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1. A review of band structure and material properties of transparent conducting and semiconducting oxides: Ga 2 O 3 , Al 2 O 3 , In 2 O 3 , ZnO, SnO 2 , CdO, NiO, CuO, and Sc 2 O 3

   https://doi.org/10.1063/5.0078037  

   Spencer, Joseph A. ; Mock, Alyssa L. ; Jacobs, Alan G. ; Schubert, Mathias ; Zhang, Yuhao ; Tadjer, Marko J. ( March 2022 , Applied Physics Reviews)

   This Review highlights basic and transition metal conducting and semiconducting oxides. We discuss their material and electronic properties with an emphasis on the crystal, electronic, and band structures. The goal of this Review is to present a current compilation of material properties and to summarize possible uses and advantages in device applications. We discuss Ga 2 O 3 , Al 2 O 3 , In 2 O 3 , SnO 2 , ZnO, CdO, NiO, CuO, and Sc 2 O 3 . We outline the crystal structure of the oxides, and we present lattice parameters of the stable phases and a discussion of the metastable polymorphs. We highlight electrical properties such as bandgap energy, carrier mobility, effective carrier masses, dielectric constants, and electrical breakdown field. Based on literature availability, we review the temperature dependence of properties such as bandgap energy and carrier mobility among the oxides. Infrared and Raman modes are presented and discussed for each oxide providing insight into the phonon properties. The phonon properties also provide an explanation as to why some of the oxide parameters experience limitations due to phonon scattering such as carrier mobility. Thermal properties of interest include the coefficient of thermal expansion, Debye temperature, thermal diffusivity, specific heat, and thermal conductivity. Anisotropy is evident in the non-cubic oxides, and its impact on bandgap energy, carrier mobility, thermal conductivity, coefficient of thermal expansion, phonon modes, and carrier effective mass is discussed. Alloys, such as AlGaO, InGaO, (Al x In y Ga 1− x− y ) 2 O 3 , ZnGa 2 O 4 , ITO, and ScGaO, were included where relevant as they have the potential to allow for the improvement and alteration of certain properties. This Review provides a fundamental material perspective on the application space of semiconducting oxide-based devices in a variety of electronic and optoelectronic applications. 

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2. The impact of site selectivity and disorder on the thermoelectric properties of Yb 21 Mn 4 Sb 18 solid solutions: Yb 21 Mn 4−x Cd x Sb 18 and Yb 21−y Ca y Mn 4 Sb 18

   https://doi.org/10.1039/d1ma00497b  

   He, Allan ; Cerretti, Giacomo ; Kauzlarich, Susan M. ( August 2021 , Materials Advances)

   Thermoelectric materials can convert heat into electricity. They are used to generate electricity when other power sources are not available or to increase energy efficiency by recycling waste heat. The Yb 21 Mn 4 Sb 18 phase was previously shown to have good thermoelectric performance due to its large Seebeck coefficient (∼290 μV K −1 ) and low thermal conductivity (0.4 W m −1 K −1 ). These characteristics stem respectively from the unique [Mn 4 Sb 10 ] 22− subunit and the large unit cell/site disorder inherent in this phase. The solid solutions, Yb 21 Mn 4− x Cd x Sb 18 ( x = 0, 0.5, 1.0, 1.5) and Yb 21− y Ca y Mn 4 Sb 18 ( y = 3, 6, 9, 10.5) have been prepared, their structures characterized and thermoelectric properties from room temperature to 800 K measured. A detailed look into the structural disorder for the Cd and Ca solid solutions was performed using synchrotron powder X-ray diffraction and pair distribution function methods and shows that these are highly disordered structures. The substitution of Cd gives rise to more metallic behavior whereas Ca substitution results in high resistivity. As both Cd and Ca are isoelectronic substitutions, the changes in properties are attributed to changes in the electronic structure. Both solid solutions show that the thermal conductivities remain extremely low (∼0.4 W m −1 K −1 ) and that the Seebeck coefficients remain high (>200 μV K −1 ). The temperature dependence of the carrier mobility with increased Ca substitution, changing from approximately T −1 to T −0.5 , suggests that another scattering mechanism is being introduced. As the bonding changes from polar covalent with Yb to ionic for Ca, polar optical phonon scattering becomes the dominant mechanism. Experimental studies of the Cd solid solutions result in a max zT of ∼1 at 800 K and, more importantly for application purposes, a ZT avg ∼ 0.6 from 300 K to 800 K. 

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3. Systematic Trends of Transformation Temperatures and Crystal Structure of Ni–Mn–Ga–Fe–Cu Alloys

   https://doi.org/10.1007/s40830-020-00273-3  

   Armstrong, Andrew ; Nilsen, Frans ; Rames, Michal ; Colman, Ross ; Vertat, Petr ; Kmjec, Tomas ; Straka, Ladislav ; Mullner, Peter ; Heczko, Oleg ( March 2020 , Shape memory and superelasticity)

   Here we report a systematic research on effects of Fe and Cu upon properties relevant for the magnetic shape memory effect of Ni–Mn–Ga ferromagnetic shape memory alloys. Fe and Cu were identified as elements with potential synergism to increase the martensite transformation temperature of Ni–Mn–Ga magnetic shape memory (MSM) alloys. Eighteen Ni–Mn–Ga–Fe–Cu alloys with different systematic trends in substituting the ternary elements with Cu and Fe have been investigated. We found a method to describe the effectiveness of Ni, Mn, and Cu upon raising the martensitic transformation temperature, lowering the saturation magnetization, and varying the Curie temperature. We find the martensite transformation temperature most influenced by the Ni content, followed by Mn, with a smaller effect of Cu. The saturation magnetization decreases with similar coefficients for Mn and Cu alloying. The Curie temperature monotonously decreases with Mn, but not Cu. The 10M martensite structure is stable for the composition Ni46.5Mn25?XGa25-X-YFe3.5CuY with X and Y range of 0–5.7, and 0.8–3.0. Used in combination with the total e/a, the elemental e/a-ratio gives some insight into the complex behavior of quinary MSM alloys and is a useful method of analyzing MSM alloys for improved functional properties. 

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4. Experimental and theoretical evidence for hydrogen doping in polymer solution-processed indium gallium oxide

   https://doi.org/10.1073/pnas.2007897117  

   Huang, Wei ; Chien, Po-Hsiu ; McMillen, Kyle ; Patel, Sawankumar ; Tedesco, Joshua ; Zeng, Li ; Mukherjee, Subhrangsu ; Wang, Binghao ; Chen, Yao ; Wang, Gang ; et al ( July 2020 , Proceedings of the National Academy of Sciences)

   The field-effect electron mobility of aqueous solution-processed indium gallium oxide (IGO) thin-film transistors (TFTs) is significantly enhanced by polyvinyl alcohol (PVA) addition to the precursor solution, a >70-fold increase to 7.9 cm²/Vs. To understand the origin of this remarkable phenomenon, microstructure, electronic structure, and charge transport of IGO:PVA film are investigated by a battery of experimental and theoretical techniques, including In K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS); resonant soft X-ray scattering (R-SoXS); ultraviolet photoelectron spectroscopy (UPS); Fourier transform-infrared (FT-IR) spectroscopy; time-of-flight secondary-ion mass spectrometry (ToF-SIMS); composition-/processing-dependent TFT properties; high-resolution solid-state¹H,⁷¹Ga, and¹¹⁵In NMR spectroscopy; and discrete Fourier transform (DFT) analysis with ab initio molecular dynamics (MD) liquid-quench simulations. The⁷¹Ga{¹H} rotational-echo double-resonance (REDOR) NMR and other data indicate that PVA achieves optimal H doping with a Ga···H distance of ∼3.4 Å and conversion from six- to four-coordinate Ga, which together suppress deep trap defect localization. This reduces metal-oxide polyhedral distortion, thereby increasing the electron mobility. Hydroxyl polymer doping thus offers a pathway for efficient H doping in green solvent-processed metal oxide films and the promise of high-performance, ultra-stable metal oxide semiconductor electronics with simple binary compositions.

    

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5. 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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