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