A method of forming an iron-doped gallium nitride for a semi-insulating GaN substrate is provided. A substrate (1), such as a (0001)-cut sapphire substrate, is placed on a susceptor of a metalorganic hydrogen chloride vapor phase apparatus (11). Next, gaseous iron compound GFe from a source (13) for an iron compound, such as ferrocene, and hydrogen chloride gas G1HCl from a hydrogen chloride source (15) are caused to react with each other in a mixing container (16) to generate gas GFeComp of an iron-containing reaction product, such as iron chloride (FeCl2). In association with the generation, the iron-containing reaction product GFeComp, first substance gas GN containing elemental nitrogen from a nitrogen source (17), and second substance gas GGa containing elemental gallium are supplied to a reaction tube (21) to form iron-doped gallium nitride (23) on the substrate (1).