Indian Journal of Engineering & Materials SciencesVol. 3, June 1990, pp. \09-113

Oxidation mechanism involved in thin tin films

Johny T Abraham, P V Thomas, K G Gopchandran, Benny Joseph & V K Vaidyan*Department of Physics, University of Kerala, Trivandrum, 095 5RI, India

Received 13 October 1995; revised R February 1990

Tin in thin film form reaches its highest oxidation state SnO~ either through direct or indirectphase transformation. The direct or indirect transformaion depends on whether SnO is in the alphaor beta phase, respectively. When Sn02 was reactively deposited, SnO~ films formed through a di-rect transformation exhibited high conductivity and optical transmission than those formed throughan indirect phase transformation. It has been observed that a direct transformation helps to reducethe substrate temperature required for the reactive deposition of Sn02•

Stannic oxide is an important material in electri-cally conducting transparent coatings and electri-cal resistors I..•• More recently, attention has beendirected' to its potential use as gas sensors'!".Several preparation techniques are available fordepositing thin films of SnO~. Although many re-ports":" have appeared on the preparation. ofSnO::, films, very little is known about the oxida-tion mechanism involved in the phase transforma-tion of Sn toSnO::,. Therefore, an attempt hasbeen made to study the oxidation process in-volved in thin films of tin.

Experimental ProcedureThin films of tin (- 100 nm thick) were depo-

sited under a vacuum of about 10 - ~ torr by resis-tive heating of tin using tantalum boats. The sub-strate used were glass and alumina. As depositedtin films were annealed at 573, 673, 773 and 873K in a muffle furnace for 90 min in an oxygen at-mosphere. A closed steel chamber with an inletand outlet holes was used to create oxygen atmos-phere. The diameter for the outlet hole was keptvery small. The samples to be annealed were keptinside the chamber and the whole setup wasplaced inside the muffle furnace, with the inlet ofthe steel chamber connected to an oxygen cylin-der through a flow meter. The X-ray diffractionstudies were conducted using a Philips PW1701diffractometer and the scanning electron micro-scope studies were carried out using a lEOL 35Celectron microscope.

Results and DiscussionXRD studies-The X-ray diffraction of as-depo-

•Author to whom correspondence sh9uld be addressed

sited and annealed tin films at 573, 673, 773 and873 K were computed and presented in Tahle I.The data on films deposited on alumina sub-strates excluding the values corresponding to alu-mina is shown in the table. It is found that SnO,films formed on alumina are better textured thanthose formed on glass. Hence, XRD data on alu-mina alone is presented. These data reveals thatthe as-deposited films contain only the beta phase

Table I-XRD data and identification of phases in as-evaporat-ed and annealed thin films

Annealingtemperature, K

d,nm III", % Identificationphase, hkl

0.2910 52 13-Sn(200)0.2794 10 I3-Sn (101)0.20\7 2 I3-Sn (211)

0.2976 3 a-SnO( 1(1)0.2909 6 I3-Sn (200)O.27R6 4 I3-SnO (004 )0.2012 2 I3-SnO ( - - - )

0.3336 I Sn02 (110)0.2976 I a-SnO (101)0.2907 3 I3-Sn (200)0.2786 2 I3-SnO (004 )0.2712 3 SnPJ(200)0.2642 7 Sn02 (101)0.200R 2 Sn20., (131)

0.3348 3 Sn02 (110)0.2909 I f\-Sn(200)0.2786 I j-i-SnO(OO4)0.2712 I Sn;OJ (200)0.2642 14 Sn02 (101)0.1'763 2 Sn02(211))

0.3336 4 Sn02 (110)0.2642 12 s-o, (101)0.1763 3 Sn02(211)

as-evaporated

573

673

773

873

110 INDIAN J. ENG. MATER. SCI., JUNE 1996

S.o-;l,0\1

snOt\\lOI

(.)

5<'~\1011

sn~\1'OI

..I (d)

11Q\~, rSi'" ~S~l #~ .(.ri> :<;<t-' ~'I>1~.l ~# ym

Ie)~'o.<I>' ~D~'<I'c#'?~v(

(b)

1<1>'v#'\do'

f/"~

t (0)

~ W ~28, degr ••

Fig. I-XRD scan of tin films (a) as evaporated, annealed at(b) 573 K, (e) 673 K, (d) 773 K and (e) R73 K

of Sn. When these films were annealed, lower oxi-dation phases of tin along with Sn02 were foundpresent, with Sn02 phases increasing in intensitywith the increase in annealing temperature. At873 K, a complete phase transformation from Snto Sn02 has been observed as only the peakscorresponding to Sn02 are found to exist. All thed-values of the identified phases matched with theASTM data (21-1250) corresponding to pureSnOz and exhibited a tetragonal structure. Whenthe oxidation process of tin, involved in the phasetransformation process of Sn to Sn02 was ana-lyzed, a certain oxidation mechanism could beformulated C''] the basis of d-values of the variousidentified phases as,

, 573K 673K~-Sn •. j3-Sn + a-SnO + j3-SnO •j3-Sn + a-SnO + j3-SnO + Sn203 +

773KSn02 873 K" j3-Sn + j3-SnO + Sn203 +Sn02 ~ Sn02

It may be inferred from the oxidation equationthat a direct phase transformation from Sn toSn02 can occur when SnO is in the alpha phase,while an indirect phase transformation occursthrough intermediate phase like Sn203 or Sn304,

when SnO was in the beta phase. The directphase transformation may be explained as due tothe similarity of tin matrix in alpha-SnO and Sn02

structures. These both have a tetragonal structurewith lattice parameters ao = 0.3802 and Co =0.4836 nm and ao = 0.4738 and Co = 0.3188

Table 2-XRlJ data and phase iuentification of reactively dep-osited tin oxide films deposited at a rate of 0.3 nm/s

Substrate d, run 1110,% Identificationtemperature, K phase, hkI

0.3338 59 Sn02 (110)823 0.2644 100 Sn02 (101)

0.2373 77 Sn02 (200)0.1761 65 Sn02(211)

7730.2979 54 a-SnO(10l)0.2902 100 I3-SnO (101)0.2779 42 Sn3Od1l2)0.2714 9 Sn20j (200)0.2636 4 Sn02 (101)0.2404 8 a-SnO(002)0.2053 14 a-SnO(102)0.2010. 20 SnpJ(131)0.1793 9 a-SnO(112)

amorphousamorphous

573373

Table 3-XRD data and phase identification of reactively depo-sited tin oxide films deposited at a rate of 0.1 nm/s

Substrate d,nm III", % Identificationtemperature, K phase, hkl

0.3361 70 s-o, (110)773 0.2995 26 c-SnO (101)

0.2685 59 a-SnO(llO)0.2649 100 Sn02 (101)0.1895 15 a-SnO(200)0.1761 44 Sn02 (211)0.1671 12 Sn02 (220)

0.3360 100 Sn02 (110)723 0.2648 99 Sn02 (101)

0.1759 85 Sn02 (211)

0.3360 100 S002 (110)698 0.2635 60 S002 (101)

0.1761 48 Sn02 (211)

673 amorphous573 amorphous

nm, respectively. Therefore, a very little deforma-tion needs to takes place with incorporation ofadditional oxygen atoms to achieve the final Sn02state. In the case of beta-SnO, it has an orthor-hombic structure with lattice parametersao= 0.5000 and Co = 1.120 nm.

A pictorial representation of the oxidationtransformation, i.e., from betll-Sn{200] toSn02[110] and beta-Snl l Ol ] to Sn02[101] is shownin Fig. 1. From the strong beta Sn peaks, interme-diate states like alpha-SnO, beta SnO and Sn203rise in intensity with the decrease in intensity ofbeta-Sn and the intensities of these intermediatestates decrease leading to the emergence of theirrespective Sn02 phases.

ABRAHAM {'/III.:0XlDATION MECHANISM IN TIN THIN FII.MS 111

vFig.2-SEM micrographs of tin thin films (a) & (b) as-evaporated, annealed at (e) 573 K, (d) 673 K, (e) 773 K

and (f) 873 K

The existence of these intermediate oxidationstates denotes that the oxidation starts with thedisplacement of the internal 0xygen with the in-corporation of external oxygen, because of thetwo possible valence states of Sn, + 2 and + 4.

Though polycrystalline Sn02 films were ob-tained by post deposition annealing of tin films,the Sn02 films exhibited high resistivity and lowoptical transmission in visible region. But, Sn02

films deposited by reactive evaporation of tin was

found to exhibit better electrical and optical pro-perties. The reactive deposition method and theinfluence of deposition parameters on the forma-tion of pure Sn02 films were discussed elsewhere u. To observe the phase transformationsduring the reactive evaporation of tin, Sn02 wasreactively deposited at a rate of 0.3 and 0.1 nm/sand the corresponding XRD data are presented inTables 2 and 3, respectively. It can be seen thatSn02 films were formed at a lower substrate tern-

112 INDIAN J. ENG. MATER. set. JUNE 1<)%

perature of 723 K when films were deposited at arate of n.l nm/s, while a higher substrate temper-ature of 823 K was required when films weredeposited at a rate of 0.3 nm/s. On lowering thesubstrate temperature, pure Sn02 was only foundto exist in the former case (Table 3), while tin anditx lower oxide phases were found to coexist withthe Sn02 phase in the latter case (Table 2). Whenthe substrate temperature was increased to 773 Kin the former case, the alpha Sn02 phases werealso found to exist along with the Sn02. This maybe attributed to the possible reduction of Sn02

occurred at higher substrate temperature. So, itwas presumed that Sn02 films which was react-ively deposited at a rate of 0.1 nrn/s must haveformed by the direct phase transformation of Snatoms to Sn02 molecules through alpha-phase.The Sn02 films deposited at a rate of 0.3 nm/smust have formed by the direct/indirect transfor-mation as the lower oxide phases of tin like beta-SnO and Sn201 was found present along withSn·O,.

During investigation of electrical and opticalproperties, the reactively deposited Sn02 films,formed by a direct phase. transformation wasfound to exhibit better electrical and optical pro-perties than that formed by an indirect phasetransformation. The SnO, films in the latter caseexhibited an electrical - resistivity of - 10 - 1

ohrn-rn with a maximum optical transmission of- 82% in the visible region while Sn02 formedthrough direct phase transformation exhibited anelectrical resistivity of - 10- 2 ohrn-rn with anoptical transmission of - 93%. This variation inphysical properties of reactively deposited Sn02

films can be attributed to the stoichiometric devi-ation of the films formed. The improvement ofphysical properties in oxide films was foundlargely due to the high electron concentration re-sulting froin deviation from stoichiometry anddoping effects 14. The stoichiometric deviation maybe due to anion vacancies or excess interstitial at-oms. Though the exact nature of the defect struc-ture in tin oxide is still uncertain, oxygen vacancieswere found to dominant because interstitial tin at-oms can easily diffuse to the film surface and getoxidized+'-". Therefore, Sn02 films formedthrough a direct phase transformation must havebeen non-stoichiometric in nature leading to bet-ter electrical and optical properties,

The reason for directly phase transformedSn02 films to form at lower substrate temperaturecan be explained at the following lines, The simi-larity of tin matrix in the Sn, alpha-SnO and Sn02

structure must have facilitated the chemisorptionof oxygen into the Sn lattice thereby lowering itsactivation energy for the formation of Sn02 andconsequently a lower substrate temperature wasfavoured.

SEM studies= The surface morphology of theas-deposited and annealed thin films on aluminasubstrates are shown in Fig. 2. Polycrystallinegrains of tin can be seen distributed over thebackground of polycrystalline alumina substratesurface in the case of as-deposited films (Figs 2a& h). As annealing temperature increases, thesmooth texture of tin film disappears and an al-most uniform islands are formed over the substr-ate (Figs 2c & d). The islands increase in size bythe coalescence of nearby islands as the tempera-ture further' increases to finally achieve the Sn02

structure at 873 K (Figs 2e & f). In the case of al-umina substrates, the cavities between the grainboundaries of alumina also gets filled with tinduring its evaporation. On annealing during theisland formation, nucleation is centered aroundthe materials filled in the cavities. This results in abetter textured 5n02 films due to the orientedovergrowth on the substrate surface.

ConclusionsThin films of tin oxide obtained by the post-

deposition annealing of thin tin films was found tobe polycrystalline and tetragonal in structure. Theoxidation mechanism observed in this transforma-tion of Sn to Sn02 films suggested that the phasetransformation can occur either through a singleand/or two oxidation states. The electrical andoptical properties of the reactively depositedSn02 formed through a direct phase transforma-tion was found to be better than those filmsformed through an indirect phase transformation.Moreover, a direct phase transformation of Sn toSnO, needs only a lower substrate temperature inthe reactive deposition of tin oxide.

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