Impurities in hydride gases part 2: Investigation of trace CO2 in the liquid and vapor phases of ultra-pure ammonia

Abstract

Ammonia (NH 3 ) as a precursor for epitaxial nitride films is required to be free of trace oxygenated impurities, such as CO 2 , that have been shown to negatively affect growth processes and device performance. Carbon dioxide can react reversibly with the NH 3 gas to form ammonium carbamate, NH 4 COONH 2 (a solid with low solubility in liquid NH 3 ) and, therefore, can be present in cylinder sources both in the free and chemically bound form. A gas chromatograph (GC)-based method has been developed to accurately quantify the total CO 2 content in both vapor-and liquid-phase NH 3 streams. A heated GC-sampling manifold is used to thermally decompose any NH 4 COONH 2 present in the sample or calibration standard so that all CO 2 is analyzed in its free form. Several commercial cylinder sources maintained at room temperature were analyzed by this method, and in all cases, equilibrium concentrations of Ͻ75 parts-per-billion by volume (ppbv) CO 2 were present in the gas phase as long as residual liquid was present. Slightly higher concentrations were found in the liquid phase, and upon exhaustion of the liquid phase and heating, CO 2 levels strongly increased to parts-per-million by volume (ppmv) levels. The excess CO 2 is likely adsorbed on the cylinder walls or dispersed in the liquid as solid NH 4 COONH 2 . These results are consistent with thermodynamic calculations based on equilibrium data for the carbamate system available in the literature. To meet the purity requirements of organo-metallic vapor-phase epitaxy processes, the performance of an adsorbent-based purifier that is capable of removing residual CO 2 in both free and chemically bound forms from NH 3 streams is discussed.

Simultaneous electrocatalytic reduction of dinitrogen and carbon dioxide on conducting polymer electrodes

“…Ammonia and carbon dioxide reacted under pressure to form ammonium carbamate: in this case ammonia was synthesized from N 2 in the solution which became carbamate via a chemical step with CO 2 [38]. In the first step a proton was reduced to an atomic hydrogen (Reaction (1)); the H ad 's were added to N 2 and the reaction product ammonia was formed (Reaction (4)).…”

Effects of intentional oxygen and carbon doping in MOVPE‐grown GaN layers on photoelectric properties

“…However, the FWHM values increased drastically in the 6000 ppb CO 2 ‐doped GaN, showing a change in crystal quality and strongly suggesting a possible change in crystal growth mechanism. According to Funke et al , CO 2 reacts with NH 3 to form ammonium carbamate (CH 6 N 2 O 2 ), a white crystalline solid. The degradation of crystal quality in the present work is thus likely due to the formation of new products as well as the high carrier density.…”