Reminder: MSE PhD Oral Examination: Shu Hu (Monday, June 6th @ 10:00 AM in Clark Auditorium)

Shu Hu shuhu at stanford.edu
Sun Jun 5 18:59:57 PDT 2011


University PhD Dissertation Defense

Nanoscale Germanium Crystal Growth and Epitaxy Control for Advanced Electronics and Solar Cells 

Shu Hu
Department of Materials Science and Engineering 

Advisor: Prof. Paul C. McIntyre 
When: Monday, June 6th 2011, 10:00 am (Refreshments at 9:45 am) 
Where: James H. Clark Center Auditorium 
http://campus-map.stanford.edu/index.cfm?ID=07-340

    Semiconductor crystal growth at the nanoscale and integration of different materials systems are central themes of materials research. They enable novel materials processes and device applications, and may shape the landscape of future technologies. A major challenge is growth of high-quality single crystal semiconductors (e.g. Ge) on large-mismatch (e.g. Si) and non-crystalline (e.g. glass) substrates, while managing the thermal constraints of the underlying substrates. As-grown vertical semiconductor nanowires have been demonstrated as sensors, and nanoelectronic and nanophotonic devices. However, little attention has been paid to their unique structural properties: vertical Ge nanowires can be epitaxially grown on (111)-oriented Ge and Si substrates. In my talk, I will focus on nanowire-seeded crystallization and metal-induced crystallization to realize three-dimensional integration and nanostructured solar cells. Fundamental aspects of crystal growth at the nanoscale will be discussed.

Three-dimensional (3-D) device stacking and heterogeneous materials integration can improve the performance and functionality of Si-based electronics. First, I will demonstrate liquid phase epitaxy seeded by Ge nanowires to grow micron-sized single crystal Ge islands on SiO2. Vertical Ge nanowires can transfer the orientation and perfection of the underlying Si lattice to overlying layers several microns above. Liquid phase epitaxy was found to eliminate random nucleation that competes with epitaxial growth from nanowire seeds. The structure and electronic properties of Ge islands will be discussed. Given a low thermal budget annealing process, this technique can be repeated to build multiple active device layers, a key requirement for the fabrication of densely interconnected 3-D integrated circuits.

Vertical, tapered Ge nanowire arrays have shown enhanced light absorption properties, promising for high-efficiency solar cells. Metal-induced crystallization is a low-temperature crystal growth process for polycrystalline semiconductor deposition on large-area, non-crystalline substrates. Then, I will demonstrate Al-induced layer exchange crystallization to form polycrystalline Ge thin films with micron-sized grains and (111)-preferred orientation at 200°C. The textured thin films can serve as growth templates for aligned nanowire arrays. Imaging nucleation, growth and coalescence of Ge crystal islands allows us to characterize, model and control Ge crystallization kinetics, by tuning the knobs such as nucleation density. 


--
Shu Hu, PhD Candidate
Department of Materials Science and Engineering

Stanford University
476 Lomita Mall, Stanford, CA 94305-4045


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