This work was funded by the Portuguese Science Foundation (FCT-MCTES) through projects PTDC/CTM/73943/2006, PTDC/EEA-ELC/64975/2006, ERC 2008 Advanced Grant (INVISIBLE Contract No. 228144) and IT R&D program of MKE (Contract No. 2006-S079-03, Smart window with transparent electronic devices) from ETRI Korea. We thank K. Nomura for the Seebeck measurements and N. Franco for the XRD analysis.Copper oxide (Cu2 O) thin films were used to produce bottom gate p-type transparent thin-film transistors (TFTs). Cu2 O was deposited by reactive rf magnetron sputtering at room temperature and the films exhibit a polycrystalline structure with a strongest orientation along (111) plane. The TFTs exhibit improved electrical performance such as a field-effect mobility of 3.9 cm2 /V s and an on/off ratio of 2× 102.publishersversionpublishe

Barquinha, Pedro Miguel Cândido

Barros, Raquel

Elamurugu, Elangovan

Figueiredo, Vitor

Fortunato, Elvira Maria Correia

Gonçalves, Gonçalo

Hwang, Chisun

Martins, Rodrigo Ferrão de Paiva

Park, Sang Hee Ko

Applied Physics Letters

English

Repositório da Universidade Nova de Lisboa

Thin-film transistors based on p-type  thin films produced at room temperatureElvira Fortunato, Vitor Figueiredo, Pedro Barquinha, Elangovan Elamurugu, Raquel Barros, GonçaloGonçalves, Sang-Hee Ko Park, Chi-Sun Hwang, and Rodrigo MartinsCitation: Appl. Phys. Lett. 96, 192102 (2010); doi: 10.1063/1.3428434View online: http://dx.doi.org/10.1063/1.3428434View Table of Contents: http://aip.scitation.org/toc/apl/96/19Published by the American Institute of PhysicsArticles you may be interested in-channel thin-film transistor using -type oxide semiconductor, SnOApplied Physics Letters 93, 032113 (2008); 10.1063/1.2964197Epitaxial growth of high mobility  thin films and application to -channel thin film transistorApplied Physics Letters 93, 202107 (2008); 10.1063/1.3026539Transparent p-type  thin film transistors produced by reactive rf magnetron sputtering followed by lowtemperature annealingApplied Physics Letters 97, 052105 (2010); 10.1063/1.3469939Fabrication of p-channel thin-film transistors using CuO active layers deposited at low temperatureApplied Physics Letters 97, 222109 (2010); 10.1063/1.3521310Sputtering formation of -type SnO thin-film transistors on glass toward oxide complimentary circuitsApplied Physics Letters 97, 072111 (2010); 10.1063/1.3478213p-channel thin-film transistors based on spray-coated Cu2O filmsApplied Physics Letters 102, 163505 (2013); 10.1063/1.4803085Thin-film transistors based on p-type Cu2O thin films producedat room temperatureElvira Fortunato,1,a Vitor Figueiredo,1 Pedro Barquinha,1 Elangovan Elamurugu,1Raquel Barros,1,2 Gonçalo Gonçalves,1 Sang-Hee Ko Park,3 Chi-Sun Hwang,3 andRodrigo Martins11Departamento de Ciência dos Materiais, CENIMAT/I3N, Faculdade de Ciências e Tecnologia, FCT,Universidade Nova de Lisboa and CEMOP-UNINOVA, 2829-516 Caparica, Portugal2Materiais Avançados, INNOVNANO, SA, 7600-095 Aljustrel, Portugal3Electronic and Telecommunications Research Institute, 138 Gajeongro, Yuseong-gu, Daejeon, 305-700,Republic of KoreaReceived 21 March 2010; accepted 18 April 2010; published online 10 May 2010Copper oxide Cu2O thin films were used to produce bottom gate p-type transparent thin-filmtransistors TFTs. Cu2O was deposited by reactive rf magnetron sputtering at room temperature andthe films exhibit a polycrystalline structure with a strongest orientation along 111 plane. The TFTsexhibit improved electrical performance such as a field-effect mobility of 3.9 cm2 /V s and anon/off ratio of 2102. © 2010 American Institute of Physics. doi:10.1063/1.3428434Solid state devices based on cuprous oxide Cu2O semi-conductors are known for more than 80 years even before theera of Si devices. Rectifier diodes based on this semiconduc-tor were used industrially as early as 1926 Ref. 1 and mostof the theory of semiconductors was developed based onCu2O devices.2–4 Besides that, Cu2O was regarded as one ofthe most promising materials for application in solar cells5–7due to its high-absorption coefficient in the visible region,nontoxicity, abundant availability, and low-cost production.8Cu2O has a cubic structure with a direct band gap of 2.1 eVRef. 9 and shows p-type conductivity  with hole mobil-ity H exceeding 100 cm2 /V s.10 The p-type character ofCu2O is attributed to the presence of negatively charged cop-per vacancies VCu, which introduce an acceptor level about0.3 eV above the valence band VB.11 It was also proposedby some authors the coexistence of both acceptor and donorintrinsic levels with a ratio slightly larger than 1 and lessthan 10.12,13 The nature of the donor levels is not completelyclear being even controversial where the simplest candidatesare oxygen vacancies.14 In contrast to the majority of metaloxides, in which the top of VB is mainly formed from local-ized and anisotropic O 2p orbitals, which leads to a low holemobility due to hopping conduction, here the top of the VB iscomposed of fully occupied hybridized orbitals Cu 3d10and O 2p with Cu d sates dominating the top of the VB.Figure 1a illustrates the chemical bond between an oxideion and a cation that has a closed-shell electronic configura-tion. Figure 1b shows a pictorial representation of the moreimportant defects in Cu2O and Fig. 1c reveals the simpleelectronic model proposed by Brattain,12 consistingof a compensated semiconductor with one acceptor level at0.3 eV and a deep donor level at 0.9 eV from VB.The development of p-n junctions and p-type thin-filmtransistors TFTs is a major goal, as this would lead to thefabrication of complementary metal oxide semiconductorCMOS structures where both n- and p-type transistors areneeded. The interest in this field was renewed in 1997 whenKawazoe et al.15 reported p-type conductivity in a highlytransparent thin film of copper aluminum oxide.Despite the fact that high quality p-type Cu-based oxidesemiconductor thin films have already been achieved,10,16–18so far there is only one report on p-type Cu2O TFTs Ref. 18but without satisfactory requirements for practical applica-tions.In this work, we report p-type Cu2O TFTs deposited byreactive rf magnetron sputtering at room temperature RT,with the final devices requiring annealing temperatures ofonly 200 °C.Cu2O films were grown by reactive rf magnetron sput-tering at RT in a homemade system. A 2 diameter Cu metaltarget with 99.997% purity was used at 15 cm from thesubstrate soda-lime glass, at a base pressure of 3.410−4 Pa. The deposition pressure PAr+PO2 and the rfpower were 0.6 Pa and 50 W, respectively. To evaluate theoptimal growing conditions for p-type Cu2O films, the oxy-gen partial pressure OPP=PO2 /PAr+PO2 was varied be-aAuthor to whom correspondence should be addressed. Electronic mail: elvira.fortunato@fct.unl.pt. Tel.: 351 212948562. FAX: 351 212948558.FIG. 1. Color online a Chemicalbond between an oxide ion and a cat-ion that has a closed-shell electronicconfiguration; b a pictorial represen-tation of the more important defects inCu2O; and c a simple electronicmodel proposed by Brattain, with acompensated semiconductor with oneacceptor level at 0.3 eV and a deepdonor level at 0.9 eV from VB.APPLIED PHYSICS LETTERS 96, 192102 20100003-6951/2010/9619/192102/3/$30.00 © 2010 American Institute of Physics96, 192102-1tween 0% and 100%, fixing the film thickness at 200 nm,measured with a surface profilometer. The films’ structurewas analyzed by x-ray diffraction XRD experiments ingrazing incidence geometry using Cu K1,2 lines. The opticaltransmittance measurements were performed with a doublebeam spectrophotometer in the wavelength from 200 to2500 nm. Atomic force microscopy AFM was done in or-der to investigate the surface topology, operated in ac mode.Electrical transport properties were examined by Hall Effectand  measurements as a function of absolute temperatureT using samples with the van der Pauw and planar gapconfigurations, respectively.Bottom-gate TFTs were fabricated using 40 nm thickCu2O thin films as semiconductors. For the gate dielectric anengineered insulator consisting of a superlattice of Al2O3 andTiO2 ATO19 with a thickness of 220 nm deposited on aglass coated with a 200 nm thick indium tin oxide ITO filmused as the gate electrode. The source-drain electrodes con-sist of an indium zinc oxide IZO film with 250 nm pro-duced by rf magnetron sputtering at RT.20,21 Both the semi-conductor and the source-drain electrodes were patterned bylift-off yielding TFTs with a width-to-length ratio W/L of3.3, with L=15 m. The final devices were annealed at200 °C for 1, 5, and 10 h in air using a tubular furnace andthe electrical characterization was performed with an Agilent4155C semiconductor parameter analyzer and a Cascade Mi-crotech M150 microprobe station inside a dark box at ambi-ent atmosphere.Typical XRD patterns obtained from films annealed for10 h is comparatively shown in Fig. 2 with the one of as-deposited films. It is found that the annealing is not effectivein inducing any change in the Cu2O phase. However, theincreasing intensity of the strongest orientation along 111with the increase in annealing time has confirmed the im-provement in crystallinity of the as-deposited films. Thegrain size dg inferred from Scherrer formula22 for as-deposited films is 8.30 nm, increasing with the annealingtime to a maximum of 15.72 nm for films annealed for 10 h.To substantiate the increase in dg the films were analyzedby AFM and the obtained microstructures are shown as aninset in Fig. 2. The surface roughness increased from 4.6 to10.4 nm corroborating the dg obtained from XRD data.The dependence of  on T is shown in Fig. 3 for thefilms produced with 9% OPP. These films exhibit the Cu2Ophase and typical semiconductor behavior. From the experi-mental data we notice that T data are fitted by two con-duction paths controlled, respectively, by an acceptor energylevel EA and by phonons of energy Wph to which activationenergies of about 0.15 eV and 0.03 eV are, respectively, as-sociated. Taking into account the equation inside Fig. 3,23where the Fermi energy level EF is assumed to be aroundthe middle of energy gap between EA and the top energy ofVB, we estimate EA2E2EF−EV0.30 eV and Wph0.06 eV, which is in agreement with the conduction pathmodel proposed by other authors.12,23–25 A p-type resistivity of 2.7103  cm, H0.65 cm2 /V s and a carrierconcentration N of 3.71015 cm−3 were obtained at RT.The p-type conduction was also confirmed by Seebeck mea-surements performed in the same sample. After annealing at200 °C for 10 h,  and H increase to 1104  cm and18.5 cm2 /V s, respectively, while N decreases to valuesaround 31013 cm−3.The optical transmittance spectrum through the entireCu2O-TFT in the wavelength range between 200 and 2500nm is shown in Fig. 4. The average optical transmission inthe visible part of the spectrum is 80% while at 550 nmmaximum sensitivity for the human eye is 85%, which in-FIG. 2. Color online X-ray pattern of the Cu2O film annealed at 200 °C inair for 10 h in comparison with the as-deposited film. Symbol representa-tions: –Cu2O phase and –CuO phase. The insets show the AFM imagesof the corresponding films.FIG. 3. Temperature dependence of dark conductivity for a Cu2O thin filmwith 270 nm. The equation represents the conduction mechanisms for thistype of semiconductors. For higher temperatures the conductivity is con-trolled by an acceptor center located 0.3 eV above the top of VB VCu whilefor low temperatures the conductivity is controlled by hopping.FIG. 4. Color online Optical transmission spectra for the entire Cu2O-TFTstructure including the glass substrate; it is possible to observe that theCu2O-TFT is fully transparent to visible light. For comparison purposes, theoptical transmission spectra for the glass substrate, the ITO, and the ITO/ATO films are also presented.192102-2 Fortunato et al. Appl. Phys. Lett. 96, 192102 2010dicates that transmission losses due to the Cu2O-TFTs arenegligible 7% when compared to the uncoated glass sub-strate. The lower transmittance at the near infrared region isassociated to the large N of the ITO and IZO films whichshifts the infrared absorption edge toward the visible, beingthis shift determined by the plasma oscillation of the freecarriers.26 The optical band gap Eop for direct allowed tran-sitions of the Cu2O thin films was calculated using Tauc’slaw as follows: 2= h	−Eop, where  represents the ab-sorption coefficient, h the Planck’s constant, and 	 the pho-ton frequency. The obtained value was 2.39 eV, which isslightly higher than the conventional value of 2.1 eV forCu2O.27Figure 5a shows the output characteristics of a p-typeCu2O TFT. The drain-to-source voltage VDS is swept from+10 to 60 V and the gate-to-source voltage VGS isstepped between +5 and 55 V. The output characteristics ofthe device shows clear linear and saturation regions and doesnot present significant current crowding for low VDS, indi-cating low series resistance in source-drain contacts. Figure5b shows the transfer characteristics with VDS=−5 V forthe same device presented in Fig. 5a. VGS is swept between60 to +10 V.The on/off ratio is 2102, the peak value of the field-effect mobility FE, calculated by the transconductance inthe linear regime is around 3.9 cm2 /V s, while the VT is12.0 V. The transfer curve presents some nonlinearity,which is reflected in a peculiar behavior of the FE−VG,mainly for high VG. This could be related with trap fillingprocesses arising, for instance, from a nonoptimizeddielectric/semiconductor interface, which should be im-proved in the future. The inset in Fig. 5b shows the struc-ture of the bottom gate TFT.We have demonstrated the possibility to produce p-typetransparent Cu2O semiconductors by RT reactive magnetronsputtering, followed by annealing treatment at 200 °C. TheCu2O films are polycrystalline with a strong orientationalong the 111 plane. After annealing in air at 200 °C for10 h, we improve H from 0.65 to 18.5 cm2 /V s, which isassociated to an increase in the grain size, from 8.30 to 15.72nm. Concerning the optical properties, the p-type filmspresent Eop2.34 eV and the films have an average trans-mittance around 85%, between 400 and 2000 nm for a thick-ness of 40 nm. Bottom gate p-type Cu2O TFTs present aFE3.9 cm2 /V s, an on/off ratio of 2102 and a VT of12.0 V. The low temperature process together with thegood electrical performance of the devices at this early re-search stage will contribute for the optimization of p-typeoxide-based devices and for their integration in CMOS struc-tures allowing their use in flexible, low cost and transparentelectronic circuits.This work was funded by the Portuguese Science Foun-dation FCT-MCTES through projects PTDC/CTM/73943/2006, PTDC/EEA-ELC/64975/2006, ERC 2008 AdvancedGrant INVISIBLE Contract No. 228144 and IT R&D pro-gram of MKE Contract No. 2006-S079-03, Smart windowwith transparent electronic devices from ETRI Korea. Wethank K. Nomura for the Seebeck measurements and N.Franco for the XRD analysis.1L. O. Grondahl, Science 64, 306 1926.2E. Duhme and W. Schottky, Naturwiss. 18, 735 1930.3W. Schottky and F. Waibel, Phys. Z. 34, 858 1933.4W. Schottky and F. Waibel, Phys. Z. 36, 912 1935.5C. Wadia, A. P. Alivisatos, and D. M. Kammen, Environ. Sci. Technol.43, 2072 2009.6R. J. Iwanowski and D. Trivich, Sol. Cells 13, 253 1985.7L. C. Olsen, R. C. Bohara, and M. W. Urie, Appl. Phys. Lett. 34, 471979.8V. Figueiredo, E. Elangovan, G. Goncalves, P. Barquinha, L. Pereira, N.Franco, E. Alves, R. Martins, and E. Fortunato, Appl. Surf. Sci. 254, 39492008.9M. A. Rafea and N. Roushdy, J. Phys. D: Appl. Phys. 42, 015413 2009.10B. S. Li, K. Akimoto, and A. Shen, J. Cryst. Growth 311, 1102 2009.11H. Raebiger, S. Lany, and A. Zunger, Phys. Rev. B 76, 045209 2007.12W. H. Brattain, Rev. Mod. Phys. 23, 203 1951.13G. P. Pollack and D. Trivich, J. Appl. Phys. 46, 163 1975.14O. Porat and I. Riess, Solid State Ionics 81, 29 1995.15H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, and H.Hosono, Nature London 389, 939 1997.16M. Ivill, M. E. Overberg, C. R. Abernathy, D. P. Norton, A. F. Hebard, N.Theodoropoulou, and J. D. Budai, Solid-State Electron. 47, 2215 2003.17S. Ishizuka, T. Maruyama, and K. Akimoto, Jpn. J. Appl. Phys., Part 2 39,L786 2000.18K. Matsuzaki, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, and H.Hosono, Appl. Phys. Lett. 93, 202107 2008.19ITO/ATO-coated substrates from Planar Systems, I.E, Finland 2004.20G. Bernardo, G. Goncalves, P. Barquinha, Q. Ferreira, G. Brotas, L.Pereira, A. Charas, J. Morgado, R. Martins, and E. Fortunato, Synth. Met.159, 1112 2009.21E. Fortunato, L. Pereira, P. Barquinha, I. Ferreira, R. Prabakaran, G. Gon-calves, A. Goncalves, and R. Martins, Philos. Mag. 89, 2741 2009.22P. Scherrer, Göttinger Nachrichten Gesell. 2, 98 1918.23N. F. Mott, Rev. Mod. Phys. 40, 677 1968.24A. Mittiga, F. Biccari, and C. Malerba, Thin Solid Films 517, 24692009.25M. Tapiero J. P. Zielinge, and C. Noguet, Phys. Status Solidi A 12, 5171972.26T. J. Coutts, D. L. Young, and X. N. Li, MRS Bull. 25, 58 2000.27Y. Nakano, S. Saeki, and T. Morikawa, Appl. Phys. Lett. 94, 0221112009.FIG. 5. Color online a Output characteristics IDS-VDS; b Transfercharacteristics IDS-VGS at VD=−5 V left axis and FE as a function ofVGS right axis, for a typical Cu2O TFT produced at RT and annealed in airat 200 °C. The inset shows the structure of the bottom gate TFT.192102-3 Fortunato et al. Appl. Phys. Lett. 96, 192102 2010

Thin-film transistors based on p-type Cu2O thin films produced at room temperature

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Thin-film transistors based on p-type Cu2O thin films produced at room temperature

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