Chemical-equilibrium description of the gap-state distribution ina-Si:H

A pool of potential energies at which defect states can be created is incorporated into a simple chemical-equilibrium description of defect formation in a-Si:H. For a wide enough defect-pool distribution, $D^{\mathrm{-}}$ lies deeper in the gap than $D^0$, in agreement with recent photoemission results, even though the correlation energy is positive. Bonded hydrogen is proposed as the physical origin of the defect pool.