[Reminder] MSE PhD Oral Examination----Hai Lin, Thur. Feb 16, 10am, CIS-X Aud.

Hai Lin hailin at stanford.edu
Sun Feb 12 17:02:18 PST 2012



Title: Growth and Characterization of GeSn and SiGeSn Alloys for Optical Interconnects
 
Speaker: Hai Lin,   Department of Materials Science and Engineering
Advisor: Prof. James. Harris

Date: Thursday, February 16th 2012, 10AM (Refreshments at 9:45AM)
Location: CIS-X 101 Allen Auditorium    (http://cis.stanford.edu/directions/)

Abstract:
    As the device size scales down, one of the major limitations in today’s silicon (Si) integrated circuits (IC) comes from the electrical interconnects. In order to increase the interconnect density and decrease the interconnect energy, optical interconnects to chip or even on chip have been widely investigated.  Devices for optical interconnects, such as modulator and detector, have been achieved using Si and germanium (Ge). A major issue now is the lack of a Si compatible light source. Because Si has very poor light emitting efficiency, current research focuses on Ge based semiconductors. Two approaches have been considered to modify the band structure of Ge and make it a direct band gap material: applying tensile strain and alloying with Tin (Sn). In previous research, we have demonstrated the photoluminescence (PL) enhancement of Ge by tensile strain. This presentation will mainly focus on GeSn and SiGeSn alloys. 
 
    GeSn alloys were grown by molecular beam epitaxy (MBE) machines at low growth temperature (150-200⁰C) on InGaAs buffer layers. GeSn alloys with up to 10.5% Sn have been demonstrated with high crystal quality. The GeSn layers were characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The composition and strain were studied by X-ray diffraction and X-ray photoelectron spectroscopy. The optical properties of GeSn alloys were measured by photoreflectance (PR) and PL. PR measured at room temperature determined the direct bandgap energies from the maxima of the ligh- and heavy-hole bands to the bottom of Γ valley. The calculated energy bowing parameter for GeSn is 2.42±0.04eV. The effect of biaxial strain on the direct band gap is described by deformation potentials. With these basic parameters, we can plot the band gap change of GeSn alloys with respect to Sn concentration and strain, which is very useful for design of future optoelectronic devices based on GeSn alloys. In addition, from low-temperature PL measurement, we showed that adding 6~7% Sn can change unstrained GeSn alloys to direct band gap materials.

    The second part of the presentation will discuss the growth of SiGeSn and GeSn/SiGeSn double heterostructure. SiGeSn alloys have been proposed to be the barrier layers for GeSn quantum wells. Because ternary alloys, such as SiGeSn, have decoupled bandgap energy and lattice constant, the quantum well design has the freedom to choose strain state while keeping a larger band gap barrier. Good crystal quality of SiGeSn alloys have been grown by MBE and annealed by RTA at 500⁰C for 2mins in forming gas ambient. The direct bandgap energies of the alloys were measured by PR at room temperature, and the relationship between composition and direct band gap were discussed.  Finally, PL of GeSn/SiGeSn double heterostructure were demonstrated.  

    Through these investigations, we obtained the basic material properties of GeSn and SiGeSn, which are valuable in design of group IV light source for on chip optical interconnects. 


Hai Lin
Graduate Student
Department of Materials Science and Engineering
Stanford University




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