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     

High-dose carbon implantations into silicon: fundamental studies for new technological tricks

Depending on the implantation temperature, the implantation of carbon ions into silicon at high doses results in the formation of either amorphous SiC_x or crystalline 3C-SiC precipitates. Various aspects of the precipitation behaviour observed, such as the impeded nucleation, the limited growth and the resulting sensitivity to ballistic destruction are attributed to the large interfacial energy between crystalline silicon and 3C-SiC. Periodically arranged amorphous SiC_x nanoclusters, which are formed at lower temperatures, are shown to promote amorphisation by their surrounding stress field and to represent sinks for silicon self-interstitials, which can be activated by annealing at 900 °C. By control of the depth distribution of equally sized, oriented 3C-SiC precipitates formed at higher implantation temperatures, it is possible to establish suitable starting conditions for the formation of buried homogeneous, single-crystalline 3C-SiC layers during a post-implantation anneal. The properties of these ion-beam-synthesised SiC layers are described and attempts to combine them with insulating and metallic layers are reviewed. A survey is given of the emerging applications of ion-beam-synthesised buried SiC layers and microstructures in electronic, optical and micromechanical devices and as large-area SiC pseudosubstrates. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-821/598-3425, E-mail: lindner@physik.uni-augsburg.de     

Interface reactions in [Fe/B]<Subscript>n</Subscript> multilayers: a way to tune from crystalline/amorphous layer sequences to homogeneous amorphous Fe<Subscript>x</Subscript>B<Subscript>100-x</Subscript> films

[Fe/B]_{n ≥2} multilayers were prepared by thermal evaporation, ion-beam sputtering and laser ablation. By applying in situ electron spectroscopies (UPS, XPS) and monitoring the electrical resistance during layer growth, evidence could be provided for the occurrence of interface reactions within the range of studied deposition temperatures (77 K ≤T ≤300 K). These reactions result in amorphous Fe_xB_100-x phases, which are spatially restricted to a width of less than 3 nm at the original interface. The amorphicity of the reacted interlayers was unequivocally proven by additional high-resolution electron microscopy (HRTEM) and their characteristically changed magnetic properties. Due to the well-defined width of the interface reaction, homogeneous amorphous Fe_xB_100-x films can be obtained by reducing the individual Fe and B layer thicknesses to below the above reaction depth, while for larger thicknesses layer sequences of the crystalline/amorphous/crystalline type will result. Received: 30 January 2002 / Accepted: 31 January 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-731/502-2963, E-mail: hans-gerd.boyen@physik.uni-ulm.de     

Magnetic properties of Ni:SiO<Subscript>2</Subscript> nanocomposites synthesized by a modified sol–gel method

Ni nanoparticles embedded in an amorphous SiO₂ matrix were produced by a modified sol–gel method. This method resulted in nanocomposites with a controlled size distribution and good dispersion of the metallic particles. The particle-size distributions were found to have an average radius of ∼3 nm, as inferred from transmission electron microscopy, X-ray-diffraction analysis, and magnetic measurements. Magnetic characterizations revealed that samples exhibit superparamagnetic behavior above the blocking temperature T_B, 20 K≤T_B≤40 K, and absence of a shift along the field axis on hysteresis loops measured at T≤T_B, indicating that the metallic nanoparticles are also free from an oxide layer. Received: 7 October 2002 / Accepted: 9 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +55-11/3091-6984, E-mail: rjardim@if.usp.br     

Photoinduced macroscopic chiral structures in a series of azobenzene copolyesters

A study of the propagation of elliptically polarized light and the resulting formation of macroscopic chiral structures in a series of azobenzene side-chain copolyesters, in which the morphology is varied from liquid crystalline to amorphous, is reported. Real-time measurements are presented, showing the dynamic behavior of the photoinduced rotation of the polarization ellipse in the different samples. The relationship between the ellipticity of the recording light and the linear birefringence induced is studied. A numerical solution that takes into account the influence of the photoinduced linear dichroism on the light propagation through the samples is presented. Received: 10 June 2002 / Revised version: 26 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +45-4677/4588, E-mail: lian.nedelchev@risoe.dk RID="**" ID="**"Permanent address: Department of Chemistry and Physics, The Nottingham Trent Unversity, Clifton Lane, Nottingham NG11 8NS, England     

Electrical switching studies on As<Subscript>40</Subscript>Te<Subscript>60-x</Subscript>Se<Subscript>x</Subscript> and As<Subscript>35</Subscript>Te<Subscript>65-x</Subscript>Se<Subscript>x</Subscript> glasses

The I–V characteristics of bulk As₄₀Te_60-xSe_x and As₃₅Te_65-xSe_x glasses have been studied with a current sweep of 0–18 mA-0, over a wide range of compositions (4≤x≤22). All the glasses studied showed a threshold electrical switching behaviour. The number of switching cycles withstood by the samples has been found to depend on the ON-state current. It is seen that the switching voltages increase with increase in selenium content. Further, the switching voltages are found to be almost independent of the thickness of the sample (d), in the range 0.18–0.3 mm. Also, the switching voltages and the number of switching cycles withstood by the samples are found to decrease with temperature. Received: 6 November 2002 / Accepted: 8 November 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +91-80/360-0135, E-mail: sasokan@isu.iisc.ernet.in     

Deposition of large-area, high-quality cubic boron nitride films by ECR-enhanced microwave-plasma CVD

Large-area, 1-μm-thick cubic boron nitride (cBN) films were deposited on (001) silicon substrates by electron-cyclotron-resonance-enhanced microwave-plasma chemical vapor deposition (ECR-MP CVD) in a mixture of He-Ar-N₂-BF₃-H₂ gases. With the assistance of fluorine chemistry in the gas phase and substrate reactions, the phase purity of the sp³-configuration was improved to over 85% at a reduced substrate bias voltage of -40 V. The grown films show clear Raman transversal optical (TO) and longitudinal optical (LO) phonon vibration modes, characteristic of cBN. Such Raman spectral characteristics are the first ever observed in cBN films prepared under ECR-MP CVD conditions. Received: 3 May 2002 / Accepted: 7 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +852-2788/7830, E-mail: apwjzh@cityu.edu.hk     

Ion-beam synthesis and growth mechanism of diamond-like materials

Opposite to most other deposition methods, the dominating nucleation and growth mechanism during ion-beam deposition of energetic ions in the range between 10 eV and 10 keV occurs in a region of a few nanometers below the surface of the growing film. This process is called ‘subplantation’ – emphasizing the implantation of ions into a subsurface region. Ordering and phase formation is a result of the interaction of the deposited ions with the solid state that takes place within the short time scale of femto- and picoseconds. This extreme non-equilibrium process can result in metastable amorphous or crystalline structures. This review will present several examples of the influence of the deposition parameters on the properties of diamond-like materials synthesized using mass-selected ion-beam deposition. Furthermore, several existing models of the deposition process will be presented and critically discussed. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-551/39-4493, E-mail: carsten.ronning@phys.uni-goettingen.de     

GaN nanotweezers

A new form of GaN nanomaterial (nanotweezers) has been obtained by chemical vapor deposition on an etched cubic MgO (100) plane. The nanotweezers consist of a bottom rod and two arms. The bottom rods have diameters of about 100–150 nm and lengths of about 200–500 nm, on which two arms grow out. The bottoms of the arms are about 40–70 nm and the tops are about 15–30 nm in diameter, and 0.8–1.5 μm in length. X-ray and electron diffractions indicate the nanotweezers are zinc blende gallium nitride. We infer that the fabrication of the GaN nanotweezers is associated with small convex hillocks on the surface of the etched cubic MgO (100) single-crystal substrates and that the nanotweezers grow by a growth mechanism that is similar to vapor-phase heteroepitaxy. Received: 23 April 2002 / Accepted: 25 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531, E-mail: xlchen@aphy.iphy.ac.cn     

Large-scale synthesis of crystalline β-SiC nanowires

Large quantities of high-purity crystalline β-SiC nanowires have been synthesized at relatively low temperature via a new simple method, the chemical-vapor-reaction approach, in a home-made graphite reaction cell. A mixture of milled Si and SiC powders and C₃H₆ were employed as the starting materials. The results show that the nanowires with diameters of about 10–35 nm are single crystalline β-SiCwithout any wrapping of amorphous material, and the nanowire axes lie along the 〈111〉 direction. Some unique properties are found in the Raman scattering from the β-SiC nanowires, which are different from previous observations of β-SiC materials. A possible growth mechanism for the β-SiC nanowires is proposed. Received: 27 August 2002 / Accepted: 28 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-29/8491-000, E-mail: zjli-sohu@sohu.com     

VUV F<Subscript>2</Subscript> laser ablation of sodium chloride

We report an investigation of the ablation of NaCl crystals at the 157-nm wavelength of the F₂ laser where there is very strong excitonic absorption. Probe-beam deflection and etch-rate measurements show that the interaction is characterised by a low ablation threshold (∼80 mJ cm^-2) and a capability for controllable material removal at the nanometer level. Scanning electron microscopy of the exposed surfaces show this to be microscopically smooth but with fine cracks present. It is demonstrated that micron-scale features can be formed in NaCl using 157-nm laser ablation, a result attributed to the strongly localised optical and thermal nature of the interaction. The results are discussed within the framework of a thermal vaporisation model. Received: 29 May 2002 / Accepted: 17 July 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1482/465606, E-mail: p.e.dyer@hull.ac.uk     

Interaction of two light beams with different polarizations in TGS

By ellipsometric measurements we have observed the polarization variations of the primary light beam, with wavelength λ₁=5145 ?, interacting with the secondary light beam of controlled variable polarization, transmitted through a TGS crystal layer. We present a theoretical explanation based on the second-order optical nonlinearity of TGS. Received: 4 December 2001 / Revised version: 28 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +48-22/660-5447, E-mail: petyk@if.pw.edu.pl     

Stress investigations of thin amorphous Ge-Se-Ga films

Thin chalcogenide films from the systems (GeSe₄)_1-xGa_x and (GeSe₅)_1-xGa_x with gallium contents up to 20 at. % have been prepared by vacuum evaporation and their stress has been investigated by a cantilever technique. The addition of gallium to the Ge-Se matrix plays an important role in stress formation in the films: films without gallium possess negligible stress, while all gallium-containing films are under compressive stress. The increase of the gallium content leads to structural changes and an increase in the density, which results in higher stress values. For all films, stress reduction with time is observed due to spontaneous relaxation. Received: 2 October 2002 / Accepted: 22 November 2002 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-561/8044-136, E-mail: popov@schottky.physik.uni-kassel.de     

Microstructure evolution in Pr-Co-C-Ti nanophase magnets

The microstructures of nanophase Pr-Co-C-(Ti) materials, which have improved magnetic properties, were investigated by means of transmission electron microscopy (TEM) to reveal their phase assemblage and grain-boundary structure. The phase assemblage was carefully controlled by the introduction of TiC nanoparticles and annealing. The optimal nanostructure contained uniformly distributed PrCo₅ and PrCo₂ nanophases without any magnetically soft phases, resulting in high coercivity and the characteristics of a single, hard magnetic phase. TEM analysis confirmed the presence of an amorphous grain-boundary phase surrounding the grains in alloys without TiC. In contrast, alloys with added TiC showed no amorphous phase and also showed higher coercivity compared to Co-Pr-C. Therefore, the variation of the grain boundary phases may be effective in changing the degree of exchange coupling. Controlling the formation of a uniform nanoscale microstructure, leading to improved magnetic properties, is discussed. Received: 5 September 2002 / Accepted: 10 September 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +1-630/252-7777, E-mail: ytang@anl.gov     

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     

Modification of thin-layer systems by swift heavy ions

The electronic energy loss of swift heavy ions (MeV/amu) within a solid results in a highly excited cylindrical zone of some nm in diameter, within which all atoms may be in motion for some tens of ps (transient local melting). After cooling down, a defect-rich or even amorphous latent track is left in many cases, especially in insulating materials. The resulting property alterations (density, micro-structure, morphology, phase composition, etc.) have been investigated for many bulk materials, while only very few experiments have been carried out with thin-film systems. In the present paper, a summary will be given of our studies on the transport of matter in thin-film packages induced by irradiation with high-energy ions. These is, on the one hand, atomic mixing at the interfaces, which is especially pronounced in ceramic systems and which seems to occur by interdiffusion in the molten ion track. On the other hand, we have discovered a self-organisation phenomenon in swift-heavy-ion-irradiated NiO layers, which at low fluences first showed periodic cracking perpendicular to the projected beam direction. After application of high fluences, the NiO layer was reorganised in 100-nm-thick and 1-μm-high NiO lamellae of the same separation distance (1–3 μm) and orientation as found for the cracks. Both effects can be attributed to transient melting of the material surrounding the ion trajectory. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-711/6853-866, E-mail: bolse@ifs.physik.uni-stuttgart.de     

Micrometer-sized Si-Sn-O novel structures with SiONWs on their surfaces

Novel micrometer-sized Si-Sn-O structures with SiO₂ nanowires (SiONWs) growing from their surfaces have been achieved at about 980 °C on Si (111) wafer catalyzed by Sn vapor generated from Sn powders. The Si wafer itself served as a silicon source in the reaction. The micrometer-sized structures, with diameters of several micrometers to several tens of micrometers consisted of Sn, Si and O. The amorphous SiONWs growing from the surface of the micrometer-sized structures were smooth, with diameters about 120 nm and with a composition close to that of SiO₂. The growth mechanism of these novel structures is discussed briefly. Received: 30 July 2002 / Accepted: 18 September 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-551/5591434, E-mail: shsuncn@hotmail.com     

Resonances and spectral properties of detuned OPOs pumped by fluctuating sources

Twin-beam fluctuations are analyzed for detuned and mismatched OPO configurations. Resonances and frequency responses to the quantum noise sources (quantum and pump amplitude/phase fluctuations) are examined as functions of cavity decay rates, excitation parameters and detuning. The dependence of self- and mutual correlations of beam amplitudes and phases on detuning, mismatch and damping parameters is discussed. Received: 25 January 2002 / Revised version: 28 May 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-81/676-346, E-mail: alberto.porzio@na.infn.it     

Pulsed laser deposition and in situ diagnostics of the process applied to shape-memory alloys

NiTi films deposited by pulsed laser ablation on Si/SiO₂ are shown to exhibit structural and functional properties related to the shape-memory effect. Film characterization suggests that relevant temperatures for the solid-to-solid transformation responsible for the shape memory are in substantial agreement with those of the bulk target material, demonstrating a good congruency of the deposition process. Besides the technological interest for this class of thin films, our findings point out the suitability of laser ablation for metal alloy deposition. An investigation based on in situ ion-mass spectroscopy and covariance mapping analysis allows us to determine the main vapor-phase processes leading to the formation of stoichiometric clusters expected to play a relevant role in assisting the growth of NiTi thin films. Received: 6 August 2001 / Accepted: 11 April 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-50/2214-333, E-mail: fuso@df.unipi.it     

High-peak-power diode-pumped passively Q-switched Nd:YVO<Subscript>4</Subscript> laser

We report on a passively Q-switched diode-pumped Nd:YVO₄ laser polarized along the a axis (corresponding to the smallest value of emission cross section at 1064 nm), generating 157-μJ pulses with 6.0-ns time duration (>20 kW peak power) and 3.6 W of average power at 1064 nm with good beam quality (M²<1.4). The selection of the polarization was performed by a novel technique relying on the birefringence of the laser crystal and on the misalignment sensitivity of the resonator. Received: 30 September 2002 / Revised version: 22 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +39-382/422583, E-mail: agnesi@ele.unipv.it