Journalof CrystalGrowth 79 (1986)403—409 403North-Holland,Amsterdam

STRUCTUREAND CRYSTALLIZATION PROCESSOF A THIN FILM PREPAREDBY VACUUM EVAPORATION OF SnO2 POWDER

Chihiro KAITO andYoshio SAITO

Departmentof Physics,Kyoro Instituteof Technology,Matsugasaki,Sakyo-kuKyoto 606, Japan

An amorphoustin oxidefilm 10 nm thick hasbeenpreparedby vacuumevaporationof Sn02 powderfrom atungstenboatonto acleaved surfaceof rock salt. Analysesof electrondiffraction patternsand higb resolution electronmicroscope(HREM) imagesshowedthat thefilm was composedof microcrystallitesof Sn03,2.5 nm in size.An electronbeamof theorderof 0.1 A/mm

2 wasusedto irradiate thefilm, causingthemicro-crystallitesto changeto therutilestructureof Sn0

2with acrystallitesizeof afew tensofnm. On furtherirradiationwith an intenseelectronbeam,SnOcrystalsabout10 nm in sizeappeared.A furtherincreasein radiationenergyresultedin largerclystallitescontainingcrystalshear(CS) structuresof two typesin reducedSnOcrystals.Thegrowthprocessof Sn02,SnO andCS structureof SnOarediscussedon thebasisof the preferedorientationof formed crystals.

1. Infroduction crystallization of a vacuum depositedtin oxidefilm [8].

We have carried out high resolution electronmicroscopic(HREM) studiesof some oxide films[1—4)which gaveamorphous haloesin electron 2. Expenmentaldiffraction, andhaveshownthat the microcrystal-lite structural model is useful in describingthe An amorphoustin oxide film 10 nm thick wasstructureof some so-calledamorphousmaterials, preparedby vacuumevaporationof Sn02 powderThe HREM imagestakenunderoptimum focus froma tungsten boatonto a cleaved surfaceofconditionhave identifiedthe structureof the mi- rock salt. After deposition, the film was wet-crocrystallites. stripped from the substrateand mounted on a

A new (100) CSplane and oxygen vacancy carboncoated,perforatedplastic film supportedwalls along (100) planesin W03 crystals, formed by a standardcopper electron microscopegrid.by the crystallization of amorphousthin films, The film thicknesswas measured usinga quartzhave beenshown byusingHREM imageswith the crystal oscillating microbalance.Electron micro-aid of computer simulation [5]. We have also scopic observationof the specimenfilm was car-found a new (110) CSplane[6], a (110)crossedCS nedout usinga JEM-200CXelectronmicroscope.planeand partial CS plane [7] preparedby the HREM imagesof CSstructuresweresimulatedbycrystallizationof amorphousTi02 films. Our re- themultislice methodusing FACOMM-382 com-suits showedthat the newCS structurescould be puterat thedataprocessingcenter, KyotoUniver-determinedby the agreementbetweenthe calcu- sity.latedimagesandexperimentalimages.

In the presentpaper,the structureof vacuumdepositedtin oxide films is revealedby HREM, 3. Resultsand discussionand a new CSstructureresulting from the crys-tallization of a thin tin oxide film by electron 3.1. HREMstudiesofthe as-depositedfilmbeamirradiationis reported.In a previouspaper,a new (110) CSstructurein SnO crystals was Fig. 1 showsan HREM optimum focusimagereported,which was producedby electron beam obtainedfrom a tin oxidefilm of thickness10 nm,

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404 C. Kaito, Y. Suito / Thinfilm prepared byvacuumevaporationof SnO,powder

4nm

Fig. 1. HREM imageandcorrespondingelectrondiffraction patternof theas-depositedfilm. Crossedfringes in therangeof 2.5 nmareseen.

and the correspondingelectron diffraction pat- functionwas calculated repeatedlyup to r = 0.6tern. Since thediffractionpatterngives halorings, nm until ghostsin the rangeof r < r0 disappeared,the vacuum deposited film has the so-called where r0 is the shortestinteratomic distancein theamorphousstructure.As indicatedby the circle substance.The curve was linear with r in thefor example,lattice fringescanbe observedwithin range r < 0.1 nm, giving the densityof the film.grains about 2.5 nm in size. We previously ob- The densityobtainedfrom the curve with r < 0.1servedsimilarcrossedfringesin amorphous metal nm was 4.16g/cm

3, which agreedwell with theoxide[1,2] andGe[9] films. The appearanceof the measureddensityof 3.78 g/cm3 for amorphouscrossedfringes in HREM images by axial ii- tin oxide films preparedby vacuumevaporationlumination gives sufficient evidence for the of 5n0

2 powder [8]. Since crystals of Sn02 aremicro-crystallitemodel, aselucidated in some re- known to have the rutile structure(tetragonalcentpapers[1—4,9,10]. a = 0.4764 nm,c = 0.319 mn),peakpositionscor-

The radial distribution function obtained by respondingto thedistanceSn—O, Sn—Snand0—0the analysis of theseelectron diffraction (ED) in the rutile structureare shownin fig. 2 by longhaloesis shownin fig. 2. The radial distribution bars. Since they are not coincidentwith those of

C. Kaito, Y. Saito / Thin film prepared by vacuum evaporation of SnO, powder 405

Fig. 2. Radial distribution function attained by the electron diffraction haloes of the as-deposited film. The film was identified as

having the ReO, type structure.

the experimental peak positions, the as-deposited film is not a rutile structure. The short bars in fig. 2 show the bonding distances of Sn-Sn, Sn-0 and O-O for ReO, type structure which is com- posed of comer-sharing anion octahedral with ca- tions at the centers. The octahedron in the present film is derived from the rutile structure of the [SnO,] octahedral arrangement. Since the peak positions correspond to the distance indicated by an ReO, structure, the film is assigned a ReO, type structure.

As elucidated in HREM observations of amorphous SiO films [l] and WO, films [2], if the picture is taken at the optimum defocus, crystal structures of the order of 1 nm is in size can be identified. Fig. 3 shows an enlarged part of the electron micrograph taken at the optimum de- focus. Since the octahedron derived from the rutile structure are composed of tetragonal bipyramids, two typical types of comer sharing structure are derived as shown in the projection images in fig. 3b. The crossed fringes in fig. 3a agree well with

those seen in fig. 3b. It can be concluded that the as-deposited film is composed of microcrystallites of SnO, structure.

b

t

0.2 b

Fig. 3. An enlarged HREM image of the as-deposited film and corresponding projection model constructed from the ReO, structure. The fringe spacings and angles agree well with the

model.

406 C. Kaito, Y. Saito/ Thinfilm prepared by vacuum evaporationofSn02 powder

If the amorphousfilms are assumedto havea nanometersas shownin fig. 4. The Sn02crystalsstructurein which spherical micro-crystallites,with showed a preferred orientation with their [111]the sameradii, are closelypacked, thenthe poros- axes parallelto the incident beam.Furthergrowthity, which meansthe ratio of the vacantvolume of Sn02 crystals was not observedafter moreenclosedby the spheresto the totalvolume of the intense electron beam irradiation. Instead, SnOfilm, is 26%.Since thedensityof bulk Sn02 is 6.24 crystalsaboutof 10 nm in sizeappeared(tetrago-g/cm

3,the observeddensitydeficit for the present nat a = 0.3802nm, c = 0.4836 nm) as showninfilm is 33%. This discrepancymaybe attributedto fig. 5. Thesealso showed apreferredorientationthe fact that the film was Sn0

3 with a corner with their [111]axesparallelto theelectronbeam.sharing structure. Since theSn03 structurewas A further increasein irradiationenergyresultedinnot given the natureof the chemicalbondingin a larger crystalof about 100 nm at 1 A/mm

2Sn0

3, some oxygenvacanciesmay be includedin irradiation. Thelarger crystals includedthe twothe Sn03 microcrystallites.Thereforethe density typesof CSstructure,i.e. the one anordered(110)deficit becomesgreater than for films of W03, CS structureas reportedin a previouspaper [8]SiO, andGe. andthe othera (011)CS structure.

Figs. 6a—6c show the HREM images and fig.3.2. Crystallizationprocess induced by intenseelec- 6d shows the corresponding diffractionpatternoftron beamradiation anda new CSstructure

As anelectronbeamof theorderof 0.1 A/mm2 _______ _________________

irradiated the as-depositedfilm, the microcrystal- Q.,j3 4 nmlites describedabove changed to have a rutile ______________________________________

structurewith crystallite sizes of a few tens of ________

__ ~Ø3~_____ _____ ~1~ ________ _________

Fig. 5. HREM imageshowing theappearanceof SnOcrystalsinducedby intenseelectronbeamirradiation.TheSn0

2crystalsFig. 4. HREM image correspondingto the Sn02 crystals which appearedat the initial stageof the irradiation are alsoinducedby electronbeamirradiationof theas-depositedfilm, seen.

C. Kaito, Y. Saito/ Thinfilmprepared by vacuum evaporationof SnO.powder 407

O6nm

p. i’l I,

f 45nm b 6Onm~ ~ ~ 75hm•• ‘ IS. 1,1 ‘IS ~HI~

Fig. 6. Throughfocusimages,simulatedimagesandcorrespondingelectrondiffraction patternof (011)CS structurederivedfromtheSnOcrystalstructure.

a SnO crystal including the (011) CSstructure. layersof the tetragonalpyramidswithin the SnOSince the (110)and(011) spotsof the SnOcrystals crystal. The pyramids around the extractedre-are clearly seen, theorientationof the film is with gions collapseto form the CS structure.The Snthe [1111 axesparallel to the incident electron atomsmove in the ~ [1001direction as schemati-beam.In fig. 6d, extraspotswith a period2 times cally shownin fig.7. The projectionof this struc-larger than thatof the (011)reflectionare clearly ture along the [1111direction is shown in fig. 7b.seen. Furthermore, the dark band-like contrast The projection images show that the Sn atomcorrespondingto the super lattice spots is seen population is increasedalong the defect planes,periodicallyin figs. 6a—6c. spacedevery 0.6 nm. The projection of perfect

As shown schematicallyin fig. 7a, the SnO SnOcrystal alongthe [111]direction is just thestructureis describedas an array of tetragonal overlappingof Sn atomsat upperandlowerapexespyramidswith Sn atomsat their apexesandwith [8]. In the presentmodel, themovementof the Snfour oxygen atoms at the bottom corners. The atomsalong the [100]direction producesthe sep-tetragonalpyramids alternately standupside down arationof the Snatomsat the defectplanein thealong the <111) axes. Since thereduction of the projectionimage.Calculatedimagesbasedon thisfilm took placeby electronbeamirradiation [5,7], model are shown asinserts in fig. 6. Because ofit is reasonableto supposethat an oxygen layer the good agreementwith experimentalimages itlocatedin a (011)planeis removedfrom every 2 can be concludedthat the crystal includes the

408 C. Kaito, Y. Saito / Thinfilmprepared by vacuum evaporationof SnO,powder

a 4. General discussionb~

Films preparedby vacuumdepositionof SnO

a powderwerecomposedof microcrystallitesof SnOwith a crystallite size of 2 nm. Electron beamcrystallizationsof the film introducedSnOcrystalsof a fewnm in size having [001]preferredorienta-tion. By further crystallizationunder the intense

D E electronbeam,the film changedto Sn and SnO.Crystals containing CS structuressuch as (011)

and (110) could not be seen.Thereforeit can be

in SnO crystals is concernedwith the preferred— - — orientationof the film, i.e. the (111)orientation.

0 ~ concludedthat the appearanceof the CS structureFilms depositedby evaporationof Sn02powder

were composedof Sn03 microcrystallites. Sincethe structureof the Sn03 films is not a stablestate, the film changesto Sn02 under electron0,60 nmbeamirradiation. The film of Sn02 becameSnOby further electronbeamirradiationin a reducingatmosphere.Since the lattice matching between

b *1 1* oSn Sn02 andSnOis so good, as iselucidatedby the

00 topotacticgrowth of the Sn02 oxides formed on0. 2 7 nm SnOfine particles[11], the transformationof the

Fig. 7. Schematicalrepresentationof SnO crystaland(011) CS (lii) Sn02 film to the (lii) SnO film easelystructure, occursby reductionof oxygen layers.

Projectionsof the structureof Sn02along thecaxis andof SnOalongthe a axis are shownin fig.

(011) CS plane. The generalorderedCS structure 8. The configurationof cationsis body centeredinbecomesSn2~02~_1.Since theobservedstructure both structures.SnO crystal consistsof stackedcorrespondsto n = 2, the composition of the layersof Sn—0—Sn— —Sn—0—Sn— —Sn— along thecrystalshownin fig. 6 is determinedasSn403. [001] direction and the Sn02 crystal consistsof

Sn0~~—.-.SnOSne

0 p

pSn

Sn-0 p

bo 0 0pf Sn

Fig. 8. Representationof crystallographicrelationshipbetweenSn02 andSnOcrystals.

C. Kaito, Y. Saito/ Thinfilm prepared by vacuumevaporationofSnO,powder 409

stackedlayersof Sn—O—Sn—O—Sn—O—along the increasein radiation energy resulted in larger[010] direction as shown in fig. 8. The distances crystallites containingthe crystal shear structurebetween the cations is the same. On theother of the two typesin a reduced SnOcrystal, i.e.hand, if the oxygen layersparallel to the (010) Sfl

2nO2n_i and~~m+i0m [8].

planeare eliminated fromthe perfectSnO2crystal

every two oxygen layers, i.e. Sn—O—Sn——Sn—O—Sn— —Sn—O—Sn—, the structurebecomes ReferencesSnO. The elimination of oxygen layers corre-spondsto the formationof oxygen vacancywalls

[1] C. Kaito and T. Shimizu, Japan.J. Appl. Phys.23 (1984)as found in the electron beam crystallizationof L7.

W03 films [5]. The strain energydue to the [2] C. Kaito, T. Shimizu, Y. NakataandY. Saito, Japan.J.

eliminationof oxygenlayersmaybe treatedas the Appl. Phys.24 (1985) 117.

activation energy of thegrowth of the CS struc- [3] C. Kaito, Y. Nakata, Y. Saito and K. Fujita, Appi.SurfaceSci. 22/23(1985) 621.ture.

[4] Y. Saito, C. Kaito, K. Nishio andT. Naiki, AppI. Surface

Sci. 22/23 (1985) 613.[5] T. Miyano, M. Iwanishi, C. Kaito and M. Shiojin, Japan.

5. Conclusion J. Appl. Phys.22 (1983) 863.[6] T. Miyano, M. Iwanishi, T. Harada,C. Kaito and M.

Films preparedby vacuumdepositionof Sn02 Shiojiri, Phil. Mag. A48(1983) 163.[7] C. Kaito, M. Iwanishi, T. Harada, T. Miyano and M.powder were composed of microcrystallites of Shiojiri, Trans. Japan.Inst. Metals24 (1983)450.

Sn03 with a crystallite size of 2.5 nm. Electron [8] C. Kaito, T. Harada andT. Miyano, Japan,J. Appl. Phys.

beamcrystallizationof the film introducedSn02 7 (1983) L394.

crystalsof a fewnm in sizehaving [111] preferred [9] Y. Saito, J. Phys.Soc. Japan51 (1984)4230.

orientation. By more crystallizationunder the [10] C. Kaito, N. Nakaniura, T. Yoshidaand M. Shiojiri, J.Crystal Growth66 (1984) 156.electron beam, the Sn02 crystallites change to [11] C. Kaito and M. Shiojiri, Japan.J. Appl. Phys.21 (1982)

SnOhaving [111]preferredorientation.A further 1404.