Frank Batten College of Engineering & Technology Old Dominion University: Pulsed Laser Deposition of Niobium Nitride Thin Films APPLIED RESEARCH CENTER Ingot Niobium Summary Workshop December 4, 2015 Ashraf H. Farha 1, 2, 3, Yüksel Ufuktepe 4, Ganapati Myneni 5 and Hani E. Elsayed-Ali 1,2 1 Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, VA 23529, USA 2 Applied Research Center, Newport News, VA 23606, USA 3 Department of Physics, Ain Shams University, Cairo 11566, Egypt 4 Department of Physics, University of Cukurova, Adana, Turkey 5 Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606

The phase diagram of niobium nitride is complex Crystallographic structures of NbN x : (a) cubic B1, (b) hexagonal Bi, (c) Tetragonal and (d) hexagonal of Nb 2 N. The bigger, dark blue spheres correspond to the metallic Nb sites; the smaller spheres represent N atoms, while the white corresponds to vacancy.

Frank Batten College of Engineering & Technology Old Dominion University: NbN x is grown on ingot Nb by reactive pulsed lase deposition Target: % Niobium Laser: Nd:YAG (wavelength 1064 nm, pulse width 40 ns, 10 Hz, 15 J/cm 2 laser fluence ) Base pressure of ~1×10 -9 Torr Nitrogen background pressure  500 mTorr, Substrate temperature  950 o C

Nitrogen background pressure effect At 10.7 Pa (80 mTorr) mainly β-Nb 2 N was observed with weak peaks due to hexagonal δ´-NbN at o (0 0 1), o (1 0 1) and o (1 1 0) For 13.4, 26.7 Pa, a cubic δ-NbN with mixture of hexagonal β-Nb 2 N For 40.0, 66.7 Pa (500 mTorr), a single-phase hexagonal β-Nb 2 N Over pressure range studied, higher nitrogen pressure reduces the N content of the NbN x film due to lower kinetic energies of ablated species and increase in the recombination rate Substrate cleaned at 900 °C Growth temperature 600 °C Laser energy density 15 J/cm 2 Films is ~120 nm Deposition rate ~ 2–3 nm/min Nb substrate was etched by the buffered chemical polishing (BCP) method (HPO 3 :HNO 3 :HF) cooled to 10 °C

Nitrogen background pressure effect Through EDX analysis and phase concentrations from XRD, the N:Nb ratio in the cubic δ-NbN phase was determined to be 0.95±0.03 to 1.19±0.02, and in the hexagonal Nb 2 N phase to be between 0.47± 0.02 to 0.53±0.02 The decrease in surface roughness at 26.7 Pa is related to the phase change of NbN x film. Otherwise, an increase in the surface roughness is expected when the N 2 background pressure is increased.

Substrate temperature effect For a substrate temperature up to 450 o C the film shows poor crystalline quality. With temperature increase the film becomes textured and for a substrate temperature 650  850 o C, mix of cubic δ-NbN and hexagonal phases (  -Nb 2 N + δ-NbN) are formed. Substrate temperature 950 o C results in the formation of  -Nb 2 N films. 100 mTorr (13.4 Pa) nitrogen background Laser energy density ∼ 15 J/cm 2 Nitride growth by heating the substrate in 100 mTorr nitrogen for 1 hr was checked and found not to affect the reported results

Substrate temperature effect Topographic AFM images of films grown at (a) 450, (b) 650, (c) 750, and (d) 850 °C RMS film roughness increased with the substrate temperature

Laser fluence effect Nitrogen background pressure 150 mTorr Substrate temperature 600 o C ● For 8 J/cm 2 film showed mostly β-Nb 2 N phase and weak reflection of δ-NbN hexagonal phase ● For 15 J/cm 2 film has mixed (cubic + hexagonal) phase of NbN x ● Film became pure hexagonal with increasing laser fluence. EDX measurement of N:Nb ratio in NbN x films

High-resolution transmission electron microscopy shows polycrystalline NbN film of 15 nm thickness grown on Si(100) at 800 o C Cross-sectional TEM image showing 15 nm NbN thin film on Si substrate.

Atomic force microscopy (AFM) images of films show island structure 200 mTorr 500 mTorr AFM image of film grown at 200 mTorr consists of triangular islands of nm sizes and heights of 15 nm. For nitrogen pressure of 500 mTorr, the size of islands increased.

X-ray diffraction of the NbN thin films XRD scan of NbN film deposited on Si substrate showing mainly textured cubic δ-NbN with tetragonal phase showing at the higher pressures. Graphite-monochromated CuK α radiation on a Bruker-AXS three-circle diffractometer, equipped with a SMART Apex II CCD detector

X-ray photoemission spectroscopy used for electronic structure analysis XPS spectra of Nb 3d core levels for NbN films. Binding energies are given with respect to the Fermi level. A strong pair of peaks due to Nb 3d3/2 and 3d 5/2 doublets are observed. Comparing NbN film with pure Nb spectra (205.5 and eV), the 3d 5/2 peak is shifted to higher binding energies as a result of Nb-N bonding, indicating the transfer of electrons from niobium to nitrogen. Background N 2 pressure (mTorr) Nb 3d 5/2 (± 0.05) (eV) Nb 3d 3/2 (± 0.05) (eV) Nb mm radius hemispherical photoelectron analyzer (VG Scienta SES-100) with Mg K  X-ray radiation (h  = eV) XPS spectra of Nb 3d core levels for NbN x films, Binding energies are given with respect to the Fermi level

Superconductivity of NbN x films T c increased from 7.66 to K by varying the nitrogen background pressure from 26.7 to 66.7 Pa while resistivity measured at 20 K increases from 60 x to 120 x Ohm-cm. For deposition at 66.7 Pa nitrogen, the film had mixed phases of δ-NbN and γ-Nb 4 N 3 with reduced vacancies. The lattice parameter is very close to the bulk (4.393 Å) of fcc δ-NbN which favors higher T c.

Summary  Reactive PLD of NbN x on Si(100) yields NbN films with highest T c of K at 500 mTorr (66.7 Pa) N 2 pressure, substrate held at 800 o C.  Varying laser fluence over 8  40 J/cm 2 ( N 2 20 Pa and substrate temperature 600 o C), the surface roughness, nitrogen content, and grain size increase with the laser fluence. The NbN x layers are formed in mixed phase (cubic and hexagonal). The ratio of hexagonal phase to cubic phase is strongly dependent on the laser fluence becoming pure hexagonal (β-Nb 2 N) at the higher flunces.  NbN x films were grown at different N 2 background pressures 10.7  66.7 Pa (laser fluence 15 J/cm 2, substrate temperature 600 °C). At low N 2 pressures both hexagonal (β-Nb 2 N) and cubic (δ-NbN) phases were formed. As N 2 pressure increased, NbN x films grew in single hexagonal (β-Nb 2 N) phase.  NbN x films were grown from RT  950 °C (N 2 pressure 13.3 Pa and laser fluence 15 J/cm 2 ). NbN x films with mixed cubic (δ-NbN), hexagonal (  -Nb 2 N), and δ-NbN phases were obtained. Films with a mainly hexagonal (β-Nb 2 N) phase was obtained, as the temperature was increased to 850 °C.