Silicon-based microprocessors and memory chips with a linewidth

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1 REVIEW doi: /nture10680 A role for grphene in silion-sed semiondutor devies Kinm Kim 1, Je-Young Choi 1, Tek Kim 1, Seong-Ho Cho 1 & Hyun-Jong Chung 1 As silion-sed eletronis pproh the limit of improvements to performne nd pity through dimensionl sling, ttention in the semiondutor field hs turned to grphene, single lyer of ron toms rrnged in honeyom lttie. Its high moility of hrge rriers (eletrons nd holes) ould led to its use in the next genertion of highperformne devies. is unlikely to reple silion ompletely, however, euse of the poor on/off urrent rtio resulting from its zero ndgp. But it ould e used to improve silion-sed devies, in prtiulr in high-speed eletronis nd optil modultors. lion-sed miroproessors nd memory hips with linewidth s smll s 20 nm n meet the demnd for low-power multifuntionl hips tht n proess nd store mssive mounts of heterogeneous dt. But hieving physil dimensions ner the deeper nnosle regime of 10 nm or eyond 1 is hllenge for existing tehnologies. It will require dimensionl sling using novel strutures, mterils nd proesses for omplementry metl oxide semiondutor (CMOS) devies, suh s three-dimensionl rhitetures for memory nd logi, high-κ/metl gte trnsistors, extreme ultrviolet lithogrphy nd new omputing rhitetures. Mterils suh s grphene, whih hs n extremely high hrge-rrier moility, re expeted to hve n importnt role in the dvnement of semiondutor tehnology 2 4. is monolyer of ron toms rrnged in twodimensionl honeyom lttie nd is si uilding lok of wellknown ron mterils suh s grphite, ron nnotues nd fullerene. ne grphene ws isolted y mehnil exfolition in 2004 (ref. 5), mny extrordinry properties hve een reported, suh s extremely high eletron moility 3 5. High moility rises euse eletrons n propgte without sttering over lrge distnes, perhps mirometres, euse of its redued phonon sttering. A reent poll onduted y the semiondutor industry for the Interntionl Tehnology Rodmp for Semiondutors (ITRS) nmed grphene s the mteril likely to hve the gretest impt on geometri sling, thnks to its high moility, whih is desirle in metl oxide semiondutor fieldeffet trnsistor (MOSFET) hnnels 6. Furthermore, grphene s strong intertions with photons 7 9 nd eletrohemil stility ould dd more funtions to silion-sed CMOS devies, suh s rdio-frequeny swithes nd photoni modultors. Here we onsider how grphene n e inorported into these semiondutor devies, exmine the requirements nd hllenges tht must e met, nd disuss possile solutions. Rdio-frequeny trnsistors The high moility of hrge rriers in grphene is idel for otining fst swithing nd high on urrent (I ON ). Zero ndgp indues lrge off urrent, however, so the on/off urrent rtios for grphene trnsistors re out 100 (ref. 5), muh lower thn the required for minstrem logi pplitions. But this reltively low on/off rtio is not signifint prolem for high-frequeny pplitions, suh s rdio-frequeny swithes. In generl, rdio-frequeny trnsistor performne is hrterized y two prmeters: the utoff frequeny (f T ) nd the mximum osilltion frequeny (f mx ), where f T nd f mx represent how fst hnnel urrent nd power trnsmission, respetively, re modulted y the gte. As shown in following eqution f T = 2π g m CG f T is determined y the gte pitne (C G ) nd the trnsondutne (g m ). Thus fr, the highest mesured f T hs een 300 GHz with Co 2 nnowire gtes using exfolited grphene 10. At wfer sle, 240 GHz hs een reported using epitxil grphene 11, ompred with 200 GHz using hemil vpour deposition (CVD) 12. As shown in Tle 1, even with lrger gte lengths, grphene hs demonstrted superior performne to silion 13 nd III V group ompounds s result of its high drift veloity of m s 1 (ref. 17). This mens tht f T for grphene trnsistors is pproximtely 1.42 THz for 56-nm gte 17, lthough this vlue exludes prsiti pitnes, whih would e hlved in suh ses. Another importnt prmeter for rdio-frequeny trnsistors is f mx : f mx f T (1) = (2) 2 ( g D (R G + R SD ) + 2πf T R G C G ) where g D is the hnnel ondutne, R G is the gte resistne nd R SD is the soure drin resistne. Studies hve shown lrge disrepnies in vlues etween f T nd f mx for grphene rdio-frequeny trnsistors. Previous work initilly ttriuted the disrepny to the high ontt resistne 18 nd high R G without onsidering issues inherent to the grphene film, in prtiulr the low output resistne. We expet tht the low output resistne is one of the key ftors in inresing f mx. Beuse f mx mesures trnsmitted power (I 2 R), it n e inresed y minimizing g D ttined t hnnel sturtion; this is pinh-off in most MOS trnsistors. In grphene, the pinh-off ondition, effetive for the drin sturtion, n e produed if grphene hs ndgp. More reserh to improve f mx through improvements in sturtion urrents is needed 19,20. As long s f mx remins t urrent levels, it will e diffiult to use grphene trnsistors in rdio-frequeny mplifiers. At present, they will e vlule for rdio-frequeny swithing pplitions only if f T exeeds the performne of urrent silion-sed devies. 1 Smsung Advned Institute of Tehnology (SAIT), Smsung Eletronis, Yongin-, Gyeonggi-Do , South Kore. 338 NATURE VOL NOVEMBER 2011

2 REVIEW INSIGHT Tle 1 Properties of rdio-frequeny trnsistors Trnsistor f T (GHz) f mx (GHz) L g (nm) W g Fingers g m (μm) (ms μm 1 ) Moility (m 2 V 1 s 1 ) Estimted 17 * 1, >10,000 Speifi ontt resistne (Ω μm 2 ) Mesured 12 * (ref. 23) CVD grown < Epitxil ,000 1,500 lion lion , ITRS ITRS III V InP >1,100 < , (ref. 16) InAs ,200 *Flke grphene; Trget speifition; Eletron moility; 100 nm 100 nm Ni/. L g, gte length; W g, gte width. As shown in Tle 1, the estimted f T for grphene is signifintly higher thn tht for silion. However, the experimentlly mesured f T for grphene is muh lower thn expeted. To inrese f T, moility hs to e inresed 11,12,21, for whih three ftors need to e onsidered: minimizing defets in grphene; minimizing impurities tht use sttering t the grphene hnnel gte dieletri interfe 22 ; nd reduing ontt resistne 23,24. Defets in grphene re highly dependent on the growth proess. The highest reported moility is 200,000 m 2 V 1 s 1, whih hs een otined using mehnilly exfolited grphene flkes without sustrte t temperture of 5 K (ref. 6). Moility remins within the sme order of mgnitude t elevted tempertures euse the sttering mehnism is minly used y eletron or hole puddles within the two-dimensionl grphene sheet 25. In relity, grphene flkes nnot e used for silion pplitions, so CVD or sulimtion of C will e needed, even though these might redue moility. The impurities tht use sttering t the grphene hnnel gte dieletri interfe hve een ttriuted to intertions with the underlying sustrte, suh s surfe hrge trps 26,27, interfil phonons 28 nd nnometre-sle deformtion or ripples To minimize these impurities, we propose new struture ontining ir gps, nothing-on-grphene rhiteture tht provides free-stnding ondition for grphene. On the other side of grphene is gte oxide tht does not redue the moility, suh s hexgonl oron nitride (), whih llows resonle hrge trnsport in grphene (up to 40,000 m 2 V 1 s 1 t room temperture) euse it is tomilly flt nd free of dngling onds nd hrge trps 32,33. Figure 1 shows the formtion of the nothing-on-grphene rhiteture. Copper gtes re formed through dmsene proess, long with the first metl line (M1), whih here is opper (Fig. 1). The next step is the trnsfer of nd grphene (Fig. 1). The ritil step in produing nothing-on-grphene is the formtion of ir gps etween the soure nd drin metl eletrodes; this is hieved y depositing srifiil film on the eletrodes (Fig. 1), whih is then ethed k to expose the eletrodes (Fig. 1d). A porous film is then deposited (Fig. 1d) over the eletrodes, nd the srifiil film is removed, reting the ir gps (Fig. 1e). The porous film must then e ptterned (Fig. 1f). This nothing-on-grphene rhiteture minimizes interfil sttering intertions nd thus mximizes moility. The ontt resistne of grphene is more thn 70 times higher thn tht of silion (see Tle 1). However, reduing the ontt resistne poses signifint hllenges euse of the intertions etween grphene nd different metls. shows wek inding with metls suh s opper, gold nd pltinum on their (111) surfes, where Gte oxide M1 S G Well Srifiil film dewll sper D ILD STI lion wfer d e f M4 M3 Porous film Porous film Air gp M2 Figure 1 Formtion of nothing-on-grphene rhiteture, whih minimizes the sttering of eletrons., A dmsene proess is used to rete lyer of opper metl (M1) tht funtions s the gte eletrode () for the grphene trnsistor. G, S nd D indite the gte, the soure nd the drin of the silion trnsistor, respetively, nd STI indites the shllow trenh isoltion. The well is for the seprtion of the p n doping regions., A lyer of hexgonl oron nitride () is introdued to t s gte oxide. This is tomilly flt nd lks dngling onds nd hrge trps, so it llows fst hrge trnsport. A lyer of grphene is pled over the., The soure (S1) nd drin (D1) metl eletrodes re ptterned, nd srifiil film is pled over the top. d, The srifiil film is then ethed k to revel the eletrodes, nd porous film is then deposited. e, The reminder of the srifiil film is removed through pores of the porous film, leving ir gps etween the eletrodes. f, One the nothing-on-grphene rhiteture is estlished, CMOS k end of the line proesses, inluding the ddition of further metl lyers, n omplete the iruit. ILD, interlyer dieletri. M1 ILD G S Well D STI 17 NOVEMBER 2011 VOL 479 NATURE 339

3 INSIGHT REVIEW Tle 2 On/Offurrent rtios for 20-nm logi proess Property Type 1* Type 2 Type 3 Type 4 On/Off rtio ~ ~ ~ ~ I ON (μa μm 1 ) ~800 ~1,000 ~1,100 ~1,300 All trnsistor types re needed for 20-nm silion CMOS logi iruits. *Low on urrent, highest on/ off rtio; Medium on urrent, high on/off rtio; Medium on urrent, medium on/off rtio; High on urrent, low on/off rtio. grphene preserves its nd struture 34,35. By ontrst, grphene inds strongly to nikel, olt nd plldium s their eletroni strutures re strongly intermixed 36,37. Although ontt resistne is expeted to e lower in strongly inding metls, inding strength hs een shown not to ffet resistne, s experimentl vlues for gold, nikel nd plldium re quite similr 24, This suggests tht even for strong metl grphene ontts, eletron trnsmission from the grphene region in ontt with the metl to the pristine grphene hnnel eomes diffiult. Creful studies re required to determine whether ontt resistnes hve intrinsi limittions. Even if f T is mximized, the tsk of improving f mx still remins. To omplish this, sturtion ( pinh-off ondition with ndgp) should e reted in grphene for high output resistne. When ndgp is effetively formed, grphene n e used in highspeed logi pplitions. So fr, there hve een no speifi guidelines on how wide the ndgp should e, ut this ould e inferred from the minimum I ON /I OFF rtio requirement. For notiele ndgp to e engineered t the grphene metl juntion, I OFF must e governed y the thermioni emission of rriers through the metl grphene Shottky rrier. Thus I OFF would e proportionl to exp( qφ rrier /kt), where q is the eletron hrge, φ rrier is the Shottky rrier height, k is the Boltzmnn onstnt nd T is the temperture. In ddition, ssuming tht the work funtions of grphene nd the metl re the sme, the resultnt Shottky rrier height would e E g /2, where E g is the ndgp energy. Hene the I ON /I OFF rtio would e proportionl to exp(e g /2kT). Among vrious trnsistor tehnologies of 20-nm CMOS logi produts, highspeed trnsistor requires the I ON /I OFF rtio to e three to four orders of mgnitude, whih is the lowest eptle vlue for high-performne logi pplitions (K. Kim, unpulished dt; see Tle 2). Beuse the I ON /I OFF rtio is proportionl to exp(e g /2kT), the minimum required ndgp would e 360 mev for high-speed CMOS pplitions. There is onsiderle reserh to improve the I ON /I OFF rtios of urrent grphene trnsistors: nnorion 40 42, ilyer 43 nd perforted grphene So fr, suh studies hve een unle to hieve this vlue. It might tke yers for grphene trnsistors to e used for high-speed logi, ut they ould still find use emedded in rdio-frequeny pplitions suh s low-noise mplifiers, mixers, frequeny multipliers nd resontors One grphene rdio-frequeny trnsistor n e integrted into silion CMOS, nd ssuming it hs very high f T nd liner gin, whih re usully oserved in grphene rdio-frequeny trnsistors, it will enhne the performne of low-noise nd low-distortion iruits. Furthermore, if f mx n lso e improved, its usge will extend to ll iruitry, inluding ll power-sensitive loks. Optil devies for silion photonis As well s the rdio-frequeny field, grphene ould find nother promising mrket in photonis euse it n filitte ultrwide ndwidths for ex-sle (10 18 ) omputing systems. The explosive dt growth on the Internet drives the need for ex-sle systems for lrge dt entres. Pet-sle (10 15 ) systems onsumed out 2.5 MW in 2008; simple estimte shows tht ex-sle systems will onsume out 2.5 GW, whih is uneptly high 50. Energy effiieny is therefore the next hllenge for the silion industry. lion photonis n provide ultrwide ndwidths through the multiplexing of numerous wvelengths 51,52 nd redued power onsumption. n find use here euse of its optil properties. It sors out 2.3% of norml inident light, despite only eing one tom thik Trnspreny is lmost independent of wvelength over spetr rnging from visile light to infrred euse there re twodimensionl gses of free eletrons in the gpless eletroni struture of grphene 53,56. These optil hrteristis, llied with its high eletron moility, mke grphene prime ndidte for ultrwide-ndwidth optil modultors nd photo-detetors. An optil modultor is one of the key omponents for ltering the properties of light, suh s its phse, mplitude or polriztion, y eletro-refrtion or eletro-sorption 57. Optil modultors, suh s Mh Zehnder interferometers 57,58, ring or disk resontors 59,60 nd germnium- or III V-sed eletro-sorption modultors 61,62, re sed on interferene, resonne nd ndgp sorption, respetively. Their operting spetr re usully nrrow A single sheet of grphene on silion wveguide hs reently een reported to provide n ultrfst response time 7. The internd trnsitions of photo-generted eletrons re modulted over rod spetrl rnges y drive voltge, so rodnd nd high-speed optil modultor n e implemented with grphene. This CMOS-omptile grphene optil modultor n provide exellent performne over rod operting spetr rnging from 1.35 μm to 1.60 μm, with n operting speed of 1.2 GHz, modultion effiieny of ~0.1 db μm 1 nd smll footprint of 25 μm 2 (ref. 7). Optil modultors n e further improved y inresing oth modultion depth nd operting speed using novel modultor struture, s shown in Fig. 2, in whih grphene is pled t the lotion of mximum light intensity of the ridge region in the wveguide, Figure 2 Struture of grphene-gted optil modultor., A ridge-type modultor., A uriedtype modultor. In eh se, two monolyer grphene sheets nd n sper 7 nm thik re used to seprte the wveguides, whih re mde of two different setions of silion sustrte nd polyrystlline silion (p-). The ridge is 600 nm wide, 250 nm high nd 35 μm long. The metl eletrode is loted 300 nm from the wveguide., d, Trnsverse eletri mode profiles t wvelength of 1.55 μm re shown for the ridge type () nd the uried type (d) modultor. The multilyer struture of grphene nd is overlid with the mode profiles. p- 0.5 d 0.5 p- Oxide y Distne (μm) y Distne (μm) x Distne (μm) x Distne (μm) 340 NATURE VOL NOVEMBER 2011

4 REVIEW INSIGHT wveguide Wveguide p- Detetor p- y Distne (μm) 0.6 x Distne (μm) d e f 0.6 wveguide Wveguide p- Detetor Figure 3 The struture of grphene photo-detetors with integrted wveguides., Butt-oupled photo-detetors., A longitudinl shemti is shown with n overlid longitudinl eletri field (E-field) tht shows the light trnsport from the wveguide to the detetor. The lyer of grphene is surrounded y to mximize moility., A ross-setion of the photo-detetor shown in. is pled etween the hlf-ethed wveguide nd the p- to minimize optil losses t the wveguide detetor interfe., A ross-setionl E-field profile showing good overlp with the grphene. d f, Evnesent-oupled photo-detetors. d, A longitudinl shemti is shown with n overlid longitudinl E-field profile. The E-field p- y Distne (μm) x Distne (μm) extends into the p-, enhning the intertion with grphene. e, A rosssetion of the photo-detetor shown in d. Unlike, grphene is pled ove the wveguide. f, A ross-setionl profile showing the E-field tiling upwrds to inrese the overlp with the grphene. For simpliity, longitudinl views omit the metl grids. The silion wveguide is 500 nm wide nd 200 nm high. The E-field mplitudes re lulted t wvelength of 1.3 µm. s thikness of nm nd sorption oeffiient of 301,655 m 1 (ref. 56) re used to lulte the normlized detetor lengths. The multilyer struture of grphene nd is overlid on the ross-setionl E-field profiles. mximizing the modultion depth. To this end, the ridge is omposed of two different regions, for exmple polyrystlline silion on the top nd single-rystlline silion on the ottom. New modultor strutures where dul grphene lyers re seprted y thin sper re shown in Fig. 2,. Their propgting mode profiles, whih onfirm good sptil overlp etween the light mode nd the grphene lyers, re shown in Fig. 2,d. Oxide deteriortes rrier moility in grphene, so is used s n insultor euse it llows grphene to mintin its high moility. The pitne-resistne (RC) time onstnt n e redued y repling high-resistne ulk silion with low-resistne grphene, nd y repling the gting sper with lowdieletri-onstnt. With two single sheets of grphene 600 nm wide on the wveguide, whih is 300 nm from the metl ontt nd 35 μm long (Fig. 2), the totl resistne, inluding the metl ontt, is redued from 600 Ω to 26 Ω, where the sheet resistne is 15 Ω nd the ontt resistne is 11 Ω, using unit sheet nd unit ontt resistnes for single-sheet grphene of 300 Ω per squre (Ω h 1 ) (ref. 64) nd 200 Ω μm (ref. 36), respetively. In ddition, the pitne is redued from 220 ff to 100 ff. The predited ndwidth of this highspeed modultor, f 3B = 1/2πRC, is intrinsilly suitle for 55-GHz modultion, whih is omprle to the est reported optil modultor ndwidth 63. In the ner future, it is likely tht oth inter-hip nd intr-hip interonnets for ultrwide-ndwidth dt networks will e possile using grphene. Another prime optil pplition for grphene is s photo-detetor. Extremely high ndwidth is possile euse the high rrier moility llows the ultrfst extrtion of light-generted rriers. The trnsit time-limited ndwidth is lulted to e 1.5 THz 4 t the reported sturtion rrier veloity 65. In omprison, the est results using germnium photo-detetors re round 30 GHz 66,67, nd the mximum ndwidth is expeted to e 80 GHz 68. The fesiility of grphene s photo-detetor hs een demonstrted in photo-generted urrent imging studies 69,70. A 10-GHz ndwidth stnd-lone grphene photo-detetor for optil ommunition hs reently een reported 71. However, its mximum responsivity is low (6.1 ma W 1 ) euse the detetor is designed to sor only smll perentge of norml inident light. The responsivity n e improved y integrting the wveguide with the photo-detetor euse the light grphene intertion length inreses s light propgtes long grphene s plne. In photo-detetor with n integrted wveguide, optil signls from wveguide n e trnsferred to detetor y either utt-oupled or evnesent-oupled shemes 72. In the utt-oupled sheme, the detetor nd the wveguide re diretly onneted to eh other on the sme plne, wheres in the evnesent sheme the detetor is on top of the wveguide. Our proposed pprohes for oth shemes re shown in Fig. 3 with their longitudinl nd ross-setionl eletri-field profiles. For the utt-oupling pproh (Fig. 3 ), grphene is sndwihed etween hlf-ethed silion nd polyrystlline silion to minimize light refletion t the interfe. In the evnesent-oupling pproh (Fig. 3d f), grphene is too thin to shift the light mode oming from the silion wveguide nd hrdly sors ny light. The polyrystlline silion lyer in Fig. 3 enles grphene to sor light y hnging the refrtive index suh tht the mode shifts upwrds. To determine whether these detetors ould dequtely sor light despite their extreme thinness, normlized detetor lengths for the sme sorption hve een ompred sed on detetor volume nd eletrifield intensity profiles. To hieve the sme sorption s tht of the widely used evnesent-oupled germnium detetor, the evnesent-oupled grphene detetor needs to e 3.5 times longer. In the utt-oupled se, however, the grphene detetor only needs to e 12% longer euse the grphene overlps with the highest eletri field. A further derese in detetor length n e hieved y using multilyer grphene euse the sorption inreses linerly with thikness for up to six lyers NOVEMBER 2011 VOL 479 NATURE 341

5 INSIGHT REVIEW Intensity (.u.) Intensity (.u.) D G 1,500 Rmn shift (m 1 ) 1,372 m 1 2D 2,000 2,500 3,000 1,000 1,500 2,000 2,500 3,000 Rmn shift (m 1 ) To inrese responsivity, effetive extrtion of photo-generted rriers should lso e onsidered. Beuse doping ontrol is diffiult, grphene requires different rrier-extrtion strutures from semiondutor photo-detetors. A novel system for using metl-indued, uiltin potentil hs een studied 35,69,70,74 nd the devie demonstrted 8,71. Assuming light sorption of 2.3%, out 21% rrier extrtion is estimted from 6.1 ma W 1 responsivity 71, nd up to 60% rrier extrtion n e ntiipted 69. Further struturl optimiztion nd grphene doping studies 75,76 re expeted to inrese the extrtion effiieny. Integrtion in semiondutor proesses Inorporting grphene into silion devies is hllenging, nd reful seletion of the right growth methods for the pplition is importnt. For grphene-sed hyrid silion-cmos pplitions, grphene films should e deposited on topologil surfes s determined y the devie rhiteture. There re generlly two methods of grphene deposition: epitxil growth on C(0001) y the sulimtion of silion toms 77,78, nd growth on tlyti metl films with susequent trnsfer onto trget sustrtes (CVD trnsfer) CVD trnsfer proesses hve lredy een demonstrted t wfer sles 82. This method llows grphene to e trnsferred onto ny type of mteril s long s it is flt, nd there re no limittions on proess tempertures. However, it is diffiult to void defets suh s holes, rks nd folds during the trnsfer Suh defets would e minimized if grphene ould e grown diretly on underlying mterils with topologil surfes t resonle proess tempertures. The strutures nd proess requirements of devies disussed in this Review re summrized in Fig. 5. For the rdio-frequeny trnsistor shown in Fig. 5, for exmple, grphene will e deposited on. d Ni Spphire Figure 4 Diret growth of grphene nd.,, The growth of grphene on film showing the Rmn spetrum () nd snning eletron mirosopy imge (). Sle r, 1 μm. The film ws grown on opper foil nd trnsferred to O 2 / wfer. The inset in shows grphene smple grown on 1 m 1 m film on 2 m 2 m O 2 / wfer. shows typil Rmn peks of D (1,346 m 1 ), G (1,582 m 1 ) nd 2D (2,700 m 1 ) nd homogeneous film struture with no prtiulte impurities. The G pek origintes from the douly degenerted virtionl mode of hexgonlly strutured grphene. The D pek is disorder-indued nd oserved in defetive grphene struture. The 2D pek is the seond order of the D nd., d, The growth of film on Al 2 O 3 (0001) sustrte showing the Rmn spetrum () nd trnsmission eletron mirosopy imge (d). Sle r, 2 nm. The inset in shows n smple grown on the Al 2 O 3 (0001) sustrte mesuring 1 m 2 m. The film shows typil Rmn pek t 1,372 m 1..u., ritrry units. However, growing presents hllenge euse growth ould tke ple on two or more different mterils suh s metls (usully opper) nd low-κ dieletris. The prolem is the high proessing temperture for grphene nd, whih exeeds the k-end-of-line proessing tempertures of less thn 500 C. The most sensile wy of prepring grphene nd is therefore y CVD on metl tlysts nd film trnsfer. An optil modultor with uried rhiteture (Fig. 5) requires multiple stking of grphene nd (/grphene//grphene/ ) on nd O 2. Beuse multiple trnsfers ould inrese the numer of trnsfer-relted defets, the entire film stk ould e divided into two different stks, whih would require only two-step trnsfer. The first stk would onsist of grphene/, nd the seond would e /grphene/. In this se, these stks ould e grown on tlyti metl film y CVD. For the ridge strutures shown in Fig. 5,, onforml stking of grphene nd is needed on topologil silion surfe: /grphene//grphene/ for optil modultors (Fig. 5) nd / grphene/ for the photo-detetor (Fig. 5). Beuse the trnsfer method is limited to flt surfes, trnsfer-free deposition on nontlyti surfes suh s in silion, grphene nd is required. Moreover, grphene nd depositions on non-tlyti surfes must e prepred elow 1,000 C to void struturl hnges in CMOS devies, suh s in doping profiles. There re two pprohes for the trnsfer-free deposition of grphene on non-tlyti surfes: on tlyti metl film/sustrte followed y eliminting the underlying metl 83 85, nd diretly on non-tlyti sustrtes y CVD 86,87. Both pprohes pose hllenges. In the first pproh, grphene n e torn during the elimintion of the underlying metl. The seond pproh is not pplile to CMOS strutures euse of the high growth tempertures required (ove 1,400 C). Moreover, this ltter proess produes grphene flkes nd dots, rther thn ontinuous films. Therefore, diret growth of defet-free grphene t low tempertures on non-tlyti surfes is required for CMOS integrtion. We hve demonstrted tht grphene film n e formed on lrge-re film y flow ontrol of the preursor gs. Figure 4 shows the experimentl results demonstrting tht few-lyered grphene n e diretly grown on t 1,000 C using the CVD method. growth on the non-tlyti film is enled y enhning the intertions of the preursor gs (C 2 H 2 ) with the film nd y optimizing the flow diretion s well s the pressure. This indites tht CVD hs the potentil to grow grphene on diretly; however, defets need to e minimized, nd ontrol of the numer of lyers should e improved. For growth, most studies hve foused on tlyti metl sustrtes To relize the suggested modultor nd detetor strutures, however, on non-tlyti surfes is required. We hve ttempted to deposit diretly on non-tlyti surfes suh s (001) nd Al 2 O 3 (0001) y therml CVD. The on ould not e grown t ll. However, rystlline hs een suessfully grown on Al 2 O 3. Figure 4d shows four-lyered on Al 2 O 3 t 1,000 C, hieved y ontrolling the flow kinetis where lrge mount of preursor gs flows vertilly reltive to the sustrte. These preliminry results suggest tht the kineti intertions of preursors with the sustrte, s well s the hemil retivity of surfe toms on the sustrte, re importnt ontrol prmeters. Therefore, ould e deposited on if the surfe is modified y dsorption of Al toms, enhning preursor retivity with the surfe. To integrte grphene in silion CMOS devies, diret growth of grphene on three-dimensionl strutures, simultneous deposition on vrious underlying mterils, nd low proessing tempertures re required. It will lso e importnt to pursue further work on film growth tht ould e used s dieletris in rdio-frequeny trnsistors nd s insulting mterils in optil modultors nd detetors. Indepth study of nuletion nd growth mehnisms on non-tlyti surfes will lso elerte the full reliztion of grphene-sed hyrid silion-cmos pplitions. 342 NATURE VOL NOVEMBER 2011

6 REVIEW INSIGHT Porous film Air gp Buried struture p- Ridge struture p- p- Oxide Mterils on on oth Cu nd low-κ dieletri Mterils on on oth nd O 2 on grphene Mterils on on on grphene Mterils on on on grphene Figure 5 Proposed devie strutures nd proess requirements for grphene nd films., Rdio-frequeny trnsistors hve flt surfe overge. Film growth tempertures should not exeed the k-end-of-line proessing tempertures round 500 C., Optil modultors with uried strutures lso hve flt surfe overge, wheres those with ridge strutures hve topologil surfe overge. Both types hve struturl stility even t high frition tempertures of up to 1,000 C., Photo-detetors hve ridge struture, giving them topologil surfe overge. Like optil modultors, they hve struturl stility nd re le to withstnd frition tempertures of up to 1,000 C. Outlook We hve reviewed grphene devies for possile pplitions of urrent CMOS tehnology, fousing on rdio-frequeny trnsistors, optil devies nd the deposition proesses. By tking dvntge of grphene s high rrier moility nd ultr-widend optil sorption, we hve proposed new rhitetures tht re most likely to provide the erliest pplitions: the nothing-on-grphene rhiteture for mximizing f T, the grphene-gting optil modultor nd the wveguide-integrted grphene photo-detetor. We hve lso onsidered the limittions of grphene, suh s the lk of ndgp through whih f mx n e improved nd its high ontt resistne. -sed memory hips nd miroproessors re unlikely to pper in the next few dedes. In the mentime, grphene will hve n importnt role in enhning the performne of, nd dding nlogue funtions to, silion-sed CMOS devies. However, for use in ommeril grphene devie pplitions, frition must e simple, reproduile nd omptile with existing semiondutor proesses. This implies tht mny of the urrent prties of grphene prodution will hve to e hnged drstilly. Diret growth of high-qulity grphene onto ommonly used mterils in semiondutor proesses, suh s nd O 2, would e desirle. Advnes onfined to grphene lone re proly insuffiient; new underlying mterils, suh s, nd proesses to otin suh mterils must e developed to fully mximize grphene s potentil. As interest in grphene ontinues to inrese, we re optimisti tht these tehnil issues will e resolved. The first pplition for grphene will proly e the rdio-frequeny swith, whih hs lredy shown potentil for improved performne, followed y optil modultors nd photo-detetors. The next rekthrough, when the ndgp energy n e inresed nd ontrolled, is likely to expnd the verstility of hyrid silion-cmos pplitions, mking it vitl prt of silion CMOS evolution. 1. Kim, K. From the future tehnology perspetive: hllenges nd opportunities. Teh. Digest Int. Eletron Devies Meet. 10, 1 9 (IEEE, 2010). 2. Novoselov, K. S. et l. Two-dimensionl tomi rystls. Pro. Ntl Ad. Si. USA 102, (IEEE, 2005). 3. Novoselov, K. S. et l. Two-dimensionl gs of mssless Dir fermions in grphene. Nture 438, (2005). 4. Zhng, Y. et l. 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Diret growth of grphene/hexgonl oron nitride stked lyers. Nno Lett. 11, (2011). Aknowledgements We thnk U.-I. Chung, H. Kim, Y. Y. Lee, S. Jeon nd S.-H. Lee for ssisting with sientifi disussions nd ontriutions. Author Informtion Reprints nd permissions informtion is ville t www. nture.om/reprints. The uthors delre no ompeting finnil interests. Reders re welome to omment on the online version of this rtile t nture. Correspondene should e ddressed to K.K. (kn_kim@smsung.om). 344 NATURE VOL NOVEMBER 2011