Optical Characterization of Ultra-Wide Bandgap Materials

Matthew Hilfiker Author

04/05/2021 Added

39 Plays

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Chips that can handle high-power switching are essential for future power electronics. An accurate determination of their bandgap properties is essential for device manufacturers. I give a brief overview on ultra-wide bandgap materials and provide a look at the bandgap properties of these materials as determined by spectroscopic ellipsometry.

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[00:00:00.840]Hi, I am Matthew Hilfiker,
[00:00:02.790]a graduate student under professor Mathias Schubert in the department of
[00:00:06.450]electrical and computer engineering. Today,
[00:00:08.700]I will be presenting on the optical characterization of ultra wide band gap
[00:00:12.570]materials. Currently there is a high demand to reduce carbon emissions.
[00:00:17.220]In particular, there is a demand to make many devices such as cars, planes,
[00:00:21.840]or trains, purely electric,
[00:00:24.330]to be able to achieve this shifts need to be able to be manufactured,
[00:00:27.960]to handle the high power needs of these electronics.
[00:00:31.560]This is why ultra wide band gap materials are considered essential research to
[00:00:35.550]developing these technological solutions.
[00:00:39.600]When we discuss ultra wide band gap materials,
[00:00:42.060]we're talking about materials with band gaps higher than common semiconductors,
[00:00:45.870]such as gallium nitride, and Silicon carbide,
[00:00:48.630]which are currently used in many high power devices and have band gaps around
[00:00:52.350]3.5 eV.
[00:00:54.390]One of the most commonly investigated ultra wide band gap materials is the
[00:00:58.410]gallium oxide,
[00:01:00.210]theoretical investigations and initial studies suggest gallium oxide,
[00:01:04.440]material parameters lead it to having relatively high figure of merit values as
[00:01:09.390]shown in this table with significant advantages over current technology
[00:01:13.590]materials.
[00:01:14.850]Another advantage is a large way for such as the one shown here are able to be
[00:01:19.110]produced at reasonable prices with low defect densities,
[00:01:23.220]and scalability has been rapidly increasing.
[00:01:27.930]Now my research focuses on using spectroscopic ellipsometry to determine
[00:01:32.790]the optical properties of these materials property,
[00:01:35.920]such as the band gap and other electronic transition parameters are essential
[00:01:40.260]knowledge for device fabrication and an accurate method to determine this is
[00:01:44.610]critical.
[00:01:45.780]Now it ellipsometry uses a light source where over a certain spectrum range,
[00:01:50.250]and by using a polarizer and compensator we're able to produce light, I mean,
[00:01:54.400]known polarization that we can reflect or transmit off the material.
[00:01:59.520]We can then analyze the resulting beam,
[00:02:01.890]knowing the material is the only thing in our system that can affect the light
[00:02:05.880]intensity and polarization.
[00:02:08.460]When we deal with isotropic samples or samples where the optical properties
[00:02:12.150]through the material are the same, regardless of direction,
[00:02:15.390]we can use the general ellipsometry equation.
[00:02:17.820]There relates to the reflection into two quantities Psi,
[00:02:21.420]the amplitude term and Delta the phase term. However,
[00:02:24.630]this can only be used for isotropic or non depolarizing media.
[00:02:29.250]If we want to deal with a sample that is anisotropic,
[00:02:32.280]we must investigate using the Mueller matrix,
[00:02:34.800]which is a four by four matrix relating the incoming and outcoming
[00:02:39.030]Stokes vector.
[00:02:41.160]Regardless of which approach we use ellipsometry is a technique where we
[00:02:45.030]collect the raw data.
[00:02:46.680]And then we have to use this regression based analysis technique where we kind
[00:02:50.760]of make an educated guess at what our material looks like.
[00:02:55.080]And we guess at these optical constants and layer thicknesses
[00:02:59.950]propose this model and see how closely it fits our data.
[00:03:03.490]This then leads to a back and forth iterative approach where we continually
[00:03:07.390]update our guests of what the material looks like until we have a model
[00:03:12.130]that best fits our data. And from there,
[00:03:14.710]we conclude that our model is representative of the material.
[00:03:19.480]Well,
[00:03:19.870]beta gallium oxide contains a band gap around 4.9eV. By alloying it
[00:03:24.850]with aluminum,
[00:03:25.630]we're able to develop samples with varying aluminum content and therefore a band
[00:03:30.370]gap between beta gallium oxide and at a much higher aluminum oxide band gap of
[00:03:35.050]8.8eV. This is very important for device fabrication. As now,
[00:03:39.670]we can physically unlock many band gap values that were previously unachievable
[00:03:44.710]with non-alloyed samples.
[00:03:47.590]So we received these batch of samples that were grown using molecular beam.
[00:03:51.310]epitaxy onto just gallium oxide substrates.
[00:03:55.330]One of the difficulties of working with beta gallium oxide. However it is,
[00:03:58.930]it has a monoclinic symmetry where here there are three lattice directions
[00:04:03.880]that are distinct from one another and an angle that is not 90 degrees.
[00:04:07.960]This leads us to having to model beta gallium oxide using the dielectric
[00:04:12.280]function, tensor that contains four distinct elements and further increasing the
[00:04:17.080]complexity of the analysis. However,
[00:04:19.900]we were able to first successfully determine the dielectric function elements,
[00:04:24.790]and we're currently in the process of using these results to determine the
[00:04:28.480]slight strain shifts that we expect from these epitaxially grown samples,
[00:04:34.120]since their epitaxial grown,
[00:04:35.590]we expect an expanding strain in the, uh,
[00:04:39.850]sample plane. And then in near the sample normal direction,
[00:04:43.450]we expect a compressive strain.
[00:04:46.480]These shifts will have a small but distinct effect on these dielectric
[00:04:49.600]functions.
[00:04:50.110]It needs to be accounted for. The figure on our left is showing the center
[00:04:54.190]energies of our band of band transitions,
[00:04:56.530]which were used to describe in our model.
[00:04:59.680]Now the center plane shows these values for the first band to band
[00:05:03.820]transitioned compared to density functional theory results by subtracting these
[00:05:08.800]results for the first transitions in both the AC and B plane,
[00:05:12.580]we're able to get the panel on the right, which shows that the AC plane has it,
[00:05:17.260]has it shift a linear upward shift as we increase in the aluminum content
[00:05:22.630]where the B direction is fairly flat across showing little or no
[00:05:27.400]shift we're in the process of working up and putting this into a manuscript.
[00:05:33.130]Now we have also looked at alpha gallium oxide samples.
[00:05:36.460]These runs were also used,
[00:05:38.140]are grown using molecular beam epitaxy and they found that by growing them on M
[00:05:42.850]plane Sapphire, you do not create any beta gallium oxide on top.
[00:05:48.040]Now they were able to grow these molecular beam epitaxy samples on the left and
[00:05:52.960]similar to using the same model approaches we used for the beta oxide
[00:05:57.680]phase. We did that for the alpha gallium oxide,
[00:06:00.530]but since it's only a uniaxial material in this, uh,
[00:06:05.630]form,
[00:06:06.620]we only have two different dielectric functions to look at our ordinary and
[00:06:10.670]extra ordinary axis.
[00:06:12.650]So we have successfully been able to describe the two distinct dielectric
[00:06:17.090]functions for all varying aluminum content samples.
[00:06:20.960]And we're in the process of seeing how those transitions shifts and also looking
[00:06:24.830]at similar effects, such as strain,
[00:06:26.930]which we do expect from these samples as well, since they're epitaxially grown.
[00:06:32.440]Finally, we have looked at zinc gallate as a potential ultra wide band gap material.
[00:06:37.630]It has gained recent attention due to its similarity of it's a large band gap to
[00:06:41.680]gallium oxide,
[00:06:42.850]but it possesses an isotropic structure which may be considered preferential for
[00:06:47.170]device designs over these intricate.
[00:06:49.750]anisotropy structural property relations in the gallium oxide,
[00:06:54.400]these samples were grown from our collaborators using this vertical gradient
[00:06:57.850]freeze method.
[00:06:58.840]And we successfully used the spectroscopic ellipsometry Psi and Delta data to
[00:07:03.490]determine the dielectric function. Importantly,
[00:07:06.430]we were able to determine even an exciton binding energy of around 15meV
[00:07:10.840]and determine the band gap energy of this, uh,
[00:07:14.470]for the first time at 5.27 eV for the direct
[00:07:16.840]In conclusion ellipsometry
[00:07:21.430]provides a powerful tool for determining the band of band transitions,
[00:07:25.030]which we were successfully able to do for various ultra wide band gap materials.
[00:07:29.560]These ultra wide band gap materials provide the potential for many technology
[00:07:33.790]applications and future work will be dedicated to exploring these materials in
[00:07:38.200]similar ones using spectroscopic ellipsometry.
[00:07:41.380]I would like to thank my lab mates because without their help,
[00:07:44.050]I would not be able to complete my work and the many collaborators we have here,
[00:07:48.310]where they were able to grow us these samples for us to investigate.
[00:07:52.180]Thank you for your attention.

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Optical Characterization of Ultra-Wide Bandgap Materials

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