Abstract
Integrated mid infra-red (MIR) photonics is of major interest for free-space communications,bio-sensing, etc. III-V antimony based lasers are efficient light sources in the MIR range that can begrown on passive Si photonics circuits [1]. However, most Si-based materials absorb a large part of theMIR spectrum above 8 μm, in contrast to germanium which has a wide MIR transparency window [2].The direct growth of III-Sb materials on Ge substrates or Ge-on-Si templates would allow developingintegrated photonic circuits operating at longer wavelengths (up to 14 μm ). Still, III-V heteroepitaxy ongroup-IV substrates is hampered by various crystal defects. In particular, anti-phase boundaries (APBs)are 2D defects particularly detrimental to devices because they act as short circuits. In this work, weinvestigated the growth of GaSb on Ge substrates and on Ge-on-Si templates grown by low-energyplasma-enhanced CVD (LEPECVD) [3].The starting surface preparation is critical for growing APB-free GaSb films on group-IVsubstrates [4]. We found that APB-free GaSb layers can be achieved only when the Ge substrates areprepared by UHV annealing in a narrow temperature window. The Ge-on-Si and SiGe/Si templates,however, exhibit cross hatch patterns arising from strain relaxation during template growth, whichprevents proper surface organization prior and during III-V growth. This, in turn, precludes APBannihilation. This effect can be alleviated either by using slightly miscut (1° - 2°) Si substrates, or bysmoothing the starting surface of on-axis Ge-on-Si templates by chemical-mechanical polishing. Underthese conditions, APB-free GaSb layers can be achieved after 500 nm growth. They exhibit a rmsroughness of 1.0 nm. Such layers can be used as templates for the direct growth of III-V devices. Theseresults represent a further step in the development of integrated MIR photonics.