Effect of thermal annealing onelectrical and optical properties ofBa-doped SrCu2O2 thin films on glass substrates

Afzal Khan1,2, Carmen Jiménez*,1, Odette Chaix-Pluchery1, Hervé Roussel1, and Jean-Luc Deschanvres1

1 Laboratoire des Matériaux et du Génie Physique, Grenoble INP, CNRS, Minatec, 3 parvis Louis Néel, 38016 Grenoble, France2 Institute of Physics and Electronics, University of Peshawar, 25120 Khyber Pakhtunkhwa, Pakistan

Received 5 June 2013, revised 12 July 2013, accepted 13 August 2013Published online 17 September 2013

Keywords Ba-doped SCO, metalorganic chemical vapour deposition, optical and electronical properties, thin films,transparent conductive oxides

* Corresponding author: e-mail carmen.jimenez@grenoble-inp.fr, Phone: þ33 456 529 334, Fax: þ33 456 529 301

Ba-doped SrCu2O2 (SCO) thin films were synthesized byannealing Ba-doped Sr–Cu–O films deposited on glasssubstrates by metalorganic chemical vapour deposition(MOCVD) and composed of SrCO3 and CuO. In order toreduce cracks formed during conventional thermal annealingand therefore, to enhance optical and electrical properties of thefilms, annealing conditions were optimized in terms of heating

rate, cooling rate and annealing time. The SCO phase wasobtained after annealing at 650 8C for 5min leading to acomplete decomposition of the initial phases via an intermedi-ate SrCuO2 phase. Optical characterizations of the SCO filmsindicate a transmittance of 75% in the visible range at 550 nmand a direct bandgap of 3.25 eV; their electrical conductivitywas measured to be 4.3� 10�2 S cm�1.

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Transparent conductive oxides(TCOs) [1–9] are fascinating materials which enter in avariety of cutting-edge applications, such as solar cells, gassensors, organic light-emitting diodes, liquid crystal andhigh definition displays, electrochromic windows andarchitectural coatings, etc. [10, 11]. In all these passiveapplications, only n-type TCOs are used as transparentelectrodes [12]. For some device applications, alternativep-type TCOs are needed in order to make a transparentpn-junction and to develop transparent electronics [1, 11].The first p-type TCO, reported in 1993, was NiO exhibitinga low transparency of 20% [13]; the second report, onCuAlO2, was published in 1997 and contained someimprovements compared to NiO [14]. This relative lack ofp-type TCOs is related to the high probability of holes to belocalized around the oxygen atoms, resulting in poorconductivity and carrier mobility in these compounds [1,14]. A proposed solution was to introduce covalency inmetal–oxygen bonding in order to induce the formation of anextended valence band structure. This method to obtain p-typeTCOs proposed by Kawazoe et al. [15] is known as chemicalmodulation of the valence band (CMVB) and based oncopper containing oxides with the well known Delafossite

compounds MCuO2 (M¼Al, Ga, Cr, In) [11] and theMCu2O2 compounds (M¼Ca, Ba, Mg and Sr) [16–18].Among the p-type TCOs proposed so far, SrCu2O2 (SCO)is considered to be one of the most promising candidatesdue to its large direct bandgap and low depositiontemperature [16, 19].

The first thermodynamic data in the Sr–Cu–O systemwas reported by Alcock et al. [20]; it is based onmeasurements of the activity of SrO and on specific heatmeasurements at low temperatures. The existence ofSrCu2O2 was confirmed and some phase compatibilitieswere reported at reduced oxygen partial pressure [21].From thermogravimetry measurements, it is known that theinter-oxide compound SrCuO2 goes from oxygen excess(SrCuO2þ x) to oxygen deficiency (SrCuO2� x) as theoxygen partial pressure decreases or temperature increases[20]. The phase diagram, based on the work of Suzukiet al. [21], shows various Sr–Cu–O phase domains as afunction of oxygen partial pressure and temperature for afixed metal ratio of Cu:Sr¼ 2:1 as in SrCu2O2 [22]. Thoughthe phase diagram of Sr–Cu–O system was set up from bulkmaterial, Sr–Cu–O thin films undergo the same phasetransformations with some deviation [23]. The integration of

Phys. Status Solidi A 210, No. 12, 2569–2574 (2013) / DOI 10.1002/pssa.201330011 p s sapplications and materials science

a

statu

s

soli

di

www.pss-a.comph

ysi

ca

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

SCO thin films into functional devices has already beenreported: transparent hetero pn-junctions have been fabri-cated by various researchers using SCO as p-type and ZnOas n-type TCO [16, 24–27]. This demonstrates the suitabilityof SCO for its use in transparent electronic devices.However, the electrical conductivity and optical transparen-cy of SCO thus obtained are yet to be improved. A suitabledopant able to improve simultaneously both propertieswould be essential for SCO. The first principles calculationsperformed on SCO by Nolan show improvements related tothe calculated bandgaps and hole effective masses inBa-doped SCO in comparison with K- and Ca-dopedSCO [9, 28, 29]. The ionic radius of Ba2þ is larger than theCuþ one, which results in the strong structural distortionaround the dopant sites. This structural distortion increasesthe Cu–Cu distance and decreases the Cu–Cu interactionwhich leads to a widening of the bandgap [28]. Thus, Badoping of SCO might be a useful alternative approach toobtain a high transparency and good p-type semiconductingproperties compared to K and Ca doping. Based on thetheoretical calculation of Nolan, Louloudakis et al. havereported Ba-doped SCO deposited by PLD with anoptical transparency as high as 60% and a bandgap of3.24 eV, but they gave no information about its electricalconductivity [9].

In a previous paper on strontium-copper oxide thin filmsdeposited on silicon substrate by metalorganic chemicalvapour deposition (MOCVD) [30], we have shown that thechemical phase transformation pathway to obtain SCO wascharacterized by the decomposition of SrCO3 and CuO intothe Sr14Cu24O41 phase when the films were annealed inoxygen atmosphere; then, the expected SCO phase wasobserved after subsequent annealing under Ar. In situ studiesrevealed that the SCO phase could be preserved at roomtemperature only when the film was cooled down veryquickly. In this paper, we report the effect of thermalannealing on Ba-doped Sr–Cu–O thin films deposited onglass substrate by MOCVD. We will mainly focus on themorphology and electrical and optical properties of the films.

2 Experimental Thin films of Sr–Cu–O were depos-ited in a hot wall pulsed injection MOCVD reactordeveloped by JIPELEC and described elsewhere [4]. Thesolution consisted of Cu(TMHD)2 and Sr(TMHD)2 triglymeadduct dissolved in metaxylene. Ba(TMHD)2 triglyme wasadded in the solution as dopant (5–15% molar as dopantpercentage related to Sr concentration). Precursors wereprovided by STREM Chemicals. The solution was injectedin the evaporator using argon as the carrier gas. Theevaporation temperature was 280 8C. The injection param-eters were set at 1Hz for the frequency and 2ms for theopening time. The deposition took place in an oxidizingatmosphere generated by an argon/oxygen mixture. The O2

partial pressure in the reactor was controlled at 333.3 Pa andthe total pressure was 666.6 Pa. The deposition temperaturewas 550 8C. SK5 and silicon (100) single crystal were usedas substrates in these experiments. SK5 is an aluminium-

borosilicate glass substrate provided by Schott with a 40%BaO weight content and able to stand temperature of 650 8C.Small pieces of 2 cm� 4 cm were used. Due to the presenceof barium in the substrate, the final Ba composition of thefilm could not be exactly quantified. However, our previousstudies indicated that the composition of films deposited byMOCVDwas not substrate-dependent, so we considered thatthe Ba-content was similar for films deposited in the sameconditions on glass substrate and on Si substrate. Thus,the barium composition of films deposited on silicon in thesame conditions as SK5 was evaluated from Electron ProbeMicroanalysis measurements performed at different electronbeam energies and analysed using the Stratagem software. Itwas found to be 4.6, 8.2 and 11% when the Ba concentrationin the solution was 5, 10 and 15%, respectively. In thefollowing, the final material will be identified as Ba-dopedSCO. In these conditions, a 200–230 nm thick film wasdeposited from 15mL solution. The cationic ratio in thesolution was adapted to obtain the stoichiometric composi-tion in the film (Cu:Sr¼ 2:1), in agreement with our previousCVD kinetics calibration [30].

A homemade conventional annealing system, formed bya tubular furnace which is able to move back and forth withincreasing heating and cooling rates under a continuous N2

gas flow was used for film annealing. A thermocouple indirect contact with the sample allowed monitoring of thelocal temperature. Once the desired phase was expected tobe formed, a rapid film cooling was achieved by moving thefurnace aside and increasing the gas flow. Rapid thermalannealing (RTA) was carried out in an annealing systemFAV4 of JIPELEC based on halogen lamps allowing fastheating and cooling rates. Here, it is to be noted that RTAwas performed on films deposited by MOCVD on siliconsubstrates, because the IR lamps used in RTA are notefficient for heating SK5 substrates due to the lowabsorbance of glass in the near-IR.

u–2u X-ray diffraction (XRD) measurements wereperformed on spinning samples at room temperatureaccording to the Bragg–Brentano configuration, using aSiemens D500 diffractometer with the Cu Ka1 radiation(l¼ 0.15406 nm) and a scintillation detector. SEM imageswere obtained with an XL30 Philips microscope coupled toan EDX system. Raman spectra were collected using a JobinYvon/Horiba LabRam spectrometer equipped with a liquidnitrogen cooled charge coupled device detector. Experi-ments were conducted in the micro-Raman mode in abackscattering geometry using a configuration of crossedpolarizer and analyser (VH) to decrease the very highintensity of the Si line when measured in unpolarizedspectra. The 488 nm line of an Arþ ion laser was used asthe excitation line. Optical measurements were performedin direct transmission using a Jasco V530 UV–Visiblespectrophotometer in the 200–1100 nm wavelength range.Conductivity measurements were carried out using a four-probe and Van der Pauwmethods on films deposited on glasssubstrate and obtained from a Ba-concentration of 10% inthe solution.

2570 A. Khan et al.: Electrical and optical properties of Ba-doped SrCu2O2 thin films

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

ph

ysic

a ssp stat

us

solid

i a

3 Results and discussion As previously reported [5,30], the as-deposited Sr–Cu–O films grown on siliconsubstrate were composed of SrCO3 and CuO giving rise tothe SCO phase after some post-deposition annealingtreatments. The Sr–Cu–O films deposited on glass substratewere also composed of SrCO3 and CuO as revealed by XRDand Raman analyses (see further on). From these results, it isclear that SCO thin film could only be obtained by annealingconditions suitable for glass substrate. So, conventionalfurnace annealing was performed.

The standard annealing conditions in our conventionalfurnace setup were a heating ramp of 30 8Cmin�1 to achieve600 8C and a cooling ramp of 3 8Cmin�1. In order to retainthe SCO phase at room temperature, the cooling rate wasincreased to 70 8Cmin�1 by using movable furnace andhigher gas flow. In conventional furnace, pure SCO thinfilms on glass substrates were first obtained after a longannealing duration of around 4 h at 600 8C under N2

atmosphere. However, film transparencies were very low ascompared to the measured optical transmittance of SCO thinfilms reported in various papers [18, 19, 29, 31, 32]. Thisfactor was easily visible to the naked eye and moreover, thetransmittance values of the as-deposited film were found tobe higher than those obtained for these SCO films, as shownin Fig. 1. This low transparency is probably due to theformation of cracks in the film as shown in the inset of Fig. 1.Reducing cracks in the film would also allow to increaseelectrical conductivity and therefore, is essential to obtainenhanced properties of the film. Fused silica is a suitablesubstrate for high temperature annealing but is not adaptedfor SCO films due to its very low thermal expansioncoefficient, leading to crack formation in the films duringannealing. SK5 glass substrate presents a good compromisebetween temperature standing and thermal expansioncoefficient (transformation temperature Tg¼ 660 8C anda20�300 8C¼ 65� 10�7K�1).

In order to establish the origin of cracks duringannealing, the film morphologies were compared for thesame annealing temperature of 600 8C after conventional andRTA annealing (Fig. 2b and c, respectively). Films weredeposited on silicon substrates to enable the RTA process. InRTA, the rate of heating (6000 8Cmin�1 or 100 8C s�1) andcooling (300 8Cmin�1) were very high and the annealingtime very short (30–45 s). In conventional furnace, filmswere annealed at 600 8C during 4 h with a heating ramp of30 8Cmin�1 and cooled down at a rate of 70 8Cmin�1. Thecomparison between Fig. 2b and c shows the formation ofcracks in the only case of conventional heating, as on glasssubstrate (see inset in Fig. 1); this means that a faster heatingis needed in such a conventional furnace in order to preventcrack formation during annealing. To achieve this require-ment, a highly pre-heated furnace is moved towards thesample as mentioned above. This working setup increasedthe heating ramp to 160 8Cmin�1. When the temperaturereached a pre-defined value (600 8C), it was kept constantduring phase transformation, i.e. until the film took a lightyellowish colour; the furnace was then moved away from thesample and the gas flow increased to allow a quick coolingdown of the film. In these annealing conditions, it was foundthat the size of cracks formed during annealing not onlydepended on heating and cooling rate but also on theannealing time, as shown in Fig. 2d and e. Cracks in the filmcould be greatly reduced by short annealing time providingthat the annealing temperature was increased. Our resultsindicated that the SCO phase was obtained after 4 h

Figure 1 Comparison of the optical transmittance spectra of as-deposited Ba-doped Sr–Cu–O films (a) and of Ba-doped SCO filmannealed in conventional furnace for long annealing time of 4 h at600 8C (b). The substrate was SK5 glass. The morphology of theannealed film is visible in the inset.

Figure 2 Comparison of the film morphology of as-deposited Ba-doped Sr–Cu–O films (a), of films annealed at 600 8C usingconventional annealing for long time of 4 h (b), using RTA (c), andof films annealed using conventional annealing with fast heatingand cooling rates for 2 h at 620 8C (d), for 5min at 650 8C (e). Filmsin (b) and (c) were deposited on Si substrate, for the other films thesubstrate was SK5 glass.

Phys. Status Solidi A 210, No. 12 (2013) 2571

www.pss-a.com � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

annealing at 600 8C or 2 h at 620 8C or only 5min at 650 8C.Annealing at temperature higher than 650 8C was notpossible because this temperature is very close to thetransformation temperature of our substrates. On the otherhand, a risk to use high annealing temperature was thediffusion of Sr into the substrate, which was indirectlyobserved with the appearance of metallic copper Cu0 in thefilm. Sr diffusion into the glass substrate was confirmedby the energy dispersive X-ray (EDX) analysis of the film-substrate interface in the cross-sectional mode usingtransmission electron microscope (TEM). The presence ofCuO in the film results in a decrease of the SCO chemicalpurity on one hand, and a degradation of the opticaltransparency of the film on the other hand. It is to be noticedthat, when successful, conventional annealing of 5min at650 8C is very interesting in comparison with annealingtimes of several hours reported in literature to obtain theSCO phase [5, 22, 31, 33, 34]. Such a short time reduces theprocessing cost. Raman spectra collected on the as-depositedfilm as well as on films annealed at 650 8C under N2 for5min and for 3min are shown in Fig. 3a, b, and c,respectively. As reported in our previous paper [30], the as-deposited film (Fig. 3a) is composed of CuO (peaks at 296,346 and 629 cm�1 [35, 36] and SrCO3 (peaks at 180, 700,1070 cm�1 [37]. In Fig. 3b, related to the film annealed for5min, all the peaks pointed at 139, 157, 182, 240, 290, 520and 576 cm�1 can be associated to the SCO phase as alreadyobserved in SCO ceramics [38] and in a polycrystalline SCOtarget [39]. When the film was annealed for only 3min, itsRaman spectrum in Fig. 3c is composed of different peaks at262, 540, 785 and 920 cm�1 assigned to the intermediatephase SrCuO2 [40–42] and at 146, 190, 218, 298, 414, 493,and 632 cm�1 corresponding to the Cu2O phase [38, 43–45].

The diffraction patterns of the as-deposited and annealedfilms are presented in Fig. 4. As expected, the identified

phases in the as-deposited films are SrCO3 and CuO(Fig. 4a). When annealed under N2 at 650 8C for 3min inconventional furnace (Fig. 4b), the SrCuO2 and Cu2Ointermediate phases were detected with small traces of CuO.These phases form after decomposition of SrCO3 under lowoxygen partial pressure (i.e. in N2 environment). However, ifthe annealing time is increased to 5min (Fig. 4c), all theinitial phases disappear and the SrCuO2 and Cu2Ointermediate phases transform into the pure SCO phase.

From our previous results [3, 5, 30], the formationpathway can be explained as follows: the 3min annealingcharacterizes the intermediate state, SrCO3 decomposes intoSrO and CO2 [46] (the non-assigned line close to 2782u inFig. 4b could be related to SrO); then, SrO reacts with CuOto form SrCuO2 and CO2 reacts itself with residual C to formCO creating a reducing atmosphere which allows totransform the remaining CuO into Cu2O. This transforma-tion pathway with SrCuO2 as intermediate phase was alsoobtained on Si substrates with RTA performed under Ar [3],together with other intermediate phases like Cu0 and Cu2O.The main difference with the present study is that for filmannealing on glass substrate we used N2 atmosphere. In theseconditions, Cu2O is obtained as an other intermediate phase,but without the presence of metallic Cu0. This differentbehaviour is related to the higher oxygen residual partialpressure in N2 atmosphere. The 5min annealing results in thecomplete decomposition and reduction of the initial phasesand SrO and Cu2O react to form SrCu2O2 (SCO). Thus,SrCuO2 can be considered as an intermediate phase betweenthe mixture CuO–SrCO3 and SCO in the chemical phasetransformation pathway. From these results it can be inferredthat the Ba content in the film does not introduce othersecondary phases.

In order to prevent the formation of cracks duringannealing, films of higher thickness were also deposited and

Figure 3 Raman spectra of as-deposited Ba-doped Sr–Cu–O films(a), and of films annealed in N2 atmosphere with fast heating andcooling rates, at 650 8C for 5min (b), and at 650 8C for 3min (c).

Figure 4 XRD pattern of as-deposited Ba-doped Sr–Cu–O films(a), and of films annealed in N2 atmosphere with fast heating andcooling rates, at 650 8C for 3min (b), and at 650 8C for 5min (c).

2572 A. Khan et al.: Electrical and optical properties of Ba-doped SrCu2O2 thin films

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

ph

ysic

a ssp stat

us

solid

i a

annealed. However, the film morphology analysis indicatedthat crack formation was not thickness dependent and thatthe annealing time to obtain the pure SCO phase wasincreased for thicker films.

Optical transmittance spectra of the SK5 glass substrate,of a film annealed at 650 8C during 5min and of thefilmþ substrate together are reported in Fig. 5a, b, and c,respectively. In order to get true value of the filmtransmittance, the substrate absorbance was subtracted fromthe filmþ substrate absorbance measured for the filmdeposited on the substrate. The film transmittance thusobtained at the middle of the visible range (550 nm) wasfound to be 75%, which gives evidence that the filmtransmittance is increased when cracks in the film arereduced. It is also to be noted that thicker Ba-SCO films areless transparent than thinner films. After subtraction of thesubstrate absorbance, (ahn)2 was plotted versus the photonenergy hn, where a is the film absorption coefficientdetermined through the expression [a¼ (1/d) ln(1/T)] whered is the film thickness and T the transmittance; the plot isshown in Fig. 5, inset. The extrapolation of the linear part ofthe graph to (ahn)2¼ 0 axis allows to obtain the bandgapvalue Eg of the film from the Tauc relation (ahn)2¼A(hn�Eg), applied to direct bandgap materials, where A is afrequency-independent constant [19]. The direct bandgap ofSCO was estimated to be close to 3.25 eV from this plot.

SCO is considered as a p-type TCO, therefore theelectrical properties of the Ba-SCO film deposited on glasssubstrate were measured by Van der Pauw method. Due tothe crack formation observed with conventional annealing,the measurement is only possible for films annealed at650 8C with high heating and cooling rates. Its electricalconductivity at room temperature was found to be4.3� 10�2 S cm�1, however, the p-type behaviour couldnot be verified. Exact values of the mobility and carrierconcentration could not be determined accurately due to the

limitation of our experimental setup at low electricalconductivity.

However, the optical and electrical properties of the Ba-doped SCO thin films thus obtained after thermal annealingof Ba-doped Sr–Cu–O films deposited by MOCVD are ingood agreement with data related to undoped, K-doped andCa-doped SCO thin films deposited by other techniques [18,19, 21, 22].

4 Conclusions Ba-doped SCO thin films have beenobtained after deposition by injection MOCVD on glasssubstrate followed by an improved conventional annealing at650 8C for 5min in optimized conditions of heating andcooling rates. In these conditions, the chemical phasetransformation pathway to obtain the SCO phase wascharacterized by the formation of an intermediate SrCuO2

phase after complete decomposition of the initial phases.Moreover, crack-free films were obtained which allows theachievement of good optical and electrical properties quitesimilar to those obtained with undoped, K-doped and Ca-doped SCO thin films deposited by other techniques, with aconductivity at room temperature of 4.3� 10�2 S cm�1, anda transmittance at 550 nm of 75%.

Acknowledgements This research work was supported bythe European Commission under a STREP project (NATCO FP6-511925).

References

[1] H. Hosono, Thin Solid Films 515(15), 6000 (2007).[2] L. Lin, F. Lai, Y. Qu, R. Gai, and Z. Huang,Mater. Sci. Eng. B

138(2), 166 (2007).[3] C. Millon, J. L. Deschanvres, C. Jiménez, N. Macsporran, B.

Servet, O. Durand, and M. Modreanu, Surf. Coat. Technol.201, 9395 (2007).

[4] J. L. Deschanvres, C. Jimenez, L. Rapenne, N. McSporran, B.Servet, O. Durand, and M. Modreanu, Thin Solid Films 516,1461 (2008).

[5] J. L. Deschanvres, C. Millon, C. Jiménez, A. Khan, H.Roussel, B. Servet, O. Durand, and M. Modreanu, Phys.Status Solidi A 205(8), 203 (2008).

[6] Z. Ji, J. Xi, L. Huo, and Y. Zhao, Phys. Status Solidi C 5(10),3364 (2008).

[7] M. L. Liu, F. Q. Huang, and L. D. Chen, Key Eng. Mater.368–372, 666 (2008).

[8] D. M. Fernandes, R. Silva, A. A. W. Hechenleitner, E.Radovanovic, M. A. C. Melo, and E. A. G. Pineda, Mater.Chem. Phys. 115(1), 447 (2009).

[9] D. Louloudakis, M. Varda, E. L. Papadopoulou, M.Kayambaki, K. Tsagaraki, V. Kambilafka, M. Modreanu,G. Huyberechts, and E. Aperathitis, Phys. Status Solidi A207(7), 1726 (2010).

[10] B. G. Lewis and D. C. Paine, MRS Bull. 25(8), 22 (2000).[11] A. N. Banerjee and K. K. Chattopadhyay, Prog. Cryst. Growth

Charact. Mater. 50(1–3), 52 (2005).[12] H. Ohta and H. Hosono, Mater. Today 7(6), 42 (2004).[13] H. Sato, T. Minami, S. Takata, and T. Yamada, Thin Solid

Films 236(1–2), 27 (1993).

Figure 5 Optical transmittance spectra of the SK5 glass substrate(a), of the SCO film without substrate (b), and of the SCO film alongwith substrate (c). The inset shows a plot of (ahn)2 versus photonenergy hn, for SCO bandgap estimation.

Phys. Status Solidi A 210, No. 12 (2013) 2573

www.pss-a.com � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

[14] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi,and H. Hosono, Nature 389, 939 (1997).

[15] H. Kawazoe, H. Yanagi, K. Ueda, and H. Hosono, MRS Bull.25(8), 28 (2000).

[16] H. Hosono, H. Ohta, K. Hayashi, M. Orita, and M. Hirano, J.Cryst. Growth 237–239, 96 (2002).

[17] X. Nie, S.-H. Wei, and S. B. Zhang, Phys. Rev. B 65(7),075111 (2002).

[18] E. Bobeico, F. Varsano, C. Minarini, and F. Roca, Thin SolidFilms 444(1–2), 70 (2003).

[19] S. Sheng, G. Fang, C. Li, Z. Chen, S. Ma, L. Fang, and X.Zhao, Semicond. Sci. Technol. 21(5), 586 (2006).

[20] C. B. Alcock, and B. Li, J. Am. Ceram. Soc. 73(5), 1176(1990).

[21] R. O. Suzuki, P. Bohac, and L. J. Gauckler, J. Am. Ceram.Soc. 75(10), 2833 (1992).

[22] A. Martinson, and D. Ginley, J. Young Investigators, Biolog.Biomed. Sci. 10(3) (2004), http://legacy.jyi.org/volumes/volume10/issue/articles/martinson.html.

[23] V. Varadarajan, D. P. Norton, and J. D. Budai, Thin SolidFilms 488(1–2), 173 (2005).

[24] A. Kudo, H. Yanagi, K. Ueda, H. Hosono, H. Kawazoe, andY. Yano, Appl. Phys. Lett. 75(18), 2851 (1999).

[25] H. Ohta, K. Kawamura, M. Orita, and M. Hirano, Appl. Phys.Lett. 77(4), 475 (2000).

[26] H. Ohta, M. Orita, andM. Hirano, J. Appl. Phys. 89(10), 5720(2001).

[27] Y. Nakamura, Y. Yasuhiro, H. Yumiko, and F. Satoru, J. Eur.Ceram. Soc. 25(12), 2167 (2005).

[28] M. N. Elliott and S. D. Elliott, Chem. Mater. 20(17), 5522(2008).

[29] M. Nolan, Thin Solid Films 516(22), 8130 (2008).[30] A. Khan, C. Jiménez, O. Chaix-Pluchery, H. Roussel, and

J. L. Deschanvres, Thin Solid Films 541, 136 (2013).[31] A. Kudo, H. Yanagi, H. Hosono, and H. Kawazoe, Appl.

Phys. Lett. 73(2), 220 (1998).[32] E. L. Papadopoulou, Z. A. Viskadourakis, A. V. Pennos, G.

Huyberechts, and E. Aperathitis, Thin Solid Films 516(7),1449 (2008).

[33] B. Roy, A. Ode, D. Readey, J. Perkins, P. Parilla, C. Teplin, T.Kaydanova, A. Miedaner, C. Curtis, A. Martinson, T. Coutts,D. Ginley, and H. Hosono, National Center for Photovoltaicsand Solar Program ReviewMeeting, Denver, Colorado, 2003,http://www.osti.gov/bridge (identifier: NREL/CP-520-33595).

[34] J. W. Liu, S.-Ch. Lee, and C.-H. Yang, Mater. Trans. 49(7),1694 (2008).

[35] S. Guha, D. Peebles, and T. J. Wieting, Phys. Rev. B 43(16),13092 (1991).

[36] J. F. Xu, W. Ji, Z. X. Shen, W. S. Li, S. H. Tang, X. R. Ye,D. Z. Jia, and X. Q. Xin, J. Raman Spectrosc. 30(5), 413(1999).

[37] C.-C. Lin and L.-G. Liu, J. Phys. Chem. Solids 8(6), 977(1997).

[38] J. Even, L. Pedesseau, O. Durand, M. Modreanu, G.Huyberechts, B. Servet, and O. Chaix-Pluchery, Thin SolidFilms 541, 113 (2013).

[39] M. Modreanu, M. Nolan, S. D. Elliott, O. Durand, B. Servet,G. Garry, H. Gehan, G. Huyberechts, E. L. Papadopoulou, M.Androulidaki, and E. Aperathitis, Thin Solid Films 515(24),8624 (2007).

[40] M. V. Abrashev, A. P. Litvinchuk, C. Thomsen, and V. N.Popov, Phys. Rev. B 55(14), 9136 (1997).

[41] Z. V. Popovic, M. J. Konstantinovic, R. Gajic, C. Thomsen,U. Kuhlmann, and A. Vietkin, Physica C 351, 386 (2001).

[42] X. Zhou, M. Cardona, W. Konig, J. Zegenhagen, and Z. X.Zha, Physica C 282–287(Part 2), 1011 (1997).

[43] H. Solache-Carranco, G. Juárez-Díaz, M. Galván-Arellano, J.Martínez-Juárez, G. Romero-Paredes, and R. Peña-Sierra, 5thInternational Conference on Electrical Engineering, Comput-ing Science and Automatic Control, Mexico City, Mexico,2008 (Institute of Electrical and Electronics Engineers (IEEE),(2008), pp. 421–424.

[44] A. Compaan and H. Z. Cummins, Phys. Rev. B 6(12), 4753(1972).

[45] P. Y. Yu, and Y. R. Shen, Phys. Rev. Lett. 32(7), 373(1974).

[46] T. Takamory and J. J. Boland, J. Appl. Phys. 64, 2130(1988).

2574 A. Khan et al.: Electrical and optical properties of Ba-doped SrCu2O2 thin films

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

ph

ysic

a ssp stat

us

solid

i a