Frank Batten College ofEngineering & TechnologyOld Dominion University: www.eng.odu.edu

Pulsed Laser Deposition of Niobium Nitride Thin Films

APPLIED RESEARCH CENTER

Ingot Niobium Summary WorkshopDecember 4, 2015

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, USA2Applied Research Center, Newport News, VA 23606, USA3Department of Physics, Ain Shams University, Cairo 11566, Egypt4Department of Physics, University of Cukurova, 01330 Adana, Turkey5Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606

The phase diagram of niobium nitride is complex

δ-NbN ε-NbN -Nb4N3

(a) Cubic B1, NaCl-type structure

(b) Hexagonal (Bi),TiP-type structure

(c)Tetragonal, deformed NaCl-type structure

-Nb2N

(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.

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

-Nb2N-NbN-NbN-NbN

N/Nb

Frank Batten College ofEngineering & TechnologyOld Dominion University: www.eng.odu.edu

NbNx is grown on ingot Nb by reactive pulsed lase deposition

Target: 99.995% NiobiumLaser: 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

Nitrogen background pressure effect

30 40 50 60 70 80

10.7 Pa

subs

trat

e

subs

trat

e

subs

trat

e

-N

bN (

2 2

0)

66.7 Pa

40.0 Pa

26.7 Pa

20.0 Pa

13.4 Pa

-N

bN (

1 1

1)

-N

bN (

2 0

0)

Inte

nsity

(ar

b.un

its)

2 Theta (degree)

-N

b 2N (

1 0

2)

-N

b 2N (

0 0

2)

-N

b 2N (

1 0

0)

-N

b 2N (

1 0

3)

-N

b 2N (

1 1

0)

-N

b 2N (

2 0

1)-

Nb 2N

(1

1 2)

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-Nb2NFor 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

Substrate cleaned at 900 °CGrowth temperature 600 °CLaser energy density 15 J/cm2

Films is ~120 nmDeposition rate ~ 2–3 nm/min

Nb substrate was etched by the buffered chemical polishing (BCP) method (HPO3:HNO3: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 Nb2N phase to be between 0.47± 0.02 to 0.53±0.02

10 20 30 40 50 60 700.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

NbN Nb

2N

N/N

b

Nitrogen pressure (Pa)

Pressure (Pa)

Ave

rage

rou

ghne

ss (

nm)

0

5

10

15

20

Length (nm)

Hei

ght

(arb

. un

its)

800 1600 2400

10.7 Pa

40.0 Pa

66.7 Pa

26.7 Pa

20.0 Pa

13.4 Pa

13 26 39 52 65

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.

Substrate temperature effect

40 60 80 100

Nb

(310

)Nb

(211

)

Nb

(200

)

Nb

(110

)

Nb-substrate

-N

b 2N (

110)

R.T.

-N

b 2N (

104)

-N

bN (

111)

-N

bN (

200)

250 OC

450 OC

950 OC

850 OC

750 OC

Inte

nsity

(ar

b. u

nits

)

2 Theta (degree)

650 OC

-N

b 2N (

202)

-N

b 2N (

201)

-N

b 2N (

112)

-N

b 2N (

103)

-N

bN (

220)

-N

b 2N (

102)

-N

b 2N (

002)

-N

b 2N (

100)

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.

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

Temperature (°C)

400 500 600 700 800 900

RM

S R

ough

ness

(nm

)

5

10

15

20

25

30

RMS film roughness increased with the substrate temperature

Laser fluence effect

40 60 80

40 Jcm-2

30 Jcm-2

15 Jcm-2

'-N

bN (

110)

'-N

bN (

104)

'-N

bN (

103)

subs

trat

e

subs

trat

e

subs

trat

e

Inte

nsity

(ar

b. u

nits

)

2 Theta (degree)

-N

b 2N (

201)

-N

b 2N (

112)

-N

b 2N (

103)

-N

b 2N (

110)

-N

b 2N (

102)

-N

b 2N (

100)

-N

b 2N (

002)

-N

bN (

220)

-N

bN (

200)

-N

bN (

111)

8 Jcm-2

Nitrogen background pressure 150 mTorrSubstrate temperature 600 oC

● 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.

10 20 30 40

0.35

0.40

0.45

0.50

0.55

N/N

bLaser fluence (Jcm-2)

EDX measurement of N:Nb ratio in NbNx films

High-resolution transmission electron microscopy shows polycrystallineNbN 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

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 CuKα 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 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

N2 pressure

(mTorr)

Nb 3d5/2 (± 0.05)

 

(eV)

Nb 3d3/2 (± 0.05)

 

(eV)

200 204.00 206.81

400 204.09 206.92

500 204.08 206.90

Nb 205.50 202.30

100-mm radius hemispherical photoelectron analyzer (VG Scienta SES-100) with Mg K X-ray radiation (h= 1253.6 eV)

XPS spectra of Nb 3d core levels for NbNx films, Binding energies are given with respect to the Fermi level

Superconductivity of NbNx films

5 10 15 20 250

20

40

60

80

100

120

Res

istiv

ity (

ohm

-cm

)

Temperature (K)

66.7 Pa 53.3 Pa 26.7 Pa

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.

20 30 40 50 60 70

8

10

12

14

16

Tc

Nitrogen pressure (Pa)

Tc

(K)

0.06

0.07

0.08

0.09

0.10

0.11

0.12

(-

cm)

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.

Summary

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.

Varying laser fluence over 840 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.

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.