High-quality Ge epilayers on Si with low threading-dislocation densities were achieved by a two-step ultrahigh vacuum/chemical-vapor-deposition process followed by cyclic thermal annealing. On large Si wafers, Ge on Si with threading-dislocation density of 2.3×107 cm−2 was obtained. Combining selective area growth with cyclic thermal annealing produced an average threading-dislocation density of 2.3×106 cm−2.We also demonstrated small mesas of Ge on Si with no threading dislocations. The process described in this letter for making high-quality Ge on Si is uncomplicated and can be easily integrated with standard Si processes.

Band gap shrinkage induced by tensile strain is shown for Ge directly grown on Si substrate. In Ge-on-Si pin diodes, photons having energy lower than the direct band gap of bulk Ge were efficiently detected. According to photoreflectance measurement, this property is due to band gap shrinkage. The origin of the shrinkage is not the Franz–Keldysh effect but rather tensile strain. It is discussed that the generation of such a tensile strain can be ascribed to the difference of thermal expansion between Ge and Si. Advantages of this tensile Ge for application to photodiode are also discussed.

In this letter, we experimentally evaluate the effect of miniaturization and surface roughness on transmission losses within a Si/SiO2 waveguide system, and explain the results using a theoretical model. Micrometer/nanometer-sized waveguides are imperative for its potential use in dense integrated optics and optical interconnection for silicon integrated circuits. A theoretical model was employed to predict the relationship between the transmission losses of the dielectric silicon waveguide and its width. This model accurately predicts that loss increases as waveguide width decreases. Furthermore, we show that a major source of loss comes from sidewall roughness. We have constructed a complete contour map showing the interdependence of sidewall roughness and transmission loss, to assist users in their design of an optimal waveguide fabrication process that minimizes loss. Additionally, users can find an effective path to reduce the scattering loss from sidewall roughness. Using this map, we confirm that nanometer-size silicon waveguides with 0.1 dB/cm transmission loss are possible with the currently available technology.

Epitaxially grown Ge layers on Si substrate are shown to reveal an enhanced absorption of near-infrared light, which is effective for the photodiode application in Si-based photonics. Ge layers as thick as 1μm were grown on Si substrate by ultrahigh-vacuum chemical-vapor deposition with a low-temperature buffer layer technique. X-ray-diffraction measurements showed that the Ge layer possesses a tensile strain as large as 0.2%, which is generated during the cooling from the high growth temperature due to the thermal-expansion mismatch between Ge and Si. Photoreflectance measurements showed that the tensile strain reduces the direct band-gap energy to 0.77 eV (c.f. 0.80 eV for unstrained Ge), as expected from the theory. Reflecting the band-gap narrowing, photodiodes fabricated using the Ge layer revealed an enhanced absorption of near-infrared light with the photon energy below 0.80 eV, i.e., with the wavelength above 1.55μm. This property is effective to apply the photodiodes to the L band (1.56–1.62μm) in the optical communications as well as the C band (1.53–1.56μm). It is shown that the experimental absorption spectrum agrees with the theoretical one taking into account the splitting of light-hole and heavy-hole valence bands accompanied by the band-gap narrowing. Based on the calculation, the performance of the photodiode using the tensile-strained Ge is discussed.

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A correlation between bulk leakage current density and threading dislocation density in silicon–germanium mesa-isolated diodes fabricated on relaxed graded buffer layers is presented. Si0.75Ge0.25 p-i-n diodes were grown on SiGe graded buffers with different grading rates. Graded buffers with different grading rates yielded “virtual substrates” with varying densities of threading dislocations. Bulk leakage current densities were differentiated from surface leakage currents by using p-i-n diodes with different areas. We demonstrate that the increase in bulk leakage current density in SiGe p-i-n diodes can be modeled by generation processes assisted by deep levels related to threading dislocations.

Thin films of SiO2 and TiO2 were used to fabricate one-dimensional photonic crystal devices using the sol-gel method: an omnidirectional reflector and microcavity resonator. The reflector consisted of six SiO2/TiO2 bilayers, designed with a stopband in the near infrared. Reflectivity over an incident angle range of 0°–80° showed an omnidirectional band of 70 nm, which agrees with theoretical predictions for this materials system. The microcavity resonator consisted of a TiO2 Fabry–Perot cavity sandwiched between two SiO2/TiO2 mirrors of three bilayers each. We have fabricated a microcavity with resonance at λ=1500 nm and achieved a quality factor of Q=35. We measured a resonance frequency modulation with a change in incident angle of light and defect layer thickness.

We demonstrate fast and efficient germanium-on-silicon p-i-n photodetectors for optical communications, with responsivities as high as 0.89 and 0.75 A/W at 1.3 and 1.55 mum, respectively, time response <200 ps and dark currents as low as 1.2 muA. Ge was epitaxially grown on Si by chemical vapor deposition, employing a low temperature buffer and cyclic thermal annealing to reduce the dislocation density. The overall performance is well suited for >2.5 Gb/s integrated receivers for the second and third fiber spectral windows. (C) 2002 American Institute of Physics