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The evaluation of polarization relaxationby means of charge holding ratioHironori Shiroto a , Akihiro Mochizuki a & Masaru Inoue a ba Functional Organic Materials Lab., Fujitsu Laboratories Ltd , 1–10Morinosato-Wakamiya, Atsugi, 243-01, Japanb Electronics Division , Toyo Corporation , 26–9 Yushima 3, Bunkyo,Tokyo, 113, JapanPublished online: 10 Feb 2011.

To cite this article: Hironori Shiroto , Akihiro Mochizuki & Masaru Inoue (1993) The evaluationof polarization relaxation by means of charge holding ratio, Ferroelectrics, 148:1, 411-423, DOI:10.1080/00150199308019967

To link to this article: http://dx.doi.org/10.1080/00150199308019967

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Ferroelecrrics, 1993, Vol. 148. pp. 411-423 Reprints available directly from the publisher Photocopying permitted by license only

0 1993 Gordon and Breach Science Publishers S.A. Printed in the United States of America

THE EVALUATION OF POLARIZATION RELAXATION BY MEANS OF CHARGE HOLDING RATIO

Hironori Shiroto, Akihiro Mochizuki and Masaru h u e * Functional Organic Materials Lab., Fujitsu Laboratories Ltd

I - I 0 Morinosato-Wakamiya, Atsugi, 243-01 Japan * Electronics Division, Toyo Corporation

26-9 Yushima 3, Bunkyo, Tokyo, 113 Japan

Abstruct A polarization switching is one of the most important factor of the characteristic properties of the surface stabilized ferroelecmc liquid crystal displays. The polarization switching behaviour is conventionally investigated by an electrooptic measurement or a polarization switching current peak. We propose here new method for the polarization behaviour using the charge holding ratio. The new method is effective for an analysis of transient behaviour of the polarization switchings.

INTRODUCTION

Polarization switching is one of the most important characteristic prpoperties of the surface stabilized ferroelecmc liquid crystal (SSFLC) cell. In an ideal SSFLC cell, once a polarization switch happens, the switched polarization stays for a long period. In an actual SSFLC cell, however, a polarization relaxation occurs after the polarization switching. This polarization relaxation of the SSFLC cell is assumed to be induced by an imperfection of the smectic layer structure and a depolarization field. Polarization switching properties are commonly investigated by an electrooptical measurement or polarization switching current peaks”. Particularly, the electrooptical measurement is effective to understand the polarization relaxation after removing the drive voltage. However, the electrooptical measurement contains many informations concerning to the polarization change. Sometimes it is very difficult to tell the detail of the polarization relaxation. Here, we propose a new measurement in conjuction with the polarization switching.

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NEW MEASUREMENT METHOD

A stability of polarization alignments after removing the drive voltage can be detected by the voltage variation which is electrically isolated from the measurement unit. A voltage of the SSFLC cell indicates the polarization switching behaviour. The perfect switching and the perfect memory effect may provide no voltage reduction after removing the drive voltage because of the perfect polarization alignment. The voltage change after removing the drive voltage is measured as a charge holding ratio. Fig. 1 shows the blockdiagram of the charge holding ratio measurement unit (Model =-I; Toyo Corp.). Model VHR-1 consists of arbitrary waveform generators, an impedance converter (including an FET switch), a high resolution digitizer and a controller. As an SSFLC cell has higher impedance than the digitizer’s input impedance, it is very difficult to observe the voltage drops directly occured at both ends of the SSFLC cell after removing the drive voltage. To solve the difficulty, an impedance converter is equipped with this system between the SSFLC cell and the digitizer (input impedance S Z , slew rate 5 V/psec). And the guarding method is used in this system to reduce leakage errors and to decrease response time.

arbitrary wave controller 1 I

I CLOSE OPEN

arbitrary wave controller 2

FIGURE 1 Charge holding measurement system (Model VHR-1; Toyo Corp.).

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THE EVALUATION OF POLALIZATION RELAXATION 187311413

EXPERlMENTAL

Definition of charge holding ratio The applied voltage to the sample cell is 2 to 10 V. The pulse width is 1 ms

which is sufficient for polarization switchings at room temperature. The charge holding ratio is defined as shown in Fig. 2. The ratio between the cell voltage being applied the switching voltage and the cell voltage 44 ms after removing the switching voltage is defined as the charge holding ratio.

I

FIGURE 2 Definition of charge holding ratio.

FLC samples Two types of FLC materials are used for this experiment. One is the naphthalene-

base mixture which has a quasi-bookshelf layer structure. The other is Merck mixture ZLI-4851-100 which has a chevron layer StructuTe. The spontaneous polarization of ZLI-4851 FLCs can be changed from 6 to 40 nC/cm2 by arranging the ZLI-4851-100 and ZLI-4851-000 which are so called two bottle system.

Sample cells A cell structure is shown in Fig. 3. Two types of insulation layers are prepared

on the IT0 electrode. One is SO2, the other is T$O,, and no insulation layer cell is also prepared. The thickness of these insulation layers is 1000 A. The cell thickness is 1.9 to 2.4 Bm.

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4144 8741 H. SHIROTO, A. MOCHIZUKI AND M. INOUE

Glass substrate 4. I

Orientation film

Cell gap : 1.9-2.4 pm I I

FIGURE 3 Structure of SSFLC cell.

RESULTS AND DISCUSSIONS

Polarization relaxation depending on the layer structure Figs. 4 and 5 present both the charge holding ratio and the optical transmittance

of the quasi-bookshelf and the chevron structure cells, respectively. These figures show that the quasi-bookshelf cell gives high charge holding ratio. In contrast, the chevron cell shows low charge holding mtio. A significant difference in the polarization switchings detected by the charge holding ratio is shown in Fig. 6. In the quasi- bookshelf cell, once a sufficient voltage for polarization switching is applied, high charge holding ratio is maintained. In the chevron cell, even a sufficient voltage is applied, the holding ratio is relatively low. Fig. 6 suggests that in the bookshelf layer geometry a switched polarization maintains the high charge holding ratio, however, in the chevron geometry a switched polarization tends to change the state. This result suggests that the bookshelf cell provides a stable memory and the chevron cell provides an unstable memory. This difference in the charge holding ratio suggests that a layer structure modification may happen in the chevron cell under application of the switching voltage.

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THE EVALUATION OF POLALIZATION RELAXATION [875]/415

100

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100 95

Time (ms)

FIGURE 4 naphthalene-base liquid crystal cell.

The charge holding ratio and memory stability of the

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FIGURE 5 100 liquid crystal cell.

The charge holding ratio and memory stability of the ZLI-4851-

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Applied voltage (V)

FIGURE 6 the quasi-bookshelf and the chevron structure cells.

Charge holding ratio 44 ms after removing the drive voltage of

Effects of the spontaneous polarization Generally a high Ps FLC provides a high depolarization. This induced

depolarization is assumed to affect on the polarization alignment stability. Figs. 7 and 8 show that a low Ps FLC gives higher charge holding ratio. The difference of the charge holding ratio between 6 nC/cm2 and 21 nC/cmz is much smaller than that between 21 nC/cmz and 40 nC/cm2. As long as the spontaneous polarization, the influence of the depolarization field becomes significant when the Ps is higher than 20 nC/cmz. The tempereture dependence of the charge holding ratio shown in Fig. 9 presents that the ratio generally decreases with the increase of tempereture. This tempereture dependence indicates that the switched polarization can keep the switched state due to high viscosity in a lower tempereture range.

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THE EVALUATION OF POLALIZATION RELAXATION (87711417

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FIGURE 7 Charge holding ratio applied 10 V voltage of the ZLI 4851- liquid crystal cell.

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Applied voltage (V)

FIGURE 8 the ZLI-4851- liquid crystal cell.

Charge holding ratio 44 ms after removing the drive voltage of

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FIGURE 9 crystal cell. The curves from above correspond to the applied voltage of 10,8, 6,4, and 2 V.

Charge holding ratio of the ZLI-4851-OOO (6 nC/cm2) liquid

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Polarization relaxation dependent on the insulation layres

The polarization relaxation caused by the depolarization is also observed in Figs. 10 and 11.

100

70

Si02

0 15 30 45

Time (rns)

FIGURE 10 with insulation layers. The applied drive voltage is 10 V.

Charge holding ratio of the naphthalene-base liquid crystal cell

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5 20 v- 0 : non

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Applied voltage (V)

FIGURE 11 the naphthalene-base liquid crystal cell.

Charge holding ratio 44 ms after removing the drive voltage of

The highest charge holding ratio is provided by the no insulatin layer cell. In contrast, the SiO, layer cell shows a low charge holding ratio. The low charge holding ratio in the SiO, layer cell is explanable by the depolarization field. This highly insulated SiO, layer may block a leakage of the depolarization field. Figs. 10 and 11 suggest that the T%05 insulation layer is effectively reducing the depolarization field. because of the its high dielectric constant. The temperature dependence of the charge holding ratio of the insulation layer cells in Figs. 12 and 13 which correspond

to Ta,O, and SiO, respectively shows that the high charge holding ratios at 35°C is obtained by the TqO, layer cell. This significant difference in the temperature dependence clarifies the suppression of the depolarization field in the T%O, layer cell.

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(2) 25°C

(3) 15°C

FIGURE 12 Charge holding ratio of the naphthalene-base liquid crystal cell with T%O, insulation layers. The cueves from above correspond to the applied voltage of 10, 8 ,6 ,4 , and 2 V.

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FIGURE 13 with SiO, insulation layer. The curves from above correspond to the applied voltage of 10, 8,6,4, and 2 V.

Charge holding ratio of the naphthalene-base liquid crystal cell

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CONCLUSION

We proposed new measurement method to understand the polarization alignment behaviours. The polarization switching behaviour can be detected by the charge holding ratio. The advantages of this method are followings.

A polarization alignment is directly detected. A time resolved polarization alignment behaviour can be obtained. An applied voltage dependence of the polarization alignment behaviour is obtained.

Using this new method, we clarified the layer structure effect, the Ps effect and the insulation layer effect on the polarization relaxations. In the bookshelf layer cell, the switched polarization keeps a stable alignment after removing the applied voltage. In the chevron cell, the switched polarization is unstable resulting in the polarization relaxation. A spontaneous polarization also affects on the polarization alignment stability in conjuction with the depolarization field. Low Ps FLC tends to show a stable polarization alignment due to low depolarization field. An insulation layer on IT0 electrode governs the leakage of the depolarization field. It is clarified that the polarization alignment stability is dependent on the leakage level of the insulation layer.

Acknowledgment

The authors would like to thank Mr. M. Kimura and Prof. T. Akahane of Nagaoka University of Tecknology for their courteous discussion.

REFERENCE

1) T. Shimasaki, S. Mizushima, S. Minezaki, K. Yano, M. Hijikigawa, Conference

record of the 14th Japanese Liquid Crystal Conference in Sendai 2B110, in Japanese,

(1988)

(Received 2 1 October 1993)

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