Vibrational population distribution and the gas temperature in the compact helicon nitrogen plasma source chewie

Summary form only given. We report preliminary results on the development of a helicon plasma source with variable activated nitrogen composition for plasma assisted molecular beam epitaxy of III/V-nitrides. The main idea is to alter the population of specific reactive nitrogen species in a helicon plasma source 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 and high ion exit flow speed (n
_e
= 10
¹³
cm
^-3
and v
_i
= 8,000 m/s for argon) should 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 active nitrogen source is a steady state, high density, helicon plasma source CHEWIE (Compact HElicon Waves and Instabilities Experiment). The helicon vacuum chamber is a 12 cm long, Pyrex tube, 6 cm in diameter, connected to a stainless steel diffusion chamber, 30 cm long, 15 cm in diameter. Three magnetic field coils surround the source and are capable of generating an axial magnetic field up to 1200 G in the source and about 100 G at the end of the expansion chamber. A 7 cm long, water cooled, Boswell saddle type antenna couples the RF energy into the plasma. RF power of up to 600 W over a frequency range of 3-28 MHz is used to create the steady state plasma in the source which expands away into a region of decreasing magnetic field. Optical emission spectroscopy investigations in the plasma source show that under certain working conditions, the N
₂
first positive system (B
³
Pi
_g
rarr A
³
Sigma
_u

⁺
) are the dominant transitions in nitrogen, helicon-generated plasma. From band head intensities a Boltzmann relative vibrational population distribution is obtained. From the fit of the Deltav=+1 (at 891.24 nm), Deltav=+2 (at 891.24 nm) and Deltav=+3 (at 687.50 nm) bands, a gas temperature of ~350 K for an input power of 300 W, a magnetic field of 800 G and N
₂
gas pressure of 20 mtorr is inferred