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May 2011

Volume 29, Issue 3, Articles (03xxxx)

Issue Cover Spotlight Figure

J. Vac. Sci. Technol. A 29, 030801 (2011); http://dx.doi.org/10.1116/1.3569757 (16 pages)

Colin A. Wolden, Juanita Kurtin, Jason B. Baxter, Ingrid Repins, Sean E. Shaheen, John T. Torvik, Angus A. Rockett, Vasilis M. Fthenakis, and Eray S. Aydil
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Improving the adhesion of Au thin films onto poly(methyl methacrylate) substrates using spun-cast organic solvents

Alan K. Mo, Thomas C. DeVore, Brian H. Augustine, Vezekile P. Zungu, Laura L. Lee, and Wm. Christopher Hughes

J. Vac. Sci. Technol. A 29, 030601 (2011); http://dx.doi.org/10.1116/1.3562167 (5 pages)

Online Publication Date: 11 March 2011

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68.35.Np Adhesion
68.55.A- Nucleation and growth
78.66.Bz Metals and metallic alloys
81.15.Cd Deposition by sputtering
78.30.Er Solid metals and alloys

Effects of vacuum ultraviolet radiation on deposited and ultraviolet-cured low-k porous organosilicate glass

H. Sinha, G. A. Antonelli, G. Jiang, Y. Nishi, and J. L. Shohet

J. Vac. Sci. Technol. A 29, 030602 (2011); http://dx.doi.org/10.1116/1.3570818 (4 pages) | Cited 1 time

Online Publication Date: 28 March 2011

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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.
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77.55.Bh Low-permittivity dielectric films
73.61.Ph Polymers; organic compounds
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.50.Pz Photoconduction and photovoltaic effects
78.66.Qn Polymers; organic compounds
71.55.Ht Other nonmetals
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Photovoltaic manufacturing: Present status, future prospects, and research needs

Colin A. Wolden, Juanita Kurtin, Jason B. Baxter, Ingrid Repins, Sean E. Shaheen, John T. Torvik, Angus A. Rockett, Vasilis M. Fthenakis, and Eray S. Aydil

J. Vac. Sci. Technol. A 29, 030801 (2011); http://dx.doi.org/10.1116/1.3569757 (16 pages) | Cited 12 times

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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88.40.jj Silicon solar cells
88.40.jm Thin film III-V and II-VI based solar cells
88.40.jn Thin film Cu-based I-III-VI2 solar cells
88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Defects in m-plane ZnO epitaxial films grown on (112) LaAlO3 substrate

Wei-Lin Wang, Chun-Yen Peng, Yen-Teng Ho, Shu-Chang Chuang, and Li Chang

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 [math2math0]ZnO∥[11math]LAO and [0001]ZnO∥[math10]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〈11math0〉 Burgers vectors are distributed on the basal plane, and BSFs have 1/6〈20math3〉 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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71.55.Gs II-VI semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.Nn Stacking faults and other planar or extended defects
68.55.ag Semiconductors
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Effect of ion bombarding energies on photocatalytic TiO2 films growing in a pulsed dual magnetron discharge

Ondřej Novák and Jaroslav Vlček

J. Vac. Sci. Technol. A 29, 031301 (2011); http://dx.doi.org/10.1116/1.3563612 (7 pages)

Online Publication Date: 10 March 2011

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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 W cm−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.
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68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
61.80.Jh Ion radiation effects
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.50.-m Photochemistry
81.15.Cd Deposition by sputtering

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

Maria Antoneta Bratescu, Junko Hieda, Tomonari Umemura, Nagahiro Saito, and Osamu Takai

J. Vac. Sci. Technol. A 29, 031302 (2011); http://dx.doi.org/10.1116/1.3569035 (7 pages)

Online Publication Date: 22 March 2011

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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.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
78.40.Me Organic compounds and polymers
78.30.Jw Organic compounds, polymers
52.77.-j Plasma applications
82.80.Bg Chromatography

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

Mrunalkumar Chaudhari and Jincheng Du

J. Vac. Sci. Technol. A 29, 031303 (2011); http://dx.doi.org/10.1116/1.3568963 (7 pages)

Online Publication Date: 24 March 2011

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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.
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82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution
82.20.Fd Collision theories; trajectory models

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

Housei Akazawa

J. Vac. Sci. Technol. A 29, 031304 (2011); http://dx.doi.org/10.1116/1.3571603 (8 pages)

Online Publication Date: 29 March 2011

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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.
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73.61.Ga II-VI semiconductors
78.66.Hf II-VI semiconductors
78.40.Ha Other nonmetallic inorganics
61.72.jd Vacancies
61.72.Mm Grain and twin boundaries
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

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

James W. Bradley, Robert Dodd, S.-D. You, Nishant Sirse, and Shantanu Kumar Karkari

J. Vac. Sci. Technol. A 29, 031305 (2011); http://dx.doi.org/10.1116/1.3580934 (7 pages)

Online Publication Date: 15 April 2011

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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 agreement in the general trends, particularly in the off-time.
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52.70.Ds Electric and magnetic measurements
52.25.Fi Transport properties
52.25.Xz Magnetized plasmas
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.80.Pi High-frequency and RF discharges
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Electrodeposition of cobalt nanowires on H-terminated conductive Si(111) surfaces using coblock polymer templating

Michael L. Curry, Kristy Crews, Vishal Warke, Martin Gerard Bakker, Kunlun Hong, Jimmy Mays, Phillip Britt, Xuefa Li, and Jin Wang

J. Vac. Sci. Technol. A 29, 031401 (2011); http://dx.doi.org/10.1116/1.3563604 (5 pages)

Online Publication Date: 16 March 2011

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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.
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81.07.Gf Nanowires
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
81.15.Pq Electrodeposition, electroplating
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

Low work function CsI coatings for enhanced field emission properties

S. B. Fairchild, T. C. Back, P. T. Murray, M. M. Cahay, and D. A. Shiffler

J. Vac. Sci. Technol. A 29, 031402 (2011); http://dx.doi.org/10.1116/1.3581058 (6 pages)

Online Publication Date: 13 April 2011

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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.
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81.15.Fg Pulsed laser ablation deposition
81.15.Dj E-beam and hot filament evaporation deposition
79.70.+q Field emission, ionization, evaporation, and desorption
79.60.Dp Adsorbed layers and thin films
73.61.Ng Insulators
73.30.+y Surface double layers, Schottky barriers, and work functions

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

Fang Fang, Robert L. Opila, Rama Venkatasubramanian, and Thomas Colpitts

J. Vac. Sci. Technol. A 29, 031403 (2011); http://dx.doi.org/10.1116/1.3581053 (5 pages)

Online Publication Date: 15 April 2011

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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.
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68.55.at Other materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
71.20.Gj Other metals and alloys
79.60.Bm Clean metal, semiconductor, and insulator surfaces
73.20.-r Electron states at surfaces and interfaces
81.65.Cf Surface cleaning, etching, patterning
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Processing-structure-property relationships in electron beam physical vapor deposited yttria stabilized zirconia coatings

D. Srinivasa Rao, Krishna Valleti, S. V. Joshi, and G. Ranga Janardhan

J. Vac. Sci. Technol. A 29, 031501 (2011); http://dx.doi.org/10.1116/1.3563600 (8 pages)

Online Publication Date: 10 March 2011

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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.
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81.15.Jj Ion and electron beam-assisted deposition; ion plating
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
68.60.Bs Mechanical and acoustical properties
68.35.bt Other materials
62.20.Qp Friction, tribology, and hardness
62.20.mt Cracks

Y thin films grown by pulsed laser ablation

A. Lorusso, V. Fasano, A. Perrone, and K. Lovchinov

J. Vac. Sci. Technol. A 29, 031502 (2011); http://dx.doi.org/10.1116/1.3563598 (5 pages) | Cited 1 time

Online Publication Date: 14 March 2011

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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.
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68.55.aj Insulators
81.15.Fg Pulsed laser ablation deposition
68.35.Np Adhesion
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

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

Bong-Jun Park, Sang-Yong Jeong, Jun-Ho Kim, Tae-Yeon Seong, and Chel-Jong Choi

J. Vac. Sci. Technol. A 29, 031503 (2011); http://dx.doi.org/10.1116/1.3565468 (5 pages)

Online Publication Date: 14 March 2011

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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.
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73.61.Ng Insulators
68.60.Dv Thermal stability; thermal effects
68.55.aj Insulators
81.40.Gh Other heat and thermomechanical treatments

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

J. G. Tobin, S. W. Yu, B. W. Chung, G. D. Waddill, and J. D. Denlinger

J. Vac. Sci. Technol. A 29, 031504 (2011); http://dx.doi.org/10.1116/1.3565490 (3 pages)

Online Publication Date: 14 March 2011

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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.
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78.70.Dm X-ray absorption spectra
78.70.En X-ray emission spectra and fluorescence
79.60.Bm Clean metal, semiconductor, and insulator surfaces

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

Yuping Wang, Jianguo Lu, Xun Bie, Li Gong, Xiang Li, Da Song, Xuyang Zhao, Wenyi Ye, and Zhizhen Ye

J. Vac. Sci. Technol. A 29, 031505 (2011); http://dx.doi.org/10.1116/1.3565462 (6 pages) | Cited 5 times

Online Publication Date: 16 March 2011

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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.
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78.66.Hf II-VI semiconductors
81.15.Cd Deposition by sputtering
79.60.Bm Clean metal, semiconductor, and insulator surfaces
78.55.Et II-VI semiconductors
68.55.-a Thin film structure and morphology
73.61.Ga II-VI semiconductors

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

Kuan-Che Lan, Jia-Hong Huang, Chi-Fong Ai, and Ge-Ping Yu

J. Vac. Sci. Technol. A 29, 031506 (2011); http://dx.doi.org/10.1116/1.3565487 (7 pages)

Online Publication Date: 17 March 2011

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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 flow rate and the stress did not showed significant dependence on the film thickness.
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68.60.Bs Mechanical and acoustical properties
81.15.Jj Ion and electron beam-assisted deposition; ion plating
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.Qp Friction, tribology, and hardness
73.61.Ng Insulators
68.55.jd Thickness

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

Jukka T. Tanskanen, Jonathan R. Bakke, Tapani A. Pakkanen, and Stacey F. Bent

J. Vac. Sci. Technol. A 29, 031507 (2011); http://dx.doi.org/10.1116/1.3572232 (6 pages) | Cited 4 times

Online Publication Date: 5 April 2011

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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.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
78.40.Fy Semiconductors
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.05.Dz II-VI semiconductors
78.66.Hf II-VI semiconductors
68.55.ag Semiconductors

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

Paul A. DeSario, Le Chen, Michael E. Graham, and Kimberly A. Gray

J. Vac. Sci. Technol. A 29, 031508 (2011); http://dx.doi.org/10.1116/1.3574350 (7 pages) | Cited 1 time

Online Publication Date: 8 April 2011

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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.
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68.55.ag Semiconductors
81.05.Hd Other semiconductors
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
81.15.Cd Deposition by sputtering
82.50.Hp Processes caused by visible and UV light
82.30.-b Specific chemical reactions; reaction mechanisms
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Microstructure evolution of Ti3SiC2 compound cathodes during reactive cathodic arc evaporation

J. Q. Zhu, A. O. Eriksson, N. Ghafoor, M. P. Johansson, G. Greczynski, L. Hultman, J. Rosén, and M. Odén

J. Vac. Sci. Technol. A 29, 031601 (2011); http://dx.doi.org/10.1116/1.3569052 (7 pages)

Online Publication Date: 23 March 2011

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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.
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68.35.bt Other materials
81.30.Fb Solidification
64.70.dj Melting of specific substances

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

Mark S. Williamsen, Shishir K. Ray, Ying Zou, John A. Dudek, Somaditya Sen, Mark Bissen, Laura Kretsch, Vaijayanti R. Palkar, Marshall F. Onellion, and Prasenjit Guptasarma

J. Vac. Sci. Technol. A 29, 031602 (2011); http://dx.doi.org/10.1116/1.3571528 (7 pages)

Online Publication Date: 29 March 2011

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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.
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82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
79.60.-i Photoemission and photoelectron spectra
41.60.Ap Synchrotron radiation
78.70.Dm X-ray absorption spectra
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

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

S. A. Mahmood and M. Z. Kabir

J. Vac. Sci. Technol. A 29, 031603 (2011); http://dx.doi.org/10.1116/1.3580902 (6 pages)

Online Publication Date: 14 April 2011

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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.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
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Versatile approach to Rb vapor cell construction

John F. Hulbert, Katie B. Hurd, Brandon T. Carroll, Aaron R. Hawkins, Bin Wu, and Holger Schmidt

J. Vac. Sci. Technol. A 29, 033001 (2011); http://dx.doi.org/10.1116/1.3568954 (4 pages)

Online Publication Date: 22 March 2011

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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.
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06.30.Ft Time and frequency
03.75.Be Atom and neutron optics
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Structural characteristics of ZnO films grown on (0001) or (11−20) sapphire substrates by atomic layer deposition

Kuang-Pi Liu, Kuo-Yi Yen, Ping-Yuan Lin, Jyh-Rong Gong, Kun-Da Wu, and Wei-Li Chen

J. Vac. Sci. Technol. A 29, 03A101 (2011); http://dx.doi.org/10.1116/1.3523289 (5 pages) | Cited 1 time

Online Publication Date: 11 January 2011

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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.
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68.55.-a Thin film structure and morphology
61.72.Nn Stacking faults and other planar or extended defects
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors

Ga-doped ZnO grown by pulsed laser deposition in H2: The roles of Ga and H

David C. Look, Timothy C. Droubay, John S. McCloy, Zihua Zhu, and Scott A. Chambers

J. Vac. Sci. Technol. A 29, 03A102 (2011); http://dx.doi.org/10.1116/1.3523296 (4 pages) | Cited 1 time

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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81.05.Dz II-VI semiconductors
81.15.Fg Pulsed laser ablation deposition
61.72.Cc Kinetics of defect formation and annealing
68.55.ag Semiconductors
72.20.Ee Mobility edges; hopping transport
73.61.Ga II-VI semiconductors

Comprehensive study of the p-type conductivity formation in radio frequency magnetron sputtered arsenic-doped ZnO film

J. C. Fan, C. Y. Zhu, B. Yang, S. Fung, C. D. Beling, G. Brauer, W. Anwand, D. Grambole, W. Skorupa, K. S. Wong, Y. C. Zhong, Z. Xie, and C. C. Ling

J. Vac. Sci. Technol. A 29, 03A103 (2011); http://dx.doi.org/10.1116/1.3525639 (4 pages) | Cited 1 time

Online Publication Date: 11 January 2011

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Arsenic doped ZnO and ZnMgO films were deposited on SiO2 using radio frequency magnetron sputtering and ZnO–Zn3As2 and ZnO–Zn3As2–MgO targets, respectively. It was found that thermal activation is required to activate the formation of p-type conductivity. Hall measurements showed that p-type films with a hole concentration of ∼ 1017 cm−3 and mobility of ∼ 8 cm2 V−1 s−1 were obtained at substrate temperatures of 400–500 °C. The shallow acceptor formation mechanism was investigated using x-ray photoelectron spectroscopy, positron annihilation, low temperature photoluminescence, and nuclear reaction analysis. The authors suggest that the thermal annealing activates the formation of the AsZn-2VZn shallow acceptor complex and removes the compensating hydrogen center.
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73.61.Ga II-VI semiconductors
78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors
81.15.Cd Deposition by sputtering
72.20.My Galvanomagnetic and other magnetotransport effects
78.66.Hf II-VI semiconductors

MgZnO/ZnO quantum well nanowire heterostructures with large confinement energies

M. Lange, C. P. Dietrich, J. Zúñiga-Pérez, H. von Wenckstern, M. Lorenz, and M. Grundmann

J. Vac. Sci. Technol. A 29, 03A104 (2011); http://dx.doi.org/10.1116/1.3531709 (5 pages) | Cited 1 time

Online Publication Date: 11 January 2011

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Mg0.25Zn0.75O/ZnO-quantum well nanowire heterostructures were grown with a three-step pulsed laser deposition process. To avoid shadowing effects during the coating, the ZnO nanowires were grown with a low area density on a ZnO buffer layer deposited on an a-plane sapphire substrate. By using spatially resolved cathodoluminescence measurements, the luminescence of axial and radial quantum wells were clearly distinguished. The large bandgap energy of the Mg0.25Zn0.75O barrier material ( ≈ 3.85 eV) made it possible to tune the energy of quantum wells from 3.4 to 3.7 eV. The homogeneity of the radial quantum well along the wire axis was probed, revealing that only small fluctuations of about 4 meV are found in the main part of the nanowire. Near the tip of the nanowire, the energy of the radial quantum well increases due to locally modified growth conditions reducing the growth rate by up to 15%. Furthermore, the growth rates of the axial and radial quantum wells were determined, showing that the one in axial direction is a factor of about 2 larger than the one in radial direction.
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81.16.Mk Laser-assisted deposition
81.15.Fg Pulsed laser ablation deposition
73.21.Hb Quantum wires
78.67.Uh Nanowires
73.21.Fg Quantum wells
78.60.Hk Cathodoluminescence, ionoluminescence

On the physical properties of In2O3 films grown on (0001) sapphire substrates by atomic layer deposition

Wei-Hsu Chi, Kuo-Yi Yen, Hsin-Lun Su, Shao-Cian Li, and Jyh-Rong Gong

J. Vac. Sci. Technol. A 29, 03A105 (2011); http://dx.doi.org/10.1116/1.3549146 (4 pages)

Online Publication Date: 26 January 2011

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The properties of In2O3 films grown on (0001) plane sapphire substrates by atomic layer deposition using trimethylindium and nitrous oxide were investigated. Using x-ray diffraction and scanning and transmission electron microscopies, In2O3 films were found to deposit on sapphire substrates with their (222) planes parallel to the (0001) planes of sapphire. It was found that there were twin structures inside the In2O3 film with twin boundaries along the {11math} planes. Most In2O3 films deposited on thermally annealed low-temperature-In2O3 buffer-layer-coated substrates exhibited high optical transmittance, low electron concentration, and high electron mobility. The best In2O3 film achieved shows an average transmittance of ∼ 90% in the visible regime with electron concentration and mobility being ∼ 2×1016 cm−3 and ∼ 60 cm2/V s, respectively, at room temperature.
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68.55.ag Semiconductors
78.66.Li Other semiconductors
73.50.Dn Low-field transport and mobility; piezoresistance
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Mm Grain and twin boundaries
73.61.Le Other inorganic semiconductors

Influence of thermally diffused aluminum atoms from sapphire substrate on the properties of ZnO epilayers grown by metal-organic chemical vapor deposition

Kun Tang, Shulin Gu, Shuzhen Li, Jiandong Ye, Shunming Zhu, Hui Chen, Jiagao Liu, Rong Zhang, Yi Shi, and Youdou Zheng

J. Vac. Sci. Technol. A 29, 03A106 (2011); http://dx.doi.org/10.1116/1.3549136 (7 pages) | Cited 2 times

Online Publication Date: 2 February 2011

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In this study, the authors investigate the evolution of the structural and electrical properties of ZnO epilayers grown by the metal-organic chemical vapor deposition method on c-sapphire substrates. The inserting of a low-temperature ZnO buffer layer not only significantly improves the structural quality of the high-temperature (HT)-grown ZnO epilayer on a sapphire substrate but also results in high background electron concentration in it from the Hall-effect measurement. After subtracting the conductive contribution from a thin degenerated layer mostly formed between the buffer layer and the substrate based on the two-layer model, the deduced electron-carrier concentration is still in the order of 1018 cm−3, which is much larger than the 1016 cm−3 obtained from capacitance-voltage measurement near the top surface. This indicates that a much thicker layer with high carrier concentration should be formed in the HT-grown ZnO epilayer, which is significantly different from that observed in GaN epitaxy, where only a thin degenerated interfacial layer is suggested to form in the GaN buffer layer. Al atoms’ distribution acquired from secondary-ion mass spectrometry shows a strong dependence on the temperature of the ZnO growth process, indicating that a thermally enhanced diffusion mechanism should be responsible for the observation of the enhanced Al atom concentration in the HT-grown ZnO epilayer. As substituted Al atoms on the Zn site act as donors in ZnO, the one-to-one correspondence between Al content and the carrier concentration, as well as the analysis of temperature-dependent Hall-effect measurement, indicates that diffusion-induced gradient-distributed AlZn shallow donors should be the main origin of the high background-carrier concentration in the HT-grown ZnO epilayers.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.ag Semiconductors
73.61.Ga II-VI semiconductors
66.30.Xj Thermal diffusivity
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

Vapor phase growth of ZnO single crystals

Xi Zhang, Frank Herklotz, Ellen Hieckmann, Jörg Weber, and Peer Schmidt

J. Vac. Sci. Technol. A 29, 03A107 (2011); http://dx.doi.org/10.1116/1.3553461 (4 pages)

Online Publication Date: 4 February 2011

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Zinc oxide is a promising wide band gap semiconductor for future optoelectronic devices. Today ZnO bulk single crystals are grown by three different techniques: hydrothermally, from the melt, and by chemical vapor transport. For our studies, the authors employed in addition a simple and low cost vapor phase method which gives us good quality crystals and flexibility in crystal doping. The as-grown needle-shaped single crystals were characterized by resistivity measurements, scanning electron microscopy, electron backscatter diffraction, and low temperature photoluminescence spectroscopy.
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81.05.Dz II-VI semiconductors
78.55.Et II-VI semiconductors
72.80.Ey III-V and II-VI semiconductors
79.20.Kz Other electron-impact emission phenomena
81.10.Bk Growth from vapor

Electrical and microstructural properties of N+ ion-implanted ZnO and ZnO:Ag thin films

Michelle A. Myers, Michael T. Myers, Chen-Fong Tsai, Joon Hwan Lee, Tianlin Lu, Lin Shao, and Haiyan Wang

J. Vac. Sci. Technol. A 29, 03A108 (2011); http://dx.doi.org/10.1116/1.3554836 (7 pages)

Online Publication Date: 14 February 2011

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ZnO and Ag-doped ZnO films were grown on sapphire (0001) substrates by pulsed-laser deposition in vacuum both with and without oxygen at 700 °C. N+ ions were implanted in these films at room temperature and at 300 °C to a dose of 1×1014 cm−2 at 50 keV. Hall measurements indicate that ZnO films deposited in vacuum without oxygen and implanted with N+ at elevated temperatures are p-type with a hole-carrier concentration of 6×1016 cm−3, a mobility of 2.1 cm2 V−1 s−1, and a resistivity of 50 Ω cm. Both scanning-electron microscopy and transmission-electron microscopy studies on the implanted films reveal microstructural differences in grain size, surface roughness, and the nature of defects, which may impact the activation of N atoms as p-type carriers. Low-energy ion implantation at elevated temperatures is shown to be an effective method to introduce p-type N dopants into ZnO, which minimizes defect clustering and promotes defect annihilation during implantation.
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81.05.Dz II-VI semiconductors
81.15.Fg Pulsed laser ablation deposition
73.61.Ga II-VI semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
68.35.bg Semiconductors
61.72.U- Doping and impurity implantation

Properties of phosphorus-doped zinc oxide films grown by pulsed laser deposition

Yuanjie Li, Zilong Liu, and Jiangbo Ren

J. Vac. Sci. Technol. A 29, 03A109 (2011); http://dx.doi.org/10.1116/1.3554838 (4 pages)

Online Publication Date: 14 February 2011

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Electrical and chemical bonding properties of P-doped ZnO thin films grown by pulsed laser deposition on sapphire substrates were systematically characterized utilizing the Hall effect and x-ray photoelectron spectroscopy (XPS) measurements. Oxygen growth pressure and postannealing processing play a great role in the properties of these films. Increasing oxygen growth pressure from 5 to 20 Pa enhanced the resistivity of P-doped ZnO films by three orders of magnitude. P-doped ZnO films grown at 700 °C under 20 Pa O2 exhibited p-type conductivity with hole concentration of 5×1017 cm−3 and hole mobility of 0.3 cm2/V s. Rapid thermal annealing processing decreased the electron density in the P-doped ZnO films. XPS binding energies of P 2s and 2p peaks showed formation of P–O bonds which increased with oxygen pressure in the films. This indicates formation of defect complexes of P dopants occupying zinc sites PZn and zinc vacancies VZn in the P-doped ZnO films.
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73.61.Ga II-VI semiconductors
61.72.Cc Kinetics of defect formation and annealing
61.72.jd Vacancies
81.15.Fg Pulsed laser ablation deposition
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
68.55.ag Semiconductors

Effect of growth temperature on a-plane ZnO formation on r-plane sapphire

Chun-Yen Peng, Jr-Sheng Tian, Wei-Lin Wang, Yen-Teng Ho, Shu-Chang Chuang, Ying-Hao Chu, and Li Chang

J. Vac. Sci. Technol. A 29, 03A110 (2011); http://dx.doi.org/10.1116/1.3549141 (4 pages)

Online Publication Date: 1 March 2011

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The effect of growth temperature on a-plane ZnO formation on r-plane sapphire has been systematically investigated by employing in situ high pressure reflection high-energy electron diffraction, atomic force microscopy, and high-resolution x-ray diffraction. For film growth above and below 600 °C, it is shown that there is a significant difference in growth rate and surface morphology due to the differences in the growth mode. Stripelike morphologies were observed on the surface of a-plane ZnO grown at low temperature (LT) because of differences in the growth rate along the c-axis and the growth rate normal to the c-axis. Furthermore, annealing of films grown at low temperature results in more pronounced stripe morphology and in improvement of crystallinity.
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68.55.ag Semiconductors
81.05.Dz II-VI semiconductors
68.37.Ps Atomic force microscopy (AFM)
68.35.bg Semiconductors
61.72.Cc Kinetics of defect formation and annealing

Effects of gallium doping on properties of a-plane ZnO films on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy

Seok Kyu Han, Hyo Sung Lee, Dong Seok Lim, Soon-Ku Hong, Nara Yoon, Dong-Cheol Oh, Byung Jun Ahn, Jung-Hoon Song, and Takafumi Yao

J. Vac. Sci. Technol. A 29, 03A111 (2011); http://dx.doi.org/10.1116/1.3562162 (7 pages) | Cited 1 time

Online Publication Date: 9 March 2011

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The authors report on the structural, optical, and electrical properties of Ga-doped a-plane (11math0) ZnO films grown by plasma-assisted molecular beam epitaxy. Ga doping level was controlled by changing the Ga cell temperatures from 350 to 470 °C with an interval of 30 °C. With up to Ga cell temperatures of 440 °C, single crystalline Ga-doped a-plane ZnO films were grown; however, the sample with a Ga cell temperature of 470 °C showed polycrystalline features. The typical striated surface morphology normally observed from undoped ZnO films disappeared with Ga doping. ZnO films doped with Ga cell temperatures up to 440 °C did not show a significant change in full width at half maximum (FWHM) values of (11math0) x-ray rocking curves by doping. The smallest FWHM values were 0.433° (ϕ = 90°) and 0.522° (ϕ = 0°) for the sample with a Ga cell temperature of 350 °C. The polycrystalline ZnO film with excessive Ga doping at the Ga cell temperature of 470 °C showed significantly increased FWHM values. Hall measurements at room temperature (RT) revealed that electron concentration began to be saturated at the Ga cell temperature of 440 °C and electron mobility was drastically reduced at the Ga cell temperature of 470 °C. The carrier concentration of Ga-doped ZnO films were controlled from 7.2×1018 to 3.6×1020 cm−3. Anisotropic electrical properties (carrier concentration and Hall mobility) were observed in measurements by the van der Pauw method depending on the direction (c- or m-direction) for the undoped sample but not observed for the doped samples. RT photoluminescence (PL) spectra from the Ga-doped single crystalline ZnO films showed dominant near band edge (NBE) emissions with negligibly deep level emission. The NBE intensity in PL spectra increases with Ga doping.
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73.61.Ga II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.05.Dz II-VI semiconductors
78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors

Effect of annealing on the electrical properties of ZnO crystals grown by chemical vapor transport

Koji Abe, Masaaki Miura, and Masanori Oiwa

J. Vac. Sci. Technol. A 29, 03A112 (2011); http://dx.doi.org/10.1116/1.3565026 (3 pages) | Cited 1 time

Online Publication Date: 10 March 2011

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Electrical properties of ZnO crystals annealed at 600 °C in H2, O2, and Ar have been investigated. The two donors (D1 and D2) with the ionization energies of ED1 ∼ 0.043 eV and ED2 = 0.08 eV and a compensating acceptor were observed in the as-grown and annealed samples. There was no significant change in electrical properties of the sample annealed in H2, while annealing in O2 and Ar affected the electron concentration. The decrease of the D1 donor concentration and the increase of the compensating acceptor concentration were observed in the samples annealed in O2. The electron mobility for the sample annealed in O2 decreased because of the increase of the compensating acceptor concentration.
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72.80.Ey III-V and II-VI semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance
61.72.Cc Kinetics of defect formation and annealing
81.10.Bk Growth from vapor

Microwave absorption properties of Mn- and Ni-doped zinc oxides

Qi-Bai Wu, Wei Zhao, Guo-Xun Zeng, Hai-Yan Zhang, Ai-Xiang Wei, and Jia Wang

J. Vac. Sci. Technol. A 29, 03A113 (2011); http://dx.doi.org/10.1116/1.3567421 (4 pages)

Online Publication Date: 17 March 2011

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Microwave absorption properties of Mn- and Ni-doped zinc oxides were assessed in this study. Samples were prepared by the decomposition of acetate solid solutions. By changing the concentration of dopant ions in the reaction solutions, zinc oxides with different amounts of dopant were obtained. The morphologies, chemical compositions, and structures of the samples were investigated by scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray powder diffraction. Electromagnetic characteristics of the doped samples were assessed by vector network analysis at a frequency range of 2–16 GHz. Both the real and imaginary parts of the complex permittivity decreased as Mn or Ni concentration increased. Results indicate that, compared with pure ZnO, Mn- and Ni-doped zinc oxides exhibit excellent microwave absorption properties. The highest level of microwave absorption observed was 80.7 dB at a frequency of 9.8 GHz, and the best frequency bandwidth was 8.6 GHz at reflection loss values below −10 dB.
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61.72.uj III-V and II-VI semiconductors
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation
78.70.Gq Microwave and radio-frequency interactions

Growth studies and optical properties of Zn1−xCdxO films grown by metal-organic chemical-vapor deposition

Corinne Sartel, Nadia Haneche, Christèle Vilar, Gaelle Amiri, Jean-Michel Laroche, François Jomard, Alain Lusson, Pierre Galtier, Vincent Sallet, Christophe Couteau, Jenny Lin, Roy Aad, and Gilles Lérondel

J. Vac. Sci. Technol. A 29, 03A114 (2011); http://dx.doi.org/10.1116/1.3567960 (4 pages)

Online Publication Date: 18 March 2011

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Heteroepitaxial growths of Zn1−xCdxO films were performed on R-oriented sapphire substrates by metal-organic chemical-vapor deposition. The authors carried out secondary ion mass spectrometry analysis, scanning electron microscopy, photoluminescence, and ellipsometric measurements to investigate the incorporation of cadmium in the layers, as well as its influence on the optical properties. Compositions up to 5.5% Cd were obtained, tuning the photoluminescence emission from 3.36 (ZnO) to 3.1 eV and increasing the refractive index at 600 nm from 1.94 to 2.
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81.05.Dz II-VI semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Transparent conductive and near-infrared reflective Ga-doped ZnO/Cu bilayer films grown at room temperature

J. G. Lu, X. Bie, Y. P. Wang, L. Gong, and Z. Z. Ye

J. Vac. Sci. Technol. A 29, 03A115 (2011); http://dx.doi.org/10.1116/1.3570864 (4 pages)

Online Publication Date: 29 March 2011

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Bilayer films consisting of Ga-doped ZnO (GZO) and Cu layers were grown at room temperature by magnetron sputtering. The structural, electrical, and optical properties of GZO/Cu bilayer films were investigated in detail. The crystallinity and transparent-conductive properties of the films were correlated with the Cu layer thickness. The GZO/Cu bilayer film with the Cu layer thickness of 7.8 nm exhibited a low resistivity of 7.6×10−5 Ω cm and an average visible transmittance of 74%. The reflectance was up to 65% in the near-infrared region for this film. The transparent conductive and near-infrared reflective GZO/Cu bilayer films could be readily deposited at room temperature. The GZO/Cu bilayer films were thermally stable when annealed at temperatures as high as 500 °C.
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68.55.A- Nucleation and growth
81.15.Cd Deposition by sputtering
73.40.Ns Metal-nonmetal contacts
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.40.Gh Other heat and thermomechanical treatments
78.66.-w Optical properties of specific thin films

Effect of substrate microstructure on the misorientation of a-plane ZnO film investigated using x-ray diffraction

Jinju Chen, Hong Deng, Hong Ji, and Yanlei Tian

J. Vac. Sci. Technol. A 29, 03A116 (2011); http://dx.doi.org/10.1116/1.3573670 (4 pages)

Online Publication Date: 5 April 2011

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(11math0) ZnO (nonpolar a-plane ZnO) films were grown on (1math02) Al2O3 (r-plane sapphire) substrates by single-source chemical vapor deposition. The misorientation in the film was investigated in detail using x-ray diffraction techniques, including 2θ-ω scan, reciprocal space mapping (RSM), and double- and triple-crystal x-ray diffraction analysis. 2θ-ω scan and RSM measurements suggest that ZnO film grows on r-plane sapphire substrate epitaxially along the [11math0] orientation, and the (11math0) plane of the ZnO film tilts with respect to the (1math02) plane of the substrate. RSM measurement for sapphire substrate shows that the diffraction intensity distribution around (2math04) reciprocal lattice point presents a curved band along the 2θ-ω scan direction, which can be attributed to a strained layer (or a lattice distortion layer) at the substrate surface. Double- and triple-crystal x-ray diffraction measurements imply that anisotropic mosaic exists in sapphire substrate. The strained layer and tilt mosaic in sapphire substrate may be the important reasons for the tilted growth of the (11math0) plane of a-plane ZnO film relative to (1math02) plane of the Al2O3 substrate.
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68.55.ag Semiconductors
81.05.Dz II-VI semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Theoretical investigations of the impurity axial displacement for ZnO:V3+

Zhi-Hong Zhang, Shao-Yi Wu, and Hua-Ming Zhang

J. Vac. Sci. Technol. A 29, 03A117 (2011); http://dx.doi.org/10.1116/1.3573978 (6 pages)

Online Publication Date: 5 April 2011

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The local structure of the V3+ center in ZnO crystal is theoretically investigated using the perturbation formulas of the spin Hamiltonian parameters for a 3d2 ion in trigonally distorted tetrahedra. From the studies, the impurity V3+ is found not to occupy the ideal Zn2+ site in ZnO but to suffer an outward displacement of about 0.08 Å away from the oxygen triangle along the C3 axis. The calculated spin Hamiltonian parameters based on the above impurity displacement show good agreement with the experimental data. The outward displacement of the impurity V3+ in ZnO can be attributed to the local tension due to the size and charge mismatching substitution of the host Zn2+ by the larger and higher charged V3+. The studies of this work would be helpful to the investigations of the structure properties of ZnO (or other similar II-VI semiconductors) doped with transition-metal ions.
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61.72.jn Color centers
61.66.Fn Inorganic compounds

Ultraviolet photodetectors based on MgZnO thin films

Cheng-Zhi Wu, Liang-Wen Ji, Chien-Hung Liu, Shi-Ming Peng, Sheng-Joue Young, Kin-Tak Lam, and Chien-Jung Huang

J. Vac. Sci. Technol. A 29, 03A118 (2011); http://dx.doi.org/10.1116/1.3575552 (5 pages) | Cited 1 time

Online Publication Date: 11 April 2011

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In this work, Ti/Au Ohmic contacts to both Mg0.24Zn0.76O and ZnO film-based metal-semiconductor-metal (MSM) photodetectors (PDs) were fabricated on glass substrates for comparative analysis. The transmittance spectra measured around the optical energy gap revealed that Mg0.24Zn0.76O films have a larger optical energy gap (3.54 eV) than ZnO films (3.25 eV). Mg0.24Zn0.76O MSM-structured ultraviolet (UV) PDs show a much higher UV-to-visible rejection ratio of 2.78×103 than those made of ZnO films. This can be attributed to the low dark current (0.08 pA) of the Mg0.24Zn0.76O UV PDs and the small full width at half maximum (0.34°) of the Mg0.24Zn0.76O (002) x-ray diffraction peak, indicating better crystal quality than that of ZnO. With an applied bias of 5 V and illuminations at 350 and 380 nm, the Mg0.24Zn0.76O and ZnO film-based MSM PDs exhibited responsivities of 0.4 and 0.32 A/W, respectively.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
84.32.Dd Connectors, relays, and switches

Magnetic properties of ZnO:V films formed by pulsed laser deposition with bias voltage application

Kei Asano, Shingo Doi, Hiroyuki Yamaguchi, Takao Komiyama, Yasunori Chonan, and Takashi Aoyama

J. Vac. Sci. Technol. A 29, 03A119 (2011); http://dx.doi.org/10.1116/1.3579416 (5 pages)

Online Publication Date: 12 April 2011

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Magnetic properties of ZnO films doped with V atoms (a 3d transition metal) were investigated by fabricating specimens using pulsed laser deposition (PLD) with bias voltage application. Electron concentrations of the ZnO:V films were changed by two methods: one was to dope with conventional n-type and p-type impurities, and the other was to change native (donor-type) defect concentrations by applying bias voltages during the film deposition. In M-H curves, smaller magnetization was observed for the film with a low electron concentration (5×1016 cm−3) and it increased linearly with the applied magnetic field, while larger magnetization was observed for the film with a medium electron concentration (5×1018 cm−3) and it exhibited a saturation at about 3 kOe. The saturation magnetization had maximum values for the electron concentrations of 1×1018–5×1018 cm−3 for the films formed by using either dopant impurities or PLD bias voltages. Anomalous Hall effect measurement of the films showed that the convergence of the Hall resistivity occurred at the same magnetic field as where the saturation of the magnetization occurred in the M-H curve.
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75.70.Ak Magnetic properties of monolayers and thin films
68.55.ag Semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
73.61.Ga II-VI semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
75.78.-n Magnetization dynamics

Anomalous optical processes in photoluminescence from ultrasmall quantum dots of ZnO

L. M. Kukreja, P. Misra, A. K. Das, J. Sartor, and H. Kalt

J. Vac. Sci. Technol. A 29, 03A120 (2011); http://dx.doi.org/10.1116/1.3578344 (8 pages)

Online Publication Date: 19 April 2011

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Ensembles of alumina capped ZnO quantum dots (ZQDs) were grown using pulsed laser deposition. The ZQDs of mean radii comparable to and smaller than the pertinent excitonic Bohr radius ( ∼ 2.34 nm), called ultrasmall quantum dots, show size dependent optical absorption edges, which follow the strong confinement model. In this model the confinement energy and Coulombic interaction energy of the localized electron-hole pairs are significantly higher than their correlation energy and the optical transitions are perceived to be nonexcitonic in nature. In photoluminescence (PL) spectra of such ZQDs of mean radius of ∼ 2.3 nm at temperatures of 6 K and above, the primary recombinations are found to be due to the surface bound and Al donor bound electron-hole pairs. The band-edge recombination peak of the PL spectra appeared at about 70 K and above, which was found to be about 166 meV Stoke and/or thermally redshifted with respect to the experimentally observed absorption edge. Almost all of the PL spectra at different temperatures conspicuously showed the LO and 2LO phonon replicas of the primary transitions, suggesting strong coupling between the recombining charge carriers and the LO phonon, which is rather unusual for nonexcitonic recombinations. The temperature dependent PL peak positions followed the well known Varshni’s relation with fitting parameters close to that of the bulk ZnO. The peak intensity of the observed PL transitions followed the normal law of thermal quenching which could be fitted with the Arrhenius equation having activation energy of about 10 meV. Temperature dependence of full width at half maximum of the PL peaks when fitted with the Hellmann and O’Neill models did not result in a close match. However, from this fit one could estimate a value of the carrier-LO phonon coupling coefficient of ∼ 980 meV, which is higher than that reported earlier for the ZQDs. These observations are hitherto unfamiliar and expected to provide further insight into the basic optical processes in alumina capped ultrasmall ZQDs.
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78.67.Hc Quantum dots
78.55.Et II-VI semiconductors
73.63.Kv Quantum dots
73.21.La Quantum dots
73.22.Lp Collective excitations
71.35.-y Excitons and related phenomena
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