SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 495È498 (1999)

Scanning Near-Ðeld CathodoluminescenceMicroscopy on Indented MgO Crystal

D. Pastre� , M. Troyon,* T. Duvaut and J. L. BeaudoinUniversite� de Reims, Unite� de Thermique et Analyse Physique, EA 2061. 21, rue cle� ment Ader, 51685 Reims Ce� dex 2, France

The cathodoluminescence (CL) emission arising from the indentation of an MgO single crystal cleaved along (100)faces is investigated by scanning near-Ðeld cathodoluminescence microscopy (SNCLM). This technique is per-formed with a hybrid instrument that is a combination of a scanning force microscope, a scanning near-Ðeld opticalmicroscope and a scanning electron microscope. The interest of this instrument is to allow one to obtain near-ÐeldCL images, the resolution of which (¿100 nm) is at least an order of magnitude better than the resolution obtainedwith a classical CL imaging system (¿1 lm) using a conventional scanning electron microscope. On the indentedMgO, the CL emission appears mainly localized in the deformed regions, especially in the slip bands created justaround the indentation. These slip bands are revealed only because of the high resolution of our hybrid system.Copyright 1999 John Wiley & Sons, Ltd.(

KEYWORDS: cathodoluminescence ; near-Ðeld microscopy ; scanning electron microscopy

INTRODUCTION

The cathodoluminescence (CL) emission from deformedionic crystals such as MgO, CaO, and SrO hasAl2O3been studied in the scanning electron microscope byseveral authors.1h5 The deformations are generallyinduced by the action of a concentrated load thatcreates a non-uniform stress state and stimulates allpossible gliding systems. One general observation ofthese previous works1h5 is the enhancement of the CLemission in the plastically deformed regions of the crys-tals. It is suggested that most of the CL in the deformedregions is due to coloured centres produced by plasticÑows. For example, in the MgO, the coloured centers (Fand F`) created by the load emit at 520 and 375 nm,respectively.4 The CL imaging mode is very interestingas a non-destructive method for studying such defects inthe crystals. However, all the classical CL detectionsystems that generally use mirrors6 to collect thephotons in the far-Ðeld mode have a resolution limitedby the energy dissipation volume, which depends on thematerials and the working conditions (electron beamenergy).7 The resolution, particularly for ionic crystals,is generally of the order of a few micrometres because ofthe di†usion length of the electron and the holes thatare created by the incident electron beam. To improvethe resolution, we have developed a new techniquebased on near-Ðeld collection of the photons emitted bythe specimen.8 This technique is scanning near-Ðeldcathodoluminescence microscopy (SNCLM), which is in

* Correspondence to : Universite� de Reims, Unite� deM. Troyon,Thermique et Analyse Physique, EA 2061, 21 rue cle� ment Ader, 51685Reims Ce� dex 2, France. E-mail : michel.troyon=univ-reims.fr

fact a combination of scanning force microscopy (SFM),scanning near-Ðeld optical microscopy (SNOM) andscanning electron microscopy (SEM). With this tech-nique a topographical SFM image, a near-Ðeld CLimage and a SEM image of the same scanned area canbe obtained simultaneously.

In this article, we present near-Ðeld CL images of anMgO(100) surface indented using a standard diamondVickers pyramid. The results obtained using SNCLMare also compared with those obtained, on the sameindentation, with a classical parabolic mirror collectionsystem.

EXPERIMENTAL

The SNCLM instrument that we have developed, theresolution of which is of the order of 100 nm, hasalready been described elsewhere,8 therefore we willbrieÑy recall only its main features.

The use of an atomic force microscopy cantilever as anear-Ðeld probe o†ers the possibility to combine SFMand SNOM;9,10 it is this scheme that we have chosen totransform the SFM head, which we previously com-bined with a scanning electron microscope,11 into ascanning near-Ðeld optical microscope. The light dif-fracted by the SFM classical tip is collected withSi3N4an optical Ðbre placed just upon the cantilever at a dis-tance of D50 lm. Although the pyramidal shape of thetip is not ideal to guide the light towards the opticalÐbre, it was the solution that we adopted because the

transparency to visible light is good (which is notSi3N4the case for silicon) and it was the easiest way, takinginto account the mechanical conception of our system.

CCC 0142È2421/99/050495È04 $17.50 Received 15 September 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 12 November 1998

496 D. PASTREŠ ET AL .

Figure 1. Schematic diagram of the experimental SNCLM set-up.

A schematic drawing of the SNCLM instrument isgiven in Fig. 1. The cantilever can be mechanicallycentred in the three directions with respect to theoptical Ðbre, which is rigidly Ðxed to the cantileverholder. The centering is done using binoculars beforemounting on the SFM head. The optical Ðbre that weused is a multimode Ðbre, the core and cladding ofwhich are 50 and 125 lm in diameter, respectively, witha numerical aperture of 0.29. In our experimental condi-tions the SEM electron beam is focused close to the tipin a Ðxed probe and the sample is scanned under thetip. The piezotube allows a scanning area of 30] 30lm2. Let us emphasize that our scanning electronmicroscope is equipped with a Ðeld emission gun.Indeed, contrary to SEM with a thermionic gun wherethe non-conducting specimens are generally metal-coated and imaged at 20È30 kV, because of the highbrightness of the Ðeld emission gun, uncoated specimenscan be examined with good resolution at much lowerenergy, thus avoiding the disturbing electrical charginge†ect at the specimen surface. Working at a low voltagealso reduces the interaction volume and thus limits thenumber of atoms excited in the far-Ðeld mode. Anotheraspect of the Ðeld emission gun is the possibility ofobtaining a high electron probe current in small probesizes (always because of high brightness), thus favouringphoton yield.

An MgO single crystal with a purity of 99.9%,cleaved along the (100) faces, was used. The indenta-tions were made using a Vickers microindenter with thefollowing procedure : the load was increased up to 200 gat a loading speed of 10 g s~1, the load was then heldconstant for 1 min and Ðnally reduced to zero. In orderto compare the results from our SNCLM system withthose from a classical CL system, the same indentationwas imaged with both systems. The classical CL systemis an Oxford instrument (ITC 502) combined with aPhilips 515 scanning electron microscope equipped witha thermionic gun ; the photons emitted by the specimenare collected with a parabolic mirror. The indentationsof the uncoated MgO crystal were Ðrst imaged using

SNCLM at an electron beam energy of 3 keV. TheSFM and CL images presented in the next section areimages of 256] 256 pixels obtained at a line frequencyof 0.1 Hz. Then the MgO crystal was coated with 30 nmof silver for imaging at 25 keV with the classical CLsystem as a comparison with the SNCLM system.

RESULTS AND DISCUSSION

Figure 2 shows the SEM secondary electron image ofthe indentation made in the uncoated MgO sample.Figure 3 shows the SFM topographic image of thisindentation and the simultaneously acquired CL image.The CL image revels very clearly, around the indenta-tion, slip bands that have been created by the crystaldeformation. There is no correlation here between thetopographical and the CL image. This demonstratesthat the luminescence contrast is only a consequence ofthe presence of coloured centres created by the plasticdeformation and is not an artefact of the surface topog-raphy ; it should be noted that the near-Ðeld collectiontechnique is often very sensitive to topographical infor-mation.12,13 Figure 4 represents a line proÐle of the CLimage of Fig. 3(b) ; slip bands less than 300È400 nm insize are clearly separated, which demonstrates the goodspatial resolution of our SNCLM instrument. Thesignal-to-background ratio of these narrowest bands isof the order of 5È10%.

Concerning the centre of the indentation, whichappears dark in the classical CL images, several authorshave interpreted the absence of luminescence in thiszone as being due to a high density of defects14 or to adisturbance of the crystal structure up to destruction.15The near-Ðeld CL image of Fig. 3(b) shows that theindented region presents contrasted zones, contrary toclassical CL images. But, because of the possible topo-graphical artefacts, it is difficult to give an interpreta-tion of this contrast here.

Figure 2. A SEM image of the MgO indentation.

Surf. Interface Anal. 27, 495È498 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.

SCANNING NEAR-FIELD CATHODOLUMINESCENCE MICROSCOPY 497

Figure 3. (a) Topographic SFM image of the MgO surfaceindented with a 200 g indentation load and (b) its simultaneouslyacquired near-field cathodoluminescence image (30 Ã30 mm)obtained with a 3 kV accelerating voltage.

Figure 5. A CL image of the same indentation as that of Fig. 3(b)obtained with a classical detection system (parabolic mirror) usingconventional SEM. The accelerating voltage is 25 kV. The sampleis coated with a 30 nm thick silver layer. Slip bands in the brightrosette area are not revealed.

Although CL images of indented MgO obtained withclassical CL imaging have already been produced in theliterature, we have also imaged the same indentationwith a classical CL system for comparison. Figure 5 isthe CL image of the same indentation as that shown inFig. 3(b) obtained with the classical CL set-up describedin the previous section. Because the image has beendone at 25 keV beam energy, the sample has beencoated with 30 nm of silver to avoid electrical chargingproblems. The fact that the scanning electron micro-scope is equipped with a thermionic gun, which haspoor brightness, imposes this high voltage because atlower voltages the sensitivity of the collection systemdoes not allow a good CL image to be obtained. Theimage of Fig. 5 shows a typical bright rosette with adark centre. The lateral resolution of this CL image isvery poor compared to the resolution obtained with

Figure 4. A CL intensity line profile along the arrow represented on Fig. 3(b). Small slip bands less than 300–400 nm in size, createdaround the indentation, are clearly resolved.

Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 495È498 (1999)

498 D. PASTREŠ ET AL .

SNCLM; the slip bands are not revealed. This is obvi-ously explained by the size of the energy dissipationvolume, which is of the order of a few micrometres at 25keV for the MgO. In principle, a lower voltage shouldgive a better resolution because of a smaller energy dis-sipation volume. Probably, using a scanning electronmicroscope with a Ðeld emission gun to obtain a clas-sical CL image at low voltage, without the presence of aconductive Ðlm on the samples surface, would have pre-sented a better resolution than that of Fig. 5. However,even then the energy transfer limitation could not beovercome. This is the advantage of near-Ðeld collection :it is not limited by the energy dissipation volume butsolely by its aptitude for detecting the evanescent waves.

The results presented demonstrate that our near-ÐeldCL imaging set-up is very suitable for a non-destructivestudy of ionic crystal structure because it can resolve

highly localized plastic deformations at the sample sub-surface.

CONCLUSION

Initial results on the near-Ðeld cathodoluminescenceimaging of an indented MgO crystal are reported. Ourscanning near-Ðeld cathodoluminescence microscopeallows the detection of crystal defects very close to thesurface (\200 nm), with a high spatial resolution that isan order of magnitude better than the resolution givenby a classical CL system with conventional SEM. Putinto perspective, spectroscopic studies should allowbetter interpretation of these images owing to the iden-tiÐcation of the coloured centres (F or F`, for example).

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Surf. Interface Anal. 27, 495È498 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.