Solid-State Electronics

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1 SolidStte Electronics 54 (2010) Contents lists ville t ScienceDirect SolidStte Electronics journl homepge: Highly durle nd flexile memory sed on resistnce switching Sungho Kim, Okty Yrimg, SungJin Choi, YngKyu Choi *,1 School of Electricl Engineering nd Computer Science, KAIST, Dejeon , Repulic of Kore rticle info strct Article history: Received 15 April 2009 Received in revised form 24 August 2009 Accepted 25 Octoer 2009 Aville online 24 Novemer 2009 The review of this pper ws rrnged y Dr. Y. Kuk Keywords: Flexile Resistnce rndom ccess memory cirrdition Resistnce rndom ccess memory (RRAM) consisting of stcked /TiO x / structure is demonstrted on flexile nd trnsprent sustrte. To improve cell to cell uniformity, TiO x formed y tomic lyer deposition is used for resistive switching mteril. The simple crossr structure of the RRAM nd good ductility of luminum electrode results in excellent flexiility nd mechnicl endurnce. Prticulrly, ipolr nd unipolr resistive switching (BRS, URS) ehvior ppered simultneously were investigted. Depending on the current complince, BRS or URS could e selectively oserved. Furthermore, the permnent trnsition from BRS to URS ws oserved with specific current complince. To understnd this trnsition ehvior, the cry irrdition effect into resistive switching is primrily investigted. Ó 2009 Elsevier Ltd. l rights reserved. 1. Introduction * Corresponding uthor. Emil ddresses: kkm226@gmil.com (S. Kim), ykchoi@ee.kist.c.kr (Y.K. Choi). 1 Tel.: ; fx: Mny flexile devices hve een developed for electronic pper, trnsistors for displys, sensors, solr cells, nd orgnic light emitting diodes [1 3]. Bsed on this technologicl trend, the need for flexile type of memory will lso increse to support these flexile electronic devices, similr to the role of flsh memory in solid stte electronics tody. However, most types of flexile memories hve een sed on orgnic mterils [4 6]. though orgnic memory shows good flexiility, its performnce cnnot mtch tht of conventionl flsh memory. Additionlly, the friction process of orgnic memory is complicted y the requirements of controlled externl conditions. These limittions require dditionl efforts to improve memory performnce nd increse processing costs. Recently, resistnce rndom ccess memory (RRAM) hs ttrcted gret ttention due to its potentil to replce flsh memory in nextgenertion nonvoltile memory pplictions [7,8]. The resistive switching effect is oserved s result of vrious insulting mterils tht consist of CMOS process comptile inorgnic mterils. In ddition, the current voltge (I V) chrcteristics of the simple metl insultor metl (MIM) structure exhiit rpid switching speeds nd distinctive chnges of the resistnce etween the high resistnce stte nd the low resistnce stte. In the present study, the friction of flexile type of RRAM is reported. In n erlier work y the uthors, plsm oxidized luminum [9] nd sol gel derived zinc oxide [10] were used s resistive switching mteril for flexile type RRAM. On the other hnd, in this study, tomic lyer deposition (ALD) process is used to improve cell to cell uniformity nd for relistic fesiility in flexile memory pplictions using existing semiconductor technology. The structurl simplicity nd good ductility of luminum electrode result in dvntges tht include good flexiility, mechnicl endurnce, nd durility. In ddition, the resistive switching mechnism is investigted y mens of permnent trnsition from ipolr resistive switching (BRS) to unipolr resistive switching (URS) in TiO x films, s understood through cry irrdition effects. 2. Device friction The flexile RRAM ws fricted on the flexile nd trnsprent sustrte of polyethersulfone (PES), s shown in Fig. 1. The PES film ws glued onto silicon wfer with polyimide. uminum with thickness of 150 nm ws used for the top nd ottom electrodes. The electrodes were ptterned y conventionl photolithogrphy rnging from 2 2 to lm 2. TiO x of 10 nm thickness ws used to formulte the resistive switching mteril. The TiO x films were deposited using plsmenhnced tomic lyer deposition t 180 C. The process temperture of the deposition is limited y the mximum working temperture of PES, which is 200 C. The thicknesses of the deposited films were confirmed y trnsmission electron microscopy imges. The silicon wfer served only s mechnicl support during the processing stge; it ws susequently peeled off mnully fter the friction of the flexile RRAM /$ see front mtter Ó 2009 Elsevier Ltd. l rights reserved. doi: /j.sse

2 S. Kim et l. / SolidStte Electronics 54 (2010) re : m 2 V RESET 0.0 V SET Current [ma] Current [A] HRS to LRS V I V I Voltge [V] Voltge [V] 10 8 t 85 o C or of TiO x Fig. 1. The process flow of flexile RRAM nd photogrph of flexile RRAM. 3. Resistive switching mechnism 3.1. Switching performnce Fig. 2 shows the typicl switching chrcteristics of /TiO x / tht produces BRS. Bis sweeps were conducted in the direction 0V? 3V? 0V? 3V? 0 V. The current incresed shrply t negtive is (V SET ) nd switched from HRS to LRS. The LRS remins during the voltge sweep ck t positive is less thn V RESET. The resistnce rtio ( / ) etween HRS nd LRS is lrger thn 50 t V READ = 0.2 V under complince current of 500 la. Fig. 2 shows the mesured retention chrcteristics of the fricted /TiO x / devices in the HRS nd in the LRS. No significnt chnges of the resistnce in either cse were oserved fter 10 4 st85 C. Additionlly, the relile endurnce showing sustined resistnce rtio lrger thn 50 is chieved even fter switching cycles of times, s shown in Fig. 2. On the one hnd, one interesting phenomenon ws oserved tht oth BRS nd URS could e ppered in TiO x films depending on the current complince. TiO x films show the BRS mode t low current complince (<500 la) while reveling the URS mode t high current complince (>10 ma) fter electroforming process, s shown in Fig. 3. Furthermore, permnent trnsition from BRS to URS ws oserved when high current (3 ma) ws pplied. After the trnsition of the switching mode, the / vlue nd distriution chnged drmticlly, s shown in Fig Oxygen vcncies in TiO x Switching Cycles Retention time [sec] Fig. 2. () Typicl I V curve of n /TiO x / device. The inset represents I V chrcteristics for Fig. 2 in doulelogrithmic plot. () Endurnce test under the current complince of 500 la. In ddition, dt retention chrcteristics for HRS nd LRS; the resistnce vlues in HRS nd LRS were red t 0.2 V t 85 C. The inset of Fig. 2 shows logrithmic plot of the I V chrcteristics of TiO x films for BRS mode. In the lowvoltge region, the current is linerly proportioned to the voltge (I / V), which is followed y I / V 2. The I / V 2 correltion cn e understood s the effect of the spcechrgelimited current (SCLC) [11 13]. From previous nlysis y the uthors [14], it ws verified tht the resistive switching of /TiO x / device ws governed y SCLC in the only TiO x lyer ner the top electrode. However, suitle physicl nlysis of TiO x film could not e provided. To crry out further detil nlysis for resistive switching, Fig. 4 shows trnsmission electron microscope (TEM) imge of the fricted /TiO x / device. As shown in Fig. 3, TiO x lyer with thickness of 10 nm ws deposited initilly. However, etween the top electrode nd the TiO x lyer, nother lyer (5 nm) ws newly generted. To identify this lyer, trnsmission electron microscopeenergy dispersive Xry spectrometry (TEM EDX) nlysis ws crried out. Fig. 4 shows the scnned tom profiling etween top nd ottom electrode y TEM EDX nlysis. From this dt, it is found tht n oxygendeficient lyer ws preferentilly produced in the TiO x lyer ner the top electrode nturlly. It ws lso oserved tht some frction of the luminum in the top electrode diffused into the TiO x lyer. The newly generted lyer (the oxygendeficient lyer) cn e considered s n doped TiO x lyer. For the doped TiO 2, 3 sustitutes for Ti 4 within the TiO x, nd oxygen vcncies cn e produced y diffused 3 [15]. It is improle tht n 3 ion in TiO x cts s trp nd cptures n electron; insted, n oxygen vcncy trps n electron [16,17]. Therefore, the mechnism of BRS mode cn e speculted tht oxygen vcncies in the TiO x lyer ner the top electrode ct s trps for electrons s shown in Fig. 5. During SET process, injected electrons from the top electrode re filled oxygen vcncies nd

3 394 S. Kim et l. / SolidStte Electronics 54 (2010) Top 10 3 doped TiO 2 Current [A] TiO 2 ntive O x Cumultive proility [%] st : BRS, 500μA 3 rd : URS 2 nd : Trnsition, 3mA Voltge [V] URS BRS LRS HRS LRS HRS , Fig. 3. () The switching mode trnsition depends on the complince current level in TiO x films. () nd distriution in oth the BRS nd URS modes. oxygen ions move to the ottom electrode, which genertes the dditionl oxygen vcncies. Consequently, distriuted oxygen vcncies induce trpcontrolled SCLC nd dominntly contriute to the resistive switching. On the other hnd, in the cse of HRS mode, defects such s oxygen vcncies tend to e ligned to form tiny conducting filments in the ulk region fter electroforming process [18], s shown in Fig. 5. though primry oxygen vcncies re loclized ner the top electrode, oxygen vcncies re re distriuted or some defects re newly generted due to the high electric field during electroforming process. These tiny conducting filments gther together to form stronger nd more conducting filments, which led to the trnsition to the LRS. During RESET process, electrons re depleted in some oxygen vcncies (especilly ner the top electrode) nd electrondepleted oxygen vcncies re recomined with O 2. It hs een still in controversy tht the HRS current of the URS mode my e trnsported through the oxide films through hopping conduction [19], Poole Frenkel emissions [20], or y the spcechrgelimited current [21] BRS nd URS trnsition nd mechnism Permnent switching mode trnsition, BRS URS, ccording to the current complince, hs een reported elsewhere [22,23] tht Intensity (r.unit) Bottom Top Oxygen Ti generted oxygendeficient TiO x 10 nm Bottom deposited TiO uminum Distnce [nm] diffusion Fig. 4. () Trnsmission electron microscopy imge of the /TiO x / structure. () TEM EDX depth profiling dt. An oxygendeficient lyer is induced y the ctivetop luminum. it ws oserved restrictedly in titnium oxide mteril. The first oservtion of the trnsition chrcteristic ws reported y Jeong et l. in Pt/TiO 2 /Pt stck [22]. At tht time, however, the only oservtion of trnsition ws reported nd physicl nlysis ws not provided. More intensive nlysis ws crried out y Wng et l. using nlyses of correltion etween RESET condition nd [23]. From correltion etween RESET current nd, the URS mode of TiO x ws in ccordnce with the therml dissolution model [24], which sttes tht, the conductive filment ws thermlly destroyed y current crowding nd locl heting effects during RE SET. In ddition, the BRS mode of TiO x cn e understood from correltion etween RESET voltge nd y the redoxrection model [25]. It implies tht resistive switching is cused y locl electrochemicl redox rection ner the top electrode (node) interfce. These explntions re well consistent with forementioned switching mechnism of /TiO x / device. In this work, to investigte trnsition chrcteristic from BRS to URS in detil, cirrdition technique ws introduced. The totl irrdition dose ws 100 krd with dose rte of 50 rd/s. The resistive switching of the TiO x films ws performed in the BRS mode t first. Then, cry ws irrdited into the device with 60 Co. Lstly, the URS mode ws chieved fter mode trnsition t 3 ma current complince. The device which showed trnsition BRS to URS without cirrdition ws lso investigted s control group. Fig. 6 shows experimentl results tht modultion of

4 S. Kim et l. / SolidStte Electronics 54 (2010) BRS URS Oxygen electron vcncies Recovered oxygen vcncies with SET RESET SET RESET nd in comprison mde with nd without cirrdition in the URS mode. In the cse of the URS mode with cirrdition, only ws decresed fter switching mode trnsition. It ws known tht the vlence of the Ti ion vries fter irrdition tht the Ti 3 ion increses in contrst with the Ti 4 ion decreses [26]. A frction of Ti 4 ions turns to Ti 3 ions nd the chemicl composition chnges s 2TiO 2 2e? Ti 2 O 3 O 2, hence excess oxygen ions (O 2 ) nd oxygen vcncies re creted in the irrdited lyer. Consequently, more conducting filments re formed y cry induced excess oxygen ions when the switching mode trnsition ws occurred ecuse defects such s oxygen vcncies tend to form conducting filments. In the cse of LRS in the URS mode, there is no significnt chnge etween with nd without cirrdition. It cn e understood tht excess filments hs not n importnt role for current flowing ecuse current minly flow thorough the min filments which connect oth electrodes, s shown in Fig. 6 (expected filments formtion is referred to simultion results y rndom circuit reker network model [27]). However, in the cse of HRS in the URS mode, remined excess filments cuse the lekge current pth, so is decresed. This ehvior from excess filments is equivlent to the cell re dependency of the HRS nd LRS in URS mode. Generlly, in the cse of URS, increses s the cell re decreses, wheres is independent of the cell re. It implies tht lrger cell re device hs the more ex Fig. 5. () The schemtic digrm for the mechnism of BRS mode. () The schemtic digrm for the mechnism of URS mode Cumultive proility [%] URS with γirrdition BRS URS without γirrdition or of TiO x Numer of ending times [X10 4 ] , 10 8 excess filments y irrditoin With irrditoin Without irrditoin or of TiOx R Fig. 6. () Irrdition effect of the nd distriution. () The schemtic digrm for the expected filments formtion with nd without cirrdition. flt Bending Rdius [mm] Fig. 7. l dt; five different rndomly selected devices were mesured ech time. () The switching chrcteristics with continuous ending of PES sustrte t reding voltge of 0.2 V. () The current rtio etween the on nd off sttes s function of the ending rdius t reding voltge of 0.2 V.

5 396 S. Kim et l. / SolidStte Electronics 54 (2010) cess filments, so more lekge current pth reduces. Therefore, during switching mode trnsition, oxygen ions nd vcncies re redistriuted nd formed conducting filments due to the enough energy from the high complince current. During this process, the mount of oxygen ions nd vcncies determines the ehvior of resistive switching fter trnsition. 4. Flexiility nd mechnicl endurnce Good mechnicl flexiility is crucil for pplictions in flexile electronics. The level of mechnicl endurnce ws evluted y performing sustrte ending test in which oth tensile nd compressive stresses were induced, s shown in Fig. 7. A virtor ws used to induce sustrte ending 4 times/s for the totl of ends. Even t ends, the / vlue ws unchnged. In ddition, the devices exhiited good flexiility, s shown in Fig. 7. In the flexiility test, severe ending of the device did not ffect memory performnce. These results indicte tht the switching chrcteristics of flexile RRAM re independent of device ending due to the good ductility of the luminum electrode nd the mechnicl endurnce rising from the simple device structure. 5. Conclusions RRAM device ws fricted nd showed relile endurnce nd retention chrcteristics, even on flexile sustrte. The trnsition ehvior from BRS to URS ws understood with the id of c irrdition. This flexile type of RRAM is ttrctive for lowcost nd werle devices nd my e suitle in flexile displys. Acknowledgements This reserch ws supported y Grnt (08K ) from the Center for Nnoscle Mechtronics & Mnufcturing, one of the 21st Century Frontier Reserch Progrms supported y the Kore Ministry of Eduction, Science nd Technology (MEST). References [1] Chen Y, Au J, Kzls P, Ritenour A, Gtes H, McCrery M. Electronic pper: flexile ctivemtrix electronic ink disply. Nture 2003;423:136. [2] Ju S, Fcchetti A, Xun Y, Liu J, Ishikw F, Ye P, et l. 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