Showing papers in "Applied Physics A in 1996"

Femtosecond, picosecond and nanosecond laser ablation of solids

Abstract: Laser ablation of solid targets by 0.2–5000 ps Ti: Sapphire laser pulses is studied. Theoretical models and qualitative explanations of experimental results are presented. Advantages of femtosecond lasers for precise material processing are discussed and demonstrated.

Scattering mechanisms of charge carriers in transparent conducting oxide films

Abstract: Scattering mechanisms of charge carriers in Transparent Conducting Oxide (TCO) films have been analyzed theoretically. For the degenerate polycrystalline TCO films with relatively large crystallite sizes and high carrier concentrations (higher than 5 × 1018 cm−3), the depletion layers between crystallites are very thin compared to the crystallite sizes, and the grain boundary scattering on electrical carriers makes a small contribution to limit the mobility of the films. Instead of thermionic emission current, a tunneling current dominates the electron transport over grain boundaries. The Petritz model which is based on thermionic emission and extensively quoted in literature should not be applicable. The main scattering mechanisms for the TCO films are ionized impurity scattering in the low-temperature range and lattice vibration scattering in the high-temperature range. The ionized impurity scattering mobility is independent of temperature and the mobility due to thermal lattice vibration scattering is inversely proportional to the temperature. The results obtained from Hall measurements on our ZnO, ITO, SnO2 and SnO2:F films prepared with various methods supports the analysis.

Fluorescence interference-contrast microscopy on oxidized silicon using a monomolecular dye layer

Abstract: A silicon chip is covered by a monomolecular film of a fluorescence dye with silicon dioxide used as a spacer. The fluorescence depends on the distance of the dye from the silicon. The modulation of the intensity is described quantitatively by an optical theory which accounts for interference of the exciting light and of the emitted light. The effect is used to obtain a microscopic picture of the surface profile with a precision of a few Angstroms. The perspectives for an application in wet systems such as neuron-silicon junctions and lipid membranes on silicon are pointed out.

A study of thermal and mechanical effects on materials induced by pulsed laser drilling

Abstract: Holes drilled in metals and silicon using different short-pulse lasers (copper vapour, Nd : YLF and titanium: sapphire) were characterized under optical and electron microscopy. The aim was to analyze and compare the thermal and mechanical effects on materials induced by laser drilling with a wide range of pulse widths (50 ns to 200 fs) and power densities (108 W/cm2 to 1015 W/cm2).

The melting of silicon nanocrystals: Submicron thin-film structures derived from nanocrystal precursors

Abstract: The size-dependent melting temperature has been shown for the group IV semiconductor silicon. A cursory comparison is made between silicon nanocrystal melting and that observed in the group II–VI material Cds and the group III–V material GaAs. Particles dispersed at a number density such that there are interactions between nanocrystals are observed to sinter before size-dependent melting occurs. Using an electron beam to selectively remove an organic mask from a substrate, this phenomenon is exploited to produce thin-film structures. This work has implications in the production of nanoelectronics, nonlinear optics and solar conversion technologies.

Picosecond laser ablation of thin copper films

Abstract: The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF thres = 172 mJ/cm2. The ablation rate per pulse is measured as a function of intensity in the range of 5 × 109 to 2 × 1011 W/cm2 and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.

Substrate-interaction, long-range order, and epitaxy of large organic adsorbates

Abstract: Large and symmetric organic molecules (>200 amu) can form highly-ordered adsorbate layers and thin films when they are deposited by vacuum sublimation on clean reactive surfaces In such cases covalent bonding often occurs via the molecular π-system leading to a parallel orientation of the adsorbate as shown for oligothiophenes and PTCDA on Ag(1 1 1) A proper choice of the substrate and/or a preadsorbate may also cause an upright orientation with bonding via a reactive group of the molecule (example: NDCA/Ni(1 11)) Most of the used molecules yield long-range ordered monolayers with large, almost defect-free domains The stronger the bonding and the smaller the molecule the more likely is the formation of commensurate superstructures which indicate site-specific adsorption even for such large molecules as PTCDA or EC4T Organic epitaxy is discussed and shown for a particular system, PTCDA on Ag(1 1 1), for which the structure of the monolayer is nearly identical to that of theβ-modification of PTCDA crystals, whereas on other substrates (eg Si(1 1 1), Ge(1 0 0)) a disordered interface and hence no true epitaxy is found

Current transport in Pd2Si/n-Si(100) Schottky barrier diodes at low temperatures

Abstract: The forward current-voltage (I–V) characteristics of Pd2Si/n-Si(100) Schottky barrier diodes are shown to follow the Thermionic Emission-Diffusion (TED) mechanism in the temperature range of 52-295 K. The evaluation of the experimentalI–V data reveals a decrease of the zero-bias barrier height (ϕb0) and an increase of the ideality factor (η) with decreasing temperature. Further, the changes in ϕb0 and η become quite significant below 148 K. It is demonstrated that the findings cannot be explained on the basis of tunneling, generation-recombination and/or image force lowering. Also, the concepts of flat band barrier height and “T0-effect” fail to account for the temperature dependence of the barrier parameters. The 1n(Is/T2) vs 1/T plot exhibits nonlinearity below 185 K with the linear portion corresponding to an activat ion energy of 0.64 eV, a value smaller than the zero-bias barrier height energy (0.735 eV) of Pd2Si/n-Si Schottky diodes. Similarly, the value of the effective Richardson constant A** turns out to be 1.17 × 104 A m−2 K−2 against the theoretical value of 1.12 × 106 A m−2 K−2. Finally, it is demonstrated that the observed trends result due to barrier height inhomogeneities prevailing at the interface which, in turn, cause extra current such that theI–V characteristics continue to remain consistent with the TED process even at low temperatures. The inhomogeneities are believed to have a Gaussian distribution with a mean barrier height of 0.80 V and a standard deviation of 0.05 V at zero-bias. Also, the effect of bias is shown to homogenize barrier heights at a slightly higher mean value.

Machining of submicron structures on metals and semiconductors by ultrashort UV-laser pulses

Abstract: Ablation of submicron structures on copper and silicon by short ultraviolet laser pulses (0.5–50 ps, 248 nm) is presented. Features like periodic line structures with a line-spacing below 400 nm, and holes with characteristic sizes well below 1 µm are produced on the sample surface by single laser shot exposure. The structures are projection printed by a Schwarzschild-objective (N.A.=0.4) in air environment. The morphology of ablation sites made with different pulse durations (0.5 ps, 5 ps, 50 ps) is discussed in terms of thermal diffusion effects.

Physical and material aspects in using visible laser pulses of nanosecond duration for ablation

Abstract: Interaction phenomena of 50 ns copper vapour laser pulses (λ = 511/578 nm) with matter are investigated. The basic ablation process is classified into four fundamental classes. On basis of this classification processing results are connected with specific material properties like the brittleness, the viscosity of the melt or the optical properties. Knowing these properties a prognosis of the expected fundamental process is possible. In order to generate a geometrically defined structure via ablation in a given material-specific process, strategies have to be developed. Typical examples for process strategies are given.

Fabrication of nano-dot- and nano-ring-arrays by nanosphere lithography

Abstract: We have applied the method of nanosphere lithography to fabricate arrays of nanometer-scale gold and cobalt particles. The individual cobalt particles were found to be in a single domain state as verified by magnetic force microscopy. By tuning the preparation conditions, we also successfully fabricated arrays of mesoscopic gold rings for the first time with potential application for persistent current experiments.

Erbium incorporation in LiNbO3 by diffusion-doping

Abstract: The erbium incorporation into LiNbO3 by diffusion doping is investigated in detail by means of Secondary Ion Mass Spectrometry, Secondary Neutral Mass Spectrometry, Rutherford Backscattering, Atomic Force Microscopy, X-ray Standing Wave technique and optical site-selective spectroscopy. The diffusion of erbium in LiNbO3 can be described by Fick’s laws of diffusion with a concentration-independent diffusion coefficient. The diffusion constants and activation energies for Z-cut (X-cut) LiNbO3 are 4.8×10-5 cm2/s (12.0×10-5 cm2/s) and 2.28 eV (2.44 eV), respectively. A limited solubility of erbium in LiNbO3 has to be taken into account increasing exponentially with rising temperature. During the first step of diffusion an Er x Nb y -oxide layer is formed at the surface of the sample acting as diffusion reservoir. Erbium is incorporated into LiNbO3 on vacant Li-sites slightly shifted from the original Li-position along the (-c)-direction. Site-selective spectroscopy found four distinguishable energetically different erbium centres at this lattice site resulting from locally different symmetries of the crystal field.

Invited paper Observation of plasma shielding by measuring transmitted and reflected laser pulse temporal profiles

Abstract: Mass ablation rate increases with laser power density following a power law dependence and a significant change occurs at 0.3 GW/cm $^2$ . A reflected laser temporal profile was measured from a brass sample. When the power density is greater than 0.3 GW/cm $^2$ +, the temporal profile changes. The transmitted laser-pulse temporal profile through a glass sample also was measured. When the power density is greater than 0.3 GW/cm $^2$ , the later part of laser pulse becomes truncated. The power density at which the laser temporal profile changes for each case is same as the power density that the mass ablation rate coefficient changes. The ablated mass can absorb incoming laser radiation through inverse Bremsstrahlung. The mass becomes thermally ionized and opaque to the incident radiation, preventing laser light from reaching the surface. A model based on thermal evaporation and inverse Bremsstrahlung absorption was developed. Calculations show that plasma shielding occurs at approximately 0.3 GW/cm $^2$ . The experiments and model suggest that the significant change observed in mass ablation rate coefficient is caused by plasma shielding.

Laser-induced damage in SiO2 and CaF2 with picosecond and femtosecond laser pulses

Abstract: Single- and multiple-shot damage thresholds and plasma-emission thresholds for fused silica and CaF2 are reported for 790 nm photons as a function of laser pulse width (190 fs – 4.5 ps). The results are compared with single-shot plasma-emission measurements [1] and with multiple-shot damage measurements [2]. Both the damage threshold and the plasma-emission threshold are shown to decrease with decreasing pulse width over the entire pulse-width range investigated.

The magnetism of nickel monolayers

Abstract: Nickel allows to study the largest variety of phenomena in the magnetism of UHV ultrathin films. The low critical temperature of ≈ 630 K for the bulk favors experiments from 0 K to aboveT c and from one monolayer to infinite thick films. The thickness dependence ofT c (d) for the (001) and the (111) orientation is compared. Susceptibility measurements in UHV are presented, and from χmax the film geometry can be deduced. Ferromagnetic resonance measures the second- and fourth-order anisotropy constants. These give a clear understanding of when and how the reorientation transition from the in-plane to the perpendicular orientation occurs and its nature. Magnetic resonance and circular X-ray dichroism measure the spin and orbital parts of the magnetic moment µ, its anisotropy Δµ, and the 3d and 4sp contributions. Finally, we show how a 4 Monolayer (ML) Ni(001) film can be transformed into NiO by controlled oxygen dosage and thermal treatment.

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Abstract: Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H2/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H2 or D2. The measured initial sticking coefficients for a gas temperature of 300K range from 10−9 to 10−5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.

Microcavity dynamics during laser-induced spallation of liquids and gels

Abstract: Photomechanical fracture induced by thermoelastic stress waves is an important mechanism of tissue ablation by short laser pulses. In this study, we present experimental investigations of the fracture process in ductile, water-containing materials and compare the results with a theoretical calculation. The model describes cavitation caused by the negative part of a bipolar thermoelastic stress wave. Pulses from aQ-switched, frequency-doubled Nd:YAG laser with 8 ns duration were used to irradiate dyed water and gelatine with variable absorption coefficient. Cavitation and ablation were observed with various time-resolved methods such as stress detection, video imaging and an optical pump-probe technique for the detection of individual cavities. Quantitative agreement between experiment and simulation could be achieved in the case of cavity lifetimes, especially at low laser fluence where the bubble density is low and no coalescence takes place. An increase of the threshold energy density for ablation with rising absorption coefficient and a distortion of the thermoelastic wave in the presence of cavitation were experimentally observed and could be qualitatively explained by use of the simulation. The results obtained in this study should facilitate the choice of the optimal laser parameters for photomechanical tissue ablation.

Reconstruction of surface topographies by scanning electron microscopy for application in fracture research

Abstract: Stereoscopic scanning electron micrographs can be used to reconstruct the microscopic topography of material surfaces. By applying a system for automatic image processing we can obtain Digital Elevation Models (DEMs) of the investigated surface. These DEMs are used to measure the degree of deformation on metallic fracture surfaces. By modelling the deformation the amount of plastic energy that is necessary to shape the microductile fracture surface can be calculated. These values are compared with experimentally obtained results.

Preparation and characterization of highly oriented, photoconducting WS2 thin films

Abstract: A novel combination of methods is shown to produce semiconducting WS2 thin films with properties close to those of a single crystal. The first step requires the deposition of a very thin Ni layer on a quartz substrate. On top of it an amorphous, sulphur rich, (WS3 +x) thin film is deposited by reactive rf sputtering. The final annealing step in an argon atmosphere yields 200 nm thick WS2 films. X-ray diffraction shows that the films crystallize in the 2H-WS2 phase and are perfectly oriented with the (002) basal planes parallel to the substrate. Residual W18O49 needles andβ-NiS grains are detected by transmission electron microscopy. The dc conductivity and its activation energy have values typical of bulk crystals. The optical absorption spectrum measured at Room Temperature (RT) shows excitonic peaks at the same energies as in a single crystal. RT photoconductivity measured as a function of wavelength is shown to result from interband transitions.

Oxygen diffusion through thin Pt films on Si(100)

Abstract: We have investigated the diffusion of oxygen through evaporated platinum films on Si(100) upon exposure to air using substrates covered with Pt films of spatially and continuously varying thickness (0–500 A). Film compositions and morphologies before and after silicidation were characterized by modified crater edge profiling using scanning Auger microscopy, energy-dispersive X-ray microanalysis, scanning tunneling microscopy, and transmission electron microscopy. We find that oxygen diffuses through a Pt layer of up to 170 A forming an oxide at the interface. In this thickness range, silicide formation during annealing is inhibited and is eventually stopped by the development of a continuous oxide layer. Since the platinum film consists of a continuous layer of nanometer-size crystallites, grain boundary diffusion of oxygen is the most probable way for oxygen incorporation. The diffusion constant is of the order of 10−19 cm2/s with the precise value depending on the film morphology.

Restructuring of the surface region in SrTiO3

Abstract: The combination of x-ray diffraction analysis with surface sensitive techniques reveals a chemical inhomogeneity in the surface region of monocrystals of SrTiO3 prepared under low and high partial pressure of oxygen at elevated temperatures. A solid state reaction leads to the formation of a multilayer-type structure. For oxidized crystals we observe AO-rich Ruddlesden-Popper phases at the surface and Magnelli phases of Ti in deeper layers of the surface region. The order of the layered structure is reversed for the reduced crystals with Ti-oxides of different oxidation levels at the surface and Ruddlesden-Popper phases in lower parts of the surface region. It is argued that this restructuring influences the electrical properties of SrTiO3.

Probing interface properties of nanocomposites by third-order nonlinear optics

Abstract: We describe the exploitation of third-order nonlinear optical response — particularly nonlinear absorption and the nonlinear index of refraction — to probe interface dynamics, modifications and relaxation processes in granular materials consisting of metal quantum dots embedded in such dielectrics as fused silica and sapphire. Many features of these materials can be interpreted in terms of the quantum-mechanical model of the “particle-in-a-box”. Electronic and thermal relaxation processes in these novel nanocomposites are dominated by interactions of conduction-band electrons at the boundary between the quantum dot and its surrounding host material. Experimental examples presented include measurements of thermal and electronic relaxation rates, dephasing due to electron collisions at the nanocluster surface, effects of local structural order, changes in the saturation parameter due to chemical modification of the substrate, and one-and two-dimensional heat-transfer effects.

Series resistance calculation for the Metal-Insulator-Semiconductor Schottky barrier diodes

Abstract: An accurate way of determining the series resistance Rs of Schottky Barrier Diodes (SBDs) with and without the interfacial oxide layer using forward current-voltage (I–V) characteristics is discussed both theoretically and experimentally by taking into account the applied voltage drop across the interfacial layerVi. For the experimental discussion, the forward biasI–V characteristics of the SBDs with and without the oxide layer fabricated by LEC (the Liquid-Encapsulated Czochralski) GaAs were performed. The SBD without the oxide layer was fabricated to confirm a novel calculation method. For the theoretical discussion, an expression ofVi was obtained by considering effects of the layer thickness and the interface state density parameters on forward biasI–V of the SBDs. The valueRs of the SBD with interfacial oxide layer was seen to be larger than that of the SBD without the interfacial oxide layer due to contribution of this layer to the series resistance. According to the obtained theoretical formula, the value ofVi for the SBD with the oxide layer was calculated and it was subtracted from the applied voltage values V and then the value ofRs was recalculated. Thus, it has been shown that this new value ofRs is in much closer agreement with that determined for the SBD without the oxide layer as predicted. Furthermore, the curves of the interface states energy distribution of each sample are determined. It was concluded that the shape of the density distribution curve and order of magnitude of the density of the interface states in the considered energy range are in close agreement with those obtained by others for Au/n-GaAs Schottky diodes by Schottky capacitance spectroscopy.

CuGaSe2 solar cells with 9.7% power conversion efficiency

Abstract: Heterojunctions, such as ZnO/CdS/CuGaSe2, were fabricated for photovoltaic applications. Optimization of device structures based on monocrystalline CuGaSe2 led to the highest-to-date power conversion efficiencies for CuGaSe2 solar cells. At room temperature under 100 mW/cm2 AM1.5 illumination a maximum cell efficiency of 9.7% was achieved, given by an open-circuit voltage of 946 mV, a short circuit current density of 15.5 mA/cm2, and a fill factor of 66.5%. Preparation and performance of the optimum device are described. Current voltage characteristics dependent on illumination intensity and temperature, spectral response and electron-beam-induced current measurements were performed to determine the device parameters as well as to analyse the current transport and loss mechanisms. Tunneling, assisted by defect levels in the CdS layer, seems to play a major role. High injection effects are observed at forward bias ofV > 0.5 V or an illumination level ofP > 10 mW/cm2. Under such conditions, as well as at low temperatures, the non-zero series resistance comes into play. Effects of the shunt resistance, however, are negligible in all cases.

Silver nanoclusters formation in ion-exchanged waveguides by annealing in hydrogen atmosphere

Abstract: Light waveguides containing silver, introduced by ion-exchange process in soda-lime glass, has been annealed in hydrogen atmosphere at temperatures in the 120–250°C range. Annealings cause a near-surface precipitation of metallic silver to form nanometer-size clusters with good uniformity in size and spatial distribution. Hydrogen permeation and ion-exchange between hydrogen and sodium (which remains in the glass matrix after silver-for-sodium exchange) are steps of the annealing process. A further step is the diffusion of silver ions towards the surface, and its eventual precipitation, with an activation energy close to that measured for silver-sodium interdiffusion in glasses of comparable composition.

Nature of the surface layer in ABO 3 -type perovskites at elevated temperatures

Abstract: X-ray diffraction measurements showed that in the surface layer of perovskite structure BaTiO3, SrTiO3, PbTiO3 and KNbO3 crystals at elevated temperatures (500–1000°C) reactions take place in the solid phase, associated with segregation in the direction to the surface of AO complexes, leading to the formation of a whole homologue series of crystalline compounds with the general formula AO · (ABO3) n ,n = l, 2,....., 10, ∞ Lowered oxygen partial pressure may inhibit the segregation processes and in the range of temperatures for which vaporisation of the AO complexes from the surface does not yet take place a suitable choice of oxygen pressure may lead to resynthesis of the compound.

Calorimetric measurement of two-photon absorption and color-center formation in ultraviolet-window materials

Abstract: Intensity-dependent absorption of 25 ns excimer laser pulses in common UV-window materials was investigated. By employing a calorimetric technique which provides greatly enhanced sensitivity compared to transmissive measurements, two-photon absorption coefficients were determined at intensities of 2–80 MW/cm2 and found to be in good agreement with previous measurements at 10–100 GW/cm2. Also, color-center formation in fused silica was observed. It was possible to quantify transient and cumulative effects as a function of intensity.

On the foundations of stochastic dislocation dynamics

Abstract: A stochastic approach to dislocation dynamics is proposed that starts off from considering the geometrically necessary fluctuations of the local stress and strain rate caused by long-range dislocation interactions during plastic flow. On a mesoscopic scale, a crystal undergoing plastic deformation is thus considered an effective fluctuating medium. The auto- and cross-correlation functions of the effective stress and the plastic strain rate are derived. The influences of dislocation multiplication, storage and cross slip on the correlation functions are discussed. Various analogies and fundamental differences to the statistical mechanics of thermodynamic equilibrium are outlined. Application of the theory of noise-induced transitions to dislocation dynamics gives new insight into the physical origin of the spontaneous formation of dislocation structures during plastic deformation. The results demonstrate the importance of the strain-rate sensitivity in dislocation patterning.

Nanostructuring with laser radiation in the nearfield of a tip from a scanning force microscope

Abstract: We report preliminary results of using an SFM(Scanning Force Microscope)/laser combination in FOLANT configuration (FOcusing of LAserradiation in Nearfield of a Tip) to perform nanostructuring on insulator and metal surfaces in air This technique is based on local intensity enhancement of laser radiation in the nearfield of a tip and enables processing of structures with a lateral resolution of approximately 10 nm The structuring has been carried out on polycarbonate and metal (gold) surfaces - appropriate materials for data storage applications - as well as on organic media to demonstrate possible applications in biotechnology Some details of nanostructuring mechanisms are discussed

Laser induced microexplosions of a photosensitive polymer

Abstract: Laser ablation of a photosensitive triazene polymer was studied in a micro region by means of a nanosecond imaging technique. The propagation of a blast wave, 100 ns after laser irradiation, sufficiently matched a planar blast wave model including the decomposed source mass which indicates characteristics of a microexplosion. The measured velocities of the fronts indicates two blast waves: an initially fast unsupported wave around the peak of the laser pulse, and a relatively slow supported wave involving the main component of the decomposition.