Effect of two-step growth process on structural, optical and electrical properties of MOVPE-grown GaP/Si

Abstract

A two-step growth process of gallium phosphide (GaP) epilayer on silicon substrate is carried out using metal organic vapor-phase epitaxy (MOVPE). This process includes the growth of a low-temperature GaP nucleating layer and a high-temperature GaP epilayer. In the first step, a GaP nucleating layer of thickness ∼80 nm with high V/III ratio ∼1725 is grown at 425 °C. This is followed by the growth of a GaP layer of thickness ∼760 nm with V/III ratio ∼100 at 770 °C. The total thickness of the epilayer is ∼845 nm as measured on a cross-section by a scanning electron microscope (SEM). A GaP layer grown by the two-step growth process shows significant improvement in morphology compared with that grown by the single-step process, as confirmed by Raman and SEM studies. High-resolution X-ray diffraction studies show that the epilayer is of single crystalline nature and structurally coherent with the silicon substrate. Epilayers grown using the two-step process show reduced dislocation density and micro-strains, with significant improvements in their structural properties. These layers also show n-type behavior with an electron density of ∼8.5×10 17 cm −3, while the single-step-grown layers show p-type behavior. This change in type is explained by a reduced/increased incorporation of silicon into phosphorus/gallium sites, respectively, i.e. a predominant donor nature of silicon in GaP grown by the two-step growth process. Photoluminescence (PL), surface photovoltage spectroscopy (SPS) and transport measurements confirm a significant reduction in sub-band gap states and carrier density.