Shallow acceptor neutralization

Besides the electrically active complexes discussed above, there is indirect evidence for the existence of neutral complexes. In close analogy to the observations in silicon and several III-V materials it appears that hydrogen passivates deep and shallow acceptors. Because of the small concentrations of these neutral centers, all attempts to detect them directly with local vibrational mode (LVM) spectroscopy or electron paramagnetic resonance (EPR) have been unsuccessful. 

Shallow acceptor levels lie close to the valence band and take up electrons from it to create holes in the valence band and produce p-type semiconductors. Interstitial nonmetal atoms often generate shallow acceptor levels because anion formation involves taking up extra electrons. Acceptor levels are said to be ionized when they take electrons from the valence band, creating holes in the process. The energy of a neutral acceptor atom is different to that of an ionized acceptor. The electrons on the ionized anions are often trapped and do not contribute to the conductivity. 

S. J. Pearton, Neutralization of Deep Levels in Silicon J. I. Pankove, Neutralization of Shallow Acceptors in Silicon... 

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