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UV-ozone cleaning prior to metal deposition of either e-beam Pt contacts or sputtered W contacts on n-type single-crystal ZnO is found to significantly improve their rectifying characteristics. Pt contacts deposited directly on the as-received ZnO surface are Ohmic but show rectifying behavior with ozone cleaning. The Schottky barrier height of these Pt contacts was 0.70 eV, with ideality factor of 1.5 and a saturation current density of 6.2 × 10−6 A·cm−2. In contrast, the as-deposited W contacts are Ohmic, independent of the use of ozone cleaning. Post-deposition annealing at 700 °C produces rectifying behavior with Schottky barrier heights of 0.45 eV for control samples and 0.49 eV for those cleaned with ozone exposure. The improvement in rectifying properties of both the Pt contacts is related to removal of surface carbon contamination from the ZnO.
Variable size nanocluster embedded in silicon substrate were obtained by low energy implantation methods. We used optical spectroscopy to measure the optical properties of the implanted samples. The implantation parameters like the ions energy, dose and sputtering rate were calculated with SRIM . Most of the implanted Zn ions (83%) clustered and oxidized during the implantation process, with the remaining 17% being oxidized during annealing in air.
GaNyAs1-x-yBix alloy lattice-matched to GaAs has been grown by molecular beam epitaxy (MBE). The lattice-matching of GaNyAs1-x-yBix to GaAs was investigated by X-ray diffraction measurements on a series of GaNyAs1-x-yBix with various GaN molar fractions. GaNyAs1-x-yBix lattice-matched to GaAs was obtained, which was confirmed by its diffraction peak overlapped with the peak of GaAs. Photoluminescence (PL) of 1.3 μm was observed from GaNyAs1-x-yBix epilayer matched to GaAs at room temperature. The temperature coefficient of the PL peak energy in a temperature range 150–300K for GaNyAs1-x-yBix was 1/3 of InGaAsP with a bandgap corresponding to 1.3-μm emission. Both lattice-matching to GaAs and bandgap adjustment to 1.3-μm waveband were achieved for GaNyAs1-x-yBix for the first time. This alloy will lead to the fabrication of laser diodes with an emission of temperature insensitive wavelength.
ZnO is a very promising material for spintronics applications, with many groups reporting room temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during PLD, we find an inverse correlation between magnetization and electron density as controlled by Sn doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for the ferromagnetism include the bound magnetic polaron model or exchange is mediated by carriers in a spin-split impurity band derived from extended donor orbitals. We will also review progress in ZnO nanowires. The large surface area of nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for micro-lasers or memory arrays. Single ZnO nanowire depletion-mode metal-oxide semiconductor field effect transistors exhibit good saturation behavior, threshold voltage of ∼-3V and a maximum transconductance of 0.3 mS/mm. Under UV illumination, the drain-source current increased by approximately a factor of 5 and the maximum transconductance was ∼ 5 mS/mm. The channel mobility is estimated to be ∼3 cm2 /V.s, comparable to that for thin film ZnO enhancement mode MOSFETs and the on/off ratio was ∼25 in the dark and ∼125 under UV illumination. Pt Schottky diodes exhibit excellent ideality factors of 1.1 at 25 °C, very low reverse currents and a strong photoresponse, with only a minor component with long decay times thought to originate from surface states. In the temperature range from 25–150 °C, the resistivity of nanorods treated in H2 at 400 °C prior to measurement showed an activation energy of 0.089 eV and was insensitive to the ambient used. By contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors. We have also made sensitive pH sensors using single ZnO nanowires.
Ultrathin silicon-on-insulator (UTSOI) technology1 has emerged as a key candidate for sub-100nm gate length CMOS devices. Recent experiments have characterized MOSFETs with silicon channels as thin as 1nm (four atomic layers of silicon),2,3 and found them to be well-behaved electrically. Quantum effects are important to the electron transport in such devices, and the penetration of the electron wavefunction into the gate oxide introduces new scattering mechanisms. We introduce here a novel method for first-principles calculation of electron mobilities in ultrathin SOI channels, including surface roughness and defect scattering. The electronic structure and scattering potentials are calculated with Density Functional Theory in the Local Density Approximation (DFT-LDA), and the mobility is calculated through Green's functions. The method requires little computational effort beyond that of the DFT-LDA calculations, and allows the calculation of temperature- and carrier concentration-dependent mobilities. Since the silicon-oxide interface is treated at the atomic-scale, the mobility contributions of different defects (e.g. suboxide bonds, oxide protrusions) and impurities (e.g. nitrogen, hydrogen) can be calculated separately, giving a precise physical picture of channel electron transport.
A theoretical and experimental investigation of electronic band structure (Γ-point) of strain balanced GaAs1-xNx/InAs1-xNx short period superlattice on InP is performed. A six-band Kane Hamiltonian and band anti-crossing models, modified for the strain effects are used to describe the electronic states of the highly strained zincblende GaAs1-xNx and InAs1-xNx ternaries. Operating wavelengths of these heterostructures are predicted to extend beyond 2 μm. Preliminary photoluminescence results of the chemical beam epitaxially grown sample are shown to be consistent with the theoretical predictions.
In this paper, we propose the design and fabrication of buried silicon optical interconnect technology, the sub-surface silicon optical bus (S3B). The proposed approach relies on engineering the dispersion properties of embedded silicon three-dimensional photonic crystals to create sub-micron routing channels and control light propagation. Further, we present a method for the fabrication of buried three-dimensional (3D) photonic-crystal structures using conventional planar silicon micromachining. The method utilizes a single planar etch mask coupled with time-multiplexed, sidewall-passivating, deep anisotropic reactive-ion etching, to create an array of spherical voids with three-dimensional symmetry. Preliminary results are presented that demonstrate the feasibility of realizing chip-scale optical interconnects using our proposed approach.
The fabrication of ZnO coated ZnS:Mn2+ nanoparticles were achieved using simple methods. ZnS:Mn2+ nanoparticles were prepared by a mechanical milling method. Coating of ZnO was then performed using a simple chemical method. Structural properties were evaluated by the X-ray powder diffraction (XRD) and the transmission electron microscope (TEM). Optical properties of the ZnO coated ZnS:Mn2+ nanoparticle were characterized by exciting the particle included solvent using an UV-LED (400 nm). Bright orange color fluorescence was observed, and the fluorescence intensity of ZnO coated ZnS:Mn2+ nanoparticles was enhanced as compared to uncoated ZnS:Mn2+ nanoparticles. The capping of the core surface has probably terminated surface defects of ZnS:Mn2+ nanoparticles, and resulted in the improved fluorescence intensity.
Zinc oxide (ZnO) is an interesting material for short-wavelength optoelectronics due to its wide band gap. The nanostructures of ZnO are also intriguing since a variety of morphology can be obtained by employing different processing parameters. In our laboratory, ZnO nanonails were successfully synthesized at low temperature using a thermal chemical vapor deposition. The morphology of the sample was studied by using scanning electron microscopy. The shape of the nail head can be controlled from hexagon to quasi-circular shape. X-ray diffraction, Raman scattering, photoluminescence spectroscopy were also performed to analyze the ZnO nanonail. Photoluminescence spectroscopy suggested that the defects in the ZnO nanonail and nanobone are of different nature.
Defect structures in ZnO thin films were studied to clarify the mechanism of charge compensation and crystallinity degradation induced by alloying. Regarding the undoped ZnO films, it was indicated that the degree of non-equilibrium behavior in the films deposited by PLD was much less than in the films prepared by the other two methods, i.e., MBE and sputtering, and, moreover, the solid-state diffusion behavior in the PLD-grown undoped ZnO was close to that of bulk ZnO. The heavily Al-doped films and alloy films with high concentrations of MgO exhibited significant non-equilibrium behavior, even for those prepared by PLD. It was indicated that the high concentration of extrinsic elements, e.g., Al and Mg, introduces non-equilibrium defects into ZnO films and those defects are the cause of the crystallinity degradation and thermal instability of the films.
We present a Raman study of pseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y grown on GaAs(100) with y ranging from 0 to 0.06. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes for y = 0.06 are represented by the strong longitudinal optic (LO1) mode at 287.4 cm-1 and the weaker transverse optic (TO1) mode at 269.0 cm-1, while the GaN-like LO2 mode is observed at 475.6 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration in the alloy increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the linearly increasing GaN-like LO2 phonon band deviates from linearity at higher nitrogen concentration (y ≥ 0.03). The reason for the deviation of the LO2 phonon from linearity is discussed.
Co-doped ZnS:(Mn, Si) films were fabricated. The ZnS was synthesized by a low-pressure thermal chemical vapor deposition. Metal Zn vapor and H2S gas were used as the CVD-precursors. Mn and Si were doped using a laser ablation technique during the ZnS growth. A solid MnSi alloy (Mn:Si = 1:1) was used as the laser ablation target. The films were deposited at the range from 650°C to 750°C. At the deposition temperature of 650°C, only a EL emission peak at 585 nm same as conventional ZnS:Mn films appeared, i.e., the Si co-doping had no effects on the EL spectrum. At the deposition temperature of 700°C, the Si co-doping to ZnS:Mn film caused the shift of the EL emission peak at 585 nm to shorter wavelength by 15 nm and provided new EL emission at 760 nm. The film deposited at 750°C exhibited new UV and blue EL emissions at 390 nm and 450 nm, respectively, although the host material of the film differed from usual ZnS.
Photoluminescence and absorption spectroscopy experiments were performed on as grown and thermally annealed GaAs1-xNx with nitrogen content in the range of 0.75–7.1%. At low temperature, the photoluminescence spectra exhibits two set of features: (i) a relatively broad peak at low energy and near to the vicinity of the predicted band gaps and (ii) a sharp excitonic feature at higher energy (about 100 meV for x>4%). Post growth thermal annealing processes systematically favor stronger excitonic emissions, and a notable intensity reduction of the deeper (defect related) luminescence. The low temperature binding energy of the higher energy excitonic peak is found to be consistent with the increase of the electronic effective masses. A careful examination of the data obtained in this work suggests that for higher nitrogen content (x>4%), the fundamental band gap of GaAsN is located at significantly higher energies than those commonly accepted for these alloys.
Quantum statistical physics methods  relate charge transport properties of small atomic clusters (or small quantum dots, QDs) to their equilibrium electronic energy level spectra. Thus, electronic energy level computations for such systems provide a foundation for realization of a virtual (i.e., fundamental theory- based, computational) approach  to synthesis of sub-nanoscale materials with pre-designed charge transport properties. In this publication the Hartee-Fock (HF) electronic energy level spectra of several pre-designed small clusters of Ga, As, In and P atoms are studied and compared to those of the corresponding clusters grown at spatially unrestricted conditions. Influence of clusters' growth conditions on formation and structure of their valence and conduction bands is discussed.
In order to develop nanoengineering methods to control electronic spectrum of self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy, we have utilized atomic force microscopy (AFM), photoluminescence (PL) and TEM methods to investigate the effects of capping layer growth on the physical/chemical properties as well as the optical/electronic performance of QD device structures. Capping layer material choice (or its absence all together) has been found to directly influence QD dimensions (size, height), and subsequently, to affect QD emission wavelength. We report results of QD lateral size and height as well as densities of InAs QDs capped with 2ML (monolayers) of AlAs or GaAs grown at various rates. Our AFM results are complemented by PL measurements, where the optical properties of capped versus non-capped QDs have been explored and direct correspondence between structural differences induced by capping and the electronic/optical properties of QDs is demonstrated. Analysis of the data shows that the results can be explained by two competing surface processes. The first of which is the redistribution of indium between QDs on top of the 2D wetting layer, resulting in the increase of QD size with time. The second effect is the diffusion of indium out of the QDs and onto the top of the capping layer. TEM with multislice image simulation has supported our AFM and PL observations with the demonstration of “indium driven” alloy intermixing in the overlayer as well as significant alloying in the InAs wetting layer.
There is considerable current interest in the synthesis of metal chalcogenide nanostructured materials1 especially for the manufacture of so called 3rd generation solar cells. The facile, large scale, synthesis of such materials is critical to enabling such technology. The synthesis of these materials, especially those of cadmium, has been widely discussed in the literature. However, whilst routes involving pyrophoric materials give high quality particles and structures2, their inherent reactivity results in complications in handling. Although the use of acetates has already been shown to give good results,3 there are, in general, problems in the synthesis of tellurium containing materials.4
This paper describes a new method providing a general synthesis of metal chalcogenide nanomaterials in a TOP/TOPO reaction system involving easy-to-handle reagents. Results for cadmium will for the basis of the discussion, which will include examples from a wider range of metals. The use of cadmium acetate in TOP and solutions of chalcogenides in TOP in the presence of suitable reducing agents provides an exceptionally reactive system. The system is flexible and may be applied to a wider range of chalcogenide based nanomaterials.
In recent years, ZnO has been proposed for new electronic and optoelectronic devices, such as transparent transistors and UV light-emitting diodes (LEDs). The LED application will require both n-type and p-type ZnO, but the latter is difficult to produce, and progress in this area will require a detailed knowledge of the various impurities and defects that affect the electrical and optical properties. The dominant donors in as-grown ZnO are usually thought to be interstitial H and substitutional AlZn, with activation energies of about 40 and 65 meV, respectively. However, interstitial Zn and its associated complexes may also contribute free electrons. The dominant acceptor, at least in vapor-phase-grown material, is the Zn vacancy; however, substitutional NO is also present, although sometimes passivated by H. To produce p-type ZnO, it is necessary to dope with acceptor-type impurities, and some success has been achieved with N, P, As, and Sb. However, only N has been proven to have simple substitutional character (NO), and more complicated acceptor structures, such as AsZn-2VZn, have been proposed for some of the other group V elements. Both homostructural and heterostructural UV LEDs have been fabricated, although not of high luminescent power so far. The main objective of this paper is to review the Hall-effect and photoluminescence results on n-type and p-type ZnO.
We report a photoreflectance (PR) characterization of InP/GaAsSb double-heterojunction bipolar transistor (DHBT) epitaxial wafers grown by metal-organic vapor-phase epitaxy (MOVPE). The origin of the Franz-Keldysh oscillations (FKOs) in the PR spectra was identified by step etching of the samples. FKOs from the InP emitter region were observed in the wafer with low recombination forward current at the emitter-base (E/B) heterojunction. In contrast, they did not appear when recombination current was dominant. The absence of the FKOs from the emitter indicates the high concentration of the recombination centers at the E/B heterojunction. We have also measured PR spectra from InAlP/GaAsSb/InP DHBT wafers. Pronounced FKOs from InAlP emitter reflect the suppression of recombination at E/B heterojunctions.
In this letter we present results on the growth of InAs nanowires (NW's) on InGaAs lattice-matched to (100) InP substrates by Chemical Beam Epitaxy. We observed that the nanostructure stability depends on the thickness of the InGaAs layer. This effect may result from two different conditions: the nanostructure strain field depth and/or compositional modulation in the buffer layer. Our investigation shows that anisotropic strain relaxation for nanowires grown on InGaAs is faster than for those grown on InP but the elastic energy in the nanostructures is no different from the InAs/InP case. These results suggest that the InAs strain relaxation does not depend significantly on the InGaAs buffer layer thickness. Nevertheless, transmission electron microscopy images show an additional stress field superimposed on that usually observed for the InAs nanostructures, which is attributed to compositional modulation in the ternary layer.
Nanoparticles of ZnO were synthesized using a sonochemical technique. Sonochemistry arises from an acoustic cavitation phenomenon, that is, the formation, growth and implosive collapse of bubbles in a liquid medium. The ultraviolet photoluminescence (PL) studies of the samples showed a strong PL intensity and a significant blue shift relative to the PL of the bulk. Shifts up to 70 meV were observed and attributed to a confinement effect. Scanning electron microscopy indicated roughly spherical particles, ∼160 nm in diameter. However, nano-platelets and rods were observed in transmission electron micrographs. Preliminary electrical measurements indicated a highly resistive nature of the nanoparticulate material.