Red, green and blue luminescence in porous silicon - A study of excitation spectra

Photoluminescence excitation spectra of red-emitting porous silicon show a quadratic rise with excitation energy, reminiscent of indirect absorption and allowing the extrapolation of a bandgap. As additional indications of the indirect nature of the material, phonon structures are observed with detection energies as high as 2.1 eV in much clearer form than in photoluminescence spectra. Green luminescing porous silicon in HF shows (at room temperature) similar excitation spectra as the red band strongly rises with excitation energy. In contrast oxidized blue-emitting material can be excited close to the detection.