Photoluminescence of boron-doped Si1−xGex epilayers grown by UHV-CVD

Low temperature photoluminescence (PL) has been used to characterize 17-nm-thick, boron-doped, Si0.865Ge0.135 epilayers on Si(001). The samples included an undoped control sample and several others for which the amount of substitutional boron, introduced as B2H6 gas during growth by UHV chemical vapor deposition at 525 °C, was varied from 1017 to 5×1018 cm−3 using flow control of the gases. Significant quantitative differences in phonon-resolved PL spectra were observed among the doped samples and with the undoped sample. At 5 K, the PL signal weakened, shifted to lower energy and broadened with increasing boron concentration. As the doping was increased, there was a greater likelihood that, instead of binding at a single acceptor, an exciton would be bound—and more strongly—to multiple acceptors. A kinetic model is presented which qualitatively explains the observed spectrum in terms of exciton diffusion by hopping from isolated acceptor centers to acceptor clusters.