Growth of GaAs on Ge/Si (001) nanovoided virtual substrate

Abstract

The integration of III-V compounds on Si substrate is very promising for photovoltaic applications. This would be an alternative to obtain low cost and high efficiency solar cells. Currently, III-V solar cells are produced on Ge substrate, which engages high production costs. However, the heteroepitaxy of these materials on silicon implies the appearance of defects and dislocations related to the difference in lattice parameter and thermal expansion coefficient. Ge is commonly employed as an intermediate buffer layer to integrate such materials. This process involves high Ge thickness and several postgrowth annealing steps to reduce the dislocation density down to 106 cm−2 which is still too high. Recently, an innovative approach using dislocation-selective electrochemical deep etching, to create nanovoid inside the germanium epilayer on silicon has shown efficiency to trap and annihilate the dislocations reducing their density down to 104 cm−2. In this work, we report on the growth of GaAs on virtual Ge/Si (001) substrate following a new approach based on direct growth of Ge buffer layer on porous Ge/Si substrate. The thermally induced reorganization of the porous Ge (PGe) leaves high density of nanoscale voids within the Ge buffer layer leading to the enhancement of the optical and structural properties compared to that directly grown on Ge/Si. Our results show that the nano-voided Ge/Si virtual substrate is potentially interesting for direct growth of III-V solar cells on Si (001) substrate.