Optically determined low-temperature, high-mobility transport in 'interface-free' GaAs heterostructures

Abstract

At low temperatures free electrons and holes condense into free excitons, which are electrically neutral, and are thus unobservable in conventional, electrical transport measurements. To study the physical limitations of low-temperature, high-mobility transport we have resorted to a new all-optical, time-resolved photoluminescence (PL) confocal imaging technique which is sensitive to both charged and neutral-particle transport. We have thus measured spatial transport of spectrally-resolved free-excitionic species at low-temperatures (1.8 - 50 K) in a series of high-quality MOCVD-prepared GaAs/Al0.3Ga0.7As double heterostructures, with varying GaAs layer thicknesses (10 μm to 50 A). We find such constants to be sensitive to, and monotonic in, laser power - with 1.8-K peak diffusion constants of > 1000 cm2/s and minimums of = 1 cm2/s. We find that this anomalous transport results from the joint diffusion of free excitons and free carriers coupled through temperature-dependent capture and ionization. We conclude that the high-mobilities for joint neutral-particle (excitonic) and free carrier (electron and/or hole) transport are the result of diminished charged-center scattering and the simulation of modulation-doping which photoexcitation creates in the screening of charged centers - and are thus limited by only by intrinsic lattice (deformation potential) scattering.