We study the persistent photoconductivity (PPC) effect observed in B-doped porous Si through first-principles pseudopotential total-energy calculations. The PPC effect is thought to result from a deep level defect associated with B in connection with the quantum confinement effect. By applying previously proposed models based on large lattice relaxation, i. e., single broken-bond and double broken-bond models, we examine the possibility that these lattice instabilities may be the origin of the PPC in porous Si.
We also discuss the effect of band gap increasing by the quantum confinement on the stability of deep level defects. In investigating configurations corresponding to large lattice relaxation models, we first examine whether the double broken-bond lattice instability can be realized in bulk Si. A quantum wire structure, which is expected to show a significant quantum confinement effect, is also employed to test the stability of both the single broken-bond and double broken-bond configurations. Our calculations show that neither of the two defect structures can be stabilized in the bulk and wire structures. While further studies are needed to explain the PPC in porous Si, we propose a quantum-dot-like structure which is expected to open up a sufficiently large band gap which could induce a large lattice relaxation.