FW: Tomorrow: EE PhD Oral Examination - Yijie Huo, Monday, July 19, 2010; 10:00 a.m.

Yijie Huo yjhuo at stanford.edu
Sun Jul 18 15:21:32 PDT 2010



From: ee-students-bounces at lists.stanford.edu
[mailto:ee-students-bounces at lists.stanford.edu] On Behalf Of Natasha Newson
Sent: Wednesday, July 14, 2010 10:51 AM
To: Subject: EE PhD Oral Examination - Yijie Huo, Monday, July 19, 2010;
10:00 a.m.


Stanford University PhD Oral Defense - Department of Electrical Engineering

Speaker: Yijie Huo

Advisor: James S. Harris

Date: Monday, July 19, 2010

Time: 10:00 a.m.

Location: CIS-X 101 Auditorium 

Title:  Group IV materials and devices for Si photonic integrated circuits 


Silicon photonics has generated much interest in the past 10 years due to
its ability to enhance the performance of CMOS integrated circuits (IC). The
interconnect bandwidth limitation becomes a more and more critical challenge
with device scaling. Optical communication has the ability to solve this
emerging problem due to its high speed, high bandwidth, and low power
consumption. Most of the key devices in Si photonic ICs have already been
demonstrated, such as waveguides, detectors, and modulators. However, a
practical silicon-compatible coherent light source is still a major


Germanium has already been demonstrated to be a promising material for
optoelectronic devices, such as photo-detectors and modulators. However, Ge
is an indirect band gap semiconductor that has strong phonon-assisted
non-radiative recombination which overcomes the radiative recombination.
This makes Ge-based light sources very inefficient and difficult to realize.
Fortunately, Ge has a direct G valley that is only 0.13eV higher in energy
than the indirect L valley, suggesting that with band-structure engineering,
Ge has the potential to become a direct band gap material and an efficient
light emitter.


In this talk, we first present the background and the key devices of Si
photonic ICs. We then focus on how band-structure engineering can be used on
Ge to achieve a direct band gap semiconductor by use of either tensile stain
or GeSn alloys. To achieve high biaxial tensile strain (up to 2.3%), Ge QWs
were grown on top of fully-relaxed InGaAs buffer layers in our MBE system
and were verified by AFM, XRD, Raman spectroscopy, and TEM. A strong
increase of photoluminescence (PL) from strained Ge layers and the
temperature-dependent PL intensity prove that a direct band gap
semiconductor was achieved. We also achieved more than 7% Sn incorporation
in Ge, which is much higher than the 1% solid solubility limit of Sn inside
Ge. Material characterization shows good crystal quality without
precipitation or phase segregation. Direct band gap narrowing is observed
with increasing Sn percentage, which is consistent with theoretical
predication. Possible applications from this work will also be discussed. 



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