  We report magnetotransport measurements of a gated InSb quantum well (QW)
with high quality Al2O3 dielectrics (40 nm thick) grown by atomic layer
deposition. The magnetoresistance data demonstrate a parallel conduction
channel in the sample at zero gate voltage (Vg). A good interface between Al2O3
and the top InSb layer ensures that the parallel channel is depleted at
negative Vg and the density of two-dimensional electrons in the QW is tuned by
Vg with a large ratio of 6.5x1014 m-2V-1 but saturates at large negative Vg.
These findings are closely related to layer structures of the QW as suggested
by self-consistent Schrodinger-Poisson simulation and two-carrier model.
