Proceedings Volume 8263

Oxide-based Materials and Devices III

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Proceedings Volume 8263

Oxide-based Materials and Devices III

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Volume Details

Date Published: 6 March 2012
Contents: 16 Sessions, 40 Papers, 0 Presentations
Conference: SPIE OPTO 2012
Volume Number: 8263

Table of Contents

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Table of Contents

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  • Front Matter: Volume 8263
  • Highly Conducting Transparent Oxides I
  • Highly Conducting Transparent Oxides II
  • Doping and Band Structure Studies
  • Material Growth
  • Light Emitters I
  • Light Emitters II
  • Material Processing
  • Band Gap Engineering
  • Optical Properties
  • Oxide-Organic Hybridation
  • Oxide-based Devices I
  • Oxide-based Devices II
  • Superconductivity
  • Growth, Properties, and Applications of Nanostructures
  • Poster Session
Front Matter: Volume 8263
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Front Matter: Volume 8263
This PDF file contains the front matter associated with SPIE Proceedings Volume 8263, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
Highly Conducting Transparent Oxides I
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Making highly conductive ZnO: creating donors and destroying acceptors
We obtain room-temperature resistivities as low as ρ =1.4 x 10-4 Ω-cm in transparent Ga-doped ZnO grown on Al2O3 by pulsed laser deposition (PLD) at 200 °C in 10 mTorr of pure Ar and then annealed in a Zn enfivironment. Donor ND and acceptor NA concentrations are calculated from a recently developed scattering theory that is valid for any degenerate semiconductor material and requires only two input parameters, mobility μ and carrier concentration n measured at any temperature in the range 5 - 300 K. By comparison with SIMS and positron annihilation measurements, it has been shown that the donors in these samples are mostly GaZn, as expected, but that the acceptors are point defects, Zn vacancies VZn. PLD growth in Ar at 200 °C produces a high concentration of donors [GaZn] = 1.4 x 1021 cm-3, but VZn acceptors are produced at the same time, due to self-compensation. Fortunately, a large fraction of the VZn can be eliminated by annealing in a Zn environment. The theory gives ND and NA, and thus [GaZn] and [VZn], at each step of the growth and annealing process. For convenience, the theory is presented graphically, as plots of μ vs n at various values of compensation ratio K = NA/ND. From the value of K corresponding to the experimental values of μ and n, it is possible to calculate ND = n/(1 - K) and NA = nK/(1 - K).
Optical characterization of high mobility polycrystalline ZnO:Al films
Florian Ruske, Mark Wimmer, Grit Köppel, et al.
Optical methods are powerful and non-destructive means to characterize highly doped transparent conducting oxide thin films. In order to describe the optical properties of high-mobility ZnO films we present a dielectric function composed of different analytic expressions to describe the different contributions to the dielectric function of the films. This allows for the correct description of measured optical spectra and reduces the complex functions to a set of fitting parameters. In a second step we compare the obtained parameters to theoretical models. The basic theories are nicely reproduced and the basic link between optical and electrical properties can be understood. The findings can help on the route to a complete presiction of optical properties from the basic material properties or vice versa.
Hydrothermal growth and characterization of bulk Ga-doped and Ga/N-codoped ZnO crystals
Buguo Wang, Matthew Mann, Michael Snure, et al.
Bulk ZnO crystals were grown by the hydrothermal technique with Ga2O3 or GaN added to the solution in an attempt to dope with Ga, or co-dope with Ga and N, respectively. Adding Ga2O3 alone to the growth solution significantly reduces the ZnO growth rate; however, the resulting crystal is highly conductive, with a resistivity approaching 0.01 Ω cm. In contrast, the addition of GaN had less effect on the growth of ZnO, but the crystal was of poor quality with a higher resistivity, about 0.1 Ω cm. Photoluminescence spectra at 4 K show Ga0-bound-exciton peak energies of 3.3604 and 3.3609 eV for the Ga- and Ga/N-doped crystals, respectively; these energies differ slightly from the literature value of 3.3598 eV, evidently due to compressive strain. Other peaks at 3.307, 3.290, 3.236, and 3.20 eV were found in the Ga/N-codoped ZnO after the crystal was annealed at 600°C in air. The 3.307 eV peak is the so-called A line, and likely arises from recombination of a free electron with a neutral N-related acceptor.
Novel fabrication method for ZnO films via nitrogen-mediated crystallization
N. Itagaki, K. Kuwahara, K. Matsushima, et al.
High quality ZnO films have been obtained by utilizing buffer layers fabricated via nitrogen mediated crystallization (NMC), where sputtering method is employed for preparation of both buffer layers and ZnO films. The crystal grain size of ZnO:Al (AZO) films with NMC-buffer layers is about 3 times larger than that of conventional films, which is considered to be due to the low nuclei density of NMC-buffer layers. As a result, the resistivity of AZO films drastically reduces from 4.76 m Ωcm for the conventional films to 0.48 m Ωcm for our films when the total film thickness is 20 nm. The NMC buffer layers also improve the spatial distribution of the resistivity, which indicates that the crystallinity at the initial stage of deposition govern the properties of AZO films. Furthermore, we have succeeded in epitaxial growth of ZnO films, whose FWHM of the rocking curve of (002) peak is as narrow as 0.061°, on c-plane sapphire substrates by using the NMC method. From these results, we conclude that our method described here is full of promise for fabrication of ZnO-based materials.
Highly Conducting Transparent Oxides II
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Properties of TCO anodes deposited by atmospheric pressure chemical vapor deposition and their application to OLED lighting
R. Y. Korotkov
Doped ZnO is one of the materials currently being considered in commercial optoelectronic applications as a potential indium tin oxide (ITO) replacement for the transparent conducting oxide (TCO). The properties of doped ZnO anodes prepared at Arkema Inc. are analyzed using spectroscopic ellipsometer (230 to 1700 nm) and Hall-effect. The modeling of the refractive indexes is conducted using a double oscillator model. The model parameters are tested on a double layer: undoped and doped structure deposited by atmospheric pressure chemical vapor deposition (APCVD) on glass substrates. Excellent correlation between calculated and experimental parameters was obtained.
Real-structure effects: Absorption edge of Mg_x Zn_{1-x} O, Cd_x Zn_{1-x} O, and n-type ZnO from ab-initio calculations
André Schleife, Friedhelm Bechstedt
The continuously increasing power of modern supercomputers renders the application of more and more accurate parameterfree models to systems of increasing complexity feasible. Consequently, it becomes possible to even treat different realstructure effects such as alloying or n-doping in systems like the technologically important transparent conducting oxides. In this paper we outline how we previously used a combination of quasiparticle calculations and a cluster expansion scheme to calculate the fundamental band gap of MgxZn1-xO and CdxZn1-xO alloys. We discuss the results in comparison to values for In2O3, SnO2, SnO, and SiO2. In addition, we discuss our extension of the Bethe-Salpeter approach that has been used to study the interplay of excitonic effects and doping in n-type ZnO. The dependence of the Burstein-Moss shift on the free-carrier concentration is analyzed.
Doping and Band Structure Studies
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Thermal process induced change of conductivity in As-doped ZnO
S. C. Su, J. C. Fan, C. C. Ling
Arsenic-doped ZnO films were fabricated by radio frequency magnetron sputtering method with different substrate temperature TS. Growing with the low substrate temperature of TS=200°C yielded n-type semi-insulating sample. Increasing the substrate temperature would yield p-type ZnO film and reproducible p-type film could be produced at TS~450°C. Post-growth annealing of the n-type As-doped ZnO sample grown at the low substrate temperature (TS=200°C) in air at 500°C also converted the film to p-type conductivity. Further increasing the post-growth annealing temperature would convert the p-type sample back to n-type. With the results obtained from the studies of positron annihilation spectroscopy (PAS), photoluminescence (PL), cathodoluminescence (CL), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and nuclear reaction analysis (NRA), we have proposed mechanisms to explain for the thermal process induced conduction type conversion as observed in the As-doped ZnO films.
Doped gallium oxide nanowires for photonics
E. Nogales, I. López, B. Méndez, et al.
Monoclinic gallium oxide, β-Ga2O3, is a transparent conducting oxide (TCO) that presents one of the widest band gaps among this family of materials. Its characteristics make it highly interesting for applications in UV - visible - IR optoelectronic and photonic devices. On the other hand, the morphology of nanowires made of this oxide presents specific advantages for light emitting nanodevices, waveguides and gas sensors. Control of doping of the nanostructures is of the utmost importance in order to tailor the behavior of these devices. In this work, the growth of the nanowires is based on the vapor-solid (VS) mechanism during thermal annealing treatment while the doping process was carried out in three different ways. In one of the cases, doping was obtained during the growth of the wires. A second method was based on thermal diffusion of the dopants after the growth of undoped nanowires, while the third method used ion implantation to introduce optically active ions into previously grown nanowires. The study of the influence of the different dopants on the luminescence properties of gallium oxide nanowires is presented. In particular, transition metals and rare earths such as Cr, Gd, Er or Eu were used as optically active dopants that allowed selection of the luminescence wavelength, spanning from the UV to the IR ranges. The benefits and drawbacks of the three different doping methods are analyzed. The waveguiding behavior of the doped nanowires has been studied by room temperature micro-photoluminescence.
Material Growth
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Growth and characterization of large-diameter, lithium-free ZnO single crystals
Shaoping Wang, Aneta Kopec, Andrew G. Timmerman
Large-diameter, lithium-free ZnO single-crystal substrates of high crystalline quality will enable development and commercialization of high-performance ZnO-based semiconductor devices, such as UV and visible light emitting diodes (LEDs), UV laser diodes and solar-blind UV detectors for variety of applications. We have recently developed a novel crystal growth technique for producing lithium-free ZnO single crystal boules of 1 inch in diameter. We also fabricated ZnO single crystal wafers in sizes up to 1 inch in diameter. Chemical purity, crystalline defects, and electrical resistivity of ZnO single crystals were analyzed. Results from crystal growth and material characterization are presented and discussed. Our research results suggest that the novel crystal growth technique is a viable production technique for producing ZnO single crystals and substrates for semiconductor device applications.
Light Emitters I
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White-light lasing in ZnO microspheres fabricated by laser ablation
Shinya Okamoto, Yosuke Minowa, Masaaki Ashida
We succeeded in fabricating ZnO microspheres with high sphericity by laser ablation in superfluid helium. Such microspheres enable efficient lasing in the whole visible region due to defects with a CW laser at room temperature. The lasing threshold is found to be around 100 W/cm2. This value is much smaller than those of the recent reports on the lasing in ZnO microwire. Cathodoluminescence of single ZnO microspheres was also measured.
White light upconversion emission in Yb3+/ Er3+/ Tm3+ codoped oxy-fluoride lithium tungsten tellurite glass ceramics
Ghizal F. Ansari, S. K. Mahajan
The bright white upconversion emission ( tri-colour UC) is generated in Er/Tm/Yb tri -doped oxy-fluoride lithium tungsten tellurite (TWLOF)glass ceramics containing crystalline phase LiYbF4 under the excitation of 980nm laser diode. The most appropriate combination of rare-earth ions (2mol% YbF3 1mol% ErF3 and 1mol%TmF3 )of glass ceramic sample has been determined to tune the primary colour (RGB and generate white light emission. By varying the pump power, intense and weak blue (487nm, 437nm), green (525nm and 545nm) and red (662nm) emission are simultaneously observed at room temperature. The dependence of upconversion emission intensity suggest that a theephoton process is responsible for the blue emission of Tm3+ ions and red emission due to both Tm3+ and Er3+ ions , while green emission originated from two photon processes in Er3+ ions. Also tri colour upconvesion and energy transfer in this glass ceramics sample were studied under 808nm laser diode excitation. The Upconversion mechanisms and Tm3+ ions plays role of both emitter and activator (transfer energy to Er) were discussed.
Optimization of photoluminescence and electroluminescence of silicon nanocrystals in a superlattice host
The use of silicon superlattices is a well established technique for creating nanocrystals. Depositing superlattices allows for adjustment of nanocrystal properties, such as the emission wavelength, by varying the silicon layer thickness. Opposed to the silicon layer, the silicon dioxide thickness effects are not documented as extensively. This study looks at superlattice films with silicon and silicon dioxide layers varying from 0.4 to 0.8 nm and 2.7 to 5.1 nm respectively, deposited via a plasma enhanced chemical vapor deposition. Photoluminescence and electroluminescence measurements were taken to show an increase in the output intensity increased oxide thickness.
Light Emitters II
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Metal oxide nanostructures and white light emission
A. Zainelabdin, O. Nur, G. Amin, et al.
We report on white light emission from zinc oxide nanostructures chemically grown on paper substrates. The effect of the growth solution pH on the morphology is discussed. The light emission form light emitting diodes based on ZnO nanorods/organic polymer hybrids on paper substrate is presented. Further copper oxide was grown on the walls of zinc oxide nanorods and the optical properties were investigated.
ZnO nanowires for tunable near-UV/blue LED
Nanowires (NWs)-based light emitting diodes (LEDs) have drawn large interest due to many advantages compared to thin film based devices. Markedly improved performances are expected from nanostructured active layers for light emission. Nanowires can act as direct waveguides and favor light extraction without the use of lenses and reflectors. Moreover, the use of wires avoids the presence of grain boundaries and then the emission efficiency should be boosted by the absence of non-radiative recombinations at the joint defects. Electrochemical deposition technique was used for the preparation of ZnO-NWs based light emitters. Nanowires of high structural and optical quality have been epitaxially grown on p-GaN single crystalline films substrates. We have shown that the emission is directional with a wavelength that was tuned and red-shifted toward the visible region by doping with Cu in ZnO NWs.
Photoresponse comparison of indium oxide (In2O3) nanomaterials
Dali Shao, Liqiao Qin, Shayla Sawyer
Optical properties of In2O3 nanoparticles coated with polyvinyl-alcohol (PVA) are studied. Compared with uncoated In2O3 nanoparticles, PVA coated sample show enhanced UV-blue emission and suppressed parasitic green emission. Ultraviolet (UV)-blue photodetectors were then fabricated by depositing aluminum (Al) as contacts on top of PVA coated and uncoated samples. The photodetector with PVA coating, exhibits lower dark current and higher responsivity than the photodetector without PVA coating. The rise and fall time of the PVA coated photodetector is about 500 s and 1600 s respectively, one half of the uncoated device. These improvements are attributed to surface passivation of In2O3 nanoparticles by PVA, which reduces the surface defects density and increase free carrier concentration of In2O3 nanoparticles.
Material Processing
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Oxide crystal-fibers grown by micro-pulling-down technique and applications for lasers and scintillators
J. Didierjean, N. Aubry, D. Perrodin, et al.
We demonstrated growth of YAG, LuAG and CALGO single crystal fibers with doping Nd, Yb, Er, and Ce by the micro-pulling-down technique. Those fibers have applications in high power lasers and scintillating detectors. For laser operation, average power of 65 W energy of 4 mJ and peak power above 7 MW have been demonstrated in various configurations. Those results push the limits of end-pumped bulk crystals in terms of average power and exceed the limits of pulsed fibers lasers in terms of energy. For scintillating applications, high density/high light yield detectors are developed for nuclear science and medical applications.
Novel process for direct bonding of GaN onto glass substrates using sacrificial ZnO template layers to chemically lift-off GaN from c-sapphire
GaN was grown on ZnO-buffered c-sapphire (c-Al2O3) substrates by Metal Organic Vapor Phase Epitaxy. The ZnO then served as a sacrificial release layer, allowing chemical lift-off of the GaN from the c-Al2O3 substrate via selective wet etching of the ZnO. The GaN was subsequently direct-wafer-bonded onto a glass substrate. X-Ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray microanalysis, Room Temperature Photoluminescence & optical microscopy confirmed bonding of several mm2 of crack-free wurtzite GaN films onto a soda lime glass microscope slide with no obvious deterioration of the GaN morphology. Using such an approach, InGaN based devices can be lifted-off expensive single crystal substrates and bonded onto supports with a better cost-performance profile. Moreover, the approach offers the possibility of reclaiming and reusing the substrate.
Band Gap Engineering
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Ion beams as a tool for the characterization of near-pseudomorphic CdZnO epilayers
A. Redondo-Cubero, M. Brandt, F. Henneberger, et al.
In this paper we show the application of Rutherford backscattering spectrometry and ion channeling (RBS/C) for the detection of compositional and strain gradients in CdZnO grown almost pseudomorphically on MgZnO. The asymmetric features revealed in X-ray diffraction studies were explained by the compositional gradient found in the first 100 nm close to the interface. Calculations of the effect of such a gradient on the strain state of the layer were developed and contrasted with RBS/C angular scans. Additionally, the substitutional behavior of Cd (and Mg) in Zn-sites was demonstrated.
Optical Properties
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Microscopic origins of the surface exciton photoluminescence in ZnO nanostructures
Mahua Biswas, Yun Suk Jung, Hong Koo Kim, et al.
Photoluminescence (PL) studies of the surface exciton peak in ZnO nanostructures at ~3.367 eV are reported to elucidate the nature and origin of the emission and its relationship to nanostructure morphology. Localised voltage application in high vacuum and different gas atmospheres show a consistent PL variation (and recovery), allowing an association of the PL to a bound excitonic transition at the ZnO surface modified by an adsorbate. Studies of samples treated by plasma and of samples exposed to UV light under high vacuum conditions show no consistent effects on the surface exciton peak indicating no involvement of oxygen species. X-ray photoelectron spectroscopy data indicate involvement of adsorbed OH species. The relationship of the surface exciton peak to the nanostructure morphology is discussed in light of x-ray diffraction, scanning and transmission electron microscopy data.
Excitonic transport in ZnO
Martin Noltemeyer, Frank Bertram, Thomas Hempel, et al.
The temperature dependence of diffusion length and lifetime or diffusivity of the free exciton is measured in a commercial ZnO-substrate and in an epitaxial ZnO quantum well using nm-spatially and ps-time resolved cathodoluminescence (CL) spectroscopy. The characteristic temperature dependence of the exciton mobility is a fingerprint of the underlying excitonic scattering processes. Since excitons are neutral particles scattering at ionized impurities should be not effective. With decreasing temperature diffusion lengths and lifetimes give rise to a monotonous increase of the excitonic mobility. Two different methods for determining the excitonic transport parameters will be presented. On the one hand we are able to perform completely pulsed excitation experiments and on the other hand a combination of cw- and pulsed excitation in two independent measurements are needed.
Electro-optical properties of barium titanate films epitaxially grown on silicon
S. Abel, D. Caimi, M. Sousa, et al.
BaTiO3 (BTO) single crystals exhibit one of the largest Pockels coefficients (r42 > 1000 pm/V) among oxides. This makes BTO an excellent active material for electro-optical (EO) devices such as switches, modulators or tuning elements. However, in order to harness these properties in silicon photonics circuits, the challenge is to integrate BTO as high quality thin films onto Si substrates. The effective Pockels coefficients can be enhanced in epitaxial films due to their tight relationship with the crystallographic symmetry and microstructure. We report on the EO properties of epitaxial BTO thin films on Si. The growth of BTO layers on Si(001) is performed by molecular beam epitaxy (MBE). A thin single-crystalline strontium titanate seed layer is grown on Si, followed by a 130 nm thick BTO layer. Electrodes to provide an electrical field parallel to the surface are patterned on the films using photolithography. Throughout this process, the BTO keeps an epitaxial relationship to the Si-substrate. Considering the tensor nature of the Pockels effect, the optical behavior of the BTO layers upon applying an electrical field is simulated, taking into account the films' crystalline multi-domain structure. An experimental way to access these EO properties is discussed, which utilizes polarization changes of a transmitted laser beam upon applying an electrical field to the film. Simulations of the measurement signals demonstrate the capability of resolving the expected EO response of the samples, which serves as a promising base for future experiments.
Tuning optical properties of complex oxides: examples of 12CaO.7Al2O3 mayenite and LaCrO3 perovskite
Peter V. Sushko
Optical absorption bands in two complex oxides are characterised using ab initio simulations and an embedded cluster method. In sub-nanoporous 12CaO.7Al2O3 the width of the optical gap can be controlled by modifying the chemical identities and relative concentration of extra-framework species. In antiferromagnetic LaCrO3, Cr 3d states split into four narrow one-electron bands and give rise to several types of the optical transitions. Their excitation energies respond differently to lattice strain, thus, providing a possibility for tuning the excitation energies in supported LaCrO3 films by selecting appropriate substrates.
Oxide-Organic Hybridation
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Functionalization of ZnO surfaces with organic molecules
N. H. Moreira, A. Garcia, A. L. Rosa, et al.
The understanding of the interaction of organic species with inorganic surfaces and nanostructures constitutes a step forward in the development of novel hybrid devices, such as sensors and solar cells. In this work the structural and electronic properties of clean and defective ZnO non-polar surfaces modified with acetic acid have been investigated by using the self-consistent charge density-functional based tight-binding method (SCC-DFTB). In particular, the adsorption of acetic acid, the role of surface defects and the relevant mechanisms acting on surface stabilization are discussed.
Oxide-based Devices I
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Reactive dual magnetron sputtering of Ta2O5 and Al2O3: optical and structural properties and thin film applications
In this paper we perform a systematic investigation and optimization of lambda controlled, reactive ion beam sputter deposition process conditions for a range of optical materials. The deposited films are compared for suitability for applications such as planar waveguides for optical interconnect, laser device manufacture, multi-layer interference filters, and precision optical mirrors. Thin films of tantalum pentoxide and aluminum oxide were deposited using a reactive dual-magnetron sputtering system (Leybold Optics Helios Pro). Deposited film quality was optimized as a function of plasma power and gas flow, and an optimum oxygen working point was determined for each material. Lambda control methods were used for the purpose of optimizing optical quality of the layer. Deposited layers were characterized by variable angle spectroscopic ellipsometry (VASE), X-ray diffraction (XRD) and SEM imaging. Waveguide losses were measured for each sample using a prism coupling arrangement. Tantalum pentoxide slab waveguides with loss as low as 1dB/cm were produced ideal for waveguide applications. Interference filters/mirrors consisting of alternating SiO2, and Ta2O5 or Al2O3 material layers were deposited and characterized. Reflectivity and transmission of the deposited mirrors was compared to the theoretical design. Good agreement between the theory and the practical filter/mirror designs was achieved confirming the material film quality.
P-type oxide-based thin film transistors produced at low temperatures
R. Martins, V. Figueiredo, R. Barros, et al.
P-type thin-film transistors (TFTs) using room temperature sputtered tin and copper oxide as a transparent oxide semiconductor have been produced on rigid and paper substrates. The SnOx films shows p-type conduction presenting a polycrystalline structure composed with a mixture of tetragonal β-Sn and α-SnOx phases, after annealing at 200 °C. These films exhibit a hole carrier concentration in the range of ≈ 1016-1018 cm-3, electrical resistivity between 101-102 Ωcm, Hall mobility of 4.8 cm2/Vs, optical band gap of 2.8 eV and average transmittance ≈ 85 % (400 to 2000 nm). Concerning copper oxide CuxO thin films they exhibit a polycrystalline structure with a strongest orientation along (111) plane. The CuxO films produced between an oxygen partial pressure of 9 to 75% showed p-type behavior, as it was measured by Hall effect and Seebeck measurements. The bottom gate p-type SnOx TFTs present field-effect mobility above 1.24 cm2/Vs (including the paper p-type oxide TFT) and an on/off modulation ratio of 103 while the CuxO TFTs exhibit a field-effect mobility of 1.3×10-3 cm2/Vs and an on/off ratio of 2×102.
Multicomponent dielectrics for oxide TFT
Luís Pereira, Pedro Barquinha, Gonçalo Gonçalves, et al.
In this work we present sputtered multicomponent dielectrics based on mixtures of HfO2 and SiO2. This way it is possible to get stable amorphous structure up to 800ºC, that does not happen for pure HfO2, for instance, that present a polycrystalline structure when deposited without any intentional substrate heating. Besides, also the band gap of the resulting films is increased when compared with pure HfO2 that theoretically is an advantage in getting a suitable band offset with the semiconductor layer on oxide TFTs. Concerning the electrical characterization, the leakage current on c-Si MIS structures is low as 10-9 Acm-2 at 10 V. The amorphous structure of the films also lead to better dielectric/semiconductor interfaces, as suggested by C-V characteristics on GIZO MIS structures, which do not present strong variation with frequency. On other hand, the dielectric constant decreases due to the incorporation of SiO2 and Al2O3. Further improvement on insulating and interface characteristics is achieved using multilayer stacks and substrate bias during deposition.
Oxide-based Devices II
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Growth and characterization of Ga2O3 on sapphire substrates for UV sensor applications
Dong-Sing Wuu, Sin-Liang Ou, Ray-Hua Horng, et al.
The β-Ga2O3 films were grown on (0001) sapphire at 500 °C by metal organic chemical vapor deposition. In the analysis of crystal structure, we found that the (-201) oriented single crystal β-Ga2O3 epilayer can be obtained under low chamber pressure of 15 torr. Moreover, a metal-semiconductor-metal solar-blind deep ultraviolet photodetector was fabricated with the β-Ga2O3 epilayer. As the bias voltage is 5 V, the photodetector exhibits a relatively low dark current about 0.2 pA, which induced by the highly resistive nature of the β-Ga2O3 thin films. From the responsivity result, it can be observed that photodetector shows a maximum responsivity at 260 nm, revealing the β-Ga2O3 photodetector was really solar-blind. The responsivity of the photodetector was as high as 20.1 A/W with an applied bias of 5 V and an incident light wavelength of 260 nm. The improved performance is attributed to the high quality of β-Ga2O3 epilayer.
Trap depth optimization to improve optical properties of diopside-based nanophosphors for medical imaging
Thomas Maldiney, Aurélie Lecointre, Bruno Viana, et al.
Regarding its ability to circumvent the autofluorescence signal, persistent luminescence was recently shown to be a powerful tool for in vivo imaging and diagnosis applications in living animal. The concept was introduced with lanthanide-doped persistent luminescence nanoparticles (PLNP), from a lanthanide-doped silicate host Ca0.2Zn0.9Mg0.9Si2O6:Eu2+, Mn2+, Dy3+ emitting in the near-infrared window. In order to improve the behaviour of these probes in vivo and favour diagnosis applications, we showed that biodistribution could be controlled by varying the hydrodynamic diameter, but also the surface charges and functional groups. Stealth PLNP, with neutral surface charge obtained by polyethylene glycol (PEG) coating, can circulate for longer time inside the mice body before being uptaken by the reticulo-endothelial system. However, the main drawback of this first generation of PLNP was the inability to witness long-term monitoring, mainly due to the decay kinetic after several decades of minutes, unveiling the need to work on new materials with improved optical characteristics. We investigated a modified silicate host, diopside CaMgSi2O6, and increased its persistent luminescence properties by studying various Ln3+ dopants (for instance Ce, Pr, Nd, Tm, Ho). Such dopants create electron traps that control the long lasting phosphorescence (LLP). We showed that Pr3+ was the most suitable Ln3+ electron trap in diopside lattice, providing optimal trap depth, and resulting in the most intense luminescence decay curve after UV irradiation. A novel composition CaMgSi2O6:Eu2+,Mn2+,Pr3+ was obtained for in vivo imaging, displaying a strong near-infrared persistent luminescence centred on 685 nm, allowing improved and sensitive detection through living tissues.
Development of functional magnetic nanoparticles for biomedical application
Y. Ichiyanagi
Magnetic nanoparticles (MNPs), including those of transition metals, such as iron oxide nanoparticles, cobalt oxide nanoparticles, and ferrite nanoparticles with diameters between 3 nm and 34 nm have been developed by a wet chemical method. These MNPs were modified with the functional groups, and introduced into cancer cells. SiO2-shelled ferrite nanoparticles have been discussed for the use in hyperthermia treatments on the basis of measurements of their AC magnetic susceptibilities.
Spectroscopic determination of the flatband potential and carrier density of ZnO nanowire array with/without hydrogen plasma treatment
Miao Zhong, Alexandra Apostoluk, Etsuo Maeda, et al.
A dense array of vertical ZnO nanowires on a-plane sapphire substrate was synthesized by a simple chemical vapor deposition method. The electrolyte-based Schottky contact of the ZnO nanowires was investigated by electrochemical impedance spectroscopy. An n-type semiconductor behavior and a flat-band potential of about 0 V (0.05 V) versus Ag/AgCl electrode were obtained for the synthesized ZnO nanowires. The ~ 0 V flat-band potential is suggested to be a balanced result of (1) the Femi-level difference induced by the Schottky contact at the ZnO/electrolyte interface and (2) the oxygen vacancy induced surface adsorption effect at the ZnO nanowire surface. Hydrogen plasma treatment was carried out to passivate the oxygen vacancies in the ZnO nanowires. An obvious shift of the flat-band potential to about - 0.6 V was obtained for the same ZnO nanowire array sample after the hydrogen plasma treatment. The negative flatband potential, indicating an electron depletion region at the surface of ZnO nanowires, is observed owing to the Fermilevel difference between the n-type ZnO nanowires and the electrolyte, without a strong influence of the oxygen vacancy-related surface adsorption effect. Moreover, the carrier density in the ZnO nanowires was increased by almost four orders of magnitude after the hydrogen plasma treatment. The increase in carrier density confirms existing reports of hydrogen atoms occupying interstitial sites in the ZnO nanowires in addition to the oxygen vacancies after the hydrogen plasma treatment.
Superconductivity
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Atomic-layer engineering of oxide superconductors
A. T Bollinger, J. N. Eckstein, G. Dubuis, et al.
Molecular beam epitaxy technique has enabled synthesis of atomically smooth thin films, multilayers, and superlattices of cuprates and other complex oxides. Such heterostructures show high temperature superconductivity and enable novel experiments that probe the basic physics of this phenomenon. For example, it was established that high temperature superconductivity and anti-ferromagnetic phases separate on Ångström scale, while the pseudo-gap state apparently mixes with high temperature superconductivity over an anomalously large length scale (the "Giant Proximity Effect"). We review some recent experiments on such films and superlattices, including X-ray diffraction, atomic force microscopy, angle-resolved time of flight ion scattering and recoil spectroscopy, transport measurements, highresolution transmission electron microscopy, resonant X-ray scattering, low-energy muon spin resonance, and ultrafast photo-induced reflection high energy electron diffraction. The results include an unambiguous demonstration of strong coupling of in-plane charge excitations to out-of-plane lattice vibrations, a discovery of interface high temperature superconductivity that occurs in a single CuO2 plane, evidence for local pairs, and establishing tight limits on the temperature range of superconducting fluctuations.
XAS study of superconducting thin film single crystals without doping
H. Oyanagi, A. Tsukada, M. Naito
The electronic state of T'-type cuprates which exhibit superconductivity without doping was investigated by polarized x-ray absorption spectroscopy (XAS) for T'-La2CuO4 and T'-(La,Y)2CuO4 thin film single-crystals. The effect of oxygenation and deoxygenation on the near-edge structures evidences the two processes create and remove apical oxygen defects that strongly suppress superconductivity. The near-edge spectra further indicate that the deoxygenation, well known as a common prerequisite for superconductivity, also creates in-plane oxygen defects, whose contribution to the n-type conduction and superconductivity without doping is not ruled out. The observed local lattice distortion consistent with the neutron scattering experiment may influence a long-range magnetic order favoring a metallic state without doping.
Growth, Properties, and Applications of Nanostructures
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Morphological effects on optical and electrical properties of ZnO nanostructures
Sang Hyun Lee, Jun Xu
Morphology control of semiconductor nanostructures is of great interest in recent years owing to their unique capabilities in achieving desired chemical and physical properties as well as enabling great potential in electronic and optoelectronic applications. In this paper, we review our recent study on morphological control of ZnO nanocones and how the optical and electrical properties of such nanostructure-based photovoltaic solar cells are affected. The nanocone shape is obtained by altering the ratio of oxygen to argon gas during thermal chemical vapor deposition. The nanocones grown on Si substrates show antireflective properties in a broad spectral range. We further found that incident light was confined in the nanocones, which enhances the antireflective properties through multi-reflection/absorption. The performance dependency of a ZnO-CdTe solar cell on the morphology of ZnO was explored by introducing the nanocones. Small junction area and strong electric field at the tip of nanocones contribute to effective charge transport across the heterojunction, resulting in improved the conversion efficiency of solar cells.
Influence of hydrothermal treatment on morphology and properties of ZnO nanostructures
Xinyi Chen, Alan M. C. Ng, Aleksandra B. Djurišić, et al.
Hydrothermal treatment at low temperature (~150 ºC) was performed on different ZnO nanostructures grown on different substrates such as Si and ITO/glass. The water vapor environment and the high pressure of water vapor at that temperature were expected to improve the optical properties of ZnO nanorods. Under such mild condition, no significant morphology changes are expected. However, significant changes were observed in both morphology and optical properties. The morphology of ZnO nanostructures has been changed in various ways, depending on the growth conditions of ZnO nanostructures as well as the substrates. In the terms of optical properties, the increase of photoluminescence (PL) intensity was observed and the UV to visible ratio of ZnO PL spectra was also improved in some cases, while in others there has been no improvement. Hydrothermal treatment was also performed on zinc oxide precursor on Si substrates to study the possible Si contamination during the procedure. Zinc nitrate solution in ethanol was used as the precursor. As a result, an increase of the intensity of the broad peak around 430 nm which is related to silicon oxide was obtained.
Engineered ZnO nanowire arrays using different nanopatterning techniques
János Volk, Zoltán Szabó, Róbert Erdélyi, et al.
The impact of various masking patterns and template layers on the wet chemically grown vertical ZnO nanowire arrays was investigated. The nanowires/nanorods were seeded at nucleation windows which were patterned in a mask layer using various techniques such as electron beam lithography, nanosphere photolithography, and atomic force microscope type nanolithography. The compared ZnO templates included single crystals, epitaxial layer, and textured polycrystalline films. Scanning electron microscopy revealed that the alignment and crystal orientation of the nanowires were dictated by the underlying seed layer, while their geometry can be tuned by the parameters of the certain nanopatterning technique and of the wet chemical process. The comparison of the alternative nanolithography techniques showed that using direct writing methods the diameter of the ordered ZnO nanowires can be as low as 30-40 nm at a density of 100- 1000 NW/μm2 in a very limited area (10 μm2-1 mm2). Nanosphere photolithography assisted growth, on the other hand, favors thicker nanopillars (~400 nm) and enables large-area, low-cost patterning (1-100 cm2). These alternative lowtemperature fabrication routes can be used for different novel optoelectronic devices, such as nanorod based ultraviolet photodiode, light emitting device, and waveguide laser.
Poster Session
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Photoconductivity and photocatalytic activity of ZnO thin films grown via thermal oxidation
J. C. Moore, R. Louder, L. R. Covington, et al.
We have investigated the photoconductivity and photocatalytic properties of polycrystalline zinc oxide (ZnO) films grown on c-plane sapphire substrates. Zinc-metal films where grown on sapphire substrates via dc-sputter deposition at room temperature with subsequent thermal annealing in air at 300°C, 600°C, 900°C, and 1200°C. Photoluminescence spectra indicate four emission bands: excitonic ultraviolet, blue, and deep-level green and yellow emission. The ratio of deep-level green emission to UV excitonic emission was observed to decrease with decreasing annealing temperature from 1200°C to 300°C. Metal-semiconductor-metal (MSM) Al:ZnO:Al planar UV photodetectors where fabricated via sputter deposition of aluminum ohmic contacts on the resulting ZnO films. Decreasing photocurrent is seen for increasing annealing temperature, which is consistent with PL studies. A responsivity of ~0.1 A/W was observed. Photocatalytic activity follows half-order reaction kinetics as determined by photodegradation of Rhodamine B.
Analysis of peculiarities of ZnO microfilms luminescence
M. V. Ryzhkov, S. I. Rumyantsev, V. M. Markushev, et al.
In luminescence spectra of certain ZnO films there is a second band in the near-ultraviolet region alongside with an exciton band. With the increasing of pumping this band intensity increases much faster than the intensity of the exciton band. It is shown that the above second band is not the so called P-line. It is rather connected with shallow level in ZnO structures. We suggest interpreting the observed effect with a due account of Burstein-Moss effect. For approximate modeling of the observed process rate equation system has been formulated. It is demonstrated that it is possible to find such parameters so that the relevant numerical solution gives a dependence of bands emission intensity ratio on pumping power that simulates quality wise experimental results.
Energy harvesting from millimetric ZnO single wire piezo-generators
D. J. Rogers, C. Carroll, P. Bove, et al.
This work reports on investigations into the possibility of harvesting energy from the piezoelectric response of millimetric ZnO rods to movement. SEM & PL studies of hydrothermally grown ZnO rods revealed sizes ranging from 1 - 3 mm x 100 - 400 microns and suggested that each was a wurtzite monocrystal. Studies of current & voltage responses as a function of time during bending with a probe arm gave responses coherent with those reported elsewhere in the literature for ZnO nanowires or micro-rod single wire generators. The larger scale of these rods provided some advantages over such nano- and microstructures in terms of contacting ease, signal level & robustness.
ZnO nanorod electrodes for hydrogen evolution and storage
S. Harinipriya, B. Usmani, D. J. Rogers, et al.
Due to the attractive combination of a relatively high specific heat of combustion with a large specific energy capacity, molecular hydrogen (H2) is being investigated for use as an alternative to fossil fuels. Energy-efficient H2 production and safe storage remain key technical obstacles to implementation of an H2 based economy, however. ZnO has been investigated for use as an alternative photocatalytic electrode to TiO2 for solarpowered photo-electro-chemical (PEC) electrolysis, in which H2 is generated by direct water splitting in a cell with a metal cathode and a semiconducting anode. In this investigation, ZnO NR grown on Si (100) substrates by pulsed laser deposition were investigated for use as electrodes in the Hydrogen Evolution Reaction (HER). The electrochemical potential and Fermi energy of the ZnO NR were estimated from the electrochemical current density in acid and alkaline solutions via phenomenological thermodynamic analysis. As well as acting as an effective electrocalytic cathode, the ZnO NR appear to operate as a hydrogen reservoir. These results indicate that the ZnO NR have excellent potential for the storage of evolved H2.