Photoexcited carrier lifetimes and spatial transport in surface-free GaAs homostructures

Abstract

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.