A process and apparatus, whereby in the process of vapor growth of a gallium nitride group semiconductor (Al.sub.x Ga.sub.1-x N; inclusive of x=0) thin film using an organometallic compound gas, a reactant gas which grows Al.sub.x Ga.sub.1-x N and a reactant gas containing a doping element are separately introduced near to a susceptor and mixed in the vicinity of a substrate held by the susceptor to grow an I-type Al.sub.x Ga.sub.1-x N thin film, are disclosed. Also, a process of vapor growth and apparatus having a mixing tube and a process and apparatus for inclining the susceptor relative to the reactant gas flow are disclosed. Moreover, a process and apparatus, whereby the Al.sub.x Ga.sub.1-x N thin film is subjected to the crystal growth using a plasma of the reactant gas under reduced pressure, under the irradiation of ultraviolet rays or laser beams, are disclosed.