# Atomic Layer Thermopile Materials: Physics and Application

Authors: P. X. Zhang; H.-U. Habermeier
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

New types of thermoelectric materials characterized by highly anisotropic Fermi surfaces and thus anisotropic Seebeck coefficients are reviewed. Early studies revealed that there is an induced voltage in highTC oxide superconductors when the surface of the films is exposed to short light pulses. Subsequent investigations proved that the effect is due to anisotropic components of the Seebeck tensor, and the type of materials is referred to atomic layer thermopile (ALT). Our recent studies indicate that multilayer thin films at the nanoscale demonstrate enhanced ALT properties. This is in agreement with the prediction in seeking the larger figure of merit (ZT) thermoelectric materials in nanostructures. The study of ALT materials provides both deep insight of anisotropic transport property of these materials and at the same time potential materials for applications, such as light detector and microcooler. By measuring the ALT properties under various perturbations, it is found that the information on anisotropic transport properties can be provided. The information sometimes is not easily obtained by other tools due to the nanoscale phase coexistence in these materials. Also, some remained open questions and future development in this research direction have been well discussed.


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## Body

## 1. Introduction

The
experiment  of laser-induced voltage (LIV) effect, in which theYBa2Cu3O7−δ (YBCO) thin films were used as a photosensor, was
performed firstly by Chang etal. [1]. It was found that there is an induced
voltage when pulsed  laser light on the surface of YBCO thin films. It is proved
that this LIV signal is induced by a thermoelectric anisotropy in the high TC superconductor (HTSC) oxides [2–5]. These films absorb incident laser
radiation, the top surface of thin film is heated rapidly, and an instant
temperature difference between the top and bottom parts of the films is
generated. This temperature gradient produces an electric voltage through the
difference between components of the Seebeck tensor. This anisotropy in
thermoelectricity originates from the anisotropic shape of the Fermi surface,
causing the different properties of atomic layers in the HTSC oxides with low
and high conductivity.They
behave like the two metals of a thermocouple. Therefore, these materials are
called atomic layer thermopile (ALT). LIV signals become even larger if the
films are grown on vicinal cut substrates because the vicinal cut structure
offers more atomic layer junctions upon the interface. Figure1 shows the
typical time response of an induced voltage with incident pulsed laser of
duration of 20 nanoseconds. The first quantitative description of this
phenomenon was due to Lengfellner etal. [3], subsequently these signals
observed in the ALT materials were defined as laser induced thermoelectric
voltages (LITVs). Since then several high TC superconductor oxides such
as doped YBCO, Bi2Sr2CaCu2O8,Tl2Ba2CaCu2O8, and so forth have been studied and all of them have shown
a similar effect [6–8]. Based on these
studies, light detection devices have been designed [9–13], this
demonstrates some superior characteristics over traditional bolometer and
photon counting devices. Most obvious advantages are that the device can
function over a broad spectral range at room temperature, has a fast time
response and no applied bias is required.Figure 1
Typical laser induced voltage responses from the as-grown and annealed LaCaMnO3 thin films grown on 15° tilted
LaAlO3 substrates, respectively.In
1998, Habermeier etal. observed a similar effect in thin films of the
quasicubicLa0.67Ca0.33MnO3 grown on vicinal cut substrates [14, 15]. This discovery
showed that the ALT property is not confined to HTSC oxide materials, but subsists
in other materials. On the other hand, Lanthanum manganites, which are
interesting for their colossal magnetoresistance (CMR) properties, have
attracted a lot of attention both from the viewpoint of basic physics as well
as potential applications. The discovery extends the field of application and
also provides new insight for the understanding the discipline of “strong-correlated
electrons.”The
LITV signal observed in ALT materials may be a general effect, and it has been
well conscious of that the effect can be applied on several devices.
However, there is no detailed relation between correlative macroscopic parameters
of the used materials and the device performances. We derived a new formula
based on a plane heat source and a thermal diffusion model [12]. This new
formula does not only correlate the used material parameters with the device
performances, but also facilitate the interpretation of physical mechanism of
ALT materials.Recently,
several new ALT materials were reported, which belong to members of
Ruddlesden-Popper family [15, 16]. Due to their different physical properties,
they are suited to function at different spectral ranges or time responses. A
new experiment shows that the superlattice like multilayer thin films grown on
vicinal cut substrates demonstrate enhanced LITV signals. We will discuss these
new ALT materials in detail in Section 3.The
first application of ALT materials is in light detection with advantages of
fast time and broad spectral response, operating at room temperature, with no
bias required. Based on the reverse effect, the Peltier effect, one can design
microcooler. Examples and device design consideration will be given in Section4.The
LITV measurements of HTSC and CMR thin films provide some important microscopic
information on these materials. The direct information obtained is the
anisotropy of the Seebeck coefficients, which are valuable for studying the
transport property. In strongly correlated electronic system, like HTSC and
CMR, currently the discussion has the focus on the nature of the phase
coexistence. The measured LITV signals from HTSC and CMR are very sensitive to
the doping, strain, oxygen content, charge- and orbital ordering, as well as
the external perturbations such as the temperature, the applied magnetic field,
and pressure. The information on anisotropic transport in these materials is
regarded as being very important and hard to obtain through other experimental methods.Although
the ALT model and the transport mechanism of CMR materials have been well
established, a number of questions in these studies are still needed to be
answered, for example, why do CMR thin films exhibit anisotropic Seebeck effect
even they are mostly cubic and in a paramagnetic phase, are there other
materials which contain even larger ALT property? These open questions will be
summarized at the end of this paper.


## 2. Basic Principles

A
thermoelectric fieldE
in a crystal can be induced by a
temperature gradient ΔT: (1)E=S⋅ΔT, where S  is Seebeck tensor. For a thin film
grown on vicinal cut substrate (See Figure 2(a)), illuminated by a laser pulse
on the surface, a temperature gradient ΔT in z-direction is
generated causing an induced voltage signal along the x-direction (Ux) due to the nonvanishing off-diagonal components of the Seebeck tensor. Based on
the experimental observations in YBa2Cu3O7−δ,
Lengfellner etal. derived a formula according to (1) [3] as
follows: (2)Ux=12d(Sab−Sc)sin(2α)∇¯zT, where Sab−Sc is the
difference of the Seebeck coefficients in the ab plane and c direction
of YBCO, respectively. l and d are the illuminated length and thickness of the film, α is the angle
between the surface normal and c-axis of the substrate, ZT the
temperature difference between the top and bottom part of the film. With this
formula, one can explain most of the observed experimental facts, and also
identify the nature of a laser-induced thermoelectric voltage from other
photoinduced effect experimentally. According to this formula, the key factor
is Sab−Sc, that is, the anisotropy of Seebeck
coefficient. Large transport anisotropy implies large induced voltage. It is
well known that the properties of atomic layers in YBa2Cu3O7 are very different: the CuO2 layer is highly conductive, while the Y(Ba)-O layer has a low conductivity, as
well as the Seebeck coefficients. Each two atomic layers form a junction like
the thermocouple. If the thin films are grown on vicinal cut substrate,
numerous of junction structures, which seem as the series thermocouples, form
spontaneously on the surface, as shown in Figure 2(c). With a pulsed light irradiating
on the film surface, the top surface is heated fleetly due to the light
absorption, and a temperature gradient is developed, thus a thermoelectric
voltage is produced in each junction. Due to the large number of junctions, the
total induced voltage is very high, and the time response is very short since
the distance for the carriers to move is very short.(a) The atomic layer thermopile thin film structure and the related coordinators. (b) The configuration of the
samples used in the LITV measurements. (c) The schematic atomic layer thermopile model.
(a)(b)(c)Figure3 shows tilting angle dependence of the LITV peak values measured in the YBCO
films grown on the SrTiO3 (STO) substrate. The results
indicate clearly that sin(2α) relation between the peak value of induced
voltage and the tilting angle of the substrate is
followed. Due to the difficulty in growing high-quality YBCO film on the
substrate with high tilting angle, the exception of the sample grown on 20°
tilted substrate does not increase with the increase tilting angle any more. ΔT depends on the incident photon fluency and the absorption of the film for
the corresponding photon energy. High absorption and small
penetration of the light in films lead to large ΔT and thus large
induced voltage. The longer the illuminated film length l, the larger
the induced voltage, that is, the more atomic scale thermocouples contributes
to the thermoelectric signals. The thinner the film thickness (smaller d),
the larger the induced voltage, this is correct when the films are relatively
thick. However, the description in (2) is proved to be unreasonable at very
thin films. Figure 4(a) shows the LITV peak values versus different thickness
in YBCO films grown on STO [5]. The increase of the peak value with reducing
thickness is followed above 250 nm, but the peak value undergoes a reduction
below 250 nm, this dependence cannot be simply ascribed to the loss of the perfection
of the thermopile arrangement. To explain this abnormal behavior, an improved
model has been derived to describe the LITV mechanism. We would like to discuss
this in the following.Figure 3
Measured LITV signals versus tilting angle of the substrate on which films are grown. The deviation from linear
relation of last point is due to the poor film quality grown on the lager lattice mismatched substrate.(a) The measured and (b) calculated LITV signals from YBCO thin films of different thicknesses are compared. The
solid lines in (a) are only used to guild eyes. Both the results show there is an optimum thickness for producing the
largest LITV signal.
(a)(b)In order to correlate the material parameters to the induced voltage and the
device performance, a new equation has been derived based on a plane heat
source and cascade power net model [12]: (3)U(t)=α0Elsin(2α)4dρc0πDt(Sab−Sc)(e−δ2/4Dt−e−d2/4Dt), where U(t) is time-dependent LITV signal, α0,ρ,δ,c0, and D are the
absorption coefficient, density, light penetration depth, specific heat, and
thermal diffusivity of the film material, respectively. E is the energy
of the incident pulsed laser integrated over the pulse duration, and all other
parameters share the same definition with that. This formula provides three important results. Firstly, the peak of LITV
signal is no more monotonic variation with the thickness d, in fact, an
optimum thickness dm which is corresponding to a maximum peak
of the induced signal does exist. Secondly, to achieve a fast time response,
materials with small thermal diffusivity D should be selected. This is
totally reasonable within the frame of this new mechanism, and the response
time now can be calculated quantitatively based on this formula. Thirdly, the
LITV signal is affected by the light absorption of the film materials. Large
absorption of the materials implies larger induced voltage. If a small penetration
depth δ for the incident light is fulfilled, then both large voltage and fast
time response can be achieved at the same time. On the other hand, Wang etal. has deduced the function of deciding the optimum thickness of film
materials by using them differential equation of (3) and some suitable boundary
conditions [17]. With inputting the material parameters of YBCO film, one can
obtain the calculated peak value Up as a function of d, as shown
in Figure 4(b). It is found that the optimum thickness of YBCO film for
producing the maximum Up is about 215 nm. The calculations agree with
the LITV experiments very well. As a result,
based on (3) and its derivative equations, it becomes easier for one to select
suitable materials to design a light detector in a specified spectral range
with desired time response and sensitivity [16–20].According
to the LITV experiments under different perturbations, one can obtain some thin
film properties, which may not be easily measured from the bulk value. Figure5
shows the experimental and calculated LITV signals of the YBCO thin films. The
film parameters of YBCO and LaCaMnO3 obtained from the fitting
process are listed in Table 1.Table 1
The parameters of materials obtained by fitting themeasured LITV curves
using (3).
ParameterSab−Sc(μV/K)D(m2/s)Δ(nm)dm(nm)YBCO107×10−796.8215LCMO0.227.8×10−9116.2196Figure 5
The measured and calculated time response curve from YBCO thin films, which demonstrate the new formula is
reasonable good in describing the time dependence of the induced voltage. The obtained parameters from the fitting
are listed in Table2.According to Mott’s band theory for non- or weakly interacting electrons, the Seebeck
coefficient is expressed as [21] (4)S=−(π23)(KB2Te){σ′(EF)σ(EF)}, where σ(EF) is the conductivity at Fermi level, and σ′(EF)=(∂σ/∂E)|Ef,KB and e are Boltzmann’s constant and the elementary charge,
respectively. As an approximation, (4) in most of metallic and
insulating system can be expressed as ΔS/S∼Δρ/ρ. It
suggests that there is a close relation between the Seebeck coefficient and the
resistivity: the larger the resistivity, the larger the Seebeck coefficients.
Equation (4) provides the basic relation between the LITV effect and the
physical properties of the strong corrected electronic materials, especially
the anisotropic transport property, which is valuable information for studying
the mechanism of HTSC and CMR materials. On the other hand, it also offers the
important hints to seek the more effective ATL materials, since high anisotropy
in conductivity links with high Seebeck anisotropy.


## 3. Alt Materials

The
ALT behavior is firstly observed in highTC superconductor
oxide YBCO [1]. Later, other HTSC materials, such as doped YBCO, Bi2Sr2CaCu2O8,Tl2Ba2CaCu2O8, and so forth, are
studied and all of them demonstrate ALT property, only the amplitude and time
response of these induced voltages are discrepant in different materials. These
oxides are known as highly anisotropic due to the special layer structure.
However, they are not stable in normal environment. Therefore, it is very
interesting to search more practical materials.The
experimental procedures for investigating ALT materials are composed of two
steps: the film preparation and the LITV measurements. The films are grown on
vicinal cut substrates by pulsed laser deposition (PLD). Targets
used are synthesized by solid-state reaction or coprecipitation, and can
be characterized using X-ray diffraction, and other methods. The selection of
substrates depends on the lattice parameter of the films to be grown on them.
The most frequently used substrates are single crystalSrTiO3 (STO), LaAlO3 (LAO), Si, LaSrGaO4, and MgO, and so forth. The
selected films have been characterized by using X-ray, Raman, TEM, and other
techniques to ensure that they are single phase grown on substrates. High-quality films are patterned using typical photolithography technique, and after
the application of electrodes, one can directly measure the LITV signals. Two
LITV measurement methods can be used according to the light source: pulsed
laser or continuous-wave light sources [10, 20]. We first report the results of
CMR thin film with ABO3 structure,
and then that of other new materials.


### 3.1.ABO3-type thin films

At
the beginning, it was a surprise to observe the LITV signal inLa0.67Ca0.33MnO3 thin
film grown on vicinal cut STO substrates, since no transport anisotropy is
expected in such manganites due to their cubic structure and showing
paramagnetic phase above TC. However, the signal is
robust and also observed at several other doping levels. We prepared the films
on substrates cut at different tilting angles to check whether these signals
were induced by thermoelectric effect. Figure 6 shows the LITV obtained from
Ag-doped LaCaMnO3 thin films grown on LAO. It
is clear that sin(2α) relation is followed the large tilting angle of
substrate, the larger peak of LITV
signal.
Therefore, the physics origin of these signals can be assigned to anisotropic
Seebeck effect, and the signals were thermoelectric voltages. These experiments
have demonstrated that these CMR thin films are a new type of ALT material
other than high TC superconductors.
Two arisen questions then have to be answered. Firstly, whether do other
manganite films with different compositions or doping levels possess strong
LITV signal, so that one can apply these new materials. Secondly, why do these
thin films show anisotropic transport properties. Inspired by the discovery,
systematic experiments were performed, including that on different doping
levels and dopant types.Figure 6
The LITV signals of Ag-doped LaCaMnO3 thin films grown on different tilting angle LaAlO3 substrates, which
demonstrate that the induced voltages are due to thermoelectric anisotropy.In
the doping experiment ofLa1−xCaxMnO3 system,
it is revealed that the decrease in the doping level of Ca leads to a large
induced voltage. According to the phase diagram, it is found that the crystal
structure change from cubic to orthorhombic with the reduction of doping level x.
Therefore, a large structural anisotropy can be expected. On the other hand,
less doping also changes the magnetic order from ferromagnetic to A-type
antiferromagnetic, this process may facilitate the anisotropic transport of
electrons at the same  time. Another interesting experimental result
arises from films grown on different types of substrates. Figure 7 shows the LITV signals from
LaCaMnO3 thin films grown on STO and
LAO substrates with the same tilting angle and prepared using identical PLD
conditions. The result reveals that the strains due to the mismatch between the
lattice parameters of the substrate and the films grown on it have strong
influence on the induced thermoelectric voltage.Figure 7
The influence of different substrate (LaAlO3 versus SrTiO3) on the LITV signals. The LaCaMnO3 thin films as the
samples are used in the measurements.Except
for theLa1−xCaxMnO3 thin films, several other
doped manganites CMR thin films have been tested and all show similar effect,
such as LaSrMnO3, LaCaSrMnO3, LaPbMnO3 (LPMO),
and LaBaMnO3 [22–25]. We also synthesized
cobaltite and other transition metal oxides with perovskite structure, which
also demonstrate ALT properties. One typical example is the La0.5Sr0.5CoO3 grown
on vicinal cut LAO substrates. La0.5Sr0.5CoO3 is a
conductive perovskite oxide and used as electrode in fuel cell and buffer layer
to grown lead-zirconate-titanate (PZT) thin films on Si substrate. We found
that the time response of La0.5Sr0.5CoO3 thin
films is extremely fast [18]. The half width of the response curve is almost
the same as the duration of the incident-pulsed laser, as shown in Figure 8(b).
Considering the limitation of the measuring method, the real time response
should be in the order of picoseconds.(a) The LITV time response curves from LaCaMnO3 and (b) LaSrCoO3 thin films. It is shown that the time response
of LaSrCoO3 is very fast (ps-ns) due to its highe conductivity.
(a)(b)The
fast time response of LITV inLa0.5Sr0.5CoO3 results
apparently from the high-thermal diffusivity, as described in (3). In case of
requirement of fast time response applications, La0.5Sr0.5CoO3 would
be a good candidate.


### 3.2. Ruddlesden-Popper family

Most
of the ABO3-type CMR thin films belongs to
the so-called infinite layer or 113 member of Ruddlesden-Popper family, in
which the anisotropy is relatively small due to their cubic structure.
Therefore, it would be interesting to test other member of Ruddlesden-Popper
family. One of the examples is La2−xSrxNiO4 (214
structure) grown on vicinal cut substrates [24]. Large LITV signals have been
measured. The first observation of LITV from this compound further enlarges the
range for seeking the more effective ALT materials. Comparing with 113 configuration,
the 214 structure demonstrates large anisotropic conductivity. Hence large LITV
signals can be expected. On the other hand, besides the cuprates, manganites,
and cobaltite, La2−xSrxNiO4 is
the first nickelate material which exhibits LITV effect. Other members of
Ruddlesden-Popper family of HTSC oxides have been measured, and all the
materials show high LITV signals [6–8].


### 3.3. Multilayer thin films

It
is well known that man-made superlattices show strong anisotropy in
conductivity. Is there the similar anisotropy in Seebeck tensors according to
(4)? Following this question, multilayer thin films were prepared by PLD. The
samples were composed with different periods (n)YBa2Cu3O7−δ and LaPbMnO3 (YBCO/LPMO) combination. The thickness of each YBCO and
LPMO layer was kept at 9 and 6 nm in these multilayer samples, respectively.The
first group multilayer thin films is grown on vicinal cut LAO substrates with
the tilting angle of0°,5°,10°,15°, and 20° between the surface normal and c-axis
by pulsed laser deposition technique. Details of the deposition process can be
found in our publication [25]. As shown in Figure 9(a), the experiments results
demonstrate clearly that the larger the tilting angles the higher the induced
voltages. Accordingly, our expectation has been confirmed in this experiment
that the observed effect originates from Seebeck effect. Furthermore, the LITV
signals are much larger than that of the individual single layer samples with
identically total thickness. The YBCO/LPMO superlattices in the second group
were deposited on LAO substrates with same tilting angle (10°),
but with different number of period as 7, 10, 15, and 20, respectively. The
LITV signals of these multilayer thin films with different periods have been
presented in Figure 9(b). In order to make a comparison, the LITV signals of
YBCO and LPMO single layer films with identical preparation process and
measuring condition are also shown in Figure 10. It is evident that the peak
value from multilayer YBCO/LPMO sample (20 periods) is about 7 and 34 times
larger than that of YBCO and LPMO single-layer films, respectively.(a) The LITV signals from the multilayer YBCO/LPMO thin films with 10 periods grown on different vicinal cut
substrates. From these curves, it has been confirmed that the LITV signals originate from thermoelectric anisotropy.
(b) The multilayer thin films with different period number n grown on vicinal cut LaAlO3(10°) substrates.
(a)(b)Figure 10
The LITV signals of the YBCO/LPMO multilayer film with 20 periods, the 400 nm thickYBa2Cu3O7−δ thin film,
and the 400 nm thick LaPbMmO3 films, respectively. The samples are fabricated and measured at the identical
experimental conditions.It
is known from (2) that the key factor for influencing the induced voltages is
the anisotropy of Seebeck tensor (Sab−Sc). In the
superlattice or multilayer film systems, it is difficult to predict the laser
absorption, penetration depth, and specific heat which also directly affect the
LITV signal, due to the large variation of these parameters in these special
systems. Therefore, one has to focus on the possibility of the enhancement from
the discrepancy of Seebeck tensor (Sab−Sc). For the multilayer
structure, it is obvious that the carrier transportation exhibits high anisotropy.
In the layer plane, the conductivity is rather high; while in the perpendicular
direction, that is relative smaller. With the increase of the period number,
the probability of carrier scattering enhances due to the increase of the
defects from the interface. As mentioned in (4), large resistivity leads to
large Seebeck coefficient, therefore, the large (Sab−Sc). Another
key influencing factor is the thermal conductivity. There are numbers of
discussion on the thermal conductivity of some manganites with layered
structures [26, 27]. Simkin and Mahan have calculated the phonon contribution
to thermal conductivity (D) in superlattices. It has been concluded that D is closely related to the number of superlattice periods (n)
and the phonon mean-free path. The heat conduction in perpendicular direction
of the layer plane is highest for the samples with small n, and rapidly
decreases with n increases, after reaching a minimum within the range of n=7–15, increases
again. The smaller the mean-free path of phonon implies the smaller the n,
at which the minimum thermal conductivity is achieved. In superlattice
materials, the electrons are confined for certain situation, and the phonons
are scattered at the interfaces. Therefore, the high-thermoelectric figure of
merit (ZT) is expected, and indeed the ZT as high as 2 to 3 have been measured
in several experiments [28–30].The
possible enhancement of(Sab−Sc) may originate from
the stressed lattices. According to X-ray diffraction, both YBCO and LPMO films
are compressively stressed in ab plane due to the smaller lattice
parameter of LAO substrate. Therefore, the lattice parameters in c-direction
were enlarged assuming preservation of the volume of the unit cell. The large
mismatch and the sandwiched structure can keep this strain in the whole sample.
This leads to the variation of intrinsic properties of each component films.
Due to the strain effects, the stressed YBCO and LPMO films must show different
transport properties, this has been verified by several experiments [16–19, 31].In a
word, there is a huge variety of strong correlated electronic materials which
are not yet studied from the viewpoint of their ALT properties. According to
our limited research so far, it is clear that most of the materials with layered
structure demonstrate ALT properties. For the first time, an enhanced LITV
signal from multilayer oxide thin films with a superlattice-like structure is
observed. Compared with single-layer film of YBCO or LPMO, the LITV signals of
the multilayer sample (the YBCO/LPMO structure with 7 periods) have
been enhanced for several times than those obtained in YBCO and LPMO
single films. This new observation is interesting for studying the transport
properties in superlattice-like multilayer thin films. The unusual enhancement
of the light-induced voltages in these multilayer films is possibly related to
the large Seebeck anisotropy and the reduction of thermal conductivity in
superlattice-like structures. The investigation of these materials and thin films provides candidates for different applications, such as light detection at different wavelengths or time responses. On the other hand, the measured LITV signals from the superlattices-likestructures provide information on the anisotropic transport property, which are hard to obtain by other experiments.


## 3.1.ABO3-type thin films

At
the beginning, it was a surprise to observe the LITV signal inLa0.67Ca0.33MnO3 thin
film grown on vicinal cut STO substrates, since no transport anisotropy is
expected in such manganites due to their cubic structure and showing
paramagnetic phase above TC. However, the signal is
robust and also observed at several other doping levels. We prepared the films
on substrates cut at different tilting angles to check whether these signals
were induced by thermoelectric effect. Figure 6 shows the LITV obtained from
Ag-doped LaCaMnO3 thin films grown on LAO. It
is clear that sin(2α) relation is followed the large tilting angle of
substrate, the larger peak of LITV
signal.
Therefore, the physics origin of these signals can be assigned to anisotropic
Seebeck effect, and the signals were thermoelectric voltages. These experiments
have demonstrated that these CMR thin films are a new type of ALT material
other than high TC superconductors.
Two arisen questions then have to be answered. Firstly, whether do other
manganite films with different compositions or doping levels possess strong
LITV signal, so that one can apply these new materials. Secondly, why do these
thin films show anisotropic transport properties. Inspired by the discovery,
systematic experiments were performed, including that on different doping
levels and dopant types.Figure 6
The LITV signals of Ag-doped LaCaMnO3 thin films grown on different tilting angle LaAlO3 substrates, which
demonstrate that the induced voltages are due to thermoelectric anisotropy.In
the doping experiment ofLa1−xCaxMnO3 system,
it is revealed that the decrease in the doping level of Ca leads to a large
induced voltage. According to the phase diagram, it is found that the crystal
structure change from cubic to orthorhombic with the reduction of doping level x.
Therefore, a large structural anisotropy can be expected. On the other hand,
less doping also changes the magnetic order from ferromagnetic to A-type
antiferromagnetic, this process may facilitate the anisotropic transport of
electrons at the same  time. Another interesting experimental result
arises from films grown on different types of substrates. Figure 7 shows the LITV signals from
LaCaMnO3 thin films grown on STO and
LAO substrates with the same tilting angle and prepared using identical PLD
conditions. The result reveals that the strains due to the mismatch between the
lattice parameters of the substrate and the films grown on it have strong
influence on the induced thermoelectric voltage.Figure 7
The influence of different substrate (LaAlO3 versus SrTiO3) on the LITV signals. The LaCaMnO3 thin films as the
samples are used in the measurements.Except
for theLa1−xCaxMnO3 thin films, several other
doped manganites CMR thin films have been tested and all show similar effect,
such as LaSrMnO3, LaCaSrMnO3, LaPbMnO3 (LPMO),
and LaBaMnO3 [22–25]. We also synthesized
cobaltite and other transition metal oxides with perovskite structure, which
also demonstrate ALT properties. One typical example is the La0.5Sr0.5CoO3 grown
on vicinal cut LAO substrates. La0.5Sr0.5CoO3 is a
conductive perovskite oxide and used as electrode in fuel cell and buffer layer
to grown lead-zirconate-titanate (PZT) thin films on Si substrate. We found
that the time response of La0.5Sr0.5CoO3 thin
films is extremely fast [18]. The half width of the response curve is almost
the same as the duration of the incident-pulsed laser, as shown in Figure 8(b).
Considering the limitation of the measuring method, the real time response
should be in the order of picoseconds.(a) The LITV time response curves from LaCaMnO3 and (b) LaSrCoO3 thin films. It is shown that the time response
of LaSrCoO3 is very fast (ps-ns) due to its highe conductivity.
(a)(b)The
fast time response of LITV inLa0.5Sr0.5CoO3 results
apparently from the high-thermal diffusivity, as described in (3). In case of
requirement of fast time response applications, La0.5Sr0.5CoO3 would
be a good candidate.


## 3.2. Ruddlesden-Popper family

Most
of the ABO3-type CMR thin films belongs to
the so-called infinite layer or 113 member of Ruddlesden-Popper family, in
which the anisotropy is relatively small due to their cubic structure.
Therefore, it would be interesting to test other member of Ruddlesden-Popper
family. One of the examples is La2−xSrxNiO4 (214
structure) grown on vicinal cut substrates [24]. Large LITV signals have been
measured. The first observation of LITV from this compound further enlarges the
range for seeking the more effective ALT materials. Comparing with 113 configuration,
the 214 structure demonstrates large anisotropic conductivity. Hence large LITV
signals can be expected. On the other hand, besides the cuprates, manganites,
and cobaltite, La2−xSrxNiO4 is
the first nickelate material which exhibits LITV effect. Other members of
Ruddlesden-Popper family of HTSC oxides have been measured, and all the
materials show high LITV signals [6–8].


## 3.3. Multilayer thin films

It
is well known that man-made superlattices show strong anisotropy in
conductivity. Is there the similar anisotropy in Seebeck tensors according to
(4)? Following this question, multilayer thin films were prepared by PLD. The
samples were composed with different periods (n)YBa2Cu3O7−δ and LaPbMnO3 (YBCO/LPMO) combination. The thickness of each YBCO and
LPMO layer was kept at 9 and 6 nm in these multilayer samples, respectively.The
first group multilayer thin films is grown on vicinal cut LAO substrates with
the tilting angle of0°,5°,10°,15°, and 20° between the surface normal and c-axis
by pulsed laser deposition technique. Details of the deposition process can be
found in our publication [25]. As shown in Figure 9(a), the experiments results
demonstrate clearly that the larger the tilting angles the higher the induced
voltages. Accordingly, our expectation has been confirmed in this experiment
that the observed effect originates from Seebeck effect. Furthermore, the LITV
signals are much larger than that of the individual single layer samples with
identically total thickness. The YBCO/LPMO superlattices in the second group
were deposited on LAO substrates with same tilting angle (10°),
but with different number of period as 7, 10, 15, and 20, respectively. The
LITV signals of these multilayer thin films with different periods have been
presented in Figure 9(b). In order to make a comparison, the LITV signals of
YBCO and LPMO single layer films with identical preparation process and
measuring condition are also shown in Figure 10. It is evident that the peak
value from multilayer YBCO/LPMO sample (20 periods) is about 7 and 34 times
larger than that of YBCO and LPMO single-layer films, respectively.(a) The LITV signals from the multilayer YBCO/LPMO thin films with 10 periods grown on different vicinal cut
substrates. From these curves, it has been confirmed that the LITV signals originate from thermoelectric anisotropy.
(b) The multilayer thin films with different period number n grown on vicinal cut LaAlO3(10°) substrates.
(a)(b)Figure 10
The LITV signals of the YBCO/LPMO multilayer film with 20 periods, the 400 nm thickYBa2Cu3O7−δ thin film,
and the 400 nm thick LaPbMmO3 films, respectively. The samples are fabricated and measured at the identical
experimental conditions.It
is known from (2) that the key factor for influencing the induced voltages is
the anisotropy of Seebeck tensor (Sab−Sc). In the
superlattice or multilayer film systems, it is difficult to predict the laser
absorption, penetration depth, and specific heat which also directly affect the
LITV signal, due to the large variation of these parameters in these special
systems. Therefore, one has to focus on the possibility of the enhancement from
the discrepancy of Seebeck tensor (Sab−Sc). For the multilayer
structure, it is obvious that the carrier transportation exhibits high anisotropy.
In the layer plane, the conductivity is rather high; while in the perpendicular
direction, that is relative smaller. With the increase of the period number,
the probability of carrier scattering enhances due to the increase of the
defects from the interface. As mentioned in (4), large resistivity leads to
large Seebeck coefficient, therefore, the large (Sab−Sc). Another
key influencing factor is the thermal conductivity. There are numbers of
discussion on the thermal conductivity of some manganites with layered
structures [26, 27]. Simkin and Mahan have calculated the phonon contribution
to thermal conductivity (D) in superlattices. It has been concluded that D is closely related to the number of superlattice periods (n)
and the phonon mean-free path. The heat conduction in perpendicular direction
of the layer plane is highest for the samples with small n, and rapidly
decreases with n increases, after reaching a minimum within the range of n=7–15, increases
again. The smaller the mean-free path of phonon implies the smaller the n,
at which the minimum thermal conductivity is achieved. In superlattice
materials, the electrons are confined for certain situation, and the phonons
are scattered at the interfaces. Therefore, the high-thermoelectric figure of
merit (ZT) is expected, and indeed the ZT as high as 2 to 3 have been measured
in several experiments [28–30].The
possible enhancement of(Sab−Sc) may originate from
the stressed lattices. According to X-ray diffraction, both YBCO and LPMO films
are compressively stressed in ab plane due to the smaller lattice
parameter of LAO substrate. Therefore, the lattice parameters in c-direction
were enlarged assuming preservation of the volume of the unit cell. The large
mismatch and the sandwiched structure can keep this strain in the whole sample.
This leads to the variation of intrinsic properties of each component films.
Due to the strain effects, the stressed YBCO and LPMO films must show different
transport properties, this has been verified by several experiments [16–19, 31].In a
word, there is a huge variety of strong correlated electronic materials which
are not yet studied from the viewpoint of their ALT properties. According to
our limited research so far, it is clear that most of the materials with layered
structure demonstrate ALT properties. For the first time, an enhanced LITV
signal from multilayer oxide thin films with a superlattice-like structure is
observed. Compared with single-layer film of YBCO or LPMO, the LITV signals of
the multilayer sample (the YBCO/LPMO structure with 7 periods) have
been enhanced for several times than those obtained in YBCO and LPMO
single films. This new observation is interesting for studying the transport
properties in superlattice-like multilayer thin films. The unusual enhancement
of the light-induced voltages in these multilayer films is possibly related to
the large Seebeck anisotropy and the reduction of thermal conductivity in
superlattice-like structures. The investigation of these materials and thin films provides candidates for different applications, such as light detection at different wavelengths or time responses. On the other hand, the measured LITV signals from the superlattices-likestructures provide information on the anisotropic transport property, which are hard to obtain by other experiments.


## 4. Application

Two
practical applications have been put forward and tested with the ALT materials.
One is light and heat detection, and the other is local cooling based on the
reverse of the Seebeck effect, the Peltier effect.The
thermoelectric response of an ALT material has a linear dependence on absorbed
radiation power. Therefore, a radiation detector was designed and tested [14, 18–20]. To compare
the ALT detectors with those fabricated by other materials, Table 2 lists the
main parameters for three types of detectors. Due to the broad absorption of
the black surfaces of most of the ALT materials, the LITV detectors can work in
broad spectrum range. The experiment shows that these detectors can response
from ultraviolet to middle infrared. According to (3), the time response
depends on the thermal diffusivity D and the penetration depth δ.
By proper selecting the materials responded at special wavelength, response
time from micro- to picoseconds is easy to be realized. Other advantages of
this type of detectors consist in that they show good performance at room
temperature without any biases.Table 2
A recapitulative comparison among the different types of photodetectors.
ParameterPhoton counting (Semiconductors)BolometersALT devices (LITV signal)Spectral rangeNarrowBroadBroadResponse timeFast (ps)Slow (ms)Fast (sub-ps to ns)SensitivityhighhighgoodBiasneedneedDo not needWorking temperature≤300 K45–110 K≥280 KConsidering
that the importance for fabricating the devices with fast time response and
high sensitivity, one can define a device figure of merit(Fm) to
describe the performance of the LITV detector [18]: (5)Fm=UPτ, where UP and τ are the peak value of
induced voltage and the full width at half maximum of the response curve of the
induced voltages. Therefore, synthesizing and searching for materials with high
LITV sensitivity and fast time response at the same time, namely, higher Fm is
interesting.We
have prepared Ag-dopedLa0.6Pb0.4MnO3 (Ag-LPMO) thin films by means of PLD, and investigated the doping
level effect on the LITV amplitude. By optimizing the doping level of Ag, it is
found that the Fm of
the LITV device is improved substantially.The
LITV signals of Ag-LPMO films with different Ag doping levels are shown in Figure11. The UP value
firstly increases with the increase of Ag doping level, then reaches a maximum
at doping level of 4–6 wt%, finally decreases.
At the same time, the time response τ varies with an opposite way,
reduce first and then increase. According to (5), one can calculate the Fm values of these
devices. Table 3 lists the measured UP, τ, and the calculated Fm at
different Ag-doping levels. It is evident that Fm value
can be enhanced to several times by optimizing doping level of Ag.Table 3
The measured conductivityδ,
peak values of LITV signals Up,
time response τ,
and the calculated figure of merit for
different Ag-doped La0.6Pb0.4MnO3 films grown on LaAlO3 substrates with 15° angle cut.
Ag-doping level (wt%)δ (S/cm)Up (mV)τ (ns)Fm0313.8251400.182567.7351060.3341302.143800.5362125.938650.588901.9481000.4810749.4201260.16Figure 11
Time dependent laser induced voltage curves obtained from different Ag-dopedLa0.6Pb0.4MnO3 films grown on
vicinal cut LaAlO3(15°) substrates. These samples are also fabricated and measured at the same experimental
conditions. It is shown that the figure of merit of the LITV device can be greatly improved with proper doping level.There
are lots of discussion on the enhancement of the ZT of thermoelectric
materials, and several new concepts to obtain materials with ZT higher than 1
have been reported [28–30]. To form
materials with “phonon glass and electron crystal” behavior become the central
issue to realize large ZT, namely, high-electronic conductivity and low-thermal
conductivity. Doping with monovalent ions, such as Ag, either in the lattice or
at grain boundary, always enhances electronic conductivity. At the same time,
the inhomogeneity is induced by doping Ag because the Ag ions form the centers
of phonon scattering. Therefore, doping with Ag in the LaPbMnO3 films
can enhance the ZT, and this may be an effective method for other material
system as well.The
reverse effect of Seebeck is Peltier effect, in which a thermoelectric cooling
can be realized by simply applying electric current. In an anisotropic
material, an applied electrical current densityJe can
produce a Peltier heat flow Jh,
which is described by (6)Jh=P⋅Je, where P is the Peltier tensor, related to Seebeck
tensor S, defined as P=S⋅T. The largest temperature difference ΔTm realized
in this effect depends on the thermoelectric figure of merit ZT and is
expressed as (7)ΔTm=(ZT)22. The cooling effect of ALT materials was firstly observed
in YBCO bulk [32]. Recently, more effective cooling effect was realized in a
Pb-Bi2Te3 multilayer
structure [33]. By applying a current in film plane, temperature difference up
to 22 K has been observed between top and bottom sides of the sample. The
cooling experiments on ALT materials are just launched. However, the renewed
interests in microcooling for various applications are pushing the related
investigations. The advantages of the type of cooling are easy to realize,
without moving parts, matching well with plane technology, and so on.


## 5. Physical Mechanisms and Open Questions

All
crystalline solid materials are composed of atomic layers, therefore, they
should have ALT behavior and can be the candidates for the application and LITV
measurements. So far, only few have been studied. Large unreached area remains
to be explored. The studies of HTSC and CMR thin films have put forward several
questions which should be answered, for example, why there is anisotropy in CMR
thin films with cubic or quasicubic crystal structure? Even more interesting is
that the LITV signals are recorded at temperature aboveTC,
the films are in paramagnetic phase. What information can be followed by
analyzing the LITV signals at different applied perturbations? Clear answer of
these questions involves in the mechanisms of charge and phonon transport in
the strongly correlated electronic systems, which are also the hot points of
condensed matter physics.We
now discuss the origin of LITV signals from CMR thin films and try to answer
the origin of the anisotropy in these films. In fact, the existence of LITV
signals is closely linked with the mixed phases of the complicated system.
Assuming that there are two competing phases nearTC in
CMR materials [34–36], the ferromagnetic
metallic (FMM) and charge-ordered antiferromagnetic (COAF) insulating phase,
the total induced voltage based on (2) is composed of the contribution of the
two: (8)UP=A[San1(1−C)+San2⋅C], where UP is
the peak value of induced voltages. San1 and San2 are defined as (Sab−Sc) in COAF and FMM phases, respectively. C is the concentration of FMM phase, which can be evaluated from the
measured temperature or applied magnetic fields dependence of magnetization (M)
and expressed as C(H,T)=M(H,T)/Msat [37].
A is a constant related to the geometric dimension of the sample and the
temperature gradient in the materials produced by the pulsed laser, which is
temperature independent in the first approximation. Considering no other phase
transition and only the two phases playing important role, two extreme cases
can be discussed. At the temperature range of T≫TC, the FMM phase disappears, C approaches to zero correspondingly. Based on (8), one can evaluate high-temperature
Seebeck anisotropy of COAF phase: San1(hT)=UP(hT)/A. While in the case of T≪TC the contribution of COAF
phase is negligible, small one can obtain Seebeck anisotropy of FMM phase at
low temperature San2(lT)=UP(lT)/A. Figure 12 shows the LITV
signals obtained from La0.67Ca0.33MnO3 thin
films grown on STO substrates [14]. According to the above analysis and
estimation, the Seebeck anisotropy of COAF phase at high temperature is San1(hT)=0.2/A, for FMM phase at low
temperature is San2(lT)=0.01/A, and
a ratio San1(hT)/San2(lT)=20 was
obtained.Figure 12
Temperature dependence of LITV signals (separated dots) and resistance (solid line) of LaCaMnO3 thin film. It is
shows that the amplitude of LITV signals is relative strong around TC.These
results are reasonable. For the FMM phase, the structure ofLa0.67Ca0.33MnO3 is
cubic, and the anisotropy is small, while the COAF phase is possibly noncubic
[34], this implies that a large anisotropy can be expected. In fact, it is well
known that the Seebeck coefficient is closely related to the conductivity. According to (4), the larger
the resistivity the larger the Seebeck coefficients. For
the two coexistent phases near and above TC,
the FMM phase is the one with higher conductivity and lower resistivity,
therefore the Seebeck coefficients are relative smaller. In addition, due to
the cubic structure, the anisotropy should be further suppressed at the same
time. On the contrary, the COAF phase is insulating (large resistivity), then a
large Seebeck coefficient is expected, and even a large structural anisotropy
occurs. With this argument and also considering the evaluated values and the ratio San1(hT)/San2(lT)=20,
one can further simplify (8) as UP=ASan1(1−c). This
approximation is valid at temperature near and above TC. Since C can be
evaluated from magnetization measurements accordingly, and based on the
measured LITV temperature dependence [8], the temperature dependence of San1 is easily estimated. Figure 13
shows the obtained San1−T dependence
from experiments, together with the used M(T) curve. Based on
these results, the concentration of FMM phase can be easily
estimated. With the decrease of temperature, it is shown that
the difference of the Seebeck coefficients San1=(Sab−Sc) or
the anisotropy of the resistivity in COAF phase increases firstly. After it
reach a maximum at around TC,
then the San1 decreases again. Since there
are significant LITV signals measured above TC, we
would like to emphasize that the anisotropic Seebeck coefficients are robust. Therefore,
an anisotropic transport behavior can be definitely observed above TC in
paramagnetic phase of La0.67Ca0.33MnO3 thin
films. This is unexpected, since the sample is cubic or quasicubic in terms of
its structure, and magnetically is in paramagnetic phase, both factors should
not lead to anisotropic transport properties. In fact, the temperature-dependent
Seebeck coefficient has been measured at temperature near and above TC in La0.67Ca0.33MnO3 bulk
by Jaime etal. [37], which is an isotropic value, and demonstrated
qualitatively the same temperature behavior as the deduced San1 from
LITV measurements. This means that the difference of Sab−Sc is roughly
proportional to the absolute value of S as a function of temperature.
The interesting point is that the obtained anisotropic components should be
correlated to the structural anisotropy as well as the conductivity anisotropy
of a possible COAF phases. Recently, there were a number of experiments which
reported the measurement of Seebeck coefficients from Pr0.67D0.33MnO3 (D = Ca, Sr, Pb, and Ba) by Venkataiah et
al. [38, 39], La0.7Sr0.3−xAgxMnO3 by Battabyal and Dey [40], these results demonstrated similar
temperature behavior as the one observed in [37]. However, all of the results of their samples are obtained in the bulk materials, and
less information on the anisotropy of the thermoelectric effects is given.
Furthermore, there is no discussion on the contribution of the competing phase in
these materials. Therefore, it is hard to tell which phase is responsible to
the anisotropy.Figure 13
Temperature dependence of Seebeck coefficient anisotropy(San1=Sab−Sc), deduced from measured LITV signal
based on two phases model. The dot line is only used to guide eyes.There
are several possible phases coexistent with the FMM phase aboveTC [41–43]. Figure 14 shows
the spin structure in A, C, and G types of antiferromagnetic phases, together
with a ferromagnetic (F) and newly proposed CE, E, and CxE1−x types of charge-ordered
antiferromagnetic phases. The parent compounds of doped La0.67Ca0.33MnO3 are LaMnO3 and CaMnO3. The undoped LaMnO3 is
orthorhombic (space group Pnma),
with magnetic ground state of A-type antiferromagnetic structure. In a-c plane,
the alignment of spins is parallel, it indicates the configuration of two-dimensional
ferromagnetism. While in the successive plane of b direction, the orientations
of the spins are antiparallel. Therefore, the transport anisotropy has been
built accordingly, that is, the electrons can move easily in the a-c plane,
but difficultly in the b direction due to the antiparallel spin
orientation. The other doping end is CaMnO3,
which is cubic in structure (space group Pm3m)
and with magnetic ground state of G-type antiferromagnetic structure. In this
phase, there is no anisotropy originated from crystal and magnetic structure.
With different doping levels, the COAF phases could be a mixture of all these
phases at temperature near and above TC.
The CE phase is most likely the candidate, which is responsible for an
anisotropic transport property. The zigzag conductive chains are in ab planes,
while the conductivity in c direction is very poor because the hopping of
spin is forbidden. The E-type of COAF phase is similar to CE-type. However, the
zigzag chains are composed of two parallel oriented spins. Therefore, there is
also anisotropic transport property. The carrier transport may be easier in the
zigzag chains than that in CE phase. Up to now, it is hard to directly
correlate which phase is responsible for the measured Seebeck anisotropy. More
theoretical and structural studies are required to make the question be clearer.The possible antiferromagnetic phases in mixed phases of CMR materials aroundTC.
(a)(b)In
summery, a new experimental method, laser induced thermoelectric voltage effect
from CMR thin films, was used for the first time to reveal the transport
anisotropy at temperature near and aboveTC in La0.67Ca0.33MnO3 thin
film. By neglecting the contribution of FMM phase, the temperature dependence
of Sab−Sc in
COAF phase was evaluated, this dependence provides clear evidence for the
existence of noncubic, highly anisotropic phase above TC.The
results show that the Seebeck coefficients differenceSab−Sc in
COAF phase increases firstly with the lowering temperature, and then reaches a
maximum at around TC,
finally decreases again. This information is unique, and may offer great help in
correlating COAF phase type as well as the mechanisms of the complicated phase
competition and coexistence in CMR materials. In fact, similar situation is
also in high TC superconductor
materials, where a T* is defined, and little is known on the region
between T* and TC.
Therefore, LITV measurements may contribute valuable information on the
anisotropic transport, and hence the physical understanding of strongly
correlated electronic systems.


## 6. Conclusion

Atomic layer thermopile (ALT) materials are a new type of materials, which
demonstrate novel thermoelectric property. The basic principle
originates from the different transport properties in different atomic layers,
this induces the anisotropic Seebeck effect. This effect can be used in light
or heat detection, and local cooling based on its inverse effect. There are
large amounts of materials which may exhibit ALT property, but not yet well
studied. The investigations of LITV in different ALT materials provide unique
information on transport properties at temperatures especially important for
study the mechanism of highTC superconductor
and CMR effect.



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*Source: 329601-2008-11-11.xml*