Transparent conducting oxide semiconductors for transparent electrodes

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INSTITUTE OF PHYSICS PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY

Semicond. Sci. Technol. 20 (2005) S35–S44 doi:10.1088/0268-1242/20/4/004

Transparent conducting oxidesemiconductors for transparent electrodesTadatsugu Minami

Optoelectronic Device System R&D Center, Kanazawa Institute of Technology,7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan

Received 6 October 2004Published 15 March 2005Online at stacks.iop.org/SST/20/S35

AbstractThe present status and prospects for further development of polycrystallineor amorphous transparent conducting oxide (TCO) semiconductors used forpractical thin-film transparent electrode applications are presented in thispaper. The important TCO semiconductors are impurity-doped ZnO, In2O3and SnO2 as well as multicomponent oxides consisting of combinations ofZnO, In2O3 and SnO2, including some ternary compounds existing in theirsystems. Development of these and other TCO semiconductors is importantbecause the expanding need for transparent electrodes for optoelectronicdevice applications is jeopardizing the availability of indium-tin-oxide(ITO), whose main constituent, indium, is a very expensive and scarcematerial. Al- and Ga-doped ZnO (AZO and GZO) semiconductors arepromising as alternatives to ITO for thin-film transparent electrodeapplications. In particular, AZO thin films, with a low resistivity of the orderof 10−5 cm and source materials that are inexpensive and non-toxic, arethe best candidates. However, further development of the depositiontechniques, such as magnetron sputtering or vacuum arc plasma evaporation,as well as of the targets is required to enable the preparation of AZO andGZO films on large area substrates with a high deposition rate.

1. Introduction

For most optoelectronic devices such as flat panel displays, itis essential to use a transparent electrode consisting of a thinfilm of a transparent conducting oxide (TCO) semiconductor.Although tin-doped indium oxide (commonly called indium-tin-oxide, or ITO) thin films deposited by magnetron sputtering(MSP) have been in practical use for most transparentelectrode applications, there are many reports on other TCOsemiconductors as well as deposition methods [1–7]. A stablesupply of ITO may be difficult to achieve for the recentlyexpanding market for optoelectronic devices because of thecost and scarcity of indium, the principal material of ITO.In addition, recent developments in optoelectronic deviceshave frequently required thin-film transparent electrodes withspecialized properties. Recent research concerning thin-filmtransparent electrodes using TCO semiconductors has focusedon resolving these problems. For example, we have proposedthe use of impurity-doped zinc oxide (ZnO) as an alternativeto ITO [4, 8, 9] as well as the use of multicomponentoxide thin films with properties suitable for specializedapplications [4, 10]. In addition, film deposition techniques

suitable for fabricating these TCO semiconductors are beingdeveloped.

This paper introduces the present status and prospectsfor further development of polycrystalline or amorphousTCO thin films for transparent electrode applications. Inparticular, approaches for resolving the above problemsand the relationship between TCO semiconductors and filmdeposition techniques are presented in detail.

2. TCO semiconductors for thin-film transparentelectrodes

In general, TCO thin films that are in practical use astransparent electrodes are polycrystalline or amorphous,except for single crystals grown epitaxially, and exhibita resistivity of the order of 10−3 cm or less andan average transmittance above 80% in the visible range.Thus, TCO semiconductors suitable for use as thin-filmtransparent electrodes should have a carrier concentrationof the order of 1020 cm−3 or higher and a band-gap energyabove approximately 3 eV: i.e., degenerated n-type or p-type

0268-1242/05/040035+10$30.00 © 2005 IOP Publishing Ltd Printed in the UK S35

T Minami

Table 1. TCO semiconductors for thin-film transparent electrodes.

Material Dopant or compound

SnO2 Sb, F, As, Nb, TaIn2O3 Sn, Ge, Mo, F, Ti, Zr, Hf, Nb, Ta, W, TeZnO Al, Ga, B, In, Y, Sc, F, V, Si, Ge, Ti, Zr, HfCdO In, SnZnO–SnO2 Zn2SnO4, ZnSnO3

ZnO–In2O3 Zn2In2O5, Zn3In2O6

In2O3–SnO2 In4Sn3O12

CdO–SnO2 Cd2SnO4, CdSnO3

CdO–In2O3 CdIn2O4

MgIn2O4

GaInO3, (Ga, In)2O3 Sn, GeCdSb2O6 YZnO–In2O3–SnO2 Zn2In2O5−In4Sn3O12

CdO–In2O3–SnO2 CdIn2O4−Cd2SnO4

ZnO–CdO–In2O3–SnO2

semiconductors. Historically, most research to develop TCOthin films as transparent electrodes has been conducted usingn-type semiconductors; in practice, TCO thin films used astransparent electrodes are n-type semiconductors consisting ofmetal oxides. On the other hand, the preparation of TCO thinfilms consisting of p-type semiconductors was first reported in1993 [11]. Since this report of p-type NiO thin films depositedby rf magnetron sputtering (rfMSP), there have been manyreports on the preparation of p-type semiconducting thin filmsusing new TCO semiconductors [12–15]. Nevertheless, therehas been no report on the preparation of a p-type TCO thinfilm suitable for use as a practical transparent electrode.

For the purpose of obtaining lower resistivities, variousTCO semiconductor materials have been developed; n-typeTCO semiconductors now available for thin-film transparentelectrodes are listed in table 1, grouped by compound type[4, 10]. One advantage of using binary compounds as TCOmaterials is the relative ease of controlling the chemicalcomposition in film depositions compared to using ternarycompounds and multicomponent oxides. Up to now, variousTCO thin films consisting of binary compounds such as SnO2,In2O3, ZnO and CdO have been developed, with impurity-doped SnO2 (SnO2:Sb and SnO2:F), impurity-doped In2O3

(In2O3:Sn, or ITO) and impurity-doped ZnO (ZnO:Al andZnO:Ga) films in practical use. In addition, it is well knownthat highly transparent and conducting thin films can alsobe prepared using metal oxides without intentional impuritydoping [1–3]. The resulting films are n-type degeneratedsemiconductors with free electron concentrations of the orderof 1020 cm−3 provided by native donors such as oxygenvacancies and/or interstitial metal atoms. However, sinceundoped oxide films were found to be unstable when used at ahigh temperature, binary compounds without impurity dopinghave proved unusable as practical transparent electrodes[9, 16]. The reported effective dopants are also listed intable 1 along with their associated binary compounds.

In addition to binary compounds, ternary compoundssuch as Cd2SnO4, CdSnO3, CdIn2O4, Zn2SnO4, MgIn2O4,CdSb2O6 and In4Sn3O12 have been developed [1–3, 17–20].However, there are few reports on the effect of impurity dopingdue to the lack of an effective dopant. As a result, TCOfilms fabricated from these ternary compounds have yet to

Figure 1. Practical TCO semiconductors for thin-film transparentelectrodes.

be used widely. In order to obtain TCO films suitable forspecialized applications, new TCO semiconductor materialshave been actively studied in recent years. In the 1990s, newTCO semiconductor materials consisting of multicomponentoxides, such as combinations of binary compound TCOs, weredeveloped [21, 22]. In addition, new TCO semiconductormaterials consisting of multicomponent oxides composed ofcombinations of ternary compound TCOs have been developed[23]. The use of multicomponent oxide materials makespossible the design of TCO films suitable for specializedapplications because the electrical, optical, chemical andphysical properties can be controlled by altering the chemicalcomposition [4, 10].

Table 1 shows that TCO semiconductors for thin-filmtransparent electrodes have been developed only with metaloxides containing at least one of the following metal elements:Zn, Cd, In and Sn. Although producing a low resistivitythin film, it should be noted that Cd-containing TCOsemiconductors such as In-doped CdO (CdO:In), Cd2SnO4,CdSnO3 and CdIn2O4 [1–3, 5, 7, 19] are of no practicaluse because of the toxicity of Cd. For thin-film transparentelectrodes, TCO semiconductors such as impurity-dopedZnO, In2O3 and SnO2 and multicomponent oxides composedof combinations of these binary compounds are the bestcandidates for practical use, as shown in figure 1.

3. Challenge to obtain lower resistivity

ITO thin films prepared by MSP with a resistivity of theorder of 1 × 10−4 cm are commercially available atpresent. Although the obtainable electrical properties arestrongly dependent on the deposition method as well as thedeposition conditions, TCO films with a low resistivity of theorder of 10−5 cm can only be prepared with impurity-dopedbinary compounds. Figure 2 shows the change in minimumresistivity of impurity-doped binary compound TCO filmsreported in recent years. The impurity-doped ZnO, In2O3 andSnO2 films were prepared on glass substrates under variousdeposition conditions using various deposition methods. Itshould be noted that the obtained minimum resistivities ofimpurity-doped ZnO films is still decreasing, whereas thoseof impurity-doped SnO2 and In2O3 films have essentiallyremained unchanged for more than the past twenty years. Asalso seen in figure 2, a resistivity of the order of 10−5 cm

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Transparent conducting oxide semiconductors for transparent electrodes

Figure 2. Reported resistivity of impurity-doped binary compoundTCO films, 1972–present: impurity-doped SnO2 (), In2O3 () andZnO (•).

Table 2. Electrical properties of ITO films with a resistivity of theorder of 10−5 cm.

Carrier DepositionResistivity concentration Hall mobility method (year)( cm) (cm−3) (cm2 (V s)−1) [Reference]

7.4 × 10−5 ∼1.38 × 1021 103 Zone confining(1996) [24]

8.9 × 10−5 1.3 × 1021 54.1 Pulsed laserdeposition(2000) [25]

7.2 × 10−5 2.5 × 1021 33.2 Pulsed laserdeposition(2001) [26]

9.5 × 10−5 1.8 × 1021 40 Spraypyrolysis(2002) [27]

8.45 × 10−5 1.38 × 1021 53.5 Pulsed laserdeposition(2002) [28]

has been recently reported in ITO and ZnO:Al (AZO) thinfilms. The obtained electrical properties (resistivity, carrierconcentration and Hall mobility) and the deposition methodused for the past ten years to produce ITO thin films with aresistivity of the order of 10−5 cm are summarized in table 2;the best results seem to result from using depositions at a hightemperature and/or a pulsed laser deposition (PLD) method.In addition, Suzuki et al have recently reported AZO thin filmsprepared by PLD with a resistivity of the order of 10−5 cm[29]: the first report of impurity-doped ZnO thin films withthis level of resistivity, the same as that found for ITO.

It should be noted that the obtained minimum resistivitiesin ITO and AZO films prepared by PLD were lowerthan those resulting from MSP, the conventional depositionmethod in practical use. As an example, table 3 showsa comparison of the obtained electrical properties betweenlow-resistivity AZO thin films prepared by PLD and MSP.The difference of obtained resistivity was ascribed to thatof Hall mobility rather than carrier concentration: PLD Hallmobility, about 40 cm2 (V s)−1 [29, 30] and MSP Hall mobility,20–30 cm2 (V s)−1 [9, 31]. It is well known that the Hallmobility of AZO films with a carrier concentration of the order

Ionized impurity scattering(B–H–D theory)

CARRIER CONCENTRATION n (cm–3)

HA

LL

MO

BIL

ITY

(cm

2/V

s)µ

103

102

101

100

1019 1020 1021 1022

Figure 3. The µ–n relationship of reported AZO films: broken lineshows the µ–n relationship calculated using the BHD theory.

Table 3. Electrical properties in low-resistivity AZO films preparedby PLD or MSP.

Carrier DepositionResistivity concentration Hall mobility method (year)( cm) (cm−3) (cm2 (V s)−1) [Reference]

1.3 × 10−4 1.31 × 1021 36.7 Pulsed laserdeposition(1999) [29]

8.5 × 10−5 1.5 × 1021 47.6 Pulsed laserdeposition(2003) [30]

1.9 × 10−4 1.5 × 1021 22.0 rf magnetronsputter(1984) [9]

2.7 × 10−4 9.0 × 1020 25.0 dc magnetronsputter(1990) [31]

of 1020–1021 cm−3 is dependent on the carrier concentrationbecause Hall mobility is mainly dominated by ionized impurityscattering [32]. Figure 3 shows the relationship betweenthe obtained Hall mobility (µ) and carrier concentration (n)for AZO films with a carrier concentration of the order of1020–1021 cm−3 and a resistivity (ρ) of 5 × 10−4 to 8.5 ×10−5 cm prepared using various deposition methods such asPLD and MSP; also shown is the µ–n relationship calculatedusing the Brooks–Herring–Dingle (BHD) theory by takinginto account degeneracy and assuming that ionized impurityscattering dominates the mobility [32]. As can be seen infigure 3, the measured mobility in AZO films with a carrierconcentration of the order of 1020–1021 cm−3 was lower thanthat calculated by the BHD theory. It has been reported that theresults of the experimental examination of the µ–n relationshipcan be explained by modified ionized impurity scattering usingthe BHD theory by taking into account degeneracy and thenon-parabolicity of the conduction band [32].

However, as can be seen in figure 3 at a carrierconcentration around 1 × 1021 cm−3, the data measured forHall mobility were scattered in the range from approximately10 to 50 cm2 (V s)−1. With the carrier concentration

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FILM THICKNESS (nm)

HA

LL

MO

BIL

ITY

(

cm2 /

Vs)

CA

RR

IER

CO

NC

EN

TR

AT

ION

n (

cm-3

)

µ

RE

SIS

TIV

ITY

(

Ω c

m)

ρ10–1

10–2

10–3

10–41019

1020

1021

1022

100

101

102

n

µ

ρ

200 600 1000 1400

Figure 4. Resistivity (), carrier concentration () andHall mobility () as functions of film thickness for AZO filmsprepared at 200 C by PLD.

approximately constant, the variation of Hall mobility cannotbe solely explained by ionized impurity scattering. Thissuggests that the mobility of AZO films is dominated by notonly ionized impurity scattering but also another scatteringmechanism. In addition, we have found the Hall mobilityof AZO films prepared with various film thicknesses by PLDexhibiting the same µ–n relationship as shown in figure 3.As an example, the thickness dependences of electricalproperties (ρ, n and µ) are shown in figure 4 for AZO filmsprepared with an Al content (Al/(Al+Zn) atomic ratio) of1.5 at% on glass substrates at 200 C [33]. The Hall mobilityincreased from approximately 20 to 50 cm2 (V s)−1 as thethickness was increased, whereas the carrier concentration ofapproximately 1 × 1021 cm−3 was relatively independent of thethickness. The prepared AZO films were polycrystalline andpredominantly c-axis oriented perpendicular to the substratesurface, as evidenced by x-ray diffraction analyses. It wasalso found that the thickness dependence of Hall mobility isclosely related to that of crystallinity estimated from the fullwidth at half maximum (FWHM) of the (0002) diffractionpeak. Figure 5 shows Hall mobility and crystallite sizeas functions of film thickness for the AZO films shown infigure 4. It should be noted that Hall mobility was estimatedby the van der Pauw method using the carrier transport in thedirection parallel to the film surface whereas the crystallitesize was estimated in the direction perpendicular to the filmsurface. However, it is usually believed that the crystallite sizeestimated in the direction perpendicular to the film surfacemay be correlated with the crystallite size in the directionparallel to the film surface, as evidenced from the observationof surface morphology by an atomic force microscope (AFM).The variation in Hall mobility shown in figures 3 and 5 suggeststhat the mobility of polycrystalline AZO thin films with acarrier concentration of the order of 1020–1021 cm−3 is notsolely dominated by ionized impurity scattering but is alsoaffected by crystallinity.

CR

YS

TA

LL

ITE

SIZ

E D

(n

m)

HA

LL

MO

BIL

ITY

(

cm2 /V

s)µ

FILM THICKNESS (nm)200

50

40

30

50

40

30

20

10

0600 1000 1400

Figure 5. Hall mobility () and crystallite size () as functions offilm thickness for AZO films prepared at 200 C by PLD.

In addition to the above, it has also been reported thatHall mobility in polycrystalline AZO and undoped ZnO filmsare affected by grain boundary scattering [32]; this scatteringis characterized by an increase in Hall mobility causedby increases in carrier concentration up to approximately1021 cm−3, resulting from increases of the potential barrierheight as well as the trapping of free electrons due to oxygenadsorbed on grain boundaries and the film surface. In contrast,grain boundary scattering may be unimportant for AZO filmswith a carrier concentration of the order of 1021 cm−3 becauseof the lowering of potential barrier height resulting from thecarrier screening effect. Nevertheless, the increase of meanfree path from approximately 2 to 10 nm, corresponding to theincrease of mobility from 10 to 50 cm2 (V s)−1 in AZO filmswith a carrier concentration of 1 × 1021 cm−3, may be assignedto the improvement of crystallinity, which is related to carrierscattering mechanisms such as grain boundary scattering anddislocation scattering [33]. However, details concerningcrystal imperfections such as point defects and dislocationsthat dominate carrier scattering as well as the relationshipbetween imperfections and scattering mechanisms have yet tobe investigated sufficiently.

From the results described above, the higher obtainedmobility in AZO films prepared by PLD than in filmsprepared by MSP may be explained by differences in thin-film crystallinity. In addition, it is clear that obtaining aresistivity of the order of 10−5 cm in AZO films with acarrier concentration of the order of 1 × 1021 cm−3 requiresan improvement of crystallinity. On the other hand, this wayof obtaining AZO thin films with a resistivity of the order of10−5 cm may also be effective for the preparation of ITOthin films.

In addition to the investigation of Hall mobility describedabove, it should be noted that the preparation of thin films witha carrier concentration of the order of 1021 cm−3 is essential toobtain a resistivity of the order of 10−5 cm. It should be notedthat the preparation of impurity-doped ZnO thin films with acarrier concentration of the order of 1021 cm−3 is much moredifficult than with ITO films. Since the oxygen adsorption on

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Transparent conducting oxide semiconductors for transparent electrodes

the surface and grain boundaries of the films is affected bythe oxidizing atmosphere during deposition, controlling theoxidizing atmosphere during the deposition is very importantfor the preparation of highly transparent and conductingAZO thin films with a carrier concentration of the order of1021 cm−3 [34]; a weaker oxidizing atmosphere is requiredthan with the deposition of other TCO materials such as ITO[4]. It should also be noted that an oxygen vacancy in theZnO lattice usually acts as a donor that generates a singlefree electron, whereas the one in the In2O3 lattice acts asa donor that generates doubly free electrons [3]. Becauseof the differences in the chemical and physical propertiesbetween ITO and AZO described above, ITO thin filmscan even be prepared by various deposition methods onhigh temperature substrates with a resistivity of the order of10−5 cm. However, in order to prepare AZO filmswith a resistivity of the order of 10−5 cm using adeposition method other than PLD, it is necessary to developdeposition techniques that provide a controlled weak oxidizingatmosphere during the deposition.

4. TCO films as alternatives to ITO

Recently, the demand for ITO thin-film transparent electrodeshas dramatically increased in the field of optoelectronicdevices. If the increase in usage of ITO films for flatpanel displays and solar cells continues, not only will theprice of ITO continue to rise but also the availability of Inmay be jeopardized in the near future. The development ofalternative TCO materials is necessary to resolve this seriousproblem. For obtaining a resistivity of the order of 10−5 cm,impurity-doped binary compounds such as Al-doped ZnO(AZO) and In-doped CdO have been proposed as alternativematerials. With consideration for the environment, it appearsthat impurity-doped ZnO, particularly AZO, is a promisingalternative because of its use of ZnO or Zn, both beinginexpensive, abundant and non-toxic materials, and its abilityto exhibit resistivity comparable to ITO, as described above.In the following, the present status and prospects for furtherdevelopment of impurity-doped ZnO films are surveyed. Inaddition, various problems and solutions associated with usingimpurity-doped ZnO as a practical alternative to ITO aredescribed.

In the 1980s, transparent conducting ZnO films witha resistivity of the order of 10−4 cm were prepared byimpurity doping [9, 35, 36]. However, a low resistivity belowapproximately 2 × 10−4 cm has only been obtained in AZOand GZO films, as shown in figure 6; table 4 summarizes theminimum resistivity and the maximum carrier concentrationobtained for typical impurity-doped ZnO films prepared withoptimal doping content for various dopants and depositionmethods.

In general, the obtainable electrical properties of impurity-doped ZnO films are strongly dependent on the depositionmethods and conditions. Impurity-doped ZnO films with aresistivity of the order of 10−4 cm have been preparedby vacuum arc plasma evaporation (VAPE), metal organicmolecular beam deposition (MOMBD) [37] and metal organicchemical vapour deposition (MOCVD) [6] as well as by MSP[9, 10] and PLD [29]. Recently, AZO and GZO films with a

Figure 6. Reported resistivity of impurity-doped ZnO films,1983–present: AZO (), GZO () and other impurity-dopedZnO (•).

Table 4. Resistivitiy, carrier concentration and dopant content fortypical ZnO films doped with various impurities.

Dopant Carriercontent Resistivity × 10−4 concentration × 1020

Dopant (wt%) ( cm) (cm−3)

Al2O3 1–2 0.85 15.0Ga2O3 2–7 1.2 14.5B2O3 2 2.0 5.4Sc2O3 2 3.1 6.7SiO2 6 4.8 8.8V2O5 0.5–3 5.0 4.9F 0.5 (at%) 4.0 5.0None 0 4.5 2.0

resistivity of the order of 1 × 10−4 cm were prepared usingPLD and VAPE [38]. Although VAPE using an Uramoto gunas the arc plasma source [39], similar to activated reactiveevaporation (ARE), has attracted much attention as a newdeposition technique with a high deposition rate on large areasubstrates, vacuum evaporation of AZO is difficult to achievebecause the vapour pressure of Al2O3 is too low in comparisonwith that of ZnO. AZO films with a resistivity of the order of10−5 cm have been prepared by PLD, but preparing films onlarge substrates with a high deposition rate is also very difficultto achieve.

When preparing highly conductive and transparentimpurity-doped ZnO films, controlling the oxidation of Znis much more difficult than with other binary compounds suchas SnO2 and In2O3, because Zn is more chemically active inan oxidizing atmosphere than either Sn or In. Because ofthis binding energy of Zn and O, the activity and amount ofoxygen must be precisely controlled during the deposition.As a result, ZnO films with low resistivity are attainableonly by depositions in atmospheres that are less oxidizingthan in depositions of In2O3 and SnO2 films. In general, thepreparation of ITO and impurity-doped SnO2 films by MSPusing oxide targets usually requires an introduction of O2 gasduring the sputter deposition in order to obtain a low resistivity

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T Minami

[3]. In contrast, the introduction of O2 gas is unnecessary forthe sputter deposition of AZO and GZO films. As a result, theobtainable resistivities in AZO and GZO films are considerablyaffected by the oxygen content in the sintered oxide pellets inPLD and VAPE and in the targets in MSP film preparations.For example, the oxygen content in the AZO target is veryimportant in the preparation of lower resistivity AZO filmsby MSP, because the oxidization occurring on the surface ofsubstrates is mainly dominated by the substrate temperatureand the activity and quantity of the oxygen generated fromthe oxide target reaching the substrate surface [37]. At thesame substrate temperatures, controlling oxidization duringVAPE impurity-doped ZnO film depositions is easier than withMSP, because there is less oxidation occurring on the substratesurface.

The preparation of AZO films on large area substrates bydc MSP (dcMSP) using an oxide target is particularly difficultbecause a spatial distribution of resistivity corresponding to thetarget erosion area pattern is created on the substrate surface[40]. This problem is mainly attributed to the distributionpattern of the activity and quantity of oxygen reaching thesubstrate surface, which depends on the sputter depositionconditions as well as the target preparation conditions. Forthe purpose of improving this distribution pattern, varioussputtering techniques have been developed. For example,lower resistivities in AZO films prepared by MSP have beenachieved by placing substrates perpendicularly to the target[8, 9] and by conducting depositions under an applied magneticfield [8, 31], but the problem has not yet been resolvedcompletely. Therefore, further development of depositiontechniques that can produce more spatially uniform lowresistivity films on large area substrates is required to overcomethe problems associated with MSP.

At present, resistivities of 2–3 × 10−4 cm, a refractiveindex of approximately 2.0 and an average transmittance above85% in the visible range can be obtained in AZO and GZOfilms prepared on large area substrates with a high depositionrate at a temperature above approximately 200 C by MSPand VAPE. The transparent conducting AZO and GZO filmshave characteristics that can be either an advantage or adisadvantage depending on the application. As an example,there are several problems at present with the use of these filmsas the transparent electrode of flat panel displays, replacingITO. For example, good patterns can be etched in amorphousITO films deposited on low temperature substrates by dopingwith Zn [41] or using a H2O [42] atmosphere. In contrast, it isdifficult to apply the presently used photolithography processwith wet treatments, because doped ZnO films are more easilyetched by both acid and alkaline solutions than ITO films. Ofcourse, in other applications their high etching rates might beadvantageous. In addition, the etching rate of doped ZnO filmsis considerably affected by not only the crystallinity and thekind of dopant used but also the deposition conditions. As anexample, figure 7 shows etching rates in HCl (0.2 mol l−1 at20 C) and KOH (3 mol l−1 at 20 C) solutions as functionsof Co content for Co-codoped AZO (AZO:Co) films preparedat 200 C by MSP [43]; the etching rate of the undoped ZnO(ZO) film in the HCl solution decreased markedly with theaddition of Al doping. The etching rate for AZO films inboth HCl and KOH solutions was further decreased with Co

CoO CONTENT (wt.%)

ZO AZO 1 20

5

10

RE(H)

RE(OH)

15

20

ET

CH

ING

RA

TE

RE (

nm

/s)

RE

SIS

TIV

ITY

(

Ω c

m)

ρ

10–1

10–2

10–3

10–4

AZO:Co

Etchant Temp. 20˚C

0.2 mol./1 HCL3 mol./1 KOH

(Al2O3:2wt.%)

RE(H)

RE(OH)

Figure 7. Resistivity (•) and etching rates in HCl () and HOH ()solutions as functions of Co content for AZO:Co films.

Figure 8. Resistivity (•) and etching rate () in HCl solution forundoped and impurity-doped ZnO films: FGO, GZO and GZO:F.

codoping. As another example, the dopant dependence ofetching rate in a HCl solution (0.2 mol l−1 at 20 C) is shown infigure 8 for various impurity-doped ZnO films prepared at100 C by VAPE [44]; the etching rate of both F- and Ga-codoped ZnO (FZO and GZO) films was also lower thanthat of the undoped ZnO (ZO) film. The decreased etchingrate of F-codoped GZO (GZO:F) film was also accompaniedby a decreased obtained resistivity. In addition, we haveproposed that using low temperature etchants is very effectivefor reducing the etching rate. On the other hand, an all dry

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Transparent conducting oxide semiconductors for transparent electrodes

Table 5. Comparison of thin-film transparent electrode propertiesbetween AZO and ITO.

Doped ZnO ITO

Low resistivity ( cm) 10−5 10−5

Practical resistivity ( cm) 2–3 × 10−4 1 × 10−4

Eg (eV) 3.3 3.7Index of refraction 2 2Work function 4.6 4.8–5.0Cost Inexpensive Very

expensiveStability

Acid solution <Good (stable)Alkali solution <Good (stable)Oxidizing atmosphere at high <Good (stable)temperature (or oxygen plasma)Reducing atmosphere at high Good (stable)>temperature (or hydrogen plasma)

process using an oxygen ashing process can be applied to AZOfilms [45].

It should be noted that for applications involvingreducing atmospheres at high temperatures and hydrogenplasma, doped ZnO films are more stable than ITO films[46]. In contrast, for use in oxidizing atmospheres athigh temperatures, ITO films are more stable [37]. Asanother specialized application example, AZO films can beadvantageously used with thin-film solar cells, because milkyZnO films with a textured surface structure are more easilyproduced on relatively low temperature substrates than milkySnO2 films [47, 48]. The use of milky AZO transparentelectrodes for solar cells can produce an improvement inefficiency resulting from a light confinement effect. Severalresearchers have reported the preparation of milky AZO,GZO and B-doped ZnO (BZO) films using various depositionmethods such as MSP and atmospheric pressure or lowpressure MOCVD and CVD [4, 49].

Using the comparison of AZO and ITO thin-filmtransparent electrode characteristics summarized in table 5,AZO and GZO films are shown to have thin-film transparentelectrode properties comparable to those of ITO films. GZOfilms prepared by VAPE and AZO films by MSP willbecome widely used in optoelectronic device applications asalternatives to ITO films. However, further development ofMSP deposition techniques is necessary in order to obtain amore uniform spatial distribution, a less oxidizing atmosphereand AZO targets with a lower oxygen content that wouldenable the preparation of lower resistivity AZO thin films.

5. TCO films suitable for specialized applications

Recent developments in optoelectronic device applicationshave frequently required improvements in the physicaland chemical properties of TCO films used as thin-filmtransparent electrodes. In order to develop TCO filmssuitable for specialized applications, previously proposedmaterial development using multicomponent oxides hasrecently been attracting much attention as a source of newTCO semiconductors [10]. Minami et al reported new TCOsemiconductors in 1994, ZnO–SnO2 multicomponent oxides,that not only had the advantages of ZnO but also those of

Figure 9. Resistivity as a function of In content for ZnO–In2O3

films prepared at RT by dcMSP (•) and at 200 C by VAPE ().

SnO2 [10, 49]. In addition, TCO films using multicomponentoxides composed of combinations of binary compoundTCO materials and/or ternary compound TCO materialswere developed [10]. Several examples of multicomponentoxides composed of combinations of binary compound TCOmaterials, ZnO–In2O3, In2O3–SnO2 and SnO2–ZnO systems,are shown in figure 1.

Figure 9 shows resistivity (ρ) and carrier concentration(n) as functions of chemical composition (In content, orIn/(In+Zn) atomic ratio) for multicomponent oxide ZnO–In2O3 thin films prepared on glass substrates at roomtemperature (RT) by dcMSP using an oxide powder target[50] and at 200 C by VAPE using a sintered oxide target[44]. With dcMSP, the surface temperature of substrates atRT rose up to about 180 C during the sputter deposition.The In content in the deposited films was approximately equalto that existing in the targets used, as measured by energydispersive x-ray (EDX) spectroscopy analyses. It shouldbe noted that transparent multicomponent oxide conductingfilms could be prepared in all possible composition ratios ofIn2O3 and ZnO. The minimum resistivity, corresponding to themaximum carrier concentration, was obtained by altering theIn content. The carriers were generated from native donorssuch as oxygen vacancies. ZnO–In2O3 films prepared atRT with an In content of about 75.5 and 90 at% by dcMSPand VAPE, respectively, exhibited a resistivity as low as 3 ×10−4 cm, comparable to ITO films. In addition, all depositedZnO–In2O3 films with thicknesses below 400 nm exhibited anaverage transmittance above 80% in the visible range.

Figure 10 shows etching rate (RE) and band-gap energy(Eg) as functions of In content for the films prepared by dcMSP[50]: RE measured using 0.2 M HCl solution at 25 C. The RE

of ZnO–In2O3 films increased as the In content was decreasedfrom about 80 at%; films prepared with In contents in the rangefrom about 80 to 100 at% were not etched. The minimumband-gap energy of dcMSP deposited films, 2.9 eV, occurredat an In content around 80 at%. As evidenced from x-raydiffraction analyses, ZnO–In2O3 films with In contents in the

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T Minami

Figure 10. Etching rate () and band-gap energy (♦) as functionsof In content for ZnO–In2O3 films prepared at RT by dcMSP.

range from about 40 to 80 at% were amorphous. Regardingthe composition dependence of the electrical, optical andchemical properties of other multicomponent oxides, resultssimilar to those described above for ZnO–In2O3 films were alsoobtained in In2O3–SnO2 and SnO2–ZnO systems; transparentconducting thin films were prepared in multicomponentoxides composed of all composition ratios of ZnO and SnO2

or of SnO2 and In2O3 [4, 10]. In addition, the chemicalstability and etching rate changed as the composition wasaltered in In2O3–SnO2 and ZnO–SnO2 films prepared by MSPand VAPE.

As mentioned above, dcMSP deposited ZnO–In2O3 filmswith In contents around 80 at% exhibited minimums inband-gap energy as well as in resistivity. It is well knownthat many compounds such as ZnmIn2O3+m (m = 2–7) existin the ZnO–In2O3 system. In order to investigate theircrystallographical properties, ZnO–In2O3 films were preparedon high temperature substrates with various In contents. Forexample, dcMSP deposited ZnO–In2O3 films prepared at350 C with In contents in the range from about 40 to 90 at%were identified as a ternary compound, Zn2In2O5, a new TCOsemiconductor, as evidenced from x-ray diffraction analyses[50]. The Zn2In2O5 thin films have a band-gap energy of2.9 eV and a refractive index of approximately 2.1 to 2.4 in thevisible range, higher than the refractive index of about 2.0 seenin conventional TCO thin films such as ZnO, ITO and SnO2.For the purpose of obtaining lower resistivity, an impurity suchas Sn was doped into the Zn2In2O5 films; however, a significantdecrease of resistivity could not be attained.

Minami et al have reported that SnO2–ZnO films preparedby MSP and VAPE with Zn contents (Zn/(Zn+Sn) atomicratio) of approximately 50 at% could be identified as a ternarycompound, ZnSnO3, another new TCO semiconductor [49].In addition, Minami et al have also reported that In2O3–SnO2 films with Sn contents (Sn/(In+Sn) atomic ratio) inthe range from 40 to 60 at% could be identified as a ternarycompound, In4Sn3O12, another new TCO semiconductor [20].A resistivity of 2 × 10−4 cm, comparable to ITO films,was obtained in In4Sn3O12 films prepared with a Sn content of

Figure 11. Resistivity (•) and etching rate () as functions ofZn2In2O5 content for In4Sn3O12–Zn2In2O5 films prepared at RT byrfMSP.

50 at% on substrates at 350 C. The In4Sn3O12 films were verystable in acid solutions and in an oxidizing atmosphere at hightemperatures. Thus, In4Sn3O12 films, which are lower in costthan ITO films with their higher In content, are very promisingas TCO films.

In addition, it has been reported that TCO semiconductorscould also be obtained using a ternary compound such as(Ga, In)2O3 in the In2O3–Ga2O3 system or MgIn2O4 in theMgO–In2O3 system, composed of a TCO semiconductor andan insulator. Un’no et al have reported that transparentconducting MgIn2O4 films were prepared by rf sputtering[18]; Phillips et al have reported that transparent conductingundoped and impurity-doped GaInO3 thin films prepared byPLD and reactive sputtering exhibited resistivities of the orderof 10−3 cm, very low optical absorption coefficients in thevisible range, and a refractive index of 1.65 [51]. In In2O3–Ga2O3 and MgO–In2O3 systems, however, the resistivity ofthe resulting films increased considerably as the insulatingmaterial (Ga2O3 or MgO) content was increased [10]. TCOsemiconductors could be prepared in all compositions of theIn2O3–(Ga, In)2O3 and the In2O3–MgIn2O4 systems when thecomponents were TCO semiconductors.

In developing TCO films suitable for specializedapplications, Minami et al prepared Zn2In2O5–MgIn2O4

(or Zn–Mg–In–O) multicomponent oxide, a new TCOsemiconductor material, in 1995 [52]. Various TCO films wereprepared by MSP using combinations of ternary compoundsemiconductors such as MgIn2O4, ZnSnO3, GaInO3, Zn2In2O5

and In4Sn3O12. As an example, figure 11 shows ρ and RE asfunctions of Zn2In2O5 content for (In4Sn3O12)1−X–(Zn2In2O5)X

(or Zn-In-Sn-O) thin films prepared on RT substrates byrfMSP [10]; the electrical and chemical properties changedmonotonically as the composition was altered. Theetching rate was measured using 0.2 M HCl solution at25 C. It should be noted that all transparent conductingthin films prepared on low temperature substrates usingmulticomponent oxides composed of combinations of ternarycompound semiconductors were amorphous, regardless of thecomposition. It was also found that the work functions of

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Transparent conducting oxide semiconductors for transparent electrodes

multicomponent oxide thin films usually decreased as theircarrier concentration was increased [4, 10].

As described above, TCO films could always be obtainedin multicomponent oxides composed of combinations, at allcomposition ratios, of ternary compound TCO semiconductorssuch as Zn2In2O5–MgIn2O4, GaInO3–Zn2In2O5, Zn2In2O5–In4Sn3O12, ZnSnO3–In4Sn3O12, ZnSnO3–Zn2In2O5 andGaInO3–In4Sn3O12 [10]. It should be noted that there are manyrecent reports on TCO thin films using ternary compounds aswell as multicomponent oxides composed of CdO [5, 7] thatare usable in rather specialized applications but not introducedhere.

By using TCO films made of multicomponent oxides,the electrical, optical and chemical properties as well as thephysical properties such as band-gap energy and work functioncan be controlled by altering the chemical composition.All the TCO developed films discussed in this paper wereprepared with metal oxides containing at least one of thefollowing metal elements: Zn, In and Sn. In fact, itis difficult to name an important TCO semiconductor thatcan exhibit improved TCO film properties other than thosecontained in the triangle shown in figure 1. Thus, only thematerials contained in the triangle shown in figure 1 are usablefor developing TCO semiconductors for practical thin-filmtransparent electrodes: impurity-doped ZnO, In2O3 and SnO2

as well as multicomponent oxides consisting of combinationsof ZnO, In2O3 and SnO2, including some ternary compoundsexisting in their systems. From the results described above, thefollowing points should be taken into account when designingTCO semiconductors for practical use [4, 10]:

• The composition dependence of resistivity inmulticomponent oxide films is similar to that in metalalloys.

• The composition dependence of optical properties inmulticomponent oxide films is similar to that in mixedfilms of dielectric materials.

• The chemical properties of multicomponent oxide filmsare basically determined by the kind and amount of metalelements they contain. For example, the etching rate in anacid solution increased as the Zn content was increased,but it decreased as the Sn content was increased.

• Importantly, the obtainable properties of TCO filmsusing multicomponent oxides can be estimated fromthose of their binary and/or ternary compound TCOsemiconductor constituents.

However, the development of deposition techniquesthat can enable reliable high rate large area depositions ofmulticomponent oxide thin films has yet to be achieved.

6. Conclusion

Further development of practical thin-film transparentelectrodes using polycrystalline or amorphous transparentconducting oxide (TCO) semiconductors is necessary sincea stable supply of indium-tin-oxide (ITO) cannot be assuredbecause indium is a very expensive and scarce material.In addition, recent developments in optoelectronic deviceapplications frequently require thin-film transparent electrodeswith specialized properties. The following are the most

important TCO semiconductors for use as constituents ofpractical thin-film transparent electrodes: impurity-dopedZnO, In2O3 and SnO2 and multicomponent oxides consistingof combinations of ZnO, In2O3 and SnO2, including someternary compounds existing in their systems. Al- and Ga-doped ZnO (AZO and GZO) semiconductors are promisingas alternatives to ITO for thin-film transparent electrodeapplications. In particular, AZO thin films, composed ofinexpensive and non-toxic source materials and comparableto ITO film with its low resistivity of the order of 10−5 cm,can be considered the best candidate. However, furtherdevelopment of deposition techniques, such as magnetronsputtering or vacuum arc plasma evaporation, as well as oftargets is required to enable the preparation of AZO andGZO films on large area substrates with a high depositionrate. On the other hand, multicomponent oxides are suitablefor specialized applications because their electrical, optical,chemical and physical properties such as band-gap energyand work function can be controlled by altering the chemicalcomposition. However, both deposition technique andsintered targets have to be developed further in order toproduce transparent conducting multicomponent oxide filmsfor specialized applications.

Acknowledgment

The author would like to thank Professor T Miyata for fruitfuldiscussions.

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