Interface roughness of pseudomorphic In0.74Ga0.26As/In0.52Al0.48As modulation-doped quantum wells (MD-QWs) grown on the (411)A and (100) InP substrates by molecular beam epitaxy was characterized by sheet,electron concentration (N-s) dependence of two-dimensional electron gas (2DEG) mobility by applying gate bias at 15 K. The (411)A MD-QW with well width of 4 nm (8 nm) showed value of 1.8-2.1 (1.5-1.6) times higher 2DEG mobility at 15 K comparing to the corresponding (100) sample in the range of N-s (1.2-2.3 X 10(12) cm(-2)), which results from reduced interface roughness scattering due to the super-flat (411)A InGaAs/InAlAs interfaces. By analyzing N-s dependence of the 2DEG mobility, we deduced wave-number dependence of the Fourier components of the lateral well-width fluctuation (Delta(q0)) arising from the interface roughness in the range of q(0)=0.55-0.7 nm(-1). Values of vertical bar Delta(q0)vertical bar(2) of the (411)A InGaAs/InAlAs interface were about half of those of the (100) interface in the whole range of q(0)=0.55-0.7 nm(-1). (c) 2005 American Vacuum Society.