1. Dielectric Function of NiO and Si from 25 meV to 6 eV: What’s the difference?
Ayana Ghosh Department of Physics, University of Michigan-Flint
Cayla M. Nelson, Travis I. Willett-Gies, Stefan Zollner Department of Physics, New Mexico State University, Las Cruces, NM
FTIR ellipsometry: 2 to 40 mm (Sandia)
NIR/VIS/QUV ellipsometry: 190 to 2500 nm, 77 to 800 K
NSF: DMR

2. Dielectric Function of NiO and Si from 25 meV to 6 eV: What’s the difference? (Outline)
Spectroscopic ellipsometry measures the absorption coefficient of materials:
Infrared light: Atomic (lattice) vibrations
Visible/UV light: Electronic excitations (VB to CB)
Infrared Results
NIR/VIS/UV Results
Comparison with band structure calculations for NiO
New Mexico State University
Stefan Zollner, 03/07/2014, APS March meeting 2

3. Ellipsometry: How does it work ?
We measure the change in the polarization state of light, when it is reflected by a flat surface.
P plane
S plane
Angle of incidence
Bulk sample
Monochromator or
Interferometer
polarizer
analyzer
detector Φ
Result:
Optical constants versus photon energy
Stefan Zollner, 03/07/2014, APS March meeting 3

4. Infrared Lattice Absorption
Silicon:
Diamond lattice
NiO or NaCl:
Rocksalt lattice
Si-Si bonds are non-polar.
Bonds: no dipole moment.
No infrared absorption.
Ni2+-O2- bonds are polar.
Ni-O vibration has dipole moment.
NiO
Typical Lorentz oscillator
e2=0
FTIR ellipsometry
New Mexico State University
Stefan Zollner, 03/07/2014, APS March meeting 4

5. Antiferromagnetism: Zone-folded TO phonon
Rocksalt Crystal Structure (FCC): Single optical phonon.
Antiferromagnetic ordering along (111).
Small rhombohedral distortion.
Lattice waves are reflected by the anti-parallel ordered spins along (111).
(Or: L-point phonon folded back to G due to doubling the crystal size.)
NiO cell
Rooksby, Nature, 1943
NiO Reststrahlen Band
Theory: e0=13.1 (Louie)
e0=11.3
e¥=5.0
Zone-folded phonon TO LO
Stefan Zollner, 03/07/2014, APS March meeting 5

6. NiO Band Structure I
Atomic electron configurations: Ni: [Ar] 3d8 4s2 O: [He] 2s2 2p4
Ni2+O2-: 4s electrons of Ni are transferred to O 2p: Ni2+: [Ar] 3d8 4s0 O2-: [He] 2s2 2p6
This should be a metal, because only 8 of 10 d-states are filled.
However:
Transmission measurements show that NiO is an insulator with a fundamental band gap of about 0.8 eV.
Compare: Newman, Phys. Rev
New Mexico State University
Stefan Zollner, 03/07/2014, APS March meeting 6

7. NiO Band Structure II
Let’s double the unit cell: 2 Ni and 2 O atoms per crystal cell: Ni(1): [Ar] 3d8 4s0 Ni(2): [Ar] 3d9 4s0 O(1): [He] 2s2 2p6 O(2): [He] 2s1 2p6
Every other O atom (ligand) transfers one electron to a Ni atom. Ligand (O) hole adds an extra d-electron to Ni.
Antiferromagnetic ordering.
Charge-transfer gap commonly assumed to be eV.
Most evidence from photoemission.
Optical spectroscopy (transmission, ellipsometry) is usually ignored.
Charge-transfer gap at 3.97 eV
Newman & Chrenko, Phys. Rev. 114, 1507 (1959). Kang, Lee, & Lee, J. Kor. Phys. Soc. 50, 632 (2007). ?
New Mexico State University
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8. NiO looks just like Silicon
NiO:Charge-transfer gap at 3.97 eV
Silicon: E1 at 3.4 eV ?
The charge-transfer gap (CTG) of NiO at 4 eV looks just like the E1 gap of Si.
NiO CTG energy decreases with increasing temperature.
NiO CTG broadening increases with temperature.
Just like Si: Phonon-related ?
Compare: Vina, PRB, 1984 (Ge); Lautenschlager, PRB, 1987 (Si).
New Mexico State University
Stefan Zollner, 03/07/2014, APS March meeting 8

9. Failures of the Charge-Transfer Model for NiO
Optical absorption between 0.8 and 3.5 eV.
Too strong to be defect-related.
Absorption decreases above room temperature.
Several weak critical points between 0.8 and 4.0 eV.
Charge-gap model is flawed.
We need a full-zone band structure for NiO to compare with optical experiments.
NiO
Derivative analysis
Charge-transfer gap at 3.97 eV
Li, Riganese, Louie, Phys. Rev. B 71, (2005).
Several critical points
Lowest gap at 1.5 eV
New Mexico State University
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10. Electron Structure: Band Gaps Peaks indicate interband transitions
Valence band (flat): Ni(3d) states with some O
Conduction band: Flat: Ni(3d) states (unfilled) Curved: Unfilled O states
Strong 4.0 eV peak goes from Ni(3d) VB to Ni(3d) CB.
Also observed in photoemission
Weak peaks are transitions from many Ni(3d) VB states to the O(4s) band, only at G.
Explains all experimental evidence.
Several critical points
Lowest gap at 1.5 eV
J.L. Li & S.G. Louie, PRB, 2005
New Mexico State University
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11. Summary: NiO looks like Si in the VIS/UV, but it’s not like Si at all.
NiO is a fascinating material.
The antiferromagnetic ordering of the electron spins doubles the size of the crystal cell (4 atoms per cell instead of 2).
This cell doubling causes a side-band of the Ni-O vibration (L-point phonon folded back to G).
The cell doubling also makes NiO a charge-transfer insulator.
A full-zone band structure is needed to understand the weak optical transitions below the charge-transfer gap at 4.0 eV.
New Mexico State University
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12. Graduate Students: Lina Abdallah, Travis Willett-Gies, Nalin Fernando, Tarek Tawalbeh (Theory) Undergraduate Students: Cesar Rodriguez, Nathan Nunley, Khadijih Mitchell, Cayla Nelson, Laura Pineda, Eric DeLong, Chris Zollner (Cornell), Amber Medina, Maria Spies, Ayana Ghosh (Michigan-Flint) Collaborators: Igal Brener (CINT), Neha Singh, Harland Tompkins (J.A. Woollam Co.), S.G. Choi (NREL) Samples: Demkov (UT Austin), Alpay (Uconn), Droopad (Motorola), MTI (LAO), SurfaceNet (NiO)
Flat & uniform films, at least 5 by 5 mm2, low surface roughness, films on single-side polished substrate
New Mexico State University