Estimation of N atom density in a nitrogen radical source for GaN growth by optical emission spectroscopy--comparison with appearance mass spectrometry

We adopted optical emission spectroscopy (OES) for estimating N atom density in a nitrogen radical source, which was used for the growth of GaN film by molecular beam epitaxy. In addition, we compared the N atom density evaluated by OES with that evaluated by appearance mass spectrometry (AMS). We measured the intensities of optical emissions from N (747 nm) and N$_2$ (337 nm, the 2nd positive band) using a monochromator combined with an ICCD camera. The ratio of the N to N$_2$ densities ($[{\rm N}]/[{\rm N}_2]$) was obtained from the emission intensity ratio, by considering the rate coefficients for electron impact excitations. The absolute N atom density was estimated from the density ratio with the help of a thermodynamic equation of state $p=([{\rm N}] + [{\rm N}_2])k_{\rm B}T_{\rm g}$, where the pressure $p$ was measured using a capacitance manometer and the gas temperature $T_{\rm g}$ was evaluated from the rotational temperature of N$_2$ 2nd positive band. We found an excellent agreement between the N atom densities evaluated by OES and AMS, when we assumed an electron temperature of 10 eV and an N$_2$ vibrational temperature of 5000 K. This work was supported by 21st Century COE (Center of Excellence) Program ```Information Nano-Devices Based on Advanced Plasma Science" of Nagoya University.