Laser Pulse Energy Density Research Articles

Kinetics of defect creation in hydrogenated amorphous silicon by light pulses

Abstract The creation of metastable defects in hydrogenated amorphous silicon subjected to laser pulses with laser pulse energy density in the range U = (2.5–33) 10−3 J cm−2 has been investigated. After a rise time nearly independent of U, saturation of the defect density N sat is observed, and N sat varies linearly with U in the range of energies considered. A model including the light-induced annealing of defects fits the present experimental data.

DEPOSITION OF DIAMOND-LIKE AND OTHER SPECIAL COATINGS BY PULSED LASER ABLATION AND THEIR POST-SYNTHESIS PROCESSING

Abstract Use of pulsed laser ablation technique for deposition of diamond-like and other special coatings is discussed here at length, establishing the correlation between the process parameters and the film quality. Process parameters dealt with sufficient detail are the laser wavelength, laser pulse width and energy density, nature of ambient and the corresponding partial pressure, substrate temperature and the electric field, if any, applied during deposition. The optimum parameter space for deposition of films having high performance features are identified in the case of diamond-like films and films of cubic boron nitride, titanium nitride and tungsten carbide. In this context, results on the characterization of the films deposited under different conditions are discussed. These include data obtained by Raman spectroscopy, IR spectroscopy, X-ray diffraction, ellipsometry, UV-VIS transmission, scanning electron microscopy. The post-synthesis of the laser deposited films using ion and laser beams from ...

LASER-DRIVEN PHYSICAL VAPOR DEPOSITION FOR THIN-FILM CdTe SOLAR CELLS

We review the use of laser-driven physical vapor deposition from solid targets for the growth of CdTe-related photovoltaic materials on glass substrates. The physical conditions in the laser-driven plume from 15 nsec pulses at λ = 308 nm from an XeCI laser have been studied by optical multichannel spectroscopy and time-of-flight methods. Film growth has been optimized in terms of substrate temperature, laser pulse energy density, and overall growth rate through the use of optical absorption, Raman scattering, x-ray diffraction, electron microscopy, energy dispersive x-ray spectroscopy, and electrical measurements on the films. Early results of device fabrication have yielded an all-LDPVD CdS/CdTe cell of 8.7% AM 1.5 efficiency.

Degradation of Transmission of Silica Glass on Excimer Laser Irradiation

The effects of prolonged irradiation of KrF (248nm) and ArF (193nm) lasers on the optical properties of silica glass were studied, with a glass made from a high purity silicon tetrachloride by the oxy-hydrogen flame hydrolysis method for sample. Experiments were conducted as an acceleration test for services in excimer laser apparatuses, and the results were examined in terms of solarization, i.e., degradation in transmission, T, or increase in the absorption coefficient at 5.8eV, k2, as a function of the per pulse laser energy density, e. It was found that k2 was related to e as in: for KrF excimer laser: k2=2.38×10-7⋅e3.15(e⋅p)1.31, and for ArF excimer laser: k2=8.71×10-8⋅e1.42(e⋅p)1.45, where p is the number of shots. It was demonstrated that these formulas were precise enough to make reasonable estimates of the service lifetime for silica glass in industrial excimer laser apparatus, where the intensity of the laser would be much smaller.

High-Mobility Bottom-Gate Thin-Film Transistors with Laser-Crystallized and Hydrogen-Radical-Annealed Polysilicon Films

High-performance bottom-gate thin-film transistors (TFTs) have been realized with excimer-laser-crystallized polysilicon films, for the first time. TFT characteristics were greatly dependent on the silicon film thickness as well as the pulsed-laser energy density. It was found also that posthydrogenation based on hydrogen-radical annealing improves the TFT characteristics drastically. The field-effect mobilities exceeded 220 cm2/Vs for electrons and 140 cm2/Vs for holes, respectively.

Calculation of ultrasonic surface waves from an extended thermoelastic laser source

A method is developed to calculate ultrasonic surface waveforms generated by an extended laser source, operating in the thermoelastic regime of laser-pulse energy density. This approach integrates over a suitably weighted distribution of point surface centers of expansion, for observation to within 1 mm of the edge of the source. Power spectra as well as both horizontal and vertical displacements are presented and discussed for ultrasonic waveforms on an aluminium surface, for incident laser pulses having Gaussian lateral profiles of various sizes. Far from the source, the waveform is dominated by a dipolar Rayleigh (R) wave, whose amplitude and spectral content depend on laser spot size. Weak, monopolar pulses also occur at the intersection of bulk pressure and shear wavefronts with the surface (denoted assP andsS, respectively). Close to the source, thesP wave amplitude approaches that for theR wave, and overlaps theR wave for large source sizes. The fall-off with distance for bothsP andR waves is given. Finally, the changes in pulse shape and amplitude are calculated when anR wave from an extended thermoelastic source is reflected or transmitted by a right-angled corner of an aluminium block.

Generation of acoustic pulses of short duration from laser pulses on metal foils

To calibrate transducers in water, a laser was used as a means to generate acoustic pulses of short duration. In the experiments, samples were chosen from the available metal foils (stainless steel and aluminum) with different thicknesses. Several conclusions can be drawn from the experiments. (1) The energy density of the incident laser pulse has a strong effect, depending on the properties of sample, on the duration of the generated acoustic pulses. (2) Thermal expansion β and thermal diffusivity length L are two important parameters that affect the generated acoustic waveforms, where L = (2Dtρ)1/2 and D = K/cρρ, K is the thermal conductivity, cρ is the specific heat, ρ is the density, and tρ is the duration of a laser pulse. (3) If the thickness of a sample is comparable to the thermal diffusivity length L, it will affect the generated waveform. (4) Laser generation of acoustic pulses from metal foils may be able to be used to calibrate receiving transducers in a liquid, at least in a relative sense. [...

Segregation of impurities in pulsed-laser-melted carbon

The segregation of 73Ge and 75As in pulsed-laser-melted carbon has been investigated. Both 73Ge and 75As were implanted into highly oriented pyrolytic graphite at a fluence of 1.0×1015 cm−2 at several energies. The implanted graphite was subsequently irradiated with a 30-ns pulsed ruby laser with laser pulse energy densities above the melt threshold for graphite. The distribution of impurities was measured before and after laser irradiation using Rutherford backscattering spectrometry to determine the redistribution of impurities resulting from diffusion in liquid carbon and segregation at the liquid-solid interface. Numerical calculations were then used to determine the diffusivity of the impurities in liquid carbon and the nonequilibrium segregation coefficient of Ge and As in carbon.

The percolation approach to the development of pulsed laser exposed positive photoresists

Results of photolithography experiments utilizing the XeCl excimer laser and positive photoresists are reported. Abrupt reciprocity loss resulting in the growth of photoresist contrast and sensitivity was found when laser pulse energy density exceeded a threshold value of about 50 mJ/cm2. Percolation theory is used to derive a model of resist thickness remaining versus dose for development of pulse exposed photoresist. The model accurately describes the change in dissolution of AZ2400 photoresist when the energy is delivered in multiple versus single pulse exposures.

Properties of thin PbF_2 films deposited by cw and pulsed laser assisted evaporation

Thin films of lead fluoride were deposited by means of cw or pulsed laser assisted evaporation and were characterized for their structural and optical properties. Continuous wave laser evaporated films had smooth morphology, good optical transmission, and oriented columnar grain structure. The crystallinity of the pulse laser evaporated films, as measured by x-ray diffraction peak intensities, was found to be higher than the cw laser or e−-beam evaporated films and was found to depend on the laser pulse energy density (fluence). These films, however, had higher absorption in the visible range.

Pulsed Laser Melting of Graphite

ABSTRACTExperimental evidence for laser melting of graphite, by irradiation with 30ns pulses from a ruby laser, is presented. RBS-channeling analysis, Raman scattering and TEM measurements reveal that the surface of graphite melts at a threshold energy density of about 0.6 J/cm2. For laser pulse energy densities above 0.6 J/cm2, the melt front penetration depth increases nearly linearly with increasing energy density. An intense emission of carbon particles during and after irradiation is observed. The thickness of the carbon layer removed in this process also increases nearly linearly with increasing pulse fluence. A dramatic redistribution of ion implanted impurities is also observed. Furthermore, the crystalline structure of the resolidified material is shown to depend on the energy density of the laser pulse. In order to explain these phenomena, a model for laser melting of graphite at high temperatures to form liquid carbon has been developed in which a free electron gas approximation is used to describe the properties of liquid carbon. The model is solved numerically to give the time and depth dependences of the temperature as a function of the laser pulse energy density. Very good agreement is found between the observed melt depth dependence on laser pulse energy density, as determined by RBS-channeling, and the model calculations. The redistribution of ion implanted impurities and the modification of the crystalline structure, caused by the pulsed laser irradiation, are also consistent with the model and permit the determination, for the first time, of interfacial segregation coefficients for impurities in liquid carbon. The model also predicts that liquid carbon at low pressure (p < 1 kbar) has metallic properties.

Nucleation of Allotropic Phases During Pulsed Laser Annealing of Manganese

ABSTRACTManganese has four allotropes with an equilibrium melting point of the high temperature δ-phase at 1517 K and calculated metastable melting points for the ϒ, β and α phases at 1501 K, 1481 K and 1395 K, respectively. Our observations for Mn irradiated with a pulsed laser and supporting estimates of maximum allotropic transition rates indicate that transformations between allotropes are suppressed during heating with ~ 25 ns laser pulses, as well as during subsequent cooling. Upon pulsed heating of β-Mn to the melt threshold, the melt is undercooled 122 K below the δ-Mn melting point. For incident laser pulse energy densities near the melting threshold, resolidification involves regrowth of β-Mn from the substrate. At energy densities well above threshold, the ϒ-Mn phase forms by separate nucleation and growth from the undercooled melt, and is retained upon rapid solidification. From these results and analyses, we conclude that significant melt undercooling, which may exceed 100 K, can occur during pulsed laser melting of metallic crystals and that the resulting crystalline structure is determined by both thermodynamics and nucleation kinetics.

Synthesis of rare gas-halide mixtures resulting in efficient XeF(C→A) laser oscillation

Significantly improved XeF(C→A) laser performance has been achieved using electron beam excitation of complex, multicomponent gas mixtures specifically tailored so as to reduce medium transient absorption in the blue-green region. Use of Ar and Kr together as the effective rare gas buffer-energy transfer species, along with a combination of NF3 and F2 to produce the desired F-donor molecule characteristics, has permitted synthesis of near optimum medium properties for which XeF(C) is produced efficiently while transient absorption is minimized. With this technique we have achieved laser pulse energy density and intrinsic efficiency of 2.2±0.3 J/l and ∼1.5%, respectively, values that are comparable to those of the B→X rare gas-halide lasers.

Efficient XeF(C→A) laser oscillation using electron-beam excitation

Significantly improved XeF(C→A) laser energy density and efficiency have been obtained using electron-beam excited Ar-Xe gas mixtures at pressures up to 10 atm which contain both NF3 and F2. Maximum blue-green laser pulse energy density in excess of 1.0 J/liter was obtained, corresponding to an intrinsic electrical-optical energy conversion efficiency estimated to be in the 0.5%–1.0% range. Comprehensive, time-resolved absolute measurements of XeF(C→A) fluorescence, laser energy, and gain were carried out for a wide variety of experimental conditions. Analysis of these data has resulted in identification of the dominant transient absorbing species in the laser medium. For the laser mixtures investigated in this work, the primary blue/green absorption processes have been identified as photoionization of the 4p, 3d, and higher lying states of Ar, and of the Xe 6p and 5d states, and photodissociation of Ar2(3∑+u) and Ar+3.

Optimization of electrically excited XeF(C→A) laser performance

Significantly improved output power and spectral characteristics have been obtained for an electron beam-pumped XeF(C→A) laser by selective tailoring of kinetic processes. Using a high pressure Ar-Xe mixture containing both NF3 and F2 in combination, a laser pulse energy density of 0.1 J/l was obtained corresponding to an electrical-optical conversion efficiency of approximately 0.1%. Modeling of kinetic processes and analysis of laser spectral output confirm that the improved laser efficiency is primarily a consequence of reduced absorption by Xe-related excited species.

Free-running ruby laser annealing of boron implanted silicon

An analytical treatment of the thermal phenomena during free-running ruby laser annealing of boron implanted silicon is presented. The heat equation was solved for a simplified shape of the pulse train consisting of a uniform succession of triangular spikes, identical in energy. During one spike the optical and thermal parameters of the sample were taken constant, but for each new spike, new values of these parameters were calculated taking into account their temperature dependence. Such a model predicts the melting of the top surface before the laser pulse has ended, for energy densities higher than 9×104 J/m2. RHEED confirms the presence of recristallization at about the same value of the laser-pulse energy densities.

Characterization of Laser Induced Backside Damage for Gettering Purposes

ABSTRACTA characterization of Q-switched Nd-YAG laser induced backside damage in various stages of a bipolar process is presented. TEM-analysis shows the occurence of microcracks, low angle boundaries and dislocations in as-irradiated wafers, of which the microcracks anneal out during an initial oxidation at 1050°C. The other damage is very stable and remains even after a heat treatment at 1200°C. This is quite in contrast with mechanically or Ar implantation induced backside damage, which anneal out completely at such high temperatures. Side effects of laser induced backside damage such as surface roughness and influence on wafer strength are found to be of no consequence within a limited range of laser pulse energy densities where the gettering efficiency is found to be very promising.

Solubility limit of impurities in silicon after laser induced melting

The solubility of several dopants (Sb, Ga, Bi, In) in laser treated silicon has been investigated. The dopants were introduced by vacuum deposition followed by ruby laser irradiation. Their solubility was determined by Rutherford backscattering spectroscopy measurements in channelling and random conditions. In all cases, a maximum solubilityCS*, much higher than the equilibrium solubility limitCS0 and independent of the pulsed laser energy density, was found. The values obtained are in good agreement with those calculated from a simple model based on phase diagram considerations, using the relationship: $$C_S^* = \frac{{C_S^0 }}{{k_0 }}k^* ,$$

Pulsed Ruby Laser Annealing of Zn, Mg, Se and Si Ion Implants in Semiconducting Gaas

ABSTRACTThe properties of p+ and n+ layers formed by pulsed ruby laser annealing (PRLA) of shallow (Rp ~ 320–680 Å) implantations of Mg, Zn, Si and Se ions in both n- and p-type semiconducting GaAs have been evaluated using a combination of SIMS and electrical properties measurements. High activation (> 80%) was obtained for high dose (5 × 1015 ions/cm2 ) implants of both Mg and Zn, within a pulsed laser energy density “window” 0.5 ≤ Eλ ≤ 0.8 J/cm2 (FWHM pulse duration = 20–25 ns). SIMS measurements following PRLA show a wellbehaved increasing penetration of dopant ions into the GaAs substrate, with dopant ion concentrations well in excess of 1020 ions/cm3 in the near-surface region. Measured hole mobilities are consistent with the values anticipated for these high concentrations of ionized impurity scattering centers.

A continuously pulsed copper halide laser with a cable-capacitor Blumlein discharge circuit

Experimental characteristics of a continuously pulsed copper halide laser with a cable-capacitor Blumlein discharge circuit are reported. Quartz laser tubes 1 m in length and 1.5 and 2.5 cm in diameter were employed to study the effects of the electrical circuit, lasant, and buffer gas on laser performance. Measured properties of the Blumlein circuit are compared with an analytic solution for an idealized circuit. Both CuCl and CuBr with neon and helium buffer gas were studied. A maximum average power of 12.5 W was obtained with a 1.5 nF capacitor charged to 8 kV and discharged at 31 kHz with CuCl and neon buffer gas at 0.7 kPa in a 2.5-cm-diam tube. A maximum efficiency of 0.72 percent was obtained at 9 W average power. Measurements of the radial distribution of the power in the laser beam and the variation of laser power at 510.6 and 578.2 nm with halide vapor density are also reported. Double and continuously pulsed laser characteristics are compared, and the role of copper metastable level atoms in limiting the laser pulse energy density is discussed.