Nanostructure and bonding of zirconium diboride thin films studied by X-ray spectroscopy

Abstract

Zirconium diboride (ZrB 2 ) is an important ceramic due to its extremely high melting temperature of 3245 • C and metallic electrical conductivity, properties that make it an ideal candidate thin film electrode material for high temperature electronics. In this report, thin films of varying B:Zr ratio ranging from 3-0.67 have been grown by e-beam evaporation from elemental sources. X-ray absorption spectra at the Zr K-edge were measured before and after annealing in ultra-high vacuum for 9 hours at 1000 • C. Films with compositions near ZrB 2 stoichiometry show x-ray absorption fine structure that can be well modeled by crystalline ZrB 2 with a small portion of a coexisting tetragonal zirconia (t-ZrO 2 ) phase. Films far from stoichiometry show substantial disorder beyond the nearest-neighbor distances, and after vacuum annealing exhibit high levels of oxidation. Contributions to the x-ray absorption fine structure from a pure Zr phase are very small compared to ZrB 2 and t-ZrO 2 phases. The fact that nearly stoichiometric (3 < B : Zr < 1.6) as-deposited amorphous films form the same crystalline ZrB 2 nanostructure after annealing is particularly encouraging for high temperature thin film electronics applications, because it would allow the production of highly stable electrodes with e-beam evaporation without the need of any high temperature heating during film growth.

Chemical bonding in epitaxial ZrB2 studied by X-ray spectroscopy

“…Comparing our recorded XANES spectra to those of Bösenberg et al [15] and Stewart et al [16], we find the same spectral features and peak shapes. Metallic Zr exhibit a main doublepeak while ZrB2 has a rather deep minimum as indicated by the arrows in Figure 4.…”

“…The high energy shift determined from the first derivative of the absorption edge (marked by the arrows) in comparison to pure α-Zr 17.994 keV (ref: 17.99273 keV) is 4 eV for the film and 6 eV for bulk. The energy shift is most pronounced for the bulk ZrB2 target reference, which is related to a higher oxygen content as supported by Stewart et al [16] for annealed e-beam co-evaporated thin films. We further note that an even larger shift of 10 eV was observed by Bösenberg et al in their investigated ZrB2 powder [15].…”

“…Contrary to a-Zr, the main peak of the ZrO2 reference spectrum from A. Nozaki et al [41] has a broad and more intense single-peak shape that occur at higher energy and does not appear in the spectrum of the epitaxial ZrB2 film. This shows that our film is of higher purity compared to ZrB2 materials investigated in previous studies [15,16]. Figure 4 displays EXAFS structure factor oscillations of the epitaxial ZrB2 film, the ZrB2 compound target from which the film was synthesized, and the a-Zr bulk reference, obtained from raw data that has not been phase shifted.…”

“…Table IV shows the results of the EXAFS fitting using the FEFF scattering paths of ZrB2, a-Zr and ZrO2 as model systems. Note that the EXAFS data of the bulk ZrB2 and a-Zr samples cannot be fitted without considering superimposed ZrO2 in the modelling as in the case of Stewart et al [16] where tetragonal ZrO2 was applied. In our study, the XRD pattern recorded from the ZrB2 compound target in Fig.…”

ZrB2 Thin Films : Growth and Characterization

“…ZrB2 thin films have been grown by CVD [6,7,11,[18][19][20][21][22][23][24], pulsed laser deposition [25,26], e-beam deposition [26][27][28], sputtering of reactive multilayers [29], and sputtering from a compound source [30][31][32][33][34][35][36][37][38][39][40][41][42][43].…”