Thermoelectric performance of transparent oxide semiconductor films and superlattices
H. N. Alshareef, Hyunho Kim, S.R. Sarath Kumar, and A. Abutaha
King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
E-mail:
The thermoelectric properties of oxide semiconductors are currently attracting significant
interest due to their stability at high-temperatures, compared to current thermoelectric
materials. In this talk, we review state of the art thermoelectrics properties of n-type
transparent and semi- transparent oxide semiconductor thin films and demonstrate three
distinct strategies to improve their thermoelectric performance, including (1) defect
engineering, (2) superlattices structures, and (3) crystal orientation in ZnO and doped
SrTiO
3
thin films and superlattices. For example, we show that record power factor can
be achieved in ALD ZnO by user the proper oxidant during ZnO deposition. A
thermoelectric power factor (σ
S
2
T
) of 0.45 W m
-1
K
-1
is obtained at 780 K for ZnO-O
films, which is nearly double the previously reported values in literature, and even
comparable with Al-doped ZnO thin films. Further, we report the successful
fabrication of superlattices (SLs) based on conducting Nb-doped SrTiO
3
(NbSTO) and Pr-
doped SrTiO
3
(SPTO) oxides, [(SPTO)
a
|(STNO)
b
]
x
, where the electrical conductivity and
absolute Seebeck coefficient, as a function of a/b ratio, of [(SPTO)
a
|(STNO)
b
]
20
SLs were
found to have a trade-off relationship. The thermal stability of [(SPTO)
a
|(STNO)
b
]
20
SLs,
as a function of time, temperature, and individual layer thickness, is reported. In
addition, we extracted the in-plane thermal conductivity of [(SPTO)
a
|(STNO)
b
]
20
SLs by
using the continuum analysis along with the measured thermal conductivities of the
individual SPTO and STNO layers. The optimum SL exhibits a high
ZT
of
0.46 at 1000 K. We demonstrate that the thermoelectric properties of highly oriented Al-
doped zinc oxide (AZO) thin films can be improved by controlling their crystal orientation.
The crystal orientation of the AZO films was changed by changing the temperature of the
laser deposition process on LaAlO
3
(100) substrates. The change in surface termination of
the LaAlO
3
substrate with temperature induces a change in AZO film orientation. The
anisotropic nature of electrical conductivity and Seebeck coefficient of the AZO films
showed a favored thermoelectric performance in c-axis oriented films. Our work
demonstrates the importance of nano-structuring approach in improving
ZT
of metal oxide
semiconductors which are useful for high temperature energy harvesting applications.
P 7
-85-
