This work uses photoreflectance to investigate the band gap, built-in electric field, and surface Fermi level of a series of lattice-matched In0.52Al0.48As surface intrinsic-n(+) structures having different undoped layer thickness. Experimental results indicate that the surface Fermi level is weakly pinned. By converting the dependence of the built-in electric field on undoped layer thickness into the dependence of surface state density on the surface Fermi level, this study defines the energy spectrum of the surface state density of InAlAs surface using a Gaussian distribution function. (C) 1999 American Institute of Physics. [S0003-6951(99)05540-0].