Pulsed Laser Deposition of Niobium Nitride Thin Films
Ashraf H. Farha1, 2, 3, Yüksel Ufuktepe4, Ganapati Myneni5 and Hani E. Elsayed-Ali1,2
1Department of Electrical and Computer Engineering, Old Dominion University,
Norfolk, VA 23529, USA
2Applied Research Center, Newport News, VA 23606, USA
3Department of Physics, Ain Shams University, Cairo 11566, Egypt
4Department of Physics, University of Cukurova, 01330 Adana, Turkey
5Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606
APPLIED RESEARCH CENTER
Frank Batten College of
Engineering & Technology
Old Dominion University: www.eng.odu.edu
Ingot Niobium Summary Workshop
December 4, 2015
The phase diagram of niobium nitride is complex
δ-NbN
ε-NbN
-Nb4N3
-Nb2N
0.3
-NbN
-NbN
-Nb2N
0.4
0.5
0.6
0.7
0.8
0.9
-NbN
1
1.1
1.2
N/Nb
(a) Cubic B1,
NaCl-type
structure
(b) Hexagonal (Bi),
TiP-type structure
(c)Tetragonal,
deformed NaCltype structure
(d) Hexagonal ,
Fe2N-type structure
Crystallographic structures of NbNx: (a) cubic B1, (b) hexagonal Bi, (c) Tetragonal and
(d) hexagonal of Nb2N. The bigger, dark blue spheres correspond to the metallic Nb
sites; the smaller spheres represent N atoms, while the white corresponds to vacancy.
NbNx is grown on ingot Nb by reactive pulsed lase deposition
Target: 99.995% Niobium
Laser: Nd:YAG (wavelength 1064 nm, pulse width 40 ns, 10 Hz, 15 J/cm2
laser fluence )
Base pressure of ~1×10-9 Torr Nitrogen background pressure  500 mTorr,
Substrate temperature  950 oC
Frank Batten College of
Engineering & Technology
Old Dominion University: www.eng.odu.edu
substrate
-Nb2N (2 0 1)
-Nb2N (1 1 2)
-Nb2N (1 0 3)
-Nb2N (1 1 0)
substrate
-Nb2N (1 0 2)
substrate
-Nb2N (1 0 0)
66.7 Pa
40.0 Pa
26.7 Pa
-NbN (2 0 0)
-NbN (2 2 0)
20.0 Pa
-NbN (1 1 1)
Substrate cleaned at 900 °C
Growth temperature 600 °C
Laser energy density 15 J/cm2
Films is ~120 nm
Deposition rate ~ 2–3 nm/min
Intensity (arb.units)
Nb substrate was etched by the buffered
chemical polishing (BCP) method
(HPO3:HNO3:HF) cooled to 10 °C
-Nb2N (0 0 2)
Nitrogen background pressure effect
13.4 Pa
10.7 Pa
30
40
50
60
70
80
2 Theta (degree)
At 10.7 Pa (80 mTorr) mainly β-Nb2N was observed with weak peaks due to hexagonal δ´-NbN at 33.22o (0 0
1), 47.94 o (1 0 1) and 62.25 o (1 1 0)
For 13.4, 26.7 Pa, a cubic δ-NbN with mixture of hexagonal β-Nb2N
For 40.0, 66.7 Pa (500 mTorr), a single-phase hexagonal β-Nb2N
Over pressure range studied, higher nitrogen pressure reduces the N content of the NbNx film due to lower
kinetic energies of ablated species and increase in the recombination rate
Nitrogen background pressure effect
66.7 Pa
40.0 Pa
Height (arb. units)
Average roughness (nm)
20
15
10
26.7 Pa
20.0 Pa
13.4 Pa
10.7 Pa
800
2400
1600
Length (nm)
5
0
13
26
39
65
52
Pressure (Pa)
1.3
1.2
NbN
Nb2N
1.1
1.0
0.9
N/Nb
The decrease in surface roughness at 26.7 Pa is
related to the phase change of NbNx film. Otherwise, an
increase in the surface roughness is expected when the
N2 background pressure is increased.
0.8
0.7
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 Nb2N phase to be between 0.47± 0.02 to
0.53±0.02
0.6
0.5
0.4
10
20
30
40
50
Nitrogen pressure (Pa)
60
70
Nb (310)
-Nb2N (104)
-Nb2N (202)
Nb (211)
-Nb2N (112)
-Nb2N (201)
-Nb2N (103)
-Nb2N (110)
Nb (200)
-Nb2N (102)
-Nb2N (002)
O
950 C
O
-NbN (220)
-NbN (200)
850 C
-NbN (111)
For a substrate temperature up to 450 oC
the film shows poor crystalline quality.
With temperature increase the film
becomes textured and for a substrate
temperature 650  850 oC, mix of cubic
δ-NbN and hexagonal phases (-Nb2N +
δ-NbN) are formed.
Substrate temperature 950 oC results in
the formation of -Nb2N films.
Intensity (arb. units)
100 mTorr (13.4 Pa) nitrogen background
Laser energy density ∼15 J/cm2
-Nb2N (100)
Nb (110)
Substrate temperature effect
O
750 C
O
650 C
O
450 C
O
250 C
R.T.
Nb-substrate
40
60
80
2 Theta (degree)
Nitride growth by heating the substrate in 100 mTorr nitrogen for 1 hr was checked and found not to
affect the reported results
100
Substrate temperature effect
RMS Roughness (nm)
30
25
20
15
10
5
400
500
600
700
800
Temperature (°C)
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
900
Laser fluence effect
0.55
40 Jcm
-2
-NbN (220)
-NbN (200)
-2
0.50
N/Nb
-Nb2N (112)
-Nb2N (201)
substrate
2
-Nb N (110)
-Nb2N (103)
substrate
'-NbN (110)
-Nb2N (102)
'-NbN (104)
'-NbN (103)
-Nb2N (002)
EDX measurement of N:Nb ratio in NbNx films
30 Jcm
-NbN (111)
Intensity (arb. units)
-Nb2N (100)
substrate
Nitrogen background pressure 150 mTorr
Substrate temperature 600 oC
0.45
0.40
15 Jcm
-2
0.35
8 Jcm
-2
10
20
30
40
-2
Laser fluence (Jcm )
40
60
80
2 Theta (degree)
● For 8 J/cm2 film showed mostly β-Nb2N phase and weak reflection of δ-NbN hexagonal phase
● For 15 J/cm2 film has mixed (cubic + hexagonal) phase of NbNx
● Film became pure hexagonal with increasing laser fluence.
High-resolution transmission electron microscopy shows polycrystalline
NbN film of 15 nm thickness grown on Si(100) at 800 oC
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 100-200 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
Graphite-monochromated CuKα radiation on a Bruker-AXS three-circle diffractometer,
equipped with a SMART Apex II CCD detector
XRD scan of NbN film deposited on Si substrate showing mainly
textured cubic δ-NbN with tetragonal phase showing at the higher
pressures.
X-ray photoemission spectroscopy used for electronic structure analysis
XPS spectra of Nb 3d core levels for NbNx films, Binding
energies are given with respect to the Fermi level
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
3d5/2 doublets are observed.
Comparing NbN film with pure Nb spectra
(205.5 and 202.3 eV), the 3d5/2 peak is shifted
to higher binding energies as a result of Nb-N
bonding, indicating the transfer of electrons
from niobium to nitrogen.
Background
Nb 3d5/2 (± 0.05)
N2 pressure
(mTorr)
(eV)
Nb 3d3/2 (± 0.05)
200
204.00
206.81
400
204.09
206.92
500
204.08
206.90
Nb
205.50
202.30
(eV)
100-mm radius hemispherical
photoelectron analyzer (VG Scienta
SES-100) with Mg Ka X-ray radiation
(hu = 1253.6 eV)
Superconductivity of NbNx films
Tc increased from 7.66 to 15.07 K by varying the nitrogen background pressure
from 26.7 to 66.7 Pa while resistivity measured at 20 K increases from 60 x 10-3 to
120 x 10-3 Ohm-cm.
16
120
100
Tc

14
0.11
80
60
12
0.09
0.08
10
40
0.07
20
8
0
5
10
15
Temperature (K)
20
25
0.06
20
30
40
50
60
70
Nitrogen pressure (Pa)
For deposition at 66.7 Pa nitrogen, the film had mixed phases of δ-NbN and γ-Nb4N3
with reduced vacancies. The lattice parameter is very close to the bulk (4.393 Å) of
fcc δ-NbN which favors higher Tc.
 (-cm)
0.10
Tc (K)
Resistivity (ohm-cm)
0.12
66.7 Pa
53.3 Pa
26.7 Pa
Summary
 NbNx films were grown at different N2 background pressures 10.7  66.7 Pa
(laser fluence 15 J/cm2, substrate temperature 600 °C). At low N2 pressures both
hexagonal (β-Nb2N) and cubic (δ-NbN) phases were formed. As N2 pressure
increased, NbNx films grew in single hexagonal (β-Nb2N) phase.
 NbNx films were grown from RT  950 °C (N2 pressure 13.3 Pa and laser fluence
15 J/cm2). NbNx films with mixed cubic (δ-NbN), hexagonal (-Nb2N), and δ-NbN
phases were obtained. Films with a mainly hexagonal (β-Nb2N) phase was
obtained, as the temperature was increased to 850 °C.
 Varying laser fluence over 840 J/cm2 ( N2 20 Pa and substrate temperature 600
oC), the surface roughness, nitrogen content, and grain size increase with the laser
fluence. The NbNx 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-Nb2N) at the higher flunces.
 Reactive PLD of NbNx on Si(100) yields NbN films with highest Tc of 15.07 K at
500 mTorr (66.7 Pa) N2 pressure, substrate held at 800 oC.