UC Santa Barbara

In virtually all semiconductors and insulators, hydrogen interstitials ([Formula: see text]) act as negative-U centers, implying that hydrogen is never stable in the neutral charge state. Using hybrid density functional calculations, we find a different behavior for [Formula: see text] in diamond and cubic BN. In diamond, [Formula: see text] is a very strong positive-U center, and the [Formula: see text] charge state is stable over a Fermi-level range of more than 2 eV. In cubic BN, a III-V compound similar to diamond, we also find positive-U behavior, though over a much smaller Fermi-level range. These results highlight the unique behavior of [Formula: see text] in these covalent wide-band-gap semiconductors.