Probing plasma-surface interactions with the transmission electron microscope or the Si-collector interface of the plasma bipolar junction transistor

Two platforms developed to probe the plasma-solid interface are briefly reviewed here. A sensitive diagnostic of the interaction between a low-temperature plasma and a silicon surface is provided by the plasma bipolar junction transistor (PBJT) in which e^--h⁺ (semiconductor) and e^--ion (gas phase) plasmas are separated by a nanoscale potential barrier but coupled by a strong (>1 V μm^-1) electric field. Electrical properties of the Si base-collector plasma interface are controlled by the bias applied to the PBJT emitter-base junction, as well as the morphology and electronic structure of the base surface facing the collector plasma. Recent experiments are described in which the Si(100) surface of the PBJT base is transformed into black Si, a nanostructured array comprising ∼3 × 10⁹ nanocones. Each cone is ∼100 nm in height and has a radius of curvature at its tip of <1 nm. Altering the base surface so as to intentionally enhance field emission results in burst-mode operation of the PBJT in which the collector current oscillates as a result of the periodic contraction (or collapse) and revival of the plasma sheath. The observed oscillation frequencies (3.6-13 kHz) are consistent with the transit time of ions across the sheath. Integration of a microplasma device with a transmission electron microscope has also been realized recently (Tai et al 2013 Sci. Rep. 3 1325). This diagnostic tool allows plasma-surface interactions to be observed in real-time with a spatial resolution <100 nm. Although initial experiments concerned the growth of Au islands in a dc plasma, the extension of this probe to nanostructured and spatially modulated surfaces is proposed.