Hafnium oxide (HfO 2 ) is one of the most promising high-k materials to replace SiO 2 as a gate dielectric. Here we report material and electrical characterization of atomic layer deposition ͑ALD͒ hafnium oxide and the correlations between the results. The HfO 2 films were deposited at 200, 300, or 370°C and annealed in a nitrogen ambient at 550, 800, and 900°C. Results indicate that deposition temperature controls both the material and the electrical properties. Materials and electrical properties of films deposited at 200°C are most affected by annealing conditions compared to films deposited at higher temperatures. These films are amorphous as deposited and become polycrystalline after 800°C anneals. Voids are observed after a 900°C anneal for the 200°C deposited films. The 200°C deposited films have charge trapping and high leakage current following anneals at 900°C. The 300°C deposited films have lower chlorine content and remain void-free following high-temperature anneals. These films show a thickness-dependent crystal structure. Annealing the films reduces leakage current by four orders of magnitude. Finally, films deposited at 370°C have the highest density, contain the least amount of impurities, and contain more of the monoclinic phase of HfO 2 than those deposited at 300 and 200°C. The best electrical performance was obtained for films deposited at 370°C.
One of the goals in understanding any new class of superconductors is to search for commonalities with other known superconductors. The present work investigates the superconducting condensation energy, U, in the iron based superconductors (IBS), and compares their U with a broad range of other distinct classes of superconductor, including conventional BCS elements and compounds and the unconventional heavy Fermion, Sr2RuO4, Li0.1ZrNCl, -(BEDT-TTF)2Cu(NCS)2 and optimally doped cuprate superconductors. Surprisingly, both the magnitude and Tc dependence (UTc 3.4±0.2 ) of U are -contrary to the previously observed behavior of the specific heat discontinuity at Tc, C, -quite similar in the IBS and BCS materials for Tc>1.4 K. In contrast, the heavy Fermion superconductors' U vs Tc are strongly (up to a factor of 100) enhanced above the IBS/BCS while the cuprate superconductors' U are strongly (factor of 8) reduced. However, scaling of U with the specific heat (or C) brings all the superconductors investigated onto one universal dependence upon Tc. This apparent universal scaling U/ Tc 2 for all superconductor classes investigated, both weak and strong coupled and both conventional and unconventional, links together extremely disparate behaviors over almost seven orders of magnitude for U and almost three orders of magnitude for Tc. Since U has not yet been explicitly calculated beyond the weak coupling limit, the present results can help direct theoretical efforts into the medium and strong coupling regimes.
Co-doped BaFe 2 As 2 has been previously shown to have an unusually significant improvement of T c (up to 2 K, or almost 10%) with annealing 1-2 weeks at 700 or 800 o C, where such annealing conditions are insufficient to allow significant atomic diffusion. While confirming similar behavior in optimally Co-doped SrFe 2 As 2 samples, the influence on T c of strain induced by grinding to ~50 μ sized particles, followed by pressing the powder into a pellet using 10 kbar pressure, was found to increase the annealed transition width of 1.5 K by approximately a factor of ten. Also, the bulk discontinuity in the specific heat at T c , ΔC, on the same pellet sample was completely suppressed by grinding. This evidence for a strong sensitivity of superconductivity to strain was used to optimize single crystal growth of Co-doped BaFe 2 As 2 . This strong dependence (both positive via annealing and negative via grinding) of superconductivity on strain in these two iron based 122 structure superconductors is compared to the unconventional heavy Fermion superconductor UPt 3 , where grinding is known to completely suppress superconductivity, and to recent reports of strong sensitivity of T c to damage induced by electron-irradiationinduced point defects in other 122 structure iron-based superconductors, Ba(Fe 0.76 Ru 0.24 ) 2 As 2 and Ba 1-x K x Fe 2 As 2 . Both the electron irradiation and the introduction of strain by grinding are believed to only introduce non-magnetic defects, and argue for unconventional superconducting pairing.
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