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1. Electron band bending of polar, semipolar and non-polar GaN surfaces

   https://doi.org/10.1063/1.4943592  

   Bartoš, I. ; Romanyuk, O. ; Houdkova, J. ; Paskov, P. P. ; Paskova, T. ; Jiříček, P. ( March 2016 , Journal of Applied Physics)
2. Mapping Polar Distortions using Nanobeam Electron Diffraction and a Cepstral Approach

   https://doi.org/10.1093/micmic/ozad070  

   Holtz, Megan E ; Padgett, Elliot ; Johnston-Peck, Aaron C ; Levin, Igor ; Muller, David A ; Herzing, Andrew A ( July 2023 , Microscopy and Microanalysis)

   Measuring local polar ordering is key to understanding ferroelectricity in thin films, especially for systems with small domains or significant disorder. Scanning nanobeam electron diffraction (NBED) provides an effective local probe of lattice parameters, local fields, polarization directions, and charge densities, which can be analyzed using a relatively low beam dose over large fields of view. However, quantitatively extracting the magnitudes and directions of polarization vectors from NBED remains challenging. Here, we use a cepstral approach, similar to a pair distribution function, to determine local polar displacements that drive ferroelectricity from NBED patterns. Because polar distortions generate asymmetry in the diffraction pattern intensity, we can efficiently recover the underlying displacements from the imaginary part of the cepstrum transform. We investigate the limits of this technique using analytical and simulated data and give experimental examples, achieving the order of 1.1 pm precision and mapping of polar displacements with nanometer resolution.

    

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3. Atomic-Level Structure Determines Electron–Phonon Scattering Rates in 2-D Polar Metal Heterostructures

   https://doi.org/10.1021/acsnano.1c05944  

   Steves, Megan A. ; Rajabpour, Siavash ; Wang, Ke ; Dong, Chengye ; He, Wen ; Quek, Su Ying ; Robinson, Joshua A. ; Knappenberger, Kenneth L. ( November 2021 , ACS Nano)
4. Dusty Space Plasma Diagnosis Using the Behavior of Polar Mesospheric Summer Echoes During Electron Precipitation Events: DUST CHARGING PROCESS IN SPACE

   https://doi.org/10.1029/2018JA025395  

   Mahmoudian, A. ; Senior, A. ; Scales, W. A. ; Kosch, M. J. ; Rietveld, M. T. ( September 2018 , Journal of Geophysical Research: Space Physics)
5. High-density polarization-induced 2D electron gases in N-polar pseudomorphic undoped GaN/Al0.85Ga0.15N heterostructures on single-crystal AlN substrates

   https://doi.org/10.1063/5.0107159  

   Zhang, Zexuan ; Encomendero, Jimy ; Kim, Eungkyun ; Singhal, Jashan ; Cho, YongJin ; Nomoto, Kazuki ; Toita, Masato ; Xing, Huili Grace ; Jena, Debdeep ( August 2022 , Applied Physics Letters)

   The polarization difference and band offset between Al(Ga)N and GaN induce two-dimensional (2D) free carriers in Al(Ga)N/GaN heterojunctions without any chemical doping. A high-density 2D electron gas (2DEG), analogous to the recently discovered 2D hole gas in a metal-polar structure, is predicted in a N-polar pseudomorphic GaN/Al(Ga)N heterostructure on unstrained AlN. We report the observation of such 2DEGs in N-polar undoped pseudomorphic GaN/AlGaN heterostructures on single-crystal AlN substrates by molecular beam epitaxy. With a high electron density of ∼4.3 ×1013/cm2 that maintains down to cryogenic temperatures and a room temperature electron mobility of ∼450 cm2/V s, a sheet resistance as low as ∼320 Ω/◻ is achieved in a structure with an 8 nm GaN layer. These results indicate significant potential of AlN platform for future high-power RF electronics based on N-polar III-nitride high electron mobility transistors.

    

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