Optical constants of B- and P-doped Ge 1-y C y alloys on Si substrates

Abstract

Ge<SUB>1-y</SUB>C<SUB>y</SUB> alloys are meta-stable and challenging to produce due to the large disparity between the atomic sizes of Ge and C. However, this same disparity results in an alloy system that potentially spans a wide range of bandgaps, refraction indices, and lattice constants. As such, it has potential as a Si lattice matched material for use in Si based waveguides, detectors, modulators, and other devices. The performance of these devices, however, depends on the refractive indices, which are not well known in these alloys. We present the results of comprehensive measurements of refractive index, energy bandgap, and free carrier absorption versus doping level. In situ B and P doped 550 nm Ge<SUB>1-y</SUB>C<SUB>y</SUB> alloy films were grown on Si (100) substrates by molecular beam epitaxy. The infrared optical transmission spectra were measured at room temperature. The indices of refraction were obtained from the amplitude of the interference fringes at sub-bandgap photon energies. Hall effect measurements were employed to measure the carrier concentrations. The refractive index of our Ge- 1-y)C<SUB>y</SUB> alloys was nominally 4.01, and decreased with increasing B and P concentration.