Optically determined minority-carrier transport in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures

We have studied minority-carrier electron and hole transport versus temperature (30–300 K) in a series of undoped, ‘‘interface-free,’’ GaAs/${\mathrm{Al}}_{0.3}$ ${\mathrm{Ga}}_{0.7}$As double heterostructures prepared by organometallic vapor-phase epitaxy, with GaAs thicknesses from 0.1 to 10 μm. This was achieved using an all-optical, time-resolved photoluminescence-imaging technique with a spatial resolution of ≲3 μm, temporal resolution of ∼50 ps, and spectral resolution of <1 ${\mathrm{cm}}^{\mathrm{-}1}$. This technique allows direct determination of minority-carrier transport properties, and is superior to electrical transport measurement techniques in that it is contactless, may distinguish between diffusive and nondiffusive carrier motion, and has high temporal and spectral resolution. We find all transport (electron and hole) in these structures to be diffusive. Specifically, transport in thick structures (≳0.5 μm) is hole-dominated ambipolar diffusion, whereas in thinner structures (≲0.5 μm) we observe a time-dependent transition from ambipolar to electron-dominated diffusion. Minority-carrier mobilities derived from these diffusion measurements, from 300 to ∼30 K, are in excellent agreement with both electron and hole majority-carrier mobilities. Furthermore, fits to the temperature-dependent mobilities yield deformation potentials in agreement with published electrically derived values.