Epitaxial thin films of group 4 transition metal diborides

• Group 4 transition metal diboride films were deposited epitaxially onto different substrates, typically at 900 °C, using direct-current magnetron sputtering from compound targets of ZrB2, TiB2, and HfB2. Epitaxial ZrB2 has been deposited on Al2O3(0001), 4H-SiC(0001), and Si(100) describing the epitaxial relationships on Si(100) for the first time, where two relationships were observed from pole-figure analysis, namely A) in-plane: ZrB2[001] ∥ Si[110] and ZrB2[110] ∥ Si[110], out-of-plane: ZrB2(100) ∥ Si(100), and B) in-plane: ZrB2[11] ∥ Si[110] and the same rotated 90° around the 102 axis, out-of- plane: ZrB2(102) ∥ Si(100). Composition analysis by time-of-flight elastic recoil detection analysis revealed typically B-rich ZrB2 and TiB2, and stoichiometric to Hf-rich HfB2. For ZrB2, the application of an additional external magnetic field during growth influenced the B-to-Zr ratio towards being stoichiometric. Rocking curve measurements of ZrB2 deposited onto Si(100) reveal a higher crystal order in the 100-oriented domains, compared to the 102-oriented domains. In ZrB2 films annealed to temperatures in the range of 1100-1500 °C, rocking curve measurements of the symmetric 001 reflection as well as the asymmetric 101 reflection reveal increased order with increased temperature. This phenomenon occurred at lower temperature when the annealing was performed in H2 compared to Ar.The morphology in plan-view transmission electron microscopy reflects the composition of the film: TiB2.5 has B-rich areas around the grain boundaries, forming an almost continuous network around the grains. ZrBx films with x between 2.0 and 2.3 also contain B-rich regions, though to a smaller extent and mostly in areas where more than two grains adjoin each other. In Hf-rich HfB1.8, no B-rich areas were observed. Scanning transmission electron micrographs and a combination of B electron energy loss spectroscopy and energy dispersive X-ray analysis Hf distribution maps revealed Hf-rich areas in a close-to single crystalline matrix. The hardness of epitaxial HfB2 films is reported to 33 and 36 GPa for films deposited onto Al2O3 and SiC, respectively. These values are slightly higher than reported for bulk HfB2. ZrB2 films on SiC decrease in hardness to 38, 37, and 30 GPa, upon annealing in Ar up to 1100, 1300, and 1500 °C, respectively. In summary, the knowledge is expanded about epitaxially grown group 4 transition metal diborides in terms of their Chemical composition, crystallographic orientations, microstructure, electrical and mechanical properties, as well as their response to heat treatment.

Subject headings

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

Publication and Content Type

vet (subject category)

dok (subject category)