Radiative and Nonradiative Recombination in Porous Silicon. What Can We Learn from Spin Resonance?

Abstract

Recent results of spin-resonance investigations of porous silicon are presented. Special attention is given to nonradiative and radiative recombination processes as detected by spin-dependent recombination (spin-dependent photoconductivity, SDPC, and optically detected magnetic resonance, ODMR). P,-like defects are identified as the main recombination centers which quench both photoconductivity and visible photoluminescence in porous Si. Strongly localized triplet excitons are found as the initial states of visible luminescence. IntroductionIn many cases, electron spin resonance (ESR) and related techniques have been instrumental in clarifying details of recombination processes in semiconductors and insulators. Particularly informative are combinations of spin resonance with photoconductivity (spindependent photoconductivity, SDPC) and luminescence (optically detected magnetic resonance, ODMR) which make use of spin-selection rules for radiative and nonradiative transitions involving paramagnetic states with electronic spin S =I= 0. In a simple picture, the final state of recombination is a doubly occupied, diamagnetic (S = 0) state according to the Pauli principle. The excited state prior to the recombination can be a parallel (S = 1, triplet state) or antiparallel (S = 0, singlet) combination of the electron and hole spins, respectively. Thus, if we neglect changes in the orbital momentum L for the sake of simplicity, nonradiative recombination can only occur out of the singlet state, since phonons have no spin, whereas radiative recombination occurs out of the triplet state to account for the orbital momentum J = 1 carried away by the emitted photon. In spin resonance the induced spin-flips of either the electron or hole state effectively change singlet into triplet combinations and vice versa, thereby also modulating the lifetime of the excited state. This modulation is finally detected as a small, resonant decrease or increase of the photoconductivity and the luminescence, which constitutes the measured signal.