Chemical Bonding in Epitaxial ZrB2 Studied by X-ray Spectroscopy

The chemical bonding in an epitaxial ZrB2 film is investigated by Zr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies and compared to the ZrB2 compound target from which the film was synthesized as well as a bulk {\alpha}-Zr reference. Quantitative analysis of X-ray Photoelectron Spectroscopy spectra reveals at the surface: ~5% O in the epitaxial ZrB2 film, ~19% O in the ZrB2 compound target and ~22% O in the bulk {\alpha}-Zr reference after completed sputter cleaning. For the ZrB2 compound target, X-ray diffraction (XRD) shows weak but visible -111, 111, and 220 peaks from monoclinic ZrO2 together with peaks from ZrB2 and where the intensity distribution for the ZrB2 peaks show a randomly oriented target material. For the bulk {\alpha}-Zr reference no peaks from any crystalline oxide were visible in the diffractogram recorded from the 0001-oriented metal. The Zr K-edge absorption from the two ZrB2 samples demonstrate more pronounced oscillations for the epitaxial ZrB2 film than in the bulk ZrB2 attributed to the high atomic ordering within the columns of the film. The XANES exhibits no pre-peak due to lack of p-d hybridization in ZrB2, but with a chemical shift towards higher energy of 4 eV in the film and 6 eV for the bulk compared to {\alpha}-Zr (17.993 keV) from the charge-transfer from Zr to B. The 2 eV larger shift in bulk ZrB2 material suggests higher oxygen content than in the epitaxial film, which is supported by XPS. In EXAFS, the modelled cell-edge in ZrB2 is slightly smaller in the thin film (a=3.165 {\AA}, c=3.520 {\AA}) in comparison to the bulk target material (a=3.175 {\AA}, c=3.540 {\AA}) while in hexagonal closest-packed metal ({\alpha}-phase, a=3.254 {\AA}, c=5.147 {\AA}).Comment: 15 pages, 5 Figures, 4 table

Reactive magnetron sputtering of tungsten target in krypton/trimethylboron atmosphere

W-B-C films were deposited on Si(100) substrates held at elevated temperature by reactive sputtering from a W target in Kr/trimethylboron (TMB) plasmas. Quantitative analysis by X-ray photoelectron spectroscopy (XPS) shows that the films are W-rich between ~ 73 and ~ 93 at.% W. The highest metal content is detected in the film deposited with 1 sccm TMB. The C and B concentrations increase with increasing TMB flow to a maximum of ~18 and ~7 at.%, respectively, while the O content remains nearly constant at 2-3 at.%. Chemical bonding structure analysis performed after samples sputter-cleaning reveals C-W and B-W bonding and no detectable W-O bonds. During film growth with 5 sccm TMB and 500 oC or with 10 sccm TMB and 300-600 oC thin film X-ray diffraction shows the formation of cubic 100-oriented WC1-x with a possible solid solution of B. Lower flows and lower growth temperatures favor growth of W and W2C, respectively. Depositions at 700 and 800 oC result in the formation of WSi2 due to a reaction with the substrate. At 900 oC, XPS analysis shows ~96 at.% Si in the film due to Si interdiffusion. Scanning electron microscopy images reveal a fine-grained microstructure for the deposited WC1-x films. Nanoindentation gives hardness values in the range from ~23 to ~31 GPa and reduced elastic moduli between ~220 and 280 GPa in the films deposited at temperatures lower than 600 oC. At higher growth temperatures the hardness decreases by a factor of 3 to 4 following the formation of WSi2 at 700-800 oC and Si-rich surface at 900 oC.Comment: 14 pages, 8 figure

Panel Three: The Roles of Juries and the Press in the Modern Judicial System

Reactive sputtering by high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS) of a Zr target in Ar/H-2 plasmas was employed to deposit Zr-H films on Si(100) substrates, and with H content up to 61 at.% and O contents typically below 0.2 at.% as determined by elastic recoil detection analysis. X-ray photoelectron spectroscopy reveals a chemical shift of similar to 0.7 eV to higher binding energies for the Zr-H films compared to pure Zr films, consistent with a charge transfer from Zr to H in a zirconium hydride. X-ray diffraction shows that the films are single-phase delta-ZrH2 (CaF2 type structure) at H content greater thansimilar to 55 at.% and pole figure measurements give a 111 preferred orientation for these films. Scanning electron microscopy cross-section images show a glasslike microstructure for the HiPIMS films, while the DCMS films are columnar. Nanoindentation yield hardness values of 5.5-7 GPa for the delta-ZrH2 films that is slightly harder than the similar to 5 GPa determined for Zr films and with coefficients of friction in the range of 0.12-0.18 to compare with the range of 0.4-0.6 obtained for Zr films. Wear resistance testing show that phase-pure delta-ZrH2 films deposited by HiPIMS exhibit up to 50 times lower wear rate compared to those containing a secondary Zr phase. Four-point probe measurements give resistivity values in the range of similar to 100-120 mu Omega cm for the delta-ZrH2 films, which is slightly higher compared to Zr films with values in the range 70-80 mu Omega cm

ZrB2 Thin Films : Growth and Characterization

Zirconium diboride, ZrB2, is a ceramic material with bulk properties such as high melting point (3245 °C), high hardness (23 GPa), and low resistivity (~8 μΩcm). Thin film growth of ZrB2 using physical vapor deposition has suffered from problems with films deviating from stoichiometry and with high levels of contaminants, especially high oxygen content. The homogeneity range of ZrB2 is very narrow, and consequently it is vital to achieve the correct stoichiometry to grow films with high crystalline order. This thesis describes a direct current magnetron sputtering process to grow stoichiometric ZrB2 thin films with a low degree of impurities. Growth of epitaxial ZrB2 films was achieved on 4H-SiC(0001), Si(111) and Al2O3(0001) substrates. The effect of deposition temperature and power applied on the sputtering target was investigated and showed that high power density (8.77 Wcm-2) and high temperature (900 °C) resulted in films with the best composition and the highest crystal quality. ZrB2 films on GaN(0001) templates exhibit an amorphous layer at the film-substrate interface and the resulting films are either polycrystalline or textured. Resistivity measurements showed that the ZrB2 thin films exhibit typical resistivity values of ~100-250 μΩcm and that the resistivity decreased with increasing deposition temperature. Nanoindentation was applied to assess the mechanical properties of the films. The epitaxial ZrB2 films exhibit high elastic recovery and a hardness of ~45-50 GPa, twice as high as the literature bulk value. In addition, evaluation of the mechanical properties was performed at high temperatures of up to 600 °C and showed that the epitaxial films retained a higher hardness, compared to textured ZrB2 films and bulk, also at these temperatures

Growth and Characterization of ZrB2 Thin Films

In this thesis, growth of ZrB2 thin films by direct current magnetron sputtering is investigatedusing a high vacuum industrial scale deposition system and an ultra-high vacuum laboratory scalesystem. The films were grown from ZrB2 compound targets at temperatures ranging from ambient (without external heating) to 900 °C and with substrate biases from -20 to -120 V. Short deposition times of typically 100 or 300 s and high growth rates of 80-180 nm/min were emphasized to yield films with thicknesses of 300-400 nm. The films were characterized by thinfilm X-ray diffraction with the techniques θ/2θ and ω scans, pole figure measurements andreciprocal space mapping, scanning and transmission electron microscopy, elastic recoil detection analysis and four point probe measurements. The substrates applied were Si(100), Si(111),4H-SiC(0001) and GaN(0001) epilayers grown on 4H-SiC. The Si(111), 4H-SiC(0001) substrates and GaN(0001) epilayers were chosen given their small lattice mismatches to ZrB2 making them suitable for epitaxial growth.The films deposited in the industrial system were found to be close to stoichiometric with a low degree of contaminants, with O being the most abundant at a level of < 1 at.%. Furthermore, the structure of the films is temperature dependent as films deposited in this system without external heating are fiber textured with a 0001-orientation while the films deposited at 550 °C exhibitrandom orientation. In contrast, epitaxial growth was demonstrated in the laboratory scale system on etched 4H-SiC(0001) and Si(111) deposited at 900 °C following outgassing of the substrates at 300 °C and in-situ heat treatment at the applied growth temperature to remove the native oxides. However, films grown on GaN(0001) were found to be 0001 textured at the applied deposition conditions, which make further studies necessary to enable epitaxial growth on this substrate material. Four point probe measurements on the films deposited in the industrial system show typical resistivity values ranging from ˜95 to 200 μΩcm with a trend to lower values for the films deposited at higher temperatures and at higher substrate bias voltages

beta-Ta and alpha-Cr thin films deposited by high power impulse magnetron sputtering and direct current magnetron sputtering in hydrogen containing plasmas

Thin films of beta-Ta and alpha-Cr were deposited on Si(1 0 0) and 1000 angstrom SiO2/Si(1 0 0), by high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (dcMS) in hydrogen-containing plasmas. The films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction, scanning electron microscopy, elastic recoil detection analysis, and four-point probe measurements. The results showed that 001-oriented beta-Ta films containing up to similar to 8 at% hydrogen were obtained with HiPIMS, albeit with no chemical shift evident in XPS. The 110 oriented alpha-Cr films display a hydrogen content less than the detection limit of 1 at%, but H-2 favors the growth of high-purity films for both metals. The beta-Ta films deposited with dcMS are columnar, which seems independent of H-2 presence in the plasma, while the films grown by HIPIMS are more fine-grained. The latter type of microstructure was present for the alpha-Cr films and found to be independent on choice of technique or hydrogen in the plasma. The beta-Ta films show a resistivity of similar to 140-180 mu Omega cm, while alpha-Cr films exhibit values around 30 mu Omega cm; the lowest values obtained for films deposited by HiPIMS and with hydrogen in the plasma for both metals

Atom probe tomography field evaporation characteristics and compositional corrections of ZrB2

The microstructure of stoichiometric ZrB2.0 and B over-stoichiometric ZrB2.5 thin films has been studied using atom probe tomography (APT), X-ray diffraction, and transmission electron microscopy. Both films consist of columnar ZrB2 grains with AlB2-type crystal structure. The narrow stoichiometry range of ZrB2 results in the presence of separate disordered B-rich boundaries even in ZrB2.0. At higher average B content, specifically ZrB2.5, the formation of a continuous network around the sides of the ZrB2 columns is promoted. In addition, the APT field evaporation characteristics of ZrB2 and its influence on the measured local composition has been studied and compared to the average composition from elastic recoil detection analysis (ERDA). Differences in the measured average compositions of the two techniques are explained by the APT detector dead-time/space. A new pile-up pairs correction procedure based on co-evaporation correlation data was thus employed here for the APT data and compared with the 10B-method (the B equivalence of the 13C-method), as well as the combination of both methods. In ZrB2.0, all of the applied compositional correction methods were found to reduce the compositional difference when appropriate isotopic abundances were used. In ZrB2.5, the inhomogeneity of the film likely increased the local APT composition to such an extent that even conservative correction procedures overestimated the B content compared to the ERDA reference. The strengths of the pile-up pairs correction compared the 10B and the combined methods are higher precision, due to it being less dependent on the accuracy of estimated isotopic abundances, and that the correction itself is not dependent on careful background correction of the mass spectrum

Cubic boron phosphide epitaxy on zirconium diboride

Cubic boron phosphide (BP) is one of the least studied III-V compound semiconductors, in part because it is difficult to prepare in high quality form. In this study, zirconium diboride (ZrB2) was studied as a potential substrate for BP epitaxial layers, because of its advantages of a low lattice constant mismatch and high thermal stability. Two types of substrates were considered: ZrB2(0001) epitaxial films on 4H-SiC (0001) and bulk ZrB2(0001) single crystals. The optimal temperature for epitaxy on these substrates was 1100 degrees C; higher and lower temperatures resulted in polycrystalline films. The BP film/ZrB2 interface was abrupt as confirmed by cross-sectional transmission electron microscopy, attesting to the stability of ZrB2 under BP deposition conditions. The BP films were under compressive and tensile strain on ZrB2 and ZrB2/4H-SiC substrates, respectively, as determined by Raman spectroscopy, due to differences in the substrate/film coefficients of thermal expansion. This study suggests that with further optimization, ZrB2 can be an excellent substrate for BP epitaxial films. (C) 2017 Elsevier B.V. All rights reserved.Funding Agencies|Department of Energy [GEGF001846]; Swedish Research Council (VR) [621-2010-3921]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]</p