BaCuQF (Q=S,Se) materials, candidate transparent p-type conductors, were prepared by solid-state reaction, and their bulk electrical and optical properties were evaluated. The room-temperature Seebeck coefficient and electrical conductivity of undoped BaCuQF pellets were +56 μV/K and 0.088 S/cm, respectively, for the sulfide fluoride, and +32 μV/K and 0.061 S/cm, respectively, for the selenide fluoride. The conductivity was greatly enhanced by the substitution of several percent of K for Ba; the highest conductivities were 82 S/cm for Ba₀.₉K₀.₁CuSF and 43 S/cm for Ba₀.₉K₀.₁SeF. The band gaps for Q=S and Q=Se were measured to be 3.2 and 3.0 eV, respectively. Undoped BaCuSF exhibits strong red luminescence near 630 nm under ultraviolet excitation.Article appears in Applied Physics Letters (http://apl.aip.org/) and is copyrighted by American Institute of Physics (http://www.aip.org/)

Keszler, Douglas A.

Park, Cheol-Hee

Park, Sangmoon

Tate, Janet

Yanagi, Hiroshi

Undetermined

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APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 17 28 APRIL 2003p-type conductivity in wide-band-gap BaCuQF „QÄS,Se…Hiroshi Yanagi and Janet Tatea)Department of Physics, Oregon State University, Corvallis, Oregon 97331Sangmoon Park, Cheol-Hee Park, and Douglas A. KeszlerDepartment of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003~Received 27 December 2002; accepted 10 March 2003!BaCuQF (Q5S,Se) materials, candidate transparentp- ype conductors, were prepared bysolid-state reaction, and their bulk electrical and optical properties were evaluated. Theroom-temperature Seebeck coefficient and electrical conductivity of undoped BaCuQF pellets were156mV/K and 0.088 S/cm, respectively, for the sulfide fluoride, and132mV/K and 0.061 S/cm,respectively, for the selenide fluoride. The conductivity was greatly enhanced by the substitution ofseveral percent of K for Ba; the highest conductivities were 82 S/cm for Ba0.9K0.1CuSF and 43 S/cmfor Ba0.9K0.1SeF. The band gaps for Q5S and Q5Se were measured to be 3.2 and 3.0 eV,respectively. Undoped BaCuSF exhibits strong red luminescence near 630 nm under ultravioletexcitation. © 2003 American Institute of Physics.@DOI: 10.1063/1.1571224#elaedo,lstict ireesttetalQrrey-e-drekxrethce.fble.ngeredac-lesenssewasthe-revercon-See-h aeTransparentp-type conducting oxides such as CuMO2(M5Al,Ga,In,Sc)1–4 and SrCu2O2 ~Ref. 5! have recently at-tracted attention as possible components of transparenttronic and optoelectronic devices. However, so far it appethat optimizing electrical conductivity in such Cu-basp-type oxides compromises the optical transparency tgreater extent than is the case forn-type conducting oxidesso that it seems sensible to search for alternate materiaband gap greater than about 3.18 eV is necessary for optransparency, and within this constraint, an improvemenhole mobility is desirable to improve conductivity. The mostrongly covalent nature of the CuuS bond compared to thCuuO bond should improve hole mobility, and suggeexamination of sulfides or other chalcogenide-based maals. BaCu2S2 has a hole mobility of 3.5 cm2/V s, which isindeed much higher than the mobilities of order 0.1 cm2/V sreported for most Cu-basedp-type transparent oxides, buthe band-gap energy is 2.3 eV.6 LaCuOS is also ap-typetransparent conducting material,7 but the mobility of0.5 cm2/V s is still rather low.8In 1994, Zhuet al.9 reported the synthesis and cryststructure of two copper chalcogenide fluorides of BaCu(Q5S,Se). Figure 1 shows the BaCuQF crystal structuwhich belongs to the space group P4/nmm. This is a layestructure with tetragonal symmetry, in which (Ba2F2)21 and(Cu2Q2)22 layers are alternately stacked along thec axis.The (Cu2Q2)22 layers, which can also be thought of as laers of edge-sharing CuS4 tetrahedra, have an obvious corrspondence to the BaCu2S2 structure, so that this is expecteto be the plane for hole conduction. The BaCuQF layestructure is the same as that of LaCuOS, which has stac(La2O2)21 and (Cu2S2)22 layers.10 The similarity is inter-esting, because LaCuOS is not only a transparentp-type con-ductor, but also an ultraviolet light emitter that exhibits ecitonic absorption and emission even at room temperatu11Experimental and theoretical analyses indicate that(Cu2S2)22 layers contribute to both the top of the valena!Electronic mail: tate@physics.orst.edu2810003-6951/2003/82(17)/2814/3/$20.00Downloaded 22 Jan 2010 to 128.193.163.2. Redistribution subject to AIPec-rsa. Aalnsri-Fe,dded-.eband and the bottom of the conduction band of LaCuOS12Zhu and co-workers9 reported semiconducting behavior oBaCuQF, but details were lacking, and other data availaare from a preliminary study by Yanagi and co-workers13Here, we report promisingp-type electrical conductivity ofbulk BaCuQF, a band gap of 3.0 eV or greater and strophotoluminescence of BaCuSF.Powder samples of undoped and K-doped BaCuQF wprepared by heating stoichiometric mixtures of BaQ~Cerac,99.5%!, Cu2Q ~Cerac, 99.5%!, BaF2 ~Cerac, 99.9%!, and KF~Alfa Aesar, 99%! at 450– 550 °C for 12 h in an evacuatesilica tube. The powders are generally stable in air, but retion is evident upon exposure to water. All of the sampwere characterized by powder x-ray diffraction on a SiemD5000 diffractometer; they were predominantly single phawith only trace amounts of unreacted BaF2 observed. Forconductivity measurements, each powder samplepressed into a pellet and annealed at 650 °C for 1 h in anevacuated silica tube. The pellets averaged 70%–80% oforetical density; as detailed in Table I.The electrical conductivities of the sintered pellets wemeasured from 300 to 77 K by a four-probe technique. Silpaste was used for electrodes and ohmic contact wasfirmed before detailed measurements were made. Thebeck coefficient was measured at room temperature wittemperature gradient of;5 K across the pellet. The diffusFIG. 1. Crystal structure of BaCuQF (Q5S,Se).4 © 2003 American Institute of Physics license or copyright; see http://apl.aip.org/apl/copyright.jspwblewe151/lieratiavhfofogheahav-dnce,e-cepta-sethema-Oent,x-ofoa–oinonga-SF-appedions offorat-orp-heeVwoo an0ffor2815Appl. Phys. Lett., Vol. 82, No. 17, 28 April 2003 Yanagi et al.reflectance at room temperature of powdered BaCuQFmeasured against MgO powder in the UV-visible region~300to 900 nm! by using an integrating sphere and a doumonochromator. Photoluminescence measurementsmade with an Oriel 300 Watt Xe lamp and Cary model-prism monochromator for excitation, and an Oriel 22500m monochromator and Hamamatsu R935 photomultiptube for detection. Each spectrum was corrected for thesponse of the system by using rhodamine B and a standized tungsten lamp.The temperature dependence of the electrical conducity of Ba12xKxCuSF and Ba12xKxCuSeF pellets is illustratedin Fig. 2. Undoped pellets of BaCuSF and BaCuSeF helectrical conductivities in the range of 1022 S/cm, and theconductivity increases with increasing K concentration. Troom-temperature conductivity of the pellets is 82 S/cmBa0.9K0.1CuSF and 43 S/cm for Ba0.9K0.1SeF. This comparesfavorably with the values of order 1–20 S/cm reportedother Cu-basedp-type transparent conductors, includinthe structurally similar LaCuOS12ySey system(1021– 100 S/cm).14 The temperature dependence of telectrical conductivity of Ba12xKxCuSF is semiconductingfor x,0.025, with an activation energy around 60 meV neTABLE I. Electrical properties~conductivity and Seebeck coefficient! anddensities of Ba12xKxCuQF (x50, 0.025, 0.05, 0.1, and Q5S,Se) pellets atroom temperature.Ba12xKxCuSF Ba12xKxCuSeFK ~at. %! 0 0.025 0.50 0.10 0 0.025 0.50 0.1s ~S/cm! 0.088 2.6 20 82 0.061 17 27 43S ~mV/K ! 156 1205 155 ,15 132 134 125 141Density ~%! 73 89 80 89 76 69 62 76FIG. 2. Electrical conductivity of Ba12xKxCuQF pellets as a function oreciprocal temperature for~a! Q5S, and~b! Q5Se. In each case,x50,0.025, 0.05, 0.1.Downloaded 22 Jan 2010 to 128.193.163.2. Redistribution subject to AIPasre8re-rd-v-eerrrroom temperature, and there is a crossover to metallic beior for x.0.05. The electrical conductivity of the undopeselenide fluoride has a very weak temperature dependeand all of the doped pellets exhibit essentially metallic bhavior. Seebeck measurements on all samples, exBa0.9K0.1CuSF, gave a positive sign indicating that these mterials arep type conductors at all doping levels. In the caof Ba0.9K0.1CuSF, the voltage was too small to measureSeebeck coefficient. The transport measurements are sumrized in Table I. BaCuSF thin films were deposited on Si2by thermal coevaporation. Undoped films are transparpolycrystalline, and insulating. K-doped films do not yet ehibit conductivities higher than the pellets.The diffuse reflectance of BaCuSeF~ ig. 3! is low atshort wavelengths and increases sharply at;400 nm. Theenergy band gap of 3.0 eV was estimated from a plot$(k/s)•hn%2 versushn, wherek ands denote absorption andscattering coefficients, andhn is the photon energy. The ratik/s was calculated from the reflectance via the KubelkMunk equation.15,16 This gap value should be sufficient ttransmit all but the deepest blue light. The band edgediffuse reflectance spectra of BaCuSF is obscured by stremission excited by ultraviolet light, but transmission mesurements on an undoped transparent thin film of BaCuyielded a value of 3.25 eV.13 As expected for a quasi twodimensional layered structure like BaCuQF, the band-gvalues are larger than for BaCu2S2 , which has a three-dimensional arrangement of the CuS4 tetrahedra.Excitation and emission spectra for undoped and doBaCuQF compositions are summarized in Fig. 3. Emissfrom each sample occurs in the orange-red to red portionthe spectrum. The onset of excitation near 425 nmBaCu0.975SeF agrees well with the reflectance data, indicing excitation and emission occurs through band-gap abstion of the compound rather than an impurity. Similarly, texcitation spectrum of BaCuSF is consistent with the 3.2band gap observed from thin-film transmission data.The emission from BaCuSF is characterized by tbroad peaks centered near 560 and 630 nm, giving rise tFIG. 3. ~Top! reflectance spectrum for BaCuSeF and excitation spectraBaCuQF and~bottom! emission spectra for BaCuQF. license or copyright; see http://apl.aip.org/apl/copyright.jspr 5snmlooerpenda-KaeVuna-evrH.ita,ett.ndys.pl.er.ys.olid2816 Appl. Phys. Lett., Vol. 82, No. 17, 28 April 2003 Yanagi et al.emission that appears orange to the eye. The peak neanm can be quenched by altering the synthesis conditionby selective doping. In the case of Mn doping, the 560band is completely quenched, giving an emission with cocoordinatesx50.66 andy50.34, similar to those (x50.65and y50.34) observed for the commercial red phosphY2O3:Eu. A more detailed accounting of the optical propties of these materials will be given elsewhere.In summary, we have prepared undoped and K-doBaCuQF, which is a promising transparentp- ype conductor.The electrical conductivity was controlled by K doping athe bulk conductivity is 82 S/cm for sintered pellets of BCuSF:K and 43 S/cm for BaCuSeF:K at a doping level ofof 10 at. %. The Seebeck coefficients were positive, indicing that undoped and K-doped BaCuQF arep-type conduc-tors. The optical band gap of BaCuQF is larger than 3and strong luminescence is observed near 630 nm.This work was supported by the National Science Fodation ~Grant No. DMR 0071727!, by the Army ResearchOffice ~MURI E-18-667-G3!, and by the Research Corportion. The authors thank Professor David McIntyre and LKilcher for assistance with the diffuse reflectance measuments.Downloaded 22 Jan 2010 to 128.193.163.2. Redistribution subject to AIP60orrr-dt-,-ie-1H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, andHosono, Nature~London! 389, 939 ~1997!.2K. Ueda, T. Hase, H. Yanagi, H. Kawazoe, H. Hosono, H. Ohta, M. Orand M. Hirano, J. Appl. Phys.89, 1790~2001!.3H. Yanagi, T. Hase, S. Ibuki, K. Ueda, and H. Hosono, Appl. Phys. L78, 1583~2001!.4N. Duan, A. W. Sleight, M. K. Jayaraj, and J. Tate, Appl. Phys. Lett.77,1325 ~2000!.5A. Kudo, H. Yanagi, H. Hosono, and H. Kawazoe, Appl. Phys. Lett.73,220 ~1998!.6S. Park, D. A. Keszler, M. M. Valencia, R. L. Hoffman, J. P. Bender, aJ. F. Wager, Appl. Phys. Lett.80, 4393~2002!.7K. Ueda, S. Inoue, S. Hirose, H. Kawazoe, and H. Hosono, Appl. PhLett. 77, 2701~2000!.8H. Hiramatsu, M. Orita, M. Hirano, K. Ueda, and H. Hosono, J. ApPhys.91, 9177~2002!.9W. J. Zhu, Y. Z. Huang, F. Wu, C. Dong, H. Chen, and Z. X. Zhao, MatRes. Bull.29, 505 ~1994!.10M. Palazzi, Acad. Sci., Paris, C. R.292, 789 ~1981!.11K. Ueda, S. Inoue, H. Hosono, N. Sarukura, and M. Hirano, Appl. PhLett. 78, 2333~2001!.12S. Inoue, K. Ueda, and H. Hosono, Phys. Rev. B64, 245211~2001!.13H. Yanagi, S. Park, A. D. Draeseke, D. A. Keszler, and J. Tate, J. SState Chem.~in press!.14K. Ueda and H. Hosono, J. Appl. Phys.91, 4768~2002!.15P. Kubelka and F. Munk, Zh. Tekh. Fiz.12, 593 ~1931!.16P. Kubelka, J. Opt. Soc. Am.38, 448 ~1948!. license or copyright; see http://apl.aip.org/apl/copyright.jsp

p-type conductivity in wide-band-gap BaCuQF (Q=S,Se)

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p-type conductivity in wide-band-gap BaCuQF (Q=S,Se)

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