By using the CLEFT process, epitaxial GaAs films can be grown on a reusable GaAs single-crystal substrate and transferred to amorphous substrates. An essential step in forming the epitaxial films is lateral growth seeded from openings in a mask on the single-crystal substrate. The lateral growth process has been studied as a function of growth conditions and crystal orientation, and optimized to give continuous films with specular surfaces. Very few defects are found in the films by cathodoluminescence and transmission electron microscopy. As an initial demonstration of film quality, solar cells having efficiencies as high as 17% at AM1 and an area of (1)/(2) cm2 have been fabricated. The CLEFT process is advantageous for device applications because amorphous or polycrystalline substrates can be much less expensive than single-crystal substrates, and also because they can have electrical and optical properties leading to improved device performance.