380 | Nature | Vol 584 | 20 August 2020
Article
ceramics with a quasi-Ohmic contact show a negligible piezoelectric
effect (Extended Data Fig. 6). More striking, the Au/Nb:BSTO junction
shows a large pyroelectric effect with a room-temperature coefficient
reaching 5.3 mC m−2 K−1 (Fig. 3b). The obtained pyroelectric coefficient
here is over one order of magnitude larger than that for conventional
ferroelectric materials, such as lithium tantalate (LiTaO 3 ) crystal
(230 μC m−2 K−1) widely used in the fabrication of pyroelectric detec-
tors^7. In addition to their large coefficients, the interface pyroelectric
effect in Schottky junctions shows another two distinctive features
compared with conventional bulk materials. First, the pyroelectric
effect in conventional ferroelectric materials has a strong temperature
dependence, that is, pyroelectric coefficients decay sharply away
from the phase transition temperature, inevitably limiting their work-
ing temperature in practical devices. In contrast, the pyroelectric
coefficient in the Au/Nb:BSTO junction shows a weak temperature
dependence across the phase transition region and retains a large
magnitude over a wide temperature range, owing to the persistence of
the depletion region (Fig. 3b). Similarly, the pyroelectric coefficients
in both Au/Nb:STO and Au/Nb:TO junctions increase monotonically
with temperature, supporting their wide working temperature range.
Second, the interface pyroelectric effect has a rapid response to the
thermal perturbation. Figure 3c, d shows the time dependence of the
pyroelectric current generated by the Au/Nb:BSTO junction and a
commercial lead titanium zirconium oxide (Pb(Ti0.8Zr0.2)O 3 ) ceramic
under the same red light pulsed illumination. Clearly, the pyroelectric
response of the Au/Nb:BSTO ceramic is over one order of magnitude
larger than that of the poled Pb(Ti0.8Zr0.2)O 3 ceramic. Moreover, the
thermal time constant is three orders of magnitude shorter (about
300 μs) than that of the bulk Pb(Ti0.8Zr0.2)O 3 ceramic (300 ms) of similar
dimensions.
We emphasis two main features of these effects arising from the
interface polar symmetry. First, both piezoelectric and pyroelectric
coefficients observed at the metal–semiconductor interface surpass
that of conventional polar materials. Although the interface piezo-
electric constants are smaller than those of ferroelectric materials
with switchable polarizations (for example, BaTiO 3 crystals), they still
rival that of non-switchable polar materials, such as zinc oxide (ZnO)
and cadmium sulfide (CdS) (Fig. 4a). For example, the piezoelectric
constant of the Au/Nb:BSTO junction is over two times larger than
that of the ZnO crystals, which have a similar electromechanical cou-
pling factor (Methods)^25. Apart from oxide semiconductors, there is
still a large space to enhance the interface piezoelectric coefficient by
exploring a wide range of semiconductors with a large electrostriction
effect, such as the organic–inorganic halide perovskites wherein the
electrostriction coefficient is over three orders of magnitude larger
than that of SrTiO 3 crystal^26. Remarkably, the interface pyroelectric
effect is much larger than that of conventional polar materials, even
the best ferroelectrics. The Schottky junction shows both a substantial
pyroelectric coefficient and a large figure of merit FV = pi/cpχ 3 , where
pi is the pyroelectric coefficient and cp is the heat capacity (Fig. 4b,
Methods). In particular, the Au/Nb:BSTO interface shows a figure of
merit of 2.11 m^2 C−1, which is one order of magnitude larger than that of
classic ferroelectric materials, such as LiTaO 3 crystal (FV = 0.17 m^2 C−1)^7.
This enhanced figure of merit in the Schottky junction originates from
the large pyroelectric coefficient and built-in field-depressed dielectric
permittivity in the depletion region.
Interface piezoelectric and pyroelectric effects are universal effects
applicable to materials of any symmetry. These effects occur in the
heterostructures wherever an electric field builds at the interface.
It is worth noting that the electric field is ubiquitous at interfaces of
dissimilar materials owing to the chemical potential inhomogeneity
across interfaces. To validate this scenario, we studied the piezoelectric
and pyroelectric effects of Schottky junctions on silicon wafer. The
Au/Si (001) junction outputs a dynamic electrical current, the ampli-
tude of which increases linearly with that of the applied stress (Fig. 4c).
This corresponds to a low but finite piezoelectric constant of about
−0.013 pC N−1. Moreover, the silicon Schottky junction shows a sizable
pyroelectric effect with a room temperature coefficient of 200 μC m−2 K−1
and a figure of merit of FV = 1.17 m^2 C−1 (Fig. 4b).
In summary, we have demonstrated interface piezoelectric and pyro-
electric effects that not only show substantial coefficients but also are
free from the symmetry limitation. They can be found in and are appli-
cable to a wide range of materials, from conventional semiconductors
and oxides, to halide perovskites and two-dimensional materials. These
features enable their practical applications in the realm of electrome-
chanical and thermal effects, such as energy conversion and infrared
sensors, with distinctive mechanisms and additional tuning feasibility
that are different from that of intrinsic non-centrosymmetric materials.
With careful design, the interface polar effects can also work concur-
rently with bulk effects arising from inherent^7 ,^8 or externally induced
polarity by, for example, strain gradients^10 –^15 , to achieve enhanced
piezoelectric and pyroelectric coefficients or even new effects.Online content
Any methods, additional references, Nature Research reporting sum-
maries, source data, extended data, supplementary information,
acknowledgements, peer review information; details of author con-
tributions and competing interests; and statements of data and code
availability are available at https://doi.org/10.1038/s41586-020-2602-4.c01234560510152025Current density (nA cm–2)Stress, V 11 (MPa)ab0.01 0.1 1100.1110100Pyroelectric coef cient(10–4 C m–2K–1)Figure of merit (m^2 C–1)Au/Nb:BSTOAu/Nb:TO
Au/SiAu/Nb:STOLiTaO 3PVDFPZT lmTGS0 0.05 0.10 0.15 0.20051015Piezoelectric constant,|d| (pC N 31–1)Electromechanical coupling, k 31Au/Nb:BSTOAu/Nb:TOAu/Nb:STO
LiNbO 3CdS ZnOFig. 4 | Giant magnitude and universal nature of the interface polar effects.
a, Comparison of the interface piezoelectric constants d 31 and the
electromechanical coupling factors k 31 of the studied devices with those of
conventional polar materials. b, Comparison of the pyroelectric coefficients
and the figures of merit of the studied devices with those of ferroelectric
materials. c, The amplitude of the current density generated in Au/Si junctions
as a function of the amplitude of the applied stress. Piezoelectric and
pyroelectric data on bulk materials are taken from refs.^7 ,^22 ,^25 ,^27. PZT, lead
zirconate titanate (Pb(Ti0.8Zr0.2)O 3 ); PVDF, polyvinylidene f luoride; TGS,
triglycine sulfate.