Cw UV Laser Research Articles

Measurement of the rate coefficient of H+O 2+M→HO 2+M for M=Ar and N 2 at high pressures

OH mole fraction profiles were measured using cw UV laser absorption in shock-heated mixtures of H 2 /O 2 /Ar/N 2 between 1260 and 1375 K at 50, 68, and 115 atm. Rate coefficients for the reaction H+O 2 +M→HO 2 +M, M=Ar and N 2 were inferred by comparison of measured OH profiles with OH profiles modeled using the GRI-Mech v1.2 mechanism. The rate coefficient data were fit with pressure-dependent expressions that use k ∞ and F cent from Cobos et al. [3]. For M=Ar, the measured third-order rate coefficients are consistent with a pressure-dependent expression that uses k 0 =7×10 17 T −0.8 [cm 6 /mol 2 /s] from GRI-Mech v1.2. For M=N 2 , the present data, when combined with the lower-temperature value of Bromly et al. [15], are consistent with a pressure-dependent expression that uses k 0 =2.6×10 19 T −1.24 [cm 6 /mol 2 /s] over the temperature range of 700–1375 K. This expression yields k 0 values that are substantially higher than the GRI-Mech v1.2 recommendation but consistent with previous flow reactor, flame, and shock tube data and the review of Baulch et al. [24].

Kinetics of the OH + C2H2 reaction in the presence of O2

Room-temperature rate constants for the removal of OH radicals in the gas-phase reaction with acetylene have been determined using pulsed H2O2 photolysis production of OH followed by cw UV-laser long-path absorption detection. The pressure dependences of the rate constants in N2, O2 and synthetic air as buffer gases have been investigated in the fall-off range between 1.5 and 100 k Pa. In N2 the pressure dependence can be described by the empirical expression of Gardiner: k={(k∞)a+(k0[M])a}1/a, using the parameter a=–⅓. This results in the rate constants k∞=(1.07 ± 0.07)× 10–12 cm3 s–1, and k0=(3.9 ± 1.6)× 10–29 cm6 s–1 for the given pressure range. Smaller effective rate constants were obtained in the presence of O2, owing to a fast regeneration channel for OH. In terms of the above expression the rate constants k∞=(4.6 ± 0.2) and (3.1 ± 0.1)× 10–13 cm3 s–1 and k0=(1.8 ± 0.2) and (1.9 ± 0.5)× 10–29 cm6 s–1 were obtained for O2 and synthetic air as buffer gases, respectively. Compared with N2 there is virtually no difference in the fall-off behaviour in O2 and synthetic air, indicating that the OH regeneration yield is independent of pressure. On the other hand, this yield was found to decrease with the O2 mixing ratio. A kinetic model is proposed that qualitatively explains this effect with different chemical properties of non-thermalized OH–acetylene adducts with regard to O2. This model is confirmed by measurements showing a significantly stronger dependence of the regeneration yield on the O2 mixing ratio in O2–He mixtures.

Shock tube study of the rate coefficient of H + O 2 → OH + O

The H + O 2 → OH + O reaction was reexamined by monitoring OH radical concentrations using cw UV laser absorption spectroscopy in H 2-O 2-Ar test gas heated by 1850-3550 K shock waves. The rate coefficient expression (1.0 ± 0.1) × 10 14 × exp(−7690 ± 250 K/T) cm 3 mol −1 s −1 was derived. Fitting the present data together with those of Shin and Michael [J. Chem. Phys. 95 (1991) 262] gave k 4 = (7.6 ± 0.7) × 10 13 exp(−7065 ± 140 K/ T) cm 3 mol −1 s −1 for the temperature range 1100–3550 K. The reverse reaction rate coefficient was found to be (9.3 ± 0.8) × 10 12 cm 3 mol −1s −1 independent of temperature from 1850 to 3550 K.

Ultraviolet laser contamination of quartz optics

Laboratory experiments were conducted to determine the cause of degradation of two quartz polarizers used in conjunction with a cw UV laser. UV transmission spectra and x-ray photoelectron spectroscopic measurements of the damaged optic and laser-induced photolysis of one of the adhesives used to hold the polarizer in position indicate that the degradation is most probably a result of UV photolysis of adhesive residue. It would therefore be prudent to avoid use of all organic adhesives in UV laser optics. An alternative method of holding a quartz beam splitter without adhesives is described and is currently being used.

Effect of the conditions during photostimulated epitaxy on the structure of the resulting films

A study has been made of how electromagnetic radiation influences the growth of epitaxial structures. Experiments on the growth of IV–VI films used a high-power xenon flashlamp and pulsed and cw UV lasers. The conditions which result in the growth of nonplanar film structures were determined. An experimental study was made of the properties of indium-doped films, including the transition layer between the film and the substrate and film peeling processes. A model for processes at the crystallization surface is examined. The kinetics of defects must be taken into consideration in an analysis of photostimulated epitaxy.

Laser‐induced deposition of aluminium on gallium arsenide from trimethylamine alane

AbstractAluminium microstructures (dots or strips) have been photodeposited on GaAs substrates via the decomposition of a solid precursor, the 2:1 hydride adduct of Al, called trimethylamine alane (TMAA).The dissociation of TMAA, the vapour pressure of which can be varied between 0.4 and 1 mbar by dilution in hydrogen carrier gas, has been achieved via a pyrolytic process using a CW Ar+ laser operating in the visible range (488–514 nm). This decomposition reaction occurs at laser powers higher than 0.3 W, i.e. at laser‐induced temperatures higher than 130°C, and leads to the formation of high‐purity aluminium strips as observed from Auger electron spectroscopy (AES) and energy‐dispersive X‐ray analysis (EDX) data. The deposition rates are in the range 50–1400 Å s−1. The minimum of line resistivity measured versus scanning speed and laser power is 6.5 μΩ cm.The photolytic decomposition of TMAA has been performed at room temperature within the laser spot area using a CW UV laser (intracavity frequency doubler emitting at 257 nm). The formation of aluminium clusters has been observed at very low deposition rates (20 Å s−1). However, these tiny clusters react quickly with oxygen and are transformed into alumina clusters.All these results have been tentatively explained on the basis of the surface reactivity of H2 and TMAA regarding GaAs or Al, as obtained by other workers through surface science techniques and mass spectrometry.

IR and UV laser-induced chemical vapor deposition: Chemical mechanism for a-Si:H and Cr (O,C) film formation

The characteristic features of laser-induced chemical vapor deposition in the parallel and perpendicular laser beam/surface configurations are discussed. Low temperature chemical processing with directed and spatially localized energy deposition in the system is investigated. Results obtained for the deposition of hydrogenated amorphous silicon (a-Si:H) films in the parallel configuration employing CO 2 and KrFlasers and SiH 4 and Si 2H 6 as precursors are presented. As a second example, the growth of oxygen- and carbon-containing chromium films Cr(O,C) from chromium hexacarbonyl as the precursor using cw and pulse uv lasers is discussed. The chemical pathways leasing to film formation are investigated in detail.

LASER ASSISTED CVD SEMICONDUCTOR FILMS

The paper reports the theoretical and experimental approches for laser assisted CVD semiconductor films such as Si and GaAs. In contrast with previous works, a special experiment design was used to separate the otpic and thermal effects of laser. Experimental results have shown that the deposition activation energy is lowered under the irradiation of cw IR or UV laser, so that the deposition rate increases by a factor which is associated with the laser frequency and the substrate temperature. Also, the technology for depositing high quality films and for LCVD in large area are discussed.

Quartz crystal microbalance measurements of absolute laser photodeposition rates: Application to 257-nm deposition from W(CO)6

A quartz crystal microbalance has been used to measure absolute rates of laser-induced photochemical deposition processes in real time. A cw UV laser (257 nm) focused at normal incidence onto the microbalance crystal surface is used to induce deposition by photodecomposition of an organometallic vapor. An equation is derived to obtain the absolute mass of the deposit. The method is demonstrated for photodeposition from W(CO)6.

Photochemical Processes in Photo-Assisted Epitaxy of CdxHg 1-xTe

ABSTRACTMechanisms for the low temperature photo-dissociation of alkyl precursors for the epitaxial growth of CdxHg1-xTe (CMT) are discussed. The roles of vapour and surface nucleation are considered in the light of the free radical model which also can provide methods for controlling the vapour phase photochemistry. Higher quality CMT has been grown at 250 °C by using dimethyl mercury as a source of free methyl radicals. Problems encountered in reducing growth temperature for multilayer epitaxy are considered for CdTe and HgTe and conditions established for epitaxial growth at 200°C.The roles of alkyl concentration, substrate temperature, UV intensity and free radical concentration are explored. Results on the first reported growth of CdTe deposition using a cw UV laser source are compared with the arc lamp grown layers.

A cw UV laser absorption study of the reactions of the hydroxy-cyclohexadienyl radical with NO 2 and NO

Rate constants for the reactions of the hydroxycyclohexadienyl (HCHD) radical, the adduct of the OH + benzene reaction, with NO 2 (1) and NO (2) in the gas phase have been determined at 298 K using long-path cw UV laser absorption to monitor HCHD. The rate constants are found to be k 1 = (8.5±2.1)×10 −12 cm 3/s and k 2 = (1.0±0.3)×10 −12 cm 3/s. The apparent rate coefficient for reaction (2) is only slightly influenced by addition of O 2.

Direct writing of regions of high doping on semiconductors by UV-laser photodeposition

The direct doping of InP with variable Cd concentrations has been demonstrated in a maskless process with micrometer-scale spatial control. The process makes use of cw UV-laser photodeposition of the dopant on a laser-heated surface.

NEW cw METAL-VAPOR LASER TRANSITIONS IN Cd, Sn, AND Zn

cw laser action has been observed at 3250 Å in Cd II, 6453 and 6844 Å in Sn II, and 7479 and 7588 Å in Zn II. Laser action occurred in He-metal-vapor discharges at pressures of several Torr He and much lower partial pressures of the metal vapor. The first four transitions have similar operating characteristics and are believed to be excited by Penning collisions between metastable He atoms and neutral metal-vapor atoms (similar to the mechanism for exciting the 4416 Å cw transition in Cd II). The 3250 Å transition is the first cw uv laser that can be built and operated with relatively simple techniques, similar to those used in He–Ne lasers.