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A visible transparent electroluminescent europium doped gallium oxide device

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Beta phase gallium oxide thin films deposited by pulsed laser deposition are efficient hosts for rare earth metals such as europium. In this study europium doped gallium oxide deposited on glass substrates is used to make red (611nm)
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  Materials Science and Engineering B 146 (2008) 252–255 A visible transparent electroluminescenteuropium doped gallium oxide device P. Wellenius a , A. Suresh a , J.V. Foreman b ,H.O. Everitt b , J.F. Muth a , ∗ a  Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27606, United States b  Department of Physics, Duke University, Durham, NC 27708, United States Abstract Beta phase gallium oxide thin films deposited by pulsed laser deposition are efficient hosts for rare earth metals such as europium. In thisstudy europium doped gallium oxide deposited on glass substrates is used to make red (611nm) electroluminescent devices that are transparentto the visible spectrum. The conducting electrodes used are indium tin oxide (ITO), and a novel indium gallium zinc oxide (IGZO) layer alsodeposited by pulsed laser deposition. The srcin of the red emission is the  5 D 0  to  7 F 2  transition and is consistent with photoluminescence andcathodoluminescence results. The turn on voltage of the device is about 45V ac, and the device appears to be robust, operating at elevated voltageswithout degradation.© 2007 Elsevier B.V. All rights reserved. Keywords:  Electroluminescence; Gallium oxide; Europium; Phosphor 1. Introduction There is a long history of the investigation of phosphormaterials with applications ranging from fluorescent lighting,to cathode ray tubes, to electroluminescent devices for displayapplications [1]. Typically for electroluminescent based devices sulfide based phosphors such as ZnS:Mn are widely used due totheir high efficiency. However in comparison with oxide basedphosphors, sulfide based materials have poor chemical stabilitywithregardtomoistureandobtainingefficientredemissionfromsulfidebasedmaterialshasbeendifficult[2].Thepoorefficiency of oxide based materials has been attributed to low hot electroncurrent densities but Xiao et al. [3] has pointed out that in oxide materials with an open tunnel crystal structure such as galliumoxide the electron transport is promoted resulting in higher hotelectron current densities such that efficient optical emissioncan be obtained. In the past few years there has been increasinginterest in the use of wide band gap oxides such gallium oxideas a host material for rare earth dopants [4–6]. Initial demon- strations of electroluminescent based devices using Eu:Ga 2 O 3 ∗ Corresponding author. Tel.: +1 919 513 2982.  E-mail address:  muth@unity.ncsu.edu (J.F. Muth). powders [4] or Eu:Ga 2 O 3  deposited by spray pyrolysis [7] have been successful.Inthisstudy,electroluminescentdeviceshavebeenfabricatedby pulsed laser deposition using a europium doped beta phasegalliumoxidelightemittinglayerandanovelamorphousindiumgallium zinc oxide (IGZO) top contact. This choice of materi-als has produced a functional red light emitter on glass that isoptically transparent throughout the visible spectrum. 2. Experimental procedure The pulsed laser deposition (PLD) target for the phosphorlayer was prepared by mixing powders of europium oxide andgallium oxide and then pressing the mixture to 5000psi. Thepressed disk is sintered for 5h at 1400 ◦ C, resulting in substan-tial densification in the case of gallium oxide targets. For thisstudy, targets were prepared with 2.4mol% europium oxide. Inprevious studies emission from the europium dopant had beenobserved with lower concentrations as well, but increased withhigher concentrations [6].Thephosphorlayerwasdepositedontoacommerciallyavail-able substrate that is composed of an aluminum oxide–titaniumoxide dielectric superlattice deposited on top of a crystallineindium tin oxide contact layer on Corning 7059 glass, as 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.mseb.2007.07.060  P. Wellenius et al. / Materials Science and Engineering B 146 (2008) 252–255  253Fig. 1. A schematic diagram of the thin film electroluminescent (TFEL) devicestructure as deposited by PLD onto the ATO/ITO/glass substrate. shown schematically in Fig. 1. The 250nm thick gallium oxide layer was deposited in 5 × 10 − 3 Torr oxygen partial pressurewith the substrate having been gradually heated to 850 ◦ C toobtain the beta phase. The high temperatures do not appearto have affected the substrates, as confirmed both by electro-luminescent device operation and also by capacitance–voltagemeasurements. The IGZO top contact is an amorphous trans-parent material [8] that has also been used in the fabrication of  amorphous thin film transistors [9]. The 200nm thick contact was deposited at room temperature and also in 5 × 10 − 3 Torroxygen partial pressure. To simplify fabrication, devices werepatterned using a micromachined silicon shadow mask duringdeposition.A Lambda Physik KrF excimer laser emitting at 248nm wasused to ablate the targets at a pulse rate of 10Hz. The laserenergy was 250 and 150mJ per pulse for the gallium oxide andthe IGZO layers, respectively. The substrates were rotated topromote uniform deposition while the targets were rastered androtated to avoid pitting.Several spectral techniques were employed to characterizethe devices, including electro-, cathodo- and photolumines-cence. Cathodoluminescence (CL) and spectral electrolumi-nescence (EL) measurements were obtained using an OxfordInstruments MonoCL 0.25 meter monochromator apparatusattachedtoaJEOL6400scanningelectronmicroscope.Thedis-persed light is detected using a Peltier cooled photomultipliertube (PMT) sensitive from 190 to 930nm. CL measurementswere obtained using a 5keV electron beam. Integrated elec-troluminescent intensity measurements were obtained usinga Hamamatsu PMT. The pulsed laser source for the time-integrated photoluminescence measurements was an opticalparametric amplifier (OPA). The OPA was pumped by a 1kHzregenerativeamplifierseededbyan80MHzTi:sapphireoscilla-tor operating at 780nm. For time-integrated photoluminescenceexcitation,theOPAwastunedto257nm.Thedatawereanalyzedwith a 0.3m focal length Acton spectrometer and detected bya liquid nitrogen cooled Princeton Instruments CCD. The exci-tation was attenuated using a series of metallic neutral densityfilters.Devices were driven using a homemade high voltage ampli-fier circuit, using an Apex PA97 operational amplifier. Awaveform generator was used as input for the amplifier. Volt-ages discussed are given as the half-wave amplitude of an acsignal. Fig. 2. Transmission spectrum of the complete device stack showing about70% transparency throughout the visible region of the spectrum. The inset isa photograph of device in operation. 3. Results and discussion Highly transparent red planar light emitters were fabricatedas described. The transmission spectrum is shown in Fig. 2, and demonstrates the high transparency through the visible spec-trum. An image of a working device is shown in the inset,demonstratingtheredemission.AcharacteristicELspectrumisshown in Fig. 3 along with a CL spectrum for comparison. Both Fig. 3. Cathodoluminescence (top) and electroluminescence (bottom) spectranormalized to the 611nm emission peak and vertically offset for visual clarity.Both techniques produced similar emission spectra.  254  P. Wellenius et al. / Materials Science and Engineering B 146 (2008) 252–255 spectra show the peak emission at 611nm that is attributed tothe  5 D 0  to  7 F 2  transition in the europium ion [6]. Other major peaks are observed at 578 and 589nm, likely corresponding totransitions from the  5 D 0  to the  7 F 0  and  7 F 1  states, respectively.The photoluminescence (PL) spectra shown in Fig. 4 agreewell with CL and EL data, though the emission lines from PLare narrower than either of the other two techniques. As thelaserpowerisincreased,the611nmemissionisfavoredoverthe615nmemission,whiletheotherpeakscontinuetoincreaseuni-formly. This is also seen by examining the electroluminescenceintensityversusvoltagecurveshowninFig.5whichindicatesthe lowerthresholdandhigherslopeofthe611nmlineascomparedto the other observed emission lines.Integrated intensity data is shown in Fig. 6 as a function of  devicebiasforseveralfrequenciesrangingfrom60Hzto2kHz.Thethresholdforinitiallightemissionisbetween40and45VasdemonstratedinFig.6withthemostefficientemissionforthese devices occurring at 2000Hz with applied voltages of 100V.The intensity discontinuity observed for 60, 100 and 200Hzexcitation is believed to be related to charge trapping, which atlow frequencies results in the accumulation of electrons. Whenthe polarity of signal is reversed and the field is sufficiently highthe electrons are swept from the traps resulting in an emissionsignal increase. The different emission thresholds for differenttransitions as a function of voltage is shown in Fig. 5.In general the devices appeared robust with no observabledegradation observed over the time periods of 3–4h that thedevices were tested in air. The devices also seem reliable in thatall the devices on a given substrate appeared to work with thesame characteristics. Fig. 4. Time-integrated photoluminescence spectra using a series of neutraldensity filters, ranging in optical densities (OD), to attenuate the source laserpower. Incident laser power with 0.0 OD was 2.2  J per pulse. Emission linesare narrower than EL or CL emission, but emission peak positions are in goodagreement. Note the relative increase in the 611nm line with increasing powerover the 615nm line.Fig. 5. Peak intensities of EL emission from the device as a function of bias.Notethe611and615nmemissionbegintoemitatalowervoltagethantheotherwavelengths.Fig. 6. Wavelength-integrated electroluminescent intensity of the device as afunction of ac bias voltage for a range of drive frequencies (60Hz–2kHz).Brightness increases strongly with drive frequency, though the 1 and 2kHzdata overlap considerably at these low output intensities. 4. Conclusions In conclusion, optically transparent electroluminescentdevices have been fabricated by pulsed laser deposition of europium doped gallium oxide thin films, using a novel amor-  P. Wellenius et al. / Materials Science and Engineering B 146 (2008) 252–255  255 phous indium gallium zinc oxide film as a conducting topcontact. EL emission was strongest at 611nm, correspondingto the  5 D 0  to  7 F 2  transition in europium and emission spectrawere also in good agreement with PL and CL measurements.The devices demonstrated an emission threshold between 40and 45V ac. Brightness increased substantially for higher drivefrequencies, with optimal emission near 2kHz for this devicegeometry. Acknowledgements John Muth would like to thank Henryk Tempkin at DARPAand the ONR Young Investigator Award Program for financialsupport of this work. References [1] W.M. Yen, S. Shionoya, H. Yamamoto, Phosphor Handbook, second ed.,CRC Press/Taylor and Francis, Boca Raton, FL, 2007.[2] T. Minami, Solid State Electron. 47 (2003) 2237–2243.[3] T.Xiao,A.H.Kitai,G.Liu,A.Nakua,J.Barbier,Appl.Phys.Lett.72(1998)3356–3358.[4] T. Miyata, T. Nakatani, T. Minami, J. Lumin. 87–89 (2000) 1183–1185.[5] J.H. Hao, M. Cocivera, J. Phys. D 35 (2002) 433–438.[6] P. Gollakota, A. Dhawan, P. Wellenius, L.M. Lunardi, J.F. Muth, Y.N. Sari-palli, H.Y. Peng, H.O. Everitt, Appl. Phys. Lett. 88 (2006) 221906.[7] J.H. Hao, Z.D. Lou, I. Renaud, M. Cocivera, Thin Solid Films 467 (2004)182–185.[8] A. Suresh, P. Gollakota, P. Wellenius, A. Dhawan, J.F. Muth, Thin SolidFilms, doi:10.1016/j.tsf.2007.03.153, in press.[9] A. Suresh, P. Wellenius, A. Dhawan, J. Muth, Appl. Phys. Lett. 90 (2007)123512.
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