Photoexcited carrier lifetimes and spatial transport in surface-free GaAs homostructures
We show that both the radiative efficiencies and lifetimes of photoexcited carriers in epitaxial GaAs may be enhanced by 3–4 orders of magnitude by the preparation of n + , doped layers at surface and substrate interfaces. Samples were prepared by organometallic vapor phase epitaxy, with n-region thicknesses of 3–10 µm, and narrow layers Si-doped to n + concentrations of 5×1018 cm–3. Time-resolved luminescence in such structures, under both surface and bulk (near-band-edge) excitation conditions, reveal near-edge-excitonic or band-to-band-dominated recombination spectra, with carrier lifetimes ranging from 1.5 ns at 1.5 K to nearly 1 µs at room temperature. This is in contrast to the subnanosecond lifetimes typical in conventionally prepared GaAs, but is comparable to the best reported for high-purity liquid phase epitaxy prepared GaAs/AlxGa1–xAs double heterostructures. The spatial distributions of photoexcited carriers in these structures are observed to expand by over an order of magnitude during their 1 µs, room temperature lifetime. The expansion is diffusive, with a measured minority carrier (hole) diffusion constant of 8 cm2/s at 300 K. This corresponds to a room temperature hole mobility of 300 cm2/V s, comparable to previously measured majority carrier (hole) mobilities in p-type GaAs of similar purity. The measured temperature dependence of the photoexcited hole mobilities show that µ~T–2.1, indicating that the spatial transport of carrier in these structures is limited by scattering with acoustic and optic phonons, just as in high-purity p-type GaAs. These results are clear evidence that the narrow, heavily doped layers effectively ``shield'' minority carriers from the interfaces, thereby virtually eliminating interface recombination.