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Interview: Dr. George Nelson, RIT

Hi George, can you introduce yourself and tell us about your research?

I am a postdoctoral fellow at RIT working under Prof. Seth Hubbard at NanoPower Research Labs. I completed my PhD in 2019 within the same group, and my postdoc is a continuation of my PhD research. Our group specializes in III-V solar cells for the satellite industry. A significant aspect of designing cells for space has to do with the effects of the space environment on cell performance. Over the course of a satellite's mission, high-energy particles continually bombard the cell's crystal structure and displace atoms, and the cell's power output degrades as these lattice defects accumulate. The particular defects that drive the performance loss are of interest to us, and a technique that I frequently use to study these defects is deep level transient spectroscopy (DLTS).

What is the principle of DLTS, and what are its potential applications?

DLTS is a non-destructive technique used to detect certain crystalline defects in semiconductor devices and characterize electronic properties of those defects such as the energy level within the bandgap and their concentration. Where applicable, it is a powerful tool to identify both well-known and never-seen-before defects present in a semiconductor material. Conventionally, it requires a device where an internal electric field can be modulated by an applied bias, such as p-n junction or Schottky diodes. The defects are typically small and numerous, with densities of at least 108 cm-3 and usually higher; they must also be electrically active.

Perhaps the most well-known historical DLTS results are those for gold-contaminated silicon or for the donor-complex in n-type AlGaAs, where DLTS helped explain the poor performance of devices made from these materials. In 2019, a group claimed that their DLTS results explain the cause of light-induced degradation in silicon solar cells, a problem that has vexed the industry for four decades.

What is the role of the Zurich Instruments MFIA Impedance Analyzer in your DLTS system?

Our DLTS system consists of a temperature controller, one of a variety of cryostats designed specifically for DLTS, a PC or laptop, and the MFIA. The MFIA replaces three of the components found in a traditional DLTS system, as it acts primarily as the capacitance meter but is also the pulse generator and the data acquisition system. It's a very economical system.

The MFIA allows for fine tuning of parameters (such as modulation frequency and amplitude) that are generally fixed in other meters. This is useful because there are many edge cases in DLTS where one-size-fits-all parameters are not ideal. Another aspect that proved useful is the small size of the unit. Because it handles so many responsibilities, our DLTS system with a laptop is surprisingly compact and portable, which helps when we need to take it to particle accelerator facilities with limited space.

Our older commercial DLTS system was driven by dozens of physical knobs and switches, and it was easy to make mistakes. It also required someone to be present to start or stop experiments. With our new system and the MFIA, everything is controlled by my own MATLAB® software; experiments can be run remotely or queued up. Writing my own software to control the MFIA was easy thanks to its documented API. By now, my efficiency must have improved by an order of magnitude over our older commercial DLTS system in terms of the amount of useful data I can collect over time.

What motivated you to upgrade your DLTS system?

I learned how to perform DLTS on an older commercial system. The hardware was completely analog, and the transient processing took place in this analog hardware. That greatly limited the type of analysis I could perform, because to make any changes to the signal processing I had to modify the circuitry. With the MFIA, the transient data is digitized: I can process the decay components within the software. I am free to perform any type of processing or fitting that I wish, and I continually update my software with new techniques.

Can you tell us a little more about the software you wrote to control your DLTS system?

At first, I was intimidated by the idea of writing my own DLTS software suite. Once I committed to it though, I found it to be a lot less difficult than expected. I already understood the physics and many of the practical considerations such as wiring. Coding for the MFIA was straightforward thanks to its documented API and example scripts. Something that was invaluable was the command log on the LabOne® Web Server, which told me the code-equivalent of whatever I clicked on in the LabOne user interface allowing me to quickly translate LabOne interactions into my own software. As far as I'm aware, my code is the most complete open-source implementation of DLTS software available. It is presently a collection of MATLAB® scripts that acquire and process capacitance transients. Unofficially, it also has functionality for impedance or admittance spectroscopy. I'm doing my best with documentation and how-to guides to make it more user-friendly, but DLTS is complicated. I'm about to release v1.0, and my plans for v2.0 are to move everything to Python and to improve the user interface. I hope this code is useful to the DLTS community, and perhaps I will receive feedback that will benefit my own experiments too.

You have developed state-of-the-art solar cells: what role can they play in reducing our carbon footprint?

III-V solar cells are indisputably the most efficient, and they can be made with much smaller mass and greater mechanical flexibility than silicon cells. One of my favorite aspects of III-V cells is that they can be designed to absorb high-energy visible and UV light while reflecting infrared light, thus preventing unnecessary heating. One could imagine these lightweight, highly efficient cells built into cars, planes, or buildings. Unfortunately, III-V cells cost ten to a hundred times more than silicon cells that perform almost as well. This led to a crash of the III-V terrestrial market; right now the satellite industry is the only one willing to pay for the extra performance. The cost problem is mostly due to the substrates and could be eliminated if someone found a way to make great virtual substrates. Many groups are working on this aspect, including ours.

What are your interests outside of the lab?

When I don't spend family time with my wife and son, I go to the gym and lift weights, run, or play tennis. I'm a weekend handyman and like fixing things such as old electronics, cars, and houses. I'm also turning into a history buff, especially the history of science and philosophy.

George Nelson

George Nelson is a postdoctoral fellow at the Rochester Institute of Technology (RIT). His work focusses on III-V solar cells for the satellite industry.

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