Hsin-Chiao Luan

High-quality Ge epilayers on Si with low threading-dislocation densities

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.

Effect of size and roughness on light transmission in a Si/SiO2 waveguide: Experiments and model

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 n...

Strain-induced band gap shrinkage in Ge grown on Si substrate

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.

SiO2/TiO2 omnidirectional reflector and microcavity resonator via the sol-gel method

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.

Correlation between leakage current density and threading dislocation density in SiGe p-i-n diodes grown on relaxed graded buffer layers

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.

Strain-induced enhancement of near-infrared absorption in Ge epitaxial layers grown on Si substrate

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) i...

High-performance p-i-n Ge on Si photodetectors for the near infrared: from model to demonstration

We have investigated the integration of Ge p-i-n and n-i-p heterojunction photodiodes on Si. Recognizing the crucial role of interface defects at the Ge-Si interface on the performance of photodetectors, we have designed and fabricated high-performance n-i-p Ge photodiodes on p/sup +/-Si substrates. These photodiodes exhibit short-circuit responsivities of 0.3 and 0.2 A/W at 1.3 and 1.55 /spl mu/m, respectively, reverse dark currents of 20 mA/cm/sup 2/ and response times of 800 ps.

Ge on Si p-i-n photodiodes operating at 10Gbit∕s

We report on fast p-i-n photodetectors operating in the near infrared and realized in pure germanium on silicon. The diodes were fabricated by chemical vapor deposition at 600°C without affecting the crystal quality and allowing the integration with standard silicon processes. We demonstrate responsivities of 0.4 and 0.2A∕W at 1.3 and 1.55μm, respectively, as well as operation at 10Gbit∕s.

Monolithic Si-based technology for optical receiver circuits

Optical communications networks must be terminated by receiver circuitry capable of converting an optical circuit to an electrical one. While current III-V technology is capable of delivering high performance, it is costly and difficult to integrate with low-cost Si based technologies. In order to overcome these barriers, we are pursuing a Si-compatible technology for integrated photodetectors. Ge, monolithically integrated with Si, offers a low-cost, high-performance materials system for photodetector integration with existing Si technology. In this paper we discuss the performance requirements and figures of merit for integrated photodetectors. We then discuss the materials issues associated with the integration of Ge on Si and show that high quality Ge films can be grown directly on Si, despite the 4% lattice mismatch. By cyclic annealing after growth, the dislocation density can be reduced to 2.3x107 cm-2, and diodes fabricated on these films show a responsivity of 300 mA/W at 1300 nm without an AR coating. Finally, we discuss the integration of waveguides with photodetectors and propose an integration scheme we believe will be capable of delivering high-performance integrated photoreceivers on a Si platform.

High efficiency photodetectors based on high quality epitaxial germanium grown on silicon substrates

We demonstrate Ge/Si heterojunction photodetectors based on high quality epitaxial germanium grown on silicon. Germanium deposited by ultra-high-vacuum chemical vapor deposition (UHV-CVD) undergoes thermal annealing cycles which reduce the number of dislocations and, thus, improve the overall quality. The photodetectors exhibit record responsivity of 0.55 A/W and a sub-ns photoresponse at 1.3 μm. We describe the fabrication process as well as a complete optoelectronic characterization of the devices.