Comparative study of InGaAs integration on bulk Ge and virtual Ge/Si(1 0 0) substrates for low-cost photovoltaic applications

Abstract

Ge virtual substrates have been developed via low-temperature CVD based on new hydride molecular chemistry routes fully compatible with conventional CMOS. These are designed to enable integration of III–V compounds directly on Si and therefore have the potential to replace expensive Ge wafers in multijunction photovoltaics grown on the conventional 4–6″ format. Here we first describe in detail the protocols needed to produce defect-free and atomically flat Ge buffers with ∼0.25–3 μm thicknesses directly on vicinal (5°, 8°) and on-axis Si (1 0 0) substrates with up to 4″ diameters. Industrial MOCVD is then used to grow Ge-matched InGaAs films with thicknesses of 0.8–2.5 μm, both on our virtual substrates and on vicinal (6°) Ge wafers. A thorough characterization of the films’ morphological, structural, and optical properties allows a meaningful comparison of our best “virtual-substrate-grown” films with a mature and commercially available InGaAs technology on bulk Ge. Our studies confirm that the InGaAs films grown on Ge wafers exhibit the highest quality, followed closely by those grown on miscut Ge buffered Si. The latter films, in contrast to their on-axis counterparts, are devoid of antiphase-boundary defects and exhibit smoother surfaces and superior crystallinity, indicating the need for misoriented substrates to successfully integrate InGaAs on large area Si platforms. Collectively our work demonstrates the promise for transitioning our virtual substrate technology to the industrial scale production of photovoltaic III–IV films on Si(1 0 0) platforms.