The invention concerns an assembly method comprising the following steps: a) providing a first substrate comprising a first face made from crystalline indium phosphide, b) providing a second substrate comprising a second crystalline face different from the indium phosphide, c) forming an intermediate layer of crystalline indium phosphide on the second face of the second substrate, d) forming an assembly, via a direct bonding step, by bringing the first face of the first substrate into contact with the intermediate layer, the direct bonding step being carried out in an atmosphere having a pressure greater than 10−4 Pa, and preferably higher than 10−3 Pa, e) subjecting the assembly formed in step d) to heat treatment.