Some Luminescence Properties of Excited Silicon at Low Temperatures

Detailed studies of the luminescence due to the recombination of nonequilibrium electrons and holes in Si are reported for the temperature range 2-16°K. The properties of the exciton and electron-hole condensate luminescence are investigated in the spectral range from 1.11μ to 1.17μ. The Si was excited by laser pumping and by electrical injection. The temperature dependence of the intensity ratio of the LO- to TO-phonon assisted exciton recombination luminescence in laser excited, high purity Si is reported. The ratio is found to differ from that observed in absorption and to vary from 0.3 at 2°K to 0.1 at 13°K. The variation of the ratio with temperature is shown to be due to the splitting of the ground state of the exciton by several tenths of a meV. The relevance of these results to the recombination from the electron-hole condensate in Si is discussed. Detailed measurements and lineshape fits of the combined LO- to TO-phonon assisted electron-hole liquid luminescence lines from laser excited, high purity Si are reported. These data permit a determination of the liquid density, chemical potential, Fermi energy, and work function as functions of temperature. Results give support to the metallic model of the electron hole liquid in Si. The zero temperature condensate density is found to be 3.33 ± .05 x 1018 cm-3 and the zero temperature work function is measured as 8.2 ± .1 meV. An investigation of luminescence from Si double injection diodes in the temperature range 5-16°K is also reported. Properties of the luminescence are compared with luminescence from laser excited, high purity Si and laser excited, Li doped Si. The peak position of the combined LO- and TO -phonon assisted electron-hole condensate luminescence is observed at 1.082 eV in each of these cases. The density of the condensate, obtained by fitting the lineshape of this Luminescence, was found to be 2 x 1018 cm-3 for both double injection and laser excitation of Li doped Si. This value is significantly smaller than the density found for the condensate in laser excited, high purity Si, 3 x 1018 cm-3. Several features of the double injection luminescence suggest the influence of Li doping in the double injection diodes. Finally, the effects of heating in the luminescence from Si double injection devices is discussed in an appendix.