Investigation of the initial stage of heteroepitaxy of GaP on silicon

Employing the real-time monitoring capability of p-polarized reflectance (PR) we have evaluated the early stage of heteroepitaxial growth of GaP on Si substrate of (100), (111), and (311) orientation by pulsed chemical beam epitaxy (PCBE) using continuous flow of tertiarybutyl-phosphine (TBP) and pulsed flow of triethylgallium (TEG) as source vapors. We have shown that the high surface sensitivity of PR makes it possible to observe and measure an incubation time, tau(i), during which the PR intensity increases continuously, which is followed upon nucleation at time t=tau(i) of GaP islands by a stepwise increase of the PR intensity. This finding is corroborated by high resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) which reveals no GaP nuclei at t<tau(i) while at t greater than or equal to tau(i) GaP nuclei are present. The incubation time is a strong function of surface orientation. For a given substrate orientation tau(i) is also a strong function of temperature. We interpret the observed shortening of tau(i) with increasing temperature as an effect of increasing rate of the surface reactions leading to both phosphorus and gallium moieties that contribute to nucleation and growth, that is, as an effect of time dependent evolution of supersaturation conditions.