Metal-induced solid-phase crystallization of hydrogenated amorphous silicon: dependence on metal type and annealing temperature

We report on metal (Cr, Ni, or Pd)-induced solid-phase crystallization (MISPC) of plasma-enhanced chemical-vapor-deposited hydrogenated amorphous silicon at annealing temperatures ≤600 °C. MISPC is found to significantly reduce the thermal budget of crystallization at annealing temperatures as low as ∼400 °C. The lowest achievable annealing temperature is found to depend on the metal type. The metal type is also found to influence grain size and the conductivity of the polycrystalline silicon. Received: 21 June 1999 / Accepted: 20 October 1999 / Published online: 23 February 2000     

Implications of the precision data for very light Higgs Boson scenarios in 2HDM(II)

We present an up-to-date analysis of the constraints the precision data impose on the (CP-conserving) Two Higgs Doublet Model of type II with emphasis on the possible existence of very light neutral (pseudo)scalar Higgs boson with mass below 20–30 GeV. We show that even in the presence of such light particles, the 2HDM(II) can describe the electroweak data with the precision comparable to that given by the SM. Particularly interesting lower limits on the mass of the lighter neutral CP-even scalar are obtained in the scenario with light CP-odd Higgs boson and large . Received: 8 June 1999 / Published online: 14 October 1999     

Characterization of photorefractive materials by spontaneous noncolinear frequency doubling

‘Spontaneous’ noncolinear frequency doubling (SNCFD) is a type of optical second-harmonic generation (SHG) that uses scattered light to provide additional fundamental beams in order to accomplish noncolinear phase matching. Based on a novel algorithm for the automated evaluation of the resulting ring patterns, we present an easy-to-apply, sensitive, and non-destructive method for the characterization of photorefractive materials, yielding two-dimensional spatial resolution. As applications of the technique, examples for the characterization of lithium niobate crystals are presented. Received: 30 October 1998 / Revised version: 17 December 1998 / Published online: 7 April 1999     

Optical properties of nickel(II)-azo complexes thin films for potential application as high-density recordable optical recording media

Smooth thin films of three kinds of nickel(II)-azo complexes were prepared by the spin-coating method. Absorption spectra of the thin films on K9 glass substrate in the 300-600 nm wavelength region were measured. Optical constants (complex refractive index ) and thickness of the thin films prepared on single-crystal silicon substrate in the 300-600 nm wavelength region were investigated on rotating analyzer-polarizer type of scanning ellipsometer, and dielectric constants , absorption coefficients α as well as reflectance R of thin films were then calculated at 405 nm. In addition, in order to examine the possible use of nickel(II)-azo complex thin film as an optical recording medium, one of the nickel(II)-azo complex thin film prepared on K9 glass substrate with an Ag reflective layer was also studied by atomic force microscopy and static optical recording. The results show that the nickel(II)-azo complex thin film is smooth and has a root mean square surface roughness of 2.25 nm, and the recording marks on the nickel(II)-azo complex thin film are very clear and circular, and their size can reach 200 nm or less.     

Fermonic zero-norm states and enlarged supersymmetries of Type II string

We calculate the NS–R fermionic zero-norm states of the type II string spectrum. The massless and some possible massive zero-norm states are seen to be responsible for the space-time supersymmetry. The existence of other fermionic massive zero-norm states with higher spinor–tensor indices correspond to new enlarged boson–fermion symmetries of the theory at high energy. We also discuss the R–R charges and R–R zero-norm states and justify the idea that the perturbative string does not carry the massless R–R charges. Received: 12 July 1999 / Revised version: 16 September 1999 / Published online: 17 March 2000     

Pulsed blue-light generation by the frequency doubling of the 4F3/2 to 4I9/2 transition in Nd:YAG and Nd:YAlO3

We report on the frequency doubling of Q-switchedNd:YAG and Nd:YAlO₃ lasers emitting at 946 and 930 nm, respectively (⁴F_3/2 to ⁴I_9/2 transition). The neodymium-doped laser host crystals were excited with a flashlamp-pumped Cr:LiSAF laser operating in a free-running mode. Blue-light pulses were obtained at both 473 nm (9 mJ, 25 ns FWHM) and 465 nm (4.4 mJ, 35 ns FWHM) by using a potassium niobate crystal as an extra-cavity frequency doubler. The second-harmonic generation conversion efficiencies reached 53% and 31%, respectively. Received: 23 June 1999 / Revised version: 8 August 1999 / Published online: 3 November 1999     

Surface-state linewidth from scanning tunnelling spectroscopy

The STM can be used to investigate lifetimes of hot holes in a surface state at low temperatures. We analysed dI/dV data from Ag(111) using detailed tunnelling calculations and a simple model and found an electron self-energy of Σ=4.9±0.6 meV. The corresponding lifetime τ= 67±8 fs is considerably higher than those determined by angle-resolved photoemission, although it remains below theoretical predictions. Spatially resolved dI/dV spectra reveal that the lifetime decreases drastically in proximity to defects such as surface steps. Received: 22 March 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999     

Dynamic light beam deflection caused by space charge waves in photorefractive crystals

During holographic recording in photorefractive crystals (BSO, BGO, and BTO) by an oscillating interference pattern we observe a strong dynamic deflection of the laser beams reflected from the crystal’s surface. The theoretical treatment shows that this new effect is associated with a nonlinear interaction of space charge gratings resulting in a quasi-homogeneous oscillating space charge field which provides deformations of the crystal via the piezoelectric effect. Received: 3 August 1999 / Accepted 5 August 1999 / Published online: 8 September 1999     

Atom lithography with a cold, metastable neon beam

We study different aspects of atom lithography with metastable neon atoms. Proximity printing of stencil masks is used to test suitable resists that are sensitive to the internal energy of the atoms, including dodecanethiols on gold and octadecyltrichlorosilanes grown on a SiO₂ surface. As an example of patterning the atomic beam with laser light, we create parallel line structures on the surface with a periodicity of half the laser wavelength by locally de-exciting the atoms in a standing quenching wave. Received: 29 June 1999 / Revised version: 30 August 1999 / Published online: 10 November 1999     

Defect passivation using ultrasound treatment: fundamentals and application

Ultrasonic vibrations introduced into semiconductor thin-films can trigger defect reactions, which are beneficial for electronic materials and devices. This type of semiconductor processing is assigned as ultrasound treatment (UST). The UST technology was initially developed in compound semiconductors and recently successfully applied to Si-based materials. The analysis of UST effects is performed within a general scenario of three-step point-defect gettering comprised of the (a) release, (b) diffusion, and (c) capture of the defects. As a demonstrating vehicle of UST mechanisms, the experimental data on ultrasonically enhanced diffusion of atomic hydrogen in thin polycrystalline Si films are discussed. UST applied to plasma-hydrogenated films improves the homogeneity of recombination and transport properties. Ultrasound promotes a passivation of grain boundary defects as revealed using scanning photoluminescence spectroscopy, contact potential difference with nano-scale resolution, and sheet resistance measurements. The results favor a model of trap-limited hydrogen diffusion facilitated by ultrasound. Illustrative examples of the UST application for electronic devices are presented. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 14 June 1999     

Collisional effects on the collective laser cooling . of trapped bosonic gases

We analyse the effects of atom–atom collisions on a collective laser cooling scheme. We derive a quantum master equation which describes the laser cooling in presence of atom–atom collisions in the weak-condensation regime. Using such equation, we perform Monte Carlo simulations of the population dynamics in one and three dimensions. We observe that the ground-state laser-induced condensation is maintained in the presence of collisions. Laser cooling causes a transition from a Bose–Einstein distribution describing collisionally induced equilibrium, to a distribution with an effective zero temperature. We analyse also the effects of atom–atom collisions on the cooling into an excited state of the trap. Received: 18 June 1999 / Revised version: 24 September 1999 / Published online: 10 November 1999     

Efficient cw sodium dimer Raman laser operation in a high-temperature sapphire cell

cw Raman lasing of Na₂ molecules generated in a heated, sealed-off, all-sapphire cell is demonstrated. Being not damaged by highly corrosive alkaline vapours, this type of cell enables operation without buffer gas in contrast to the normal heatpipe operation of these lasers. This allows us to study Raman lasers in alkaline vapours in new regimes and under ideal conditions. With an argon ion pump laser at 488 nm, Raman laser operation at 525 nm with more than 10% efficiency and thresholds below 0.2 mW for a cell without buffer gas (length 9 cm) have been obtained so far. The low thresholds, being a factor of 10 less than for comparable heatpipe operation, gives us the chance to use low-power diode lasers as pump sources and to realize compact reliable Raman laser systems. Received: 17 May 1999 / Published online: 25 August 1999     

Output from an atom laser: theory vs. experiment

Atom lasers based on rf-outcoupling can be described by a set of coupled generalized Gross–Pitaevskii equations (GPE). We compare the theoretical predictions obtained by numerically integrating the time-dependent GPE of an effective one-dimensional model with recently measured experimental data for the F=2 and F=1 states of Rb-87. We conclude that the output of a rf atom-laser can be described by this model in a satisfactory way. Received: 15 June 1999 / Revised version: 9 September 1999 / Published online: 10 November 1999     

Electrically conducting ion tracks in diamond-like carbon films for field emission

Electrically conducting channels in an insulating carbon matrix were produced by 140-MeV Xe ion irradiation. The high local energy deposition of the individual ions along their pathes causes a rearrangement of the carbon atoms and leads to a transformation of the insulating, diamond-like (sp³-bonding) form of carbon into the conducting, graphitic (sp²-bonding) configuration. The conducting ion tracks are clearly seen in the current mapping performed with an atomic force microscope (AFM). These conducting tracks are of possible use in field emission applications. Received: 4 May 1999 / Accepted: 5 May 1999 / Published online: 1 July 1999     

Application of scanning force microscopy in nanotube science

Recent developments in the application of scanning force microscopy in nanotube science are reviewed. The non-destructive character of this technique allows the structural characterisation of (chemically modified) single- and multi-wall nanotubes deposited on substrates for further investigations such as electrical transport measurements. Furthermore, SFM is now an established tool for manipulation of nanotubes, which allows position control and determination of elastic constants such as the Young’s modulus. Finally it is shown that very sharp and stable probes for scanning force microscopy can be made from nanotubes due to their excellent stability and aspect ratio. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 29 July 1999     

Solid-phase production of carbon nanotubes

4 C₃) are heated to 1000–1100 °C for 2–4 hr in an argon flow. Carbon nanotubes are formed when metal powders, e.g. Fe, Co, Y and Ni, are also present. Received: 2 February 1999 / Accepted: 3 February 1999     

Holographic recording in amorphous side-chain polymers: a comparison of two different design philosophies

Recently, Bieringer et al. [1] reported on the optical properties of liquid-crystalline side-chain polymers. These photoaddressable systems showed light-induced modulations Δn of their refractive index of up to 0.06. We present similar materials and compare them with a novel ansatz reaching birefringence levels of up to 0.2 [2]. This was achieved by donor–acceptor substitutions of the chromophores and by introducing a new type of second side chain. We discuss the different photophysical properties of the two materials under the point of view of potential applications. Received: 16 November 1998 / Revised version: 18 December 1998 / Published online: 7 April 1999     

Non-steady-state photocurrents in SnS2 crystals

We report on non-steady-state photocurrent measurements of SnS₂ grown by vapor transport method. The main characteristics of photoinduced carriers, such as type of photoconductivity, average diffusion length, dielectric relaxation time, and the Debye screening length, were estimated for the excitation wavelengths in the red and blue region of the spectrum. Received: 12 November 1998 / Revised version: 22 December 1998 / Published online: 31 March 1999     

III-V compounds for solar cell applications

Research activities in the field of III-V solar cells are reviewed. III-V compound semiconductors are used for space solar cells, concentrator solar cells, and in thermophotovoltaic generators. The epitaxial growth of ternary and quaternary material by MOVPE and LPE allows us to realize various band gaps. Multi-junction solar cells with different band gaps are necessary to obtain efficiencies larger than 30%. Recent results of the III-V solar cell research at the Fraunhofer ISE are presented. A mechanically stacked GaAs/GaSb tandem concentrator solar cell achieved an efficiency of 31.1% under 100×AM1.5d. An efficiency of 23% for a two-terminal concentrator module (486 cm²) with Fresnel lenses has been measured under realistic outdoor conditions. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Superluminal pulse propagation in an erbium fiber laser

It is well known that saturating gain media can give rise to superluminal pulse propagation velocity. In most mode-locked lasers the effect is unmeasurably small and has not yet been directly demonstrated. We present experiments on the initial transient of an erbium fiber laser that show a dynamic shift in the propagation velocity to a value larger than the medium’s linear group velocity as the pulse builds up. Received: 14 December 1998 / Revised version: 21 May 1999 / Published online: 25 August 1999