ESR investigations of modulation-doped Si/SiGe quantum wells

In standard electron spin resonance (ESR) spectroscopy on modulation-doped SiGe/Si quantum wells, we observe a very sharp ESR due to the 2D electron gas and also cyclotron resonance (CR) of free carriers in the well. Both signals increase persistently after illumination and the 2D CR becomes sharper by one order of magnitude. We explain these effects in terms of potential fluctuations due to the ionized donors which are partially neutralized and effectively screened by photogenerated carriers. By measuring the integrated ESR absorption we are able to determine the density of states (DOS) at the Fermi level for different carrier concentrations in the well. We observe a potential fluctuation-induced tail in the 2D-DOS instead of an ideal step-like behaviour. The amplitude of these fluctuations can directly be evaluated from the DOS at the Fermi level.