Giant bulk photovoltaic effect in thin ferroelectric BaTiO3 films

RAPID COMMUNICATIONS

PHYSICAL REVIEW B 90, 161409(R) (2014)

Giant bulk photovoltaic effect in thin ferroelectric BaTiO3 films A. Zenkevich,1,2 Yu. Matveyev,1,2 K. Maksimova,3 R. Gaynutdinov,4 A. Tolstikhina,4 and V. Fridkin4,* 1

Moscow Institute of Physics and Technology, 9 Institutskiy lane, Dolgoprudny, Moscow Region 141700, Russia 2 National Research Nuclear University MEPhI, 31 Kashirskoe chaussee, Moscow 115409, Russia 3 Deutsches Electronen Synchrotron, 85 Notkestraße, Hamburg D-22607, Germany 4 Institute of Crystallography, Russian Academy of Sciences, 59 Leninskii prospekt, Moscow 119333, Russia (Received 1 July 2014; revised manuscript received 12 September 2014; published 28 October 2014)

The voltage generated in a noncentrosymmetric crystal due to the bulk photovoltaic effect (BPE) can greatly exceed the energy gap, however, the light energy conversion efficiency is extremely low. Here we show that the BPE is remarkably enhanced in the case of thin films. The measurements of the BPE in heteroepitaxial single domain ferroelectric BaTiO3 thin films reveal the enhancement of both photoinduced electric field and conversion efficiencies of the BPE by more than 4 orders of magnitude. Besides the fundamental aspect, our results indicate the potential for the use of the BPE in photovoltaic applications. DOI: 10.1103/PhysRevB.90.161409

PACS number(s): 72.40.+w, 73.50.Pz, 84.60.Jt

The conversion of light energy to electricity in photovoltaic devices implies the presence of the built-in potential, which facilitates the separation of electrons and holes excited in the absorbing layer. In a classical solid-state photovoltaic device the charge is separated due to the potential developed at the p-n junction. Polar materials, and particularly ferroelectrics, present an alternative pathway, which provides charge separation to generate a photovoltaic effect. In the past, the photovoltaic current was observed in the paraelectric (centrosymmetric) phase of BaTiO3 , however, the origin of these currents remained unclear [1]. In the beginning of the 1970s, the anomalous photovoltaic effect, which was called the bulk photovoltaic effect (BPE), was experimentally discovered in bulk crystals without a symmetry center [2,3]. The first model of the BPE was suggested in Ref. [4], and the theoretical approach was developed in Refs. [5–7]. Unlike classical semiconductor-based photovoltaics, the voltage generated in the BPE can be much higher than the band gap of a bulk ferroelectric, however, the energy conversion efficiency is extremely low. Over the past decade, photovoltaic effects in polar materials have attracted renewed attention [9–19]. The prevailing point of view on the nature of the photovoltaic effects observed in the devices comprising thin ferroelectric films suggests the combined effects of the remaining unscreened depolarizing electric field and the formation of the Schottky barriers at the metal contact/ferroelectric interface [9–12]. Alternatively, the above-band-gap voltages generated in ferroelectric crystals were discovered and were explained by the charge separation at the nanometer-wide steps of the electrostatic potential in the domain walls [13]. The abnormal photovoltaic effect and several orders of magnitude local enhancement of the photocurrent density and quantum efficiency have been reported while using an atomic force microscopy tip as a collector of photoexcited carriers [19]. In the present Rapid Communication, we report on the abnormal photovoltaic effect in the devices comprising thin ferroelectric BaTiO3 films. We show that the observed several orders of magnitude enhancement of both the photoinduced electric field

*

[email protected]

1098-0121/2014/90(16)/161409(5)

and the light energy conversion efficiency as compared to the bulk crystals can be explained in terms of the BPE strongly manifesting itself in thin films. The origin of the BPE considered in this Rapid Communication has been previously shown [5] to be fundamentally different from that of the classical photovoltaic effect and is analogous to the well-known parity nonconservation effect in weak interaction [8]. If a homogeneous medium without a symmetry center with short-circuited electrodes is subjected to a uniform illumination, it leads to the generation of a steady-state current jpv , whose value depends on the light polarization. If the electrodes are disconnected, i.e., in the open-circuit conditions, the current jpv generates the photovoltage, Upv =

jpv l , σd + σpv

(1)

where σd and σpv are the dark conductivity and photoconductivity, respectively, and l is the distance between electrodes. If we suppose that σpv σd , the electric field Epv photoinduced due to the BPE is given by Epv =

jpv , σpv

(2)

where σpv = eI0 αϕ(ω)−1 (μτ )pν . Here μpv and τpv are the mobility and the lifetime of nonequilibrium carriers, I0 is the incident light intensity, α is the absorption coefficient, ω is the photon energy, ϕ is the quantum yield, and e is the elementary charge. If σpv is small enough (which is the case for most known ferroelectrics and piezoelectrics), the photovoltage Upv in a crystal can exceed the energy gap by several orders of magnitude. Since the BPE exists only in noncentrosymmetric crystals, it can be observed in 20 point symmetry groups of ferroelectrics and piezoelectrics. The tensor properties of the linear BPE current are described by i jpv = αGij l ej el I0 ,

(3)

where ej and el are the components of the light polarization vector and Gij l is the corresponding third-rank piezoelectric tensor. 161409-1

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A. ZENKEVICH et al.


(a)

Vacuum

(b)

light absorbed by the unit surface Qpv = αI0 l,

Vacuum

Ec

η=

Ec

hν

hν Ev

Ev

η = GEpv

There are corresponding scalar relations [5], Epv =

ω G . ϕ(μτ )pv e

(4)

It is seen from (1) that for σpv σd the photovoltaic field Epv does not depend on the light intensity I0 . From the microscopic point of view, the “ballistic” and “shift” mechanisms of the BPE have been previously identified [5]. The ballistic mechanism is associated with the excitation of nonthermalized (hot) carriers in a crystal and is caused by the violation of the detailed balance principle. Figure 1 illustrates the internal photoeffect in (a) centrosymmetric and (b) noncentrosymmetric crystals. The fundamental difference is that the excitation of a photoelectron in a noncentrosymmetric crystal provides an asymmetric momentum distribution of nonthermalized carriers in the conduction band. The photoexcited nonthermalized carriers lose their energy and descend to the bottom of the band, which results in the shift in space l 0 . The alternative shift mechanism of the BPE [5–7] is of quantum-mechanical nature. It is obtained by taking into account the nondiagonal elements of the density matrix. The BPE in this case is caused not by the carrier movement in the band, but by the virtual shift R in the real space following the carrier band-band transition. The values of both l 0 and R are estimated to be on the order of 10–100 nm [5]. The specific ballistic and shift contributions to the BPE can be distinguished by performing Hall-effect measurements [20,21]. However, irrespective of the particular mechanism, both characteristic shifts in excited carriers l 0 and R, which affect the photoinduced Epv , are of the same order of magnitude. It is beyond the scope of the current Rapid Communication to establish the particular microscopic BPE mechanism, and the ballistic one, considered further below and illustrated in Fig. 1, is chosen only for the sake of clarity. The amplitude of the tensor component G has the form [5] G = el0 ξ ϕ(ω)−1 ,

(6)

Taking into account (4), the conversion efficiency is as follows:

FIG. 1. (Color online) (a) Isotropic and (b) anisotropic nonequilibrium carriers momentum distribution in centrosymmetric and noncentrosymmetric crystals corresponding to the classical and bulk photovoltaic effects, respectively.

jpv = αGI0 ,

2 2 jpv jpv QR = = Qpv αI σpv αI0 (μτ )pv

(5)

where ξ is the parameter, characterizing the excitation asymmetry. The energy conversion efficiency η for the unit surface is determined by the ratio between the power dissipated in the 2 −1 −1 R, where R = σpv l and σpv ∼ (μτ )−1 load QR = jpv pv and the

(7)

where G is the corresponding component of the BPE tensor (3) and Epv is the BPE photoinduced electric field (4). The values of G and Epv have been measured for the most known ferroelectric and piezoelectric crystals [5,22,23]. It was previously shown for bulk crystals that the conversion efficiency in the BPE is extremely low. For example, BaTiO3 bulk crystals (symmetry group C4v ) reveal G31 ∼ (3 − 6)×10−9 cm/V, Ep ∼ 102 V/cm, and η ∼ 10−7 [5,24]. It is worth noting that these values do not depend on the light intensity I 0 . However, at the nanoscale, when the thickness of the ferroelectric crystal l is comparable to or less than the shift in the nonthermalized electron l 0 , the photoinduced electric field and the conversion efficiency can be expected to become much larger. For l ≈ l0 all excited carriers are nonthermalized and contribute to the photovoltaic current jpv . The value of the shift l 0 depends on the asymmetry parameter ξ as well as on the energy of the exciting light. In Ref. [5] it was shown that in the frame of band description l 0 can be in the range of l0 ∼ 101 −102 nm, although this estimation was very rough. The growth procedure of Pt/BaTiO3 heterostructures used in this Rapid Communication and their structural properties have been described in detail elsewhere [25–27]. Briefly, heteroepitaxial Pt and BaTiO3 layers are successively grown on a single-crystalline MgO(001) substrate in a single vacuum cycle by a pulsed laser deposition technique in an O2 atmosphere (PO2 = 10−2 mbar) at elevated temperatures of T = 300 °C and T = 500 °C, respectively. Due to the favorable lattice mismatch (aBaTiO3 − aPt )/aPt ≈ 1.8%, a thin heteroepitaxial BaTiO3 layer on Pt is under biaxial in-plane (out-ofplane) compressive (tensile) strain as was previously revealed by x-ray diffraction (XRD) measurements (see Ref. [27] and Supplemental Material [28]). The stress in the BaTiO3 layer produces additional tetragonal distortion and ensures that the ferroelectric polarization is aligned in the direction perpendicular to the surface. Indeed, as-grown heteroepitaxial BaTiO3 (100) films are monodomain ferroelectrics with the spontaneous polarization direction perpendicular to the surface of the film as revealed by piezoresponse force microscopy (see Refs. [26,29]). The obtained hysteresis loops are symmetric with respect to zero bias (see Refs. [28,29]) ruling out any internal unscreened depolarizing electric field in our thin-film BaTiO3 . To further improve the structural quality of and subsequently to minimize the leakage paths across as-grown BaTiO3 (100) films, they were subjected to postannealing in oxygen at the atmospheric pressure of T = 700 °C, 2 h prior to the top electrode formation. The resistivity across thus prepared BaTiO3 films 20 and 50 nm in thickness is R ≈ 5×105 /cm2 . The series of lithographically patterned semitransparent (10–15-nm-thick) Pt electrodes 10–50 μm in diameter were deposited on top of the BaTiO3 film surface. The set of light-emitting diodes with the emission band around

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GIANT BULK PHOTOVOLTAIC EFFECT IN THIN . . .


Top Pt contact

10-50 µm

hν = 3.4 eV

100 μm

A 10-15 nm

Pt BaTiO3 (001)

Photocurrent (μA/cm2)

2

-4

3 4

-6

5

dBaTiO = 50 nm

-0.5

-1

-1.5

-2 500

0.2

0.4

0.6

0.8

1 1

600

Time (s) FIG. 3. (Color online) Time evolution of the photovoltaic current generated in a thin ferroelectric BaTiO3 film.


0

400

0

Voltage (V)

0

300

-2

3

(b)

λ ≈ 360 nm (E ≈ 3.4 eV) and the controlled overall output light power up to 50 mW were used for the illumination, yielding the incident light power density on the sample surface up to 0.5W/cm2 . The absorption in the top 10–15-nm-thick Pt electrode at our experimental conditions was measured to be ∼3, thus decreasing the actual light intensity illuminating the BaTiO3 layer. The reflection of the light from the sample surface was neglected in the calculations thus somewhat underestimating the energy conversion efficiency. The photoelectrical measurements were performed using the probe station Cascade Summit 1100 coupled with the Agilent Semiconductor Device Analyzer B1500A allowing measurement of currents down to 10−14 A. The Pt underlayer used as a bottom electrode was contacted by the probe through the protrusion made in the BaTiO3 layer with an ion beam. The scheme of the experiment is shown in Fig. 2.

200

1

-0.2

FIG. 2. (Color online) Scheme of measurements of the bulk photovoltaic effect in thin-film ferroelectric BaTiO3 films (inset: optical microscopy view of the sample under investigation).

100

0

-10

MgO (001)

0

2

-8

20, 50 nm 20 nm

Pt


(a)

-1

2

-2

3

-3 4

-4 5

dBaTiO = 20 nm

-5

3

-6 -0.2

0

0.2

0.4

0.6

Voltage (V)

FIG. 4. (Color online) Photovoltaic current for different light intensities measured on a thin-film heteroepitaxial Pt/BaTiO3 /Pt sample for (a) l = 50 nm and (b) l = 20 nm: 1: dark current; 2: I = 1.5×10−1 W/cm2 ; 3: I = 3×10−1 W/cm2 ; 4: I = 4.5×10−1 W/cm2 ; and 5: I = 7.5×10−1 W/cm2 .

Figure 3 displays time evolution of the photocurrent across the Pt/BaTiO3 /Pt trilayer (lBaTiO3 = 20 nm) after switching on the illumination. Except for the initial pyroelectric signal [4], the photovoltaic current reveals the steady-state character. Figure 4 displays I -V plots during the illumination of the Pt/BaTiO3 /Pt stack at different intensities along with the dark current (measured without any illumination). I -V measurements in the Upv range from −0.1 to +0.1 V were performed for both 20-nm- [Fig. 4(a)] and 50-nm- [Fig. 4(b)] thick BaTiO3 films. The sets of linear I -V characteristics obtained for both film thicknesses and different illumination intensities are then linearly extrapolated to the crossing with the U axis to extract the photovoltage [which, in accordance with (4), does not depend on the light intensity]. From these measurements, we get similar values Upv ≈ 0.60(5) and Upv ≈ 0.65(5) V for 20- and 50-nm-thick films, respectively. The corresponding photoinduced electric field is Epv ≈ 3×105 (1.3×105 ) V/cm for 20- (50-) nm-thick films. We are ruling out the presence of the internal electric field in our single domain isolating ferroelectric BaTiO3 films sandwiched between symmetric Pt electrodes, related to the unscreened depolarizing field, domain walls, or space charges in the film. For this reason, we further treat the observed photocurrents in terms of the BPE persisting without any internal electric field. We do not distinguish the particular

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microscopic mechanism of the BPE since both yield the same effect. The obtained Epv values exceed by 4 orders of magnitude those for the bulk crystals [5] and thick films of BaTiO3 [13]. It is worth noting that the obtained values of Epv for both thicknesses of BaTiO3 are less than the corresponding coercive field Ec . It is because, unlike bulk crystals, the previously obtained values of Ec for ultrathin BaTiO3 films [30,31] are very close to the intrinsic Landau-Ginzburg value Ec ∼ 106 V/cm for BaTiO3 [31]. Furthermore, assuming the bulk absorption coefficient for BaTiO3 in our spectral range α = 5×102 cm−1 [32], the calculated tensor component is equal to G31 ≈ 2×10−8 V/cm, which is somewhat larger than the literature data for the BaTiO3 crystal [5]. Finally, for the energy conversion coefficient we get η = Epν G31 ∼ 6×10−3 , which is 4.5 orders of magnitude larger as compared to the bulk crystals. The fact that Upv is almost the same for different thicknesses of the absorbing BaTiO3 layer points at approximately two times variation in the tensor component G31 value as the film thickness is decreased from 50 to 20 nm. The difference in the obtained G31 values as compared to the bulk crystal and for different thicknesses can be attributed to the in-plane compressive strain observed in the BaTiO3 films heteroepitaxially grown on Pt underlayers [27], the magnitude of the stress varying depending on the thickness. This may also mean that the shift in nonthermalized electrons l 0 defining the magnitude of the BPE is somewhat smaller than the thickness of the absorbing layer (20–50 nm). Thus, both Epv and η = Epν G31 are not fully saturated. To determine more accurately the value of l 0 one needs to perform the same experiments for the films with noncentrosymmetric crystal structure down to nanometer thickness. Such experiments for few nanometer-thick single domain ferroelectric BaTiO3 films are under way, and the results on the effect of the film thickness on the values of Epv and η will be reported in a forthcoming paper. It is worth noting that if the value of l 0 approaches the nanometer scale, the photoinduced electric field can even lead to the photobreakdown of the film.

The observed enhancement of the energy conversion efficiency η in BaTiO3 thin films due to the BPE up to η ≈ 1% is still much lower than the commercial solar cells exploiting classical photovoltaic effect. However, considering other thin-film materials with noncentrosymmetric crystal structures which exhibit different Epv and G parameters, such as ferroelectric PbTiO3 , LiNbO3 :Fe, and BiFeO3 [5], one can expect that the described nonclassical photovoltaic effect can be further enhanced. It might then provide a viable alternative for the effective use of the BPE in photovoltaic applications. In particular, recently reported tip-enhanced anomalous photovoltaic effects in bismuth ferrite were claimed to have implications for commercial photovoltaics [19]. We believe that the observed effects can be explained by the BPE manifesting itself in the region close to the tip. In conclusion, the bulk photovoltaic effect observed in the noncentrosymmetric crystals depends on the relative contribution of nonthermalized excited carriers. We have shown that once the thickness of the crystal is reduced so that it is comparable to the characteristic shift in the nonthermalized carriers yielding the photoinduced electric field, the effect is greatly enhanced. The measurements performed on the ferroelectric BaTiO3 films few tens of nanometers in thickness reveal the sharp, by 4 orders of magnitude, increase in the photoinduced electric field generated in the film and the efficiency of the light energy conversion as compared to the bulk material. Besides the fundamental importance, such enhancement indicates the potential for the use of the BPE in photovoltaic applications.

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Help from M. Spiridonov (MIPT) with piezoresponse force microscopy characterization of as-grown samples is acknowledged. This work was performed using equipment at the Baltic Federal University and MIPT Centers of Collective Usage and with financial support from the Ministry of Education and Science of the Russian Federation (Grant No. RFMEFI59414X0009).

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