Intracavity Laser Spectroscopy: Chemical Vapor Deposition And Supersonic Jet Expansions

Intracavity laser spectroscopy: chemical vapor deposition and supersonic jet expansionsJ.J. O'Brien, N. Goldstein and G.H. AtkinsonDepartment of Chemistry and Optical Sciences Center, University of ArizonaTucson, Arizona 85721AbstractThe high sensitivity of intracavity laser spectroscopy (ILS) for direct, quantitativedetection of species at low concentrations and in special environments has been wellestablished. In this paper, we demonstrate the use of ILS to obtain rotationally -resolvedabsorption spectra of (i) reactive radical species in situ during chemical vapordeposition (CVD) processes used to produce silicon films and (ii) ultra -cold gas phasemolecules and van der Waals (vdW) complexes produced in supersonic jet expansions.Information on the chemistry of CVD processes for a number of silicon source materials andthe formation efficiencies, stability and excited -state dissociation dynamics for a seriesof I2 -noble gas vdW complexes is presented.IntroductionThe research areas involving the quantitative control of CVD and the preparation ofultracold beams of atoms, molecules, and ions both have become increasingly utilized inthe development of new, high -technology materials. Of particular interest has been thefabrication of thin film materials which display new optical and electronic properties. Acritical dimension to such studies has been the quantitative, in situ detection of the lowconcentrations of species formed during the reactive chemistry. This provides a basis foranalyzing the reaction dynamics and a real -time diagnostic method in the optimization ofprocessing conditions.Laser induced fluorescence (LIF) often has been the detection scheme of choice forobserving species present in low concentrations. This is primarily because of its highsensitivity, but also because LIF provides the capabilities for the time resolution ofinternal energy flow and excited -state dynamics. LIF is an indirect detection method,however, in that it relies upon the emission properties of a species prepared by theabsorption of radiation. If emission does not occur, (e.g., due to predissociation) or ifthe species is in an environment where the emission is quenched or dominated by backgroundradiation, LIF is inapplicable. Multiphoton ionization (MPI) and photoacoustic techniquesare other sensitive detection methods that have been employed successfully to monitorspecies at low concentration. Both provide information that complements that obtainedfrom LIF. These methods, however, also derive from phenomena that occur subsequent to theabsorption of radiation.Absorption spectroscopy in principle affords the most widely applicable spectroscopictechnique for detection because it only requires the absorption of radiation by a speciesin its ground state. Traditionally its scope has been limited by its low sensitivity.Since their inception over a decade ago,1,2 a number of techniques based on intracavitylaser absorption have provided tremendous enhancements in detection sensitivity overconventional absorption spectroscopy.3 Until the last few years, however, an incompleteunderstanding of the intracavity absorption effect, a proliferation of experimentalmethods giving disparate results, and the nonquantitative nature of many of the reporteddata prevented the wide adoption of intracavity laser absorption as a reliable detectionmethod. Over the last few years this has changed. There is now a much improvedunderstanding of the intracavity laser effect and of the experimental parameters that areimportant for obtaining reliable, quantitative results. " -6 The most recently developedtechniques based on intracavity laser spectroscopy (ILS) have exhibited capabilities forextremely sensitive and quantitative detection over a broad dynamic range in a number of