On the dynamic nature of microscopy specimens at lattice resolution

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On the dynamic nature of microscopy specimens at lattice resolution

J. Washburn, Z. Liliental-Weber

To cite this version:

J. Washburn, Z. Liliental-Weber. On the dynamic nature of microscopy specimens at lattice res- olution. Journal de Physique, 1989, 50 (24), pp.3431-3440. �10.1051/jphys:0198900500240343100�.

�jpa-00211152�

3431

LE JOURNAL DE PHYSIQUE

Short Communication

On the dynamic ^{nature of} microscopy specimens ^{at lattice}

resolution(*)

J. Washburn and Z. Liliental-Weber

Center for Advanced Materials, ^Lawrence Berkeley Laboratory 62/203, ¹ Cyclotron Road, Berkeley,

CA 94720, ^U.S.A

(Reçu ^le 4 juillet 1989, ^{révisé le 13 octobre} 1989, accepté le 16 octobre 1989)

Résumé.

Nous donnons des exemples illustrant les changements rapides qui peuvent ^{avoir lieu} lorsqu’un échantillon en fine feuille est observé dans un microscope à ^haut voltage. ^Ces exemples, exceptionnellement spectaculaires, soulignent ^{le fait} que, pour ^une caractérisation correcte de détails

structuraux de n’importe quel matériau cristallin par microscopie d’imagerie (particulièrement ^des surfaces, interfaces et autres défauts), ^{il est} essentiel de ne jamais négliger ^la possibilité que ^{des ef-} fets structuraux locaux ou des défauts à l’échelle atomique ^aient pu ^être modifiés ^{dans le} microscope

avant la prise ^{de la} micrographie ^ou pendant ^la préparation ^de l’ échantillon en fine feuille. La mobi- lité atomique ^induite par le faisceau électronique ^{alliée à} l’imagerie séquentielle à ^{haute résolution}

conduisent à l’observation d’intéressants détails structuraux sur des surfaces d’or et de GaAs dans différentes orientations cristallographiques.

Abstract.

Examples ^are shown that illustrate rapid changes ^{that can take} place ^{while a thin foil} specimen is under observation in a high-voltage microscope. ^These unusually ^dramatic examples ^em- phasize the fact that for correct characterization of structural details of any crystalline ^material by

lattice imaging microscopy particularly ^at surfaces, interfaces, ^{and other} defects, it is essential never to overlook the possibility ^{that local} structural features or atomic-scale defects may ^{have been} changed

in the microscope prior ^{to the} recording ^{of a} micrograph ^or during preparation ^{of the thin foil} speci-

ment. Electron beam induced atomic mobility combined with sequential high resolution imaging ^are

shown to provide interesting ^details concerning ^{the structure of} gold and GaAs surfaces of different

crystallographic orientations.

Tome 50 N°24 ¹⁵ DÉCEMBRE 1989

J. Phys. ^{France SO} (1989) ^{3431-3440 15} DÉCEMBRE _1989,

Classification

Pjrysics Abstracts

61.80F - 73.60D - 73.60F

(*) ^{Zhis paper} is dedicated ^to Jacques ^{Friedel on} the occasion of his retirement. It ^was unfortunately ^sub-

mitted ^too late to be included in the special issue of Journal de Physique, ^{vol. 50, n° 18.}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198900500240343100

Introduction.

Within the last decade, ^electron microscopes ^{have been} improved ^{to the extent that} point- to-point resolution is smaller than nearest-neighbor ^{distances in} crystalline materials. Lattice-

imaging microscopy has become a powerful ^{tool for} identifiying ^of phases, revealing ^the presence of long-range order, ^and determining crystal ^{structure and the structure of} interfaces, grain ^bound- aries, dislocations, ^and precipitates. ^{A lattice} image reveals the periodicity ^{of the} structure, ^{and it} appears ^that individual columns of atoms are being "seen". However, it has become well recog- nized that interpretation ^{of lattice} images is far from intuitive, ^and serious mistakes can be made unless a through-focus ^{series of} micrographs ^is compared ^{with a series of} computer-simulated ^im-

ages ^{for the same set of} parameters.

Two steps ^{inherent to} high-resolution microscopy ^can affect the atomic scale structures being investigated particularly ^{near defects and} interfaces as well as introduce new defects or features that were not originally present : (1) ^{the final} preparation ^of ultra thin foils necessary ^for good

lattice images, ^and (2) interaction with the electron beam when the specimen ^{is in the} microscope.

These problems ^{have not been} sufficiently emphasized by ^{most authors.} Structural changes ^can

occur because the best resolution requires accelerating voltages ^{in excess of 200 kV} and because many specimens ^{can be} prepared only by ^ion thinning. Any defect of structural detail that can act as a sink for vacancies, interstitials, ^or impurities introduced or mobilized by this radiation damage

may therefore be altered before or during observation. Even though ^{the electron} energy may ^be below the threshold energy ^{needed to create a Frenkel} pair, ^{it may} still be able to displace ^atoms.

Sub-threshold displacement ^{of the} heavy ^{element in an} alloy which also contains a lighter ^element

while under observation in a high voltage ^electron microscope ^{has been} well known for many years

[1-3]. ^Such displacements ^{are caused} by transfer of sufficient energy from the electron collisions

to the light ^{element atoms so that the} resulting energetic ^{ions in turn} displace ^the heavy ^element through ^{a distance} great enough ^{to form a stable} vacancy-interstitial pair. Sub-threshold displace-

ments near the exit surface which create a stable vacancy because the interstitial is ejected ^{to the}

surface have also been discussed by Cherns et ail [4]. ^These mechanisms could contribute to dra- matic changes ^{such as those in the} gold specimens descnbed in the next section. However, ^there

is another possibility ^that could also explain ^low temperature mobility ^{of surface atoms. Transfer}

of energy from the electrons in the beam to carbon atoms or other light elements in the contam- ination layers ^could produce ^ion damage ^which would be localized to near surface layers. ^This

radiation induced surface diffusion combined with the tendency ^{of the thin foil to} reduce total surface free energy could also contribute to structural changes. ^{These effects are more trouble-}

some for high-resolution ^lattice imaging microscopy, ^not only ^{because of the} higher accelerating voltages normally employed, ^{but also} because ^the precise alignment required ^{for a} good ^lattice image generally ^{results in a} relatively long exposure ^{to the} electron beam compared ^{with conven-}

tional bright-field ^or dark-field imaging techniques. ^Also, the atomic scale structural details being

studied are the ones most likely ^{to be} significantly changed by ^these mechanisms. Before definite conclusions are made concerning ^{the structure} that existed prior ^to observation, particularly ^at

the surfaces and edges ^{of a thin} foil, ^{it is} necessary ^to try ^to determine which features may ^have been altered by radiation-damage effects. In this paper ^several unusually ^dramatic examples ^will

be shown that illustrate the dynamic ^nature of certain specimens beeing ^observed by high-voltage, high-resolution ^electron microscopy.

Au samples.

Au samples ^were prepared by electron-beam evaporation ^{of 100 nm of} pure Au in situ in ultra-

high ^{vacuum on cleaved} (110) ^GaAs substrates. These samples ^{were annealed in N2 at 405° C for}

3433 10 min. Cross sections were prepared by cutting ^{1-2 mm} long ^{slabs and} gluing ^two samples together

with the thin Au layers facing each other. The samples ^were mechanically polished ^{to achieve a}

thickness of about 40 mm ^and then ion milled to achieve thin foils transparent ^to électrons. No Au-Ga-As phases ^{were observed after} annealing, however, approximately ^{5% of Ga was dissolved}

in the Au. Figure ^{1 shows a series of three} micrographs ^{of the same area of} gold ^{at the} edge

Fig.1.- Three lattice image micrographs ^{of the same area of a} gold ^foil exposed ^for increasing ^{time to 800 keV}

electrons and a beam current density ^{of 1 A} cm-2. Foil thinning occured in some areas while other areas

increased in thickness. Note that one of the ^areas that thinned and eventually ^{became a hole was at the site}

of a dislocation. As soon as a hole appeared ^the edges ^retreated rapidly until the thickness at the edge ^was

of the order of 5 to 10 nm (estimated ^from contrast). ^{In this} image the thickness at the edge ^is comparable

to the diameter of the holes. Therefore the { 100} ^and { 111} facets that are seen edge-on ^are longest ^{in the}

thickness dimension.

of one of these specimens, chosen from ^a through-focus ^{series taken on} the Atomic Resolution

Microscope ^{at Lawrence} Berkeley Laboratory ^{with an} accelerating voltage ^{of 800 kV and beam}

current of 1 A cm-2. The estimated electron flux was 5 ^x 1013 electrons/s. ^The time interval between figures ^{la and lc was about 50 s. As can be seen} from these figures, ^{some areas} of the film

get ^thinner during observation and eventually become holes while other areas grow thicker. ^{It is}

clear that there is ^a continuous loss and/or surface migration ^of gold atoms, probably by ejection

from the exit surface and surface diffusion. The electron energy is well below the 1300 keV that would have been required for Frenkel pair production ^{in the interior of the} gold [1]. ^{Therefore the}

atomic mobility is initiated by electron beam interaction with lighter ^{elements and} possibly ^with

near surface gold ^{atoms. Under the} conditions necessary ^for transmission electron microscopy ^at

the highest resolution very few specimens ^{would be free of some radiation} damage ^effects by ^the possible mechanisms. The series of images ⁱⁿ figure 1 shows that. { 111 } ^and { 100} ^{facets were de-} veloped ^{at the} edges of the holes that formed and grew. The ^holes appear ^to have increased in size

by preferential ^{removal of atoms at} steps that had the smallest number of nearest neighbors. ^The { 111 } ^and { 100} ^{facets are} particularly well developed ⁱⁿ figure ^2. All other surface orientations tended to be composed of altemating strips ^{of the two closest} { 111} ^or { 100} planes ; ^{or for those} nearly parallel ^to { 111 } ^or { 100}, they ^{show a} terrace-step structure, reminiscent of ^{the structure} that develops during ^field evaporation ^{on the} tip of a field-ion microscope specimen [5]. ^{In addi-}

tion, ^{where the} foil thickens in some places around the holes, ^{there are} abrupt changes ^{in contrast}

that show that the top and bottom foil surfaces normal to the beam direction also developed ^{a se-}

ries of terraces and steps (Figs. ^{lc, 2b, and 3, indicated} by arrows) ^{that, like the} edges, ^follow (110)

directions. One of the thin areas that eventually ^{became a hole in} figure ^{1 formed at the} original

site of a dislocation loop. ^This may ^indicate preferential ejection ^{of atoms from the} disordered re-

gion ^{at the} intersection of a dislocation core with the exit surface. However, the locations of most holes seem likely ^to have been determined by local variations in the initial thickness of the gold

combined with possible local variations in the thickness of the amorphous carbon contamination

layer. ^{It is} interesting ^{that, even after a hole had} developed ^{in the} gold, ^the contamination layer initially remained continuous. Only ^{later and} only ^for very large ^{holes did} gaps begin ^to appear ⁱⁿ this layer, ^{which can} be assumed to cover both the top and bottom surfaces (Figs. ^{2a, b, and} 3).

The effects of the amorphous ^{carbon layer on} the details of the gold ^{surface structure in these} specimens ^{did not seem to be as} important ^{as has been} suggested by previous ^authors [6-8]. ^How-

ever, there is ^one feature that may ^{indicate a} structural interaction between the gold ^{and the carbon} layers. ^{At several} places along ^the edge ^{surfaces of the} gold, ^{there was a} tendency for the last { 111 } layer ^of gold ^to separate ^{near its end from the} underlying layer ^{to more} than the usual interpla-

nar distance. Effects of this kind have been interpreted ^as "reconstruction" of gold ^surfaces [6-8].

However, ^{in these} pictures ^{it seems} likely ^{that this was anomalous} separation, which occured in

some places ^{but not in others, because} of local variations in the interaction between the gold ^and

the contamination layers. In all the areas where the contamination layer ^{was absent, such} layer- by-layer separation ^{did not occur} (see Figs. ^{2 and} 3). ^In many materials the contamination layers

would be expected ^{to consist not} only ^of hydrocarbons, ^{carbons, or other} contamination deposited

in the microscope, ^{but also} of oxides, sulfides, ^{or other} products ^of reaction with the ambient. This effect may ^{also have} implications ^{for the} interpretation of localized atomic arrangements ^{in lattice} images ⁱⁿ general.

Figure ^{4 shows a} vacancy tetrahedron and a planar stacking ^fault loop features that ^are not

likely ^{to have been} present ^{in this} evaporated ^{gold foil} prior ^{to ion} thinning. Any defect of this

type ^{that can be formed} by ^a small cluster of vacancies ^or interstitial atoms should be suspect Before concluding that the defect existed before thinning it would be necessary ^to prepare ^other

specimens ^without resorting ^{to ion} thinning, ^{and to use an} accelerating voltage ^for observation

that is less than the threshold for Frenkel pair formation, ^{or for surface} sputtering.

3435

Fig. ^{2.- Two} images ^{of the same area} showing growth of holes with increasing length ^of exposure ^{to the} electron beam. Note that the carbonaceous contamination layer ^at first remains continuous over holes but

eventually develops gaps. ^{In some} places ^it appears ^that interaction between the contamination layer ^{and the}

columns of gold ^{atoms near the end} of a { 111} layer ^{at an} edge has caused anomalously ^wide separation ^from

the next underlying 11 ll} layer.

Fig. ^3.- Development ^of planar { 111 } ^and {100} ^{facets is} clearly ^{shown on both concave and convex} edge

surfaces. Where the general orientation of the surface is far from parallel ^{to one of these} planes ^{two différent}

{ 111 } ^or { 111 } ^and { 100} strips ^{alternate to} give ^a zig-zag ^{structure to} the surface. Where the orientation is

close but not exactly parallel ^{to one} of the close packed orientations the surface has a terrace-step ^structure.

3437

Fig. 4.- Defects probably ^caused by ^ion damage, vacancy type stacking fault tetrahedron and stackinc ^fault

half loop ^{on an inclined} plane.

0

GaAs samples.

Another illustration of the dynamic ^{nature of a} specimen under observation in an electron mi- croscope ^is provided by ^{a recent} study ^of stoichiometry ^{in GaAs. Under a metal} contact to GaAs, it has recently been shown by analytical ^electron microscopy [9] ^{that an excess of As} frequently

exists. However, ^{this excess is} extremely ^unstable during observation. The excess As atoms, being

less strongly ^bound migrate away rapidly ^{under the} influence ^{of the electron beam. In order to}

compare ^the concentration that originally existed in a set of spécimens, ^{it was} necessary ^{to make} observations on each set under increasing ^beam exposure time in order to extrapolate ^{to the con-}

centration that would have coriesponded ^{to zero} observation time.

To further investigate the effect of contamination layers ^and specimen-preparation procedure

on the stability ^{of TEM} specimens ^under observation, ^{a set of GaAs} specimens ^were prepared by

three différent methods : (1) crushing, ^where thin foil surfaces were prepared by cleavage ^with only

a short exposure ^to dry ^{air ;} (2) ^chemical thinning, ^which required exposure ^{to a} polishing ^solution

followed by drying, resulting ^{in a} thicker and perhaps ^{less uniform oxide} contamination layer; and,

(3) mechanical thinning ^followed by ^ion milling. The last method was likely ^{to introduce} point

defects associated ^{with ion} damage ^{near the} surfaces, ^{in addition to a thin} oxide contamination

layer.

For these three specimen-preparation techniques, ^those prepared by crushing ^{were the most}

stable under electron-beam illumination, ^even when the accelerating voltage ^{was increased to}

1500 kV, ^{and the electron beam current increased to 1 A cm-2.} These GaAs samples ^could stay

under the beam for 20 min without obvious visible changes. ^The least stable samples ^{were those} prepared by ^ion milling using ^an accelerating voltage ^{above 5 kV and an} ion incidence angle ^above

10° .

Stability ^also depended ^on specimen orientation. Samples ^with { 111 } exposed ^{surfaces were}

most sensitive to electron damage, ^followed by ^{those with} { 110} exposed ^{surfaces. lhe} samples

with {100} ^{surfaces were} comparatively less sensitive. There are suggestions ⁱⁿ the literature [10]

that Ar+ ion milling ^with voltages below 10 keV can produce up ^{to 10 nm of} amorphous ^GaAs.

Samples ^{in this} study prepared by ^ion milling ^{at 5 keV and finished} briefly (- 10 min) ^{at 2 keV were}

relatively ^{stable but the} dependence of electron damage ^on sample orientation remained the same.

In both cases a cold stage ^with nitrogen cooling ^{was used. The behavior of the} chemically prepared samples ^{varied : some} areas were as stable as crushed samples ^{and some were} comparable ^with

ion-milled samples. ^This behavior _suggests that reaction products ^{were not} completely ^removed

from the sample ^{surface and that} they adversely affected resistance to electron-beam damage ⁱⁿ

some areas.

GaAs samples prepared by Ar+ ^ion milling (5 ^keV 10°) ^with (110) parallel ^{to the electron}

beam were observed in the JEOL JEM electron microscope ^{with a 200 kV} accelerating voltage

for of 20 min. No electron-beam damage ^{was observed} (Fig. 5b). ^{A second} sample prepared ⁱⁿ

the same way ^{was then} observed in the Atomic Resolution Microscope ^{at 800 keV for 20 min} (Fig.

5a). ^A monocrystalline-to-amorphous transition was observed in the thinnest areas in this sample.

Amorphization of GaAs has been observed to occur after (Kr+) ^and (art ^ion implantation [11] ^{at low} temperatures (30 K) at 50 keV and with a dose range ^{from 2 x} 1011 to 5 x 1013 ions

cm-2. These amorphous layers recrystallize ^{at room} temperature. ^{It was} observed that increasing

the dose rate from 2 to 70 mA cm" ^{at an} irradiation temperature ^{of 373 K} produced amorphous

zones that were resistant to room-temperature recovery [11].

This increased stability ^{of the} amorphous ^{zones was} apparently correlated with a difference in the structure of the zones.

In the present experiment, ^stable amorphous zones were also formed at increased electron- beam irradiation times. A transition zone between the monocrystalline ^{material and the amor-} phous region ^was highly ^faulted, developing ^a high density of microtwins.

Conclusions.

The results described in this paper illustrate the dynamic ^{nature of some} specimens ^under

the conditions necessary ^for high resolution microscopy. ^Carbon contamination layers ^{which are}

almost always present ^{on electron} microscope ^foils during observation are thought ^to play ^{an im-} portant role. Some local structural features also suggest structural interaction between this layer

and the specimen.

Electron beam induced atomic mobility allowed observation of the development ^of atomically planar {111} ^and {100} ^{facets at the} edges ^{of a} gold ^foiL These observations contradict some

previous interpretations suggesting ^{that similar} features should be observed only after carbon-

layer ^{removal and are not altered} by ^the electron beam [6-8].

Observations of GaAs samples ^{indicated that} specimen-preparation technique ^{has a} strong effect on sample stability in the electron beam. Crushed samples ^{which are} expected ^{to be the}

most free of contamination and ion damage ^{were most resistant to} electron beam changes. ^The apparent sensitivity ^to electron-beam induced structural change ^{was also shown to} depend ^{on the} crystallographic orientation of the thon-fil

The examples ^{descnbed in this} paper ^{are intended to} illustrate the dynamic ^{nature of several}

diffèrent kinds of specimen ^{under the} conditions necessary ^for high-resolution ^electron microscopy imaging ^{and the} difficulties of interpretation ^with respect ^to deciding ^{whether defects or features}

were originally present ⁱⁿ the material or were the result of high-energy ^electron irradiation or ion-

beam thinning. ^These observations suggest that further systematic study ^{of the factors} that affect

3439

Fig. 5.- Electron beam amorphization of GaAs. Disorder first developed ^as heavily faulted material which

eventually ^became completely amorphous, a) edge ^{of a} foil observed for 20 min at 800 key and electron current density ^{of 1 A} cm-2, b) edge of a foil observed for 20 min at 200 keV, and électron current density ^of

2

10-3 Acm-2.

stability ^of transmission electron microscopy specimens prepared ^for high resolution observations would be useful but also that electron beam induced atomic mobility ^{can be} extremely helpful ⁱⁿ

the study of surface structure in some cases.

Acknowledgments.

The authors ^{want to} thank W Swider for TEM sample preparation. ^{Use of} the electron mi- croscopes ^{at the} National Center for Electron Microscopy ^{at Lawrence} Berkeley Laboratory ^is acknowledged. ^{This work was} supported by ^{the Director, Office of} Energy ^Research, Office of Ba- sic Energy Sciences, Materials Science Division of the U. S. Department ^of Energy ^{under Contract}

No DEAC03-76SF00098

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