickness, which could be related to microstructure change of the thin films. Residual stress of the ZrN phase in the ZrNxOy thin films, measured using the modified sin2 ψ XRD method, decreased with increasing oxygen flow rate. The thickness dependence of the residual stress in ZrN was different with different oxygen flow rates. The average residual stress of the ZrNxOy thin films also decreased with increasing oxygen - low rate and the stress did not showed significant dependence on the film thickness. View full abstract»

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  • Influence of organozinc ligand design on growth and material properties of ZnS and ZnO deposited by atomic layer deposition

    Page(s): 031507 - 031507-6
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    Deposition of ZnS and ZnO by the atomic layer deposition technique is performed using both dimethylzinc (DMZn) and diethylzinc (DEZn) as the metal source and H2S or H2O as the counter-reactant. The deposited films are characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy, and ultraviolet-visible measurements, and particular emphasis is placed on the influence of the metal precursor on material growth and properties. The use of DMZn as the Zn source results in faster material deposition than growth with DEZn due to a less significant steric factor with DMZn. The material properties of the deposited ZnS films are nearly identical for the DMZn/H2S and DEZn/H2S processes, whereas XRD provided evidence for slight variations in the material properties of the DMZn/H2O and DEZn/H2O grown films. Overall, pure and crystalline ZnS and ZnO films can be deposited via either DMZn or DEZn, and ZnO growth is more affected by the modification of the ligand of the Zn precursor from methyl to ethyl. View full abstract»

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  • Effect of oxygen deficiency on the photoresponse and reactivity of mixed phase titania thin films

    Page(s): 031508 - 031508-7
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    Nonstoichiometric mixed phased titania nanocomposites (TiO2-x) were deposited by reactive direct current magnetron sputtering. The authors explored the role of nonstoichiometry (as defined by oxygen deficiency in synthesis) in mixed phase titania thin films and its effects on the photoresponse and photocatalytic performance for CO2 reduction to methane under UV and visible light. Oxygen partial pressure was varied during film deposition, yielding different levels of oxygen deficiency in the films. Optimized nonstoichiometric films showed a strong redshift. The authors have identified an optimum set of synthesis conditions for TiO2-x films that produce a relative maximum in photocatalytically produced methane under both UV and visible light. View full abstract»

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  • Microstructure evolution of Ti3SiC2 compound cathodes during reactive cathodic arc evaporation

    Page(s): 031601 - 031601-7
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    The microstructure evolution and compositional variation of Ti3SiC2 cathode surfaces during reactive cathodic arc evaporation are presented for different process conditions. The results show that phase decomposition takes place in the near-surface region, resulting in a 5–50 μm thick converted layer that is affected by the presence of nitrogen in the deposition chamber. This layer consists of two different sublayers, i.e., 1–20 μm thick top layer with a melted and resolidified microstructure, followed by a 4–30 μm thick transition layer with a decomposed microstructure. The converted layer contains a polycrystalline TiCx phase and trace quantities of Si-rich domains with Ti5Si3(C) at their interface. The arc discharge causes Si redistribution in the two regions of the layer, whose Si/(Ti+Si) ratio is higher in the top region and lower in the transition region compared to the virgin material. View full abstract»

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  • Ultrahigh vacuum sample mount for x-ray photoelectron spectroscopy up to very high temperature (150–1400 K)

    Page(s): 031602 - 031602-7
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    Spectroscopic studies are rarely performed at very high temperature, especially when combined with light from a synchrotron source. Demanding conditions of maintaining ultrahigh vacuum (UHV) during heating, together with the typically brief access to beam time at multiuser synchrotron end stations, may contribute to some of the reasons for the difficulty of such experiments. Consequently, a large number of materials with interesting properties and industrial applications at high temperature remain unexplored. The authors describe here a simple portable sample mount assembly that can be easily utilized at a beamline, with potential utility for a variety of spectroscopic measurements requiring elevated temperatures and an UHV environment. In the specific application described here, the authors use a resistive cartridge heater interfaced with a standard manipulator previously designed for cooling by liquid nitrogen with an UHV chamber and a cylindrical mirror analyzer for x-ray photoemission spectroscopy (XPS) [also known as electron spectroscopy for chemical analysis (ESCA)] at the Synchrotron Radiation Center in Stoughton, WI. The heater cartridge required only modest power to reach target temperatures using an open-loop temperature control. Finally, the authors describe the measurements of XPS (ESCA) and total-electron yield x-ray absorption spectroscopy on nanopowders and on single crystals grown by them. They emphasize the simplicity of the setup, which they believe would be of interest to groups performing measurements at large facilities, where access and time are both limited. View full abstract»

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  • Dark current mechanisms in stabilized amorphous selenium based n-i detectors for x-ray imaging applications

    Page(s): 031603 - 031603-6
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    The dark current behavior under operating bias is one of the important selection criteria for an x-ray photoconductor to be usable in a practical x-ray image detector. The authors have developed an analytical model for describing the transient and steady-state behavior of dark current in n-i-type amorphous selenium (a-Se) detectors by considering carrier injections from the metal contacts and thermally generated carriers. It has been found that the thermal generation current is almost two orders of magnitude smaller than the total steady-state dark current in n-i-type a-Se detectors. The main source of dark current is the injection of holes from the metal/n-layer interface which is described by the diffusion theory. The hole injection from the metal depends on the blocking layer (n-layer) thickness, the concentration of trap centers in the blocking layer, the characteristic carrier release time, and the effective barrier height. The fitting of the first principles model with the experimental results estimates the concentration of deep hole trap center in the n-layer, the trap depth from the valence band edge, and the effective barrier heights for the injecting carriers. The electron injection varies with the work function of the contact metal. View full abstract»

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  • Versatile approach to Rb vapor cell construction

    Page(s): 033001 - 033001-4
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    A versatile approach to Rb atomic vapor cell construction is proposed and tested. The construction method employs pinch-off copper cold-welds and epoxy to create hermetic seals between dissimilar geometries and materials. Accelerated testing revealed expected lifetimes of 3 days at 90 °C operation and in excess of 1 yr at 25 °C operation. The reaction of Rb with epoxy was determined to be the largest contributor to failure. View full abstract»

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  • Structural characteristics of ZnO films grown on (0001) or (11-20) sapphire substrates by atomic layer deposition

    Page(s): 03A101 - 03A101-5
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    The structural characteristics of zinc oxide (ZnO) films deposited on the (0001)- or (11-20)-oriented sapphire substrates were investigated. ZnO films having low temperature ZnO buffer layers were prepared by atomic layer deposition using diethylzinc and nitrous oxide. The ZnO films were analyzed using cross-sectional transmission electron microscopy, high-resolution transmission electron microscopy and x-ray diffractometry. Cross-sectional high-resolution transmission electron microscopic observations showed the presence of extrinsic stacking faults along with Frankel partial dislocations in ZnO near the ZnO/sapphire heterointerface. Based on the results of cross-sectional transmission electron microscopic observations and double-crystal x-ray rocking curve measurements, it was found that buffer-layer annealing treatment tended to reduce the density of threading dislocations in a ZnO film efficiently so that the crystalline quality of the film was greatly improved. View full abstract»

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The Journal of Vacuum Science and Technology A is devoted to reports of original research, review articles, and Critical Review articles.

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G. Lucovsky
North Carolina State University