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1. Comparison of Hafnium Dioxide and Zirconium Dioxide Grown by Plasma-Enhanced Atomic Layer Deposition for the Application of Electronic Materials

   https://doi.org/10.3390/cryst10020136  

   Xiao, Zhigang; Kisslinger, Kim; Chance, Sam; Banks, Samuel (February 2020, Crystals)

   We report the growth of nanoscale hafnium dioxide (HfO2) and zirconium dioxide (ZrO2) thin films using remote plasma-enhanced atomic layer deposition (PE-ALD), and the fabrication of complementary metal-oxide semiconductor (CMOS) integrated circuits using the HfO2 and ZrO2 thin films as the gate oxide. Tetrakis (dimethylamino) hafnium (Hf[N(CH3)2]4) and tetrakis (dimethylamino) zirconium (IV) (Zr[N(CH3)2]4) were used as the precursors, while O2 gas was used as the reactive gas. The PE-ALD-grown HfO2 and ZrO2 thin films were analyzed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The XPS measurements show that the ZrO2 film has the atomic concentrations of 34% Zr, 2% C, and 64% O while the HfO2 film has the atomic concentrations of 29% Hf, 11% C, and 60% O. The HRTEM and XRD measurements show both HfO2 and ZrO2 films have polycrystalline structures. n-channel and p-channel metal-oxide semiconductor field-effect transistors (nFETs and pFETs), CMOS inverters, and CMOS ring oscillators were fabricated to test the quality of the HfO2 and ZrO2 thin films as the gate oxide. Current-voltage (IV) curves, transfer characteristics, and oscillation waveforms were measured from the fabricated transistors, inverters, and oscillators, respectively. The experimental results measured from the HfO2 and ZrO2 thin films were compared. 

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2. Carbides and Nitrides of Zirconium and Hafnium

   https://doi.org/10.3390/ma12172728  

   Ushakov, Sergey V.; Navrotsky, Alexandra; Hong, Qi-Jun; van de Walle, Axel (September 2019, Materials)

   Among transition metal carbides and nitrides, zirconium, and hafnium compounds are the most stable and have the highest melting temperatures. Here we review published data on phases and phase equilibria in Hf-Zr-C-N-O system, from experiment and ab initio computations with focus on rocksalt Zr and Hf carbides and nitrides, their solid solutions and oxygen solubility limits. The systematic experimental studies on phase equilibria and thermodynamics were performed mainly 40–60 years ago, mostly for binary systems of Zr and Hf with C and N. Since then, synthesis of several oxynitrides was reported in the fluorite-derivative type of structures, of orthorhombic and cubic higher nitrides Zr3N4 and Hf3N4. An ever-increasing stream of data is provided by ab initio computations, and one of the testable predictions is that the rocksalt HfC0.75N0.22 phase would have the highest known melting temperature. Experimental data on melting temperatures of hafnium carbonitrides are absent, but minimum in heat capacity and maximum in hardness were reported for Hf(C,N) solid solutions. New methods, such as electrical pulse heating and laser melting, can fill the gaps in experimental data and validate ab initio predictions. 

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3. HF-Free synthesis of colloidal Cs ₂ ZrF ₆ and (NH ₄ ) ₂ ZrF ₆ nanocrystals

   https://doi.org/10.1039/d3cc00374d  

   Tzanetopoulos, Eden; Schwartz, Julie; Gamelin, Daniel R. (May 2023, Chemical Communications)

   A solution-phase synthesis of colloidally stable A 2 BF 6 nanocrystals is reported for the first time, focusing on A + = Cs + , NH 4 + and B 4+ = Zr 4+ . Handling hypertoxic HF is avoided by using NH 4 F and a low-boiling-point alcohol, representing the first synthesis of any A 2 BF 6 nanocrystals without HF addition. The chemical incompatability of Zr 4+ with other common fluoride sources is discussed. 

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4. Crystal structure of tetrakis(1,1,1,5,5,5-hexafluoroacetylacetonato)hafnium(IV)

   https://doi.org/10.1107/S2056989018010514  

   Schwandt, W. Franklin; Woods, Toby J.; Girolami, Gregory S. (August 2018, Acta Crystallographica Section E Crystallographic Communications)

   The crystal structure of the title compound, [Hf(C 5 HF 6 O 2 ) 4 ], has been determined. The asymmetric unit contains two Hf(hfac) 4 molecules (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate); both are located on general positions and have identical structures apart from the disorder involving three CF 3 groups in one of the two molecules. The molecules of Hf(hfac) 4 are arranged in layers that are parallel to the ab plane, and the coordination geometry of each hafnium(IV) center is a distorted square antiprism. An interesting aspect of the structure is that the hfac ligands are arranged so that the Hf(hfac) 4 molecules have idealized 2 point symmetry, in which two of the hfac groups bridge between the two squares. Although all other M (β-diketonate) 4 compounds of Hf (and Zr) also have square-antiprismatic geometries; in almost all of them the ligands are arranged so that the molecules have 222 point symmetry (in which none of the hfac ligands bridges between the two squares). The factors that favor one structure over another are not clear. 

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5. Synthesis, dynamics and redox properties of eight-coordinate zirconium catecholate complexes

   https://doi.org/10.1039/d0dt02487b  

   Do, Thomas H.; Brown, Seth N. (August 2020, Dalton Transactions)

   Reaction of the 9,9-dimethylxanthene-bis(imine)-bis(catechol) ligand XbicH 4 with half an equivalent of Zr(acac) 4 affords the neutral tetracatecholate complex (XbicH 2 ) 2 Zr, containing four iminium ions hydrogen bonded to the catecholates. The heteroleptic bis(catecholate)-tetraphenylporphyrin complex (TPP)Zr(XbicH 2 ) is formed from reaction of (TPP)Zr(OAc) 2 with XbicH 4 in the presence of base. Both compounds adopt an eight-coordinate square antiprismatic geometry around the zirconium center. NMR spectra of (TPP)Zr(XbicH 2 ) show that it is fluxional at room temperature, with homoleptic (XbicH 2 ) 2 Zr showing fluxionality at higher temperatures. Calculations and kinetic isotope effect measurements suggest that the motions involve dissociation of a single catecholate oxygen and subsequent twisting of the seven-coordinate species. The compounds show reversible one-electron oxidations of each of the bound catecholates to bound semiquinones. 

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