Improving the Quality of Virtual Ge-on-Si Substrates

Sang M. Han, University of New Mexico, DMR 0907112

This award supports a combined experimental and computational investigation aimed at substantially lowering the manufacturing cost of highly-efficient solar cells (>40%) based on chemical elements from Group III and V of the periodic table. Currently, the manufacturing costs of these cells are prohibitive for everyday usage on rooftops. By replacing the conventional use of expensive Ge wafers with high quality Ge-on-Si virtual substrates, we aim to increase the application scope of this exciting technology. In order to improve the quality of Ge-on-Si virtual substrates, we are employing state-of-the-art atomic scale simulations of a recently-developed thin-film deposition process for growing Ge layers on cheap Si wafers using a barrier silicon dioxide (SiO2) layer. Shown in the Figure (left side) are high-resolution microscopy images of Ge islands annealed on SiO2 at different temperatures. On the right are the results of a molecular dynamics simulation in which a Ge sphere anneals on SiO2. The change in equilibrium island shape as a function of temperature provides insight into the nature of the Ge-SiO2 interaction, which is a key factor in virtual substrate quality. Comparing the results of the simulations to experiment gives direct assessment of model validity.

Ge island morphology on amorphous SiO2. Left – microscopy; right – computer simulation

(green – Ge, red – Si, blue – O)

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Undergraduate Research Training in Atomistic Simulations

Sang M. Han, University of New Mexico, DMR 0907112

Research opportunities are increasingly a critical aspect of undergraduate education. A new program entitled The Rachleff Scholars Program at Penn seeks to increase the opportunities for undergraduate students to carry out fruitful research. Over the summer, Ms. Yungchi Chuang, a Graduate Research Assistant in the Sinno Group at Penn, was actively involved in the research mentoring of a rising junior in Chemical and Biomolecular Engineering, Mr. Daniel Consoli. Daniel used atomistic simulations to study the diffusion of oxygen atoms within crystalline silicon. As part of the Rachleff Scholars Program, he presented his research at the end of the summer session to a large audience comprised of faculty, student peers, and other attendees. He is continuing his research in the Sinno group during the Fall and is developing simulation tools to study diffusion pathways. These codes will be used by others within the group in a variety of applications.

Consistent with the theme emphasizing undergraduate research, Han recruited Jose Bonilla, a 2nd year undergraduate student in Chemical Engineering, to work on a well-defined semester-long project related to Ge-on-Si strain engineering. Han directly mentors Mr. Bonilla through weekly discussions, while a graduate research assistant, Swapnadip Ghosh, supervises him for daily activities in the laboratory. This is one of the efforts by Han to recruit and train underrepresented students to perform undergraduate/graduate research.