Optical and electrical properties of SnOx thin films made by reactive r.f. magnetron sputtering

704

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OIrI'ICAI+ A N D E I . i ! C T R I ( ' A L P R O P E R T I E S O F Sn(), T H I N F I L M S MADE BY REACFIVE R.F. M A G N E T R O N SPUTTERIN(~ 1{. SI'JtiRNA AND ('. (;. (iRAN(~VISI Phv+t~ s I)cparlmcJtl. ('ha/mcr.x I ttlt('r,,tll ~17~'t/m,J/o.k'+ +rod ~ tlltt't',itl tq (iotlwnt,utV .S'-412 ~,'tj ( i~*tlu'nhur.~ ' ,%;c
Sn()~ l}lms were made by reactive r.f. magnetron sputtering of tin in Ar O , onto t, nheated glass. At a well-defined O , - t o - A r gas flow ratio, one could obtain an electrical resistivit} o f about 10 2 Q c m . a lunfinous transmittance of about 75". and a deposition rate of about 3 nm s i. The optical and electrical properties could be quantitati+ely understood from a theoretical model for wide-band-gap semiconductors, heavily n-type doped b.v d o u b l y ionized oxygen vacancies, that accot, nted for ionized impurity scattering of tile free electrons.

1. INFR(II)IRII()N AND St

MMARY

We discuss transparenl and electrically conducting thin lihns of nonstoichiometric tin oxide (SnO,). Tile paper reports on preparation b~ reactive sputtering and a quantitative theoretical model for the optical properties. I)oped ~ i d e - b a n d - g a p oxide semiconductors hit',e nunlerous applications in technolog}., and thin tiln1~, o f such tnaterials can provide Iox~ emittance and hence thernlal insulation in architectural x~indov, s ~. extended transparent electrodes in large-area chromogenic devices 2, electrical heating of transparent bodies, passive frost prevention for sk.v-tacing windows, antistatic surfaces, electromagnetic shielding, surface hardness, antirellection, diffusion harriers etc. I:ollowing our earlier ~ o r k on Ine()3:Sn3 and Z n O : A l a ~se now stud>. SnO, made b\ reactive sputtering at reasonably high rates ~. ()ur technology has certain advat:tages o~er tile more conventional methods based on chemical v a p o u r deposition <- in that the SnO., coating can be applied to tetnperature-sensitixe substrates ~ and can be conveniently integrated in multilayer configurations. Section 2 deals with thin lilm deposition using reactive r.f. magnetron sputtering of tin in Ar O 2 and optical properties o f the ensuing SnO~ films studied by spectrophotometry. Optimization of tile sputter parameters is discussed in Section 3, where it is shown that, at a well-defined O2-to-Ar gas tlow ratio, one can have films 0.3 I.tm thick with a resistance per square o f about 1()()~ or less and a ltlminous transmittance of 75",,. Deposition rates o f u p to a b o u t 3 n m s t were found in films with a resistance per square o f about 300Q. Section4 outlines a 0040-6090'90 $3511

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O P T I C A L A N D E L E C T R I C A l . P R O P E R T I E S OF S n O x T H I N FILMS

705

theoretical model for the optical and electrical properties of a wide-band-gap semiconductor that is strongly n-type doped through doubly ionized oxygen vacancies serving as donors. The model, which follows earlier work of ours 3'4, accounts for ionized impurity scattering of the free electrons. In Section 5 we compare theory and experiment for the optical properties and verify the applicability of the theoretical model. 2.

SPUTrl:R DEPOSITION AND OPTICAL PROPERTIES

The SnO~ films studied in this work were made by reactive r.f. magnetron sputtering in a versatile box-type stainless steel coater described in some detail elsewhere '~. The system was evacuated by cryopumping to an ultimate pressure in the low 1 0 - 6 T o r r range. Magnetron sputtering was done from a planar cathode 10cm in diameter powered by a 600W supply, operating at 13.56 MHz, connected via a manual impedance-matching network. Substrates were clamped onto an unheated support table located 5 cm below the target. Sputter gases were supplied via mass-flow-controlled gas inlets, run via a mixing chamber and distributed through a perforated toroidal tube surrounding the target. The target material was 99.999'!o pure tin. The sputter gases were 99.997",~, pure argon and 99.998!~, pure 02. The substrates were 2.5 cm × 2.5 cm × 0.1 cm Corning 7059 glass plates. Each of the samples was produced with constant sputter parameters. Specifically, the r.f. power delivered to the cathode was in the 10 W ~< Pr.r. ~< 220 W range, the sputter gas pressure was in the 2 m T o r r ~< p ~< 6 mTorr range, and the 0 2 to-Ar gas flow ratio was in the 41~, ~< F ~< 35°0 range. The film thickness t, determined by surface profilometry, was kept at 0.3 +0.06 p.m. Spectral optical properties were measured on double-beam spectrophotometers. The normal transmittance T was recorded in the 0.3p.m<)~<2.5p.m wavelength range by use o f a Perkin-Elmer Lambda 9 instrument. The reflectance R at 0.35 p.m < 2 < 2.5 p.m was recorded on a Beckman Acta MVII instrument with integrating sphere using a BaSO4-coated surface as reflectance standard. For the thermal IR (2.5 p.m < 2 < 50 p.m), a Perkin-Elmer 580 B instrument was used to record near-normal reflectance with an evaporated gold film as standard. The spectral absorptance A was evaluated from A(,~) = I - T(;.)- R(;.)

(l)

Figure I shows T(2), R(2) and A(2) for two films chosen so that they illustrate characteristic features. One of the films (upper part) was produced under nearoptimum conditions. Its T(2) is large for 0.4 p . m < £ < 2 p . m with maxima up to about 87"4, R(,;.) oscillates between about 10!',oand about 20~o for 2 < 2 p.m: at longer wavelengths, R(2) increases and approaches 59'~, at 2 = 50 p,m. A(2) has a U-shaped characteristic with a magnitude as low as a few per cent at 2 ~ 0.8 p.m. The other film (lower part of Fig. 1) was produced under conditions that deviated significantly from ideal: now A(2) was significantly enhanced at 2 < 0 . 8 p.m and the increase in R(2) at long wavelengths was much less apparent. For both films, the oscillations in T(2) and R(2) at 2 < 2 p.m originate from optical interference. The increase in A(2) at the shortest wavelengths is influenced by absorption in the glass substrate. This

706

B. S I J I ! R N A A N D ( . (;. (;RANQVIST

latter effect also accounts for the minima in R(2) at ,;. ~>6 IJm, particularly for o f f optimum sputtering parameters. The only difference in the preparative conditions for the two films in Fig. I is a 4". change in the O2-to-Ar gas flow ratio, and it is evident that accurate process control is imperative if optimized optical and electrical properties arc to bc achieved.

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3. ( ) I)F I M I Z A ' F I O N OF SPUTTER PARAMETERS

The goal was to make SnOx films with low electrical resistance and high optical transmittance. Hence we investigated the resistance REq per square as well as the luminous transmittance 7-].m and absorptance Ajum as a function of sputter parameters. RtS_I was obtained by pressing a resistance probe against the tilm surface. The integrated optical properties were derived from spectral data according

707

OPTICAL AND ELECTRICAL PROPERTIES OF S n O x THIN FILMS

to

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Id;. v(),) where V(2) is the luminous efficiency for photopic vision '° (qL the shaded region in Fig. 1). Figure 2 shows T~um,A~,m and R R cs. F for two values of Pr.r. and with almost identical sputter gas pressures. It appears that the resistance has a minimum value RU3* at a rather well-defined magnitude F* of the gas flow ratio. From measurements at several r.f. powers it was found that both R;q* and F* increase with increasing P,.f.. An approximate relation can be written as RE]* = 9 0 + 0 . 9 Pr.f.

(4)

with RE3* in ohms and Pr.~. in watts. REI increases sharply on either side o f F * . T~,m increases monotonically, and A~,m decreases monotonically, with increasing F. At o/ minimum resistivity we observed T,,m = (75 + i.5)'~ and A~,m = (9 + 2.5),~, with a tendency that a low P,.f. gave a high T~,m and a small A~,m- At F < F * the films appeared yellowish, thus indicating the presence of non-conducting SnO" at F > F* the films were highly transparent and we expect the stoichiometry to approach SnOz. These indications concerning the stoichiometry were substantiated by M6ssbauer and Hall effect data that will be discussed elsewhere.

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708

B. S'FJERNA AND ( . (;. (.;RANOVISI

The optical and electrical properties of the SnO., tilms were somewhat dependent on post-treatment. The role of aging is illustrated in the upper parts of Fig. 2. where the broken curves refer to as-deposited tih'ns and the full curves refer to films stored in the laboratory for about I year. At low P , , (10W. sa~). aging consistently yielded a decrease m R I :: ill intermediate I',.,. (60W, say), aging lowered R M only I b r / ' > I'* but left R,._2 unchanged for I < I'*: at high P r , (I10 data shown), aging consistently yielded an increase in R.

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(5)

with r in nanometres per second and P,.,. in ~.atts gives a good representation of the data. The highest observed rate. with/',, t .-. 220 W. is 2.85 nm s

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The model for the electromagnetic properties of SnO, fl~llows our earlier work 3"'* and in fact strongly supports the usefulness of the theoretical framework originally put forward for ln20.~:Sn. The frequency-dependent complex dielectric function +:(<,>)is obtained as a sum of three additive contributions due to free carriers (labelled by' a superscript FC)-..which are electrons in the case of SnO., valence electrons and phonons. Well away from the semiconductor band gap (in the UV)

OPTICAL AND ELECTRICAL PROPERTIES OF

SnOx

709

THIN FILMS

and phonon resonances (in the mid-thermal range), one can write e(0)) = 8~ + zFC(0))

(6)

with 8~ being the high-frequency dielectric constant. The free-electron contribution is conveniently discussed in terms of a dynamic resistivity P(0)) = P1(0)) + ip2(0))

(7)

which is related to the free-electron susceptibility by the general relation xvc(0)) = i / c o o ) p ( 0 ) )

(8)

with eo being the permittivity of free space. Quantitative expressions for p(0)) can be obtained from the equivalence of Joule heat and energy loss ~~. For the case of electrons scattered against Coulomb-like ion potentials, and taking the semiconductor to be non-polar, one obtains ~2 •

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nce2

(10)

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with e being the electronic charge and me* the effective conduction band mass. We note that eqn. 9 does not account for displacement polarization of the atoms and is not easily generalized so as to incorporate this effect ~3. A more complete--and computationally much more demanding--theory for p(0)) can be derived from the K ubo formalism and diagrammatic pertubation expansion TMt4. 5. COMPARISON OF THEORY AND EXPERIMENT FOR THE OPTICAL PROPERTIES

Quantitative data for the optical properties were computed under the assumption that doubly ionized oxygen vacancies in the SnO 2 lattice serve as donors ~5. Thus we put

(11)

z=2 and n e

Ni = S

(12)

in eqn. (9). 8cg was computed for a degenerate electron gas using the random phase approximation16 as extended by Hubbard17 to include exchange effects. Explicit formulae for 8es are given in ref. 3. The high frequency dielectric constant was set to7.15

~:~ = 4

(13)

II. S I J E R N A A N D ( . (i. t ; R A N Q V I S T

710

and the effective conduction band mass #l*#c ~

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whh #no being the free-electron mass. The value in eqn. 14 which was found empirically to give the best tit to the electrical d.c. resistivity is only inarginally {2.5",) lower than the polaron effective mass determined from electrical measurements ts. Once ~:(,J) was fully specified, the transmittance and reflectance were computed flom Fresnel's formuhie ~<~for a thin tihn on a substrate. The substrate was represented by the dieleclric function for Corning 7059 glass tit ,;. < 2.5 him (ref. 20) and for SiO_, at ,:. > 2.5 I-un (ref. 9). Figure 4 shows a comparison between theory and experiment for the optical properties. The broken curves reproduce T(),) and R().) for the SnO, films 0.344 nm thick earlier reported in the upper part of Fig. I. The electron density, determined by our Hall effect measurements, was 1.2 x 10e°cm ~. The full curves were obtained by computation using the earlier mentioned values of t, n~.. ~:, and m * . The overall agreement between theory and experiment is vet 5 good, which attests to the applicability of the theory ~ outlined above. The It|el that the experimental transmittance drops increasingly below the theoretical prediction as one tippreaches the shortest wavelengths is due to absorption in the glass substrate as well as to some residual absorption perhaps due to SnO-like inclusions that are not accounted for by the theory. The lack of detailed agreement for the phonon-induced absorption features in the thermal I R is not unexpected since the glass composition deviates from SiO2.

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[-'ig. 4. S p e c t r a l trllllSllliltance "I' lind r c l l e c l a n c e R for a S n O , lihn v,.'tlh the shov, t+ t h i c k n e s s t, e l e c t r o n d e n s i t y n<+a n d effecti,..e c o n d u c t i o n b ~ l l l d 111ab,:4#H *. . theor3 : . e x p e r m m n t a l c u r v e s shov. n earlier in the u p p e r p a r t o f F i g I.

OPTICAL AND ELECTRICAL PROPERTIES OF S n O x THIN FILMS

711

The theory used here to explain the optical properties of a wide-band-gap semiconductor that is strongly n-type doped through doubly ionized oxygen vacancies, was in earlier work 3"4 successfully applied to substitutionally n-type doped In203 and ZnO with singly ionized impurities. In the latter cases, one has Z --- 1 and Ni = ne. Thus it appears that a consistent theory can be used to model the optical and electrical properties of several types of heavily doped semiconductors and to assess their usefulness for example in energy-efficient windows. ACKNOWLEDGMENTS

This work was financially supported by grants from the Swedish Natural Science Research Council and the National Swedish Board for Technical Development. REFERENCES 1 C . G . Granqvist, Spectrally Selective Surfaces jor Heating and Cooling Appfications, Society of Photo-optical Instrumentation Engineers, Bellingham, WA, 1989. 2 C.M. Lampert and C. G. Granqvist (eds.), Large-area Chromogenics." Materials and Devices for Transmittance Control, Society of Photo-optical Instrumentation Engineers, Bellingham, WA, 1990. 3 I. HambcrgandC. G. Granqvist, J. Appl. Phys.,60(1986) RI23.

4 Z.-C. Jin, I. HambergandC. G. Granqvist, J. AppI. Phys.,64(1988) 5117. 5 B. StjernaandCGGranqvist, SoI. EnergyMater.,20(1990)225. 6 J.C. Manifacier, L. Szepessy, J. F. Bresse and M. Perotin, Mater. Res. Bull., 14 (1979) 163. 7 H. Haitjema, J. J. P. Elich and C. J. Hoogendoorn, Sol. Energy Mater., 18 (1989) 283. 8 B. Stjerna and C. G. Granqvist, Appl. Opt., 29 (1990) 447. 9 T.S. Eriksson and C. G. Granqvist, J. Appl. Phys., 60 (1986) 208 I. I0 G. Wyszecki and W. S. Stiles, Color Science, Wiley, New York, 1982, 2nd edn., p. 256. 11 M.G. Calkin and P. J. Nicholson, Rev. Mod. Phys., 39 (1967) 361. 12 E. Gerlach, J. Phys. C, 19(1986)4585. 13 B. E Sernelius and M. Morling, Thin Solid Films, 177 (1989) 69. 14 B.E. Sernelius, Phys. Rev. B, 36 (1987) 1080. 15 Z.M. Jarzebsky and J. P. Marton, J. Electrochem. Soc., 123 (1976) 199c, 299c, 333c. 16 J. Lindbard, K. Dan. Vidensk. Selsk., Mat.-Fys. Medd., 28(8) (1954). 17 J. Hubbard, Proc. R. Soc. London, Ser. A, 243 (1957) 336. 18 C.G. Fonstad and R. H. Rediker, J. Appl. Phys., 42 (1971) 291 I. 19 M. Born and E. Wolf, Principles of Optics, Pergamon, Oxford, 1983, 6th edn. 20 Information, Coming Glass Works, Corning, NY.