FOCUSED ION BEAM TREATMENT OF ZnO NANOWIRES. G.Sh. Shmavonyan

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1 FOCUSED ION BEAM TREATMENT OF ZnO NANOWIRES G.Sh. Shmvonyn Stte Engineering University of Armeni, 105, Teryn str., 0009, Yerevn, Armeni E-mil: (Reeived 29 Septemer 2009) We investigted vpour-liquid-solid (VLS)-grown ZnO nnowires (NWs) on Si sustrte y snning eletron mirosopy (SEM). Nnowires re found in vriety of shpes, surfe properties nd fetures, ut seem to e ll homogeneous inside. Drkish spots nd res on the surfe re esily removed y gentle foused ion em (FIB) tretment. To unveil their deeper interior, we lso pplied FIB tretment. An overview of our results (SEM imges) is given in this pper. 1. Introdution Nnostrutures hve ttrted muh ttention in reent yers [1-6]. New methods hve een developed to frite welth of strutures [3, 4, 6-9] whose prtiulr fetures depend on the preise growth onditions, sustrtes, nd tlysis. Espeilly, self-ssemled NWs offer lrge vriety of interesting nd unique properties sine they turned out to grow in rther exellent qulity even in reltively esily ontrollle proesses nd to eome nerly strin-free under these onditions. In prtiulr, ZnO NWs hve ttrted renewed ttention in reent yers due to their peulir optil properties [2, 6, 10-14], euse this mteril system is promising ndidte for short wvelength optoeletroni devies. The ttention to ZnO hs onsiderly enhned sine the first report on p-doped ZnO light emitting diodes ws pulished in 2005 [15]. Mny room-temperture pplitions hve lredy een shown, suh s single-nw field-effet trnsistors, light emitting diodes, lser diodes, logi gtes omining oth n-type nd p-type NWs, solr ells, nd sensors [1, 5, 15-19]. Self-orgnized ZnO NWs hve sustntil dvntges ompred to other mteril systems, e. g., sene of surfe oxidtion, high exiton inding energy nd ferromgneti properties when doped with trnsition elements. A key property is the unique verstility in terms of geometril dimensions nd omposition. However, for ny pplition detiled knowledge of the optil properties, their mirosopi origin, surfe morphology, nd ross-setion re of fundmentl importne. In this pper, we report on investigtions of the surfe nd ross-setion of VLS-grown ZnO NWs on Si sustrte y SEM fter foused ion em (FIB) tretment. FIB systems hve een produed ommerilly for pproximtely ten yers, primrily for lrge semiondutor mnufturers; they operte in similr fshion to SEM exept, rther thn em of eletrons, they use finely foused em of gllium ions for the milling of smll holes into the smple t well lolized sites, so tht ross-setionl imges of the struture n e otined or tht the strutures n e polished. The pplitions of FIB inlude: rosssetionl imging through semiondutor lyered strutures nd devies, modifition of the eletril routing on semiondutor devies, iruit modifition, filure nlysis, defet nlysis, preprtion for physil-hemil nlysis, preprtion of the smples for TEM of site speifi lotions on integrted iruits, miro-mhining, msk repir, nd vrious nonsemiondutor pplitions.

2 Moldvin Journl of the Physil Sienes, Vol.8, N2, Experiments The experiments were rried out in Dul Bem foused ion em system (model: NOVA 200 NnoL, ompny: FEI), whih disposes omined SEM/FIB hmer, t ~ mr. For SEM imging n elertion high voltge of 5 kv nd em urrent of 0.4 na ws used resulting in n e-em dimeter of 67.8 nm; for FIB proessing, n elertion high voltge of 30 kv nd n G + -ion-em urrent of 1 pa nd 3 na re used, with em dimeter of 7.0 nm nd 81.0 nm, respetively. The milling time depends on re of the smple to e milled, depth, nd em energy; for the investigtions in this pper we employed times from few seonds down to frtions of seond. The ZnO NW smples were grown on Si sustrte y the VLS-tehnique [20] y S. Börner in the group of Prof. W. Shde t the TU Clusthl, Germny. The geometril struture of the otined NWs depends strongly on the growth onditions. Tempertures t the soure mteril nd the sustrte re of min influene s well s the pressure nd gs flow. Due to the ft tht individul wires of the s-grown ensemle nnot e diretly ddressed y foused lser em or SEM, single NWs hve to e extrted from the ensemle, whih ws done in n ultrsoni dissolver th, efore they investigted the optil properties of ZnO single nd ensemle NWs [12-14]. To e le to hrterize ZnO NWs y their surfe properties, SEM smples with high densities of NWs were frited fter the proedure desried ove. After proessing different NWs y FIB tretment the surfes nd ross-setions of the NWs were imged y SEM. In the following, FIB polishing stnds for gentle FIB-proessing, using very short times, just to remove the top lyers of the wires leving the overll integrity undmged, wheres FIB milling denotes the proessing t longer exposure times not only to remove the top lyers, ut to drill, ut, nd disintegrte the wires to gin ess nd insight into their interior, the overll integrity here is ertinly not onserved. As the ion urrents nd exposure times re not unique for milling nd polishing, they re expliitly given in the figures. While polishing normlly ffets the entire imge re, milling only ffets smll prts of it, s n e seen y the presene of dmged regions ( rters ) in the sustrte SEM on s-grown NWs 3. Results nd disussion Here we present the results of SEM investigtions of VLS-grown ZnO NWs on Si sustrte efore ny FIB tretment. SEM imges of ZnO NWs in Figs. 1 nd 2 show tht there re single NWs nd ensemles of NWs, mong whih we found stright nd end, perfet (regulr fets, smooth surfes) nd non-perfet (irregulrly fetted, rough surfes, vrile widths, dmged or disintegrted) NWs, s well s NWs with len surfes nd surfes with drk spots nd fetures property y whih we tegorized into Figs. 1 nd 2. The sizes of the NWs re: out 2 24 µm in length nd out nm in width nd height. The hexgonl fets oserved emphsize the good rystlline qulity of the NWs (Fig. 1) Verifying surfe fetures To investigte now whether these surfe speilties re propgting into the NWs, their surfes nd ross-setions hve een investigted y FIB polishing (Figs. 3 nd 4) nd milling (Fig. 4). Here, the removl of prominent drk spot on NW is niely shown in Figs

3 G.Sh. Shmvonyn nd 3, together with FIB-shdow effet on the Si sustrte in Fig. 3, the NWs re not lying fltly on the ground. The ross-setion of the NW t the left-hnd side in Fig. 4 (rrow) is shown in Fig. 4, with no evidene of ny protrusion of the originl drk spot in Fig µm 10 µm 2 µm d e f 500 nm Fig. 1. SEM imges of ensemle (ove) nd single (elow) perfet ZnO NWs on Si sustrte 5 kv. high voltge; top view (0 tilt) in d, 45 tilt in e, f. 2 µm d e f Fig. 2. SEM imges of ZnO drkishly spotted single NWs on Si sustrte, imge d is zoomin of imge ; 5 kv. high voltge, 45 tilt in f. 203

Moldvin Journl of the Physil Sienes, Vol.8, N2, 2009 d Fig. 3.

for split seond; 5 kv. high voltge, 52 tilt in d. 4

polishing () nd FIB milling () t 3 na G + ion urrent; 5 kv. high voltge, 52 tilt in.

3, upper row) on the surfes nd fets of the NWs re removed fter FIB polishing (Fig.

Their sizes were lso mesured: length 200 nm, width 5 100 nm. 3.

4 Moldvin Journl of the Physil Sienes, Vol.8, N2, 2009 d Fig. 3. SEM imges of the s-prepred (, ) ZnO NWs nd fter FIB polishing (, d) t 3 na of G + ion urrent for split seond; 5 kv. high voltge, 52 tilt in d. 4 µm 4 µm 500 nm Fig. 4. SEM imges of the s-prepred ZnO NWs efore FIB tretment (), fter FIB polishing () nd FIB milling () t 3 na G + ion urrent; 5 kv. high voltge, 52 tilt in. The exmintion of the surfes of the NWs y SEM showed tht the surfe spots nd fetures (Fig. 3, upper row) on the surfes nd fets of the NWs re removed fter FIB polishing (Fig. 3, lower row), showing they re not propgting within the NWs, whih is gin onfirmed y the ross-setion imge in Fig. 4. Thus, ll those drk spots nd fetures on the NWs re rel surfe fetures. Their sizes were lso mesured: length 200 nm, width nm Ruling out uried inhomogeneities So, surfe fetures re not propgting into the NWs, ut wht out, e.g., uried strutures? For the ross-setionl investigtion the NWs were gin FIB milled (ut with muh lower energy to gently thin the wires insted of rute ut s in setion 3.2), whih yields informtion of the deeper interior of the NWs. The suessive ut of single NW (Fig. 5) gin shows no evidenes for ny hidden inhomogeneities, n expeted result s from setion 3.2, ut here only one wire ws exmined. 204

G.Sh. Shmvonyn Fig. 5. SEM imges of the ZnO NWs efore () nd fter suessive FIB milling ( nd ) t 1 pa G + ion urrent; 5 kv. high voltge, 52 tilt in. 4.

Surfe hrteriztion of the ZnO NWs y SEM showed tht the surfe of the ZnO NWs is not smooth nd there re different drkish spots nd fetures on it, whih re esily removed y gentle FIB tretment nd do not

The question, how those surfe fetures develop nd if they influene the optil properties nd thus show ny tehnologil relevne, remins field of reserh for the future. 5.

5 G.Sh. Shmvonyn Fig. 5. SEM imges of the ZnO NWs efore () nd fter suessive FIB milling ( nd ) t 1 pa G + ion urrent; 5 kv. high voltge, 52 tilt in. 4. Conlusions SEM investigtions show tht there re single nd ensemle of NWs, mong whih we found perfet nd non-perfet NWs. Surfe hrteriztion of the ZnO NWs y SEM showed tht the surfe of the ZnO NWs is not smooth nd there re different drkish spots nd fetures on it, whih re esily removed y gentle FIB tretment nd do not protrude into the wires. By more gentle ross-setionl nlysis we found out tht uried strutures re lso totlly sent. The question, how those surfe fetures develop nd if they influene the optil properties nd thus show ny tehnologil relevne, remins field of reserh for the future. 5. Aknowledgements We wish to thnk Prof. Dr. Jens Flt nd his group t the Bremen University for ollortion, S. Börner nd the group of Prof. Shde t the TU Clusthl for providing the rw nnowires, nd L. Wishmeier nd the group of Prof. Gutowski t the university of Bremen for ultrsoni preprtion of the SEM-smples. Referenes [1] P. Yng, H. Yn, S. Mo, R. Russo, J. Johnston, R. Syklly, N. Morris, J. Phm, R. He, nd H.-J. Choi, Adv. Funt. Mter., 12, 323, (2002). [2] U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshhikov, S. Dogn, V. Avrutin, S.-J. Cho, nd H. Mrkos, J. Appl. Phys., 98, , (2005). [3] M.H. Hung, S. Mo, H. Feik, H. Yn, Y. Wu, E. Weer, R. Russo, nd P. Yng, Siene, 292, 1897, (2001). [4] G.C. Yi, C. Wng, nd W.I. Prk, Semion. Si. Tehnol., 20, S22, (2005). [5] X. Dun, Y. Hung, R. Agrwl, nd C.M. Lieer, Nture, 421, 241, (2003). [6] Z.L. Wng, Mter. Tody, 6, 6, 26, (2004). [7] L. Smuelson, Mter. Tody, 6, 10, 22, (2003). [8] H.J. Fn, W. Lee, R. Sholz, A. Ddgr, A. Krost, K. Nielsh, nd M. Zhris, Nnotehnology, 16, 913, (2005). [9] H.J. Fn, P. Werner, nd M. Zhris, Smll, 2, 6, 700, (2006). [10] C. Klingshirn, R. Hushild, H. Priller, M. Deker, J. Keller, nd H. Klt, Superltties nd mirostrutures, 38, 209, (2005). 205

6 Moldvin Journl of the Physil Sienes, Vol.8, N2, 2009 [11] C. Klingshirn, H. Priller, M. Deker, J. Bruekner, H. Klt, R. Hushild, J. Zeller, A. Wg, A. Bkin, H. Wehmnn, K. Thonke, R. Suer, R. Kling, F. Reuss, nd Ch. Kirhner, Adv. in Solid Stte Phys., 45, 261, (2005). [12] L. Wishmeier, T. Voss, S. Borner, nd W. Shde, Appl. Phys. A, 84, 111, (2006). [13] L. Wishmeier, C. Bekeny, T. Voss, S. Borner, nd W. Shde, Phys. Stt. Sol. (), 243, 4, 919, (2006). [14] T. Voss, C. Bekeny, L. Wishmeier, H. Gfsi, S. Börner, W. Shde, A.C. Mofor, A. Bkin, nd A. Wg, Appl. Phys. Lett., 89, , (2006). [15] A. Tsukzki et l., Nture Mterils, 4, 42, (2005). [16] J. Xing, W. Lu, Y. Hu, Y. Wu, H. Yn, nd C.M. Lieer, Nture, 441, 489, (2006). [17] B.S. Kng, Y.W. Heo, L.C. Tien, D.P. Norton, F. Ren, B.P. Gil, nd S.J. Perton, Appl. Phys. A, 80, 1029, (2005). [18] C. Thelnder, T. Mrtensson, M.T. Bjork, B.J. Ohlsson, M.W. Lrsson, L.R. Wllenerg, nd L. Smuelson, Appl. Phys. Lett., 83, 2052, (2003). [19] C. Levy-Clement, R. Ten-Zer, M.A. Ryn, A. Ktty, nd G. Hodes, Adv. Mter., 17, 1512, (2005). [20] A.C. Mofor, A.S. Bkin, A. Elsher, D. Fuhrmnn, F. Bertrm, A. Hngleiter, J. Christen, nd A. Wg, Phys. Stt. Sol. (), 3, 1046, (2006). 206

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