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

Abstract

Ammonia (NH₃) as a precursor for epitaxial nitride films is required to be free of trace oxygenated impurities, such as CO₂, that have been shown to negatively affect growth processes and device performance. Carbon dioxide can react reversibly with the NH₃ gas to form ammonium carbamate, NH₄COONH₂ (a solid with low solubility in liquid NH₃) 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 CO₂ content in both vapor- and liquid-phase NH₃ streams. A heated GC-sampling manifold is used to thermally decompose any NH₄COONH₂ present in the sample or calibration standard so that all CO₂ 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 <75 parts-per-billion by volume (ppbv) CO₂ 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, CO₂ levels strongly increased to parts-per-million by volume (ppmv) levels. The excess CO₂ is likely adsorbed on the cylinder walls or dispersed in the liquid as solid NH₄COONH₂. 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 CO₂ in both free and chemically bound forms from NH₃ streams is discussed.