Dynamics of the formation and loss of boron atoms in a H$_2$/B$_2$H$_6$ microwave plasma

For further improvements in doped-diamond deposition technology, an understanding of the complex chemistry in H$_2$/CH$_4$/B$_2$H$_6$ plasmas is of general importance. In this context, a H$_2$/B$_2$H$_6$ plasma ignited by microwave power in a near resonant cavity at high pressure (100-200~mbar) is studied to measure the B-atom density in the ground state. The discharge is ignited in the gas mixture (0-135~ppm B$_2$H$_6$ in H$_2$) by a 2.45~GHz microwave generator, leading to the formation of a hemispheric plasma core, surrounded by a faint discharge halo filling the remaining reactor volume. Measurements with both laser induced fluorescence and resonant absoption with a boron hollow cathode lamp indicate that the B-atom density is higher in the halo than in the plasma core. When the absorption line-of-sight is positioned in the halo, the absorption is so strong that the upper detection limit is reached. To understand the mechanisms of creation and loss of boron atoms, time-resolved absorption measurements have been carried out in a pulsed plasma regime (10 Hz, duty cycle 50~\%). The study focuses on the influence of the total pressure, the partial pressure of B$_2$H$_6$, as well as the source power, on the growth and decay rates of boron atoms when the plasma is turned off.