PHYSICS OF THE SOLID STATE VOLUME 40, NUMBER 4 APRIL 1998

Anomalously high photovoltages in terbium molybdateB. K. Ponomarev, V. D. Negri , G. M. Vizdrik, and B. S. Red’kin

Institute of Solid-State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Province,Russia

I. A. Kornev

Novgorod State University, 173003 Novgorod, Russia~Submitted October 22, 1997!Fiz. Tverd. Tela~St. Petersburg! 40, 719–721~April 1998!

In terbium molybdate Tb2(MoO4)3 at room temperature, a laser beam of wavelengthl154880Å induces an electric polarization that lasts for several days after the irradiation is turnedoff. The photoinduced polarization has the same sign as the spontaneous polarization. At a laserpower of 0.5 W and an exposure time of 4.5 h, the photoinduced polarization exceeds thespontaneous polarization by an order of magnitude. The corresponding electric field is2.53106 V/cm. Under the influence of radiation withl154880 Å, terbium molybdateluminesces in the green part of the spectrum nearl2'5425 Å. The luminescence quantum yieldis 20%. © 1998 American Institute of Physics.@S1063-7834~98!03104-9#

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The phenomenon of anomalously high photovoltag~AHP effect! has been observed experimentally in a numof ferroelectrics: SbSi0.35Br0.65, BaTiO3, LiNbO3, SbNbO3,KNbO3, anda-LiIO3 ~Refs. 1–6!. The highest photoinducefield E'105V/cm was obtained in LiNbO3 ~Ref. 3!.

Here we report an investigation of the AHP effect in tmetastable, orthorhombic, ferroelectricb8 phase of terbiummolybdate Tb2(MoO4)3. A Tb2(MoO4)3 single crystal wasgrown by the Czochralski method described in Ref. 7. Tsamples comprised polished 33332 mm3 rectangular par-allelepipeds. The square faces of the samples were perdicular to the polar@001# axis, which was the direction ospontaneous electric polarization. The edges of the sqfaces were parallel to the@110# and@11̄0# axes. The potentiadifference between the~001! faces was measured. The mesurements were performed by means of a Cary 31 electreter. The latter had a dc input resistance of at least 1014 V.To safeguard against electrometer overload, the sampleshunted by an additional capacitance. For a sample captanceCsam50.36310212 F, the additional capacitance waCadd56.4631029 F. The photoinduced potential differencdecreased 18000-fold in this case. The time constant ofelectrometer input circuit was of the order of 102 h. Spring-loaded brass and sprayed-on aluminum contacts were usthe measurements.

Prior to the measurements the sample was made a sdomain by a procedure described in Ref. 8. The rootemperature value of the spontaneous polarizationPs and theprofile of its temperature curvePs(T) agreed with publisheddata.

During the measurements the sample was containeddouble-walled copper container. The container serveddual role of electrostatic shield and thermostat. A heaterenclosed in the container. The container walls had openfor the passage of a laser beam. This structure was suitfor electric polarization measurements in the tempera

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range from room temperature to 220 °C. The temperatwas measured by means of a Chromel-Alumel thermocouThe laser had an output power of 0.5 W and an emisswavelength of 4880 Å. The latter corresponded to the intrsic absorption of Tb2(MoO4)3 due to the dipole-forbiddentransitions7F6⇒5D4 ~Ref. 9!. The laser beam was paralleto the orthorhombic@110# axis and passed through thmiddle of the sample at various distances from the~001!faces. The polarization plane of the laser radiation was ppendicular to the~001! faces. At the given radiation powelevel and wavelength the equilibrium temperature of tsample in the container with the heater off was appromately 40 °C. The equilibrium temperature was determinexperimentally prior to the start of the photoinduced voltameasurements.

Before the measurements the temperature of the shosample was raised to 40 °C by the heater without radiatOnce this temperature had been established, the laserturned on, the heater was turned off, the shorting contactopened, and the sample was connected to the electromThe measurements were thus performed in thermal equrium between the sample and the laser beam. This condeliminated any contribution from the pyroelectric effect. Tmeasured potential differenceV(t) at timet was proportionalto the photoinduced electric polarization of the samPind(t)5P(t)2P(t0). Here P is the electric polarization,and t0 is the start time of the measurements. The measment results are shown in Fig. 1. The various curves wmeasured at the same temperature of 40 °C but with diffeshapes of the laser beam cross section.

Curve1 was measured while the sample was illuminatwith an unfocused beam having a circular cross sectiondiameter'1 mm. We see that the illumination of the sampcreates a potential differenceV(t) between its~001! facesthat increases with time. The average photoinduced elecpolarizationPind(t) over the volume of the sample is dete

© 1998 American Institute of Physics

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662 Phys. Solid State 40 (4), April 1998 Ponomarev et al.

mined from the measured quantityV(t) according to the re-lation Pind(t)5@V(t)•Csam1Cadd)]/S. HereS50.09 cm2 isthe area of the~001! faces of the sample, between which tpotential differenceV(t) was measured. The rate of increaof the potential difference decreases with time. The voltaafter four-hour illumination of the sample is 6.2 V. It corrsponds toPind544531029 C/cm2, which is two and a halftimes the room-temperature spontaneous polarizationTb2(MoO4)3: Ps5187.53109 C/cm2 ~Ref. 10!. The photo-induced polarization has the same sign as the spontanpolarization. The potential differenceV(t) remains almostconstant after removal of the laser beam, decreasing by apercent after 24 h. After shorting of the photopolarizsample the temperature dependence of its spontaneous pization Ps(T) does not differ more than a few percent frothe initial value. In Tb2(MoO4)3 radiation at the wavelengthl154880 Å stimulates strong luminescence in the greenof the spectrum nearl2'5425 Å. The luminescence quantum yield is'20%.

For the measurements in curve2, the laser beam wafocused by a cylindrical lens. The beam cross section wathe shape of a narrow stripe approximately 0.1 mm wideapproximately 2 mm long, oriented along the polar@001#axis of the crystal and passing through the middle of~110! face, its ends almost touching the~001! faces of thesample. Clearly, when the beam is focused in this way,Pind

increases more rapidly than without focusing. The electroeter input voltage after 4.5-h illumination of the sample28 V. This voltage corresponds toPind'231026 C/cm2,which is an order of magnitude higher than the spontanepolarization.

For the measurements in curve3, the beam formed astripe parallel to the~001! faces of the sample and was drected onto the middle of the~110! face of the sample. It isevident that for this orientation of the laser spotPind in-

FIG. 1. Photovoltage in Tb2(MoO4)3 versus time for various laser beamcross sections.1! Circular cross section of diameter 1 mm;2! stripe of width0.1 mm and length 2 mm, parallel to the@001# axis of the crystal;3! thesame cross section as for curve2, but with the stripe perpendicular to th@001# axis;4! the same cross section as for curve2, but from later measure-ment.

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creases far more slowly with time than in the two precedcases.

For the measurements corresponding to curve4, the la-ser spot was oriented as in the curve-2 measurements. Acomparison of curves 2 and 4 shows that the effect depeon the previous irradiations of the sample.

The values of the photocurrent can be determined frthe curves in the figure by means of the relationi (t)52(Csam1Cadd)(dV/dt). The values of the current at thstart of illumination arei'10211 A. The current depends onthe distance between the illuminated spot and the electroThe larger the distance, the lower is the current.

The value attained by us for the photoinduced fieEind52.53106 V/cm is two orders of magnitude higher thathe valueEind51.053104 V/cm for SbSi0.35Br0.65 in Ref. 1and is an order of magnitude higher than the valueEind

5105V/cm for LiNbO3 in Ref. 3. In Ref. 3Eind was evalu-ated by direct measurements. In the present study and in1 it was determined from measurements of the photopoization Pind according to the relationEind54pPind /«. Thevalues of« are 10 for Tb2(MoO4)3 ~Ref. 11! and 23103 forSbSi0.35Br0.65 ~Ref. 1!.

According to Fridkin,12 there are two possible causesthe onset of anomalously high photovoltages: 1! the photo-voltaic effect; 2! transition photocurrents.

The photovoltaic effect refers to the generation ofsteady current~the photovoltaic current! in a single-domainferroelectric single crystal when it is uniformly illuminatedthe induced current can be attributed to the asymmetry ofpotential distribution of impurity centers. As a result of thasymmetry the photoexcitation of an impurity center cauan electron to be ejected into the conduction band, produca steady current. In our case the role of such centers cafilled by Tb31 ions. The asymmetric potential distribution othese centers also causes them to move when ion~Franck–Condon relaxation!. This mechanism also contributes to the steady photovoltaic current.

The AHP effect in ferroelectrics can be identified nonly with the steady photovoltaic current, but also with trasition photocurrents. According to Fridkin,12 transition pho-tocurrents can be induced as a result of shielding the sptaneous polarization by nonequilibrium carriers and frominfluence of nonequilibrium carriers on the magnitude of tspontaneous polarization.

The presence of transition photocurrents in our expment is obvious from theV(t) curves. The derivativedV/dt,which is proportional to the photocurrent, decreases along timest. This behavior is typical of a transition currenHowever, the nature of the transition current is vague. Ifobserved values ofV(t) were associated with the influencof nonequilibrium carriers on the spontaneous polarizatPs(T), the long persistence ofV(t) on the irradiated, un-shorted sample after the removal of illumination would indcate that the photoinduced polarizationPind was equal to thechangeDPs(T) induced in the spontaneous polarizationnonequilibrium carriers. In that event, however, the curobtained from measurements of the temperature dependof the spontaneous polarization of the sample after it hbeen illuminated and shorted would have the fo

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663Phys. Solid State 40 (4), April 1998 Ponomarev et al.

Psind(T)5Ps(T)1DPs(T)5Ps(T)1Pind , which woulddiffer from the initialPs(T) curve by an amount equal to thphotoinduced polarizationDPs(T)5Pind . As mentioned,Pind is an order of magnitude larger thanPs(T). Our mea-surements of the temperature dependence of the spontanpolarization of the sample after its illumination and shortihave shown that thePsind(T) curve does not differ morethan a few percent from the initialPs(T) curve. Conse-quently, DPs(T)!Pind , and the measured effect is not rlated to the influence of nonequilibrium carriers on the sptaneous polarization.

Shielding of the spontaneous polarizationPs cannot pro-duce the observed values ofPind , for two reasons: 1! theshielding ofPs cannot alter the polarization of the samplean amount in excess ofPs ; 2! the sign of Pind must beopposite that ofPs when the latter is shielded. It followstherefore, that the nature of the transition currents inexperiment is not the same as that described in Ref. 12.solution of the steady-current problem will require additionmeasurements, which are now in the preparation stage.

The authors are indebted to V. V. Kveder and V.Sinitsyn for valuable discussions.

This work has received financial support from the Intnational Association for the Promotion of Cooperation wScientists from the Independent States of the Former SoUnion ~INTAS Project No. 94-0935!, from the InternationalScience Foundation~ISF! in collaboration with the Russian

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government~Project No. JET100!, and from the StipendFund of the President of the Russian Federation.

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Translated by James S. Wood