Use of a beam of biradicaloid such as dialkyl group-III nitrene produced by the method of photodissociation of energetic compound beams (PDECB) is expected to be advantageous to the low-temperature thin-film growth of stoichiometric group-III nitride. Through the clarification of fundamental processes involved in pyrolysis as well as photolysis of dimethylgallium azide [(CH3)(2)GaN3], dimethylaluminum azide [(CH3)(2)AlN3], and dimethylboron azide [(CH3)(2)BN3] based on post-self-consistent field ab initio molecular orbital methods, we discuss the suitability of these possible source compounds for the production of beams of dimethylgalliumnitrene [(CH3)(2)GaN], dimethylaluminumnitrene [(CH3)(2)AlN], and dimethylboronnitrene [(CH3)(2)BN] by the PDECB method. The theoretical results suggested that (CH3)(2)GaN3 is a promising PDECB source material in that this molecule possesses the nature of unimolecular metastable dye.