  We report ultrafast transient-grating experiments on heavily p-type InP at 15
K. Our measurement reveals the dynamics and diffusion of photoexcited electrons
and holes as a function of their density n in the range 2E16 to 6E17 cm-3.
After the first few picoseconds the grating decays primarily due to ambipolar
diffusion. While at low density we observe a regime in which the ambipolar
diffusion is electron-dominated and increases rapidly with n, at high n it
appears to saturate at 34 cm2/s. We present a simple calculation that
reproduces the main results of our measurements as well as of previously
published measurements that had shown diffusion to be a flat or decreasing
function of n. By accounting for effect of density on charge susceptibility we
show that, in p-type semiconductors, the regime we observe of increasing
ambipolar diffusion is unique to heavy doping and low temperature, where both
the holes and electrons are degenerate; in this regime the electronic and
ambipolar diffusion are nearly equal. The saturation is identified as a
crossover to ambipolar diffusion dominated by the majority carriers, the holes.
At short times the transient-grating signal rises gradually. This rise reveals
cooling of hot electrons and, at high photocarrier density, allows us to
measure ambipolar diffusion of 110 cm2/s in the hot-carrier regime.
