InGaN multiple quantum well laser diode (LD) wafer that lased at 400 nm. was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit, However, the wells exhibited a Stokes-like shift (SS) of 49 meV and an effective localization depth E-0 was estimated by time-resolved photoluminescence (TRPL) measurement to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells is estimated to be as small as 286 kV/cm, SS is considered to originate from an effective bandgap inhomogeneity. Because the well thickness fluctuation was insufficient to produce SS or E-0, the exciton localization is considered to be an intrinsic phenomenon in InGaN material. Indeed, bulk cubic In0.1Ga0.9N, which does not suffer any polarization field or thickness fluctuation effect, exhibited an SS of 140 meV at 77 K and similar TRPL results. The origin of the localization is considered to be due to the large bandgap bowing and In clustering in InGaN material. Such shallow and low density localized states are leveled by injecting high density carriers under the lasing conditions for the 400 nm LDS. (C) 2002 Elsevier Science B.V. All rights reserved.