Hydrogen in Silicon-Unanswered Questions
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HYDROGEN IN SILICON-UNANSWERED QUESTIONS S. J. PEARTON AT & T Bell Laboratories, Murray Hill, NJ
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The study of hydrogen in crystalline Si has enjoyed a resurgence of interest in recent times, principally because hydrogen is a common constituent of many semiconductor reagents, and also because of its ability to passivate the electrically activity of shallow acceptor dopants, and many deep contaminating centers in Si. This enables its use as a sensitive probe of the defect chemistry occurring in Si at relatively low temperatures
-the
presence of hydrogen can be detected electrically through the passivation of impurities and defects, and isotopic substitution with deuterium enables chemical analysis by secondary ion mass spectrometry. This recent flurry of activity concerning the effects and state of hydrogen has contributed a great deal to the understanding of this simplest impurity in Si. However much is unclear, including exactly how hydrogen passivates many of the deep level centers it is observed to do. For the substitutional Au impurity, for example, it is generally accepted that there is no defective or 'dangling' bond for hydrogen to attach to, and passivation must proceed by some form of chemically driven reconstruction, a term first used by J. W. Corbett and co2 1 workers,O ) and conjectured independently by G. A. Baraff.( ) Much more optical characterization of such samples is needed, because the observation of hydrogen-related IR bands might shed new light on the bonding mechanisms involved in hydrogen passivation of defects. A related area of confusion is the charge state(s) of hydrogen under equilibrium conditions. It has generally been assumed that interstitial atomic hydrogen is neutral (Hw), but recent results on the drift of hydrogen under the action of the electric field in diode 3 structures can only be explained by a positive charge state (H') - a protonic model.0 ) Numerous theoretical treatments have raised the possibility of charge states in the bandgap, 4 depending on the lattice position of the hydrogen.( -') The diffusivity of these possible charge states is also unknown, although there is some evidence that capture of an electron by
a positively charged hydrogen leads to more rapid motion of this species through the lattice, in agreement with what one would expect intuitively for the smaller H° atom. On the question of the diffusivity of hydrogen, a good qualitative understanding of the mechanisms involved now exists. The postulation of the formation of relatively immobile molecules (H_2)during hydrogenation leads to the concept of an effective diffusion coefficient
Mat. Res. Soc. Syup. Proc. Vol. 59. 1 1986 Materials Research Society
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for hydrogen dependent on the total amount present.(8) The effect of trapping of hydrogen at defects or impurities is another mechanism by which the hydrogen permeation is retarded, and while the appropriate modelling is still somewhat incomplete, reasonable agreement is obtained between experimental values for
DH
and those obtained from the
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