  The amplitude V_0 of unidirectional periodic potential modulation introduced
by a surface grating into a two-dimensional electron gas (2DEG) formed at
AlGaAs/GaAs heterointerface is measured as a function of electron density n_e
by analyzing commensurability oscillation of the magnetoresistance. The
electron density is varied either by applying a bias to a metallic back gate or
by illumination. The amplitude decreases with increasing density, with the rate
|dV_0/dn_e| roughly an order of magnitude larger for the former method. The
result is interpreted in terms of the rate, dE_1/d(delta E_c), of the change in
the first subband level E_1 in response to the variation of the conduction-band
edge delta E_c above the heterointerface. The rate crucially depends on the
thickness of the 2DEG.
