Surface nanostructuring of TiO2 thin films by ion beam irradiation

1 Su ace Nanos uc u ing o TiO2Thin Films by Ion Beam I adia ion P. Rome o-Gomez(a, A. Palme o(a,1, F. Yube o(a, M. Vinnichenko(b, A. Koli sch(b, A.R. Gonzalez-Elipe(a a) Ins i u o de Ciencia de Ma e iales de Se illa (CSIC-Uni e sidad de Se illa). c/ Ame ico Vespucio 49, 41092 Se illa (Spain) b) Ins i u e o Ion Beam Physics and Ma e ials Resea ch. Fo schungszen um D esden- Rossendo , POB 510119, 01314 D esden (Ge many). Abs ac . This wo k epo s a p ocedu e o modi y he su ace nanos uc u e o TiO2ana ase hin ilms h ough ion beam i adia ion wi h ene gies in he keV ange. The i adia ion wi h N+ions leads o he o ma ion o a laye wi h oids a a dep h simila o he ion p ojec ed ange. By se ing he ion p ojec ed ange ew ens o nanome es below he su ace o he ilm, well o de ed nano ods appea aligned wi h he angle o incidence o he ion beam. Sligh ly di e en esul s we e ob ained by using hea ie (S+) and ligh e (B+) ions unde simila condi ions. 1)Email: [email p o ec ed]sic.es * Tex only Click he e o iew linked Re e ences 2 Ti anium dioxide (TiO2) hin ilms possess ou s anding p ope ies ha make hem sui able o many echnological applica ions. Thei excellen op ical ansmi ance, high e ac i e index, pho oac i i y, chemical s abili y, e c. ha e mo i a ed he s udy o his ma e ial as well as he op imiza ion o i s p ope ies and unc ionali ies [1-2]. Wi hin his con ex , su ace nanos uc u ing o TiO2 hin ilms u ns ou o be o g ea impo ance as he mo phology o his ma e ial de e mines in a g ea ex en many o i s p ope ies. The e a e many examples o ion-beam-induced modi ica ions o hin ilms in he li e a u e [3]. Fo ins ance, Si and Ge hin ilms unde He ion i adia ion in he keV ange de elop mic oca i ies in he ma e ial [4]. On he o he hand, Ge, GaSb o InSb hin ilms show a so-called anomalous beha iou , i.e., hey expe ience a clea swelling du ing he ion beam i adia ion due o he o ma ion o oids in he bulk [5-7]. These oids a e o med as a esul o he long- ange mig a ion o in e s i ial a oms and he mo emen o ion-induced poin de ec s in he ilm [5]. Ano he ema kable e ec has been epo ed o GaN hin ilms unde ion beam i adia ion. He e, an impo an s oichiome ic imbalance akes place in he ma e ial due o he mass di e ence be ween 70Ga and 14N, p oducing Ga- ich and N- ich egions. Since many o hese N a oms can no be easily inco po a ed in o he ne wo k o he ma e ial, N2molecules a e gene a ed leading o he o ma ion o nanome e-size bubbles embedded in he ilm [8-10]. Fu he mo e, he e is e idence o mig a ing Ga in e s i ial a oms du ing he ion i adia ion [11], hus sugges ing a possible ela ion be ween he bubble o ma ion mechanism and he ion beam induced mo emen o poin de ec s. The mechanism o he bubble o ma ion is s ill unde esea ch as he e a e cases o hin ilms con aining andomly sca e ed N2molecules ha do no come oge he and o m a bubble. 3 Fo ma ion o sca e ed N2molecules embedded in hin ilms subjec ed o low ene gy N+ implan a ion has been epo ed o di e en ma e ials, such as GaN [12], SiOX[13] and Al2O3[14-15]. The beha iou o c ys alline TiO2 hin ilms unde se e al ens keV ion beam i adia ion has been ex ensi ely s udied in he li e a u e. I is known, o ins ance, ha he TiO2 ne wo k expe iences an impo an deple ion o oxygen due o he p e e en ial spu e ing [16-18]. To ou knowledge, no indica ion o mic oca i ies, anomalous swelling o bubble o ma ion has been epo ed in he li e a u e o TiO2. The main aim o his le e is o s udy he in e ac ion o ions wi h ene gies in he o de o se e al ens keV wi h a TiO2ana ase hin ilm o de elop a sys ema ic me hod o modi y i s mo phology and nanos uc u e. In he cou se o his in es iga ion, i has been shown ha his echnique can be used as a no el app oach o he su ace nanos uc u ing o his ma e ial. TiO2 hin ilms wi h hicknesses be ween 300 and 500 nm we e deposi ed a 523 K using he plasma enhanced chemical apou deposi ion echnique. The ilms, depic ing he ana ase s uc u e, showed a high deg ee o c ys allini y and a clea columna mic os uc u e pe pendicula o he subs a e. The de ails o he s uc u e, mic os uc u e and o he cha ac e is ics o he ma e ial as well as he desc ip ion o he deposi ion echnique appea in e . [19]. In igu e 1a-b, he c oss-sec ional and no mal- iew images o an as-deposi ed TiO2 hin ilm ob ained by scanning elec on mic oscopy (SEM) a e depic ed. I consis s o an a angemen o diso de ed g ains agglome a ed in he o m o columns ha ex end om he subs a e up o he su ace. This mic os uc u e will se e as a e e ence o compa e wi h he esul s o he subsequen expe imen s. The ion i adia ions we e ca ied ou a he Ins i u e o Ion 4 Beam Physics and Ma e ials Resea ch, Fo schungszen um D esden-Rossendo . The ion luence was always se o 17 2 2.4 10 cm  , whe eas he ion pene a ion dep h and he ion p ojec ed ange (RP) we e calcula ed using he SRIM so wa e [20]. In he cases whe e he i adia ion was ca ied ou a oom empe a u e, he ion cu en was se low enough o keep he ilm empe a u e below 70º C (340 K). The TiO2 hin ilm was i s ly i adia ed using a 60 keV N+ion beam impinging he su ace a 45° o he no mal. These condi ions imply an ion PD o a ound 200 nm and a Rpo a ound 100 nm. This means an a e age concen a ion o implan ed ni ogen o ~1015 ions cm-2 nm-1 in he ilm and ew ens o nanome es o ma e ial emo ed due o he spu e ing. The no mal and c oss-sec ional SEM image o he TiO2 hin ilm a e i adia ion appea s in igu e 2a-b. In igu e 2b we ha e also depic ed he calcula ed dis ibu ion o implan ed ions in he ilm. Figu es 2a-b e idence h ee impo an di e ences compa ed wi h igu es 1a-b: i) he columna mic os uc u e disappea s in he implan a ion egion, ii) he su ace shows a e y la mo phology, and iii) an in e media e laye con aining sphe ical oids, wi h a diame e up o 10 nm, appea s in he bulk o he ma e ial. The i s ea u e can be unde s ood by aking in o accoun he ion-induced diso de in he ma e ial along he ion acks, which also causes an impo an dec ease o he c ys allini y. This was co obo a ed by he loss o he X- ay di ac ion (XRD) peaks obse ed o he o iginal ilms (no shown). I is wo h men ioning ha he o iginal columna mic os uc u e su i es benea h he implan ed egion. The second ea u e indica es ha he o iginal pa e n on he su ace has been la ened: his phenomenon is well documen ed in he li e a u e [21-22], whe e i has been explained by aking in o conside a ion he 5 compe i ion among ballis ic e ec s, su ace di usion and he mal spike-induced olume iscous low. The hi d ea u e in igu e 2a-b, he appea ance o a laye wi h nanome e-sized oids, is a ema kable phenomenon ha , o ou knowledge, is epo ed o he i s ime in he li e a u e on TiO2 hin ilms. Figu e 2a shows a clea ela ion be ween he appea ance o oids and he ion p ojec ed ange (RPdep h) in he ma e ial. Thus, i seems ha he mig a ion o ion-gene a ed poin de ec s ( esponsible o he men ioned anomalous swelling o Ge, GaSb o InSb hin ilms) and/o he o ma ion o N2bubbles in o he ma e ial migh play an impo an ole in he o ma ion o hese oids. This issue will be discussed la e on. Figu es 2a-b indica e ha he mic os uc u e o TiO2ana ase hin ilms can be hea ily modi ied h ough ion beam i adia ion. Rega ding he h ee abo emen ioned modi ica ions p oduced by he N+ions, we p opose a me hod o modi y he ilm su ace mo phology: i he ion i adia ion ene gy is se in such a way ha RPis close o he su ace, apa om he well known p ocesses esponsible o he la ening o he su ace (ballis ic e ec s, su ace di usion and he mal spike-induced olume iscous low), we will in oduce a new compe ing mechanism ( oid o ma ion) wi hin he same spa ial egion. Thus, i is expec ed ha his addi ional nanos uc u ing ac o loca ed in he op-mos su ace laye s can a ec he mo phology o he ilm su ace. This hypo hesis is es ed in he ollowing expe imen s. In igu e 2c we depic he op- iew and c oss-sec ional SEM images o a ilm i adia ed wi h N+ions wi h an ene gy o 15 keV and an angle o incidence o 45 deg ees, keeping he ilm empe a u e below 340 K du ing he i adia ion. These condi ions imply an ion pene a ion dep h o 100 nm, and a RPo abou 50 nm. Figu e 2c depic s a e y di e en 6 mo phology o ha in igu e 2a. In his case, a ypical sponge-like diso de ed mic os uc u e o med by nanome ic elemen s ha pa ially ollow he di ec ion o he ion beam is ound. The c oss-sec ional SEM image ( igu e 2d) shows ha he ypical columna s uc u e o he o iginal ilm emains in he non-implan ed pa , whe eas il ed and diso de ed nano ods a e o med in he su ace egion. The Raman spec um o his ilm is shown in igu e 3 along wi h he spec um o a non-i adia ed ana ase TiO2 ma e ial o he equency ange whe e he mos in ense TiO2ana ase peak appea s (i.e., 143 cm-1 co esponding o he Eg ib a ional mode) [23]. This egion is o pa icula in e es because i p o ides impo an in o ma ion abou he s uc u al changes o he ma e ial. The emaining pa o he spec a is no shown as i p o ides edundan in o ma ion abou he ana ase s uc u e and has no ele ance in his s udy. Compa ing he wo spec a in igu e 3, we ind wo impo an di e ences: i) he peak co esponding o he i adia ed ilm is b oade han ha o he non-i adia ed one, and ii) he peak associa ed o he i adia ed ilm shows a small shi owa ds highe wa enumbe . In gene al, he a ia ions o Raman peak shape a e usually caused by su ace ension o g ains, nons oichiome y, diso de induced by mino phases, o phonon con inemen e ec s wi h he g ain size a ia ion. Howe e , o TiO2i was demons a ed ha he changes obse ed in he Raman spec a we e no ela ed o any in e nal s ess o g ain size e ec s [23]. Thus, he edshi o he spec um can only be linked o he inc ease o la ice pa ame e (loss o c ys alini y), and he subsequen dec ease o he phonon equency, as well as o he p esence o dangling bonds associa ed o he oxygen deple ion [23]. This la e e ec is expec ed due o he a o emen ioned p e e en ial spu e ing o oxygen a oms du ing i adia ion, and ag ees wi h he ac ha he ilm becomes da k colou ed and shows a high elec ical conduc i i y, bo h ea u es ypical o pa ially educed i anium oxide ma e ials. 7 In o de o allow he ilm o ec ys allize du ing he ion bomba dmen , we pe o med an expe imen simila o he p e ious one, i.e. we i adia ed wi h N+ions wi h an ene gy o 15 keV and an angle o incidence o 45º, bu his ime we se he ilm empe a u e o 700 K du ing he ion i adia ion. These condi ions ensu e he ec ys alliza ion o he ilm in o he ana ase phase. The no mal and c oss-sec ional SEM images depic ed in igu es 4a-b co espond o his expe imen . As in he p e ious case, he c oss-sec ion iew depic s a ypical columna mic os uc u e in he bo om pa o he ilm. Howe e , now he ilm e mina ion consis s o well o de ed nano ods o med in he ion pene a ion dep h egion. Thus, in igu e 4a, p esen ing he no mal- iew in wo scales, a egula pa e n o nano ods wi h simila wid h anging be ween 20 and 30 nm can be obse ed on he su ace. The c oss-sec ional iew ( igu e 4b) shows ha he nano ods ha e a leng h o app oxima ely 100 nm and ha hey a e qui e well aligned along he ion beam di ec ion. In igu e 3, he Raman spec um o his ilm shows a sha pe peak han ha o he equi alen sample i adia ed a oom empe a u e, indica ing a be e c ys allini y o he ilm [23], which is expec ed due o he highe ilm empe a u e du ing i adia ion. This ag ees wi h XRD measu emen s whe e he peaks o he ana ase s uc u e o i anium oxide could be de ec ed (no shown). To know mo e abou he p oposed me hod o he modi ica ion o he su ace opog aphy, we pe o med ano he expe imen using an ion wi h a simila a omic mass as ni ogen bu ha in no way can p oduce gas bubbles in o he laye . Thus, we i adia ed he TiO2 hin ilm wi h a B+ion beam wi h an ene gy o 30 keV and an angle o incidence o 45º. The ilm empe a u e was kep equal o 700 K du ing he i adia ion o allow ec ys alliza ion. Unde hese condi ions, he ion pene a ion dep h was 120 nm 8 and RP60 nm, qui e simila o he p e ious expe imen s wi h ni ogen. The no mal- iew SEM image o he i adia ed ilm is depic ed in igu e 4c, showing di e en ea u es han in he p e ious cases. This ime small ca i ies a e o med on he su ace o he ilm, and no hin o nano od o ma ion is ound. This becomes mo e e iden in igu e 4d, whe e we depic he c oss-sec ional SEM image. Thus, his si ua ion seems o ep esen an in e media e s a e whe e oids a e o med on he su ace o he ma e ial while o he mechanisms a e esponsible o he la ening o he emaining su ace egions. The Raman spec um o his ilm also appea s in igu e 3, and is cha ac e ized by a la ge shi o he peak owa ds he igh when compa ed wi h he non-i adia ed spec um. This poin s ou ha he educ ion o he ma e ial due o he p e e en ial spu e ing o oxygen is much la ge han ha du ing ni ogen i adia ion. The dis inc chemical cha ac e o ni ogen and bo on in he implan ed s a e may also con ibu e o his di e ence: ni ogen is a e y elec onega i e a om ha ends o o m ni ide species when i is combined wi h me als, whe eas bo on may ac as a ca ion when i is bonded o oxygen. Hence, di e en chemical s uc u es a e expec ed when hey a e implan ed in TiO2. In a las expe imen we employed a hea ie ion, S+, pe o ming he i adia ion wi h an angle o incidence o 45° and an ene gy o 45 keV, keeping he ilm empe a u e equal o 700 K. Thus, he ion pene a ion dep h was abou 50 nm, and RP30 nm. The no mal- iew SEM image o he ilm a e i adia ion wi h he S+ion is shown in igu e 4e, whe e we can ind spaced s uc u es simila o hose in igu e 4a. The c oss-sec ional SEM iew o he ilm ( igu e 4 ) shows ha he su ace is comple ely co e ed wi h nano ods aligned wi h he ion beam, al hough his ime hey a e sha pe and sligh ly less o de ed han hose o med by he ni ogen i adia ion. Rega ding he Raman spec um 9 ( igu e 3), we ind a clea shi o he peak owa ds he igh as compa ed wi h he non- i adia ed case. As we no ed p e iously, his shi e eals a signi ican educ ion o he oxide hin ilm [23]. This expe imen wi h S+ions sugges s ha he o ma ion o nano ods on he su ace o he ilm, and hus he o ma ion o oids du ing ion i adia ion, is mainly associa ed o he mig a ion o poin de ec s du ing he i adia ion and no o he o ma ion o bubbles. This is suppo ed by he ac ha implan ed S+canno be eleased as a gas and mus emain as de ec ions wi hin he TiO2s uc u e. The di e en beha iou ound wi h B+ sugges s ha a ce ain h eshold o he momen um and ene gy deposi ed on he TiO2 and he di e en chemical cha ac e o he species a e also impo an ac o s con olling he o ma ion o nano ods. Ou p e ious esul s, showing ha o de ed nano ods a e only o med a 700 K also sugges ha he mally ac i a ed di usion is also impo an o hese p ocesses. Al hough his empe a u e is no e y high o he mobilisa ion o s oichiome ic TiO2, i can be su icien o he mobilisa ion o an oxygen deple ed i anium oxide: he so called Magnelli phases (i.e., Ti anium Oxides wi h a a iable s oichiome y o he ype TiOx, x<2) a e well known o hei high mobili y a no e y high empe a u es [24]. Simila ly, o ana ase ilms, a high mobili y o Ti3+ species om he bulk o he su ace has been di ec ly obse ed using he scanning unneling mic oscopy echnique on pa ially educed single c ys als o his ma e ial hea ed in acuum [25]. No u he conclusion on he undamen al mechanisms esponsible o he su ace nanos uc u ing unde ion i adia ion can be achie ed ega ding ou expe imen s, and mo e esea ch, ou o he scope o his le e , is equi ed. Howe e , om a phenomenological poin o iew, i seems ha he pa ame e s ha de e mine he appea ance o nano ods on he su ace o he ilm a e he ion pene a ion dep h, he Figu e(s) Click he e o download high esolu ion image Figu e(s) Click he e o download high esolu ion image Figu e(s) Click he e o download high esolu ion image