Ž .Thin Solid Films 394 2001 219�223

A new transparent conductive thin film In O :Mo2 3

Yang Meng, Xi-liang Yang, Hua-xian Chen, Jie Shen, Yi-ming Jiang,Zhuang-jian Zhang�, Zhong-yi Hua

Department of Materials Science, Fudan Uni�ersity, Shanghai 200433, PR China

Received 23 June 2000; received in revised form 28 March 2001; accepted 25 April 2001

Abstract

Ž .A new high quality transparent conductive thin film In O :Mo IMO was prepared by conventional thermal reactive2 3Ž .evaporation at the substrate temperature of approximately 350�C. From X-ray photoelectron spectroscopy XPS and X-ray

Ž . 6� 3�diffraction XRD analysis of IMO films, it was confirmed that Mo substituted In without changing the cubic bixbyitestructure of In O and there were no new compounds in IMO as well. One atom of dopant contributes with more electrons to2 3the electrical conductivity and at the same carrier concentration there is fewer dopant in IMO than in other doped oxides. So, theIMO film exhibits simultaneously higher values of Hall mobility, electric conductivity, visible light transmittance, infraredreflectance and plasma wavelength. An electrical resistivity as low as 1.7�10�4 � cm was obtained, while the infraredreflectance above 4 �m and the average total visible light transmittance of the IMO film plus the glass substrate were both over80%, and the plasma wavelength was at approximately 2.2 �m. IMO is more suitable for the energy efficient windows used incold climates or even for optoelectronic device applications. � 2001 Elsevier Science B.V. All rights reserved.

Keywords: Evaporation; Indium oxide; Molybdenum oxide

1. Introduction

Transparent conductive films exhibit very impressiveproperties, i.e. high electrical conductivity combinedwith high optical transparency and high infrared re-flectivity, and have been used for many applications� � Ž .1,2 . Although In O :Sn ITO , SnO :F and ZnO:Al2 3 2are the most commonly used materials at present,however, looking for new materials still remains inter-

� �esting and attractive 3�7 .Some stoichiometric metal oxide films are transpar-

ent because their band gaps are wider than 3 eV, buttheir resistivity is very high at room temperature. Someof them become n-type semiconductors in the case of

� �oxygen deficiency 8,9 . Their conductivity depends on

� Corresponding author. Tel.: �86-21-65642682.Ž .E-mail address: yxzhangc@online.sh.cn Z. Zhang .

the extent of oxygen vacancies. The widely studied andused transparent conductive films, such as In O :Sn,2 3SnO :F and ZnO:Al, are doped semiconductor films.2Each doped atom, such as Sn, F and Al, may contributewith one electron to the free carriers. So, there aresome excessive electrons in the conductive band. Their

�3 �5 � �resistivity may lower than 10 to 10 � cm 10�12 .We found that the resistivity of the In O film2 3

decreased dramatically when doped with Mo, but nochanges were detected in the spectral transmittance,although the compound MO itself was yellowish. The3experimental results had confirmed that the molybde-

Ž .num doped indium oxide film In O Mo IMO was a2 3novel transparent conductive film.

2. Experimental setup

The IMO films were deposited by a conventionalthermal reactive evaporation method. The metal In and

0040-6090�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 1 0 1 1 4 2 - 7

( )Y. Meng et al. � Thin Solid Films 394 2001 219�223220

MoO were evaporated from two separate sources,3respectively. The residual pressure of the depositionchamber was lower than 7�10�3 Pa. The working gaswas pure O . During the deposition process, the oxygen2pressure was kept constant in the range of 9�10�2 to4�10�1 Pa. The substrates were normal glass micro-scope slides of 1.2 mm thickness. Their temperatureswere in the range of room temperature to 350�C. Thedistances between the source and the substrate wereapproximately 25 cm. The deposition rate, which wasadjusted by the temperature of the evaporator forindium, was approximately 0.2�0.4 nm s�1.

In order to prevent the ions in the glass from diffus-ing into the IMO films, the start time of doping molyb-denum was usually later than that of evaporating in-dium properly. Therefore, an indium oxide layer of afew hundred Angstrom thickness between the IMOand the substrate may be formed and work as a bufferlayer.

The spectral reflectance and the total spectral trans-mittance of the IMO film plus the glass substrate weremeasured by a Perkin-Elmer � 9 spectrometer. Thethickness of the IMO film was calculated from the

� �spectral transmittance 13 . The sheet resistance wasmeasured with a four-point probe. The carrier concen-tration and the Hall mobility were calculated from theHall effect measured with the van-der-Pauw techniquein a magnetic field of 5000 Gauss. The component wasanalyzed by a Phi-5702 X-ray photoelectron spectro-

Ž .scope XPS . The crystal construct was analyzed by aŽ .Rigaku D�Max-rB X-ray diffractometer XRD . The

weight proportion of the Mo to In was calculated fromŽ .the proton induced X-ray emission PIXE .

3. Experimental result

Without any special treatments, high quality IMOfilms with thickness of 250�400 nm were depositedduring 20 min at 350�C. Their sheet resistances werelower than 6.5 � ��1. The reflectance above 4 �mwas over 80%. The average total spectral transmittanceof the IMO film plus the glass substrate in the visible

Ž .region was over 80% Fig. 1 . This was a good resultsince the glass substrate itself only transmitted 92%.Their electrical resistivity, carrier concentration andHall mobility were in the range of 1.7�10�4 to 2.1�10�4 � cm, 2.5�1020 to 3.5�1020 cm�3 and 80 to130 cm2 V�1 s�1, respectively. The weight proportionof Mo to In was approximately 4%.

Fig. 1 shows the spectral transmittance, reflectanceand absorptance of a 370-nm-thick IMO film on a1.2-mm-thick glass substrate. Its electrical resistivityand sheet resistance were 1.8�10�4 � cm and 5 ���1, respectively. Its carrier concentration N andhHall mobility � were approximately 2.6�1020 cm�3

hand 1.3�102 cm2 V�1 s�1, respectively. The cutoff

Ž .Fig. 1. Spectra of In O :Mo IMO film on glass substrate: T, tran-2 3smittance; R, reflectance; A, absorptance.

wavelength � of transmittance in short wave was atgapproximately 0.3 �m. The plasma wavelength � waspat approximately 2.2 �m.

Figs. 2 and 3 show XPS profile spectra and XRDspectra, respectively, of an IMO film. In Fig. 2, twomain bind energies of molybdenum were 232.8 eV and236.2 eV, coincided with those of Mo6� in MoO . In3Fig. 3, the background was from the amorphous glasssubstrate. The XRD spectra of the film coincided verywell with those of In O of cubic bixbyite structure.2 3The lattice constant of the IMO film was 0.10135 nm.Similar to that of Sn doped In O , it was slightly larger2 3than that of pure In O which was 0.10118 nm. It could2 3be observed that Mo6� substituted In3� in In O and2 3there were no other new compounds or solutions ofmolybdenum with In O in the IMO films.2 3

The properties of the IMO films produced dependon the temperatures of the substrate and of the twosources, the oxygen partial pressure and the distancesbetween the substrate and the two sources.

The temperature of the substrate was one of the keyprocess parameters. Fig. 4 shows its effects on theresistivity and on the average transmittance in thevisible region. The films were prepared keeping theoxygen partial pressure at approximately 1.1�10�1 Paand the deposition rates in the range of 0.2�0.3 nms�1. The thickness was controlled by time and the filmtransmittance was in the range of 380�400 nm.

It was apparent that the spectral transmittance ofthe films obviously lowered when the temperature waslower than 230�C, and the resistivity increased drasti-cally when the substrate temperature was lower than100�C. The deterioration of the films was mainly due to

� �insufficient oxidization of indium 14,15 . Although thedeterioration of the films may be reduced slightly bydecreasing the deposition rate, this was not an efficientand economical method to use.

The partial pressure of oxygen was another keyparameter. When the partial pressure ranged from9�10�2 to 4�10�1 Pa, high quality IMO films were

( )Y. Meng et al. � Thin Solid Films 394 2001 219�223 221

Ž .Fig. 2. XPS profile spectra of In O :Mo IMO film.2 3

prepared. Once the pressure was lower than 9�10�2

Pa, the spectral transmittance immediately decreased,due to the presence of metal indium in the film. Onlywhen the duration of low oxygen content was shortenough, could the transmittance be regained by in-creasing the oxygen partial pressure within the deposi-tion chamber. Of course, if both evaporated sourceswere oxide, the dependency of the film transmittanceon the oxygen partial pressure would be weakened to agreat extent.

4. Discussion

To prepare doped semiconductor films, firstly, theionic radius of the dopant should be the same as or

Ž .Fig. 3. XRD spectra of In O :Mo IMO film.2 3

smaller than the ion been substituted. Secondly, nonew compounds or solid solutions of dopant oxide withthe host oxide would be formed. Thirdly, there shouldbe a proper technique to dope the ions.

The ionic radius of In3� in In O is 8.1�10�11 m,2 3and that of Mo6� is 6.2�10�11 m, so it is possible forMo6� to substitute In3� into In O . The vapor pres-2 3sure of MoO is over 2�102 Pa at 740�C. Therefore, it3is easy to mix MoO in In O , to deposit films pro-3 2 3duced by conventional thermal evaporation. XPS andXRD analysis showed that there were no other newcompounds or solutions of molybdenum with In O .2 3Thus, IMO can be thought of as a well doped oxide.

Most of the doped transparent conductive oxide filmsstudied up to now were impurity doped indium oxides,tin oxides and zinc oxides. Most of their valence dif-ferences between the dopant and the substituted ionare 1 or 2. However, in In O :Mo, there is a valence2 3difference of 3 between Mo6� and In3�. This means

Fig. 4. Effects of substrate temperature on resistivity and transmit-Ž .tance of In O :Mo IMO film.2 3

( )Y. Meng et al. � Thin Solid Films 394 2001 219�223222

that one atom of dopant contributes with more elec-trons to the electrical conductivity or at the samecarrier concentration there is fewer dopant in IMOthan in other doped oxides. The Hall mobility of de-generated semiconductors with carrier concentrationsof 1020 to 1021 cm�3 was mainly dominated by ionized

� �impurity scattering 5 , so the IMO film may have ahigh mobility due to its lower impurity.

The mobility may also be found from optical spectra.In Fig. 1, the plasma wavelength � was at approxi-pmately 2.2 �m. The relation between � and the car-p

� �rier concentration N was given by 7p

2 �� � m2�c � 0 Ž .N � 1p 2ž /� ep

where c is the velocity of light, � is high frequency�

permittivity, � is the permittivity of free space, m� is0the effective mass of the free electron in the conduc-tion band, and e the electronic charge. If � is as-�

� � �sumed to be 4, and m is assumed to be 0.3 m 7 ore� �0.35 m 16 , where m is the electron mass, then Ne e p

would be 2.8�1020 cm�3 or 3.3�1020 cm�3, respec-tively. So the mobility � was correspondingly approxi-pmately 1.2�102 cm2 V�1 s�1 or 1.0�102 cm2 V�1

s�1. The difference between � from plasma wave-plength and � from the Hall effect measurement washapproximately 8% or 26%. This agreement was better

� �than that of Hamberg and Granqvist 16 .One of the main applications of transparent conduc-

tive oxides is the energy efficient window. The mostimportant properties of the energy efficient window arethe maximum values obtainable for the infrared re-flectance R, the visible transmittance T and the associ-ated cut off wavelength � .p

The solar irradiance spectrum is mainly in the rangeof 0.3�3 �m; the retinal spectral sensitivity for humanbeing ranges from 0.4 to 0.7 �m, and the emittancespectrum of a blackbody at 50�C is within the range of3�50 �m. Therefore, the energy efficient windows usedin a cold climate should have a high transmittance inthe range of 0.4�3 �m, and a high reflectance in the

� �range of 3�50 �m 16 . The plasma wavelength � waspat approximately 3 �m. Thus, the carrier concentration

20 �3 Ž .should be approximately 1.5�10 cm from Eq. 1 .Unfortunately, � of most transparent conductivep

films increases approximately in the same order as the� �resistivity 1 , and the visible light transmittance and

the infrared reflectivity were both affected by the resis-tivity. The spectral absorptance A in the visible regionand the reflectance R in the infrared region can be

� �approximated as 17,18

�2e2 t Ž .A� 22 3 � 2 24� � c nm �0

1 Ž .R� 32Ž Ž ..1� 2� c�t0

where t is the thickness, n is the refraction constant,and is the electrical resistivity. In practice, the mobil-ity � should be as high as possible while keeping theresistivity as low as possible. So, it was a betterstrategy to increase conductivity without compromisingthe optical properties by increasing the mobility rather

� �than the carrier concentration 7 .Comparing with the other transparent conductive

film electrical resistivities, IMO exhibited simultane-ously a higher plasma wavelength, a lower spectralabsorptance and a higher infrared reflectance due to itshigher mobility and lower resistivity, making them moresuitable for the energy efficient windows used in cold

� �climates 1 .The most common substrates for transparent con-

ductive films were various glasses. Some inexhaustibleions in glasses may diffuse into films. Unfortunately,the most common and most mobile ions in glasseswould be acceptors or scattering centers in indium

Žoxide films, even at substitutional positions such as� 2� 2� 2� 2� .Li , Mg , Mn , Cu and Zn , etc. or interstitial

Ž � 2� 2� 2� .positions such as Na , Ca , Sr , and Ba , etc. .Because the diffusing rate increases with temperature,it was necessary to prevent the diffusing process for thehigh quality films prepared, treated or used in a hightemperature. One of the effective methods was todeposit a buffer layer between the film and the glass,for example, the SiO buffer layer was popularly used2between the ITO thin film and the glass substrate� �1,19 . Because In O could be used as an efficient2 3

� �buffer layer 20 , a thin buffer layer of In O can be2 3easily formed between the IMO film and the glass bydelaying the start time of doping molybdenum. Theexperimental results have proved this statement. In Fig.2, sodium from the glass substrate could not be foundin the IMO film and molybdenum from IMO could notbe found in the substrate either. Therefore, the indiumoxide film with the thickness of a few hundredAngstroms was an effective buffer layer; meanwhilethere was no influence on the crystal structure, trans-parence and adherence to the glass substrate of theIMO film. Another experimental result has providedeloquent proof, i.e. an annealing process in vacuum attemperatures as high as 600�C for 3 h, did not onlydeteriorate but also improved the conductivity of theIMO thin films. During the annealing process, the glasssubstrate was gradually softened and bent to a certainextent due to gravity.

The properties of doped oxide transparent conduc-tive films can be improved by some postdeposition

� �treatments such as annealing 14,16,21,22 . Though theIMO films mentioned in this paper were not annealedintentionally, there was approximately 5�15 min when

( )Y. Meng et al. � Thin Solid Films 394 2001 219�223 223

the temperature of the IMO films was over 300�C invacuum after the deposition. It may be that they hadbeen annealed incompletely and the resistivity haddecreased during the process.

5. Conclusion

Without any special treatment, high quality IMOfilms with thickness of 250�400 nm were depositedduring 20 min at 350�C by conventional thermal reac-tive evaporation. Their sheet resistances were lowerthan 6.5 � ��1. The average total spectral transmit-tance of the IMO film plus the glass substrate in thevisible region was over 80%. The cutoff wavelength oftransmittance at short waves was at approximately 0.3�m. The plasma wavelength was at approximately 2.2�m. The reflectance above 4 �m was over 80%. Theirelectrical resistivity, carrier concentration and Hallmobility were in the range of 1.7�10�4 to 2.1�10�4

� cm, 2.5�1020 cm�3 to 3.5�1020 cm�3 and 80 to130 cm�2 V�1 s�1, respectively. The weight proportionof Mo to In was approximately 4%.

Ž .From X-ray photoelectron spectroscopy XPS andŽ .X-ray diffraction XRD analysis of IMO films, it was

confirmed that Mo6� substituted In3� without chang-ing the cubic bixbyite structure of In O and there2 3were no new compounds in IMO as well.

One atom of dopant contributed with more electronsto the electrical conductivity and at the same carrierconcentration there was fewer dopants in IMO than inother doped oxides. So the IMO film exhibited simulta-neously higher values of Hall mobility, electric conduc-tivity, visible light transmittance, infrared reflectanceand plasma wavelength. This makes the IMO filmsmore suitable for energy efficient use in cold climates,

or in optoelectronic devices as window layers, where itis required to use films that are highly transparent andconductive.

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