Forbidden optical transitions between impurity levels in silicon and gallium phosphide

Experimental estimates are made of absorption cross sections for forbidden optical transitions from the ground state to long-lived excited states of P, As, Sb, In, and Ga impurities in silicon and Te impurities in gallium phosphide. The results can be used to predict the possibility of long-wavelength stimulated emission being excited as a result of the population inversion of long-lived impurity states in these materials.