  Modulation-doped oxide two-dimensional electron gas (2DEG) formed at the
LaMnO3 (LMO) buffered disorderd-LaAlO3/SrTiO3 (d-LAO/LMO/STO) heterointerface,
provides new opportunities for electronics as well as quantum physics. Herein,
we studied the dependence of Sr-doping of La1-xSrxMnO3 (LSMO, x=0, 1/8, 1/3,
1/2, and 1) thus the filling of the Mn eg subbands as well as the LSMO polarity
on the transport properties of d-LAO/LSMO/STO. Upon increasing the LSMO film
thickness from 1 unit cell (uc) to 2 uc, a sharp metal to insulator transition
of interface conduction was observed, independent of x. The resultant electron
mobility is often higher than 1900 cm2V-1s-1 at 2 K, which increases upon
decreasing x. The sheet carrier density, on the other hand, is in the range of
6.9E1012~1.8E1013 cm-2 (0.01~0.03 e/uc) and is largely independent on x for all
the metallic d-LAO/LSMO (1 uc)/STO interfaces. These results are consistent
with the charge transfer induced modulation doping scheme and clarify that the
polarity of the buffer layer plays a trivial role on the modulation doping. The
negligible tunability of the carrier density could result from the reduction of
LSMO during the deposition of disordered LAO or that the energy levels of Mn 3d
electrons at the interface of LSMO/STO are hardly varied even when changing the
LSMO composition from LMO to SrMnO3.
