On the spectroscopic detection of neutral species in a low-pressure plasma containing boron and hydrogen

Spectroscopic studies of microwave discharges in H₂–Ar–B₂H₆ gas mixtures (f = 2.45 GHz, P = 1.2–3.5 kW, p = 1–8 mbar) have been performed to improve the possibilities of diagnostics of non-equilibrium, low-pressure plasmas containing boron and hydrogen. For this purpose, UV–VIS optical emission spectroscopy and infrared absorption spectroscopy with tunable diode lasers (TDLAS) have been applied. It is shown that information about neutral species and the gas temperature may be obtained by means of new and modified spectroscopic methods. A method for the determination of the absolute number density of boron atoms from measured relative intensities of the components of the boron resonance doublet (distorted by reabsorption) is proposed and tested for validity. The maximum of the density was found to be 3.8×10¹¹ atoms cm⁻³ at an admixture of diborane of about 2%. The gas temperature was determined from the intensity distributions in the rotational structure of the emission bands of BH and H₂ and from Doppler broadening of the absorption line profiles of the BH molecule. It was observed that values of the gas temperature obtained from the rotational intensity distributions are in good agreement with those obtained from Doppler widths (T_g = 700–1070 K). Based on measurements of the relative line intensities of atomic and molecular hydrogen and the gas temperature, and using a simple excitation–deactivation model, the density of molecular hydrogen was found to be about 40 times higher than the density of atomic hydrogen. It is shown that some absorption lines of boron hydrides (B₂H₆, BH₃ and BH) detected by TDLAS may be used for plasma diagnostics.