RF plasma sources are now the active nitrogen source of choice for III-nitrides growth by molecular beam epitaxy. One of the key process controls for III-nitrides film growth is the III:N flux ratio. Particularly for GaN growth, this has been shown to have a critical bearing on the morphology and electronic properties of films. However, due to the combination of species emitted from an RF plasma source, these ratios can be difficult to interpret. Active nitrogen species can include neutral and ionic forms of both atomic and excited molecular nitrogen. Factors which can influence which species are produced and in what quantity include, flow rate and RF power, as well as device features such as the aperture plate and cavity design. In this study we monitor the optical emission from an Oxford Applied Research HD-25 RF source at various powers and flow rates in the 500 – 900 nm range. Indium nitride films grown on GaN buffers atop sapphire substrates were grown under a range of plasma operating conditions to investigate the effect of active nitrogen species on film properties. Hall effect measurements revealed a carrier concentration of 5 × 10¹⁸ cm^-3 for films grown where molecular nitrogen transitions dominated the spectrum as opposed to 5 × 10¹⁹ cm^-3 when atomic nitrogen dominated. In-situ monitoring by reflection high energy electron diffraction showed that the a plane lattice constant relaxed fully within 2 nm of growth for high atomic flux, but does not fully relax until films are approximately 30 nm thick for high molecular flux. Emission spectra from the HD-25 are compared to similar measurements from the Veeco UNI-Bulb. It is found that both sources produce relatively more atomic nitrogen at high RF powers and low flow rates. However, for given operating conditions the HD-25 is found to produce a higher content of atomic and 2nd – positive series excited molecular nitrogen than the UNI-Bulb.

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