Pyroelectric thin films—Past, present, and future Cite as: APL Mater. 9, 010702 (2021); https://doi.org/10.1063/5.0035735 Submitted: 31 October 2020 . Accepted: 27 December 2020 . Published Online: 22 January 2021 Gabriel Velarde, Shishir Pandya, J. Karthik, David Pesquera, and Lane W. Martin COLLECTIONS Paper published as part of the special topic on 100 Years of Ferroelectricity - a Celebration ARTICLES YOU MAY BE INTERESTED IN A new era in ferroelectrics APL Materials 8, 120902 (2020); https://doi.org/10.1063/5.0034914 Mild and wild ferroelectrics and their potential role in neuromorphic computation APL Materials 9, 010903 (2021); https://doi.org/10.1063/5.0035250 Expanding the application space for piezoelectric materials APL Materials 9, 010901 (2021); https://doi.org/10.1063/5.0035416 APL Mater. 9, 010702 (2021); https://doi.org/10.1063/5.0035735 9, 010702 © 2021 Author(s). APL Materials RESEARCH UPDATE scitation.org/journal/apm Pyroelectric thin films—Past, present, and future Cite as: APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 Submitted: 31 October 2020 • Accepted: 27 December 2020 • Published Online: 22 January 2021 Gabriel Velarde,1 Shishir Pandya,1,a) J. Karthik,2,b) David Pesquera,1,3 and Lane W. Martin1,4,c) AFFILIATIONS 1 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, USA 2 Microsoft, One Microsoft Way, Redmond, Washington 98052, USA 3 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain 4 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Note: This paper is part of the Special Topic on 100 Years of Ferroelectricity—A Celebration. a) Formerly at: Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA 94720, USA. b) Formerly at: Department of Materials Science and Engineering, University of Illinois, Urbana–Champaign, Urbana, IL 61801, USA. c) Author to whom correspondence should be addressed: lwmartin@berkeley.edu ABSTRACT Pyroelectrics are a material class that undergoes a change in polarization as the temperature of the system is varied. This effect can be utilized for applications ranging from thermal imaging and sensing to waste-heat energy conversion to thermally driven electron emission. Here, we review recent advances in the study and utilization of thin-film pyroelectrics. Leveraging advances in modeling, synthesis, and characteriza- tion has provided a pathway forward in one of the more poorly developed subfields of ferroelectricity. We introduce the complex physical phenomena of pyroelectricity, briefly explore the history of work in this space, and highlight not only new advances in the direct measure- ment of such effects but also how our ability to control thin-film materials is changing our understanding of this response. Finally, we discuss recent advances in thin-film pyroelectric devices and introduce a number of potentially new directions the field may follow in the coming years. © 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0035735 I. PHYSICS OF PYROELECTRICS imaging/sensing to waste-heat energy conversion to electron emis- sion.3–6 Despite its widespread use, our fundamental understanding While we are celebrating 100 years of ferroelectricity in this of the pyroelectric effect has remained underdeveloped compared to special edition, the first known report of the pyroelectric effect dielectric (E) and piezoelectric (X) effects (Fig. 1). While the under- dates back to more than 2000 years ago when the Greek philoso- lying thermodynamics appears to be straightforward, the literature pher Theophrastus observed that small particles of straw and wood on this subject provides a rather confusing picture. became attracted to a heated pyroelectric mineral.1 Our physi- Recently, advances in epitaxial thin-film synthesis and char- cal understanding of the pyroelectric effect has evolved greatly, acterization have provided researchers access to model versions of of course, and today, pyroelectricity is defined as the variation ferroelectric materials, which have ushered in deeper understand- of the spontaneous polarization (P) as a function of temperature ing of the limits of polarization, the role of domain walls in vari- (T). When measured at a constant electric field (E) and stress ous responses, and more.7 Yet, while our understanding of dielec- (X), this variation gives the so-called pyroelectric coefficient π tric, piezoelectric, and ferroelectric responses have blossomed, little = (∂P/∂T)E,X .2 Thus, when a polar material in a capacitor structure progress has been made in understanding pyroelectric effects. This is subjected to a change in temperature, the change in polarization is, in part, due to the complexity of this effect and the challenge drives a current in the surrounding closed circuit or a voltage in an of accurately measuring both the temperature and the often con- open circuit, both of which can be utilized. As a result, ferroelectrics voluted pyroelectric response in thin films. To fully quantify and (because of their strong polarization) have been widely studied control pyroelectricity, one must understand several contributions as pyroelectrics and used in applications ranging from infrared to the overall response. The total pyroelectric response πtotal can be APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-1 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm material properties such as the piezoelectric coefficient, elastic stiff- ness, and the thermal-expansion coefficient tensors.8,12,13 Further- more, under applied electric fields, a dielectric contribution (πε , fourth term) arises from the temperature and electric-field depen- dence of εr . Finally, the tertiary contribution (πtertiary , fifth term) arises via the piezoelectric effect from inhomogeneous heating that induces nonuniform stresses.14 This effect may be neglected if uni- form heating is achieved, which is typically the case in thin films. To further complicate matters, each of these contributions exhibits not only various magnitudes but also varying signs depending on the nature of the system and, until recently, few attempts to mea- sure such contributions had been undertaken. Developing method- ologies to understand and quantify these complex (and competing) contributions remains a clear challenge. However, with advances in modeling and characterization, our understanding of these contri- butions has expanded greatly and this review aims to capture the recent history of this evolution. II. ADVANTAGES AND DISADVANTAGES OF PYROELECTRIC THIN FILMS FIG. 1. Heckmann diagram depicting the various thermodynamic coupled electri- cal, mechanical, and thermal susceptibilities of crystal materials. Here, E, X, T, P, While the work on pyroelectrics is, in general, quite exten- x, and S are the electric field, stress, temperature, polarization, strain, and entropy, sive, studies specifically on thin films are limited. Initially moti- respectively. In addition to the direct pyroelectric effect that relates polarization and vated by the enhanced performance and compatibility with the temperature, other indirect thermodynamic paths are evident (e.g., pyroelectricity ever-shrinking dimensionality of integrated microelectronics of the via temperature induced piezoelectricity). time, prominent researchers (e.g., Lang, Muralt, and Whatmore) drove advances in both fundamental material understanding and commercial applications in the 1980s and 1990s.15–17 As it pertains defined as follows: to thin-film pyroelectrics specifically, much early work focused on micrometer-thick micro-machined systems15,18 for infrared sensing πtotal = πprimary + πsecondary + πε + πteriary (Fig. 2). However, even as early as 1992, researchers at Texas Instru- ∂P ∂ϕ ∂P ∂X ∂x ments, Inc. had developed uncooled infrared focal plane arrays (IRF- = ϕ( ) + P( ) +( ) ( ) ( ) PAs) based on pyroelectric Ba1−x Srx TiO3 thin films with excellent ∂T x ∂T x ∂X T ∂x T ∂T X performance.19–21 This work showed the potential utility and impact ∂εr (E) of thin-film pyroelectrics and paved the way for the key develop- + ε0 E( ) + πtertiary , ∂T ments of the past decade on nanometer-scale thin-film pyroelectric materials, which has become increasingly attractive due to the con- where ϕ is the fraction of ferroelectric domains contributing to trol of materials afforded by manipulating epitaxial strain, compo- the pyroelectric response, x is the strain, ε0 is the permittivity of sition, thickness, and electrostatic and mechanical boundary condi- free space, and εr is the relative permittivity. Many prior studies tions therein.7 Thin films also open up the possibility of leveraging report the primary contribution as a single value, but in reality, it enhanced susceptibilities, low thermal mass, and microfabrication is made up of a combination of both the intrinsic (πintrinsic , first techniques for large-scale device integration, which could provide term) and extrinsic (πextrinsic , second term) effects, similar to that pathways to performance well beyond the theoretical limits of bulk of dielectrics.8–10 The intrinsic contribution arises from the tem- ceramics.22 Simultaneously, researchers have been developing exotic perature dependence of P (i.e., within the ferroelectric domains), new thin-film heterostructures—from single-layer films to superlat- while the extrinsic contribution arises from the temperature depen- tices to compositionally graded structures—which afford previously dence of the domain structure (i.e., thermally induced domain-wall unimagined potential for enhanced material properties. For exam- motion).8,11 Only within the past ten years has dedicated effort ple, large gradients in electric potential, strain, and composition can been applied to elucidating the nature of extrinsic effects, specifi- be obtained, which induce polar phenomena in thin-film devices cally in thin films where deterministic control of domain architec- that are simply not attainable in bulk counterparts.23 Such advances tures can be achieved. The secondary contribution (πsecondary , third have resulted in dramatic theoretical and experimental demonstra- term) arises from the fact that all pyroelectric materials are also tions of high efficiency pyroelectric energy conversion (PEC) and piezoelectric. Thus, temperature-induced volume (shape) changes large-scale infrared detection arrays.22,24 are commensurate with piezoelectric-induced polarization changes. While thin-film engineering offers a fertile opportunity for This becomes more complex in thin films, where in-plane thermal- redefining the limits of pyroelectricity, it also results in sig- expansion mismatch with the underlying substrate induces addi- nificant challenges. For example, taking advantage of the new tional changes in polarization via the piezoelectric effect. To encom- nanoscale design approaches, including utilization of thin-film pass all these coupled subtleties, the secondary contribution includes strain, exotic polarization domain structures, and compositional APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-2 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm FIG. 2. (a) Three components of an early 1 × 12 micromachined pyroelectric array. The 0.4 × 0.9 mm2 device structure consists of a 1.6 μm-thick lead–zirconate titanate- based thin film layer capped with a black IR adsorbing platinum layer.18 (b) Pyroelectric linear array with two lines of lead–zirconate titanate-based active elements (black rows) capable of operating in different wavebands, thus permitting two-color IR imaging.15 (c) A 320 × 240 pyroelectric barium–strontium titanate-based pixel array on an integrated circuit with 48.5 μm centers.21 gradients, also requires advanced capabilities in thin-film growth, change in the compensating charges of the electrodes. By monitor- pyroelectric characterization, and materials modeling. The mate- ing the pyroelectric current in response to the applied temperature, ip rial characterization and modeling techniques that are typically the pyroelectric coefficient can be extracted directly as π = A(dT/dt) , suited to bulk pyroelectric materials are inadequate for thin films where ip is the pyroelectric current, A is the capacitor area, and t is and provide an incomplete picture of the underlying mecha- the time. In contrast, indirect measurements are accomplished via nisms that impact pyroelectricity.8,25 In addition to the devel- multiple isothermal measurements of polarization-electric field hys- oped Ginzburg–Landau–Devonshire (GLD) formalisms for thin- teresis loops as a function of temperature. The temperature depen- film pyroelectrics, the development of more rigorous characteriza- dence of polarization (∂P/∂T) is then extracted to obtain π. Indirect tion methods has been pivotal in providing comprehensive under- measurements are simpler to implement but can lead to an over- standing of pyroelectric response. With key advancements in the estimation of π due to electric-field-induced leakage currents and development of thin film models and characterization methods insensitivity to various contributions to pyroelectricity (e.g., extrin- over the past decade, the 2000 year-old field of pyroelectrics now sic, secondary, or field-dependent effects), which can be particularly stands poised to enter the new frontier of sub-micrometer thin-film significant for thin films. response. III. MEASURING PYROELECTRIC EFFECTS B. The evolution of direct measurements IN THIN FILMS While direct measurements offer a higher degree of accu- Since the 1950s, more than 20 measurement protocols have racy, they are also more difficult to implement. Rapidly and uni- been developed taking advantage of various excitation/measurement formly applying temperature variations, measuring those temper- signals and required device and material specifications (i.e., area, ature changes accurately in complex material stacks, and measur- thermal conductivity, and specific heat capacity) that have offered ing the potentially small currents (voltages) that result from the varying abilities to separate out spurious effects such as thermally pyroelectric effect are non-trivial. In thin films, these challenges stimulated currents (TSCs).26 The relative lack of advanced study are further exacerbated due to the reduced lateral sizes of devices on thin-film pyroelectrics is primarily related to the fact that direct (rendering thermocouple-based studies nearly impossible) and the (and accurate) measurements of applied temperature changes are small signal pyroelectric response itself, which can be beyond the difficult. The small thermal mass/area of thin-film devices and the resolution of off-the-shelf instruments. The key, however, is that presence of underlying substrates lead to non-trivial boundary con- the approach must be able to address potentially spurious contri- ditions where the film loses heat to the substrate at relatively short butions to the response, including thermally stimulated or leak- time scales.27 age currents. Fortunately, significant work on the development of direct measurements, particularly on dynamic or phase-sensitive techniques, has incorporated microfabricated devices to overcome A. Direct vs indirect measurements these metrology challenges and shed new light on the origin of Pyroelectric measurements can be classified as either “direct” pyroelectricity. or “indirect” depending on how the temperature dependence of P These advances, described below, build upon several impor- is extracted. Direct measurements involve characterization of the tant contributions to the field of pyroelectric characterization, which temperature dependence of P directly via the pyroelectric current merit mention. Early pioneering work by Chynoweth utilized fil- or voltage. In the case of a closed-circuit configuration of a parallel- tered tungsten projection lamps to illuminate and heat BaTiO3 plate device, the temperature stimulus and the concomitant change crystals and collect the resulting transient currents from which the in P result in a flow of current through the external circuit due to a pyroelectric response was extracted.28 Although still limited by the APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-3 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm accuracy and homogeneity of the applied temperature changes, this C. New approaches for thin films work, unlike static methods of the time, provided a pathway whereby researchers could separate out non-pyroelectric contributions to Building from this idea, various studies9,30,33 have employed the measured response. From here, Byer and Roundy developed a sinusoidally oscillating temperatures and phase-sensitive measure- direct method that extracted the pyroelectric current using only a ments to extract pyroelectric currents. For example, researchers were data acquisition system, amperemeter, and temperature-controlled able to apply such phase-sensitive approaches to probe pyroelec- oven.29 While this simplicity was a step in the right direction, tric effects in thin films by oscillating the temperature of the entire depending on the material and temperature regime, this approach film stack (substrate and films).9,34 The challenge in this approach could not fully separate out currents that were not of pyroelec- was multifold but was primarily limited by the small temperature tric origin. In turn, Garn and Sharp applied sinusoidal temper- magnitudes (∼1.25 K) and slow oscillations (0.125 rad/s) needed to ature excitations where phase sensitivity could be maintained.30 maintain homogeneous heating. Unsurprisingly, these applied tem- Due to the time-derivative dependence of temperature, one can perature variations on small device structures (circular capacitors effectively separate out the true pyroelectric current (which is 90○ with just 25 μm–100 μm diameter) resulted in small sub-picoampere out-of-phase with the input heating signal) from thermally stimu- response. To do this more accurately, localized thin-film heating lated currents (TSCs)31,32 that arise from thermally excited trapped would be required. As a result, researchers explored three compli- charges (which are in-phase with the input heating signal). As mentary approaches to locally heat and probe thin-film pyroelec- compared to the pyroelectric current, the TSC has a large relax- tric response across unprecedented frequency ranges [0.02 Hz to ation time (orders of magnitude longer than pyroelectric currents) 1.3 MHz;33 Fig. 3(a)]. One important approach introduced at this and therefore depends more on the actual temperature and less time, and subsequently utilized extensively,11,22,35–37 made use of on its time derivative. This distinguishing feature arms the pyro- a microfabricated thin-film resistive heater patterned directly on electric measurement with phase selectivity during periodic (AC) top of the pyroelectric to oscillate its temperature [Fig. 3(b)] while heating. simultaneously measuring pyroelectric currents [Fig. 3(c)].33,35 The FIG. 3. (a) Pyroelectric current response across eight heating frequency decades only achievable in thin films using low frequency periodic (LFP), 2ω, and laser intensity modulation (LIM) characterization techniques.30 (b) A nanofabricated thin film [100 nm 0.68Pb(Mg1/3 Nb2/3 )O3 –0.32PbTiO3 (PMN–0.32PT)] pyroelectric electrothermal test platform. With the utilization of thin film resistive heating and adapted 3-Omega temperature characterization methods, AC phase-sensitive pyroelectric measurements can effectively minimize spurious thermally stimulated current contributions to the total measured response.19 Pyroelectric measurements of the PbZr0.2 Ti0.8 O3 thin film including (c) the phase relationship of the measured current (at 2ωH ) and the temperature oscillation (also at 2ωH ) with respect to the phase of the applied heating current depicting that the measured current leads the temperature oscillation by ∼90○ . (d) Measured pyroelectric current and pyroelectric coefficient as a function of heating frequency. At low frequencies (<4 Hz), π → ( ∂T ∂P )E,X , while at high frequencies (>500 Hz), π → ( ∂T ∂P )E,x due to secondary contribution to pyroelectricity (reduction by ∼30%). (e) Pyroelectric hysteresis loops as a function of the background DC electric field at 1 kHz.32 APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-4 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm biggest advantage of this technique is that the applied tempera- suitability for non-equilibrium phenomena such as switching and ture oscillation frequency was no longer limited to the sub-hertz leakage. These limitations of the equilibrium thermodynamic regime but could be pushed out to the order of kilohertz, sig- descriptions must be kept in mind as we use these approaches to nificantly enhancing the pyroelectric current (ip ≈ dTdt ) [Fig. 3(d)]. understand and optimize thin-film materials. More broadly, this Additionally, researchers developed another approach that utilized means that such formalisms are incapable of uncovering new pyro- a modulated laser to locally heat the pyroelectric at frequencies up electric materials and are further limited only to well-known mate- to the order of tens of megahertz.38 Both of these AC techniques rial systems (e.g., BaTiO3 and PbZr1−x Tix O3 ), which have previ- leverage advances from the thermal-sciences community (e.g., 3- ously defined bulk descriptions. As a result, the development of omega methods39 and time-domain thermoreflectance40,41 ) as well first-principles/ab initio approaches to both understand the effects as sophisticated nanoscale heat-transport modeling42 to accurately in materials and design new materials represents a clear need for predict the temperature change in the pyroelectric. Another advan- investment by the community. What little work that does exist has tage of these high-frequency techniques is their ability to separate only begun to scratch the surface of the connection between micro- out the secondary pyroelectric contribution from the primary con- scopic and macroscopic response in pyroelectrics and has not yet tribution by controlling the thermal penetration depth using the addressed these issues in the classical systems and geometries the frequency of the temperature oscillation, all the while controlling broader community might desire.52,53 the background DC electric field [Fig. 3(e)]. In all, the localized high-frequency techniques enabled more accurate and more detailed B. Experimental studies characterization of pyroelectricity in thin films. Such techniques now provide scientists with some of the most advanced and accurate The past decade has seen numerous advances in experimen- measurements of these effects and a way to quantify and differentiate tal probes of pyroelectricity in thin films. Unlike bulk ceram- various contributions to pyroelectricity. ics, thin films can maintain large strain states >±1% imposed by the underlying substrate and thus provide researchers consider- able ability to manipulate and control polar order and domain IV. RECENT ADVANCES IN PYROELECTRIC THIN FILMS structures. This is particularly interesting since all properties (i.e., A. Models and simulations dielectric, piezoelectric, and pyroelectric) can be strongly affected The equilibrium properties of ferroelectrics and pyroelectrics in by changes in the transition temperatures, magnitude of polar- addition to their dependence on temperature, electric fields, and epi- ization, and domain structures. The first work to probe such taxial stress/strain have generally been described by mean-field GLD effects—namely, how ferroelectric domain structures impact prop- thermodynamic models.43 There, the free energy of a ferroelectric erty evolution—focused on PbZr0.2 Ti0.8 O3 thin films where growth is expanded as a power series of the order parameter (i.e., polariza- on different substrates (i.e., different lattice mismatches) produced tion) and then minimized to identify the properties at equilibrium. domain structures with varying fractions of out-of-plane oriented c The effects of stress/strain, temperature, and electric fields are evalu- domains and in-plane oriented a domains. In turn, researchers com- ated by adding appropriate coupling terms to the free energy. In the bined both GLD approaches and experimental studies to directly reference frame of the paraelectric phase, the free energy (F) may be quantify the extrinsic contribution of both dielectric10 and pyroelec- written as44 tric9 response [Fig. 4(a)]. This represented the first attempt to estab- lish and directly measure an analogous extrinsic contribution to 1 pyroelectricity and concurrently demonstrated that secondary con- F = α1 (P12 + P22 + P32 ) + ⋅ ⋅ ⋅ s11 (X12 + X22 + X32 ) − ⋅ ⋅ ⋅ E1 P1 2 tribution arising from the thermal-expansion mismatch of the film − E2 P2 − E3 P3 , and substrate could be substantial (accounting for 25%–50% of the response). Subsequent work has built from these early studies, lever- where αi are the material dependent stiffness coefficients, Pi are aging more advanced measurement techniques, to further quan- the polarization components, and sij is the compliance tensor that tify extrinsic contributions to pyroelectricity. For example, again describes the coupling between polarization and stress. The sim- using PbZr0.2 Ti0.8 O3 as a model system, researchers used a range plest such model assumes a monodomain polarization state and of substrates and strain states together with temperature-dependent adequately captures the phenomenological features of pyroelectric- scanning-probe microscopy and x-ray diffraction to observe simi- ity. Such approaches are, however, inadequate for thin films where lar trends.37 Here, by referencing a purely c-oriented monodomain epitaxial strain can induce complex domain structures, depolariza- film and adjusting for the strain-induced variation of polarization, tion fields, thermal-expansion mismatch with the substrate, etc., all researchers were able to quantify both the intrinsic and extrinsic of which impact the equilibrium polarization. Recent work on phe- effects, showing that the latter could be as much as 35% of the nomenological models has included increasing complexity, bringing total measured pyroelectric response and that the strain state (com- such models closer to the realities of experiment and polycrystalline pressive strains diminished and tensile strains enhanced) affects the thin films where non-trivial domain structures exist.25,45–51 Such temperature-driven domain interconversion and thus the overall approaches have uncovered various effects that can impact pyro- pyroelectric response [Fig. 4(b)]. electricity in thin films and allow for systematic optimization of In turn, researchers undertook further studies aimed at illumi- pyroelectric figures of merit (FoMs). nating the various contributions to pyroelectric response. Specifi- These mean-field approaches, however, provide a purely cally, work combining both GLD models and experiments focused macroscopic picture of the phenomena and do not predict on PbZr0.52 Ti0.48 O3 (perched at the morphotropic phase bound- the microscopic (atomistic) nature of the physics, thus limiting ary) films was also able to directly quantify the magnitude and APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-5 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm FIG. 4. (a) The measured pyroelectric coefficient (filled orange squares) as a function of a domain percentage in polydomain PbZr0.2 Ti0.8 O3 thin films. The red line is the intrinsic response, while the blue line depicts the primary response (intrinsic + extrinsic) calculated using polydomain GLD theory. The open green squares indicate the sum of primary + secondary contributions to the pyroelectric coefficient for each film–substrate combination and the green line indicates the trend expected assuming an average thermal expansion coefficient of 10.9 × 10−6 K−1 for all the substrates.9 (b) First reported experimental quantification of extrinsic domain-wall contributions to pyroelectricity. Through the systematic control of mechanical strain, the ferroelastic domain structure may be tuned in thin film pyroelectrics to elucidate not only the magnitude of extrinsic contributions but also the sign (i.e., >0 or <0).34 (c) Summary of the intrinsic, extrinsic, secondary, and total pyroelectric response as a function of the composition across the various PbZr1−x Tix O3 chemistries. Due to the opposing signs of the various contributions, the total net pyroelectric coefficient value is indicated via a solid line for each composition. The dielectric contribution displayed here is that measured at an applied dc-electric field of 100 kV cm−1 .11 direction of the secondary and field-induced pyroelectric effects.54 and the dielectric permittivity to increase the material perfor- In a similar spirit, recent work has quantified the intrinsic, extrin- mance and FoMs for various applications [Fig. 5(b), see details in sic, dielectric, and secondary pyroelectric contributions for the Sec. V A].58 The compositional gradients result in large strain gradi- PbZr1−x Tix O3 (x = 0.40, 0.48, 0.60, and 0.80) system without the ents (∼105 m−1 to 106 m−1 ; not achievable in bulk), which, in turn, need for a monodomain reference material.11 Using a combination generate large built-in potentials that can reduce the permittivity of direct high frequency and high applied DC voltage measure- while maintaining large pyroelectric response. In the end, the work ments, researchers managed to systematically suppress and indi- showed a route to enhance the pyroelectric FoMs of materials by vidually quantify all contributions [Fig. 4(c)]. It was found that 3–12 times in comparison to standard materials. the DC voltage could be used to suppress both the dielectric and Recent work has also explored pyroelectricity in some of the extrinsic contributions, while high frequency heating allowed for the most complex ferroic materials in the community, the so-called suppression of secondary contributions. Not only was the intrinsic relaxor ferroelectrics. These materials exhibit a characteristic broad pyroelectric response measured this way, but the researchers also dielectric response with temperature and strong dielectric relax- explored the evolution of both the magnitude and the sign of the ation with frequency, which are generally attributed to the forma- dielectric and extrinsic contributions across the morphotropic phase tion of polar nano-regions/domains. Leveraging advances in thin- boundary of the PbZr1−x Tix O3 system. Similar observations have film synthesis of these materials59,60 and building from the already recently been seen in Pb0.99 [(Zr0.52 Ti0.48 )0.98 Nb0.02 ]O3 thin films robust performance demonstrated in micrometer-thick pyroelec- on both nickel foils and silicon wafers where dramatic differences tric ceramic relaxor ferroelectrics,61 new studies have exploited in room temperature pyroelectric response were measured due to strong field- and temperature-induced polarization susceptibilities differences in the various pyroelectric contributions on these sub- in 0.68Pb(Mg1/3 Nb2/3 )O3 –0.32PbTiO3 to produce record-breaking strates.55 In all, such reports of pyroelectric contribution extrac- PEC (see Sec. V A).22 Electric-field-driven enhancement of the tion have elucidated the difference in susceptibility enhancement pyroelectric response [which was measured to be as large as −550 design rules between pyroelectrics and their dielectric/piezoelectric μC m−2 K−1 , Fig. 5(c)] and suppression of the dielectric response (by counterparts. up to 72%) yielded substantial FoMs for PEC [Fig. 5(d)]. Field- and temperature-dependent pyroelectric measurements highlighted the role of polarization rotation and field-induced polarization in medi- C. New thin-film pyroelectric materials ating these effects. This opens a new class of materials that could The advent of new and better measurement approaches has have increasingly interesting effects, especially under field-driven opened the potential for researchers to explore new systems. For conditions for pyroelectric applications. example, researchers have probed compositionally graded ferro- Finally, recent years have seen a dramatic investment in the electrics in which the chemistry of the material is varied across the study of novel ferroelectric materials such as the hafnia-based sys- thickness of the film. Recent studies of epitaxially grown composi- tems, including the work to probe pyroelectricity. Early efforts62 tionally graded films revealed the potential for exotic domain struc- using indirect measurements claimed (surprisingly) large pyro- tures and properties,23,56,57 which were further extended to pyroelec- electric effects in silicon-doped HfO2 , which was attributed to a tricity. Focusing on compositionally graded films transitioning from temperature-driven phase transition. This work, however, came PbZr0.2 Ti0.8 O3 to PbZr0.8 Ti0.2 O3 and vice versa, researchers were into question when researchers applied phase-sensitive detection able to independently control the pyroelectric coefficient [Fig. 5(a)] approaches on zirconium-63 and silicon-doped HfO2 ,64,65 which APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-6 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm FIG. 5. (a) Top to bottom: sinusoidal temperature variation applied to extract pyroelectric responses of single-layer PbZr0.2 Ti0.8 O3 (20:80), single-layer PbZr0.8 Ti0.2 O3 (80:20), and compositionally up-graded and compositionally down-graded heterostructures. (b) Table summarizing the pyroelectric coefficients (π), dielectric permittivity (εr ), heat capacity (Cp ), thermal imaging figure of merit (FoMIR ), and pyroelectric energy conversion figure of merit (FoMPEC ) for compositionally graded and non-graded pyroelectric materials.54 (c) Pyroelectric coefficient and (d) FoMPEC as a function of the dc electric field for a 0.68PbMg1/3 Nb2/3 O3 -0.32PbTiO3 (PMN-PT) and a PbZr0.2 Ti0.8 O3 thin film measured in the same configuration.19 (e) Pyroelectric coefficient measured as a function of the background dc electric field at 1 kHz for a silicon-doped HfO2 thin film. The black arrow and red filled data point show the direction and starting point of the measurements.33 suggested that the effects are rather modest. Using an advanced is π/Cp ε0 εr and Cp is the specific heat capacity15,68 and (2) PEC where microfabricated pyroelectric measurement setup, researchers were the FoM is π2 /ε0 εr or the closely related electrothermal coupling able to undertake a comprehensive zero-field and field-dependent coefficient k2 = π2 T/Cp ε0 εr , where T is the temperature of opera- study of pyroelectricity to show that thin films of silicon-doped tion.69–71 Such FoMs provide a simple but informative description HfO2 exhibit pyroelectric response with a “wake-up” behavior akin of the tradeoffs at play in typical applications and provide material to the wake-up phenomenon observed in the ferroelectric polar- scientists guidance on how to control materials in useful ways to ization with electric-field cycling. This pyroelectric response was improve the overall performance. From these FoMs, the design chal- found to be rather small (∼25 μC m−2 K−1 ), which was consistent lenges for pyroelectric materials are clear—produce a large pyroelec- with other phase-sensitive measurements [Fig. 5(e)]. Surprisingly, tric coefficient, a small heat capacity, and a small dielectric permittiv- however, simultaneous direct electrocaloric measurements revealed ity for all applications. Many of the highest performance pyroelectric an electrocaloric coefficient ∼4 times larger in magnitude than that materials are complex oxide systems with generic ABO3 chemistry, expected for the measured pyroelectric coefficient. This enhance- which generally possess heat capacity values that lie within a rela- ment was explained using the plausible role played by defect dipoles tively small range (2 J/cm3 K–3.2 J/cm3 K),17,70 making it an inef- that not only contributed to additional configurational or dipolar fective tuning parameter. This leaves independently enhancing the entropy but also dictated the wake-up behavior in polarization and pyroelectric coefficient and suppressing the dielectric permittivity; pyroelectricity.36 an optimization process that is difficult in conventional materials where dielectric and pyroelectric responses are generally enhanced V. APPLICATIONS OF PYROELECTRIC THIN FILMS by the same generic features. Some approaches (see Sec. IV C) have been successful in addressing this design tradeoff. Here, we A. Figures of merit for pyroelectric applications highlight two areas where recent work on thin films has made Since the 1960s, pyroelectrics have found their way into vari- impact. ous thermal imaging, thermal sensing, and gas monitoring applica- tions, owing mostly to their intrinsic insensitivity to a wide wave- length spectrum, speed, and room-temperature performance.15,66,67 B. Pyroelectric energy conversion The relative potential of a material for a given application can be The need for PEC is clear—nearly two-thirds of the primary described by an appropriate FoM. For pyroelectrics, these include energy produced in the U.S. is lost as waste heat; a significant por- (1) thermal (infrared) sensors and electron emitters where the FoM tion of which is considered as low-grade (i.e., 100 ○ C–230 ○ C).72 APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-7 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm Unlike thermoelectrics that utilize a spatial-temperature gradient, thin-film geometries are promising for future pyroelectric device PEC requires a temporal variation in temperature (∂T/∂t), mak- design. ing PEC highly desirable in instances where temperature gradi- ents are either difficult to establish or the temperature of the heat source is fluctuating. These temperature changes result in a pyro- C. Electron emission electric current ip = πA( ∂T ∂t ), which equates to performance scaling For a detailed description and review of pyroelectric (thermally with the area, not the volume. This leads to a number of poten- driven) electron emission, see Ref. 5. Here, we briefly provide a recap tial advantages for thin-film devices and thus motivates the focus of the field and the efforts within thin films. Defined via the open- on materials in such geometries here. To harvest waste heat, a circuit configuration V = π dΔT, where d and εσ33 refer to thickness PEC device mimics a thermodynamic heat engine leveraging a vari- εσ33 ety of cycles73,74 distinguished by their polarization vs electric-field and dielectric permittivity at constant stress, respectively, pyroelec- pathways [Figs. 6(a)–6(e)]. The power density is governed by the tric emission is a special case of field emission, where the energy nec- combined electrical and thermal-time constant of the device and essary for electrons to tunnel into vacuum is provided by internally thus scales with the frequency of temperature oscillation. Often, the bound charges rather than an external electric field. Provided a rapid thermal-time constant can be much larger than the electrical-time enough temperature change, a pyroelectric material can generate a constant and is, therefore, the limiting factor in achieving a large large field (typically 102 V/μm–104 V/μm5,68,76 ), which can eject elec- power density. It has been shown22 that thin-film devices inherently trons into vacuum. Rather than depending on the absolute tempera- have a better power density than bulk devices.75 For example, in ture of the system, pyroelectric electron emission is uniquely deter- thin-film relaxors, researchers have achieved a power density that is mined by the rate of change of temperature when operated below three orders of magnitude larger than that of bulk versions, despite the phase-transition temperature.77–80 Compared to thermionic or having an energy density that is only one order of magnitude larger. field emitters, pyroelectric emitters can operate with a temperature It should be noted that the trend in thin-film research is to calculate variation of only a few degrees, emit high-energy electrons with no the energy and power densities for the volume of the thin film only acceleration, and produce large current densities. Most work, how- (excluding the substrate) in an attempt to allow for direct compar- ever, has focused on pyroelectric materials with thicknesses >50 μm, ison to bulk materials. Regardless, looking forward, it appears that thus limiting their integration into modern microelectronics. To FIG. 6. (a) Depiction of the temperature dependence of the spontaneous polarization (P) and associated electrical dipole displacements at the unit cell level of the perovskite structure. (b) Carnot, (c) Stirling, (d) Brayton, and (e) Ericsson (Olsen) cycles use for pyroelectric waste-heat energy conversion.68 (f) Pyroelectric emission from a PbZr0.8 Ti0.2 O3 thin film with varying electric field strengths. The noise arises from surface flashover events and discharge from the anode to the pyroemitters (bottom) The sample was actively heated at 100 ○ C/min but passively cooled for two cycles. Inset: experimental setup for pyroelectric electron emission. Tip emitter samples are tested in a vacuum chamber at 10−6 Torr to 10−7 Torr. The samples were secured to an alumina plate using silver paint and separated from the anode using a Kapton tape of 150 μm thick. A ceramic heater heated the samples from the bottom, while the anode generated a macroscopic electric field using a pos- itive voltage bias. A SEM micrograph of a microfabricated tip emitter array coated with a 30-nm-thick epitaxial pyroelectric film. The tips have a mean tip radius of 30 nm.78 APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-8 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm achieve pyroelectric emission from films in the 20 nm–20 000 nm a temperature change in a system with polar vortices. Today, we thickness range, heating rates of up to 109 ○ C/min are required to await a careful study of these effects. overcome other electric compensation mechanisms (e.g., leakage). Recent attempts to miniaturize these effects have focused on inte- C. Pyromagnetism and pyro-electric-magnetic effects grating thin films of pyroelectrics with microelectromechanical sys- The pyromagnetic effect is analogous to the pyroelectric tems (MEMS) where it is possible to produce such heating rates. For effect—it is the change in the spontaneous magnetization of a mate- example, researchers were able to directly synthesize 30-nm-thick rial with a change in temperature (Chynoweth also developed a films of PbZrx Ti1−x O3 (of various compositions) on arrays of sili- dynamic method to measure this effect).88 The question then arises: con nanoemitters and achieve both field81 and thermal emission82 Can we use the pyromagnetic and pyroelectric effects in tandem to [Fig. 6(f)]. In the latter, the emitted charge was 7% of that expected achieve something new? Leveraging advances in materials, pyroelec- for a perfect thin film, and the pyroelectric emission was calcu- tric measurements, and related fields such as multiferroics and mag- lated to occur without an applied field if the heating rate exceeded 4 × 107 ○ C/min. netoelectrics, the community seems poised to address this question. What one might call pyro-electric–magnetic effects could be pos- sible in multiferroic/magnetoelectric systems wherein one obtains VI. LOOKING TO THE FUTURE—PYROELECTRICITY both pyroelectric (from the polar order) and pyromagnetic (from AND BEYOND the magnetic order) effects from a single or composite system. This could, in turn, produce larger values of pyroelectric current, pro- A. Large and novel pyroelectric effects and FoMs vide for new coupling effects, or open the way for novel appli- While we know how to produce large pyroelectric response cations. For example, the field of thermomagnetic conversion has (e.g., by perching a material near a chemically or structurally been studied for some years89 and is, again, analogous to PEC. The induced phase boundary), such routes are, in general, highly tem- idea is that temperature oscillations drive a time-varying magnetic perature dependent and shown to simultaneously enhance dielec- flux that induces a current in a solenoid. Could a composite struc- tric permittivity, which drives down FoMs. While initial studies are ture be designed that leverages these two effects (pyroelectric energy promising in addressing some of these concerns, there is consid- and thermomagnetic conversion) to create new record-breaking erable work to be done. A number of important questions remain: conversion efficiencies and energy/power densities? Can we make new materials (or even new versions of known materials) with record-breaking pyroelectric coefficients? Can we D. Electrocaloric effects: Leveraging new develop additional routes to increase pyroelectric currents while measurement platforms suppressing the dielectric permittivity? The community has not applied the same diligence and dedication given to the design of The measurement protocols developed in the past few years for dielectric, piezoelectric, ferroelectric, or multiferroic materials. One pyroelectrics can also be applied to other effects, such as the elec- can image design approaches working not only at the ab initio trocaloric effect. The small thermal mass of thin films with respect and unit-cell levels in identifying materials with exotic tempera- to the supporting substrates has so far prevented the direct mea- ture dependence for their polar order but also at the mesoscale surement of these temperature changes, which are (like pyroelec- level identifying materials with designer domain structures that pro- tricity) primarily obtained indirectly by measuring the temperature duce larger than expected composite pyroelectric effects. The design dependence of the order parameter and applying Maxwell rela- space is immense and requires specific approaches to address the tions to obtain temperature and entropy changes. In order to be dearth of high-performance materials. The values of pyroelectricity applied correctly, these relations require knowledge of the temper- (away from a phase transition) for most materials studied to date ature and field dependence of heat capacity and careful attention lie within about one order of magnitude of each other—begging to the boundary conditions (i.e., the stress/strain state of the film) the question of how larger effects near room temperature can be in order to obtain accurate results. The novel electrothermal test achieved? devices developed for the accurate measurement of pyroelectric cur- rent also provide new direct probe capabilities for electrocaloric temperature changes. Indeed, researchers have35 registered tem- B. Novel pyroelectric phenomena from emergent perature changes as small as millikelvins for a 150-nm-thick fer- topologies roelectric thin film in response to an electric field. Further mea- Progress in the synthesis of nearly perfect oxide interfaces with surements near phase transitions are required in order to verify unit-cell control has allowed the creation of superlattice heterostruc- the giant electrocaloric response reported for thin films via indi- tures where emergent phenomena such as new polar topologies rect methods. Other techniques such as infrared lock-in thermog- (skyrmions and vortex structures)83,84 are characterized by an addi- raphy90 may also enable non-contact and spatially resolved studies tional order parameter: the electric toroidal moment. The toroidal of caloric responses, provided that the heat transfer to the sub- moment, originally proposed for zero-dimensional ferroelectrics,85 strate can be reduced by fabricating suspended ferroelectric mem- should give rise to new phases (ferrotoroidic and antiferrotoroidic), branes.91,92 Such “mechanically free” membranes could also enable as well as new electromechanical (piezotoroidic) and electrothermal the study of piezocaloric93 (polarization-dependent), elastocaloric94 (pyrotoroidic) couplings. The latter emerges from the temperature (strain-dependent), and flexocaloric95 (strain gradient-dependent) dependence of the toroidal moment described by the correspond- effects where strain is no longer dominated by underlying sub- ing pyrotoroidic tensor.86 The converse effect, the toroidocaloric strates, but by the elastic compliance, thickness, and lattice misfit effect,87 is also possible, where a curl of the electric field produces of the various constituent material layers of the heterostructure96 APL Mater. 9, 010702 (2021); doi: 10.1063/5.0035735 9, 010702-9 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm Such approaches could also allow for unprecedented function of Pyroelectrics have substantially progressed in recent years. pyroelectric-based thermal imaging/sensing devices—the full impli- While compared to dielectric, piezoelectric, and even electrocaloric cations of these observations, however, remains to be seen as more effects, pyroelectricity has received less attention, the potential for work is undertaken. pyroelectrics to make impact on modern applications is as real as any in these areas. Continued attention to understanding the fundamen- E. Ferroelectric thermoelectrics tal nature of these materials and their physical mechanisms while simultaneously developing ways to utilize them in real applications is In comparison to pyroelectric, thermomagnetic, and thermoe- needed. Pyroelectrics have, over the past few decades, already played lastic energy conversion, thermoelectrics have received wider atten- a key role in a range of applications we use on a daily basis and, tion.97 Several strategies exploiting the physics of ferroelectrics for looking to the future, the advent of advanced thin-film pyroelectrics thermoelectric applications have been proposed.98–100 For exam- stands poised to open up a new generation of applications. ple, the coupling of soft-polar phonons associated with ferroelec- tric instabilities with heat carrying acoustic phonons was shown to induce scattering, effectively reducing the lattice thermal con- ACKNOWLEDGMENTS ductivity in tellurides without suppressing electrical transport.101,102 We write this review on behalf of numerous collaborators and Ferroelastic domain engineering could also be effective in opti- colleagues who have provided critical insights, support, and feed- mizing FoMs103 in addition to increased interfacial thermal resis- back. In particular, we acknowledge the support of Professor David tance (Kapitza resistance) with respect to the typical values obtained Cahill and Professor William King and their respective groups at in grain boundaries.104 Moreover, the ability to manipulate the the University of Illinois, Urbana–Champaign, as well as Profes- domains with electric fields enables the electrical manipulation of sor Chris Dames and his group at the University of California, thermal transport,105,106 demonstrating the potential of ferroelectrics Berkeley, for their contributions to the development and imple- for thermal control in microelectronic devices. mentation of novel pyroelectric measurements and devices based on thin-film materials. Over the years, our team received sup- F. Pyroelectric devices of tomorrow port from numerous sources for the development of these tech- Building from this work, we are beginning to see new ideas of niques and scientific studies, including the Air Force Office of Sci- how these materials will be used in the coming years. For example, entific Research (Grant No. AF FA 9550-11-1-0073), the Army within the realm of PEC, one of the challenges has been to envision Research Office (Grant Nos. W911NF-10-1-0482 and W911NF-14- how one generates the required time-varying temperature profile. 1-0104), the Office of Naval Research (Grant No. N00014-10-1- One approach focused on the use of a modulated laser heat source 0525), the National Science Foundation (Grant Nos. DMR-1149062, that provides both high frequency thermal cycles and long-distance DMR-1451219, and DMR-1708615), the European Union’s Hori- energy transfer.107 Key to this work was developing a low-reflectivity zon 2020 (Marie Skłodowska-Curie Grant No. 797123), and the U.S. nanostructured IrO2 top electrode to absorb the incident radiation. Department of Energy, Office of Basic Energy Sciences [Grant Nos. In other work, researchers showed the potential for a portable power DEFG02-07ER46459 and DE-AC02-05-CH11231 (Materials Project concept using PEC driven by the heat from on-chip catalytic com- Program No. KC23MP)]. bustion of methanol. This ingenious approach takes advantage of the high energy density of liquid fuels (22 MJ/kg for methanol) and DATA AVAILABILITY the ability to directly apply and combust such fuels to demonstrate a pathway to energy densities comparable to lithium-ion batter- Data sharing is not applicable to this article as no new data were ies (∼0.8 MJ/kg).108 Other available routes to impact the needs of created or analyzed in this study. PEC include addressing the fact that nearly all pyroelectric materi- als have high specific heats. Identification of new classes of materials REFERENCES with lower specific heat but similar pyroelectric performance would 1 E. R. Caley and J. F. C. Richards, Theophrastus on Stones (The Ohio State result in more effective utilization of heat sources. Additionally, new University, Columbus, 1956). efforts focused on regenerative schemes employing multi-staged 2 M. E. Lines and A. M. 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