Surface induced liquidcrystal alignment studied by optical secondharmonicgenerationM. Barmentlo, N. A. J. M. van Aerle, R. W. J. Hollering, and J. P. M. Damen Citation: Journal of Applied Physics 71, 4799 (1992); doi: 10.1063/1.350620 View online: http://dx.doi.org/10.1063/1.350620 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/71/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Microscopic calculation of surface-induced second-harmonic generation in crystals of para-nitroaniline J. Chem. Phys. 112, 6757 (2000); 10.1063/1.481251 Liquid-crystal alignment on polytetrafluoroethylene and high-density polyethylene thin films studied byoptical second-harmonic generation J. Appl. Phys. 83, 5195 (1998); 10.1063/1.367339 Ultrafast optical switching of secondharmonic generation at the C60 singlecrystal surface J. Appl. Phys. 79, 3781 (1996); 10.1063/1.361213 The measurement of secondharmonic generation in novel ferroelectric liquid crystal materials J. Appl. Phys. 70, 3426 (1991); 10.1063/1.350330 ERRATUM: OPTICAL SECONDHARMONIC GENERATION IN CRYSTALS OF ORGANIC DYES Appl. Phys. Lett. 16, 224 (1970); 10.1063/1.1653170

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Surface induced liquid-crystal alignment studied by optical second-harmonic generation

M. Barmentlo, N. A. J. M. van Aerle, R. W. J. Hollering, and J. P. M. Damen Philips Research Laboratories, P. 0. Box 80 000, 5600 J A Eindhoven, The Netherlands

(Received 27 September 1991; accepted for publication 3 1 January 1992)

The orientational order induced in a liquid crystal (LC) monolayer of adsorbed 4-n-octyl-4’- cyanobiphenyl molecules on rubbed substrates is studied by measuring the second-harmonic response. By systematically varying the rubbing conditions and measuring the induced LC bulk and monolayer alignment, we can conclude that the homogeneity of the LC bulk alignment and the surface in-plane order parameter of the LC monolayer are closely related. The polar and azimuthal ordering of the LC monolayer, however, are found to be decoupled. By rubbing polyimide films twice and studying other rubbed surfaces, we show that two mechanisms are responsible for the LC alignment, depending on the kind of substrate that is used. For rubbed bare glass substrates long-range elastic interactions, which do not affect the monolayer alignment, are dominant. In the case of rubbed polyimide films molecular short-range interactions are responsible for the alignment of both the LC bulk and the LC monolayer.

1. INTRODUCTION

It is well known that different bulk liquid crystal (LC) alignments can be induced by several types of surface treat- ments.’ Homogeneous alignment of LC molecules (i.e., av- erage molecular orientation parallel to the surface along some easy axis) can be achieved by modifying substrates by oblique evaporation of inorganic materials2 or the use of stretched polymers,3 photolithographic gratings,4 rubbed polymer film~,~ or Langmuir-Blodgett films6 as orienting medium. Preparation of homogeneous, defect-free LC lay- ers of a large area sandwiched between two surface-treated glass substrates is a prerequisite for optimal electro-optical performance of an LC display (LCD). From the surface preparation methods mentioned above, the use of rubbed polymers is of particular interest because of its feasibility for large area treatment and mass production.

Usually, the rubbing is performed by translating the substrates underneath a rotating cylinder, covered with a particular cloth. Although this method is mostly used in large-scale LCD production, the physical mechanism re- sponsible for the LC alignment is not yet clearly under- stood. Berreman proposed elastic interactions between rub- bing-induced microgrooves and the LC molecules as the underlying mechanism.’ On the other hand, Castellano adopted the concept of LC alignment induced by orienta- tion of the polymer chains.8 Recently various polymers were found to induce LC alignment perpendicular to the rubbing direction. In addition the alignment decreased as the grooved substrates were covered with a thin metal overlayer.9~10

Until recently the interaction between modified sur- faces and LC molecules was studied only by extrapolating data of bulk LC phenomena because the experimental tech- niques lacked enough surface specificity.” Optical second- harmonic generation (SHG) has proven to be a highly surface-specific technique. The surface specificity stems from the fact that, in the electric dipole approximation, SHG is a symmetry-forbidden process in the bulk of cen- trosymmetric media. Only at the surface/interface, where

this symmetry is broken, is efficient generation of a polar- ization at the second-harmonic frequency possible. Espe- cially in situations where the frequencies of the light waves involved are near a resonance frequency for the adsorbed molecules, the technique has a submonolayer sensitivity.r2 In recent publications it has been exploited as a very sen- sitive probe for determination of molecular tilt angles of LC molecules on isotropic (unrubbed) surfactant-coated substrates.‘3’14 In addition, it has been shown that aniso- tropic molecular orientation distributions of LC monolay- ers on rubbed polyimide’tllms can be determined.“‘16

In this paper we present a systematic investigation of the influence of rubbing on molecular alignment by study- ing LC monolayers applied to these substrates and LC cells made of these substrates. With the use of LC monolayers we study the orienting influence of the 8CB-solid and the 8CB-air interface. The influence of the bulk LC is absent. Following Geary et al.,’ the rubbed polymer films were analyzed by measuring the rubbing-induced birefringence. Our results show a strong correlation between the in-plane surface order parameter and the homogeneity of the LC bulk alignment in LCDs. We also find a decoupling be- tween the polar and the azimuthal anchoring of LC mole- cules. By varying the rubbing directions and the substrates, we are able to distinguish .between two different mecha- nisms in LC alignment. In the case of rubbed bare glass substrates, long-range elastic interactions are responsible for the bulk alignment, while the LC monolayer is unaf- fected. In the case of rubbed polyimide layers, a short- range molecular interaction aligns both the LC bulk and monolayer.

II. THEORY

For surface SHG, the signal at the second-harmonic frequency 2w is generated by the induced surface nonlinear polarization:

Pc2)(2wj =xc2’( - 2w;w,w):E(w)E(w).

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Here, E(w) is the excitation field at the fundamental fre- quency w and xc2) is the surface nonlinear susceptibility tensor. Starting from this expression, it can be derived that the SH intensity 1(2w) reflected from a surface is given by:”

I( 20) a sec2S1 1 e20*x(2):e,e, 1 212(o). (1)

Here, Q is the angle of reflectance, which equals the angle of incidence and e20, e, denote the output and input po- larization vectors at frequency 2w and o, respectively, which contain products of linear Fresnel factors,” and I(w) denotes the laser intensity at frequency o. In this paper, the polarization directions are denoted by p^ and $ which are in the plane of incidence and normal to it, re- spectively.

For LC molecules adsorbed on clean centrosymmetric substrates often the nonlinear susceptibility of the substrate is overwhelmed by that of the polar ordered molecules. Assuming the interaction between molecules to be negligi- ble, the xc’) tensor can be related to the hyperpolarizability tensor ac2) of the adsorbed molecules by:

# = N,(G $&a$. (2)

Here N, is the surface density of molecules and (G $$) denotes an appropriate average over molecular orientations of the transformation matrix from the molecular coordi- nate axes ($,,;i,t> to the laboratory system (z$).

For molecules of rod-like structure often a(‘) is dom- inated by a single component, ag6& (2) along the long molec- ular axis 8. The tensor ,y$ then takes the.form:

~$2 = N,( (?f) (j’a (f;‘g))ag$. (3)

Here (?$& refer to the unit vectors ($$a, which are in the plane of the sample ($7 and normal to it (a, respec- tively. Note that the assumption of ac2) being dominated by a& yields index permutation symmetry for the ele- ments of xc2). For surfaces exhibiting C,, symmetry along 2, with independent polar 0 and azimuthal $ distributions of ‘& we tind the following nonvanishing elements of xi2):

x (2) x.xX = N,(sin3 0) (cos3 4)agk,

Y @I _ (2) _ (2)

j XYY - xyxy - xyyx

= N,(sin3 0) (cos 4 sin2 4)a&,

xg = xg = xz

= N,(sin 0 cos’ 0) (cos #)a&

(4)

(5)

(6)

(7)

(8)

I

FIG. 1. Definition of all relevant vectors for the description of SH re- sponse of anisotropic LC monolayers according to the notation of Ref. 17.

(9)

= N,(cos 0 sin” 0) (cos2 +)agl, (10)

X (2) _ (2) _ (2) ZYY - xv, - XYYZ (11)

= N,(cos 0 sin’ 0) (sin2 $)a& (12)

xg = Ns(cos3 @)a&. (13)

The angular brackets denote an average over all mo- lecular orientations within the (macroscopic) probed area. The six independent elements of ,$‘) are determined by measuring the SH response in reflection from the LC monolayer as a function of sample rotation about its sur- face normal (3 for four different polarization combina- tions I, IsP, &., $,. Here, i,, denotes the zpolarized SH signal under p-polarized excitation. In order to describe the dependence of the SH signal on the rotation angle for the different polarization combinations we closely follow the notation of Mizrahi and Sipe.” Since we are interested in anisotropic SH signals we introduce the following expres- sions for our polarization vectors $, 3, containing the rota- tion angle $J:

LG.= (:y,),= (:s~~:~).

Here, the angle rj is zero if the easy axis of the LC mono- layer is in the plane of incidence (see Fig. 1). By properly transforming the polarization vectors, explictly incorporat- ing the equalities between the different elements of <y(‘), the following expressions for the SH fields can be deduced:

E,asecfi(&)2(R~,+ l)(sin3~~~~+3sin1C,cos2~x~~~),

Epsasecdz($,,)2C(R& - 1) cos ~[cos3 $yyJ..$ + sin2 7j cos 7&&L - .y:;) I + (Rg, + Ih Wn2 l(xg + cm2 +@“lh

Esp a set Cl ( z&)~( Rim + 1) {cos2 fi, [ sin 3/ cos2 $,YZ + ( sin3 rj - 2~0s” rj sin Ilf)x$$]

+ cos C12, sin fi2, cos $ sin 9[ 2x% - 2xJ$]e(w) + sin2 Cl, sin &~E~(w)},

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Eppasec Cl(t&J2{(R& - 1) [cos’ fi2, cos R(cos3 7&g: + 3~0s y5 sin’ Z/X:$) + cos a cos a, sin n,e(w)

X (2~0s’ $-yg + 2sin2 @j$) + E2(o)sin* fi, cos fi cos $x:2] + (R& + 1) [sin” a, sin QxgE2(w)

+ cos2 Cl, sin 8 ( cos2 t&g: + sin2q!y$i > + 2c(a > sin R sin ,R, cosfiz, cos &g$. (14)

Here, the linear Fresnel factors are labeled as follows:” f”o, stands for the transmission coefficient of the gpolarized electric field at’ the fundamental frequency when going from vacuum (0) to the medium (m), and R&,, denotes the reflection coefficient of the &polarized electric field at the SH frequency at the vacuum/medium interface. fi2, is the angle of incidence in the medium and E(O) denotes the dielectric constant of the substrate at the fundamental fre- quency. Here the linear dielectric constant of the mono- layer is taken to be equal to one.

Since for all SH signals the same orientational distri- bution of molecules is probed, all six independent elements of xc2) can be deduced by fitting all signals in one cycle. According to Eqs. (4)-( 13) the elements of xc2) contain information on both the polar and azimuthal distribution functions of the adsorbed LC molecules.

If we assume the polar molecular orientation function to be represented by a Gaussian distribution centered at 0 with standard deviation a, these two values can be deduced from the following linear combinations of x(21 elements:

p + p xxx * XYY (sin” 0)

xg +x:;; -t .&j =-

(sin @> ’

.p) + p + ,p >ZXX zuv__fu_ =

(cos 0)

xi2 (cos3 O} *

(15)

(16)

The azimuthal 4 distribution function f (q5) can be built up from a Fourier series f(4) = &an cos n$. From the remaining independent combinations of the elements of x (2) the Fourier coefficients a, can be determined up to third order. Finally, from this distribution a surface in- plane order parameter for the LC monolayer

QF i cos 246 sin2 0

7 sin- 0 1 (17)

can be calculated.r8 Since we have assumed independent distributions for 4 and 0 Eq. ( 17) reduces to

Q,= (cos24) +2Exz--*~ xi2 + A!$ *

(18)

As follows from Eq. (18) Q, = 0 for an isotropic 4 distri- bution and Qs = 1 for complete ordering. Note that the in-plane surface order parameter is averaged over the probed (macroscopic) area.

III. EXPERIMENT

The experiments were performed using the frequency- doubled output at a wavelength of 532 nm of an active- passive mode-locked Nd:YAG laser producing pulses of 30

4801 J. Appl. Phys., Vol. 71, No. 10, 15 May 1992

ps duration at a repetition rate of 10 Hz, with a pulse energy of 3 mJ. The unfocused beam with a diameter of approximately 4 mm was directed onto the sample at an angle of incidence of 45”. After blocking the reflected 532 nm beam, the SH output was selected with the use of a home-built prism monochromator with a high transmis- sion for ultraviolet (UV) light. The SH output was de- tected for both p^ and $polarization using an analyzer, a solar-blind photomultiplier and gated electronics.

The polymer films were produced by spin-coating a solution of polyimide onto 1. l-mm-thick Corning-7059 glass plates. The average thickness of the resulting polymer films was 110 nm after a thermal curing step. The prefer- ential orientation of the polyimide surface was effectuated by rubbing. It was carried out on a rubbing machine sim- ilar to the one published by Becker et a1.l’ In this way the rubbing process is carried out with excellent reproducibil- ity. By varying the rubbing force, the polyimide films were oriented and grooved to a different extent. The rubbing- induced birefringence of the polymer layer was determined using a home-built linear-optical setup*’ to quantify the result of the rubbing similar to Geary et ai.’

The liquid crystal 4-n-octyl-4’-cyanobiphenyl ( 8CB), obtained from BDH, was used without further purification. The 8CB monolayers were deposited either by evapora- tion’3y’4 or with the use of a computer-controlled Lauda Langmuir-Blodgett trough. In the latter case the 8CB monolayers were spread from a 10 - 3 M hexane solution. At a surface pressure Z- = 3.5 mN/m the closely packed monolayer was transferred to the substrate, resulting in a surface density of the LC molecules of N, = 1.3 X 1014 cme2.

In the SHG experiment the polyimide and LC-covered Corning-7059 glass samples were mounted on a l-cm-thick glass substrate with index-matching fluid in order to sepa- rate front and rear reflection of the fundamental beam. The sample was rotated, about its surface normal by a com- puter-controlled stepper motor at a rate of 1” per second. The signals were averaged over several rotations and nor- malized with respect to a reference signal obtained from quartz to reduce the influence of the shot-to-shot intensity noise of the laser. .The SH wavelength (266 nm) is close to resonance for the 8CB molecules and therefore in all cases the SH background of the glass-polyimide substrates is negligible.

IV. RESULTS AND DISCUSSION

A. Rubbing condition variation

The anisotropic SH response under rotation of the sample over an angle $ about its surface normal directly

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270” 270”

270” 270”

FIG. 2. SH field E(2o) upon rotation about its surface normal of 8CB molecules adsorbed on both um-ubbed (B) and rubbed (0 ) polyimide substrates for different input/output polarization combinations, E, Ep Epn Ew Here, E+, denotes the gpolarized SH field under p^-polarized excitation. $=O corresponds to the rubbing direction. The solid line represents a theoretical fit to the data.

reflects the symmetry properties of the LC monolayers un- der investigation. *%I6 Figure 2 shows for different polariza- tion combinations the normalized SH electric fields ob- tained from 8CB monolayers evaporated on polyimide substrates. For unrubbed polyimide substrates Epp and Eps show an isotropic response, and the symmetry forbid- den Esp and Es signals are found to be zero. For the rubbed polyimide substrate all signals exhibit a substantial anisot- ropy, and the data reveal a Cr, symmetric arrangement of the 8CB molecules. The different SH responses for $ = 0 and $ = 180” directly reflect the preferential alignment of the individual molecules along the rubbing direction. This means that the 8CB biphenyl cores tilt more in the rubbing direction, while the polar cyano groups point towards the substrate. From a fit to the data (solid line Fig. 2) we determine the relative values of ;y(‘) elements to be: XxXx (‘I = 1.00, x$; = 0.25, X;z = 0.039,x2 = 3.35,~;;; = 1.77,x% = 0.23.

Substitution of these values in Eqs. (15) and (16) yields a tilt angle distribution centered at 0 = 81” with respect to the surface normal and with a standard devia- tion of (T = 5“. After determination of the Fourier coeffi- cients a,, from the x (2) elements, the Cl, symmetric in- plane orientational distribution function f(4) = 1 + 0.08~0~4 +O.GOcos 24 + 0.02~0s 34, shown in Fig. 3,

is obtained. Figure 4 shows the effect of a strongly reduced rubbing

force on the spolarized SH response of the LC molecules. The surface-induced Cl, symmetry in f(4) is still present (Fig. 3), but the deviation from isotropy for the $polar- ized SH signals is greatly reduced in this case. The effect of rubbing on the surface in-plane orderparameter Q, as

180” 0” *

270”

FIG. 3. Anisotropic molecular distribution function of 8CB monolayers on rubbed polyimide substrates. The solid and dotted lines refer to the standard (Fig. 2) and lightly (Fig. 4) rubbed substrates, respectively.

given in Eq. ( 18)) is plotted as a function of rubbing- induced birefringence A# (mrad) in Fig. 5. For low rub- bing forces a nearly isotropic SH response from the ad- sorbed LC molecules is observed, hence Q, is close to zero. -4s the rubbing force increases, A$ as well as Q, increases. The LC cells made with substrates which showed anisot- ropy in the SH response all showed planar alignment along the rubbing direction. However, the uniformity of the LC cells was improved by increasing the amount of rubbing work. Similar results were obtained recently by Feller and colleagues.16 We see that the homogeneity of the bulk LC alignment and surface in-plane order parameter (Q,) are closely related.

It is known that the bulk pretilt angle of an LC layer depends on the rubbing conditions orienting the polyimide surfaces between which the LC layer is sandwiched.‘9P21 From the analysis of the SHG data, however, we do not find any systematic change in 0 and (T for the monolayer by varying the rubbing pressure (Fig. 6)) as was also found in Ref. 16. From these data the following can be con- cluded:

Eps EPP

180’ 0” 180” 0”

270” 270”

FIG. 4. p?polarized SH fields obtained from 8CB adsorbed on a lightly rubbed polyimide layer as in Fig. 2. The solid line represents a theoretical Et to the data.

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OG’ h

f 3 P t =P 0 8

f! z

0.0 0.0 0.5 1 .O 1.5 2.0 2.5

A cp (mrad)

FIG. 5. Surface in-plane order parameter (Q,) as determined from the anisotropic SH response (see Figs. 2 and 4) vs rubbing-induced bireti-in- gence Ad (mrad) of the polyimide layer. The solid line is merely a guide to the eye. :

(i) The polar surface anchoring for the LC monolayer is strong and independent of the azimuthal alignment. This is in agreement with the results for bulk LC alignment on rubbed polymer layers”V23 and justifies the assumption of an independent 0 and 4 distribution made earlier [IQ. (5)-(14)l.

(ii) By extrapolating 0 and a to the unrubbed case, we see that the orientational distribution of 8CB on the un- rubbed polyimide surface is not very sharply peaked, but has the same spread of approximately lo” and an average tilt angle of 78”. This is in agreement with the SH response from 8CB on unrubbed polyimide layers [Fig. 2).

B. Aligning mechanism

To examine the influence of (micro)grooves we rubbed polyimide films sequentially in two directions, both in op- posite and orthogonal directions. In both cases we deter-

80 ? =. . .

= .

0.. 0. . . . . . . ,~~~1~~~~1~~~11~~~11~~~ll~~~lll l t l l1 0 1 2 3 4 5 6 7

A cp (mrad)

FIG. 6. Variation of the average tilt angle 0 (a) and the standard devi- ation m (0) as a function of rubbing-induced birefringence A+ (mrad).

180° O0 180'

--I---

Q" G K

270" 270"

180” 0” 180" b-- 0" w1

z

270' 270"

FIG. 7. SH response IPP of 8CB evaporatedionto sequentially rubbed polyimide films (a) once rubbed (W,) (m), and antiparallel rerubbed (W, + W,) (0), (b) once rubbed (WI) (IU), and the orthogonally re- rubbed (W, + W,) (0).

mined that the last rubbing direction dominated the LC monolayer alignment (Fig. 7). This remained unchanged even if the amount of rubbing work in the last direction was an order of magnitude less than during the llrst rub- bing cycle. In all cases the cells also produced from these substrates showed orientation of the LC layer along the last rubbing direction. Similar results for the bulk LC layer were reported by Mosley et a1.24 From these surface SHG and bulk LCD alignment experiments it is clear that rub- bing-induced (micro)grooves are not dominant in either LC bulk or LC monolayer alignment.

For other systems bulk and monolayer alignment may behave differently. This is nicely demonstrated by using bare glass substrates. We rubbed these substrates and sub- sequently UV-ozone cleaned them in order to remove every possible organic material of the rubbing cloth that might have been deposited onto the glass during the rubbing pro- cess. We observed a homogeneous alignment in the LCD made from these substrates. In this case, the observed LC bulk alignment can only stem from elastic interactions of the rod-like LC molecules and the grooved substrate.’ On the other hand, we obtained an isotropic distribution of the LC monolayer from SHG data of adsorbed 8CB monolay- ers on these substrates.

We conclude that two different aligning mechanisms are dominant. For the case of rubbed bare glass substrates long-range elastic interactions, which do not aEect the LC monolayer alignment, are responsible for the bulk align- ment. In the case of rubbed polyimide layers a short-range molecular interaction aligns both the LC bulk and mono- layer.

To investigate the nature of the short-range interaction for rubbed polymer layers we also deposited monolayers of 8CB onto rubbed polyimide substrates by means of a

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Langmuir-Blodgett technique. This yielded the same 0 and 4 distributions as obtained for evaporated 8CB. The possibility of electrostatic “charging” of the substrate as the nature of the molecular short-range interaction can be excluded for the polyimide case since this effect should have been neutralized in the water subphase. In the case of rubbed isotactic polystyrene, we observed that the bulk LC is aligned perpendicular to the rubbing direction. Applica- tion of a monolayer of 8CB on these substrates leads to very weak SH signals, which indicates the absence of polar sites to which the LC molecules may become attached.14 Thus, a dipolar interaction between the surface and the molecules, which is present in the polyimide-8CB interac- tion, is not a general mechanism for all rubbed polymer layers. Van der Waals interactions, which are also pro- posed in the literature, may account for the observed phe- nomena.* In this case the strongest interactions are ex- pected to stem from the phenyl structures in both the polyimide film and the LC molecules.25126 It is known that the phenyl structures in the used polyimide are part of the oriented polymer backbone and in polystyrene, as a side- group, more or less perpendicular to the chain axis, as is the induced orientation of bulk LC in devices produced with these substrates. Further research on modified poly- mer layers will be necessary for a better understanding of the origin of the polymer/LC interactions on the molecular level.

V. CONCLUSIONS

Our experimental data on the anisotropic SH response of 8CB monolayers deposited onto rubbed polyimide layers allow us to determine the molecular orientation at the sur- face. By systematically varying the rubbing conditions and measuring the LC monolayer and LC bulk alignment we conclude that:

(i) The homogeneity of the bulk LC alignment is closely related to the surface in-plane orderparameter of a LC monolayer.

(ii) The polar surface anchoring of the LC molecules is strong and independent of the azimuthal alignment.

(iii) The polar orientation of an 8CB monolayer on both rubbed and unrubbed polyimide is not sharply peaked.

In addition, by rerubbing polyimide films in different directions and studying other rubbed surfaces we conclude that two aligning mechanisms are effective in LC align- ment:

(i) For rubbed bare glass substrates a long-range elas- tic interaction aligns the bulk LC, but does not affect the LC monolayer alignment.

(ii) For rubbed polyimide layers a molecular short- range interaction aligns both LC bulk and monolayer.

ACKNOWLEDGMENTS

We would like to thank J. I. M. Verbakel for carefully preparing the polyimide layers and Dr. F. R. Hoekstra for his assistance in writing the computer program. We are also indebted to J. C. Jans for the determination of the optical constants of the polyimide layers. Finally, we would like to thank Dr. C. J. Gerritsma and Professor Dr. Q. H. F. Vrehen for their contribution to this work.

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4804 J. Appl. Phys., Vol. 71, No. 10, 15 May 1992 Barmentlo et a/. 4804 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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