Yu-Hsuan Kuo

Strong quantum-confined Stark effect in germanium quantum-well structures on silicon

Silicon is the dominant semiconductor for electronics, but there is now a growing need to integrate such components with optoelectronics for telecommunications and computer interconnections. Silicon-based optical modulators have recently been successfully demonstrated; but because the light modulation mechanisms in silicon are relatively weak, long (for example, several millimetres) devices or sophisticated high-quality-factor resonators have been necessary. Thin quantum-well structures made from III-V semiconductors such as GaAs, InP and their alloys exhibit the much stronger quantum-confined Stark effect (QCSE) mechanism, which allows modulator structures with only micrometres of optical path length. Such III-V materials are unfortunately difficult to integrate with silicon electronic devices. Germanium is routinely integrated with silicon in electronics, but previous silicon–germanium structures have also not shown strong modulation effects. Here we report the discovery of the QCSE, at room temperature, in thin germanium quantum-well structures grown on silicon. The QCSE here has strengths comparable to that in III-V materials. Its clarity and strength are particularly surprising because germanium is an indirect gap semiconductor; such semiconductors often display much weaker optical effects than direct gap materials (such as the III-V materials typically used for optoelectronics). This discovery is very promising for small, high-speed, low-power optical output devices fully compatible with silicon electronics manufacture.

Optical modulator on silicon employing germanium quantum wells

We demonstrate an electroabsorption modulator on a silicon substrate based on the quantum confined Stark effect in strained germanium quantum wells with silicon-germanium barriers. The peak contrast ratio is 7.3 dB at 1457 nm for a 10 V swing, and exceeds 3 dB from 1441 nm to 1461 nm. The novel side-entry structure employs an asymmetric Fabry-Perot resonator at oblique incidence. Unlike waveguide modulators, the design is insensitive to positional misalignment, maintaining > 3 dB contrast while translating the incident beam 87 mum and 460 mum in orthogonal directions. Since the optical ports are on the substrate edges, the wafer top and bottom are left free for electrical interconnections and thermal management.

Quantum-Confined Stark Effect in Ge/SiGe Quantum Wells on Si for Optical Modulators

We present observations of quantum confinement and quantum-confined Stark effect (QCSE) electroabsorption in Ge quantum wells with SiGe barriers grown on Si substrates, in good agreement with theoretical calculations. Though Ge is an indirect gap semiconductor, the resulting effects are at least as clear and strong as seen in typical III-V quantum well structures at similar wavelengths. We also demonstrate that the effect can be seen over the C-band around 1.55-mum wavelength in structures heated to 90degC, similar to the operating temperature of silicon electronic chips. The physics of the effects are discussed, including the effects of strain, electron and hole confinement, and exciton binding, and the reasons why the effects should be observable at all in such an indirect gap material. This effect is very promising for practical high-speed, low-power optical modulators fabricated compatible with mainstream silicon electronic integrated circuits

Ge–SiGe Quantum-Well Waveguide Photodetectors on Silicon for the Near-Infrared

We demonstrate near-infrared waveguide photodetectors using Ge-SiGe quantum wells epitaxially grown on a silicon substrate. The diodes exhibit a low dark current of 17.9 mA/cm2 at 5-V reverse bias. The photodetectors are designed to work optimally at 1480 nm, where the external responsivity is 170 mA/W, which is mainly limited by the fiber-to-waveguide coupling loss. The 1480-nm wavelength matches the optimum wavelength for quantum-well electroabsorption modulators built on the same epitaxy, but these photodetectors also exhibit performance comparable to the demonstrated Ge-based detectors at longer wavelengths. At 1530 nm, we see open eye diagrams at 2.5-Gb/s operation and the external responsivity is as high as 66 mA/W. The technology is potentially integrable with the standard complementary metal-oxide-semiconductor process and offers an efficient solution for on-chip optical interconnects.

Optical Link on Silicon Employing Ge/SiGe Quantum Well Structures

We demonstrate an optical link on silicon employing Ge/SiGe quantum well waveguide modulators and photodetectors. Modulators show >3 dB contrast ratio for <6 V drive for a 22 nm wavelength range. Photodetectors show external responsivity up to 0.26 A/W and high-speed detection at 2.5 Gb/s.

Waveguide Electroabsorption Modulator on Si Employing Ge/SiGe Quantum Wells

We report the first waveguide optical modulator on Si that employs the quantum-confined Stark effect. For a 6 V swing, the contrast ratio is 7.72 dB at 1476nm, and exceeds 3 dB over 14nm bandwidth.

The Quantum Confined Stark Effect in Ge/SiGe Quantum Wells: An efficient electroabsorption mechanism for silicon-based applications

The recent discovery of the quantum confined Stark effect in Ge/SiGe quantum wells with absorption coefficient modulation comparable to III-V materials will permit compact, low-power photonics components densely integrated with silicon electronics.

Ge/SiGe Quantum Confined Stark Effect Modulators on Silicon

We have demonstrated efficient QCSE in silicon-based structures, using strained Ge MQWs. The behavior of the exciton peaks, the band edge shift and the shift in absorption coefficient are comparable to those observed in III-V materials at similar wavelengths. Our materials and fabrication processes are completely CMOS compatible and suitable for mass production. This approach is therefore very promising for silicon-based electro-absorption modulators operating at high speed, low power, and low operating voltage and with small device areas

Germanium electroabsorption devices on silicon for optical interconnects

Monolithic integration of both electronic and optic components into a silicon-based platform will provide high-speed optical interconnects and solve the power-bandwidth limitations. However, the lack of strong optical effects in silicon has limited the progress in the transmitter-end applications. Recently our research had demonstrated strong quantum-confined Stark effect (QCSE) in germanium quantum-well modulators on silicon. This first strong physical mechanism for group-IV photonics has a comparable behavior to III-V material systems. With proper quantum well structure design, we also demonstrated QCSE in C-band for long distance communications with CMOS-operational temperatures. The device fabrication is also compatible with standard silicon chip processes. Since the QCSE, a type of electroabsorption effect, requires much shorter optical length, it is suitable for device miniaturizations and possible for use in both lateral and vertical modulator configurations. Moreover, silicon-germanium electroabsorption modulators are inherently photodetectors, this advantage will enable efficient transmitter/receiver applications for optical interconnects.

Extended InGaAs/InGaAs quantum structures for near infrared photodetection beyond 1.9 /spl mu/m

Extended InGaAs quantum structures were grown on InP substrate using two strain compensation techniques by molecular beam epitaxy. The material quality was characterized and the absorption coefficient was measured with transmission spectroscopy.