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1. High-density polarization-induced 2D electron gases in N-polar pseudomorphic undoped GaN/Al _0.85 Ga _0.15 N 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 [Formula: see text]/cm 2 that maintains down to cryogenic temperatures and a room temperature electron mobility of ∼450 cm 2 /V s, a sheet resistance as low as ∼320 [Formula: see text] 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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2. Room temperature two-dimensional electron gas scattering time, effective mass, and mobility parameters in Al x Ga1− x N/GaN heterostructures (0.07 ≤  x  ≤ 0.42)

   https://doi.org/10.1063/5.0163754  

   Knight, Sean; Richter, Steffen; Papamichail, Alexis; Kühne, Philipp; Armakavicius, Nerijus; Guo, Shiqi; Persson, Axel R; Stanishev, Vallery; Rindert, Viktor; Persson, Per_O Å; et al (November 2023, Journal of Applied Physics)

   AlxGa1−xN/GaN high-electron-mobility transistor (HEMT) structures are key components in electronic devices operating at gigahertz or higher frequencies. In order to optimize such HEMT structures, understanding their electronic response at high frequencies and room temperature is required. Here, we present a study of the room temperature free charge carrier properties of the two-dimensional electron gas (2DEG) in HEMT structures with varying Al content in the AlxGa1−xN barrier layers between x=0.07 and x=0.42. We discuss and compare 2DEG sheet density, mobility, effective mass, sheet resistance, and scattering times, which are determined by theoretical calculations, contactless Hall effect, capacitance-voltage, Eddy current, and cavity-enhanced terahertz optical Hall effect (THz-OHE) measurements using a low-field permanent magnet (0.6 T). From our THz-OHE results, we observe that the measured mobility reduction from x=0.13 to x=0.42 is driven by the decrease in 2DEG scattering time, and not the change in effective mass. For x<0.42, the 2DEG effective mass is found to be larger than for electrons in bulk GaN, which in turn, contributes to a decrease in the principally achievable mobility. From our theoretical calculations, we find that values close to 0.3m0 can be explained by the combined effects of conduction band nonparabolicity, polarons, and hybridization of the electron wavefunction through penetration into the barrier layer. 

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3. Toward AlGaN channel HEMTs on AlN: Polarization-induced 2DEGs in AlN/AlGaN/AlN heterostructures

   https://doi.org/10.1063/5.0121195  

   Singhal, Jashan; Chaudhuri, Reet; Hickman, Austin; Protasenko, Vladimir; Xing, Huili Grace; Jena, Debdeep (November 2022, APL Materials)

   Due to its high breakdown electric field, the ultra-wide bandgap semiconductor AlGaN has garnered much attention recently as a promising channel material for next-generation high electron mobility transistors (HEMTs). A comprehensive experimental study of the effects of Al composition x on the transport and structural properties is lacking. We report the charge control and transport properties of polarization-induced 2D electron gases (2DEGs) in strained AlGaN quantum well channels in molecular-beam-epitaxy-grown AlN/Al x Ga 1− x N/AlN double heterostructures by systematically varying the Al content from x = 0 (GaN) to x = 0.74, spanning energy bandgaps of the conducting HEMT channels from 3.49 to 4.9 eV measured by photoluminescence. This results in a tunable 2DEG density from 0 to 3.7 × 10 13 cm 2 . The room temperature mobilities of x ≥ 0.25 AlGaN channel HEMTs were limited by alloy disorder scattering to below 50 cm 2 /(V.s) for these 2DEG densities, leaving ample room for further heterostructure design improvements to boost mobilities. A characteristic alloy fluctuation energy of [Formula: see text] eV for electron scattering in AlGaN alloy is estimated based on the temperature dependent electron transport experiments. 

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4. Controlled Si doping of β -Ga ₂ O ₃ by molecular beam epitaxy

   https://doi.org/10.1063/5.0101132  

   McCandless, J. P.; Protasenko, V.; Morell, B. W.; Steinbrunner, E.; Neal, A. T.; Tanen, N.; Cho, Y.; Asel, T. J.; Mou, S.; Vogt, P.; et al (August 2022, Applied Physics Letters)

   We report controlled silicon doping of Ga 2 O 3 grown in plasma-assisted molecular beam epitaxy. Adding an endplate to the Si effusion cell enables the control of the mobile carrier density, leading to over 5-orders of magnitude change in the electrical resistivity. Room temperature mobilities [Formula: see text] are achieved, with a peak value [Formula: see text] at a doping density of low-[Formula: see text]. Temperature-dependent Hall effect measurements exhibit carrier freeze out for samples doped below the Mott criterion. A mobility of [Formula: see text] is observed at [Formula: see text]. 

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5. Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in α -(Al _x Ga _1−x ) ₂ O ₃ (0≤x≤1)

   https://doi.org/10.1063/5.0087602  

   Hilfiker, Matthew; Kilic, Ufuk; Stokey, Megan; Jinno, Riena; Cho, Yongjin; Xing, Huili Grace; Jena, Debdeep; Korlacki, Rafał; Schubert, Mathias (August 2022, Applied Physics Letters)

   Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(Al x Ga 1− x ) 2 O 3 thin films (0 [Formula: see text] x [Formula: see text] 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular ([Formula: see text] = 1.31 eV) and parallel ([Formula: see text] = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between α-Ga 2 O 3 and α-Al 2 O 3 , respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in α-Ga 2 O 3 whereas for α-Al 2 O 3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M 1 type in α-Ga 2 O 3 to M 0 type van Hove singularity in α-Al 2 O 3 . 

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