PhD Oral Exam - Yu-Hsuan Kuo (Jan 17, Tuesday 9:30 AM)

Yu-Hsuan Kuo yhkuo at
Thu Jan 12 19:15:44 PST 2006

University Ph.D. Oral Examination

Yu-Hsuan Kuo
Department of Electrical Engineering
Stanford University

9:30 AM, Tuesday, January 17, 2006
CIS-X Auditorium (Refreshments served at 9:15 AM)

"Germanium-Silicon Electroabsorption Modulators for Optical

Optical interconnects between silicon electronics have attracted
researchers’ attentions for a long time because optical links have
potential advantages for higher speed, lower power, and
interference-immunity. Successful, monolithic integration of photonics and
electronics will significantly reduce the cost of optical components and
further combine the functionalities of chips on the same or different
boards or systems. Electro-optic and thermo-optic effects have been
previously utilized to fabricate modulators - the fundamental building
blocks for optical interconnects; however, the inefficiency of these
mechanisms in silicon has hampered silicon-based group-IV optical
transmitters. Since germanium has a local minimum at the zone center of
band structure and a high absorption coefficient, it is possible to realize
a strong electroabsorption effect in germanium.

In this talk, I will present germanium-silicon electroabsorption modulators
for optical interconnects. SiGe p-i-n devices with strained Ge/SiGe
multi-quantum-well (MQW) structures in the i-region are grown on relaxed
Ge-rich SiGe buffer layers on silicon substrates. The device fabrication is
based on processes for standard silicon electronics and is suitable for
mass-production with complementary metal-oxide-semiconductor (CMOS) chips.
Quantum-confined Stark effect (QCSE) - the strongest electroabsorption
effect and optical modulation mechanism - is observed in these Ge MQW
devices on silicon. The absorption spectra show clear exciton peaks related
to carriers inside the Ge QWs. The exciton peaks are red-shifted and their
absorption coefficients are reduced under increased reverse biases. The
magnitude of QCSE is comparable to that of III-V materials at similar
wavelengths. With proper MQW structure design, we also demonstrated strong
QCSE over the entire C-band wavelength region, making these modulators
suitable for telecommunications and also compatible with typical
CMOS-chip-operational temperatures (~90°C). The electroabsorption devices
can also be used as photodetectors under high reverse bias.
Germanium-silicon electroabsorption devices will enable efficient
high-speed modulators and photodetectors for optical interconnects.

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