Journal of the Korean Physical Society, Vol. 42, April 2003, pp. S1382∼S1385

Photoinduced Ferroelectric Hysteresis Curve in OrganicPhotoconductor/Inorganic Ferroelectric Heterojunction Photomemory

Young-Geun Park, Hea-Yeon Lee, Hidekazu Tanaka, Hitoshi Tabata and Tomoji Kawai∗

Institute of Scientific and Industrial Research, Osaka University,8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

Switching of ferroelectric P-V hysteresis curve by light irradiation has been achieved in organicphotoconductor (PC) copper phthalocyanine (CuPc)/inorganic ferroelectric (FE) BaTiO3 (BTO)heterojunction photomemory. The main mechanisms of this PC/FE photomemory device are theresistance decrease of photoconductor by light irradiation and the increase of ferroelectric remanentpolarization by increasing applied bias. We have also investigated the effect of CuPc thickness on thephotoinduced change of ferroelectric P-V hysteresis curve, and found that the intrinsic insulatingdark resistance of organic photoconductor CuPc film layer plays an important role for high efficiencyof PC/FE heterojunction photomemory. Light irradiation could switch the remanent polarizationof ferroelectric BTO in this heterojunction up to 3 µC/cm2, almost full remanent polarization ofBTO single layer ferroelectric, whereas remanent polarization of that without light irradiation wasalmost zero. The efficiency of this heterojunction photomemory reached up to 500 %.

PACS numbers: 42.79.V, 73.50.P, 85.50, 85.30.TKeywords: Optical memory devices, Photoconductivity, Ferroelectric devices, Field effect transistors

I. INTRODUCTION

Conversion of light information to electric polariza-tion in ferroelectric material is very interesting from thepoint of view as optical-data storage. A photoconduc-tor (PC)/ferroelectric (FE) heterojunction is promisingfor this type of nonvolatile photomemory. This PC/FEheterojunction photomemory was proposed by Schaffertin IBM [1]. Experiments on this heterojunction weredone to utilize PC to detect and FE to store optical in-formation [2-8]. The main mechanisms of this PC/FEdevice are the resistance change of photoconductor bylight irradiation, which enhances ferroelectric remanentpolarization through the increase of effective applied biason ferroelectrics. Light information saved in ferroelectriccan be read out optically by using transmittance and re-flectance property difference depending on polarizationstates of ferroelectric [2-4]. The switching current differ-ence depending on the ferroelectric polarization statesalso can be used to read out electrically the optical in-formation in PC/FE device [5,6]. However, electricalreading out of optically stored data is essential to usethe information as a signal in electronic devices.

In a previous experiment on electrical reading out ofoptical information in PC(CdSe)/FE[Pb0.92Bi0.07La0.01

(Fe0.405Nb0.325Zr0.27)O3] using inorganic photoconduc-tor layer [5], they could obtain only somewhat of modu-lation of ferroelectric P-V hysteresis curve by light, and

∗E-mail: kawai@sanken.osaka-u.ac.jp

polarization without light [Pr(dark)] could not be sup-pressed up to zero, which means low photomemory ef-ficiency. This result comes from the intrinsic semicon-ductive property of inorganic photoconductor. The effi-ciency of this PC/FE device is dominated by the ratioof resistance between photoconductor and ferroelectrics.Because inorganic photoconductors show semiconductiveproperty, the dark resistivity of photoconductor is muchsmaller than that of insulating ferroelectrics as have beenestimated in Ref. 5. Therefore, Pr(dark) of PC/FE pho-tomemory in their work could not be suppressed by insuf-ficient dark resistivity of inorganic photoconductor layer.

Usually, dark conductivity of organic photoconductorsis insulating or at most semiconducting, the reason beingthat these systems consists of well-separated molecules[9]. The resistivity of copper phthalocyanine (CuPc),especially, can be increased up to that of insulating fer-roelectric by crystallization. In addition, the photocon-

Fig. 1. (a) Schematic illustration and (b) simple equivalentcircuit of the CuPc/BTO heterojunction photomemory.

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Photoinduced Ferroelectric Hysteresis Curve in Organic Photoconductor/Inorganic· · · – Young-Geun Park et al. -S1383-

ductivity of CuPc is 3 orders of magnitude greater thandark conductivity of that, and this change arise in vis-ible ray region, and this organic photoconductor alsohas good stability at somewhat high temperature thanother organic material. We applied this organic pho-toconductor, CuPc, to photoconductor layer of PC/FEheterojunction. BaTiO3 (BTO), a well-known ferroelec-tric material, is used as an inorganic ferroelectric layerto save light information detected by organic photocon-ductor layer.

In this paper, we will discuss about the effect of darkresistance of CuPc photoconductor layer on the effi-ciency of CuPc(PC)/BaTiO3(FE) photomemory, and re-port the drastic photoinduced hysteresis curve in thisheterojunction photomemory by visible light irradiationand bias combination.

II. EXPERIMENTAL

The PC/FE device consists of a semi-transparent elec-trode on a photoconductor covering a ferroelectric is fab-ricated on a conductive substrate, Nb(1 wt%)-SrTiO3

(NSTO). CuPc/BTO/NSTO heterojunction, as shownin Fig. 1(a), was fabricated by pulsed laser deposition(PLD) and thermal evaporation for fabrication of inor-ganic and organic layer, respectively. BTO ferroelectricthin film is fabricated on NSTO substrate at temperatureof 580 C and oxygen (including 8 % ozone) pressure of1 Pa. Organic photoconductor CuPc is fabricated at asubstrate temperature of 100 C and K cell source tem-perature of 450 C. For electric measurement, gold topelectrode which is thin enough to transmit incident light(∼300 A) was deposited with thermal vacuum evapora-

Fig. 2. Typical XRD patterns of (a) CuPc and (b) BTOthin films. Crystallized CuPc organic photoconductor andc-axis oriented BTO ferroelectrics can be fabricated on eachsubstrate.

tion using metal mask of 200×200 µm2. The ferroelec-tric property of fabricated device is characterized withSawyer tower circuit [10], and the leakage current of thedevice was estimated with a Keithley 235 Source MeasureUnit. A Xe-lamp was used as light source to measurethe photomemory property of CuPc/BTO heterojunc-tion device. To avoid the photo-induced characteristicsof BTO whose band gap of 3.2 eV (∼380 nm), only whoselight wavelength is less than 520 nm was directed ontothe sample by using cutoff filter. The intensity of lightwas fixed at 310 mW/cm2.

III. RESULTS AND DISCUSSION

Before the fabrication of heterojunction, we confirmedthe structural and electrical properties of each singlephase films. Figure 2(a) and (b) shows the typical x-raydiffraction (XRD) θ-2θ scan profile of CuPc and BTOthin films, respectively. As shown in figure, c-axis per-pendicular BTO on NSTO substrate is fabricated, whichhas a lattice constant of 4.092 A, and crystallized CuPcin α-phase can be fabricated on highly doped p-type Sisubstrate with an a-lattice parameter of 25.749 A. Asshown in Figure 2 (c), dark resistivity of the crystallizedCuPc organic photoconductor was 2×1011 Ω cm, and itchanged down to 2×1010 Ω cm by light irradiation. InBTO ferroelectric films, typical remanent polarizationwas 3 µC/cm2, as shown in Figure 2(d). The leakagecurrent was 3×10−7 A/cm2 at 1 V, and the calculatedresistivity from this I-V data was 9×1010 Ω cm. Fromthese data, we could confirm the dark resistivity of crys-tallized CuPc thin film is increased up to the resistivityof insulating BTO ferroelectrics, and this can be changed

Fig. 3. Resistivity of CuPc in dark and light irradiation,and that of BTO thin film. Dark resistivity of crystallizedCuPc photoconductor thin film can be increased up to theresistivity of BTO ferroelectric insulator, and that can bechanged down to the value lower than that of BTO.

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down to the value lower than that of BTO, as shown inFigure 3.

Figure 4 shows an x-ray diffraction (XRD) θ-2θ scanprofile of a PC/FE heterojunction, where the thicknessof CuPc layer is 100 nm and 600 nm on the BTO of 650nm fixed thickness. As shown in figure, well-crystallizedCuPc and BTO thin films could be deposited on NSTOsubstrate. The dependence of remanent polarizationof PC/FE heterojunction on light irradiation was mea-sured. In the PC/FE heterojunction, the main mech-anism of change in ferroelectric remanent polarizationfrom Pr(dark) to Pr(light) comes from the change in ef-fective applied bias on ferroelectric by light irradiationon photoconductor layer, and this value is decided byvoltage division rule of serially connected resistance asdescribed in Fig. 1(b). The photomemory efficiency, ηcan be defined as η = [(Pr(light) - Pr(dark))/Pr(dark)] ×100 [%]. Consequently, the resistance of photoconductorlayer in dark should be larger, and that in light shouldbe lower than the resistance of ferroelectrics to increaseefficiency of PC/FE photomemory device. To elucidatethe effect of resistance ratio of photoconductor and ferro-electrics on the efficiency of PC/FE device, we fabricatedtwo types of PC/FE device, those are 100 nm or 600 nmof CuPc thickness on the BTO with 650 nm fixed thick-ness.

Photo-induced effect on P-V hysteresis loop curve ofPC/FE photomemory device is shown in Figure 5. Asshown in Figure 5(a), at CuPc thickness of 100 nm, lightirradiation has little effect on the remanent polarizationchange, although the ferroelectric property of BTO canbe confirmed from the P-V hysteresis curve. The ob-tained η is only 15 %. On the other hand, light irradi-ation onto the sample with CuPc thickness of 600 nmcaused a large remanent polarization change, as shownin Figure 5(b). While Pr(dark) was almost zero, Pr(light)

Fig. 4. XRD pattern of CuPc/BTO heterojunction de-posited on Nb(1 wt%)-STO substrate. S means the peak ofsubstrate.

was 3 µC/cm2, that is, almost same value of BTO singlelayer. Namely, effective photo-induced P-V hysteresiscurve was achieved in PC/FE device with the structureof 600 nm of CuPc thickness and 650 nm of BTO thick-ness, and its photomemory efficiency η reached up to 500%. In detail, we could observe asymmetric P-V hystere-sis curve, that is, the remanent polarization after positivebias on top electrode was decreased to zero, and only thatafter negative bias on top electrode was retained. Thisphenomenon can be explained by the depolarization fieldcaused by incomplete compensation of the polarizationin a ferroelectric film due to nonidentical electrodes likeas metal/ferroelectric/semiconductor structure [11].

Now, let us consider these experimental results withthe concept of resistance ratio between photoconductorand ferroelectrics. The resistance can be derived fromthe physical property like carrier density (n) and carriermobility (µ), and geometrical parameters like thickness(l) and cross-section area (A). The resistance ratio ofphotoconductor and ferroelectric in dark is controllablewith the thickness (l) of each layer, and only the resis-tivity of photoconductor can be decreased by increase incarrier density (n) by light irradiation. The dark resis-

Fig. 5. P-V hysteresis loop traced in the dark and in thelight irradiation of CuPc/BTO heterojunction photomemorywith (a) CuPc(100 nm)/BTO(650 nm) and (b) CuPc(600nm)/BTO(650 nm), respectively. The photomemory effi-ciency η reached up to 500 %.

Photoinduced Ferroelectric Hysteresis Curve in Organic Photoconductor/Inorganic· · · – Young-Geun Park et al. -S1385-

Fig. 6. Thickness dependence of CuPc on effective appliedbias change on ferroelectric BTO layer in PC/FE heterojunc-tion. This result is in good agreement with the experimen-tal data of photo-induced ferroelectric curve of PC/FE pho-tomemory.

tance of photoconductor can be calculated by an equa-tion of R = ρd(l/A), where ρd is resistivity in dark. Typ-ical dark resistivity, 2×1011 Ω cm of CuPc is obtainedfrom the I-V measurement of crystallized film. Resis-tance change by light irradiation can be found, if weknow the photo-induced carrier density in photoconduc-tor. Exciton carrier distribution in CuPc is calculatedwith light absorption coefficient, α and exciton diffu-sion length, Lp. Exciton density can be expressed asnexc(x) = N [exp(−αx) − exp(−x/Lp)], where N and xis the normalization coefficient and the distance fromthe light incident surface, respectively [12]. We calcu-lated exciton density profile nexc(x) with values of α =0.012 nm−1 and Lp = 68 nm. The value of absorptioncoefficient, α was obtained from the UV-Vis measure-ment, and we used exciton diffusion length, Lp in Ref.9. Thickness dependence of dark and light resistanceof CuPc can be calculated by integrating the modula-tion of resistivity by exciton carrier distribution in CuPcwith equation ρl = 1/(qnexcµ), where q, nexc and µ iselementary charge, carrier density and mobility, respec-tively. Normalization coefficient N is obtained from thecalculated resistance with exciton density profile and ex-perimental value of 2×1010 Ω cm in light irradiation.Finally, we obtained thickness dependence of CuPc on ef-fective bias change on ferroelectric BTO layer in PC/FEheterostructure as shown in Figure 6.

This calculation result shows, when CuPc layer ofthickness below 200 nm, the bias on BTO cannot be sup-pressed in dark state, leading to poor η value. However,the photo-induced bias change on BTO in PC/FE can becontrolled effectively with CuPc layer of thickness above400 nm, leading to large η. This result agrees well withthe experimental data on the change of photo-inducedP-V hysteresis loop curve of PC/FE samples by increas-

ing the thickness of CuPc photoconductor from 100 nmto 600 nm, and point out the importance of resistanceratio between FE layer and PC layer as photomemory.

IV. CONCLUSIONS

Organic/inorganic, photoconductor/ferroelectric het-erojunction bilayer is fabricated for application as a pho-tomemory. The dependence of remanent polarization inferroelectric layer can be controlled by the change of pho-toconductivity in organic layer. The remanent polariza-tion of 3 µC/cm2 could be achieved when the light isdirected onto this heterojunction photomemory, and thedark remanent polarization could be suppressed down toalmost zero. This remanent polarization value in lightirradiation is almost same with that of the BTO fer-roelectric single layer. Because of these eminent pho-toinduced switching of ferroelectric P-V hysteresis loop,this organic photoconductor/inorganic ferroelectric het-erojunction structure is eminently suitable for use in pho-tomemory devices.

ACKNOWLEDGMENTS

The authors would like to thank the financial supportby the Center of Excellence (COE) program by the Min-istry of Education, Culture, Sports, Science and Tech-nology, Japan.

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