Optical studies of semiconductors under intense femtosecond laser pulse excitation suggest that an ultrafast phase transition takes places before the electronic system has time to thermally equilibrate with the lattice. The excitation of a critical density of valence band electrons destabilizes the covalent bonding in the crystal, resulting in a structural phase transition. The deformation of the lattice leads to a decrease in the average bonding-antibonding splitting and a collapse of the band-gap. Direct optical measurements of the dielectric constant and second-order nonlinear susceptibility are used to determine the time evolution of the phase transition.

Degenerate pump and probe measurements on bulk GaAs <100> surfaces with sub-100 fs near infrared laser pulses have been performed in the 0.8 − 3 × 1017 carriers cm-3 excitation range at room temperature. Transient reflectivity data reveal the progressive extinction of the LO-phonon emission channel when the excess excitation energy is decreased. At these wavelengths and for low excitation levels, acoustic phonon absorption from conduction band minimum is observed. Surface recombination rates are deduced from the picosecond evolution of the reflectivity.

Pulsed laser induced crystallization of amorphous GeSb films has been studied as a function of the laser pulse duration. The energy density crystallization threshold has been determined for pulses in the range from 400 fs to 8 ns. The threshold is observed to increase substantially for pulse durations in the ns range due to the existence of heat flow through the substrate while the pulse is still being absorbed. The energy density crystallization threshold remains constant within the experimental resolution as the pulse duration is decreased down to 800 fs. For the shortest pulse length used, (400 fs), a decrease in the threshold is observed, suggesting the possible existence of electronic excitation enhanced crystallization.

Although MgO is much more resistant to radiolysis by 248-nm photons than NaNO3, the ion emission processes at low fluence have much in common: both materials yield high energy ions (> 5 eV kinetic energy) with a strongly nonlinear fluence dependence. We report time-of-flight measurements of quadrupole mass-selected Mg+ from polished, single crystal MgO and Na+ from cleaved, single crystal NaNO3. At fluences between 10 and 1000 mJ/cm2, the Mg+ intensities show a strongly nonlinear fluence dependence which decreases to roughly second order behavior at fluences above 100 mJ/cm2. The Na+ intensities display third or fourth order emission kinetics throughout the experimental range of fluences. We attribute these emissions to cations adsorbed atop surface electron traps where the cation is ejected when the underlying trap is photo-ionized. The potential energy of this defect configuration accounts for the observed ion kinetic energies. However, the direct photo-ionization of surface vacancy/adsorbed ion defects with 5 eV photons should not be possible. Thus we propose that emission requires the photo-ionization of nearby electron traps, followed by photo-induced charge transfer to the empty traps. We show that a sequence of single-photon absorption events [involving photo-ionization, charge transfer, and electron retrapping] accounts for the strongly nonlinear fluence dependence.

A two temperature finite difference model has been developed and is used to describe the response of materials under ultrafast femtosecond laser pulses in the energy regime where melting and vaporization can occur. In applying this model to silicon it is observed that, for 800 nm light, laser pulse intensities that are just sufficient to achieve threshold for vaporization are also at the level of optical electric field strength where electron avalanche breakdown at the surface of the material can occur. For sub-picosecond pulses the physical response of the material is associated with a strongly temperature dependent coupling coefficient connecting electron and phonon thermal distributions. The results of these analyses demonstrate that a very thin near solid density plasma, caused by avalanche ionization, is responsible for the surface heating and subsequent thermodynamic response of the material. This interpretation is consistent throughout the pulse duration range from 80 femtoseconds to 0.2 nanoseconds. The proposed mechanism for absorption, at the near infra-red wavelength being used here, is very different from the types of mechanisms usually considered for nanosecond laser heating of semiconductors. Surface damage threshold is determined by atomic force microscopy and the threshold for plasma optical emission by photomultiplier detection . Melt deths are probed with SIMS impurity diffusion profiles and high resolution cross sectional TEM.

Gated photon counting spectroscopy and species-resolved ICCD photography have been applied to study the weak plasma luminescence which occurs following the propagation of the initial ablation plume in vacuum and during the ‘rebound’ of the plume with a substrate during pulsed laser deposition of amorphous diamond. These time- and spatially-resolved spectroscopic techniques were required in order to investigate notable differences between amorphous diamond-like carbon films formed by pulsed laser deposition from ArF (193 nm) and KrF (248 nm) irradiation of pyrolytic graphite in vacuum. Three principal regions of plume emission have been characterized: (1) a bright luminescent ball (v ∼3-5 cm/(μ.s) displaying nearly entirely C+ emission which appears to result from laser interaction with the initial ejecta, (2) a spherical ball of emission (v ∼1 cm/μs) displaying neutral carbon atomic emission lines and, at early times, jets of excited C2, and (3) a well-defined region of broadband emission (v ∼ 0.3 cm/μs) near the target surface first containing emission bands from C2, then weak, continuum emission thought to result from C3 and higher clusters and/or blackbody emission from hot clusters or nanoparticles. For both lasers, the measurements reveal an explosive interaction within the plume which results in a variety of new gas dynamic observations in vacuum:. These include (a) generation of instabilities or jets, (b) confinement of a residual part of the plume near the pellet surface, (c) cluster formation in the collisional, confined regions of the plume, and (d) reflection of the confined region backward to splash and redeposit on the pellet surface. Evidence for gas-phase formation of these clusters in vacuum is indicated from the dynamic evolution of the same cluster bands observed during the collision of the plume with the substrate surface during film growth. Addition of background gases strongly enhances the third (cluster) component, in accordance with plume-splitting phenomena. The combination of sensitive imaging and photon-counting diagnostic techniques permit an understanding of the importance of gas dynamic effects, such as clustering, on the time-of-flight distributions of species arriving during the deposition of thin films in both vacuum and background gases.

Laser ablation of silicon and germanium was carried out in moderate vacuum with l00fs to 400fs pulses at 248nm and intensities up to 3x1013 W/cm2. Evidence for non-thermal material removal was found. Imaged multishot ablation patterns display the intensity dependent self-structuring effect, forming well-known columnar structures. It is shown that continued irradiation of these structures eventually results in comparatively clean ablation. An increase of ablation rate with depth was observed. The reason is an intensity enhancement inside the pits by reflective focussing to a level where bond-breaking takes place. Furthermore, it was noticed that ablation contours can be significantly improved by electrically grounding the target.

An axisymmetric two-dimensional modeling of plasma expansion in background gas is carried out to provide a better understanding of pulsed laser deposition (PLD) processes. This model takes into account the plasma plume dynamics just after the laser ablation pulse. It is shown that hydrodynamic behaviour and thermal relaxation of the plasma are strongly dependent on the nature and the pressure of background gas. In good agreement with experimental results backward movement of particles is clearly identified at a late expansion stage (≥ 2 μs).

This paper presents the main results of an experimental study of the dependence of some important plasma parameters (plasma flux density, multicharge ion content, degree of ionization, energy distribution of the ions) on some laser parameters (light flux density, wavelength) for two laser types (TEA-CO2- and excimer laser). A special experimental method has been developed for an absolute and simultaneous measurement of the plasma parameters. The results obtained show the possibility to control the parameters of plasma fluxes over the deposition conditions and herewith to control the properties of the deposited films.

Pulsed laser deposition may be accomplished by irradiating a target with a high intensity laser pulse. The flux of particles moving from the target to the substrate obeys gas-dynamic laws when the density of the emitted particles exceeds ∼1 monolayer/(10 ns). The angular distribution of the plume, which depends on various factors, will strongly affect the deposit characteristics. It has been found that an intensified CCD camera can be successfully used to analyze laterally in two orthogonal directions the profile of the expanding plume. A marked influence of laser spot dimension and shape on plume dynamics is observed for laser ablation of a number of materials such as metals, semiconductors and high Tc superconductors.

The observation of a slow optically excited component in the excimer laser-ablated YBCO plume due to the presence of a biased ring electrode is reported. The temporal dynamics of the plume were investigated by using time-of-flight (TOF) optical emission spectroscopy (OES). Time-resolved emission signals reveal excitation and resultant fluorescence from slow-moving plume species in the presence of the discharge.

Simultaneous temporal and spatial resolution of the monochromatic vacuum ultraviolet emission from an excimer laser ablated YBa2Cu3O7 plasma has yielded quantitative information on the plasma generation and velocity. From the time resolved spectral data, plasma velocities were measured and found to be ∼2.3x106 cm/s. It was observed that vacuum ultraviolet emission from above the YBa2Cu3O7 surface does not occur until ˜20 ns after the beginning of the laser pulse. Energetic material continues to be ejected from the target for more than 100 ns after the end of the laser pulse.

Pulsed laser ablation (PLA) has several characteristics that are potentially attractive for the growth and doping of chemically complex compound semiconductors including (1) stoichiometric (congruent) transfer of composition from target to film, (2) the use of reactive gases to control film composition and/or doping via energetic-beam-induced reactions, and (3) low-temperature nonequilibrium phase formation in the laser-generated plasma “plume.” However, the electrical properties of compound semiconductors are far more sensitive to low concentrations of defects than are the oxide metals/ceramics for which PLA has been so successful. Only recently have doped epitaxial compound semiconductor films been grown by PLA. Fundamental studies are being carried out to relate film electrical and microstructural properties to the energy distribution of ablated species, to the temporal evolution of the ablation pulse in ambient gases, and to beam-assisted surface and/or gas-phase reactions. In this paper we describe results of ex situ Hall effect, high-resolution x-ray diffraction, transmission electron microscopy, and Rutherford backscattering measurements that are being used in combination with in situ RHEED and time-resolved ion probe measurements to evaluate PLA for growth of doped epitaxial compound semiconductor films and heterostructures. Examples are presented and results analyzed for doped II–VI, I–III–VI, and column-Ill nitride materials grown recently in this and other laboratories.

Epitaxial thin films of nitrogen-doped p-ZnTe were grown on single-crystal, semi-insulating GaAs substrates via pulsed laser ablation of a stoichiometric ZnTe target. Both low pressure nitrogen ambients and high vacuum were used. Results of in situ reflection high energy electron diffraction (RHEED) and time-resolved ion probe measurements have been compared with ex situ Hall effect and transmission electron microscopy (TEM) measurements. A strong correlation was observed between the nature of the film's surface during growth (2-D vs. 3-D, assessed via RHEED) and the ambient gas pressures employed during deposition. The extended defect content (assessed via cross-sectional TEM) in the region >150 nm from the film/substrate interface was found to increase with the ambient gas pressure during deposition, which could not be explained by lattice mismatch alone. At sufficiently high pressures, misoriented, columnar grains developed which were not only consistent with the RHEED observations but also were correlated with a marked decrease in Hall mobility and a slight decrease in hole concentration. Ion probe measurements, which monitored the attenuation and slowing of the ion current arriving at the substrate surface, indicated that for increasing nitrogen pressure the fast (vacuum) velocity-distribution splits into a distinct fast and two collisionally-slowed components or modes. Gas-controlled variations in these components mirrored trends in electrical properties and microstructural measurements.

Laser ablation technique has been successfully used for the deposition of CdSe and CdTe/CdSe multilayers on Si(100) and Si(l11) substrates. X-ray analysis showed that CdSe/Si films were highly oriented. Their orientation changed from (100) to (002) by varying the substrate temperature from 473 to 673K. High orientation was also obtained on multilayered polycrystalline structures of CdSe and CdTe on Si(lll). Photoluminescence experiments have also been carried out on the deposited films.

We report new results on continuous wave Nd: YAG laser deposition of Cadmium Sulfide (CdS) thin films. Substrates were soda-lime silicate (SLS) glass, silica glass, silicon, alumina, and copper coated formvar sheets. As-deposited films were characteristically mixtures of cubic and hexagonal phases. X-ray diffraction analysis reveals that two different grain size groups are present. As revealed by SEM micrographs, films had smooth surface morphology. Transmission electron microscopy analysis reveals that grain sizes were extremely small. Also, semiconductive behavior was noted.

Pulsed laser deposition (PLD ) is known for its capacity to reproduce a target composition on a substrate. We have used this deposition technique to produce thin films of transition metal chalcogenides. However, the deposits were always deficient in Te relative to the starting material ( composed by a refractory metal ( niobium ) and a chalcogene ( tellurium ) ). Variations of the interreticular distances have been observed with respect to fluence and substrate temperature. We show that spatial composition of the films is determined by a degree of crystallinity of deposit and by the reaction of formation of Te2 molecule within laser induced plume Two kinds of deposits have been obtained : Nb5Te4-type thin films which have a one-dimensional structure and NbTe2-type thin films which have a two-dimensional structure. While NbTe2 films have been realized by sputtering, it is the first time that Nb5Te4 films have been deposited.

Laser MBE is a process especially useful for epitaxial layer-by-layer growth of ceramic thin films directly from sintered ceramic targets. By employing high vacuum MBE conditions, the process has a restriction in the controllability of chemical composition, e.g. nonstoi chiometry in oxides and nitrides, as compared with conventional pulsed laser deposition, but instead gains the possibility of in situ monitoring of surface reaction on an atomic scale by RHEED. Ever since our first success in observing RHEED intensity oscillation for CeO2 film growth on Si(l11), we have verified the molecular layer epitaxy by laser MBE for perovskite oxides (SrTiO3, BaTiO3, SrVO3 ,etc) and infinite-layer cuprates MCuO2 (M= Sr, Ba, Ca) on SrTiO3 substrates as well as for oxide and nitride films on Si substrates. Key factors to design the laser MBE system, operation parameters, and recent experimental results are presented and discussed.

A detailed comparison of the ZnO film microstructure prepared with both targets at two deposition pressures (0.01 and 0.1 torr) and a variety of substrate temperatures from 50-500°C was performed. The crystallographic and morphological results suggest that the condensation of highly electronically excited state particles, both ions and long-lived excited state neutrals likely affects film growth, and can enhance desirable surface processes, such as athermal recrystallizalion and surface mobility, that favor oriented crystal growth at lower substrate temperatures.

The visible plume, induced during pulsed-laser deposition (PLD) of epitaxial La0.5.jSr0.5 5CoO3/Ba0.4Sr0.6TiO3/La0.5Sr05CoO3/YBa2Cu3O7/YSZ heterostructures on silicon (100) wafers, was studied by optical-emission spectroscopy (OES). These films are suitable for the fabrication of ferroelectric capacitors and pyroelectric-sensor devices. A YAG laser, at 266 nm, is used for ablation. A collection lens transfers the PLD-plume emission into an optical fiber and onto a diffraction grating and a CCD array, for time-averaged spectroscopy from 410 to 640 nm. Plume emissions from ablated targets in the presence of an oxygen ambient, due to various atomic (Ba, Co, Cu, Sr, Ti, Y, Zr), ionic (Ba+, La+, Sr+, Y+), and a diatomic oxide (YO) species were identified. Emission intensity and evolution of ablated species are reported for distance away from the target surface, oxygen pressures, and laser fluences (1 to 4 J/cm2). The behavior of reactive-product species, especially YO for plumes from yttria-stabilized zirconia (YSZ) and YBCO targets, is discussed. This simple and inexpensive OES system is suitable for use as a plume-quality monitor on routine PLD film synthesis.