  Since the discovery of two-dimensional electron gas (2DEG) at the oxide
interface of LaAlO3/SrTiO3, improving carrier mobility has become an important
issue for device applications. In this paper, by using an alternate polar
perovskite insulator (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) for reducing lattice
mismatch from 3.0% to 1.0%, the low-temperature carrier mobility has been
increased 30 fold to 35,000 cm2V-1s-1. Moreover, two critical thicknesses for
the LSAT/SrTiO3 (001) interface are found: one at 5 unit cell for appearance of
the 2DEG, the other at 12 unit cell for a peak in the carrier mobility. By
contrast, the conducting (110) and (111) LSAT/STO interfaces only show a single
critical thickness of 8 unit cells. This can be explained in terms of polar
fluctuation arising from LSAT chemical composition. In addition to lattice
mismatch and crystal symmetry at the interface, polar fluctuation arising from
composition has been identified as an important variable to be tailored at the
oxide interfaces to optimise the 2DEG transport.
