ESR investigations of modulation-doped Si/SiGe quantum wells

Electron spin resonance (ESR) was measured on high-mobility two-dimensional electron gases (2DEG) in modulation-doped SiGe/Si quantum wells. We observ e a resonance line as narrow as 40 mG, which allows the detection of as few as 10(9) spins in a standard microwave absorption configuration. Longitudi nal spin relaxation times T-i of up to 30 mu s were found, which is six to seven orders of magnitude longer than the momentum relaxation times in thes e samples. Using either gated samples or samples where the 2D carrier densi ty n(s) can be adjusted persistently by illumination, the ESR signal was re corded as a function of the carrier density. By measuring the integrated ES R absorption the density of states (DOS) at the Fermi level can be derived as a function of n(s). The 2D DOS shows a tail, which allows a quantitative extraction of the potential fluctuations. (C) 2000 Elsevier Science S.A. A ll rights reserved.