Lee Wienkes is a graduate student working with Professor Jim Kakalios on mixed-phase silicon thin films. The materials they study have two main applications, solar cells and thin film transistors (which help make LCD screens possible). Amorphous silicon can be easily deposited over a large area , which combined with its strong light absorption, make it an excellent candidate for cost effective solar cells. Unfortunately, due to the Staebler-Wronski effect, the efficiency of the cells degrade in the presence of light. “Amorphous silicon is a solar cell which works well in the dark."
The microscopic mechanism that causes the degradation isn’t fully understood despite over 30 years of study. He is currently working on a line of research in which silicon nanocrystals are embedded in an amorphous silicon film during the deposition. Other researchers have suggested that the inclusion of nanocrystals leads to a reduction in light-induced degradation. Kakalios and his collaborator in Mechanical Engineering, Uwe Kortshagen, have built a dual plasma co-deposition system for growing such mixed-phase thin films and have produced a novel way to study the combination of amorphous silicon and nanocrystals.
Amorphous silicon is a disordered material without a well defined crystal structure, while the nanocrystalline silicon is small enough that the fundamental assumption of an infinite lattice begins to break down, making the transport in these materials both interesting and difficult to study. “We do very basic physics, looking at electronic transport channels that describes how the electron traverses the material,” Wienkes says. There are a number of techniques used to probe the transport; Wienkes’ first major experiment on this project was to study the conductivity as a function of temperature from 10 Kelvin up to 470 K (~200 C). “The thing I did, which was new for our lab and had a bit of a learning curve, was cryogenic stuff; most of our measurements are above room temperature.”
Wienkes said that after careful analysis they found three distinct modes of the transport. “The types of transport mechanisms we saw were not necessarily new. What was new was the fact that we saw all three in one set of samples and that we could very clearly identify each type of conduction.” These results have recently been published in Applied Physics Letters.
The group is currently studying noise spectroscopy on the thin films; noise spectroscopy examines the small fluctuations of the conductivity, which are normally averaged out in mean-value measurements, to extract transport information. Amorphous silicon is a very noisy material, but the noise drops off as you begin to introduce nanocrystallites into the film. “What was a surprise was that the noise power was dropping off even in conditions were the conductivity measurements suggested we were seeing conduction through the amorphous silicon. The fact that the noise power was decreasing as the nanocrystalline fraction increased wasn’t surprising, it was the temperature range where it was happening. It hints that either the nanocrystals are active players in altering the surrounding amorphous matrix or still participating somehow in the transport of electrons.”
Aside from examining the noise magnitudes, Wienkes’ research also looks at the correlation between different frequencies in the noise. In most materials, different frequencies are completely uncorrelated, which leads to Gaussian noise. In amorphous silicon you do see correlations, which has lead to speculations that there may be a handful of fluctuators that dominate the noise. Wienkes compares these critical fluctuators to one or two boisterous people at a party dominating conversations between groups of people. Understanding these important fluctuators is key to understanding the electronic transport properties of these technologically important materials.
Lee Wienkes' research is currently supported by a University of Minnesota Doctoral Dissertation Fellowship