Diffractive optical elements are designed and demonstrated as elemental units in photonic gas sensors. Diffraction gratings are written on specially designed photosensitive polymers using photolithographic techniques, as well as on multilayer metal/metal oxide thin film structures. Photonic sensors are implemented using grating structures as the elemental units for the detection of the external agent. These gratings are designed from such materials that show response to the external agent and the sensitivity is increased through the design of the grating. The principle of operation is based on the grating's diffraction efficiency variations due to index of refraction alterations and/or geometrical changes of the grating structure (e.g., groove depth, groove spacing) to external factors. The advantageous characteristics of the presented integrated sensor are the fully reversible behavior at ambient operating conditions, without the need for additional heating or light exposure. Applications of these sensitive photonic sensors so far include water vapor, hydrocarbons, and alcohol detection. The optical designs are based on diffraction efficiency measurements, and incorporate a monochromatic optical source and simple optoelectronic detection components. The photonic sensor integration is based on bulk optics approach.
Materials involved in gas sensing applications have been deposited by various methods in thin film form. Variation of the optical properties of the thin films were investigated under butane and ozone exposure using the m-lines technique. Effective index variations down to 10<sup>-4</sup> were observed. Concentrations of 100 ppm of butane diluted in the air or in nitrogen were detected.
III-V nitrides group are promising materials for technological applications such as: semiconductor lasers, light emitting diodes, optical detectors and refractory materials. In addition, transistors based on the group III nitrides should operate at higher temperatures and under more adverse conditions than similar devices on silicon, II- VI materials, or other III-V materials, due to the high band gap, the strong chemical bonds and the high chemical inertness of the nitrides. The paper reports the first results concerning the deposition of InN thin films by reactive laser ablation of indium target in nitrogen atmosphere. A XeCl excimer laser ((lambda) equals308 nm, (tau) equals30 ns) was used as laser source. The laser beam was incident on the target with an angle of 45 degrees, laser fluency was set at 5 J/cm<SUP>2</SUP>. In order to achieve uniform irradiation condition and to avoid fast drilling, the target was rotated with 180 rpm. KBr, Si and sapphire substrates were positioned at 3.5 cm from the target and parallel to it. The nitrogen pressure during deposition was set at 1*10<SUP>-4</SUP>, 5*10<SUP>-3</SUP> and 5*10<SUP>-1</SUP> mbar, respectively. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDAX), and X-ray Diffraction (XRD) analysis were carried out in order to check the composition, structure and the surface aspect of the deposited layers.
We have designed and built an ultra high vacuum chamber which allows thin film depositions on large area flat substrates and on 3D substrates by the pulsed laser deposition and reactive pulsed laser deposition techniques. Heating of substrates during and after film deposition is possible by using either resistive heaters or a lamp array. Metal and metal nitride and carbide were deposited on Si wafers, 3D steel substrates, teflon plates and paper sheets.
We report on parametric studies of CN<SUB>x</SUB> films deposited by excimer laser ablation of graphite targets in molecular nitrogen atmosphere as a function of gas pressure and laser fluence values. Substrates were Si single crystals at room temperature. Deposition rates decrease with increasing nitrogen pressure. The N/C atomic ratio generally increases with increasing nitrogen pressure and laser fluence, N atoms are mainly bonded to C atoms in the sp<SUP>2</SUP> and sp<SUP>3</SUP> bonding states. At relatively high pressure and laser fluences about 40 percent of the C atoms and about 50 percent of the N atoms are bounded in the C-N single bonds, generally attributed to the (beta) -C<SUB>3</SUB>N<SUB>4</SUB> compound.
Carbon nitride films were deposited on Si substrates at room temperature by XeCl laser ablation of graphite targets in low pressure N<SUB>2</SUB> atmosphere at the fluence of 12 J/cm<SUP>2</SUP>. The films have been submitted to x-ray diffraction and transmission electron microscopy investigations in order to study the structure of the films. Results showed that the samples are constituted of a continuous amorphous film inside which microcrystals are dispersed. A new CN<SUB>x</SUB> phase has been identified. This new phase has a triclinic crystallographic cell with lattice parameters a equals b equals 0.384 nm, c equals 0.438 +/- 0.007 nm, (alpha) equals 110 +/- 1 degrees, (beta) equals 105 +/- 1 degrees, and (gamma) equals 120 degrees. It coherently grows on the Si plane with the following orientation relationships: (001)<SUB>CNx</SUB> (parallel) (111)<SUB>Si</SUB>; (100)<SUB>CNx</SUB> (parallel) (1-10)<SUB>Si</SUB> and (010)<SUB>CNx</SUB> (parallel) (01-1)<SUB>Si</SUB>.
The effects of laser fluence on the structure and properties of the reactive pulsed laser deposited carbon nitride (CNx) thin films prepared at different gas pressures of N<SUB>2</SUB> and NH<SUB>3</SUB> were investigated. The structure, the morphology and the chemical composition of the films were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Rutherford backscattering and Fourier transform infrared. The films were plane, adhesive and relatively hard, with a low droplet density. The deposition rate decreases with increasing gas pressure and laser fluence, while the N/C atomic ratio increases. The nitrogen concentration in the deposited films drops when the substrate temperature is increased, indicating a desorption process of the CN radicals. Spectroscopic studies of the plasma plume indicates an interesting correlation between the CN band emission intensity and the nitrogen concentration in the samples.
Carbon nitride films were deposited on <111> Si substrates by pulsed laser ablation of graphite targets in low pressure (1 - 50 Pa) N<SUB>2</SUB> atmosphere. The irradiations were performed with an XeCl excimer laser at the fluences of 12 and 16 J/cm<SUP>2</SUP>. Many different diagnostic techniques (SEM, RBS, XPS, XRD, TEM) have been applied to characterize the deposited layers. The deposition rate decreases with increasing ambient pressure. The N/C atomic ratio into the deposited films generally increases with increasing ambient pressure and laser fluence. N/C values up to 0.7 were inferred from the RBS spectra. There are evidences of the formation of quite large crystals, which have grown almost epitaxially on the <111> Si substrate. Heating of the substrates during depositions causes a reduction of the N/C ratio. Optical emission spectra of the laser plasma plume have been recorded and analyzed, to try to correlate plasma characteristics with the composition of the deposited films.
Thin amorphous C-N films were deposited on <100> Si and KBr substrates at room temperature by XeCl laser ablation of graphite in low pressure (0.01 - 2.5 mbar) nitrogen atmosphere. Laser fluences were 3, 6, and 12 J/cm<SUP>2</SUP>. Scanning electron microscopy, energy dispersion spectroscopy, x-ray diffraction spectroscopy, Rutherford backscattering spectrometry, Fourier transform infrared spectroscopy were used to characterize the deposited films, which result homogeneous, hard, amorphous and present a high electrical resistivity. The deposition rate decreases with increasing ambient pressure. The N/C atomic ratio into the deposited films generally increases with increasing ambient pressure and laser fluence. N/C values up to 0.5 were measured. Heating of the substrates during film deposition causes a reduction of the N/C ratio.
Time-integrated optical emission spectra were recorded during XeCl laser ablation of graphite targets in vacuo and in low pressure (up to 2.5 mbar) N<SUB>2</SUB> and NH<SUB>3</SUB> atmospheres. The emission spectra of the laser induced plasma plume recorded during ablation in vacuo are dominated by the bands of the C<SUB>2</SUB> Swan system. Weaker bands from CN are well evident. Spectra recorded during ablation in NH<SUB>3</SUB> are also dominated by the bands of the C<SUB>2</SUB> Swan system. During ablation in N<SUB>2</SUB> (10<SUP>-4</SUP> - 2.5 mbar), strong bands of the CN violet system are detected. Their intensities increase with increasing N<SUB>2</SUB> pressure and with increasing distance from the graphite target. Mass spectra were recorded during ablation in vacuo (10<SUP>-6</SUP> mbar) and in N<SUB>2</SUB> at 1 multiplied by 10<SUP>-4</SUP> mbar. In vacuo, during the first stage of ablation, the mass spectra are characterized by the peaks at 12 and 26 a.m.u. (C and CN). The CN peak intensity decreases with the laser pulse number, to disappear after approximately 3000 laser pulses. In contrast, the peak at 26 a.m.u. is permanent during the whole ablation when N<SUB>2</SUB> is introduced in the chamber.
Thin amorphous C-N films were deposited on silicon substrates at room temperature by XeCl laser ablation of graphite in low pressure nitrogen and ammonia atmospheres. The deposition rates decrease with increasing ambient pressure. Films deposited in NH<SUB>3</SUB> are thinner than films deposited in N<SUB>2</SUB> at the same ambient pressure, their N/C atomic ratios are higher, but they present a lower quantity of N atoms bound to C atoms. Different diagnostic techniques were used to characterize the deposited films, which result homogeneous, hard and present a high electrical resistivity.
We report a study of the characteristics of thin films deposited at room temperature on Si and KBr substrates by XeCl laser ablation of graphite in low pressure (0.25-2.5 mbar) nitrogen and ammonia atmospheres. Very hard films, with a very high electrical resistivity were obtained. The deposition rates decrease with increasing ambient pressure. N/C atomic ratios up to 0.6 were calculated from backscattering measurements. Different diagnostic techniques (XPS, IR absorption spectroscopy, etc.) demonstrate the formation of carbon nitride with a prevalent graphite-like structure. Films deposited in NH<SUB>3</SUB> are thinner and present a lower quantity of N atoms bound to C atoms than films deposited in N<SUB>2</SUB> at the same ambient pressure.
Titanium nitride and titanium carbide films were deposited on silicon substrates by XeCl excimer laser reactive ablation of titanium in nitrogen and methane atmospheres, respectively. A series of 10,000 pulses at the fluence of approximately 5 J/cm<SUP>2</SUP> and repetition rate of 10 Hz were directed to the target. The pressure in the chamber was fixed, during every irradiation series, at a given value within the range 6 X 10<SUP>-4</SUP> - 10 mbar of N<SUB>2</SUB> or CH<SUB>4</SUB>. Very flat films with thickness exceeding 1 micrometers were deposited. The structural characteristics of the deposited films were investigated by Rutherford backscattering spectrometry, scanning electron microscopy, and by x-ray diffraction. Under specific experimental conditions very pure nitride films were deposited.