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 i s readily extendable to quantum wires. The top-gated device architecture av oids the disorder inherent in conventional modulation-doped devices and all ows 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 mo bility, which is attributed to the low level of impurities. The variation o f the linewidth at small filling factor is also consistent with a low conce ntration of impurities. These results suggest that the filling-factor-depen dent oscillations observed in linewidth are not due to the screening of ion ized impurities. Filling-factor-dependent oscillations in photoconductivity intensity are also observed, with maxima occurring at even filling factors .