The invention relates to a method of manufacturing a rare-earth doped alkaline-earth silicon nitride phosphor of a stoichiometric composition. Said method comprising the step of selecting one or more compounds each comprising at least one element of the group comprising the rare-earth elements (RE), the alkaline-earth elements (AE), silicon (Si) and nitrogen (N) and together comprising the necessary elements to form the rare-earth doped alkaline-earth silicon nitride phosphor (AE2Si5N8:RE). The method further comprises the step of bringing the compounds at an elevated temperature in reaction for forming the rare-earth doped alkaline-earth silicon nitride phosphor (AE2Si5N8:RE). In such a method normally a small amount of oxygen, whether intentionally or not-intentionally added, will be incorporated in the rare-earth doped alkaline-earth silicon nitride phosphor (AE2Si5N8:RE). According to the invention the creation of defects by formation of a non-stoichiometric oxygen containing phosphor is at least partly prevented by partly substituting for the ions (AE, Si, N) of the alkaline-earth silicon nitride phosphor (AE2Si5N8:RE) suitable further elements of the periodic system by which vacancies are created, filled or annihilated resulting in the formation of a modified alkaline-earth silicon nitride phosphor (AE2Si5N8:RE) having a stoichiometric composition. In this way a modified phosphor is obtained having excellent and stable optical properties. The invention further relates to a modified phosphor obtainable by the above-mentioned method and a radiation converting device comprising such a phosphor.