Electric and magnetic field fluctuations in modulation doped Si/Ge quantum wells

The recently observed electron spin resonance (ESR) of conduction electrons in modulation doped Si/Ge quantum wells is utilized to determine the density of states (DOS) of the two-dimensional electron gas (2DEG). Instead of the constant DOS expected for an ideal 2DEG we find tails extending into the energy gap. With increasing carrier density (adjusted by persistent photoconductivity or applied gate voltage in our experiment) the tail broadening decreases. We attribute the appearance of these tails to the screened potential fluctuations caused by fluctuations in the charge distribution of ionised donors in the doping layer. For low carrier concentration we observe a tendency for a divergence of these potential fluctuations.