DX centres, conduction band offsets and Si-dopant segregation in AlxGa1-xAs/GaAs heterostructures

4 K Hall measurements of MBE-grown delta-doped AlxGa1-xAs/GaAs heterostructures are presented for nearly the whole composition range. The persistent photoconductivity effect is used to determine the DX level and the conduction band offset independently of each other by comparing the experimental results with self-consistent calculations. The variation of the DX level energy with composition x is found to be E(DX) = 1.38X - 0.25 (eV) for x < 0.38 in agreement with the negative-U model of DX centres. A ratio of conduction band to valence band offset of Delta E(C)/Delta E(V) = 68 : 32 for x < 0.38 is determined. The valence band offset is proportional to the composition x for the whole x range. We obtain the best fit for our data with Delta E(V) = 0.544x (eV). Secondary ion mass spectrometry measurements reveal a strong enhancement of the Si-dopant segregation towards the surface with increasing Al concentration. The full width at half-maximum of the Si delta-doping layers (I-S) was found to be I-S = 67 + 13.7x (nm) at a growth temperature of 620 degrees C.