Low temperature photoluminescence (PL) has been used to characterize 17-nm-thick, boron-doped, Si 0.865Ge 0.135 epilayers on Si(001). The samples included an undoped control sample and several others for which the amount of substitutional boron, introduced as B 2H 6 gas during growth by UHV chemical vapor deposition at 525 °C, was varied from 10 17 to 5×10 18 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.