Journal of Alloys and Compounds 600 (2014) 159–161

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Journal of Alloys and Compounds

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Letter

Highly conductive n- and p-type CuInO thin films by reactiveevaporation

http://dx.doi.org/10.1016/j.jallcom.2014.02.1130925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 484 2605072; fax: +91 484 2607534.E-mail address: reenatara@rediffmail.com (R.R. Philip).

Surya A. Mary a, Bindu G. Nair a, Johns Naduvath b, G.S. Okram c, Stephen K. Remillard d,P.V. Sreenivasan a, Rachel R. Philip a,⇑a Thin Film Research Lab, Union Christian College, Aluva, Cochin, Kerala, Indiab Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, Indiac UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452 001, MP, Indiad Department of Physics, Hope College, Holland, MI 49423, USA

a r t i c l e i n f o

Article history:Received 27 January 2014Received in revised form 19 February 2014Accepted 19 February 2014Available online 28 February 2014

Keywords:Oxide materialsThin filmsElectrical transport

a b s t r a c t

Undoped copper indium oxide films with conductivity three to four orders of magnitude higher than sofar reported in this system, exhibiting both n- and p-type polarity are successfully prepared by reactiveevaporation method. Hall coefficient and hot probe technique in conjunction with thermopower mea-surements ascertain the conductivity type of the films to be changing from n- to p-type, as the film com-position assessed by energy dispersive analysis of X-rays changed from In rich to Cu rich. Theconductivity is found to be �2.1 � 101 S/cm in In-rich n-type films where as it is �1.13 � 102 S/cm inCu-rich p-type films with a corresponding increase in carrier concentration and carrier mobility in thelatter. Scanning electron micrographs show close assembling of nanograins and X-ray diffraction revealsa change in orientation of crystallites from (006) to (600) as the composition goes from In rich to Cu rich.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The search for Transparent Conductive Oxide (TCO) films, themajority carriers of which can be tuned n-type and p-type byintrinsic doping is actively under way, with the view of extendedapplications of such compounds in various devices like solar cellsand LED’s, where homojunction pn junctions can realize latticematching and hence better device properties than heterojunctions[1–11]. Delafossite metal oxides ABO2 are recognized as good can-didates for realizing this bipolarity due to their unique structure,where the layered structure with O–A–O dumbbell layer is conjec-tured to act as a better conduction path for holes and edge sharingBO6 layers for carrier electrons, but their poor conductivity inhibittheir extensive usage [4,12–15]. CuInO2 is a typical delafossitemetal oxide which is expected to behave in the same way, as theytoo have an alternate stacking of edge shared InO�2 octahedral layersand Cu+ ions coordinated by two O2� ions forming perpendiculardumbbell layers, but whose reported conductivity is only ofthe order of 10�2 S/cm limiting its application [15–18]. Reportson successful fabrication of bipolar CuInO2 junction in which

substitution of In3+ with Ca2+ and Sn4+ resulting in p-type andn-type conduction respectively, with conductivity raised to amaximum of �103 S/cm due to doping, are found in the literature[19–21]. This increase in conductivity on extrinsic doping withSn4+ has been attributed to the easier movement of carrier electronsalong the InO6 layer in the doped films with preferential orientationalong (006), compared to the undoped films obtained with (110)orientation [17].

This communication reports successful deposition and charac-terization of both n-type (In rich) and p-type (Cu rich) thin filmsamples of CuInO showing very high conductivity, irrespective oftheir structural orientation, the conductivity being more than threeto four orders of magnitude higher than the values so far reportedin these undoped films.

2. Experimental

Copper indium oxide thin films are prepared by reactive evaporation, atechnique which seemingly, has not been used so far in the preparation of thesecompounds. Here, 99.999% pure copper and 99.9% pure indium are evaporatedsimultaneously from independently heated molybdenum boats in a vacuum of�10�5 mbar and deposited on glass substrates kept at a temperature of 523 ± 5 K,after creating oxygen plasma (plasma current �2 A) between two stainlesssteel discs of 20 cm diameter and kept at a spacing of 10 cm. The samplecomposition has been changed from In rich to Cu rich by varying the elementalflux.

160 S.A. Mary et al. / Journal of Alloys and Compounds 600 (2014) 159–161

The preliminary structural, morphological and compositional characterizationhas been done by X-ray diffraction (XRD) using a BRUKER D8 advance X-ray diffrac-tometer and JEOL JSM 7600F Field Emission Scanning Electron Microscope (FE-SEM)equipped with EDAX instrument respectively. Thermopower measurements aretaken using a nanovoltmeter, by placing samples between two oxygen free highconductivity copper blocks with silver paste for ohmic contact and conductivity ismeasured using a Keithley 2611A source meter. Hot probe technique and Hallcoefficient measurement are done to ascertain conductivity type of the films. TheHall voltage measured for varying currents by applying a constant magnetic fieldof �3580 Gauss perpendicular to it is used to determine the carrier concentrationand mobility of carriers. Optical characterization is done using a Hitachi U-3410UV Vis–NIR spectrometer and thickness measurement is done using Fizeau’sinterferometric technique.

Fig. 2. Scanning electron micrograph of (a) typical n-type film with (Cu/In) atomic%ratio 0.8 and (b) typical p-type film with (Cu/In) atomic% ratio 1.4.

3. Results and discussion

The XRD patterns of the nearly stoichiometric films depicted inFig. 1 show that In rich (Cu/In �0.8) samples have rhomb centredrhombohedral structure with (006) and (104) orientations [JCPDSfile No. 530954] where as Cu rich films (Cu/In = 1.4) seem to under-go a structural change with the grains going into an orientationalong the (600) plane [JCPDS file No. 70-1082]. The crystallitesize determined by Scherrer’s formula is �15 nm for n-type and�10 nm for p-type film.

Scanning electron micrographs (Fig. 2) reveal nanocrystallitesassembling closely together to form agglomerated grain structures.The closely packed grains evidenced by SEM might be a contribut-ing factor to the high conductivity shown by these samples, as itcan reduce the grain boundary effect lowering the sheet resistanceof the thin film samples. The particular growth of the films must bethe result of the new technique used here for sample preparation.

The conductivity type ascertained by thermoelectric power(TEP) measurement in conjunction with Hall coefficient measure-ment and hot probe technique shows the In rich films to be n-typeand Cu rich to be p-type. The Seebeck coefficient at 300 K is mea-sured to be � �41.8 lV/K in the n-type and +1.68 lV/K in thep-type films which manifest a room temperature-conductivity ofthe order of 2.1 � 101 S/cm and 1.13 � 102 S/cm respectively. It isto be noted that the conductivity is around three orders higherthan the reported values in n-type and four orders higher in thep-type films [15–18]. Room temperature Hall coefficient measure-ments show that the hole concentration and mobility in p-type are1.4 � 1018/cm3 and 4.97 � 102 cm2/Vs respectively where as theelectron concentration and mobility in n-type are 2.7 � 1017/cm3

and 4.7 � 102 cm2/Vs respectively, probably indicating that greaterthe deviation of composition from stoichiometry, greater is the car-rier concentration. The ln(r) versus 1000/T graphs that are plottedmeasuring the conductivity of the samples in the range 303–385 K(Fig. 3 inset), show defect levels with activation energy (Ea)

30 40 50 60 70

30 40 50 60 70

(6 0

0)

Inte

nsity

(arb

. uni

ts)

2 Theta (Degrees)

(1 0

4)

(0 0

6)

Cu rich

In rich

Fig. 1. XRD patterns of Cu rich and In rich films.

�0.17 eV for n-type samples and Ea �0.06 eV for p-type samples,indicating occurrence of shallow defects. The change in conductiv-ity from n-type in In-rich films to p-type in Cu-rich could be ex-plained as due to the substitution of In3+ at Cu1+ sites (InCu) inthe former and Cu1+ at In3+ sites (CuIn) in the latter, which agreeswith the EDAX observation mentioned earlier. In the previous sec-tion we reported that the n-type films have excess indium (Cu/Inatomic% ratio = 0.8 instead of the expected value of 1in stoichiom-etric CuInO2) and p-type films are deficient in indium (Cu/In atom-ic% ratio 1.4) leading to the possibility of InCu in the n-type and CuIn

in p-type.Electron concentration in the n-type increases with InCu occur-

rence as the films turn In rich where as hole concentration is en-hanced as CuIn occurs in Cu-rich films. In addition, variousinvestigators [17,19] have suggested that the preferential growthof the samples along (006) plane could offer an easy path for con-duction of electrons and the plane perpendicular to it can act as aneasy path for hole conduction. While this, along with the closepacking of grains and possible removal of structural defects dueto the particular preparation technique employed could promoteeasy conduction of the carriers and high mobility, thus enhancingthe conductivity in these samples, the manifestation of suchunusually high conductivity observed in all the CuInO thin filmsprepared by this technique calls for much detailed investigations,which are in progress.

The optical absorbance versus wavelength depicted in Fig. 3shows that there is a blue shift in band gap for the n-typefilms(Eg � 3.56 eV) when compared to the p-type (Eg � 3.27 eV),with least absorbance in the visible region. The greater band gap of

400 600 800 1000 12000.0

0.5

1.0

1.5

2.73 2.94 3.15 3.36

3

4

5

6

ln(σ

)

(b) Ea=0.06eV

(a) Ea=0.17eV

1000/T (K-1)

(a) In rich (n-type)(b) Cu rich (p-type)

(b)

Wavelength (nm)

Abs

orba

nce

(a)

Fig. 3. Absorbance spectra showing the blue shift in the fundamental absorptionedge of In rich films. Inset: ln(r) versus 1000/T graphs for CuInO films with r inS/cm. (For interpretation of the references to colour in this figure legend, the readeris referred to the web version of this article.)

S.A. Mary et al. / Journal of Alloys and Compounds 600 (2014) 159–161 161

n-type when compared to the p-type is desirable when homojunc-tions are fabricated using these films for solar cell devices. Thefinding that CuInO samples with high conductivity can be preparedby reactive evaporation, as p-type and n-type by suitably control-ling the elemental flux, while so far doping with Ca and Sn hasbeen adopted for polarity change, can open up a new vista forbipolar device fabrication using CuInO.

4. Conclusion

To conclude, highly conductive undoped copper indium oxidefilms whose conductivity is three to four orders higher than thatreported in these films so far, are prepared as both n- and p-typeby reactive evaporation technique, simply by controlling the ele-mental flux. The achievement of such unusually high conductivityalong with the observation that transport and optical propertiescan be varied simply on changing the Cu/In ratio of the films, with-out any extrinsic doping, offers a solution to the difficulty so far

encountered in using this material to fabricate pn junctions foroptoelectronic devices.

Acknowledgements

The corresponding author acknowledges DST and UGC, India forfunding respectively through major and minor research projectsand United Board, NY for UB fellowship. The authors acknowledgeSTIC, Cochin University for carrying out XRD and SAIF, IIT Bombayfor carrying out SEM analysis.

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