Control of Nitrogen Dissociation Degree in a Helicon Discharge Used for Plasma Assisted Molecular Beam Epitaxy of GaN

Summary form only given. Plasma assisted molecular beam epitaxy (PAMBE) of III/V-group nitrides is a potential alternative to metal-organic chemical vapor deposition (MOCVD) for fabrication of high quality wide band gap semiconductor devices. The main advantage of PAMBE over MOCVD is the use of solid gallium evaporated in a Knudsen cell in conjunction with a pure nitrogen plasma source to generate the reactive nitrogen species. Consequently, elevated substrate temperatures are not needed and low temperature nitridation dramatically improves the optical and structural qualities of the GaN epitaxial layers. A helicon discharge could be an attractive alternative plasma source for reactive nitrogen species creation since it allows control of the population of specific reactive N₂ species by modifying the electron energy distribution function through the resonant wave-particle interaction arising from electrons traveling at same velocity as the phase velocity of the helicon wave. The high plasma density (Gt10¹³ cm^-3) and high ion exit flow speed (Mach 2.5 for argon) would yield significantly higher fluxes at the substrate surface and consequently an improved deposition rate over existing MBE plasma sources. Epilayer quality could also be improved by lowering kinetic energy of reactive species. The helicon source will also be less contaminating since, unlike conventional RF or other plasma sources, it does not require an antenna, a plasma filtering aperture or a nozzle inside the chamber. We report on control of nitrogen dissociation degree in a compact helicon plasma source by adjustment of the external working parameters such as the source magnetic field, gas pressure, driving frequency, and input power. The dissociation degree has been inferred from the ratio of nitrogen atomic line (746.8 nm) and the molecular transition (B³P_g, v\´=4 fi A³S_u ⁺, v"=2 at 750.4 nm) by taking into ac- ount electron impact excitation rate coefficients