Mobility enhancement of modulation-doped materials by low-temperature optical annealing of spacer-layer defect charge state

We report extremely strong mobility (μ) enhancement accompanying moderate increases in the two-dimensional electron gas density ($N_s$) in certain GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As heterostructures when they are illuminated at low temperatures (T<4.2 K) by short (∼50 μs) light pulses from a (x=0.23,0.24) light-emitting diode (LED). The samples in which this is observed differ from typical modulation-doped heterostructures in that the undoped spacer layers are wider (15 and 20 nm for the samples in this study) than the doped ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As electron supply layer (which is 10 nm). In these cases the 4.2-K mobilities in the dark begin at low values ∼${10}^4$ ${\mathrm{cm}}^2$/V s and increase to 350 000 ${\mathrm{cm}}^2$/V s as $N_s$ changes by a factor of 2. The dependence of mobility on number density in these samples obeys a power law with an exponent 3.5±0.5. The net increase in mobility with number density is about three times the earlier results which showed a 1–1.5-power law ascribed to the DX center in the doped ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As layer.