Lifetimes of Free and Bound Excitons in High-Purity GaAs

An analysis has been carried out for obtaining the free- and bound-exciton lifetimes from optical phase shifts measured on crystals with surface loss. It is found that the bound-exciton lifetime can be obtained directly from the measured phase shifts associated with the free- and bound-exciton lines, regardless of the surface condition. The optical phase shift of the free-exciton line from a "surface-free" crystal should be larger than that measured on the same crystal but with a lossy surface. Experiments to measure the phase shifts of free excitons and excitons bound to neutral and ionized donors ($D^0$ and $D^{+}$) and neutral acceptors ($A^0$) have been carried out and the various lifetimes have been determined from the above analysis. It was found that ${\tau }_{D^0}=1.07\mathrm{}\pm 0.1$ nsec, ${\tau }_{D^{+}}=0.8\mathrm{}\pm 0.08$ nsec, and ${\tau }_{A^0}=1.6\mathrm{}\pm 0.6$ nsec at 1.6 °K. The free-exciton lifetime decreases with increasing impurity concentration in the material (the longest lifetime obtained being 2.9±1.3 nsec), indicating that the decay of free excitons is not governed by the direct annihilation of the electron-hole pairs.