Long-range, minority-carrier transport in high quality ‘‘surface-free’’ GaAs/AlGaAs double heterostructures

Using a novel time-resolved optical photoluminescence imaging technique, analogous to the electrical Haynes–Shockley experiment, we have measured room-temperature minority-carrier transport in a series of ‘‘surface-free’’ GaAs/Al0.3Ga0.7As double heterostructures. These measurements are only possible in ‘‘surface-free’’ samples in which the band-to-band radiative recombination lifetimes are long−here up to 2.5 μs. We find minority-carrier transport to be ‘‘diffusive,’’ with diffusion lengths of up to ∼140 μm. We also find transport in thick (≳1 μm) structures to be mediated by hole-dominated ambipolar diffusion, whereas for thinner structures a transition from ambipolar to free-electron-dominated diffusion is observed. These results demonstrate that our heterostructures become effectively modulation doped for GaAs thicknesses ≲1 μm.