Cyclotron resonance in undoped, top-gated heterostructures

We report on cyclotron resonance detected by far-infrared photoconductivity of the two-dimensional electron gas formed in undoped, top-gated GaAs/Al0.3Ga0.7As heterostructures. The photoconductivity method demonstrated here is readily extendable to quantum wires. The top-gated device architecture avoids the disorder inherent in conventional modulation-doped devices and allows precise in situ tuning of carrier density over two orders of magnitude. We observe very sharp resonances (6 mT at 1.5 K) indicating a very high mobility, which is attributed to the low level of impurities. The variation of the linewidth at small filling factor is also consistent with a low concentration of impurities. These results suggest that the filling-factor-dependent oscillations observed in linewidth are not due to the screening of ionized impurities. Filling-factor-dependent oscillations in photoconductivity intensity are also observed, with maxima occurring at even filling factors.