Microstructures and optical properties of Cu-doped ZnO films prepared by radio frequency reactive magnetron sputtering

Pure and Cu-doped ZnO (ZnO:Cu) thin films were deposited on glass substrates using radio frequency (RF) reactive magnetron sputtering. The effect of substrate temperature on the crystallization behavior and optical properties of the ZnO:Cu films have been studied. The crystal structures, surface morphology and optical properties of the films were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and a fluorescence spectrophotometer, respectively. The results indicated that ZnO films showed a stronger preferred orientation toward the c-axis and a more uniform grain size after Cu-doping. As for ZnO:Cu films, the full width at half maxima (FWHM) of (0 0 2) diffraction peaks decreased first and then increased, reaching a minimum of about 0.42° at 350 °C and the compressive stress of ZnO:Cu decreased gradually with the increase of substrate temperature. The photoluminescence (PL) spectra measured at room temperature revealed two blue and two green emissions. Intense blue-green luminescence was obtained from the sample deposited at higher substrate temperature. Finally, we discussed the influence of annealing temperature on the structural and optical properties of ZnO:Cu films. The quality of ZnO:Cu film was markedly improved and the intensity of blue peak (∼485 nm) and green peak (∼527 nm) increased noticeably after annealing. The origin of these emissions was discussed.     

Structural and optical properties of Au-implanted ZnO films

The structural and luminescence related optical behaviours of Au ion implanted ZnO films grown by magnetic sputtering and their post implantation annealing behaviours in the temperature range of 100-700 °C have been investigated. Optical absorption and transmittance spectra of the films indicate that band edge of Au-implanted ZnO has shifted to high energy range and optical band gap has increased, because the sharp difference of thermal expansion induces the lattice mismatch between ZnO and SiO₂. PL spectra reveal that UV and visible luminescence bands of ZnO films can be improved after thermal annealing due to recovery of defects and Au ions incorporation. Importantly, green luminescence band of 530 nm has been only observed in the Au-implanted and subsequently annealed ZnO films and it enhances with the increasing annealing temperature, which can be related to Au atoms or clusters in ZnO films. Furthermore, X-ray photoelectron spectroscopy measurements reveal that the Au⁰ is dominant state in Au implanted and annealed ZnO films. Possible mechanisms, such as optical transitions of Au atoms or clusters and deep level luminescence of ZnO, have been proposed for green emission.     

Au deposits on graphite: On the nature of high temperature desorption peaks in CO thermal desorption spectra

Due to the discovery of Au as a catalyst for low temperature CO oxidation, the adsorption of CO on Au surfaces has attracted a lot of attention recently. On stepped and rough single crystal surfaces as well as on deposited particles two characteristic desorption states above 100 K have been observed via TPD. We have studied Au deposits on graphite in order to elucidate the nature of these desorption peaks in more detail. For this purpose, Au was deposited at 100 K and 300 K on HOPG as a weakly interacting support. In analogy to other supports, we obtain two desorption states (∼140 K and ∼170 K) whose relative intensities depend strongly on the deposition temperature with the high temperature peak being much more pronounced for the 100 K deposits. After annealing to 600 K, both states drastically lose intensity. XP spectra, on the other hand, show virtually no decrease of the Au 4f intensity as would be expected for desorption or significant changes of the particle morphologies. We conclude that both desorption peaks are defect-related and connected with under-coordinated Au atoms that are lost for the most part upon annealing. These sites could be located at the perimeter of dendritic islands or on small, defect-rich particles in addition to larger particles not adsorbing CO at 100 K. Preliminary STM results are in favour of the second interpretation.     

Oxidation studies of niobium thin films at room temperature by X-ray reflectivity

We report the results of growth kinetics of oxidation process on niobium thin film surfaces exposed to air at room temperature by using a surface sensitive non-destructive X-ray reflectivity technique. The oxidation process follows a modified Cabrera-Mott model of thin films. We have shown that the oxide growth is limited by the internal field due to the contact potential which develops during the initial stage of oxidation. The calculated contact potential for 100 and 230 Å thick films is 0.81 ± 0.14 and 1.20 ± 0.11 V respectively. We report that 40% increase in the contact potential increases the growth rate for the first few mono layers of Nb₂O₅ from ∼2.18 to ∼2790 Å/s. The growth rates of oxidation on these samples become similar after the oxide thicknesses of ∼25 Å are reached. We report on the basis of our studies that a protective layer should be grown in situ to avoid oxidation of Nb thin film surface of Nb/Cu cavities.     

Annealing effects on the plasmonic excitations of metal/metal interfaces

The effects of the annealing procedure at 400-450 K on the electronic properties of nanoscale thin films of Ca, Au and Ag grown on Cu(1 1 1) at room temperature were probed by high-resolution electron energy loss spectroscopy measurements. Ca surface plasmon underwent to a significant red-shift upon annealing, due to the oxidation of the topmost Ca layer. Water strongly interacted with the CaO interface at room temperature. Au surface plasmon disappeared upon annealing the gold film, as a consequence of the formation of an Au-Cu alloy. Ag surface plasmon red-shifted both in the annealed adlayer and with increasing temperature compared with the frequency recorded for the as-deposited silver film.     

Interfacial differences in enhanced schottky barrier height Au/n-GaAs diodes deposited at 77 K

The use of cryogenic temperatures (∼77 K) during Au Schottky contact deposition onto n-GaAs produces an increase in barrier height from 0.73 eV for room temperature diodes to 0.82 eV. Not all Schottky metals show this enhancement—for example Pt and Ti do not show any significant change in barrier height whereas Au, Pd and Ni show increases between 7 and 18%. We used X-ray reflectivity to show that the main difference between Au deposited at 77 K and room temperature is a decreased metal roughness while the interfacial roughness between the Au and GaAs is basically the same. As the diodes are annealed to 300 °C both the difference in barrier height and interfacial roughness is lost. This is a simple method with potential for improving the performance of GaAs metal-semiconductor-field-effect-transistors (MESFETs).     

Structure and electronic properties of gold adsorbed on Ti(0 0 0 1)

The Au/Ti(0 0 0 1) adsorption system was studied by low energy electron diffraction (LEED) and photoemission spectroscopy with synchrotron radiation after step-wise Au evaporation onto the Ti(0 0 0 1) surface. For adsorption of Au at 300 K, no additional superstructures were observed and the (1 × 1) pattern of the clean surface simply became diffuse. Annealing of gold layers more than 1 ML thick resulted in the formation of an ordered Au-Ti surface alloy. Depending on the temperature and annealing time, three surface reconstructions were observed by LEED: (√3 × √3) R30°, (2 × 2) and a one-dimensional incommensurate (√3 × √3) rectangular pattern. The Au 4f core level and valence band photoemission spectra provided evidence of a strong chemical interaction between gold and titanium. The data indicated formation of an intermetallic interface and associated valence orbital hybridization, together with diffusion of gold into the bulk. Au core-level shifts were found to be dependent on the surface alloy stoichiometry.     

Influence of Li dopants on thermoluminescence spectra of CaSO4 doped with Dy or Tm

Thermoluminescence emission spectra are presented for lithium doped variants of CaSO₄:Dy or CaSO₄:Tm dosimetry material. All three dopants (Li, Dy and Tm) variously introduce different changes in both the glow peak temperatures and the luminescence efficiency. In every case the emission signals display the line emission characteristic of the rare earth ions. At temperatures below ∼50 K the relative peak intensities differ for Dy and Tm doped samples, and there are small temperature shifts between the Dy:Li and Tm:Li co-doped materials. Above room temperature the rare earth ions do not show peak temperature movements when co-doped with lithium. However they do influence the peak temperature by ∼5 °C when they are the sole dopant. Inclusion of lithium dramatically moves the high temperature glow peak from ∼200 °C down to 120 °C. All these changes are consistent with a single defect model in which the trapping sites and luminescence occur within the complexes formed of the rare earth ion, an intrinsic sulphate defect and lithium. The evidence and rationale for such a model are presented. There is discussion which suggests that such defect complexes are the norm in thermoluminescence.     

The effects of annealing on Au/pyronine-B/MD n-InP Schottky structure

Thin film of non-polymeric organic compound pyronine-B has been fabricated on moderately doped (MD) n-InP substrate as an interfacial layer using spin coating technique for the electronic modification of Au/MD n-InP Schottky contact. The electrical characteristics have been determined at room temperature. The barrier height and the ideality factor values for Au/pyronine-B/MD n-InP Schottky diode have been obtained from the forward bias I-V characteristics at room temperature as 0.60 eV and 1.041; 0.571 and 1.253 eV after annealing at 100 and 250 °C, respectively. An increase in annealing temperature at the Au/n-InP Schottky junction is shown to increase the reverse bias leakage current by about one order of magnitude and decrease the Schottky barrier height by 0.027 eV. Furthermore, the barrier height values for the Au/pyronine-B/MD n-InP Schottky diode have also been obtained from the C-V characteristics at room temperature as 1.001 and 0.709 eV after annealing at 100 and 250 °C, respectively. Finally, it was seen that the diode parameters changed with increase in the annealing temperature.     

Characterization of Cu additive FePt-C granular films

FePt-C granular films doped with different Cu atomic fractions (x_Cu) were fabricated using facing-target sputtering at room temperature and subsequently annealed at 650 °C. Structural analyses reveal that the as-deposited films are in amorphous state. Appropriate Cu addition (x_Cu = 14) can improve the ordering of L1₀ FePt phase, and excessive Cu doping destroys the formation of ordered L1₀ phase with the appearance of Fe₃C and CuPt phases. Besides, preferential graphitization of amorphous carbon (a-C) occurs near large metal particles upon annealing. Annealing turns the as-deposited superparamagnetic films into ferromagnetic associated with coercivity peaks at x_Cu = 14, drops from ∼11.2 kOe at 5 K to ∼7.2 kOe at 300 K in a 50 kOe field.     

Carbon monoxide adsorption on the metastable fcc-Co/Cu(1 0 0) surface

The adsorption properties of CO on the epitaxial five-monolayer Co/Cu(1 0 0) system, where the Co overlayer has stabilized in the metastable fcc-phase, are reported. This system is known to exhibit metallic quantum well (MQW) states at energies 1 eV or greater above the Fermi level, which may influence CO adsorption. The CO/fcc-Co/Cu(1 0 0) system was explored with low energy electron diffraction (LEED), inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Upon CO adsorption, a new feature is observed in IPE at 4.4 eV above E_F and is interpreted as the CO 2π^∗ level. When adsorbed at room temperature, TPD exhibits a CO desorption peak at ∼355 K, while low temperature adsorption reveals additional binding configurations with TPD features at ∼220 K and ∼265 K. These TPD peak temperatures are correlated with different C-O stretch vibrational frequencies observed in the IR spectra. The adsorption properties of this surface are compared to those of the surfaces of single crystal hcp-Co, as well as other metastable thin film systems.     

The kinetics of exciton photoluminescence in mercuric iodide

Time-resolved photoluminescence (TRPL) of red mercuric iodide single crystal is measured at low temperatures and its two-photon luminescence is measured at room temperature. Sharp near band-gap luminescence is observed around 530 nm and was ascribed to radiative annihilation of free and bound excitons; the phonon replica of exciton luminescence are found between 533 and 540 nm at low temperatures. TRPL experiment reveals that near band-gap luminescence comprises fast and slow decay components and shows the different relaxation processes between free and bound exciton annihilation. Luminescence of bound excitons steeply lowers with increasing temperature and disappears about 40 K. A luminescence tail band is observed around 540 nm that is ascribed to defects in the anion sublattice. The temporal behavior of the tail band is described by rate equations very well. A broad luminescent band appears at 630 nm. The decay curves suggest that the luminescence is ascribed to the radiative recombination of donor-acceptor pairs and there are two kinds of mechanisms to control the decay. At room temperature, a luminescent band appears at the band-gap region, which shows the band-gap at room temperature is about 2.125 eV.     

Buffer effects of Ag layers on magneto-optical Co/Ge(1 0 0) ultrathin films

Magnetic properties of the Co/Ag/Ge(1 0 0) films grown at room temperature and 200 K were studied by the surface magneto-optical Kerr effect (SMOKE). More than 1.5 monolayer Ag buffer layers not only effectively block the interdiffusion between the capped Co layers and the Ge(1 0 0) substrate but also stabilize the magnetic phase. The temperature and thickness dependence on coercivity measurements show that interactions upon the interfaces are strongly correlated to the microstructures.     

Thermal reaction and desorption behaviors of 2,5-diiodothiophene on clean and passivated Au surfaces

Thermal reactions and desorption behaviors of 2,5-diiodothiophene on Au were studied with temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Diiodo-substituted hetero-cyclic molecules are an important precursor molecule for photochemical production of conjugated polymers. This paper describes the surface reactions and multilayer structure of 2,5-diiodothiophene in the absence of photon irradiation. 2,5-Diiodothiophene adsorbs molecularly on Au at 100 K. At 200-300 K, the C-I bond of the molecule dissociates producing atomic iodine. The C-I bond cleavage appears to induce further dissociation of the thiophene ring structure. The iodine species desorb at 600-750 K from the surface. The dissociated carbon and sulfur remain on the Au surface even at 800 K. The desorption of thin multilayers occurs at ∼220 K. During the desorption of these layers, a clustering process seems to occur. The desorption of thick multilayer occurs at ∼235 K.     

Structural phase transitions in Au thin films on Si (1 1 0): An in situ temperature dependent transmission electron microscopy study

We present a review on the formation of gold silicide nanostructures using in situ temperature dependent transmission electron microscopy (TEM) measurements. Thin Au films of two thicknesses (2.0 nm and 5.0 nm) were deposited on Si (1 1 0) substrate under ultra-high vacuum (UHV) conditions in a molecular beam epitaxy (MBE) system. Also a 2.0 nm thick Au film was deposited under high vacuum condition (with the native oxide at the interface of Au and Si) using thermal evaporation. In situ TEM measurements (for planar samples) were made at various temperatures (from room temperature, RT to 950 °C). We show that, in the presence of native oxide (UHV-MBE) at the interface, high aspect ratio (≈15.0) aligned gold silicide nanorods were observed. For the films that were grown with UHV conditions, a small aspect ratio (∼1.38) nanogold silicide was observed. For 5.0 nm thick gold thin film, thicker and lesser aspect ratio silicides were observed. Selected area diffraction pattern taken at RT after the sample for the case of 5.0 nm Au on Si (1 1 0)-MBE was annealed at 475 °C show the signature of gold silicide formation.     

Dielectric and optical behaviors in relaxor ferroelectric Pb(In1/2Nb1/2)1−xTixO3 crystal

Dielectric permittivities (ε′,ε″) have been measured as functions of temperature (140-535 K) and frequency (500 Hz-2.0 MHz) in a (001)-cut Pb(In_1/2Nb_1/2)_0.7Ti_0.3O₃ (PINT30%) single crystal grown by the modified Bridgman method with Pb(Mg_1/3Nb_2/3)_0.71Ti_0.29O₃ (PMNT29%) seed crystal. A diffused phase transition was observed in the temperature region of ∼430-460 K with strong frequency dispersion. Above the Burns temperature T_B≅510 K, the dielectric permittivity was found to follow the Curie-Weiss behavior, ε′=C/(T−T_C), with parameters C=3.9×10⁵ and T_C=472 K. Below T_B≅510 K, polar nanoclusters are considered to appear and are responsible for the diffused dielectric anomaly. Optical transmission, refractive indices, and the Cauchy equations were obtained as a function of wavelength at room temperature. The unpoled crystal shows almost no birefringence, indicating that the average structural symmetry is optically isotropic. The crystal exhibits a broad transparency in the wavelength range of ∼0.4-6.0 μm.     

Morphological and structural characterizations of CrSi2 nanometric films deposited by laser ablation

The structure and morphology of chromium disilicide (CrSi₂) nanometric films grown on 〈1 0 0〉 silicon substrates both at room temperature (RT) and at 740 K by pulsed laser ablation are reported. A pure CrSi₂ crystal target was ablated with a KrF excimer laser in vacuum (∼3 × 10⁻⁵ Pa). Morphological and structural properties of the deposited films were investigated using Rutherford backscattering spectrometry (RBS), grazing incidence X-ray diffraction (GID), X-ray reflectivity (XRR), scanning (SEM) and transmission electron microscopy (TEM). From RBS analysis, the films’ thickness resulted of ∼40 nm. This value is in agreement with the value obtained from XRR and TEM analysis (∼42 and ∼38 nm, respectively). The films’ composition, as inferred from Rutherford Universal Manipulation Program simulation of experimental spectra, is close to stoichiometric CrSi₂. GID analysis showed that the film deposited at 740 K is composed only by the CrSi₂ phase. The RT deposited sample is amorphous, while GID and TEM analyses evidenced that the film deposited at 740 K is poorly crystallised. The RT deposited film exhibited a metallic behaviour, while that one deposited at 740 K showed a semiconductor behaviour down to 227 K.     

Wide band polarizer with suspended silicon grating

In this paper, we propose an ultra broadband polarizer operating in the telecommunication wavelength band; this device consisting a single silicon suspended resonant grating layer is designed with using the inverse mathematical method and rigorous vector diffraction theory. It is shown from our calculations that the device presents extremely high reflection (R > 98%) for TE polarization light and high transmission (T > 98%) for TM polarization over ∼330 nm wavelength range; moreover, the extinction ratio is ∼100 in the central wavelength 1550 nm. Furthermore, it is found with Rigorous Coupled Wave Analysis (RCWA) and near field distribution that the extremely wide band property for TE polarization is due to the excitation of strong modulation guided modes in the design wavelength range.     

Property changes of electroplated Cu/Co alloys and multilayers by organic additives

We investigated the change of magnetic properties of the electroplated Cu/Co alloys and multilayers caused by organic additives and high temperature annealing. When plated with a pure Cu/Co electrolyte, the alloy contained ∼25% of Cu and ∼75% of Co. The alloy was made of hcp-Co, fcc-Co and Cu(1 1 1) and was super-paramagnetic at room temperature. As we add a few organic additives in the plating electrolyte, the hcp-Co of the films disappeared. The organic additives contained in the electrolytes changed paramagnetic Cu/Co multilayers to ferromagnets. High-temperature thermal annealing increased coercivity due to the growth of the Co grains.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.