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

The gas phase reaction of trimethylgallium (TMG) with ammonia was investiga ted because of its importance in the GaN chemical vapour deposition process . Water is the most important impurity in ammonia and therefore its reactio n 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 temperat ure range 294 to 1000 K by molecular beam sampling and mass spectrometric d etection. Two ionisation methods were applied: VUV radiation at 118 nm and electron ionisation at 20 eV. The ionisation pattern of TMG was studied, an d the results were used in the interpretation of the TMG-ammonia system. Th e reaction of deuterated ammonia with TMG was used to evaluate the sum form ula of the detected compounds. In the TMG-ammonia system, two signal groups were found mainly at low temperatures, indicating TMG-ammonia complexes (T MG : NH3,TMG(NH3)(2)). Also, species with two gallium atoms and nitrogen we re detected in an extended temperature range and interpreted to be fragment s of (CH3)(4)Ga2NH, and (CH3)(4)Ga-2(NH2)(2). Higher mass species were not found. In the reaction between TMG and water, several signals of volatile r eaction products were detected up to 670 K. Some of them are consistent wit h a trimeric dimethylgallium hydroxide, but other compounds might also be p resent. From the temperature dependence, strategies for the reduction of ox ygen in GaN films can be worked out.