Hydrogen immobilization in siliconp−njunctions

Abstract

The spatial distribution of hydrogen that results from its diffusion through a $p-n$ junction in single-crystal silicon at moderate temperatures (e.g., 200°C) can be highly structured with features that depend on a reverse bias applied during H diffusion. In $n^{+}-p$ junctions, a prominent peak in the distribution appears at the edge of the bias-dependent depletion layer in the $p$-type material, and in reverse-biased diodes, a sharp step also appears within the depletion layer. Both features represent accumulations of hydrogen in excess of the local boron concentration (∼1×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$). Most of the accumulation is in the form of highly immobile neutral entities. We propose that these are hydrogen pairs, ${\mathrm{H}}_2$. Formation of ${\mathrm{H}}_2$ by the conventional reaction 2${\mathrm{H}}^0$→${\mathrm{H}}_2$ is, by itself, incapable of accounting for the structure in the depth profiles, and it seems necessary to conclude that the competing reaction ${\mathrm{H}}^{+}{\mathrm{H}}^0\to {\mathrm{H}}_2+h^{+}$, where $h^{+}$ denotes a free hole, becomes dominant in $p$-type regions, at least at 200°C. Local electronic equilibration of ${\mathrm{H}}^0$ with ${\mathrm{H}}^{+}$ seems to occur much more rapidly than the time scale (1 h) of our experiments.