An experimental study of the reactions of trimethylgallium with ammonia and water over a wide temperature range

Abstract

The gas phase reaction of trimethylgallium (TMG) with ammonia was investigated because of its importance in the GaN chemical vapour deposition process. Water is the most important impurity in ammonia and therefore its reaction with TMG was investigated as a source of oxygen impurities in GaN films. Gas phase intermediates were studied in a flow tube reactor in the temperature range 294 to 1000 K by molecular beam sampling and mass spectrometric detection. Two ionisation methods were applied: VUV radiation at 118 nm and electron ionisation at 20 eV. The ionisation pattern of TMG was studied, and the results were used in the interpretation of the TMG–ammonia system. The reaction of deuterated ammonia with TMG was used to evaluate the sum formula of the detected compounds. In the TMG–ammonia system, two signal groups were found mainly at low temperatures, indicating TMG–ammonia complexes (TMG:NH3, TMG(NH3)2). Also, species with two gallium atoms and nitrogen were detected in an extended temperature range and interpreted to be fragments of (CH3)4Ga2NH, and (CH3)4Ga2(NH2)2. Higher mass species were not found. In the reaction between TMG and water, several signals of volatile reaction products were detected up to 670 K. Some of them are consistent with a trimeric dimethylgallium hydroxide, but other compounds might also be present. From the temperature dependence, strategies for the reduction of oxygen in GaN films can be worked out.