Corona discharge supersonic free-jet for III–V nitride growth via A 3Σu+ metastable nitrogen molecules

Abstract

A corona discharge supersonic free-jet is shown to be a nearly pure source of A 3Σu+ metastable molecular nitrogen, an electronically excited and chemically active form of N2 that is ideally suited to nitride semiconductor growth. Optical emission spectroscopy at various distances downstream of the supersonic nozzle reveals a cascade through the excited state manifold of N2 triplet states to populate the A 3Σu+ state. Appearance potential spectroscopy (mass spectrometer electron bombardment ionization yield, measured as a function of electron impact energy) delivers the composition of the terminal molecular beam. A 3Σu+ molecules are the dominant activated species in the beam, which otherwise contains only nonreactive X 1Σg+ ground state nitrogen molecules plus a minor amount of S04 nitrogen atoms. Up to 1.56% number fraction of the beam is A 3Σu+, providing 1.0×1017 metastables sr−1 s−1.

Microcapillary Supersonic Free-jet Injection: Plasma Diagnostics

“…The wavelengths of persistent band heads are well known for these series [13] and a selection has been plotted in the figures. As has been previously documented [3,4] and as is evident in the top panel of these three figures, the relative ratio of Second Positive to First Positive emission decreases rapidly with increasing distance from the nozzle, reflecting the electronic relaxation that accompanies expansion cooling of the free-jet. At a distance of 3 cm from the CDSFJ nozzle, the Second Positive emission has largely vanished, indicating nearly full depletion of the C 3 P u state.…”

“…Voltage and current were supplied to the wire from a high voltage supply (Spellman SL6PN150, 6 kV, 150 W) through a 250 kX ballast resistor. In the measurements described here, the corona wire was always held negative relative to the grounded cylindrical in order to achieve a stable, steady corona discharge having no ''streamers'' [3,6]. With 400 Torr of nitrogen supplied to the CDSFJ nozzle and a negative corona discharge running at 16 mA and 5.2-5.4 kV, the pressure in the nozzle chamber (18,000 l/s pumping speed for air) was 1 9 10 -6 Torr.…”

“…These measurements were conducted in an experimental apparatus that has been described previously [11], using a custom-made CDSFJ nozzle that we have characterized in considerable detail [3,4]. The CDSFJ nozzles were formed by heating and drawing fused silica tubing (6 mm OD 9 4 mm ID) to closure, then grinding back (600 and 180 SiC grit) until an aperture of 100-200 lm diameter was obtained.…”

“…This emission decreases with distance from the nozzle but persists well into the outer region, far beyond the cessation of collisions. Indeed, spectroscopic study verifies the glow to arise through optical relaxation of excited electronic states on time scales set by intrinsic lifetimes [3,4]. Significant quantities of metastable and other long-lived states can therefore survive into the terminal expansion, providing a useful flux of such species for scientific and technological use [9,10].…”

“…' For demonstration purposes, we employ as the plasma source a corona discharge supersonic free-jet (CDSFJ) of nitrogen. A CDSFJ is formed by striking a steady, currentlimited, corona discharge at the sharp tip of an electrode placed within a supersonic free-jet nozzle [3][4][5]. The properties of any corona discharge are dominated by the greatly enhanced electric field in the vicinity of the corona electrode [6].…”