Neutralization of Shallow Acceptors in Silicon

CENTER FOR OPTOELECTRONIC COMPUTING SYSTEMS DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING UNIVERSITY OF COLORADO AT BOULDER BOULDER, COLORADO 

SOLAR ENERGY RESEARCH INSTITUTE GOLDEN, COLORADO 

As was pointed out in previous chapters, the role of hydrogen in silicon is to passivate all the Si dangling bonds. It is the realization that there is a Si dangling bond near every acceptor that led us to search for the possibility that H might neutralize the acceptor. 

The earliest evidence for acceptor neutralization was found in the work of Sah et al. (1983, 1984), who attributed the neutralization effect to a bonding between H and B. This hypothesis inspired our search for the B—H vibrational mode, to be described in Section V. Our model of H binding to Si near a B atom aroused some controversy from Pearton (1984) that required additional definitive tests by Pankove et al. (1984b). 

---

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

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. 

For the vacancy mechanism, the single vacancy in silicon is believed to exist in four charge states V+, Vs, V , and V=, where + refers to a donor level, x is a neutral level, and - is an acceptor level (5, 6). The creation of a vacancy introduces a new lattice site and, thus, four new valence band states in the crystal. These states are available as acceptors but are not shallow. The lattice distortion associated with the vacancy splits states from the valence and conduction bands of the surrounding atoms by a few tenths of an electronvolt into the forbidden cap. States split from the valence band become donors, and those split from the conduction band become acceptors. 

---