Investigating the Carrier Lifetimes of Lithium Tetraborate

Zoe Marzouk Author

04/01/2021 Added

14 Plays

Description

This research considers current voltage characteristics for differently cut samples of lithium tetraborate in calculations of the drift carrier lifetimes

Searchable Transcript

Search:

[00:00:01.380]Hello everyone.
[00:00:02.280]My name is Zoe Marzouk and I participated in the UCARE program for
[00:00:07.050]2020, 20, 21. And I want to share my student research days
[00:00:10.440]presentation with you all. I am a mechanical engineering student,
[00:00:14.430]but I did my research this year in the physics department.
[00:00:17.160]And I investigated carrier lifetimes in lithium tetraborate.
[00:00:22.620]I will in this presentation go over first,
[00:00:25.560]my motivation for doing this research,
[00:00:28.440]including the first phase of my research.
[00:00:31.020]Then I'll talk about the objective with continuing the research and how I went
[00:00:34.920]about it and how I calculated my results.
[00:00:38.880]I'll go over the results.
[00:00:40.140]And then I will also share with you what that means for the future.
[00:00:45.270]In the first phase of my research,
[00:00:46.680]I analyze data from a neutron detector called Danson,
[00:00:49.860]and this was designed by a group at UNL and placed on the international space
[00:00:54.480]station a few years ago. From the incident neutron count,
[00:00:58.210]so how many neutrons were hitting the,
[00:01:00.570]how many neutrons hit the detector over the time it was active.
[00:01:03.990]I determined the mean energy of the neutrons arriving from the sun.
[00:01:07.650]I found that a low energy steady state solar neutron flux from the sun is a
[00:01:11.790]possibility that cannot be discounted. A neutron flux
[00:01:16.320]such as this would explain a lot of the harm that comes to astronauts from long
[00:01:20.220]exposure to space because low-energy neutrons are going to be
[00:01:25.170]moving a lot slower than neutrons we would encounter otherwise.
[00:01:29.820]And this means that the neutrons have more time to interact with and disrupt
[00:01:34.170]systems in the body
[00:01:38.900]Knowing whether or not there is a steady state solar neutron flux would also
[00:01:42.470]provide a pathway for new calculations regarding some cosmic mysteries,
[00:01:47.000]including but not limited to calculations regarding the quadripolar moment of
[00:01:51.080]the sun.
[00:01:54.110]The problem is Danson was not a real time neutron detector.
[00:01:57.410]So the analysis is based only on counts over a period of time.
[00:02:01.850]It's not impossible that solar flares are responsible for the measured flux.
[00:02:07.130]However, neutrons from solar flares
[00:02:10.580]should be at energies much higher than the mean energy that I found,
[00:02:15.650]which was 1 to 2 MeV.
[00:02:18.020]My conclusion was that a real-time space-based detector should be pursued.
[00:02:22.670]And that leads to the question of how it should be made.
[00:02:28.970]Lithium Tetraborate was the material of choice for the Danson
[00:02:33.740]detector, and actually in the detector. If you look at this figure,
[00:02:38.720]it shows that it shows the different layers of the detector.
[00:02:42.380]And each of these layers was created to capture neutrons at a certain energy.
[00:02:47.270]And that's how I was able to determine which energy was the most prevalent
[00:02:51.620]lithium Tetraborate was chosen because it is highly anisotropic in
[00:02:55.220]nature.
[00:02:56.000]And what that means is that if you take measurements from different directions
[00:02:59.410]going to get largely different properties.
[00:03:03.010]The charge carriers responsible for conduction in the material are principally
[00:03:06.790]lithium ions,
[00:03:08.710]as opposed to electrons in a lot of other semiconductors in the
[00:03:13.540]principal, conductive crystallographic direction,
[00:03:15.670]which is the (001) direction,
[00:03:18.610]which in this research is called Z cut,
[00:03:21.310]the charge carriers are expected to last longer than in the direction
[00:03:25.630]perpendicular to this. So the (100) direction,
[00:03:29.590]which here is called X cut the difference in conduction.
[00:03:33.520]But the difference in the conduction means between these directions.
[00:03:37.930]It gives this semiconductor properties that are actually appealing for a solid
[00:03:42.430]state neutron detector.
[00:03:44.920]My objective with the research is to quantitatively evaluate whether
[00:03:49.780]this material is going to be a good choice to pursue for further neutron
[00:03:53.650]detector applications, such as a real-time space based neutron detector.
[00:04:00.240]The first one property to investigate with this material was drift carrier
[00:04:03.600]lifetime. And that was the focus of the second part of my research. This year,
[00:04:07.950]I compare the drift carrier lifetime between the different directions
[00:04:12.630]Charge carriers in materials are in a constant state of moving between what are
[00:04:17.310]called holes. When they're out of the holes, they are transporting charge,
[00:04:21.660]but when they're in the holes, they're not transporting chart,
[00:04:24.720]Drift carrier lifetime is just a measure of how long the carriers are out of the
[00:04:28.350]holes or how long they're able to transport charge.
[00:04:33.300]To collect data, I
[00:04:35.850]I analyzed current voltage characteristics,
[00:04:38.790]which are also called CVC,
[00:04:41.250]and these were collected for lithium tetraborate with platinum contact.
[00:04:45.010]So either X or Z cut lithium Tetraborate was coated on each side with
[00:04:49.860]a thin film platinum contact. In this testing,
[00:04:54.390]a voltage was applied and then that voltage was stepped up at a fixed rate for
[00:04:59.250]each of the cuts.
[00:05:00.990]The test was run separately for an X cut sample at 300 Kelvin and X cut sample
[00:05:05.070]at 400 Kelvin and a Z cut sample at 400 Kelvin. The parameter
[00:05:09.840]analyzer, which was just the tool that was used, recorded
[00:05:13.350]the current response as each voltage was applied.
[00:05:17.760]I then used a plot digitizer to extract the values as data points from the graph
[00:05:22.200]in order to calculate the drift carrier lifetime in each direction.
[00:05:27.300]And if you look at the axes, the range of the axes
[00:05:32.580]for each of the figures,
[00:05:34.470]it's pretty clear that the response in each direction was quite different.
[00:05:41.150]Drift carrier lifetime calculations are not uncommon in the field of materials
[00:05:46.140]physics. So there were existing relations that I was able to use.
[00:05:49.590]The drift carrier lifetime is dependent on the changing current versus the
[00:05:53.340]change in voltage. And that's known as G naught.
[00:05:56.250]It's also important know the drift capacitance, which,
[00:06:00.620]which is calculated as the current times to change in voltage over the change in
[00:06:04.580]time. But also drift capacitance is related to drift carrier lifetime,
[00:06:09.200]which is Tao. From the parameter analyzer,
[00:06:13.370]the rate of collection was a change in voltage over change in time.
[00:06:16.850]So I was able to use that rate to easily calculate the drift capacitance
[00:06:22.520]Drift carrier lifetime is going to be somewhat different for each applied
[00:06:26.060]voltage, even, even within the same sample.
[00:06:30.050]So the results for each of the samples is going to be a graph of the drift
[00:06:34.880]carrier lifetime versus the applied voltage.
[00:06:38.480]And that is exactly what we created here.
[00:06:41.120]The main takeaway is that carrier lifetime in the Z cut direction is six times
[00:06:46.070]longer than the carrier lifetime, the X cut and that temperature,
[00:06:51.830]the temperature also has little effect on the carrier lifetime.
[00:06:55.880]So the bigger effect is the direction. In normal semiconductors drift
[00:07:00.590]carrier lifetime is new usually no more than a single second,
[00:07:04.580]but here the largest value for carrier lifetime is over 600 seconds.
[00:07:10.030]This is, this is
[00:07:12.880]unusual. However,
[00:07:14.380]it is consistent with ion transport rather than electron transport and other
[00:07:19.060]semiconductors electrons are going to be what's transported,
[00:07:22.360]but in something such as lithium tetraborate ions,
[00:07:27.070]the lithium ions are going to be more prevalent.
[00:07:31.270]Because the carrier lifetime.
[00:07:32.800]in the Z cut data is six times larger than the lifetime in the X cut data.
[00:07:37.960]Lithium tetraborate can be considered a good candidate for
[00:07:41.070]further research in neutron detectors.
[00:07:44.020]It already worked well as a space-based detector in Danson,
[00:07:47.110]but it's going to be a little bit more complicated to create a real-time neutron
[00:07:51.370]detector.
[00:07:53.530]Long carrier
[00:07:54.310]lifetimes, as I said, are not usual for semiconductors,
[00:07:58.060]but knowing these material properties gives a little bit of promise for
[00:08:02.920]more research into lithium tetraborate for a variety of uses.
[00:08:07.510]So for example,
[00:08:08.440]not only can the material be used for additional neutron detector applications,
[00:08:13.660]but it can also be expanded into the field of already existing lensless
[00:08:18.030]pyroelectric infrared sensors.
[00:08:22.990]Thank you all for taking the time to view my research.
[00:08:25.510]I'd like to thank all of the sponsors that make the UCARE program possible.
[00:08:28.900]So students like me can, keep doing things like this.
[00:08:32.320]I would also like to thank my research advisor, Dr.
[00:08:34.600]Peter Dowben for his trust in me with this work.
[00:08:37.510]And I also want to thank Archit Dhingra, a PhD student in Dr.
[00:08:41.470]Dowben's research group.
[00:08:42.660]And co-author on a paper that we wrote during this research.
[00:08:46.810]Both of them have guided me through this research and I truly appreciate their
[00:08:50.920]time and attention. Thank you again for listening. I hope you enjoyed.

The screen size you are trying to search captions on is too small!

You can always jump over to MediaHub and check it out there.

Comments

0 Comments

Investigating the Carrier Lifetimes of Lithium Tetraborate

Description

Searchable Transcript

Comments icon comment

Related Channels

Comments