Competition between homogeneous and inhomogeneous broadening of orbital transitions in Si: Bi

We present results for the lifetime of the orbital transitions of Bi donors in Si, measured using both frequency-domain and time-domain techniques, allowing us to distinguish between homogeneous and inhomogeneous processes. The proximity of the energy of the optically allowed transitions to the optical phonon energy means that there is an unusually wide variation in the lifetimes and broadening mechanisms for this impurity, from fully homogeneous lifetime-broadened transitions to fully inhomogeneously broadened lines. The relaxation lifetime (T1) of the states ranges from the low 10's to 100's of ps, and we find that there is little extra dephasing (so that T1 is of the order of T2/2) in each case.