A method of forming an iron-doped gallium nitride for a semi-insulating GaN substrate is provided. A substrate 1, such as a sapphire substrate having the (0001) plane, 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.