18 Philips tech. Rev. 36, 18-25, 1976, No. 1

Observations of domains in ferroelectrics and ferromagneticswith a scanning electron microscope

necessitate the destruction of the material, as in tech-niques such as chemical surface etching [41.

The resolution of voltage-contrast imaging is in prin-ciple only limited by the diameter of the electron beam(0.02 !Lm). This value represents a considerable im-provement: the conventional direct methods such aspolarization microscopy and X-ray topography have aresolution of about I urn, because they depend on theuse of an ordinary optical microscope.It is also possible to use the SEM for a kind of

'dynamic' observation of ferroelectric materials. Theelectron beam then gradually builds up a surface chargethat induces 'domain flipping'. This induced polariza-tion switching and the associated redistribution of theferroelectric domains can be observed with the samehigh resolution while it is actually taking place.

In the following sections some of our work in ferro-electricity, with both single-crystal and polycrystalline(ceramic) materials, will be discussed. The experimentsclearly illustrate the feasibility of voltage-contrastimaging. A few preliminary details are given of thesamples used and of the instrument settings. The finalsection of the article contains materialon the - ratherlimited - possibilities of observing ferromagneticdomains by the somewhat analogous method ofmagnetic-contrast imaging.

C. Michel

The case described in this article - work with a scanning electron microscope -possesses two contrasting aspects. It shows, for both ferroelectricity and the electronmicroscope, that a scientific and technical subject can reach a stage of renaissance evenwithout ever having been a true classic. Forferromagnetism, however, exactly the reverseis true. The subject really is a classic, but SEM observations by means of magnetic-contrast do not appear to be a very hopeful prospect for the future.

Detection of surface potentials

The scanning electron microscope offers about adozen different ways of obtaining contrast in theimages it can produce [11. One of these is imageformation by voltage contrast. In this method thesecondary electrons emitted by the sample under in-vestigation are detected. These secondary electrons areproduced by the scanning (primary) beam of the SEM.The number of secondary electrons reaching the de-tector will vary with the electrostatic potential at thesurface of the sample, giving a corresponding contrastin the image. This method of imaging has been widelyused in the study of semiconductors and of micro-electronic circuits [21.

The method can also be used to give a directly visiblepresentation of the domain structure in ferroelectricmaterials. This application is important because usefulelectro-optic phenomena such as electrically controlledbirefringence and light scattering have been shown tobe related to the ferroelectric domain configuration inthe material concerned [31. The study of the behaviourof ferroelectric domains - in which the object isto find ways of controlling them - is by no meanseasy. Certain interactions, such as those between elec-trical and mechanical quantities, make the treatmentof ferroelectrics more complex than that of ferro-magnetics.

In a ferroelectric the electrical polarization inducessurface charges. These charges produce the potential:fields on which image formation by voltage contrastdepends. The various domains in general have differentpolarization directions, giving sufficient contrast fordirect study of the domains with almost zero delay.Observation of domain structure by SEM does not

Dr C. Michel, formerly with Philips Laboratories, BriarcliffManor, N. Y., is now with Stauffer Eastern Research Center, DobbsFerry, N.Y.,·U.S.A.

[1) See for example D. B. Holt, M. D. Muir, P. R. Grant and1. M. Boswarva (eds), Quantitative scanning electron micro-scopy, Academic Press, London 1974; L. Reimer and G.Pfefferkorn, Raster-Elektronenmikroskopie, Springer, Berlin1973; and W. Kuypers and J. C. Tiemeijer, The PhilipsPSEM 500 scanning electron microscope, Philips tech.Rev. 35, 153-165, 1975 (No. 6).

[2) P. R. Thornton, Scanning electron microscopy, Chapmanand Hall, London 1968. See also W. H. Hackett, Jr, R. H.Saul, R. W. Dixon and G. W. Kammlott, J. appl. Phys, 43,2857, 1972.

[3) C. E. Land, P. D. Thacher and G. H. Haertling, Electroopticceramics, in: R. Wolfe (ed.), Applied solid state science 4,137-233, Academic Press, New York 1974.

[4) J. A. Hooton and W. J. Merz, Phys. Rev. 98, 409, 1955.

Philips tech. Rev. 36, No. I DOMAIN OBSERVATION BY SEM 19

Equipment and materials

The four ferroelectrics that we investigated areshown in Table I with some of their characteristic data.The values for the spontaneous polarization apply inthe region of room temperature. The first three corn-pounds were investigated in the single-crystal form.The fourth compound, lanthanum-doped lead zircon-

Statie imaging of domains by voltage contrast

The first ferroelectric sample observed by the voltage-contrast (VC) technique was an uncoated optically fiatsurface of a single crystal of triglycine sulfate. A domainpattern is clearly visible infig. 1. The domains -- whichare elongated along a preferential direction - are foundto be separated by 1800 walls, i.e. the adjacent domains

Table I. The ferroelectrics that were investigated in a scanning electron microscope by means of voltage contrast.The column 'domain-wall type' gives the angle between the polarization vectors of adjacent domains.

Compound Crystal class Density Spontaneous Domain-wall Transitionpolarization type température

(g/crn") ([J-C(cmZ) (DC)

Triglycine sulfate Monoclinic 1.69 2.8 1800 49(CHzNH2COOH)3.H2S04

Barium titanate Tetragonal 6.4 26 180°,90°,450 I 130BaTi03

Tetra bisrnu th-t rititan iurn-12 oxide Monoclinic 6.6 4 180°,900 626Bi4Ti3012

Lanthanum-doped Tetragonal ((X) properties vary with thelead zirconate-titanate anel 7.6 28 (X and (3 phase 19J

8(65/35 PLZT rhombohedral ((3)

ate-titanate was the ceramic material studied. Thismaterial contains two crystal structures: ex, which istetragonal, and (3, which is rhombic. Both depend onthe thermal and electrical history of the sample.The SEM used for the experiments was a standard

production instrument whose voltage-contrast signalcould be displayed on a television monitor. Imagescould also be recorded on a video recorder. Withsamples of soft material such as triglycine sulfate theSEM was operated at a low beam intensity (10-11 toJO-IO A) and a low accelerating voltage (2 to 6 kV).The surface then remained undamaged by the incidentelectrons.

In the 'dynamic' mode of observation a beam inten-sity of as much as 10-9 A was permitted. Under suchconditions the electric field from the surface chargegenerated by the primary beam can easily reach severalkilovolts per cm, at least for non-conducting materials.However, if some existing domain structure is beinginvestigated - by observation in the static mode -the generation of static charge must be avoided. Non-conducting ferroelectrics must then be coated with aconducting film. Our samples were coated with a car-bon film of thickness between 0.01 and 0.05 fl-m. Sucha film has no significant effect on the resolution of theSEM. Thicker films could not be used because theywould give too much screening of the electric field- responsible for the contrast - at the surface of thesample.

Fig, 1. The surface of a single crystal of the ferroelectric TGS(triglycine sulfate). Adjacent domains have opposite polarizations.The image, obtained with a scanning electron microscope. isproduced by the effect of the electric field from the variousdomains on the secondary electrons emitted by the surface('voltage contrast'). The surface of the sample was optically flatand uncoated. The small white spots denote the build-up ofsurfacecharge.

20

have opposite polarizations. The fact that 1800 wallsare easily observable represents a specific advantage ofusing the SEM. The conventional method of obser-vation, with an optical microscope and polarized light,will not normally make such walls directly visible [51.

To confirm that fig. I is indeed due to voltage-con-trast imaging we made two control experiments. Firstlywe investigated the same uncoated samples in the SEMagain, by using back-scattered electrons. However, nocontrast could be seen. This control experiment provesthat the pattern offig. I cannot be due to topographicalfeatures of the surface. We also found that an uncoatedsample with a screening gold layer of excessive thick-ness (more than 0.1 urn) did not produce any second-ary-electron imaging. From these two experiments itcan be safely concluded that fig. I does indeed representa voltage-contrast image.

The domain patterns observed were found not to beparticularly stable. They included white spots, whichare characteristic of the build-up of surface charge.

After the triglycine-sulfate crystals we examinedbarium titanate. These crystals are much harder andeasily withstand primary beams up to lQ-lO A ataccelerating voltages to a maximum of 24 kV. Fig.2shows domains with 90° walls, delineated on a cleavedsection of the crystal.

Fig.3 shows a cleavage section of the third kind ofsingle crystal: Bi4Ti3ü12. The typical striped structureis similar to that of fig. 2, although the domain con-figuration is more complex, owing to the rather unusualcrystallographic structure of the material. In the crystalof fig. 3, which had the shape of a wafer, the directionof the spontaneous polarization of all domains wasparallel to the surface of the sample. This meant thatthere was no contrast at the upper surface of thesample, which is consistent with the expected con-stant surface potential there.

Dynamic imaging by voltage contrast

Beam-induced polarization switching

The domain flipping in ferroelectrics and its inter-action with the crystal properties is a complex phenom-enon [61. It is known, for example, that when themacroscopie polarization of a ferroelectric crystal isreversed by an external electric field, the domain con-figuration undergoes drastic changes. To obtain anunderstanding of the process much work has been doneon various compounds, including the material weselected, tri glycine sulfate. Using the SEM we wereable to directly observe beam-induced domain flippingin an uncoated triglycine-sulfate crystal by means ofthe television monitor coupled to the microscope. Thethickness of the wafer was about 50 (.Lm, and as stated

C. MICHEL Philips tech. Rev. 36, No. I

Fig.2. Domain structure of an unccated cleavage surface of asingle crystal of BaTi03. The polarization vectors in adjacentdomains are perpend icular to each other. The image was madewith a scanning electron microscope (SEM), using voltagecontrast.

earl ier the primary electron beam had an intensity of upto 10-9 A. At these current levels the observed domainpattern was found to be unstable and to be extremelysensitive to the beam. Granular white spots would sud-denly appear on the monitor screen, indicating a build-up of surface charges.

When a strong field, of the order of several kilovolts,is present, the polarization switches very rapidly - in a

Fig.3. Domain structure of an uncoated cleavage surface of asingle crystal of Bi"Ti:10'2.

Philips tech. Rev. 36, No. I DOMAIN OBSERVATION BY SEM 21

a

b

c

Fig.4. Domain imaging in the voltage-contrast 'dynamic' mode.It is called 'dynamic' because the scanning beam induces domainflipping. The sample is an uncoated single crystal of TGS. Thepictures are observed without any significant time delay on atelevision monitor coupled to the SEM. a) The incident primaryelectrons rapidly neutralize the existing surface charge. After afew scans the contrast between adjacent domains (with 1800

walls) has decreased to such an extent that only the domain wallsare still clearly delineated. b) After a few more scans the inducedfields become sufficiently strong to reverse individual polarizationvectors; for example, a domain near the center has been switchedand now appears as a dark island. cl After switching charge corn-pensation occurs and the contrast disappears again. Note theincreased domain size.

few microseconds. The velocities at which the wallsmove across the wafer are of the order of meters persecond. From this it follows that the switching of anindividual domain takes place too quickly for it to beobserved directly. The time necessary for switching thepolarization of the entire surface scanned by the beamcan be controlled within a certain range by varyingthe setting of the SEM: the time increases when thescanned area is increased or the intensity of thebeam is reduced.

Fig. 4 shows various stages of the switching processoccurring at the crystal surface: these images were ob-served on the monitor screen without any significantdelay. It can be seen that initially the contrast betweenadjacent domains with J 80° walls decreased rapidly.Fig. 4a shows an example of such flat contrast: onlythe domain walls are clearly delineated. The rapidreduction in the contrast results from the fast neutrali-zation of the surface charges originally present by theelectron beam. After the sample has been scanned a fewtimes the applied charge is large enough for its field toreverse the polarization. This switching process createsthe dark islands shown in fig. 4b. Somewhat later chargecompensation occurs and the contrast disappears again,

Fig. 5. An intermediate stage in the charge-compensation process(see Ag. 4c). The areas inside the domain walls are still partlyblack, indicating that charge compensation is not yet complete.

leaving domains of larger size (fig. 4c). Fig. 5 illustratesanother interrned iate stage of the charge-compensationprocess: it can clearly be seen that the process is onlypartially completed [7]. The contrast between domainsof opposite polarization lasted for no longer than a few

t51 See for example F. Jona and G. Shirane, Ferroelectriccrystals, Pergamon Press, Oxford 1962.

[6J See for example J. R. Maldonado and A. H. Meitzler, IEEETrans. EO-17, 148, 1970.

[7] A discussion of charging effects is given in Thornton's book(see note [2) above), p. III and following.

22

Fig.6. Photomicrograph of a single crystal of TGS, showing'beam writing' produced by an SEM. The 'writing' consists ofarow of dashes formed in non-adjacent domains of the samepolarization. The dashes represent local reversals brought aboutby individual scans. The beam current was IO-u A.

scans in this experiment; this period was so short thatthe contrast could only be observed in slow-motionplayback on the video recorder.

By increasing the beam current to 10-9 A we wereable to observe local switching induced by the individ-ual scans (fig. 6). The parallellines inside the domainsindicate the individual scans. The lines are of courseonly visible in domains that have a polarization of thesarne sign, which means that they cannot be adjacent.Such a technique of 'beam writing' makes the SEM aconvenient instrument for characterizing the polariza-tion states and the domain structure at the surface of aferroelectric sample.

The domain structure; nucleation effects

If high beam currents are used, local heating arisingduring the scan may be sufficient to cause the transitiontemperature of the ferroelectric material to be ex-ceeded. In such a case a transition to the para-electric

Fig.7. Photomicrograph of the edge of a strained single crystalofTGS. The sample was in the form of a thin wafer. Two typicaldomain patterns can be seen.

C. MICHEL Philips tech. Rev. 36, No. r

state takes place in the sample, and the domain patternis erased. If a sample is externally heated and thenallowed to cool, new domains are created on passingthe transition temperature. The way in which this hap-pens is of course important for the understanding of adomain pattern that appears in a particular crystal.The creation of domains at the transition ternperatureis linked to nucleation, which depends on the overallstate of the crystal. Most crystals are in a state of non-uniform strain, as a consequence of the conditionsduring their growth. The domain configuration in acrystal is therefore a result of a comprornise betweenthe energy requirements of a perfect crystal and theperturbing effects of strain, defects, and conductivity inthe actual crystal. The number and the distribution ofvacancies, dislocations, impurities and the doping affectthe uniformity of the polarization.

Our experiments with the SEM showed that aftercooling the sample to room température the domainpattern had the same overall shape and the samepolarization directions as before heating. However, ifthe sample is strained during the cooling the domainpattern can change drastically; seefig. 7. The domainpattern that forms during the change of state can ofcourse also be affected by an external electric field. Ina particular experiment an electric field was appliedparallel to the axis of spontaneous polarization in thesample. This was done with the aid of a gold contactdeposited on the sample by vapour deposition. Obser-vations showed that the number of points at whichdomains started to grow and the velocities of themoving walls both increase with the strength of theapplied electric field. So far the discussion of voltage-contrast observation in the dynamic mode has beenlimited to its use with single-crystal materials. Neverthe-less, an extension of the technique to the observationof domains in ferroelectric ceramic materials - whichare polycrystalline - is of great practical importance.In the next part of this article results will be discussed

Philips tech. Rev. 36, No. I DOMAIN OBSERVATION BY SEM

that were obtained with the SEM in the study of oneof the ceramic materials mentioned earlier, lead zircon-ate-titanate with lanthanum.

Domains in a polycrystal/ine ferroe/ectric

Lead zirconate-titanate doped with lanthanum(PLZT) is a ferroelectric ceramic whose crystals are ofthe perovskite type [5]. This type of crystal can bepolarized along several axes: in the unpolarized state,above the transition ternperature, these axes are allequivalent. The material that is mostly studied, onaccount of its unusual electro-optic properties - whichmake it suitable for shutters, light modulators, imagestorage and display, as well as for holographic memo-ries [8] - contains a doping concentration of eightatomic per cent of lanthanum to give transparency; thelanthanum is substituted for lead. Zinc and titaniumare present in the ratio 65 : 35. This compound pos-sesses a number of physically cornplicated but gener-ally useful electrical states involving some rather un-usual phase transitions [9]. The actual state in a par-ticular sample depends on the thermal and electricalhistory. The best material is produced by hot pressingand has small grains (1-2 urn). The conventional tech-niques for the observation of the microstructure havetoo small a resolution for these grain dimensions.Again, using the SEM in the voltage-contrast dynamicmode proved to be a successful alternative. The sampleswere polished wafers with a thickness of about 250 urn.The material is hard and suffers no beam damage froma current of 10-10 A and accelerating voltages up to25 kV. However, there was a charging effect, visible onthe television monitor, which seriously limited the ob-servation of domain structures. J n fact, after a fewscans, the electric field induced across the wafer by theexcess of surface charge was so high that it generatedsmall intergranular cracks in the ceramic. The variousstages of the attack of the surface and the propagationof the cracks across it could all be observed on themonitor as they took place. In one extreme case (a scanof more than 10 seconds) the transverse field due to thesurface charge was high enough to eject a crystal grainfrom its site, leaving a cavity with a well defined prefer-ential crystallographic orientation (fig. 8). This effectwhich has only been observed above the transition tem-perature, could perhaps be due to the strong electro-mechanical force that arises on the polarization reversalof each grain. By coating the sample with a conductinglayer, the electric transverse field could be eliminated,and degradation of the surface did not then occur.Under these conditions voltage-contrast imaging was

[8] H. N. Roberts, Appl. Optics 11, 397, 1972.W. R. Salaneck, J. appl. Phys. 43, 4468, 1972.

[9] E. T. Keve and A. D. Armis, Ferroelectrics 5, 77, 1973.

Fig. 8. Beam-induced degradation of the surface of an uncoatedsample of PLZT, and the propagation of the cracks. The imageswere produced with no significant time delay. The scanning timewas more than 10 seconds. The small cracks arise between theseparate grains of the material and are caused by the high electricfield-strengths induced by the surface charges. G, b) Formationof the cracks. c) After a crystal grain has been ejected from itssite, a crystallographically well defined cavity remains. If thesurface is coated with a conducting layer surface degradationdoes not occur.

23

a

b

c

24 C. MICHEL Philips tech. Rev. 36, No. 1

a b c

Fig.9. Images of chemically etched lanthanum-doped lead zirconate-titanate (PLZT) of corn-position 8/65/35 (see text). a) Coarse-grained material, electrically poled. b) Coarse-grainedmaterial, electrically depoled. c) Fine-grained material, thermally depoled.

also possible: differences in the electrical and thermalhistory of the material were found to give clearly ob-servable differences in the microstructure of the grains.The absence of grain structure in thermally depoledmaterial supports E. T. Keve's model of a non-polarcubic structure [91. For material that was electricallypolarized or strained, striations with a spacing gener-ally smaller than] fLm were observed. Such a resultsuggests a preferential orientation of the polarization.For electrically depoled material, the images obtainedby voltage contrast clearly suggested the presence ofrandomly distributed domains. The contrast of thedifferent patterns was very poor and found to dependcritically on the thickness of the coating. Better resultswere in fact obtained with chemically etched ceramicwafers with no coating. Surface microstructures ofthreedifferent states, electrically poled, electrically depoled,and thermally depoled, are shown in fig. 9. The grainpattern of a strain-induced phase shown in fig. IDashows complex superimposed striated patterns, with ahigh degree of preferential orientation that can nolonger be satisfactorily explained by considering anordered ferroelectric domain structure alone. In furtherinvestigations with surface replicas of the same samplewith a transmission electron microscope, the greaterresolution revealed the presence of a twin band struc-ture about 0.01 fLm wide (fig. lOb). Preliminary in-vestigations suggest an ordered distribution of ex-tended defects. In this case, voltage-contrast imaging inthe dynamic mode, combined with information ob-tained by static etching techniques, might lead to abetter understanding of the unusual phase transitionsthat arise here.

Magnetic-contrast imaging

It seems rather obvious to try to extend the samemethods to ferromagnetic materials. In this case

imaging is produced by means of magnetic contrast,i.e. by the operation of the Lorentz force, which willdeflect the secondary electrons emitted from the sampleas they travel through the magnetic fringing fields. Inour study of the possibil ities that this method mightoffer we have examined single crystals of magneto-plumbite. This material has uniaxial anisotropy, whichimplies that the domain structure is rather simple. Thesaturation magnetization is lower than that of cobalt,but higher than that of the orthoferrites. Fig. J J showsthe domain structure at the basal surface of a magneto-plumbite sample. The black and white areas reveal thepresence of opposing internal demagnetizing fields. Themagnetic-contrast response can be maximized byorienting the sample in such a way that the internaldomains are directed towards the SEM collector: thisindicates that the fringing fields contain a componentparallel to the surface and perpendicular to the linebetween sample and collector [11. In effect the Lorentzforce tilts the distribution of secondary electronsleaving the surface. This tilt occurs about an axis paral-lel to the component of the fringing field mentionedabove. Magnetic-contrast imaging can therefore beimproved by increasing the directional sensitivity ofthe detector [lOl.

The work on magnetoplumbite has led us to the viewthat it will be difficult to obtain good contrast, evenwith optimum orientation conditions. The techniquecan of course only be used with materials that have asufficiently high anisotropy to produce fringing fieldsoutside the surface. These various factors set a limita-tion to the usefulness of the technique, particularly ifwe compare it with the analogous case of voltage con-trast. This was why no further studies of magnetic-contrast imaging were made.

Our experience with this particular case does how-ever enable us to make a few further comments on thismethod of imaging. We found that the observed

Philips tech. Rev. 36, No. I DOMAIN OBSERVATION BY SEM

a b

Fig. 10. Microstructure of an individual grain ofPLZT, ofcomposition 8/65/35 and chemicallyetched. The crystals have the rhornbic structure ({J, see also Table 1), as a result of strain. a)Sets of clearly delineated overlapping striations, as observed by scanning electron microscopy,b) Two images of the same striations, but now greatly magnified. These two pictures wereobserved by transmission electron microscopy, in a conventional electron microscope, ofreplicas of the surface. The spacing of the striations is less than 0.0 I fLm. The high degree ofpreferential orientation cannot be explained from the ferroelectric domain structure alone.

Fig. 11. An example of a magnetic-contrast image made with anSEM. The darker and lighter regions represent ferromagneticdomain structure on the basal surface of a single crystal ofmagnetoplumbite. Because of Lorentz forces, the fringing fieldsdeflect the secondary electrons emitted from the sample. Thesedeflections produce the contrast in the image.

[10] G. A. Wardly, J. appl. Phys. 42,376, 1971.[11] R. Carey and E. D. Isaac, Magnetic domains and techniques

for their observation, English Univ. Press, London 1966.

magnetic-domain pattern is not affected by the surface-charge accumulation during the SEM observation.Since the interaction between the magnetic field and thesecondary electrons is direction-dependent, the contrastis related to the orientation of the sample. Finally,magnetic contrast with a good signal-to-noise ratiorequires materials with a strong magnetization, such asmagnetoplumbite, the example in our study.

An attractive feature of the technique is that nospecial preparation of the sample is required, as inLorentz microscopy [111. The information is obtainedalmost instantaneously and with a much higher spatialresolution than with the conventional Kerr-effect tech-nique, or the colloidalor X-ray-topography tech-niques - both of which are unsuitable for dynamicexperiments.

Summary. The feasibility of voltage-contrast imaging in scanningelectron microscopy is demonstrated by a study of ferroelectricdomain configurations. The resolution limit is 0.02 fLm. Themethod is non-destructive. Domain imaging is possible in a staticmode, with thinly coated samples, and in a dynamic mode, inwhich the beam itself induces switching in the domains and thesamples are uncoated. Single crystals of TGS (triglycine sulfate),BaTi03, Bi4Ti3012 and the small-grained polycrystalline (ceram-ic) material 8/65/35 PLZT, which is also ferroelectric, have beenstudied. Domain patterns with 1800 walls and 900 walls areclearly visible. On increasing the beam current (up to 10-9 A)local heating beyond the transition ternperature can occur and'writing' with the beam is possible. The analogous magnetic-contrast imaging with ferromagnetic materials seems to be oflimited usefulness.

25

.~.

26 Philips tech. Rev. 36, No. 1

Recent scientific publicationsThese publications are contributed by staff of laboratories and plants which form part ofor cooperate with enterprises of the Philips group of companies, particularly by staff ofthe following research laboratories:

Philips Research Laboratories, Eindhoven, The Netherlands EMullard Research Laboratories, Redhill, Surrey, England Mlaboratoires d'Electronique et de Physique Appliquée, 3 avenue Descartes,

94450 Limeil-Brévannes, France LPhilips GmbH Forschungslaboratorium Aachen, WeiBhausstraBe, 51 Aachen,

~m~ APhilips GmbH Forschungslaboratorium Hamburg, Vogt-Kölln-StraBe 30,

2000 Hamburg 54, Germany HMBLE Laboratoire de Recherches, 2 avenue Van Becelaere, 1170 Brussels

(Boitsfort), Belgium BPhilips Laboratories, 345 Scarborough Road, Briarcliff Manor, N.Y. 10510,

U.S.A. (by contract with the North American Philips Corp.) N

Reprints of most of these publications will be available in the near future. Requests forreprints should be addressed to the respective laboratories (see the code letter) or to PhilipsResearch Laboratories, Eindhoven, The Netherlands.

W. Albers & L.A. H. van Hoof: Enhanced conversionof x-rays into visible light in thin composite layers(composite anthracene-PbClg screens).J. Electrochem. Sac.12l, 1341-1345, 1974 (No. 10). E

H. Bex: New broadband balun.Electronics Letters 11, 47-48, 1975 (No. 2). A

J.H. den Boef & J. C. M. Henning: A strain-modulatedelectron spin resonance spectrometer.Rev. sci. Instr. 45, 1199-1202, 1974 (No. 10). E

J. Bootsma: The gas liquid interface and the loadcapacity of helical grooved journal bearings.Trans. ASME F (J. Lubr. Techno!.) 95, 94-100, 1973(No. I). E

J. Bootsma: The gas-to-liquid interface of spiral groovejournal bearings and its effect on stability.Trans. ASME F (J. Lubr. Techno!.) 96, 337-345, 1974(No.3). E

J. Bootsma: Spherical and conical spiral groove bear-ings: Part 1. Theory, Part 11. Load capacity and sta-bility.Trans. AS ME F (J. Lubr. Techno!.) 97, 236-242,243-249, 1975 (No. 2). E

J. J. v.d. Broek: Calculation of the formation volumesof alloys.3rd Int. Conf. on Chemical thermodynamics, Badennear Vienna 1973, pp. 52-58; 1974. E

K. H. J. Buschow: Note on the magnetic properties ofsome Fe2P-type rare-earth intermetallic compounds.J. less-common Met. 39, 185-188, 1975 (No. I). E

C. Crevecoeur & H. J. de Wit: The growth of anodic/ aluminum oxide layers after a heat-treatment.

J. Electrochem. Soc. 121, 1465-1474, 1974 (No. 11). E

P. A. Devijver: Entropie quadratique et reconnaissancedes formes.Proc. NATO ASI Computer oriented learning processes,Bonas 1974, pp. 39-55. B

H. Durand: L'avenir des cellules solaires au silicium àusage terrestre.Onde électr. 55, 161-166, 1975 (No. 3). L

M. J. C. van Gemert & W. H. de Jeu: On the thin cellmethod in time domain spectroscopy.Chem. Phys. Letters 29, 287-289, 1974 (No. 2). E

A. A. van der Giessen: Advances in magnetic recordingmaterials.Rev. Phys. app!. 9, 869-876, 1974 (No. 5). E

J. Haisma, G. Bartels, W. F. Druyvesteyn, U. Enz, J.-P.Krumme & A. G. H. VerhuIst: Observation of varioustypes of bubbles bounded by a compensation wall.IEEE Trans. MAG-lO, 630-633, 1974 (No. 3). E, H

C. M. Hart, A. Slob & H. E. J. Wulms: Bipolar LSItakes a new direction with integrated injection logic.Electronics 47, No. 20, 111-118, Oct. 3, 1974. E

G. Jötten, K. Kyser (both with C. H. F. Müller GmbH,Hamburg) & W. J. Oosterkamp: X-ray source formammography.Medicamundi 19, 25-27, 1974 (No. 1). E

D. Kasperkovitz: Analysis and improvement of a staticshift register.Microelectronics and Reliability 13, 501-515, 1974(No.6). E

J. A. Kerr (Mullard Hazel Grove, Ltd., Stockport,Cheshire, England) & F. Berz: The effect of emitterdoping gradient onlT in microwave bipolar transistors.IEEE Trans. ED-22, 15-20, 1975 (No. I). M

Philips tech. Rev. 36, No. 1 RECENT SCIENTIFIC PUBLICATIONS 27

J. E. Knowies: The origin of the increase in magneticloss induced by machining ferrites.IEEE Trans. MAG-U, 44-50, 1975 (No. I). M

E. Krätzig &M. Rosenkranz: Ultrasonic determinationof the energy gap in superconducting layers.Phys. Stat. sol. (b) 67, K 19-21, 1975 (No. I). H

F. E. J. Kruseman Aretz: Doelstellingen en achtergron-den van de operating systems PICO, MICRO en MILLI.Informatie 16, 672-678, 1974 (No. 12). E

H. K. Kuiken: The cooling of a low-heat-resistancesheet moving through a fluid.Proc. Roy. Soc. London A 341, 233-252, 1974 (No.1625). E

M. Laguës & J. L. Domange (E.N.S.C.P., Paris): Sur-face segregation. Comparison between theory andexperiment.Surface Sci. 47, 77-85, 1975 (No. I). L

F. Meyer & J. J. Vrakking: In-depth information fromAuger electron spectroscopy.Surface Sci. 45, 409-418, 1974 (No. 2). E

F. Meyer & J. J. Vrakking: Comment on 'The adsorp-tion of oxygen on silicon (111) surfaces' by R. Dom,H. Lüth and H. Ibach.Surface Sci. 46, 287-289, 1974 (No. I). E

D. Meyer-Ebrecht & G. Schröder: Rechneranpassungfrequenzanaloger MeBsysteme.Acta IMEKO 1973, pp. 701-712; 1975. H

A. Mircea, E. Constant (Université de Lille) & R. Per-richon (Univ. de LiIle): FM noise of high-efficiencyGaAs IMPATT oscillators and amplifiers.Appl. Phys. Letters 26, 245-248, 1975(No. 5). L

A. van Oostrom : Influence of adsorbates and electricfield on the nucleation and growth of microtips in avacuum gap.Proc. 6th Int. Symp. on Discharges and electrical in-sulation in vacuum, Swansea 1974, pp. 49-70. E

K. J. van Oostrum & H. F. Premsela: Visualisation ofdiffraction information in a TEM image using colourtelevision display.8th Int. Congress on Electron microscopy, Canberra1974, Vol. I, pp. 100-101. E

J. Pockrand & J. Verweel: Magnetic domains in thinsputtered FeSi films, I. Edge effects and influence ofthe substrate temperature and the Ar pressure duringsputtering.Phys. Stat. sol. (a) 27, 413-427, 1975 (No. 2). H

H. Rau: Vapour composition and Van der Waals con-stants of arsenic. .J. chem. Thermodyn. 7, 27-32, 1975 (No. I). A

D. Rossier & F. Dumont: Dependence of the charac-teristics of a photoconductor-dielectric optical converteron photogeneration and charge transfer parameters ofthe photoconductor.J. appl. Phys. 46, 849-854, 1975 (No. 2). L

E. Roza: Analysis of phase-locked timing extractioncircuits for pulse code transmission.IEEE Trans. COM-22, 1236-1249, 1974 (No. 9). E

T. E. Rozzi & J. H. C. vanHeuven: Quasi-power alge-braic invariants of linear networks.IEEE Trans. CAS-21, 722-728, 1974 (No. 6). E

G. B. Scort, D. E. Lacklison & J. L. Page: The effectsof octahedral Fe3+ and tetrahedral Fe3+ dilution onthe Faraday spectra of bismuth-doped iron garnets.J. Physics C 8, 519-529, 1975 (No. 4). M

J. L. Sommerdijk, A. Bril & A. W. de Jager: Lumines-cence of Pr3+-activated fluorides.J. Luminescence 9, 288-296, 1974 (No. 4). E

A. L. N. Stevels & W. Kühl: New phosphors for X-rayimage intensifier tubes.Medicamundi 19, 3-7, 1974 (No. I). E

B. J. Stocker: AES and LEED study of the activationof GaAs-Cs-O negative electron affinity surfaces.Surface Sci. 47, 501-513, 1975 (No. 2). M

T. J. B. Swanenburg & J. Wolter: Frequency depend-ence of the transmission of high-frequency phononsfrom a solid into liquid helium.Phys. Rev. Letters 33, 882-885, 1974 (No. 15). E

T. L. Tansley: AC profiling by Schottky gated clover-leaf.J. Physics E 8, 52-54, 1975 (No. I). M

M. J. J. Theunissen: Charge transfer devices: I. Physi-cal principles, 11.Applications.Onde électr. 54, 317-324, 405-413,1974 (Nos. 7 & 8). E

H. J. Tolle & R. Memming: Fast imaging process inPbh films.Appl. Phys. Letters 26, 349-351, 1975 (No. 6). H

T. S. te Velde: Mathematical analysis of a heterojunc-tion, applied to the copper sulphide - cadmium sul-phide solar cell.Solid-State Electronics 16, 1305-1314,1973 (No. 12). E

J. M. P. J. Verstegen (Philips Lighting Division, Eind-hoven): A survey of a group of phosphors, based onhexagonal aluminate and gallate host lattices.J. Electrochem. Soc. 121, 1623-1627, 1974 (No. 12).

J. M. P. J. Verstegen (Philips Lighting Division, Eind-hoven): The luminescence of Tb3+ in borates of thecomposition X2Z(B03)2 (X=Ba, Sr, Ca; Z=Ca, Mg).J. Electrochem. Soc. 121, 1631-1633, 1974 (No. 12).

J. M. P. J. Verstegen, D. Radielovié & L. E. Vrenken(Philips Lighting Division, Eindhoven): A new gen-eration of 'deluxe' fluorescent lamps, combining anefficacy of 80 lumens/W or more with a color renderingindex of approximately 85.J. Electrochem. Soc. 121, 1627-1631, 1974 (No. 12),& 122, 843, 1975 (No. 6) (reply to discussion).

J. M. P. J. Verstegen (Philips Lighting Division,Eindhoven) & J. L. Sommerdijk: Line emission ofSrBe2Si207 :Eu2+and BaBe2Si207 :Eu2+.J. Luminescence 9, 297-301, 1974 (No. 4). E

28 RECENT SCIENTIFIC PUBLICATIONS Philips tech. Rev. 36, No. I

J. F. Verwey & R. P. Kramer: Atmos - an electricallyreprogrammable read-only memory device.IEEE Trans. ED-2l, 631-636, 1974 (No. 10). E

J. O. Voorman: Ideal frequency modulator.Electronics Letters 10, 387-388, 1974 (No. 18).

P. van der Wurf: On the spectral density of a cyclo-stationary process.IEEE Trans. COM-22, 1727-1730, 1974 (No. 10). E

F. Zernike: Luneburg lens for optical waveguide use.E Optics Comm. 12, 379-381, 1974 (No. 4). N

Contents ofPhilips Research Reports 30, Special issue in honour of C. J. Bouwkamp, 1975:

H. B. G. Casimir: Laser modes; an imperfect tribute toC. J. Bouwkamp (pp. 1*-4*).

O. Bottema (Delft): A lesson in elementary geometry(pp. 5*-13*).

C. H. Papas (U.S.A.): On the equation of motion inelectrodynamics (pp. 14*-19*).

M. Kac (U.S.A.): An example of 'counting withoutcounting' (pp. 20*-22*).

A. Erdélyi (U.K.): Fourier transforms of integrablegeneralized functions (pp. 23*-30*).

M. S. Klamkin (Canada): Asymptotic heat conductionin arbitrary bodies (pp. 31*-39*).

J. H. van Lint (T.H. Eindhoven) & H. O. Pollak(U.S.A.) : An asymmetrie contest for properties ofarbitrary value (pp. 40*-55*).

J. Kay (U.S.A.): Near and far field HF radar groundwave return from the sea (pp. 56*-64*).

N. G. van Kampen (Utrecht): The collapse of the wavefunction (pp. 65*-73*).

B. R. A. Nijboer (Utrecht): On a relation between thescattering cross-section in dense media and the energyof a dilute electron gas (pp. 74*-82*).

J. B. Keiler (U.S.A.): Effective conductivity, dielectricconstant and permeability of a dilute suspension(pp. 83*-90*).

P. Delsarte, J. M. Goethals & J. J. Seidel (T.H. Eind-hoven): Bounds for systems of lines, and Jacobi poly-nomials (pp. 91*-105*). B

V. Belevitch & Y. Genin: Reciprocity invariants inequivalent networks (pp. 106*-121 *). B

J. B. Alblas (T.H. Eindhoven): Relaxation phenomenain electro-rnagneto-elasticity (pp. 122*-139*).

J. W. Miles (U.S.A.): Asymptotic approximations foroblate spheroidal wave functions (pp. 140*-160*).

J. Boersma (T.H. Eindhoven): Analysis of Weinstein'sdiffraction function (pp. 161*-170*).

J. A. Geurst: Continuum theory for type-A smecticliquid crystals (pp. 171*-186*). E

L. B. Felsen (U.S.A.): Complex rays (pp. 187*-195*).

H. Freudenthal (Utrecht): On the cardinality of finiteTits geometries (pp. 196*-204*).

R. L. Brooks (U.K.), C. A. B. Smith (U.K.), A.H. Stone(U.S.A.) &W. T. Tutte (Canada): Leaky electricity andtriangulated triangles (pp. 205*-219*).

P. J. Federico (U.S.A.): The number of polyhedra(pp. 220*-231 *).

J. Meixner (Aachen) & S. Sche (Aachen): Some re-marks on the treatment of the diffraction through acircular aperture (pp. 232*-239*).

H. Levine (U.S.A.): Acoustical diffraction radiation(pp. 240*-276*).

K. M. Adams (Delft): The non-amplification propertyof networks consisting of n-terminal resistive devices(pp. 277*-287*).

F. E. J. Kruseman Aretz & J. A. Zonneveld: FFT algo-rithms (pp. 288*-301 *). E

A. T. de Hoop (Delft): The N-port receiving antennaand its equivalent electrical network (pp. 302*-315*).

H. J. Butlerweck (T.H. Eindhoven): Noise voltages ofbulk resistors due to random fluctuations of conduc-tivity (pp. 316*-321 *).

A. J. Dekkers: N-omino enumeration (pp. 322*-328*).E

A. J. W. Duijvestijn (Enschede): Fast calculation ofinverse matrices occurring in squared-rectangle cal-culation (pp. 329*-336*).

N. G. de Bruijn (T.H. Eindhoven) & D. A. Klarner(U.S.A.) : A finite basis theorem for packing boxes withbricks (pp. 337*-343*).

F. L. H. M. Stumpers (Bochum): Some notes on thecorrespondence between Sir Edward Appleton andBalth. van der Pol (pp. 344*-356*).

N. Marcuvitz (U.S.A.): Eigenmodes, quasimodes andquasiparticles (pp. 357*-375*).

Vol urne 36, 1976, No. 1 pages 1-28 Published 1st July 1976