Optical characterization of thin thermal oxide films on copper by ellipsometry

A complete optical characterization in the visible region of thin copper oxide films has been performed by ellipsometry. Copper oxide films of various thicknesses were grown on thick copper films by low temperature thermal oxidation at 125 °C in air for different time intervals. The thickness and optical constants of the copper oxide films were determined in the visible region by ellipsometric measurements. It was found that a linear time law is valid for the oxide growth in air at 125 °C. The spectral behaviour of the optical constants and the value of the band gap in the oxide films determined by ellipsometry in this study are in agreement with the behaviour of those of Cu₂O, which have been obtained elsewhere through reflectance and transmittance methods. The band gap of copper oxide, determined from the spectral behaviour of the absorption coefficient was about 2 eV, which is the generally accepted value for Cu₂O. It was therefore concluded that the oxide composition of the surface film grown on copper is in the form of Cu₂O (cuprous oxide). It was also shown that the reflectance spectra of the copper oxide–copper structures exhibit behaviour expected from a single layer antireflection coating of Cu₂O on Cu. Received: 19 July 2001 / Accepted: 27 July 2001 / Published online: 17 October 2001     

Temperature dependence of aluminum nitride reflectance spectra in vacuum ultraviolet region

Temperature dependence of optical reflectance spectra in vacuum ultraviolet region for aluminum nitride has been measured on high-quality single crystal with synchrotron radiation. The dominant structure due to the interband transition is observed at photon energy around 7.7 eV. With decreasing temperature, the energy position of the dominant structure in the reflectance spectra shifts towards higher energy. The experimental data has been fitted to the Bose-Einstein expression and the obtained parameter related to the strength of the electron-phonon interactions is much smaller than that for the peak at 6.2 eV, suggesting that the higher-lying interband transition energy decreases more slowly with increasing temperature in aluminum nitride (AlN).     

Large-scale synthesis of GaN nanorods and their photoluminescence

Large quantities of gallium nitride (GaN) nanorods have been synthesized via direct reaction of metallic gallium vapor with flowing ammonia at 970 °C in a quartz tube. The nanorods have been confirmed as crystalline wurzite GaN by powder X-ray diffraction, selected-area electron diffraction and X-ray photoelectron spectrometry. Transmission electron microscopy and scanning electron microscopy reveal that the nanorods are straight and uniform, with diameters ranging from 40 nm to 150 nm and lengths up to hundreds of micrometers. The growth mechanism is discussed briefly. Photoluminescence measurements on bulk GaN nanorods at room temperature show two strong peaks at 377 nm (3.28 eV) and 360 nm (3.44 eV) attributed to the zero-phonon donor-acceptor pair transition and the donor-bound exciton, respectively. Received: 19 April 2001 / Accepted: 10 May 2001 / Published online: 20 June 2001     

The early oxynitridation stages of hydrogen-terminated (100) silicon after exposure to N2:N2O. Nitrogen bonding states

Nitridation of hydrogen-terminated silicon in a diluted N₂:N₂O atmosphere was studied by X-ray photoemission spectroscopy and high-resolution electron microscopy. Our analysis showed that the broad N(1s) peak of width 1.5 eV at 398–399 eV, usually reported in the literature, is preceded by the formation of a narrow peak of width around 1.0 eV at 397.5 eV, attributed to the moiety Si₃N in which silicon is only marginally oxidized, and two other peaks at 400.0 eV and 401.5 eV, attributed to the moieties Si₂NOSi and SiNO, respectively. Received: 11 July 2001 / Accepted: 19 September 2001 / Published online: 20 December 2001     

Optical diffuse reflectance spectra of GaSb nanocrystals embedded in SiO2 matrix by radio-frequency magnetron co-sputtering

Nanocrystalline GaSb embedded in SiO₂ films was grown by radio-frequency magnetron co-sputtering. X-ray diffraction pattern and transmission electron microscopy (TEM) confirm the existence of GaSb nanocrystals in the SiO₂ matrix. The average size of GaSb nanoparticles is in the range of 3 to 11 nm. Diffuse reflectance spectra were used to characterize the small change of the band gap of the semiconductor. The diffuse reflectance spectra shows that the absorption peaks have a large blueshift of about 4.0 eV of the absorption relative to that of bulk GaSb. It has been explained by quantum confinement effects. Room temperature optical transmission spectra show that the absorption edge exhibits a very large blueshift of about 2.1 eV with respect to that of bulk GaSb in agreement with quantum confinement. Received: 28 July 1999 / Accepted: 27 October 1999 / Published online: 1 March 2000     

A model for the interaction of near-infrared laser pulses with metal powders in selective laser sintering

A thermal model of the interaction of pulsed near-infrared laser radiation from a Nd:YAG laser was made, taking the measured powder properties such as reflectance, optical penetration depth and thermal conductivity into account. It allows an estimation of the evolution of two different temperatures: the average temperature of the powder (taken over the grains in a volume given by the laser beam diameter and the optical penetration depth) and the temperature distinction within a single grain. It showed that in pulsed mode consolidation can be achieved at much lower average power as the surface of the powder particles are molten but their cores remain at nearly room temperature. This leads to a much lower average temperature and therefore a dramatic decrease in residual thermal stresses in the finished piece. The results of the model were experimentally tested and confirmed. Received: 26 July 2001 / Accepted: 23 November 2001 / Published online: 23 January 2002     

Energy band gap and oscillator parameters of Ga4Se3S single crystals

The optical properties of the Bridgman method grown Ga₄Se₃S crystals have been investigated by means of room temperature, transmittance and reflectance spectral analysis. The optical data have revealed an indirect allowed transition band gap of 2.08 eV. The room temperature refractive index, which was calculated from the reflectance and transmittance data, allowed the identification of the dispersion and oscillator energies, static dielectric constant and static refractive index as 21.08 and 3.85 eV, 6.48 and 2.55, respectively.     

Nonlinear optical spectroscopy of electronic transitions in hexagonal manganites

The hexagonal rare-earth manganites RMnO₃ (R = Sc, Y, Ho, Er, Tm, Yb, Lu) are a group of materials with an unusual combination of magnetic, electric and optical properties. The electronic structure of these materials was studied by second harmonic (SH) spectroscopy in the range from 1.2 to 3.0 eV. Faraday rotation and absorption spectra were measured in the range from 1.0 to 1.6 eV. Broad bands at ∼1.7 eV and ∼2.7 eV are assigned to electronic transitions between Mn³⁺(3d⁴) levels. The SH spectra are discussed on the basis of a recently developed microscopic theory. Received: 26 April 2001 / Published online: 18 July 2001     

Optical characterization of laser-induced crystallized silicon films

Optical absorption coefficient spectra of thin silicon films were precisely investigated using a simple reflectance system with total reflectance mirrors placed on the rear side of samples in order to cancel an interference effect in a range between 1.1 eV and 3 eV. The absorption coefficient decreased according to crystallization as the laser energy increased and it got similar to that of single crystalline silicon in the range of 1.7 eV 3 eV. However, the absorption coefficient was higher than 10² cm^–1 in the photon energy lower than 1.3 eV. This probably results from band tail states caused by defect states localized at grain boundaries in the crystallized films. 2.5%-phosphorus doped laser crystallized silicon films had a high optical absorption coefficient ( > 10⁴ cm^–1) in the low photon energy range (1.1 eV 1.7 eV) caused by free carriers produced from the dopant atoms activated in the silicon films. The experimental results gave the carrier density of 1.3 × 10²¹ cm³ and the carrier mobility of 20 cm²/Vs.     

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq3) by spectroscopic ellipsometry

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq₃) have been studied in the spectral range from 1.55 eV to 5 eV using spectroscopic ellipsometry. The samples have been deposited by thermal evaporation on glass substrates. Optical functions of Alq₃ deposited on unheated substrates and on substrates kept at 100 °C have been determined. The optical functions have been modeled using point-to-point fitting, with the conventional oscillator model and modified oscillator model. It has been found that point-to-point fitting gives the best agreement with the experimental data, and that the modified oscillator model yields better agreement with the experimental data than the conventional oscillator model. Received: 3 September 2001 / Accepted: 22 March 2002 / Published online: 5 July 2002 RID="*" ID="*"Corresponding author. Fax: +852-2559/8738, E-mail: dalek@eee.hku.hk     

Photoemission study of clean and c(4×2)-2CO-covered Pt (111) using high-harmonic radiation

An extreme ultraviolet (EUV) laser light source based on high-harmonic generation is presented. Coherent radiation in the photon energy range hν=20–120 eV is produced in the conversion media argon, neon and helium. High-harmonic radiation in the energy range 20–50 eV is applied to investigate photoemission spectra of Pt (111) and CO/Pt (111). In the photoemission spectra of the clean surface, new secondary electron emission structures are found which influence the cross section analysis of the CO states. When taking these Pt resonances into consideration, the 4σ and 5σ CO shape resonances are found at photon energies of 37 eV and 28 eV, respectively. Additionally, a resonance at hν=31 eV is also observed for the CO 1π state, in contrast to formerly published experimental data. Experimental and theoretical data suggest that this resonance is not connected to the well-known shape resonances in the σ-channel. Based on theoretical approaches, it is identified as an autoionization resonance. Received: 8 April 2002 / Accepted: 22 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-251/833-3604, E-mail: kutzner@uni-muenster.de     

Vibrational dynamics of fullerene molecules adsorbed on metal surfaces studied with synchrotron infrared radiation

Infrared (IR) spectroscopy of chemisorbed C₆₀ on Ag (111), Au (110) and Cu (100) reveals that a non-IR-active mode becomes active upon adsorption, and that its frequency shifts proportionally with the charge transferred from the metal to the molecule by about 5 cm^-1 per electron. The temperature dependence of the frequency and the width of this IR feature have also been followed for C₆₀/Cu (100) and were found to agree well with a weak anharmonic coupling (dephasing) to a low-frequency mode, which we suggest to be the frustrated translational mode of the adsorbed molecules. Additionally, the adsorption is accompanied by a broadband reflectance change, which is interpreted as due to the scattering of conduction electrons of the metal surface by the adsorbate. The reflectance change allows determination of the friction coefficient of the C₆₀ molecules, which results in rather small values (∼2×10⁹ s^-1 for Ag and Au, and ∼1.6×10⁹ s^-1for Cu), consistent with a marked metallic character of the adsorbed molecules. Pre-dosing of alkali atoms onto the metal substrates drastically changes the IR spectra recorded during subsequent C₆₀ deposition: anti-absorption bands, as well as an increase of the broadband reflectance, occur and are interpreted as due to strong electron–phonon coupling with induced surface states. Received: 6 June 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

Surface plasmon assisted photoemission from Au nanoparticles on graphite

The resonant multiple excitation of collective modes in metallic nanoparticles using ultrashort laser pulses leads to an enhanced multiphoton photoemission from the particles. This effect is here demonstrated for the surface-plasmon resonance of Au nanoparticles on graphite. The shape of the photoemission spectra is explained by multiphoton photo-assisted thermionic emission from the nanoparticles and resonant emission via the image-potential state on graphite. Tuning the photon energy between 1.7 eV and 3.2 eV allows the identification of an enhancement of the photoemission yield at 2.1±0.1-eV photon energy that is attributed to the resonant excitation of the surface plasmon in the Au nanoparticles. This identification of the surface-plasmon excitation in this energy range is also supported by electron energy loss spectroscopy. Received: 8 August 2001 / Revised version: 13 September 2001 / Published online: 10 October 2001     

Radiative cooling of fullerene anions in a storage ring

Thermionic emission from hot fullerene anions, C_N ^-, has been measured in an electrostatic storage ring for even N values from 36 to 96. The decay is quenched by radiative cooling and hence the observations give information on the intensity of thermal radiation from fullerenes. The experiments are analysed by comparison with a simulation which includes the quantisation of photon energy and the statistics of emission. Experiments with heating of the molecules with a laser beam confirm the interpretation of the observations in terms of radiative cooling and give an independent estimate of the cooling rate for C₆₀ ^-. The measured cooling rates agree in general within a factor of two with the prediction from a classical dielectric model of a thermal radiation intensity of ∼ 300 eV/s for C₆₀ at 1 400 K, scaling approximately with the 6th power of the temperature and with the number of atoms in the molecule. Received 12 March 2001 and Received in final form 12 June 2001     

Kilohertz extreme-ultraviolet light source based on femtosecond high-order harmonic generation from noble gases

A compact, versatile table-top kilohertz source of coherent extreme-ultraviolet (XUV) radiation in the wavelength region 18–100 nm, based on high-order harmonic generation from noble gases induced by a 40-femtosecond Ti:sapphire laser system, is presented. The XUV beamline delivers at its output 10⁸ photons/s at a wavelength of 23 nm. The monochromatized XUV radiation is directly focused onto a 10^-2-mm² spot by a toroidal grating, allowing one to reach intensities higher than 10⁶ W/cm². Optimization results are presented for a new XUV-generating geometry, utilizing a ‘semi-infinite’ quasi-static gas cell and strong focusing. In those conditions, we observe an anomalous inversion between the cutoffs of argon and krypton, with the krypton spectrum extending to much higher orders than expected in an adiabatic limit. Received: 9 July 2001 / Revised version: 1 August 2001 / Published online: 7 November 2001     

Micromechanical properties of carbon nitride films deposited by radio-frequency-assisted filtered cathodic vacuum arc

Super-hard and elastic carbon nitride films have been synthesized by using an off-plane double-bend filtered cathodic vacuum arc combined with a radio-frequency nitrogen-ion beam source. A nanoindenter was used to determine the micromechanical properties of the deposited films. X-ray photoelectron spectroscopy was used to study the composition and bonding structure of the deposited films. The influence of nitrogen ion energy on the structure and micromechanical properties of the deposited films was systematically studied. As the nitrogen ion energy is increased, the microhardness, Young’s modulus and elastic recovery also increase, reaching a maximum of 47 GPa, 400 GPa, and 87.5%, respectively, at a nitrogen ion energy of 100 eV. Further increase in nitrogen ion energy results in a decrease in microhardness, Young’s modulus and elastic recovery of the deposited films. The formation of five-membered rings, as indicated by XPS, which causes bending of the basal planes and forms a three-dimensional rigid covalent bond network, contributes to the super-hardness, Young’ s modulus and high elastic recovery of the films deposited at a nitrogen ion energy of 100 eV. Revised version: 29 October 2001 / Accepted: 7 November 2001 / Published online: 2 May 2002     

High Fe2+/3+ trap concentration in heavily compensated implanted InP

High Fe concentrations (up to 2×10¹⁹ cm^-3) have been implanted in n-doped InP to compensate the substrate donors. The resulting semi-insulating layers have been investigated by current–voltage (I-V) measurements and photo-induced current transient spectroscopy (PICTS) analyses to characterise the Fe activation process and to study the Fe related deep levels. The activation of the Fe^2+/3+ trap has been assessed by the identification of the deep level located at E_C-0.64 eV. The outcomes of the PICTS measurements have been correlated with the electrically active Fe concentration calculated from a numerical simulation of the I-V characteristics. We observe an increasing linear relation between the electrically active Fe concentration and the substrate doping density, with a maximum active Fe concentration as high as 2×10¹⁸ cm^-3, i.e. more than an order of magnitude above the equilibrium Fe solid solubility. These data are presented and their implications discussed. Received: 4 September 2000 / Accepted: 7 February 2001 / Published online: 23 May 2001     

A laser system providing tunable, narrow-band radiation from 35 nm to 2 μm

We describe a laser system that readily provides radiation tunable from 2 μm in the infra-red to 35 nm in the extreme ultraviolet spectral range. The broad spectral range is covered through a range of non-linear processes such as Raman shifting and high-order harmonic generation. Pulses with duration of tens of picoseconds are obtained. The relative bandwidth of the radiation is δλ/λ=10^-4, comparable with what can be achieved by using high-resolution monochromators at state-of-the-art synchrotron beamlines. We discuss different methods for characterising the radiation in this wide wavelength regime. We also discuss the capabilities of the system from the measured parameters. Received: 12 December 2000 / Revised version: 8 March 2001 / Published online: 27 April 2001     

The color of finely dispersed nanoparticles

We discuss the dependence of the color of low-concentrated nanoparticle systems on particle size and mass concentration for Ag, Au and TiN nanoparticles, which exhibit a surface plasmon polariton resonance in extinction spectra. Comparison is made with color data obtained for Ag and Au colloidal suspensions. When particles lump into aggregates, the splitting of the surface plasmon resonance into new resonances affects the extinction of light and, hence, the color of the particle assembly. This is demonstrated for aggregated colloidal suspensions of Ag and Au nanoparticles. Finally, for highly concentrated assemblies such as pigment films, we discuss the dependence of the color in diffuse reflectance and transmittance according to Kubelka and Munk (P. Kubelka, F. Munk: Z. Techn. Phys. 12, 593 (1931)), and extend this model by using optical properties of aggregates of spheres. Received: 2 July 2001 / Published online: 10 October 2001     

III–V semiconductor interface properties as a knowledge basis for modern heterostructure devices

Some examples of interface studies are reported which show their close link with progress in III–V modern semiconductor device physics and technology. The surface electronic properties investigated in-situ by reflectance anisotropy spectroscopy during InGaP/InP growth (metal-organic vapor-phase epitaxy) are essential for the control of ordering phenomena in these layers, which is relevant for high-performance optoelectronic devices. Studies of electronic interface states at metal/narrow-gap III–V semiconductors are presented, which enabled the successful preparation of semiconductor/superconductor hybrid devices. For group-III nitrides with wurtzite structure the presence of fixed polarization interface charges yields new challenges in order to understand and control Schottky-barrier heights, band offsets and 2D confinement in heterostructure field-effect transistors. Received: 26 April 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002