Why are green LEDs made from gallium phosphide treated with nitrogen?

In the context of light-emitting diodes (LEDs), nitrogen is added to gallium phosphide to specifically enable the emission of green light.1

While gallium phosphide is a semiconductor capable of producing visible light through electroluminescence, its natural efficiency for light emission is relatively low. By introducing nitrogen as a dopant into the gallium phosphide crystal structure, manufacturers can modify the material's electronic properties to facilitate the desired green light emission. Similarly, nitrogen is also added to gallium arsenide phosphide to produce orange and yellow light.21