Photoluminescence from Coherently Strained Si1−xGex Alloys

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Abstract
An intense, broad photoluminescence PL peak, with an internal quantum efficiency as high as 31%, has been observed from a variety of structures containing Si1−xGex strained layers on Si(100) substrates; i.e. Si1−xGex thick random alloy layers, single quantum wells (SQW) and multiple quantum wells (MQW) with layers thick enough so that zone folding effects were not relevant. This peak, which shifted consistently and predictably with Ge concentration( 0.06 < × < 0.53), had its high energy edge near the established band gap for strained SiGe. PL excitation spectroscopy indicated that no phonons were involved in the process causing the SiGe PL peak. Samples deposited at ~ 400 °C exhibited low PL intensity, whereas annealing at ~ 600 °C enhanced the intensity by as much as two orders of magnitude. This anneal treatment was found to remove grown-in defect complexes without creating a significant density of misfit dislocations. The PL peak energy at 4.2 K varied from 620 to 990 meV for Ge fractions x from 0.53 to 0.06. When the samples were forced to relax, e.g. by higher temperature annealing, the luminescence of this peak either shifted to near the relaxed bandgap or was quenched by deep, dislocation related states. Prior to such relaxation, the efficient PL was due to exciton accumulation in the strained Si1−xGex layers of single and multiple quantum wells, where the bandgap was locally reduced. It is suggested that the recombination of electrons and holes occuring within a high-density electron hole condensate (EHC) can cause the observed spectrum.