Electroluminescence, photoluminescence, and photocurrent studies of Si/SiGe p-i-n heterostructures

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Abstract
Comparative electroluminescence and photoluminescence measurements were performed on Si/Si0.7Ge0.3 p-i-n single quantum well structures, and on one p-i-n and one undoped multiple quantum well structure in a wide temperature range. The samples were grown pseudomorphically by molecular beam epitaxy, and mesa diodes for electroluminescence and photocurrent measurements were fabricated. In electroluminescence, optical emission comes primarily from the SiGe quantum wells whereas no emission from Si is observed except for high temperatures of ≊200 K and up. All p-i-n structures exhibit maximum emission intensities in a temperature range between 80 K and 220 K, depending on the quantum well width. This temperature characteristic is very different from undoped quantum well samples. A model is discussed that accounts satisfactorily for all observed temperature dependent data. As an essential feature, the model includes Auger recombination in addition to radiative recombination in the n+ and p+ sides of the junctions and in the SiGe quantum well due to the high electron or hole densities in these regions. Photocurrent spectra due to single quantum wells are measured showing the SiGe absorption threshold in addition to the Si threshold. Quantitative fits to these spectra yield threshold energies for SiGe and Si consistent with the electroluminescence spectra. The question of how photogenerated excess holes that are bound in a quantum well can escape the well at 4.2 K to yield the measured photocurrents is discussed.