ZnO is a wide and direct band-gap material (3.37 eV at room temperature) making this compound very suitable for UV
photodetector applications as well as for UV and blue light emitting devices. As an electronic conductor, ZnO may be
used as transparent and conducting electrodes for flat panel displays and solar cells. ZnO doped with various atoms can
also lead to new or enhanced functional properties. For example, doping with Al, Ga, In, Si or H allows decreasing its
resistivity to below 10<sup>-4</sup> Ω.cm, while keeping the high optical transparency. Rare-earth doped ZnO thin films have been
studied for optics and optoelectronics such as visible or infrared emitting devices, planar optical waveguide amplifiers.
Ferromagnetic semiconductors can be obtained by doping ZnO with transition metal atoms (Mn, Co, Ni...) that could be
used as spin injectors in spintronics.
These new and exciting properties of pure and doped ZnO request the use of thin films or multilayer structures. ZnO thin
film growth by pulsed-laser deposition (PLD) with or without any dopants or alloyed atoms has been intensively studied.
In this paper, we will review the aspects of ZnO thin films grown by PLD, in order to prepare dense, stoichiometric and
crystalline epitaxied ZnO layers or to form nanocrystalline films. Then, the optical and electrical properties will be
discussed with a special emphasis on the growth conditions in relation to the physical properties for applications in p-n
junctions, light emission devices, spintronics and bandgap tuning.
In recent years Pulsed-Laser Deposition (PLD) has became increasingly popular as a viable deposition process for
numerous materials. To overcome the main drawback of this method (macroscopic particles on the surface of the films),
femtosecond (fs) lasers have been thought to be an ideal tool to obtain high quality thin films. However, it appeared that
the nature of films grown by fs PLD strongly depends on the material and growth conditions. Indeed droplets are often
observed evidencing the presence of violent thermal effects during the fs PLD process. In addition, the films are
generally constituted by the random stacking of clusters in the 10-100 nm range that may be interesting for applications
especially in the field of sensors and catalysis. In this paper, the experimental conditions leading to the formation of
films composed of cluster piles without droplets will be presented. The results will be discussed as well as the possible
explanations of the formation of clusters during fs PLD at the light of the literature produced in this field.
A high number of papers were published on the simulation of laser/surface interaction at the level of nanosecond scale.
Several assumptions on thermal properties data, laser spot homogeneity, were assumed for describing as well as possible
the boundary conditions, the mathematical writing and finally the numerical or the analytical results. A few tentative of
surface temperature monitoring during laser processing were proposed for the numerical validation. Also, simulation of
the melting kinetics is rarely directly compared to in situ experiments. It is very hard to determine the time duration of a
melting pool by in situ experiments. It should be the same for the surface temperature.
A new method to plot the thermal history of the surface by using a combination of the Time Resolved Reflectivity (TRR)
and the Pulsed Photo-Thermal (PPT) or Infrared Radiometry (IR) methods is proposed in this paper. Surface
temperature, melting kinetics, threshold of melting and threshold of plasma formation are determined in the case of KrF
laser spot in interaction with several materials. In the first step, the experimental setup including fast detectors (IR, UV,
Vis.) and related optical devices is described. In the second step, typical results (TRR and IR spectra) for monocrystaline
silicon are presented and discussed. Namely, phase change transitions (melting and resolidification) are detected versus
fluence change and number of laser shots change. TRR and IR spectra of metallic surfaces (Cu, Mo, Ni, Stainless steel
15330 and 17246, Sn, Ti), are measured. For each sample the surface temperature during heating, the threshold of
melting, melting duration and the threshold of plasma formation are directly deduced.
Femtosecond and nanosecond lasers are used to produce oxide nanoparticles by laser ablation of steel. The deposition of those particles on the surface strongly modifies its properties. The aim of this study is the understanding of the nanoparticle formation. The dynamics of the plume expansion and of the nanoparticle deposition processes are investigated by means of in-situ time resolved optical analysis. Scanning electron microscopy and atomic force microscopy are used to characterise the particle film morphology deposited on the surface. The influence of laser parameters such as pulse duration (ns, fs), wavelength (UV, visible, IR) and background gas pressure (10 mbar - 1 bar) on the processes of nanoparticle formation is studied. It is shown that a high density plasma favours the particle formation, and that the high temperature of the plume obtained with nanosecond JR irradiation impedes the nanoclusters nucleation and prevents an efficient nanoparticle formation.
Laser treatments of various metals are studying depending on the laser wavelength, pulse time duration and shape, and fluence (laser/metal interaction regime). Low fluence excimer UV laser melting process of gold layer is shown to improve the corrosion resistance of multilayer (Au/Ni/Cu alloy) electrical contacts. For this application the homogenity of the laser beam as well as the initial Cu substrate roughness are found to be limiting parameters of the process. Carburization of Al alloy, performed in C<sub>3</sub>H<sub>6</sub> atmosphere with a KrF laser induces the incorporation of carbon atoms over about 4 μm depth. The crystalline Al<sub>4</sub>C<sub>3</sub> synthesized at the surface leads to a strengthening of the light Al alloy, which is of great interest for application in car industry. The study shows that diffusion of C atom in the target is possible because of a plasma presence on the surface which supports the molten bath life time and induces dissociation of the ambient gas. In the last example of laser metal surface treatment presented in that paper, a commonly used steel is treated in air with different lasers at a fluence above the plasma formation threshold. It is seen that the machining oils covering the surface before the treatment can be efficiently removed and that new compounds (nitride, carbide and oxides) are formed at the surface.
The laser surface treatment is applied to a multilayer component (copper alloy plated with two thin coatings, nickel and gold). The aim of the study is to melt the whole gold layer (thick < micrometers ) without damaging the underlying layers. The gold melting must be homogeneous and the process must be fast to avoid heat diffusion in the depths. For these reasons, the laser has been chosen for surface treatment. The application of this laser surface treatment is to improve the corrosion of resistance of electrical contacts due to columnar microstructure of gold deposited by electrolytic process. Tests of corrosion are carried out in the humid synthetic air containing low contents of pollutants (NO<SUB>2</SUB>, SO<SUB>2</SUB> and Cl<SUB>2</SUB>). An numerical study has been realized to find the best laser conditions to melt the whole gold layer.
The excimer laser nitriding and carburizing process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1 bar nitrogen or propylene gas. Vapor plasma expands from the surface then dissociates and ionizes ambient gas. Nitrogen or carbon atoms from plasma in contact with the surface penetrate in depth due to plasma recoil action onto the target surface heated by the plasma. It is thus necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The nitrogen or carbon concentration profiles are determined from nuclear analysis. Crystalline quality is evidenced by X Ray Diffraction (XRD) technique. Transmission Electron Microscopy (TEM) gives the in-depth microstructure. Fretting coefficient measurements exhibit a satisfying behavior for some experimental conditions. The polycrystalline nitride or carbide layer obtained is several micrometers thick and composed of pure A1N or Al<SUB>4</SUB>C<SUB>3</SUB> (columnar microstructure) top layer standing on a diffusion layer.
The surface morphology of single crystal (100) Si wafers irradiated by 266 nm and 1064 nm laser pulses emitted by a solid state Nd:YAG laser has been investigated. The morphology of the bottom of craters remained flat and almost featureless after 266 nm single or multipulse laser irradiation up to the maximum fluence of 18 J/cm<SUP>2</SUP> used in this study. The rims of the craters showed signs of radial liquid flow but it was apparent that the vaporization process was confined to the surface region. A different morphology was observed on the bottom of the craters formed by the 1064 nm wavelength laser pulses. Because this wavelength is absorbed in volume, (alpha) <10<SUP>4</SUP>cm<SUP>-1</SUP>, a rather thick liquid Si pool formed at the surface. For laser fluences higher than 3-5 J/cm<SUP>2</SUP> evidence of boiling sites were observed on the bottom of the crater, especially for multipulse irradiation. An evolution of surface morphology, from waves towards deep cavity was observed with the increase of pulse number. By analyzing the cavity formation mechanisms, their density and shape, we suggest that they were induced by heterogeneous boiling and not homogeneous boiling.
The typical uniform rectangular beam produced by excimer laser is attractive for surface treatment. These process are developed for to change crystalline structure, surface morphology and chemical composition by doping or to remove a contamination layer. The simple modeling of mono-dimensional heat flow equation is often used to describe these processes. Depending on the deposited laser power density, different physical and chemical processes occur, from transformation in solid or liquid phase to vapor production and plasma formation. Below laser ablation fluence, the crystalline and chemical surface modifications of polymer surface are achieved for metallization and treatment on silicon surfaces are carried out for microelectronics applications and thin film transistor devices. Over laser ablation threshold, a plasma is created in the vapor induced by laser-material interaction. At low presure (mbar) this plasma can transport species from target to a substrate for thin film growth. For high pressure ambient gas (1 bar) the plasma stays confined on the target surface and react. As chemical reactions occur there is a chemical composition transform of the target surface.
AlN nitride films are grown by reactive pulsed laser ablation of aluminum target in N<SUB>2</SUB> atmosphere. The influence of process parameters such as N<SUB>2</SUB> pressure and laser fluence is investigated. Films are characterized by Rutherford Backscattering Spectroscopy, Nuclear Reaction Analysis, X Ray Diffraction and X Ray Photoelectron Spectroscopy. O contamination appears in the film and its origin is discussed. To enhance N<SUB>2</SUB> dissociation, a RF discharge device is coupled to the deposition chamber. Its effect on thin film composition is studied. Emission spectroscopy is performed in order to find the best RF working point for N<SUB>2</SUB> molecule dissociation and to understand species transport from the target towards the substrate as a function of process parameters. Thin film with a stoichiometry near to Al<SUB>1</SUB>N<SUB>1</SUB> can be obtained with low O contamination working with 6 J/cm<SUP>2</SUP> laser fluence, 0.01 mbar N<SUB>2</SUB> with RF discharge added.
The growth of aluminum nitride films by reactive laser ablation has been studied. The influence of process parameters such as laser energy density, nitrogen pressure on the composition, chemical nature and structure of the films has been investigated. Rutherford backscattering spectrometry, nuclear reaction analysis, x-ray diffraction were used to characterize the films. The main problem in AlN film growth was the oxygen incorporation. The origin of this contamination and the mechanisms of incorporation were studied, and the crucial parameter was found to be the residual pressure during ablation. Due to the difference in chemical reactivity between oxygen and nitrogen atomic species, it is necessary to increase the density of atomic nitrogen to obtain pure AlN films. Thus, ar radio-frequency discharge device was added allowing a better nitrogen molecule dissociation. Finally, despite 10 percent O composition deviations, the AlN phase was obtained in the laser deposited films.
The excimer laser nitriding process reported is developed to enhance mechanical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1 bar nitrogen gas. A vapor plasma is expanding from the surface and shock wave dissociates and ionizes nitrogen. Then vapor and gas species stay several hundreds of microsecond(s) in contact with the surface and nitrogen diffuses in depth. Thus it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent from a too large laser-induced surface roughness. The nitrogen concentration profiles are determined from RBS and SEM coupled to EDX probe analysis. The roughness and surface state are studied to define the best irradiation conditions corresponding to a smooth and homogeneous surface.
Vaporisation of condensed substances heated by laser or electron beams is widely used for various practical purposes. Theoretical description of these processes is complicated, in particular, due to non-equilibrium nature of intense evaporation which manifests itself in metastable states of condense and vapour phases as well as in Knudsen layer (KL) jump adjacent to evaporating surface. In the framework of fluid dynamics KL is considered as discontinuity where appropriate boundary conditions should be formulated to describe phase transition kinetics which depends on gas flow dynamics.
In order to get a better inside in the reactive Pulsed Laser Deposition of nitride thin films, we performed time- and space-resolved plasma diagnostics during ablation of Ti, Al and C targets in low pressure nitrogen containing atmospheres using pulsed nanosecond UV lasers. In the case of carbon, thin films of C<SUB>x</SUB>N<SUB>y</SUB> were deposited on silicon substrates and characterized by Rutherford Backscattering Spectroscopy and Nuclear Reaction Analysis. With respect to irradiation of metal targets, during which a dense and highly ionized plasma was induced for laser intensities >= 100 MWcm<SUP>-2</SUP>, much higher values >= 1 GWcm<SUP>-2</SUP> were necessary to induce significant plasma ionization on carbon. To increase the plasma reactivity in the case of carbon ablation, a radiofrequency discharge was added to excite and preionize the ambient gas. From correlation between the plasma characteristics and thin film analyses, conclusions could be made about the C<SUB>x</SUB>N<SUB>y</SUB> deposition process.
Crystalline GaN thin films have been deposited by laser ablation of liquid Ga target in nitrogen reactive atmosphere. A Nd-YAG laser (l equals 1.06 mm, t<SUB>FWHM</SUB> equals 10 ns) able to deliver an energy of 0.35 J/pulse was used as laser source. The nitrogen pressure was varied in the range of 10<SUP>-2</SUP> - 10<SUP>-1</SUP> mbar. As substrates sapphire plates, heated below 300 degree(s)C, were used. The characteristics of the deposited films were evidenced by different techniques as XPS, SIMS, X-ray diffraction, optical absorption spectroscopy. The Nls region of the XPS spectrum contains as main peak the one centered at 397.3 eV and corresponding to Ga-N bond. From the distance between the photoelectron Ga 3d peak and the Auger Ga LMM peak, the calculated Auger parameter of 1083.9 eV corresponds to the one reported in literature for GaN compound (1084.05 eV). SIMS profiles corresponding to N and Ga in depth- distribution carried out the presence of layers of the order of 130 - 150 nm, with uniform distribution of Ga and N inside the layer. Both techniques evidenced an oxygen contamination below 5%. XRD recorded spectra show the presence of a strong peak assigned to (002) GaN crystalline orientation. Optical absorption spectroscopy studies in the UV and visible range evidenced a high transparency for the deposited films.
The present work deals with a new nitriding method applied to titanium: the Ti surface nitriding is carried out by direct laser irradiation in the presence of ambient nitrogen. The experimental procedure is performed in a chamber containing N<SUB>2</SUB> gas, allowing plasma study by emission spectroscopy. Two pulsed laser types are used, a TEA-CO<SUB>2</SUB> ((lambda) equals 10.6 micrometers ) and a XeCl excimer ((lambda) equals 308 nm) in order to compare the laser- material coupling influence on the layer synthesis process. The laser beam is focused perpendicularly to the Ti samples. Different experimental conditions are achieved to investigate the influence of laser and gas parameters on the process. Using the CO<SUB>2</SUB> laser, a N<SUB>2</SUB> plasma is created on the Ti surface. With the XeCl excimer laser, a Ti plasma on the sample appears. After treatment, the surface state of the samples is studied and chemical analysis of the targets are carried out. The TiN synthesis is evidenced. Presence of oxinitride in the compound and native surface oxygen reduction by hydrogen plasma are examined.
We describe recent advances in the development of the electronic holographic display system at MIT. These include the use of an array of galvanometric scanners as the horizontal scanning element, the use of tandem 18-channel acousto-optical modulators as the display media, and the use of a bank of custom-designed high-bandwidth framebuffers for signal drive. We also describe recent progress on rapid hologram computation.