The characterization of thin film GaAs/Si solar cell structures grown by low-temperature atomic hydrogen-assisted molecular beam epitaxy (H-MBE) is presented. With assistance of atomic hydrogen irradiation ill MBE growth, all enhanced two-dimensional growth and sharp photoluminescence peaks with linewidths of less than 10 meV at 77 K were obtained. We also found that GaAs/Si layers grown it low temperatures were strained, and gradually relaxed with increasing layer thickness. Further, by introduced, an optimally designed Brag reflector (BR) superlattice structure. remarkable improvements of spectral response were observed in GaAs/Si solar cells near the bandedge region of 750-850 nm. which is often degraded ill common GaAs/Si solar Cells. suppression of threading dislocation densities due to lattice mismatch and thermal expansion coefficient difference was achieved by the combined approach of BR superlattice insertion and low-temperature growth schemec by H-MBE. (C) 2004 Elsevier B.V. All rights reserved.