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Physica B 352 (2004) 200–205

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Pyroelectric electron emissions and domain inversion ofLiNbO3 single crystals

Dong-Wook Kim�, E.M. Bourim, Soo-Hwan Jeong, In K. Yoo

Samsung Advanced Institute of Technology, U-Team, P.O.Box 111, Suwon, Kyongki 440-600, Korea

Received 7 May 2004; received in revised form 10 July 2004; accepted 16 July 2004

Abstract

We investigated the electron emissions from a congruent LiNbO3 single crystal with variation in temperature. When

there was a small gap between the crystal and detector (o2mm), we observed abrupt drops in the emission current andpolarization domain inversion of the crystal. The current burst was distributed in tree-like patterns that suggested

plasma generation. A sufficient gap and a crystal with a high coercive field appear to be factors that allow reproducible

electron emissions from pyroelectric materials.

r 2004 Elsevier B.V. All rights reserved.

PACS: 77.70.+a; 77.84.Dy; 79.90.+b

Keywords: Pyroelectricity; Electron emission; Polarization domain

1. Introduction

Ferroelectric materials can emit electrons inresponse to variation in spontaneous polarizationand this is called ferroelectric electron emission(FEE) [1–5]. FEE can be induced by variousexternal perturbations, such as variation in tem-perature (pyroelectric effect), mechanical stress(piezoelectric effect), or spontaneous polarization

e front matter r 2004 Elsevier B.V. All rights reserve

ysb.2004.07.011

ng author. Tel.: +1-858-822-3953; fax: +1-

sses: Peterkim@ucsd.edu,

sung.com (D.-W. Kim).

switching [1]. Pyroelectrically-induced electronemission (PEE) has renewed interest in FEE, sinceapplications, such as X-ray generation, infraredsensing, and pattern replication, have been sug-gested [6–8].An uncompensated surface charge creates an

electric field at the surface, which can induce FEE[1]. An enormous electric field at a surface(4106V/cm) may cause not only electron emissionbut also plasma generation [1–3]. Much of theresearch on FEE owing to ‘polarization switching’has examined the role of plasma in electronemissions and related phenomena [1,2]. Plasmacan enhance electron emissions, while erosion of

d.

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I PD(a

rb. u

nits

)

Gap = 0.5 mm

Gap = 1.5 mm

D.-W. Kim et al. / Physica B 352 (2004) 200–205 201

the material can reduce the emissions. It has beenreported that plasma formation affects PEE [3,7],although plasma-related issues have not beenaddressed carefully in the case of PEE. This paperreports plasma behavior during PEE of a pyro-electric LiNbO3 single crystal. We also discussfactors that allow production of a stable PEEcurrent.

60 80 10040

Temperature (oC)

Gap = 2.5 mm

120

Fig. 1. PEE current curves of a +Z-faced CLN crystal at gaps

from 0.5 to 2.5mm.

2. Experimental

We used polished Z-cut plates of congruentLiNbO3 single crystals (CLN), provided byCrystal Technology, Inc., and heated the crystalsto 140 1C at a rate of 10 1C/min in a high-vacuumchamber. The chamber was equipped with a turbo-molecular pump system with a base pressure lowerthan 10�7 Torr. After PEE, we examined thepolarization domain distribution of the crystalsby etching them with hydrofluoric acid (HF),which reveals domain patterns, as �Z-facedsurfaces are etched much faster than +Z-facedsurfaces [9].We measured the PEE current using a Si p–n

junction photodiode (PD) (AXUV-100, Interna-tional Radiation Detectors, Inc.). The PD has anamplification factor of at least 102 without the aidof an external high voltage [8]. We used electron-beam resist (ER) (ZEP-520, Nippon Zeon Co.Ltd.) to determine the spatial distribution of thecurrent emitted from the crystal. Our PD and ERare very simple to use and have high tolerance overa wide range of pressures.

3. Results and discussion

Fig. 1 shows the PEE behavior of CLN withdifferent gaps between the PD and the crystalsurface. (For a clearer comparison, we used a newsample for each gap.) When the gap exceeded2mm, the emission current showed a relativelysmooth curve. As the gap decreased below 2mm,the current signal fluctuated and decreasedabruptly. This rapid drop in the PEE currentseems to be a signature of discharge.

Figs. 2 (a)–(c) show clear striations on devel-oped ER that was exposed to CLN while varyingthe temperature. We placed an ER-coated waferover CLN by inserting a 90-mm-thick spacer, asshown in Fig. 2(d). As the gap decreased, thedensity of the striation increased and the line widthdecreased. We also obtained ER patterns fromwafers that were placed parallel to CLN surfacesat various gaps from 0.5 to 2.5mm. We did notfind a clear line pattern on the developed ER whenthe gap exceeded 1mm. These results all show thatboth the emission current and ER pattern arehighly dependent on the gap.The magnitude of the electric field in the gap,

Egap, is given by (Eq. (12) in Ref. [1]):

Egap ¼DPS

�0�

1

1þdgapdcr

�crð1Þ

where DPS denotes the variation in spontaneouspolarization, e0 is the permittivity of free space,dgap is the gap between the crystal surface and thedetector, dcr is the thickness of the crystal, and ecris the dielectric permittivity of the crystal [1]. DPSequals gDT and g is the pyroelectric coefficient(g of CLN=8.2 nC cm�2K�1). For DT=20K, theDPS of CLN can be as large as 0.16 mCcm

�2 [1].Egap is estimated to be about 106V/cm fordgap=2mm and increases as the gap decreases.

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Fig. 2. (a)–(c) The ER patterns when CLN was subjected to temperature variation. (d) The experimental set up, with the locations of

the images.

D.-W. Kim et al. / Physica B 352 (2004) 200–205202

This explains why a larger PEE current appears ata smaller gap, as shown in Fig. 1.The electron emissions from a flat surface are

not uniform, since the difference in the workfunction and slight variation in the surfacemorphology may result in local field enhancement[10]. Therefore, PEE may not be uniform owing tothe roughness of the crystal, chemical inhomo-geneity (such as vacancies), contamination on thesurface, and so on. However, no direct experi-mental study has shown non-uniformity of thePEE current. This might be owing to limitedspatial resolution. If local electron emissionappears, then it can induce an inhomogeneousdistribution of uncompensated surface charges.The charges can generate a tangential electric field,Et, between adjacent electron-emitting and non-emitting regions.According to Paschen’s law, the breakdown

voltage of air is a function of the product of thegas pressure and the gap distance [11]. In a highvacuum of 10�7 Torr, the threshold field ofbreakdown might exceed Egap or Et. However,the local pressure near the sample might exceed thebackground pressure because of outgassing fromthe sample and poor conductance in the gap.1 The

1For example, the LN surface could be covered by water,

which was adsorbed before insertion into the vacuum chamber.

The water desorbed during heating in a vacuum and increased

plasma can supply compensation charges to thecharged surface of the LN instantaneously andPEE will cease. A small gap increases Egap (andalso Et) according to Eq. (1) and lowers thethreshold voltage for breakdown according toPaschen’s law [11]. Therefore, small gaps canreadily induce plasma formation. This readilyexplains how noticeable drops in the PEE currentcan appear at small gaps, as shown in Fig. 1.Note that our PD and its electronics cannot

measure the discharge current directly; the dura-tion of the current burst is very short, less than 1 ms[11]. PD can show only the plasma signature, i.e.,an abrupt drop in the emission current. ER canshow the current distribution if its magnitudeexceeds the sensitivity of �1 mC/cm2 for positivetone area exposures to 1 keV electrons [12]. Thesensitivity is much larger than the total emittedcharge density, �1 nC/cm2, of CLN [1,4,8]. Thisindicates that all the ER patterns should be formedby the current burst owing to the plasma, not bythe PEE current.As shown in Fig. 3, we examined the polariza-

tion domains of the CLN surface after PEE at a

(footnote continued)

the local pressure near the LN surface. We used a dry pump

system for evacuation, i.e., a turbo-molecular pump and a

diaphragm backing pump, in order to minimize possible

contamination of our samples.

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Fig. 3. Optical microscope images of an etched CLN surface

after PEE. The gap between the CLN and the detector was

0.5mm.

D.-W. Kim et al. / Physica B 352 (2004) 200–205 203

gap of 0.5mm. Etching with HF:H2O (1:1) for100min. clearly revealed inverted domains,although the crystals were uniformly polarizedbefore PEE. The domain inversion (DI) areaconsisted of straight lines, which seemed to followthe crystal symmetry of the LiNbO3. As shown inFig. 3(b), a magnified view of the striation showsclear dendriform patterns. In general, dischargeaccompanies such tree-like patterns. In addition,note that the DI striation in Fig. 3 is quite similarto the ER pattern in Fig. 2. This implies thatplasma formation is related to inversion of thepolarization domains.Discharge can generate many charges (IION) and

some of them will bombard the LN surface,depending on their charge and the electric field(Egap and Et). Electron beam bombardment cancause DI of CLN [9]. Although the mechanism isnot clear, an electric field caused by the penetrat-ing electrons is believed to act as a local poling

field [9]. Similarly, a plasma-generated current,IION, can also induce the observed DI. This issupported by our finding that the developed ERshowed that the pattern in the area where plasmawas generated appeared similar to that in the DIregions. There is another possibility. Local elec-tron emission will deplete screening charges andgenerate a strong depolarization field. To reducethe electrostatic energy, a multi-domain might beformed.Although complete understanding of this is

lacking, we suggest the mechanisms shown sche-matically in Fig. 4. When we raise the temperatureof a +Z-faced LiNbO3 crystal, an uncompensatedsurface charge (sSC�PS) and resulting Egap canappear at the surface (Fig. 4(a)). Egap can induceelectron emission, IPEE (Fig. 4(b)). If the non-uniform PEE is significant, a tangential electricfield (Et) can appear (Fig. 4(c)). When either Egapor Et exceeds the threshold field, plasma (IION) canbe generated, as discussed above (Fig. 4 (c)). Theplasma-induced charge bombardment or depletionof the screening charge due to local PEE caninduce DI of the crystal (Fig. 4 (d)).We compared stoichiometric LiNbO3 crystals

(SLN) supplied by Oxide Corporation with CLN,as shown in Fig. 5. CLN does not show DI at agap of 2.5mm, but SLN shows DI at a gap of upto 4mm. The coercive field of lithium-deficientCLN (�20 kV/mm) is much higher than that ofSLN (o4 kV/mm) [13]. Whether the mechanism ofDI is the electron/ion bombardment duringdischarge or depletion of the screening charge, itis clear that a smaller coercive field lowers thethreshold field of DI. Moreover, note that thehexagonal shape of the SLN DI pattern differsfrom the CLN striation patterns. Although theydiffer, both the patterns and crystal symmetry arein good agreement. In an e-beam bombardmentstudy of DI, the inverted domains on the +Z facewere broadened hexagonally, which correspondsto the hexagonal unit cell of LiNbO3 [9]. All theseresults suggest a tendency for domain walls toform along the natural preferred domain wallorientation of the crystal, but the details of theorigin remain unclear.As the above comparison implies, the PEE of

CLN was more stable during repeated thermal

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Fig. 4. Schematic diagram of the charge and electric field distribution during PEE. (a) Decreased spontaneous polarization (PS) of a +Z-

faced LiNbO3 crystal generates an uncompensated surface charge (sSC�PS) and results in an electric field at the gap, Egap. (b) Egap induces

electron emission, IPEE. (c) Local charge depletion owing to the electron emission or strong electric field at the gap can generate plasma. (d)

Bombardment of energetic charges (IION) or depletion of a screening charge (sSC) might invert the polarization domains of the crystal.

D.-W. Kim et al. / Physica B 352 (2004) 200–205204

cycles than that of SLN. When heating andcooling the CLN crystals at a moderate rate of10 1C/min, a reproducible PEE current was ob-tained after sufficient relaxation, e.g., 4 h [8].However, the PEE current of SLN decreasedmarkedly with repeated thermal cycles and didnot recover after prolonged relaxation for up to24 h. Therefore, we could find that the polarizationdomains of the SLN were too sensitive to generatea stable PEE current.

4. Conclusions

We investigated the pyroelectric electron emis-sion (PEE) behavior of congruent LiNbO3 (CLN)crystals. When the gap between the CLN anddetector was less than 2mm, fluctuation anddrastic drops in the PEE current were noticeable.

Fig. 5. Domain inversion patterns of (a) congruent and (b)

stoichiometric LiNbO3 crystals with gaps of 2.5 and 4mm,

respectively.

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D.-W. Kim et al. / Physica B 352 (2004) 200–205 205

We also observed striation of the current burst one-beam resist and accompanying domain inversionof the crystal. These results all suggest that a highelectric field ignites plasma and induces inversionof the polarization domains during PEE. Plasmabehavior is likely determined by both materialparameters (pyroelectric coefficient, conductivity,and so on) and experimental conditions (surfacetreatments, gas species/pressure of the vacuumsystem, and geometry of the setup). We found thata larger gap and a higher coercive field couldsuppress the domain inversion and produce areproducible emission current.

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