Observation and Mitigation of RF-Plasma-Induced Damage to III-Nitrides Grown by Molecular Beam Epitaxy

In this work, radio-frequency (RF) plasma-induced damage to III-nitride surfaces and bulk defects is observed and mitigated. It is shown that for InN films, the surface is more sensitive to plasma-induced damage than GaN films, as observed via atomic force microscopy and reflection high energy electron diffraction. In order to isolate any possible plasma-induced damage, a growth window for InN is established, and temperature ranges are determined for other damaging effects which include roughening due to low adatom mobility, InN decomposition, and indium desorption. In situ plasma monitoring and optimization are accomplished with a combination of optical emission spectroscopy as well as a remote Langmuir probe. It is shown that by increasing the plasma nitrogen flow, the positive ion content increases; however, the ion acceleration potential reduces. Additionally, a reduced RF plasma power results in a reduction of atomic nitrogen species. These plasma species and energetic variations result in variations in the bulk unintentional background electron concentrations observed by room temperature Hall effect measurements of ∼1 μm thick InN films. By increasing the nitrogen flow from 2.5 to 7.5 sccm for a constant RF power of 350 W, the background electron concentration decreases by 74% from 1.36 × 10¹⁹cm^-3 to 3.54 × 10¹⁸cm^-3, while maintaining a smooth surface morphology. Additionally, photoluminescence spectra indicate optical emission energies shift from ∼0.81 to 0.71 eV (closer to the fundamental bandgap of InN) by limiting the damaging plasma species. Finally, conditions are presented to further minimize plasma-induced damage in III-nitride devices.