Enhancement of L-band optical absorption in strained epitaxial Ge on Si-on-quartz wafer: Toward extended Ge photodetectors

Abstract

Enhanced optical absorption in the L band (1.565–1.625 μm) of optical communication is reported for a Ge epitaxial layer grown on a Si-on-quartz (SOQ) wafer toward an extended range Ge photodetector (PD) in Si photonics. Ge epitaxial layers are grown using ultrahigh vacuum chemical vapor deposition at 600 °C on three different wafers of bonded SOQ, bonded Si-on-sapphire (SOS), and ordinary bonded Si-on-insulator (SOI). In the Ge layer, depending on thermal expansion mismatch between the Ge layer and base substrate, different amounts of in-plane biaxial lattice strain are generated. X-ray diffraction shows that an enhanced tensile strain of 0.32 ± 0.02% is generated in Ge on SOQ, which is approximately two times higher than 0.17 ± 0.02% in Ge on SOI, whereas the strain in Ge on SOS shows a slightly compressive value of −0.06 ± 0.02%. Photoreflectance spectra for these Ge layers show a change in the direct bandgap energy in accordance with the strain. In particular, the direct bandgap energy reduces from 0.775 ± 0.003 eV for Ge on SOI to 0.747 ± 0.003 eV for strain-enhanced Ge on SOQ, being comparable to 0.74 eV for the standard III–V PD material of In0.53Ga0.47As on InP. Furthermore, enhanced optical absorption in the L band is realized for Ge on SOQ, as measured using free-space pin PDs. These results indicate that the operating wavelength range of Ge PD on SOQ extends from C (1.530–1.565 μm) to the L band.