Ultrafast Photovoltaic Response in Ferroelectric Nanolayers Page: 4 of 17
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pulse duration. Thin films of PTO were deposited on STO (001) and DSO (110) substrates at
930-990 K via metal-organic chemical vapor deposition, following procedures described
elsewhere . Prior to growth, TiO2-terminated surfaces of STO were created via a standard HF
etch . DSO surfaces were prepared with ScO2 termination by annealing in 02, followed by a
NaOH etch .
When grown on STO (001), PTO forms out-of-plane, c-axis-oriented domains below the
Curie temperature, Tc, due to compressive strain from lattice mismatch to the STO [2,3,9].
Under zero-field heating conditions, the c-axis of PTO contracts as Tc is approached,
corresponding to a negative thermal expansion coefficient. This contraction indicates a decrease
in the tetragonality and the dipole moment of the unit cell, and causes out-of-plane diffraction
peaks to shift to higher scattering angles. In contrast to the expected thermally induced
responses, we observe significantly more complex dynamics. Time-dependent rocking curves of
photoexcited, monodomain PTO on STO, with polarization pointing out of the film , taken at
room temperature [Fig. 1(a) and Fig. 1(b)] indicate a symmetric shift of the Bragg peak to higher
Q [where Q = (47r sin 0)/k] within 5 ps. This is consistent with a uniform contraction of the unit
cell in the out-of-plane direction and occurs on an acoustically limited timescale, determined by
the film thickness over the sound velocity (vs ~ 4,000 m/s) . Subsequently, we observe a
long-lived, symmetric shift of the diffraction peak to lower Q occurring on a 10 ps timescale, in
contrast to what would be expected from simple heating of a ferroelectric. Following the low-Q
shift, the nanolayer relaxes to its initial state before excitation on single-nanosecond timescales
[Fig. 1(c)]. We note that the in-plane lattice constants are expected to evolve on acoustically
limited timescales determined by the 1-mm laser spot size, and therefore do not change on the
picosecond timescales here.
In order to further elucidate the time-dependent changes in the ferroelectric unit cell, we
measured the response of the system at 5350 C, where PTO on STO enters a stripe domain
phase , characterized by neighboring regions of opposite polarization, with a period A of order
10 nm. This period is determined by a trade-off between the energy of the uncompensated
depolarization field and the domain walls . At higher temperatures, the volatile adsorbate
species that stabilize the monodomain phase [1,5] are removed. The periodicity of the stripe
domains gives rise to in-plane satellites, displaced by AQr = 27/A, which appear as a ring of
Page 4 of 17
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Daranciang, Dan. Ultrafast Photovoltaic Response in Ferroelectric Nanolayers, article, February 15, 2012; United States. (digital.library.unt.edu/ark:/67531/metadc836961/m1/4/: accessed March 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.