MSE PhD Defense, Szu-Lin Cheng, Wednesday Oct 20th 2010, 10:00am, CISX Auditorium (101X)

Chia-Yu Chen yu0528 at
Wed Oct 13 21:56:24 PDT 2010

Good luck :)

----- Original Message -----
From: "Szu-Lin Cheng" <slcheng at>
To: "mse-students" <mse-students at>, "ee-students" <ee-students at>, "labmembers" <labmembers at>
Sent: Wednesday, October 13, 2010 5:24:30 PM
Subject: MSE PhD Defense, Szu-Lin Cheng, Wednesday Oct 20th 2010, 10:00am, CISX Auditorium (101X)

Stanford University PhD Dissertation Defense 

PhD Candidate : Szu-Lin Cheng 

Title : Germanium as an Infrared Optical Emitter for Monolithic Integration on Silicon 

Research Advisor : Prof. Yoshio Nishi 

Date : Wednesday, Oct 20th, 2010 

Time : 10 am (Refreshments served at 9:40 am) 

Location : CISX Auditorium (101X) 

A silicon (Si) compatible laser for applications in telecommunication and optical interconnect systems has been an interesting topic for several years now, but has yet to be practically demonstrated. The main problem is finding an appropriate lasing material at 1550 nm which can be monolithically integrated on silicon with conventional CMOS processes. Germanium (Ge) is compatible with Si and has a direct band gap of 0.8 eV, corresponding to the required optical communication wavelength of 1550 nm. The small difference of 0.134 eV between the direct and indirect band gaps of Ge suggests the possibility of a radiative direct band gap transition. Strategies to improve the luminescence properties of germanium have included large tensile strain, tin alloying, and electron band filling. In this talk, we focus on the last approach since the emission wavelength from such a method stays near the desired 1550 nm. 

We first show how Ge direct band emission can be improved by using electron band filling of the conduction band. To achieve high electron band filling, an in-situ doping technique was applied during the growth of epitaxial Ge on Si. A strong enhancement from direct band photoluminescence (PL) was observed from highly-doped (1E19 cm -3 ) n-type epi-Ge, demonstrating that electron band filling improves the direct band emission strength. We then successfully demonstrate room temperature direct band electroluminescence (EL) from Ge n+/p light emitting diodes (LED) on a Si substrate, which is a key step towards a CMOS-compatible laser. The contribution of electron band filling and the temperature dependence to the device efficiency will also be discussed. Additionally, we fabricate and optically characterize epitaxial Ge microdisks on Si. These mircodisk resonators are successfully coupled to fiber tapers and display clear whispering gallery modes (WGM) in transmission as well as photoluminescence. Finally, we combined the LED structure and the microdisk cavity to demonstrate an electrically-pumped Ge resonator diode. Both our optical and electrical resonators are currently limited by the Ge doping concentration, which prevents sufficient electron band filling to allow material gain or lasing. Possible solutions to this problem will also be discussed. 

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