Photoluminescence (PL) spectra and Raman spectra have been utilized to study the optical properties of epitaxial InGaP grown on (0 0 1) GaAs substrate by solid-source molecular beam epitaxy with a GaP decomposition source. The 15 K PL spectrum of 3 mum-thick InGaP epilayer grown under optimized conditions (T-s = 460degreesC, V/III 15) shows that the PL peak energy is as high as 1.998 ev and the PL full-width at half-maximum is 5.26 meV, which is the narrowest value reported. The temperature dependence of PL spectrum exhibits normal behavior. The Raman scattering spectrum at room temperature shows the ratio b/a, where b and a represent the valley depth between the GaP-and InP-like LO peak and the InNike LO peak height, respectively, is as high as 0.45, which is near the largest value ever reported. All these results demonstrate that the InGaP grown under optimized conditions (T-S = 460degreesC, V/III = 15) by solid-source molecular beam epitaxy with a GaP decomposition source is highly disordered. (C) 2003 Elsevier B.V. All rights reserved.