Optical spectroscopies: Raman, infrared (IR) and photoluminescence (PL) were used to investigate the influence of the substrate temperature on the him formation of the organic molecule 3,4,9,10-perylenetetracarboxyl dianhydride (PTCDA) on hydrogen-passivated silicon(1 0 0) substrates. Raman spectra exhibit four phonon bands below 125 cm(-1), indicating the crystalline nature of the films. The spectral changes of both Raman- and infrared-active modes reflect that the size of the individual crystals increases with the substrate temperature during growth. Moreover, they support an increase in the content of the alpha -phase at the expense of the beta -phase. The rising background in the high frequency range of the Raman spectra is related to an enhancement of the PL efficiency connected to a reduced number of non-radiative centres of recombination. Time-resolved PL measurements reveal that the PL decay time increases with the substrate temperature, approaching the value characteristic for a PTCDA single crystal.