Electronic Structure of Chromium based p-type Transparent Conducting Oxides
studied by X-ray Spectroscopy
E. Norton,
1,2
L. Farrell,
1,2
S. D. Callaghan,
1
C. McGuinness,
1
I. V. Shvets,
1,2
, K. Fleischer
1,2
1
School of Physics, Trinity College, University of Dublin, Dublin 2, Ireland
2
Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN)
E-mail:
A large range of chromium based p-type TCOs have been reported in the last 5 years. Two
corundum lattice structures— Mg
x
Cr
2−
x
O
3
[1,2] and Ni
x
Cr
2−x
O
3
[3]—have been shown to be
p-type TCOs. Several other reports of high figure of merit (FOM) p-type TCOs also contain
chromium in an octahedral coordination. Besides the prototypical Mg
x
Cr
2−
x
O
3
and
Ni
x
Cr
2−x
O
3
,these include LaCrO
3
:Sr [4] and spinel structures such as Cr
2
MnO
4
:Li [5].
However, all such chromium based TCOs have shown poor mobility, well below the Hall
limit, while maintaining comparable conductivity with higher mobility p-type TCOs.
In this study, we present a spectroscopy method, Resonant Valence Band Photoemission
Spectroscopy (RVBPES), as an experimental technique to probe the composition of the
valence band. RVBPES measurements show that the valence band of the
p
-type transparent
oxides—crystalline Mg
x
Cr
2−
x
O
3
and nanocrystalline Cu
x
CrO
2
—show striking similarities to
measurements on crystalline CuCrO
2
:Mg [6, 7]. All films studied show that chromium states
compose the top of the valence band, suggesting that the valence-band structure is dominated
by the presence of the (Cr-O)
6
octahedra. A comparison of the valence band between the best
performing
p
-type, crystalline CuCrO
2
:Mg, with crystalline Mg
x
Cr
2−
x
O
3
and nanocrystalline
Cu
x
CrO
2
shows that the chromium 3
d
states are fixed irrespective of changes in long-range
crystallographic order. This indicates little spatial overlap between adjacent Cr 3
d
states. This
further confirms the conduction mechanism via hopping for chromium based
p
-type TCOs as
the Cr 3
d
states are localised within the (Cr-O)
6
octahedra. [7] At the same time this explains
why films with poor crystalline order can still perform comparable to epitaxial material, as
the long range order is less important for a strongly localised valence band state.
[1] L. Farrell et al. Phys. Rev. B
91
, 125202 (2015)
[2] E. Arca et al. Appl. Phys. Lett.
99
,111910 (2011)
[3] N. Uekawa and K. Kaneko, J. Phys. Chem.
100
, 4193 (1996)
[4] K.H.L. Zhang et al. Adv. Mater.
27
, 5191 (2015)
[5] H. Peng et al. Adv. Func. Mater.
23
, 5267 (2013)
[6] Yokobori et al. Phys. Rev. B
87
, 195124 (2013)
[7] E. Norton et al. Phys. Rev. B
93
, 115302 (2016)
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