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The quantum cascade laser, which uses electronic transitions within a single band of a semiconductor, constitutes a possible way to integrate active optical components into silicon-based technology. This concept necessitates a transition with a narrow linewidth and an upper state with a sufficiently long lifetime. We report the observation of intersubband electroluminescence from a p-type silicon/silicon-germanium quantum cascade structure, centered at 130 millielectron volts with a width of 22 millielectron volts, with the expected polarization, and discernible up to 180 kelvin. The nonradiative lifetime is found to depend strongly on the design of the quantum well structure, and is shown to reach values comparable to that of an equivalent GaInAs/AlInAs laser structure.
A semiconductor laser typically relies on conduction band electrons and valence band holes being injected through a forward-biased p-n junction into a common region in space, where the probability for a radiative recombination across the band gap is high. The active layer usually consists of a quantum well that is realized using heteroepitaxy of III-V material systems such as InGaAs/InP. A long-standing wish for microelectronics technology has been the integration of semiconductor lasers onto a silicon chip. Silicongermanium alloys have brought many of the advantages of heterostructures into the domain of the dominating Si technology, but the indirect band gap of the group IV materials is a hindrance to Si-based active optical components. This implies that radiative transitions across the band gap must be accompanied by a large momentum transfer, and are therefore unlikely. Moreover, Si and Sil,Gex alloys usually form heterostructures with staggered band offsets, in such a way that the electrons and holes are spatially separated. These problems are circumvented in the unipolar quantum cascade laser (QC laser) (1), where transitions between quantum well levels within a band (so-called intersubband transitions) are used to obtain laser light in the mid- to far-infrared wavelength region. Carriers stream down a potential staircase of coupled quantum wells, passing a sequence of active...