Dopant states ina-Si: H. III. Triply coordinated boron

Boron atoms at trigonal, alloying sites are shown to be electronically active in $a$-Si: H. Alone among the dopant elements of groups III and V, boron produces deep gap states in this configuration. The position of a level depends on the bond angle of the site and thereby its occupation. We predict that there will be a conduction tail of empty levels and a long valence tail due to paramagnetic ${\mathrm{B}}_3^{-}$ sites. We discuss evidence that such states have been seen in luminescence, optical-absorption, and transport experiments.