N+BF<Subscript>2</Subscript> and N+C+BF<Subscript>2</Subscript> high-dose co-implantation in silicon

Nitrogen and boron BF₂, and nitrogen, carbon, and boron BF₂ high-dose (6×10¹⁶–3×10¹⁷ cm^-2) co-implantation were performed at energies of about 21–77 keV. Subsequent high-temperature annealing processes (600, 850, and 1200 °C) lead to the formation of three and two surface layers respectively. The outer layer mainly consists of polycrystalline silicon and some amorphous material and Si₃N₄ inclusions. The inner layer is highly defective crystalline silicon, with some inclusions of Si₃N₄ too. In the N+B-implanted sample the intermediate layer is amorphous. Co-implantation of boron with nitrogen and with nitrogen and carbon prevents the excessive diffusivity of B and leads to a lattice-parameter reduction of 0.7–1.0%. Received: 10 January 2002 / Accepted: 30 May 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/3974895; E-mail: Lucia.Barbadillo@uam.es     

A high-sensitivity in situ optical diagnostic technique for laser cleaning of transparent substrates

A differential optical transmission technique has been used to monitor in situ the efficiency of laser cleaning for the removal of sub-micrometer-sized particles on substrates transparent at the monitoring wavelength. This technique has been applied to the removal of sub-micrometer polystyrene particles on polyimide substrates using laser pulses of 30 ps duration at 292 nm while probing the material transmission at 633 nm. The sensitivity achieved -1/10⁴ for the transmission changes induced upon single-pulse laser exposure – allows us to monitor the removal of just a few sub-micron-sized particles from the probed region inside the irradiated area. Received: 2 October 2002 / Accepted: 7 October 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +33-3/87844082, E-mail: nchaoui@iut.univ-metz.fr RID="**" ID="**"Present address: Laboratoire de Chimie et Applications, Institut Universitaire de Technologie de Metz, Département Chimie, Rue Victor Demange, 57500 Saint-Avold, France     

Tritium recovery from co-deposited layers using 193-nm laser

Recovery of tritium from co-deposited layers formed in deuterium–tritium plasma operations of the TFTR (Tokamak Fusion Test Reactor) was investigated by the use of an ArF excimer laser operating at the wavelength of 193 nm. At the laser energy density of 0.1 J/cm², a transient spike of the tritium-release rate was observed at initial irradiation. Hydrogen isotopes were released in the form of hydrogen-isotope molecules during the laser irradiation in vacuum, suggesting that tritium can be recovered readily from the released gases. In a second experiment, hydrogen (tritium) recovery from the co-deposited layers on JT-60 tiles that had experienced hydrogen-plasma operations was investigated by laser ablation with a focused beam of the excimer laser. The removal rate of the co-deposited layers was quite low when the laser energy density was smaller than the ablation threshold (1.0 J/cm²), but reached 1.1 μm/pulse at the laser energy density of 7.6 J/cm². The effective absorption coefficient in the co-deposited layers at the laser wavelength was determined to be 1.9 μm^-1. The temperature of the surface during the irradiation at the laser energy density of 0.5 J/cm² was measured on the basis of Planck’s law of radiation, and the maximum temperature during the irradiation decreased from 3570 K at the initial irradiation to 2550 K at the 1000th pulse of the irradiation. Received: 5 August 2002 / Accepted: 7 August 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +81-29/2825917, E-mail: shu@tpl.tokai.jaeri.go.jp     

High refractive index and transparent heavy metal oxide glassy thin films

Heavy metal oxide thin films of the ternary system Nb₂O₅–GeO₂–PbO have been prepared by pulsed laser deposition in an O₂ environment from either glassy or crystalline bulk samples. The range of ([Pb]+[Nb]) content in which the films are optically homogeneous and transparent is much broader (0.5–1.0) than that of the bulk samples considered in the present work (0.55–0.62). The imaginary part of the refractive index is very low in all cases (k<10^-3), whereas the real part increases linearly with the ([Pb]+[Nb]) content up to values as high as 2.35. The optical energy gap has been found to be strongly dependent on [Pb], whereas it is almost independent of [Nb]. This dependence is discussed in terms of the role of Pb and Nb as network modifiers or formers. Received: 5 August 2002 / Accepted: 8 August 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/564-5557, E-mail: j.gonzalo@io.cfmac.csic.es     

Electrical characterization of impurity-free disordering-induced defects in n-GaAs using native oxide layers

Defects created in rapid thermally annealed n-GaAs epilayers capped with native oxide layers have been investigated using deep-level transient spectroscopy (DLTS). The native oxide layers were formed at room temperature using pulsed anodic oxidation. A hole trap H0, due to either interface states or injection of interstitials, is observed around the detection limit of DLTS in oxidized samples. Rapid thermal annealing introduces three additional minority-carrier traps H1 (E_V+0.44 eV), H2 (E_V+0.73 eV), and H3 (E_V+0.76 eV). These hole traps are introduced in conjunction with electron traps S1 (E_C-0.23 eV) and S2 (E_C-0.45 eV), which are observed in the same epilayers following disordering using SiO₂ capping layers. We also provide evidence that a hole trap whose DLTS peak overlaps with that of EL2 is present in the disordered n-GaAs layers. The mechanisms through which these hole traps are created are discussed. Capacitance–voltage measurements reveal that impurity-free disordering using native oxides of GaAs produced higher free-carrier compensation compared to SiO₂ capping layers. Received: 12 March 2002 / Accepted: 15 July 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-0381, E-mail: pnk109@rsphysse.anu.edu.au     

Nonlinear electrical characteristics and dielectric properties of Ca,Ta-doped TiO<Subscript>2</Subscript> varistors

Ca,Ta-doped TiO₂ varistors with high nonlinear coefficients are obtained by a ceramic sintering. The nonlinear electrical and dielectric properties of the samples doped with 0.5mol% Ca and various concentrations of Ta (0.05∼2.0mol%) were investigated. The samples sintered at 1350 °C have nonlinear coefficients of α=5.1∼42.1 and high relative dielectric constants approach 10⁵. The effects of Ta-doping on the nonlinear and dielectric properties of the Ca,Ta-doped TiO₂ varistors are studied in greater detail. When the concentration of Ta is 0.5mol%, the sample possesses the highest nonlinear coefficient and a comparatively lower dielectric constant. The effects of Ta and the nonlinear electrical behavior of the TiO₂ system are explained by analogy to a grain-boundary atomic defect model. Received: 24 October 2001 / Accepted: 8 January 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/826-49531, E-mail: wangwanyan@yahoo.com.cn     

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H₂ atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model. Received: 11 July 2002 / Accepted: 7 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86/10-62751615, E-mail: yudp@pku.edu.cn     

Photoacoustic gas sensing employing fundamental and frequency-doubled radiation of a continuously tunable high-pressure CO2 laser

We present the first photoacoustic spectrometer for gas sensing employing both the fundamental and the frequency-doubled radiation of a continuously tunable high-pressure CO₂ laser with room temperature operation. A quasi-phase-matched diffusion-bonded GaAs crystal is used in the system for second-harmonic generation. A pulsed photoacoustic detection scheme with a non-resonant cell, equipped with an 80-microphone array, is employed. The wide continuous tuning range in the fundamental (9.2–10.7 μm) and the frequency-doubled (4.6–5.35 μm) regimes, together with the narrow linewidth of 540 MHz (0.018 cm^-1) for the 10-μm region and of 1050 MHz (0.0315 cm^-1) for the 5-μm region, allow the measurement of gas mixtures, individual species and isotope discrimination. This is illustrated with measurements on NO and CO₂. The measured isotope ratio ¹⁵ NO/¹⁴ NO=(3.58±0.55)×10^-3 agrees well with the literature (3.700×10^-3) and demonstrates the good selectivity of the system. Received: 30 April 2002 / Revised version: 10 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1077, E-mail: sigrist@iqe.phys.ethz.ch     

Removal of oxygen atoms from a SiO2 surface by incoherent vacuum ultraviolet excimer irradiation

We have demonstrated the non-thermal removal of oxygen atoms from an oxidized silicon surface (SiO₂) on a silicon wafer by the use of a low-power (0.3 mW cm^-2) incoherent vacuum ultraviolet (VUV) light source at 126 nm. X-ray photoelectron spectroscopy (XPS) has shown that a maximum Si concentration of 80% appears at the surface after a 20-h irradiation with 9.8 eV photons, as a result of oxygen removal from the SiO₂ matrix. The surface morphology, however, indicates no damage or melting on the surface even after the irradiation. Received: 15 April 2002 / Accepted: 17 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Permanent address: Nano-Tech Photon Inc., Shimotomita 4132-1, Shintomi, Miyazaki, 889-1404, Japan RID="**" ID="**"Corresponding author. Fax: +81-985/583-899, E-mail: kubodera@opt.miyazaki-u.jp     

Property study of Si + -ion-implanted Nd:YVO 4 waveguides

Planar waveguides were formed in Nd:YVO₄ crystals by 3.0-MeV Si⁺-ion implantation at doses of 1×10¹³–1.5×10¹⁵ ions/cm² at room temperature. The effective refractive indices of the waveguide propagation modes were measured by using a prism-coupling method. It was found that the number of the propagation modes is dependent on the doses for the waveguides in Nd:YVO₄. The atom displacement in the near-surface region (about 2 μm beneath the surface) of the Nd:YVO₄ crystal induced by the implantation was simulated by using the TRIM 98 (transport and range of ions in matter) code. The possible reasons for the waveguide formation are discussed in a primary way. Received: 17 July 2002 / Revised version: 20 September 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-531-8565167, E-mail: drfchen@sdu.edu.cn     

Current instabilities in dynamic random access memory storage capacitor formed with electron beam deposited Y2O3 dielectric

Metal-oxide-semiconductor (MOS) storage capacitors based on electron beam deposited Y₂O₃ extrinsic dielectric on Si show changes in capacitance density depending on the amorphous and crystalline phases. Bias stress cycle-dependent changes in capacitance density occur due to the non-equilibrium nature of defect states at the Y₂O₃/Si interface after O₂ annealing as a result of the emergence of a 4–8 nm thick SiO₂ film at the interface. Leakage currents show instability under repeated dc bias stress, the nature and extent of which depend upon the structure of the Y₂O₃ gate dielectric and the polarity of dc bias. With amorphous Y₂O₃, leakage currents drift to lower values under gate injection due to electron trapping, and to higher values under Si-injection due to the generation of holes. Though leakage current drift is minimal for crystalline Y₂O₃, its magnitude increases as the energy of injected electrons from mid-gap states is low and the local field due to asperity is high. The emergence of interfacial SiO₂ reduces the magnitude of Si-injection leakage current substantially, but causes transient changes resulting in switching to higher values at a threshold dc bias. Thermal detrapping of holes and reverse bias stress studies confirm that the instability of current is caused by an increase in the cathodic field from hole trapping at interface states. Leakage current instability limits the application of extrinsic high dielectric constant dielectrics in a high density DRAM storage capacitor, unless a new interface layer scheme other than SiO₂ and a method to form a defect-free dielectric layer can be implemented. Received: 29 October 2001 / Accepted: 22 April 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-413/545-4611, E-mail: rastogi@ecs.umass.edu     

Ion irradiation of exchange bias systems for magnetic sensor applications

It has been demonstrated that He⁺ ion irradiation is an excellent tool for modifying magnetic properties, like the magnetic anisotropy, the interlayer exchange coupling strength and the exchange bias field of ultra-thin magnetic layered systems. This paper summarizes the effects of ion irradiation on exchange bias systems. As a first example, for possible applications of the ion induced magnetic effects, the realization of an angle sensing device is described. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-631-205-4095, E-mail: fassbend@physik.uni-kl.de RID="**" ID="**"Present address: Université de Rouen, Rouen, France     

A fast-response near-infrared tunable diode laser absorption spectrometer for in situ measurements of CH4 in the upper troposphere and lower stratosphere

We describe a near-infrared in situ tunable diode laser spectrometer developed for atmospheric measurements of CH₄ in the upper troposphere and lower stratosphere (UT/LS). The instrument is designed to provide fast-response (0.5–1 Hz) measurements and operate autonomously on the NASA WB-57F high-altitude aircraft. A single-mode InGaAsP distributed feedback laser diode operating at 1.6537 μm scans continuously over the R(3) rotation–vibration transition in the 2ν₃ band. We use a direct absorption technique incorporating a custom-designed long path length (252 m) low-volume (3.6 L) astigmatic Herriott cell. The present detection sensitivity is 5×10¹⁰ molecules cm^-3, corresponding to ∼20 ppbv in the UT/LS, with the main limit to instrument precision being background optical interference fringes. In-flight performance is demonstrated by presentation of recent data. Received: 25 January 2002 / Revised version: 5 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-5373, E-mail: richard@al.noaa.gov     

Dielectric breakdown of rubber materials by femtosecond irradiation

We report the reversible micro-structuring of a synthetic rubber polymer (cis1,4-polybutadiene (PB)) by femtosecond laser illumination. Visco-elastic relaxation of the optically damaged region was observed. The recovery time, typically 10²–10⁴ ms, can be varied by changing the irradiation pulse energy. Multi-shot-induced damage recovers on the much longer scale of 10¹–10² s. It was found that the doping of PB by 4 wt. % of pentazadiene ([4-NO₂]–phenyl–N=N–N(C₃H₇)–N=N–phenyl–[4-NO₂]) reduces the threshold of light-induced photo-modification by 20%. This is explained by photo-induced (homolytic) cleavage of the pentazadiene bonds and formation of gaseous N₂, which facilitates material failure at the irradiated spot. The recovery of optical transmission can be applied to optical memory, optical and micro-mechanical applications. The underlying mechanism of the phenomenon is discussed in terms of anelastic α- and β-relaxation (polymer backbone and chains/coils relaxation, respectively). Received: 11 October 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +81-88/656-7598, E-mail: misawa@eco.tokushima-u.ac.jp     

Recrystallization behavior of high-dose Mn+-implanted GaAs

Raman spectroscopy was used to study the evolution of host lattice recrystallization in Mn⁺-implanted GaAs. A high dose of Mn⁺-ions (>10¹⁵ cm^-2) was introduced into semi-insulating GaAs by the combinatorial implantation method. Subsequent thermal annealing at 920 °C was carried out to re-grow the implantation-induced amorphous layers. The dependence of the recrystallization behavior on the Mn content was systematically observed. The lattice orientation of recrystallized layers in the surface changed after high-dose implantation (>1.6×10¹⁵ cm^-2) and annealing. The size of the recrystallized crystallites decreased with increasing Mn⁺ dose, as indicated by images from atomic force microscopy. The decrease in the phonon frequency of the Raman lines with the size reduction of microcrystals was in good agreement with the spatial correlation model. However, at higher doses (>7×10¹⁶ cm^-2), a blue shift of the frequency was observed due to the compressive stress exerted on the microcrystals. Received: 5 March 2002 / Accepted: 29 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-21/658-30734, E-mail: jqwang@mail.sitp.ac.cn     

Sub-part-per-billion detection of nitric oxide in air using a thermoelectrically cooled mid-infrared quantum cascade laser spectrometer

Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1 nmole/mole (<1 ppbv) using a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26 μm and coupled to a 210-m path length multiple-pass absorption cell at reduced pressure (50 Torr). The sensitivity of the system is enhanced by operating under pulsing conditions which reduce the laser line width to 0.010 cm^-1 (300 MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector. The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode laser spectrometer sampling from a common inlet. A detection precision of 0.12 ppb Hz^-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10^-5 Hz^-1/2 or an absorbance precision per unit path length of 5×10^-10 cm^-1 Hz^-1/2. A precision of 0.3 ppb Hz^-1/2 is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods with a totally cryogen-free instrument. Received: 1 May 2002 / Revised version: 6 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/663-4918, E-mail: ddn@aerodyne.com     

InAsSb/InAsSbP current-tunable laser with narrow spectral line width

The article describes the manufacture and testing of a new type of semiconductor laser working at low temperatures (12–100 K) in the wavelength range 3200–3300 cm^-1. This kind of laser can be tuned in the modal range up to 6 cm^-1 and is characterized by a narrow spectral line width (about 7 MHz). Received: 12 September 2002 / Final version: 29 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +420-286/591-766, E-mail: civis@jh-inst.cas.cz     

Group-IV nanocluster formation by ion-beam synthesis

A short review of our investigations devoted to the use of ion-beam-synthesized nanoclusters for silicon-based light emission and nonvolatile memory effects is presented. Blue-violet light emission is demonstrated based on Ge-implanted silicon dioxide layers thermally grown on silicon substrates. This version of silicon-based light emission relies on Ge-related defects in the amorphous ≡Si–O–Si≡ network. The photoluminescence and electroluminescence are excited by a singlet S₀–S₁ transition of a neutral oxygen vacancy and by electron injection from the silicon substrate into the silicon dioxide layer, respectively. Whereas the photoluminescence excitation is a well-known mechanism, for the case of electroluminescence an interpretation was performed for the first time in the course of our studies. It was found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. The application of the silicon-based light-emitting devices for an integrated optocoupler arrangement is described. Another application of nanoclusters is based on the investigation of thin Si-implanted silicon dioxide layers for nonvolatile memory devices. First promising results demonstrate that the observed programming window can reach several volts and the devices exhibit excellent retention behavior. A 256 K-nv-SRAM is demonstrated showing a programming window of >1 V for write pulses of 12 V/8 ms. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: w.skorupa@fz-rossendorf.de     

Thermal and carrier transport originating from photon recycling and non-radiative recombination in laser micromachining of GaAs thin films

Coupled thermal and carrier transports (electron/hole generation, recombination, diffusion and drifting) in laser photoetching of GaAs thin film is investigated. A new volumetric heating mechanism originating from SRH (Shockley–Read–Hall) non-radiative recombination and photon recycling is proposed and modeled based on recent experimental findings. Both volumetric SRH heating and Joule heating are found to be important in the carrier transport, as well as the etching process. SRH heating and Joule heating are primarily confined within the space-charge region, which is about 20 nm from the GaAs surface. The surface temperature rises rapidly as the laser intensity exceeds 10⁵ W/m². Below a laser intensity of 10⁵ W/m², the thermal effect is negligible. The etch rate is found to be dependent on the competition between photovoltaic and photothermal effects on surface potential. At high laser intensity, the etch rate is increased by more than 100%, due to SRH and Joule heating. Received: 24 January 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-310/206-2302, E-mail: xiang@seas.ucla.edu     

Properties of an ultrashort-pumped near-IR high gain dye amplifier

An ultra-short-pumped optical dye amplifier operating in the near-IR (720 nm) has been developed. We present an experimental study of the input–output characteristics of this simple device in a traveling wave collinear configuration. In essence, the combined effects of the ultrafast nature of the anisotropy induced by the exciting laser pulse along with the short transit time across the length of the cell allow for optimal output conditions given the number of molecules in the active volume. Two distinct amplifying regimes have been observed depending on photon density. Typical gain values of 10⁴ of the narrow bandwidth (∼9 nm) subpicosecond output signal were measured. The 8-mm² beam cross section enables this photon amplifier to be utilized in time-gated imaging applications. Received: 12 December 2001 / Revised version: 13 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-819/564-5442, E-mail: Daniel.Houde@Usherbrooke.ca