Optical second harmonic generation imaging for visualizing in-plane electric field distribution

Optical second harmonic generationimaging for visualizing in-plane electric

field distribution

Takaaki Manaka, Motoharu Nakao, Daisuke Yamada,Eunju Lim and Mitsumasa Iwamoto

Department of Physical Electronics, Graduate School of Science and Engineering, TokyoInstitute of Technology,

2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan

[email protected], [email protected]

Abstract: The electric field distribution in electronic devices, particularlyin the organic devices, was visualized by the optical second harmonicgeneration (SHG) imaging technique on the basis of electric field inducedSHG (EFISHG). Two-dimensional SHG images from organic field effecttransistor using pentacene were taken with a cooled CCD camera, andthe SHG images showed the electric field was successfully visualizedwith a resolution of 1 μm. The SHG imaging method provides us a noveltechnique for visualizing the electric field distribution in actual devicesunder device operation.

© 2007 Optical Society of America

OCIS codes: (110.0110) Imaging systems; (180.4315) Nonlinear microscopy; (190.1900) Di-agnostic applications of nonlinear optics; (190.4710) Optical nonlinearities in organic materials

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Lett. 58, 2921-2923 (1991).3. L. Burgi, H. Sirringhaus and R. H. Friend, “Noncontact potentiometry of polymer field-effect transistors,” Appl.

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8. T. Manaka, E. Lim, R. Tamura, D. Yamada and M. Iwamoto, “Probing of the electric field distribution in organicfield effect transistor channel by microscopic second-harmonic generation,” Appl. Phys. Lett. 89, 072113 (2006).

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#87941 - $15.00 USD Received 26 Sep 2007; revised 14 Nov 2007; accepted 14 Nov 2007; published 16 Nov 2007

(C) 2007 OSA 26 November 2007 / Vol. 15, No. 24 / OPTICS EXPRESS 15964

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inkjet printing of all-polymer transistor circuits,” Science 290, 2123–2126 (2000).

1. Introduction

The basic equation that expresses a current flowing in materials, J = enμE, implies that theelectric field distribution, E, in electronic devices plays a significant role in determining thedevice characteristics, where e, n and μ represent fundamental charge, carrier density and mo-bility, respectively. Also, the electric field in materials dominates a variety of carrier transportphenomena such as space charge limited current (SCLC), where zero electric field at an injec-tion point is a-priori boundary condition for the SCLC [1]. The development of the scanningprobe technique enables us to evaluate the potential distribution in devices with high resolu-tion [2, 3, 4]. For inorganic semiconductor devices, such as silicon-based transistors, thermalequilibrium of carriers is assumed to be established in the devices. Under the establishment ofthe thermal equilibrium, carrier distribution is ruled by Fermi-Dirac Statistics, and its density isgiven as a function of electrostatic potential; thus we perceive the importance for the direct eval-uation of a potential distribution in inorganic semiconductor devices. On the other hand, directevaluation of the electric field distribution rather than the potential distribution is an essentialissue for low conductive materials such as organic materials with a large energy gap. This isbecause the thermal equilibrium is not completely established in the devices composed of suchlow conductive materials, but the current equation, J = enμE, is always valid. In this sense, itis worthwhile developing a technique, which can evaluate directly an electric field distributionin devices.

Electric field induced second harmonic generation (EFISHG) is one of the third-order non-linear optical processes, and it generates second harmonic light of the fundamental light inthe presence of a dc electric field. For the EFISHG process, applied electric field induces theeffective polarization in materials. In a typical EFISHG experiment, the molecules with non-zero dipole moments are dissolved in a solvent and the dc electric field is applied to estimatethe molecular hyperpolarizability [5, 6]. Also, SH activation in poled polymers is well known,where the dipolar side chains of polymers align along the electric field. Therefore, orientationalpolarization contributes to the SHG in the case of poled polymer. For the symmetric mole-cules such as pentacene and phthalocyanine, there is no orientational polarization induced bythe external electric field. In such cases, the distribution of delocalized π-electron is distortedby the external field and effective polarization is induced, i.e., electronic polarization. Accord-ing to the symmetric consideration of the susceptibility tensor, the SHG signal vanishes fromcentrosymmetric media under the electric dipole approximation. However, the external electricfield breaks such centrosymmetry, and we can observe the SHG signal from the centrosymmet-ric media.

For the EFISHG, the SHG intensity is proportional to the external field as,

I(2ω) ∝∣∣∣χ (3)(2ω ;0,ω ,ω)E(0)E(ω)E(ω)

∣∣∣

2(1)

where χ (3)(−2ω ;0,ω ,ω) represents a third order nonlinear optical (NLO) susceptibility forthe EFISHG process, and E(0) and E(ω) represent the static electric field and electric field oflight, respectively. Thus the information about electric field in materials can be obtained by theEFISHG measurement, and the EFISHG was effectively employed as a probe of the electricfield near a Au-silicon Schottky interface [7]. We have also proposed methods for evaluating



the electric field distribution in organic devices on the basis of the scanning EFISHG measure-ments [8]. As mentioned in our previous paper, to obtain the electric field distribution from theSHG intensity distribution requires a deconvolution process taking into consideration the beamprofile of fundamental laser. Such deconvolution is, in general, a laborious task, because a beamprofile and the SHG intensity profile are not expressed as simple mathematical functions, andthe evaluated electric field from a scanning measurement sometimes loses accuracy, e.g. spatialresolution. In this paper, we introduce an SHG imaging technique for visualizing the in-planedistribution of the electric field in the organic devices. The evaluation of the electric field dis-tribution from SHG images does not require the complicated deconvolution process, and thespatial resolution is significantly improved. Moreover, omission of the scanning process resultsin the reduction of the measurement time.

2. Experiment

Samples used in the experiments were top-contact field effect transistor (FET) structure.Heavily-doped Si wafers were used as the base substrate, and they were covered with a 500 nmthick silicon dioxide (SiO2) insulating layer. The material for the organic semiconductor layerwas pentacene (C22H14) purchased from Tokyo Chemical Industry Co., Ltd., and was usedas received. Recently, pentacene is one of the standard materials for preparing organic FET(OFET) devices [9, 10, 11]. The pentacene layer, approximately 100 nm thick, was depositedon a SiO2 surface. The process pressure during deposition of pentacene was kept at less than1×10−4 Pa, and the deposition rate was controlled at approximately 3 nm/min. After the depo-sition of pentacene, top-Au electrodes (source and drain electrodes) with a thickness of 100 nmwere deposited on the pentacene surface. The channel length (L) and width (W ) were 50 μmand 3 mm, respectively. For the FET structure, the Si substrate was used as the gate electrode.Figure 1(a) represents the schematic images of the sample structure and electrical connection.

S D

pentacene(100 nm)

SiO2

(500 nm)

heavilydoped-Si

50μm

G

3mmS

Vpulse

D

G

(a)

YAG laser + 3ω generator

OPO

SH-cut

IR-cut

bandpass

pol.

cooledCCD

CCD

objective

pol.

ND

D

S

G

FET

beamexpander tube

lens

(b)

Fig. 1. (a) Schematic images of the sample strucrure and electrical connection. (b) Opticalsetup for the SHG imaging.

The light source for the SHG measurement was an optical parametric oscillator (OPO: Con-



tinuum Surelite OPO), pumped by a third-harmonic light of Q-switched Nd-YAG laser (Con-tinuum: SureliteII-10). The use of an appropriate wavelength is important to observe the SHGsignal from pentacene effectively, because the SHG intensity strongly depends on the funda-mental wavelength. For the EFISHG process, resonance enhancement occurs at both forbiddenand allowed excited states. In our previous paper [12], we discussed the EFISHG spectrum ofvacuum deposited pentacene films. As a result, the EFISHG peaks were located at 1120 nmand 1320 nm, and a fundamental wavelength was fixed at 1120 nm in this study. Fundamentallight from the OPO passed through a prism polarizer, long-pass filters and a beam expander(see Fig. 1(b)). Then it was focused on the channel region of the FET with normal incidence,using a long working distance objective lens (Mitsutoyo: M Plan Apo SL20×, NA = 0.28,W.D. = 20.5 mm). SH light generated from the FET was filtered by a fundamental-cut filterand an interference filter to remove fundamental and other unnecessary light. Finally, SH lightwas detected by a cooled CCD camera (Andor technology: BV420-DR). In this configuration,the polarization direction of the light was chosen in the direction corresponding to the channeldirection (source-drain direction).

Note that, there were some differences in the optical setup between scanning measurementsand SHG imaging. One is the detector for the SHG observation. For the scanning measurement,a photomultiplier tube was used to detect the SHG light, whereas a cooled CCD camera wasused to take an SHG image. Another difference was the spot size of the fundamental light. Toobtain the SHG intensity distribution on the basis of the scanning measurement, we needed toreduce the spot size because the spatial resolution depends on the spot size. On the other hand,the fundamental light should irradiate a large area of the channel region for SHG imaging. Thuswe used a beam expander for the SHG imaging.

All measurements were performed in laboratory ambient atmosphere. To avoid unexpectedinjection of carriers from the electrode, the pentacene FET was operated by pulsed voltage.Under the negative bias application to the drain electrode, electron injection is difficult be-cause of the large injection barrier for electron from the Au electrode to pentacene (see Fig.1(a)). Pulsed voltage was applied to the drain electrode using a function generator (NF Corp.:WF1974) amplified by a high-speed bipolar amplifier (NF Corp.: HSA 4101), and the sourceand gate electrodes were connected to the ground. The pulse width, repetition rate and ampli-tude applied to the FET were 20 μs, 10 Hz and 70 V, respectively. The external Q-switch triggerof the Nd-YAG laser was supplied simultaneously from the function generator. The time delaybetween the pulse voltage applied to the FET and the Q-switch trigger was controlled preciselyby the function generator, and was fixed at 100 ns.

3. Scanning SHG measurement

Because EFISHG is a third-order NLO process, susceptibility tensor does not vanish in thecentrosymmetric media. Since the SHG intensity is related to the internal electric field in mate-rials as expressed in Eq.(1), the electric field distribution can be evaluated using SHG intensitydistribution. To obtain SHG intensity distribution in OFET, microscopic SHG measurementwas employed [8]. Here, we introduce briefly scanning SHG measurement for evaluating theelectric field distribution. For the microscopic SHG measurement, fundamental light is focusedonto the sample surface using microscopic objectives, and the SHG signals from this spot areacquired. The sample stage is moved sequentially to change the spot position, and the SHGintensity was acquired at each position. Accordingly, we can obtain the SHG intensity distribu-tion. SHG intensity at position x, I2ω(x), is expressed by a convolution of the beam profile ofthe fundamental laser and the actual electric field distribution as,

I2ω(x) ∝∣∣∣∣

∫ ∞

−∞E(ξ )Iω(x− ξ )dξ

∣∣∣∣

2

(2)



where E(ξ ) and Iω(ξ ) represent the electric field distribution and the intensity profile of thefundamental laser, respectively. The boundary conditions for the calculation are

∫ L0 E(ξ )dξ =

Vds and E(ξ ) = 0(ξ < 0,ξ > L). Nevertheless, the SHG distribution strongly depends on theintensity profile of the fundamental laser.

30

20

10

0

-10

-20

-30

-30 -20 -10 0 10 20 30x position [μm]

y p

ositio

n [

μm]

30

20

10

0

-10

-20

-30

-30 -20 -10 0 10 20 30x position [μm]

y p

ositio

n [

μm]

position [μm]

0

1

2

3

4

10 20 30 40 50 60 70 80 900

SH

inte

nsity [arb

. unit]

10 20 30 40 50 60 70 80 900position [μm]

SH

inte

nsity [arb

. unit]

0

1

2

3

4

(a) (b)

(c) (d)

S D S D

Fig. 2. SHG profiles along the pentacene FET channel obtained using (a) 20× and (b) 50×objective lens. Two-dimensional intensity distrubution of fundamental light at a focal pointusing (c) 20× and (d) 50× objectives.

Figures 2(a) and 2(b), respectively, represent the SHG profiles along the pentacene FETchannel obtained using the 20× and 50× objective lenses, together with the in-plane electricfield distribution in the pentacene layer calculated based on a finite element method. Figures2(c) and 2(d) show the two-dimensional intensity distribution of the fundamental light at a fo-cal point using the 20× and 50× objectives, respectively. In these measurements, SHG signalswere acquired at an interval of 5 μm along the channel, and the region from 20 μm to 70 μmcorresponding to the channel (L = 50 μm). Open circles and solid line represent the SHG inten-sity at each spot position and electric field distribution. Remarkable SHG signals were observedon the drain side as shown in the figure. High electric fields between drain and gate electrodesalso produced a large in-plane component of the electric field around the drain electrode due tothe edge effect. Such large in-plane component of the static field effectively contributes to theSHG because of the normal incidence of the fundamental light. For both figures, the SHG pro-files are widely distributed compared with the electric field distribution. Moreover, comparingthe SHG distribution obtained using the 20× objective with that obtained using 50× objective,large magnification lens clearly causes the sharp SHG distribution.

4. SHG imaging using cooled CCD

As mentioned above, the SHG intensity profile obtained on the basis of a scanning measurementdoes not display an actual spread of SHG emission in the channel. Direct observation of the



SHG image in the channel can show a more realistic distribution of the SHG emission withinthe limits of the system resolution. Figure 3(a) shows the SHG image from the channel ofpentacene FET under the application of negative pulse. As shown in the figure, strong SHGemission was observed at an edge of drain electrode. As well as the scanning SHG measurementwhere a remarkable SHG peak was observed on the drain side, high electric fields betweenthe drain and gate electrodes produced a large in-plane component of the electric field, and itactivated the SHG at the drain edge. It is noteworthy that the width of the SHG emission inthe channel clearly decreases compared with a distribution obtained on the basis of a scanningmeasurement.

(b)

50μm

D

S

channel

(a)

50μm

D

S

channel

A B

Fig. 3. (a) SHG image from the channel of pentacene FET under the application of negativepulse. Channel region lies between two gold electrodes, and edges of the electrode areindicated by dashed lines. SHG emission was observed at the edge of the drain electrode.(b) Microscopic image of the channel between two electrodes. This picture was taken underthe illumination of visible light.

Figure 4 shows the line scan of the SHG intensity profile across the channel (representedas open squares and filled diamonds) and the in-plane component of the in-plane electric fielddistribution (solid line) in the pentacene layer. Open squares and filled diamonds, respectively,represent the SHG intensity profile at line scan A and B as shown in Fig. 3. The electric fielddistribution represented here is similar to that represented in Fig. 2, though this image is mag-nified. The edge of the electrode is located at a position of 70 μm in these figures. As shown, itis found that the SHG intensity profile is quite sharp compared with the result of the scanningmeasurement (see Fig. 2). The sharpness of emission is quantitatively estimated using full widthat half maximum (FWHM) values of the profile. FWHM values of SHG profile and the electricfield distribution are evaluated as 0.9 μm and 0.7 μm, respectively. Note that FWHM values ofthe SHG profile obtained based on the scanning measurement depended on the magnificationof the objective lens and was approximately 15 μm for 20× objective.

For the electric field calculation, there were no excess charges in the device, and only theelectrode configuration and potential were taken into account. In such case, Laplacian electricfield is formed in the device. Laplacian field is the electric field in an insulator caused bythe electrodes in the absence of any charges between the electrodes. Under the negative bias,carrier injection from the electrode into pentacene is prohibited because the injection barrier forelectrons at the pentacene/Au interface is quite high, i.e., energy difference between the lowestunoccupied molecular orbital (LUMO) of pentacene and the work function of Au electrodewas evaluated as 2.7 eV [13]. Thus the strong electric field around the edge of the electrode



0

2

4

6

0

50

100

60 65 70 75 80

line A

line B

FWHM =0.7 μm

FWHM =0.9 μm

position [μm]

SH

G in

ten

sity

[arb

un

it]

ele

ctr

ic fie

ld[a

rb. u

nit]

Fig. 4. Top figure represents the in-plane component of the in-plane electric field distribu-tion in pentacene layer. Bottom one shows the line scan of the SHG intensity profile acrossthe channel. Open squares and filled diamonds, respectively, represent SHG intensity pro-file at line scan A and B as shown in Fig. 3.

was maintained during bias application, and the SHG emission concentrated around the edgeof the drain electrode as shown. In other words, the sharp emission of the SHG at the edge ofthe electrode indicated that the SHG imaging technique successfully visualized the electric fieldin the device with a spatial resolution of approximately 1 μm. Futhermore, it should be notedthat the intensity correction of the SHG images based on the spatial distribution of fundamentallight can improve the accuracy of electric field evaluation.

The tensor origin of the EFISHG from the pentacene FET should be briefly discussed. Ac-cording to AFM images of our samples, vacuum evaporated pentacene films were composedof many small grains with a size of less than 400 nm (not shown). Thus we can reasonablyconsider the C∞v symmetry for a pentacene layer. Under the C∞v symmetry, there are 7 inde-pendent components of the NLO susceptibility for the EFISHG process. Since the x-directionalcomponent of the electric field composed a fundamental light with a normal incidence, only theχxxxx component contributes to the SHG signal. This indicates that x-component of the internalstatic electric field can be selectively evaluated.

The electronic devices using organic materials such as OFET and organic light emittingdiode have attracted much research attention due to their potential for the practical applica-tion [14]. In particular, recent focus in the field of the organic electronics is to employ an easyprocess such as ink-jet printing method for fabricating devices [15]. π-Conjugated polymersis the strong candidate for the appropriate materials using in such so-called wet-process, and



the development of well-modified π-conjugated polymers with high mobilities becomes im-portant. Nevertheless, the sensitivity of the SHG imaging method depends on the third-orderNLO susceptibility, χ (3). π-Conjugated polymers show, in general, large χ (3) values becauseof the presence of extensively delocalized π-electrons. In this sense, the SHG imaging methodfor evaluating the electric field distribution also contributes to the development of polymerelectronics.

5. Conclusion

Electric field distribution in electronic devices, particularly in organic devices, was successfullyvisualized by the optical SHG imaging technique on the basis of electric field induced SHG.Two-dimensional SHG images from OFET using pentacene were taken with a cooled CCD,and the SHG images showing the electric field was successfully evaluated with a resolution of1 μm.

Acknowledgement

This work is support by the Grants-in-Aid for Scientific Research (Grant No.18686029,19206034) from Ministry of Education, Culture, Sports, Science and Technology and The NewEnergy and Industrial Technology Development Organization (NEDO).



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Optical second harmonic generation imaging for visualizing in-plane electric field distribution