Study of the formation of a microrelief on ZnSe- and CdSe-crystal surfaces ablated by excimer KrF-laser radiaton

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1 Quntum Electronics () 9 9 () Kvntovy Elektronik nd Turpion Ltd DOI:.7/QEL Study of the formtion of microrelief on ZnSe- nd CdSe-crystl surfces lted y excimer KrF-lser rditon S.K. Vrtpetov, A.V. Zkhryp, V.I. Kozlovsky, Yu.V. Korostelin, V.A. Mikhilov, Yu.P. Podmr kov, I.Yu. Porofeev, D.E. Sviridov, Y.K. Sksyrsky, M.P. Frolov, I.M. Yutkin Astrct. One-dimensionl grtings with period of.. mm re formed on the surfces of CdSe nd ZnSe crystls lted y two interfering rdition ems of nnosecond excimer KrF lser. Investigted re the dependences of the shpe nd depth of grtings on the energy density under irrdition y single pulse, nd on the numer of pulses t given energy density. The mximum grting depth is estimted s ~.7 of the period. By forming one-dimensionl grting with period of. mm nd depth of. mm on the CdSe-crystl surfce, this surfce ecomes ntireflective t wvelength of mm. The surfce reflectivity is reduced y %. A possiility of forming two-dimensionl grtings hving periods of nd. mm is demonstrted. Keywords: ntireflection surfce micro-relief, mid-ir rnge, II VI crystls, lser ltion, excimer KrF lser.. Introduction Formtion of n ntireflection (AR) microrelief on the surfce of opticl elements is of considerle interest for numer of pplictions, prticulrly in high-power mid-ir lsers. If we compre this method with conventionl deposition of thin-film AR cotings on the surfces of opticl elements, the surfce relief AR microstructures cn significntly increse the rdition strength of the AR surfce nd its resistnce to externl operting conditions, nd lso increse the ngulr nd spectrl width of the AR curve [ ]. AR microstructures re commonly formed y using multistge expensive technology, which involves photolithogrphy nd dry etching in plsm. Another method for producing microrelief on the opticl element surfce is lser ltion. This method is well studied nd tested on numer of mterils, including metls, lloys, glsses nd some semiconductors [ ]. To S.K. Vrtpetov, V.A. Mikhilov, I.Yu. Porofeev Physics Instrumenttion Centre, A.M. Prokhorov Generl Physics Institute, Russin Acdemy of Sciences, 9 Moscow, Troitsk, Russi; A.V. Zkhryp, I.M. Yutkin Russin Federl Nucler Center All-Russin Reserch Institute of Experimentl Physics, prosp. Mir 7, 7 Srov, Nizhnii Novgorod region, Russi; V.I. Kozlovsky P.N. Leedev Physics Institute, Russin Acdemy of Sciences, Leninsky prosp., 999 Moscow, Russi; Ntionl Reserch Nucler University MEPhI, Kshirskoe sh., 9 Moscow, Russi; e-mil: vikoz@sci.leedev.ru; Yu.V. Korostelin, Yu.P. Podmr kov, D.E. Sviridov, Y.K. Sksyrsky, M.P. Frolov P.N. Leedev Physics Institute, Russin Acdemy of Sciences, Leninsky prosp., 999 Moscow, Russi Received 9 June Kvntovy Elektronik () 9 9 () Trnslted y M.A. Monstyrskiy lesser extent, lser ltion hs een investigted for ZnSe crystls nd other II VI compounds [, ]. Recently, using vriety of compounds of this type, doped y the divlent ions of trnsition metls (ZnSe : Cr, ZnSe : Fe, CdSe : Cr, etc.), efficient opticlly pumped mid-ir lsers hve een developed, operting oth in the pulsed nd cw regimes [ 9] nd possessing significnt potentil for prcticl pplictions [, ]. In this regrd, of undouted interest is the development of the methods of deposition of rodnd AR cotings on the surfces of lser elements on the sis of II VI crystls, which llow the losses to e minimised oth t the pump wvelength nd in rod (~ mm) spectrl rnge of lsing, while mintining the rdition resistnce of the mteril. The im of this work is to study the shpe nd depth of one-dimensionl grtings formed in the ltion of the mteril under the ction of two interfering ems of n excimer KrF lser.. Experiment One-dimensionl sptilly periodic structures with n ritrry period re formed on the smple ccording to the scheme shown in Fig.. Figure. Schemtic of the interferometer: ( ) cylindricl lens; ( ) phse msk; ( ) nd ( ) reflecting mirrors; ( ) smple. The lser em psses through cylindricl lens ( ) nd splits into two ems y phse grting ( ). Then, using reflecting mirrors ( ) nd ( ), the ems re converged in the plne of smple ( ) t n ngle. The lser energy density t the smple is vried y moving the cylindricl lens long the opticl xis. The interference pttern period p in the smple plne is vried y selecting the ngle in ccordnce with the formul

2 9 S.K. Vrtpetov, A.V. Zkhryp, V.I. Kozlovsky, et l. p l =. () sin( / ) As rdition source, we hve used СL7 excimer lser system (Optosystems Ltd.), whose opticl scheme is shown in Fig.. It consists of two identicl gs-dischrge modules ( ) nd ( ) (KrF lsers of CL7 series) nd opticl locks ( ) nd ( ) plced on common opticl tle, s well s of control nd synchronistion systems. Module ( ) opertes s mster oscilltor (MO), nd module ( ) s power mplifier (PA). As dischrge chmer windows of the lser modules, use is mde of monocrystlline MgF, which is plced t the Brewster ngle to the opticl xis of the lser module to minimise the intrcvity losses. The ctive medium length of single module is mm, nd the height is mm. 9 The MO resontor consists of two-prism telescope (, 7 ) with mgnifiction fctor of, Echelle diffrction grting ( ) ( lines mm, lzing ngle of.9 ) nd output mirror ( ) with reflection coefficient of %. For etter sptil selection of rdition, diphrgms (, 9, ) hve een instlled. The use of diffrction grting s one of the mirrors llows the width of the genertion line to e reduced down to. nm (incresing the coherence length up to mm). The unstle PA resontor consists of concve sphericl mirror ( ) with rdius R =. m nd output convex sphericl mirror ( ) with rdius r =. m. A through hole with dimeter of. mm is mde in the concve mirror to introduce the MO rdition into the PA ctive medium. A em with dimeter of. mm, when pssing through the interelectrode gp, is incident on the convex sphericl mirror of the cvity nd, hving reflected from it, psses gin through the PA ctive medium. The em is lredy divergent t this stge, so tht it completely overlps the cross section of the ctive medium on the concve mirror of the cvity. Then, the em is reflected from the concve mirror, nd then, fter the lst mplifiction within the ctive gseous medium, comes out of the lser system through perture ( ). An insignificnt frction of the lser em, fter reflection from em splitter ( 7 ), flls on energy meter ( ) Figure. Opticl scheme of SL7 excimer lser system: ( ) MO; ( ) PA; (, ) opticl locks; ( ) grting; (, 7 ) prism telescope; (, 9, ) pertures; ( ) output mirror; (,, 9 ) deflecting mirrors; ( ) concve sphericl mirror; ( ) convex sphericl mirror; ( ) exit window; ( 7 ) em splitter; ( ) energy meter. The distnce etween the PA resontor mirrors is clculted y the formul R r = -, () which ensures the plnrity of the output em front. In our cse, = mm. The rdii of the mirrors re chosen to optimlly fill the lser s ctive volume y rdition. The importnt resontor chrcteristic is lso the telescoping rtio M defined s M R =. () r The greter the М, the smller the divergence nd the greter the length of sptil coherence of the lser em, ut, t the sme time, the lower the output energy of the mplified pulse. In our cse, М». The use of telescopic cvity llowed reducing the rdition divergence down to. mrd []. The totl pulse energy constituted mj, the durtion ws ns, the pulse repetition rte reched Hz nd the lser genertion wvelength ws equl to nm. The mximum size of the irrdited re on the smple surfce ws mm. To decrese the re size, the em ws diphrgmed. We hve used undoped ZnSe single-crystl pltes of different crystllogrphic orienttion, nd lso ZnSe : Fe nd CdSe : Cr pltes with the dopnt concentrtion of ~ cm. The plte thickness ws. mm. The plte surfces were mechniclly polished. In some of the pltes, the dmged lyer, fter mechnicl polishing, ws removed using polishing etchnt on the sis of CrO solution in HCl. The surfces of the pltes with microrelief were studied y mens of Solver P-7 Pro (NTMDT) scnning proe microscope. We hve lso used HA-NC (NTMDT) proes hving cone-tpered tip with length of. mm nd n pex ngle of. Figure shows scn of the TGZ reference grting with verticl wlls. It is seen tht the proe width is less thn nm t height of nm from the tip. The rtio of the proe height to its width exceeds the corresponding rtio for the grooves in the microstructures otined. This indictes tht the proe shpe does not ffect the resulting imges. All mesurements were performed in the contct scnning mode. Figure. Scnned imge of reference grting with period of mm nd depth of nm.

3 Study of the formtion of microrelief on ZnSe- nd CdSe-crystl surfces lted 9 The trnsmission spectr of the smples were mesured using n FSM IR Fourier spectrometer (Infrspek Ltd.).. Results of the experiment Figure shows the shpes of grting formed on the surfce of CdSe : Cr crystl under single-pulse irrdition t different energy densities of lser rdition. The interference irrdition zone ws mm in length nd. mm in width. The energy density distriution within the irrdition zone ws nonuniform. In prticulr, modultion rising from diffrction on the perture edge ws oserved t the zone edges. In ddition, the grting contrst ( rtio of the depth of dips to the grting period) ws vried irregulrly over the surfce of the irrdition zone (pproximtely y. times). A possile reson for such nonuniformity is mcro-nonuniform intensity distriution within the lser em. However, y selecting ech time the imge res with mximum contrst, it is possile to revel certin regulrity in the vrying grting shpe nd contrst depending on the irrdition energy density. The imge of the originl surfce (Fig. ) contins the trces of mechnicl polishing. These trces dispper fter chemicl polishing. At smll excess of the ltion threshold, the grting hs firly nrrow grooves (of width. mm in Fig. ) with the side ris protruding ove the originl surfce. The ppernce of ris is ssocited with extrusion of the ner-surfce molten crystl lyer from the zone with high density of sored energy into the zone with lower energy density y the pressure of the ner-surfce plsm formed in the process of crystl ltion []. With further increse in lser energy density, the width of the grooves increses, nd the height of ris grows (Fig. c) until they merge to form grting vertices (Fig. d). One cn see from Fig. tht the volume of ris is sustntilly less thn tht of grooves. Given the verticl loction of the smple, the remining ltion products re mostly dissolved in surrounding ir nd only prtilly deposited on the grting surfce. The mximum grting depth is chieved t the rdition energy density of F» mj cm nd constitutes ~ nm. A further increse in energy density reduces the grting depth nd contrst; therefore, the grting profile remins close to sinusoidl. At F = mj cm, the grting depth is only few tens of nnometers ( nm in Fig. e). The contrst decrese is proly explined y the fct tht, due to insufficiently high contrst of the interference pttern of two lser ems, the ltion threshold is lso chieved t the minim of the interference pttern. On the other hnd, the plsm formed t the mxim of the interference pttern strts to shield the crystl surfce from incident rdition, therey reducing the ltion rte. In the cse of smll depth of the grting formed t lrge F, the nnoscle hillocks emerging on the lser-treted surfce ecome visile. This is prticulrly well seen in the imge presented in Fig. otined in the regime of lterl force microscopy (friction forces), when, long with the surfce topogrphy (Fig. ), torsionl ending of the cntilever is recorded. The chrcteristic trnsverse size of hillocks is nm, nd their height mounts to nm. The surfce density of hillocks is estimted s cm. The origin of these hillocks is not yet cler. It is possile tht the deposition of ltion products results in the formtion of nno-ojects of type of quntum dots [7]. Becuse the grting groove depth otined in single pulse is limited to ~ nm t period of. mm, we hve tried to increse tht depth y using multiple pulses. Figure presents the shpes of grtings t different numers of pulses for the irrdition energy density of mj cm. The pulses followed with repetition rte of Hz. The mximum grting depth ws otined under irrdition y five pulses nd mounted to nm, which exceeds one-third of the grting period. We should lso note tht this depth is nerly five times greter thn the grting depth resulting from single-pulse irrdition. However, the depth strts to decrese when the numer of pulses increses. The min reson for tht is pprently temporl drift of the smple position reltive to the interference pttern. In ddition, the shpe of the groove ottom is rther complicted nd vries long the grting grooves (see the imge in Fig. c). It is likely tht certin prolems with ejection of ltion products from the grooves rise in the cse of sufficiently lrge depth [9]. Similr results were otined in the formtion of grtings on the ZnSe : Fe nd ZnSe surfces. Figure 7 shows the dependence of the depth of grting with period of. mm on the numer of pulses irrditing the ZnSe surfce nd following with repetition rte of Hz. As in the cse of CdSe : Cr, rpid sturtion of this dependence is oserved, lthough we hve not reveled ny noticele decline in it when the numer of pulses mounts to. It is possile tht the drift of the interference pttern long the smple surfce irrdited y lrge numer of pulses t pulse repetition rte of Hz hs lesser influence on the grting contrst thn t pulse repetition rte of Hz (Fig. ). The mximum grting depth ws chieved under irrdition employing pulses with F = mj cm nd constituted nm. Note tht we hve not found ny significnt differences in the microrelief of the grtings formed on the surfce of () nd () crystls. An exmple of grting with smller (. mm) period, formed on the CdSe : Cr surfce using five pulses t F = mj cm, is presented in Fig.. Here, the groove depth ws ~ nm. The grtings with period of. mm were lso formed. In the cse of single-pulse irrdition with F = mj cm, the groove depth on the ZnSe surfce ws nm. Figure 9 shows three-dimensionl imge of twodimensionl grting on the ZnSe surfce. Initilly, onedimensionl grting with period of. mm ws formed under single-pulse irrdition with F = mj cm ; then, the smple ws rotted y 9 nd second one-dimensionl grting ws formed. The height difference in the thus formed microrelief ws ~ nm. To increse this difference, it is necessry to increse the numer of pulses which re used in the formtion of one-dimensionl grting. For exmple, Fig. 9 shows similr grting formed y two crossed grtings with period of mm ech, which were formed y ten pulses t F = J cm. The height difference in this two-dimensionl grting exceeds nm.. Discussion of the results An AR microrelief cn e formed in two wys. The first wy is sed on the working mechnism of single-lyer AR cotings with refrctive index n r = n cr nd thickness l/(n r ), where n r nd n cr re the refrctive indices of the AR coting nd crystl, respectively []. In this cse, lyer with n pproximtely constnt thickness nd refrctive index n r verged long the lyer is formed on the crystl surfce. In

4 9 S.K. Vrtpetov, A.V. Zkhryp, V.I. Kozlovsky, et l. c d e f Figure. Imge frgments of grting on the CdSe : Cr surfce, formed under irrdition y single pulse with n energy density F = (), (), (c), (d), (e) nd (f) mj cm. To the right of imges, the corresponding, verticlly-verged, grting profiles re presented.

5 Study of the formtion of microrelief on ZnSe- nd CdSe-crystl surfces lted 97 Figure. () Imge of the CdSe : Cr crystl surfce, otined in the regime of lterl force microscopy nd () the corresponding topogrphic imge fter irrdition y pulse with F = mj cm. Drk peks re reveled ginst the ckground of the low-contrst grting with period of. mm. c.. d Figure. Imge frgments of the grtings formed on the CdSe : Cr surfce irrdited y (), (), (c) nd (d) pulses with F = mj cm, which follow with repetition rte of Hz. To the right of the imges, the corresponding, verticlly-verged, grting profiles re presented.

6 9 S.K. Vrtpetov, A.V. Zkhryp, V.I. Kozlovsky, et l. h/nm N Figure 7. Grting depth h with period of. mm on the ZnSe crystl surfce s function of the numer of pulses N t F = mj cm nd repetition rte of Hz. prticulr, this lyer my contin holes of vrious shpes or grooves with verticl wlls, the chrcteristic sizes of which re smller thn the AR microrelief wvelength. In this cse, the proportion g of the crystl surfce occupied y these holes or grooves must stisfy the condition n = n ( - g) + n g () cr cr nd for n cr =. (ZnSe), n = (ir) is equl to.. Besides, to void the diffrction nd scttering losses (including those inside the crystl), the holes or grooves must e rrnged regulrly on the surfce, with period less thn l/n cr. In this cse, we will hve one-dimensionl or two-dimensionl grting in which the depth h of grooves nd holes my only constitute n cr /(n r ) =.9 of the period p. However, the formtion of holes nd grooves with verticl wlls nd strictly defined depth (with flt ottom) is chllenging technologicl tsk. A sustntil devition from these requirements increses the rdition scttering loss. However, one my count on significnt, though not perfect, ntireflection. In prticulr, the grting shown in Fig. (period is p =. mm, depth is h =. mm, h/p =.) nd Fig. ( p =. mm, h =. mm, h/p =.) my exhiit the AR effect. The mximum grting Figure. () Imge of the grting with period of. mm on the CdSe : Cr surfce, formed y five pulses t F = mj cm, nd () verticllyverged grting profile. z/mm Figure 9. Imges of two-dimensionl grting on the ZnSe surfce, resulting from superposition of () two one-dimensionl grtings with period of. mm, formed y single pulse t F = mj cm, nd () grtings with period of mm, formed y ten pulses with F = mj cm.

7 Study of the formtion of microrelief on ZnSe- nd CdSe-crystl surfces lted 99 contrst h/p =.7 ws chieved on the ZnSe surfce (see Fig. 7). Figure shows the IR trnsmission spectr of the originl CdSe : Cr crystl nd the crystl with grting (p =. mm) on one of its surfces (see Fig. ). One cn see significnt difference etween these spectr, which is ssocited with prtilly ntireflective crystl surfce with grting t wvelengths ner mm. In the long-wvelength region the spectr virtully coincide, since the microrelief corresponding to the long-wvelength rdition is not deep enough. In the short-wvelength region, t l <. mm, the losses re incresed due to diffrction of incident rdition on the grting. In the cse of n idel AR microstructure, the trnsmittnce in the re of mm should increse y. % (see the inset in Fig. ). For crystl with grting under considertion, we oserve n increse of.9 % in trnsmittnce, which mens tht the crystl hs firly strong, though not idel, AR microrelief.. T.... DT... We should note tht the microrelief descried llows incresing the rdition strength of n ctive lser crystl, ut, s in the cse of conventionl single-lyer AR cotings, the corresponding spectrl nd ngulr widths re rther smll. The second wy is sed on the formtion of twodimensionl rry of pyrmids (Motheye microstructure) [ ]. The shpe of the pyrmids is lso importnt, lthough the most importnt fctor is the lrge depth of the relief (up to the douled period of the structure). As hs een shown y experiments, n increse in the grting depth y more thn / of the grting period cnnot e ttined y incresing the irrdition energy density in single pulse. More promising is to increse the numer of pulses t smll excess of the ltion threshold in ech pulse. Preliminry experiments hve demonstrted tht the relief contrst h/p =.7 is rechle within the frmework of this method. A further increse in contrst is proly ssocited with the prolem of removl of ltion products from deep grooves. l/mm l/mm Figure. Trnsmittnce spectr T of ( ) the originl CdSe : Cr crystl nd ( ) the crystl with grting on one of its surfces, nd lso the clculted trnsmittnce spectr for ( ) pure CdSe crystl with Fresnel reflection from oth surfces nd ( ) crystl with n idel AR coting from one of its sides with llownce for refrctive index dispersion. The rrows indicte the position of Cr sorption lines. The inset shows the corresponding difference DT in the trnsmission spectr of CdS : Cr crystls with grting nd without it.. Conclusions Using the lser ltion method, we hve formed periodic onedimensionl grtings with period of.. mm on the surfces of CdSe : Cr, ZnSe : Fe nd ZnSe crystls. We hve used the interference of two lser ems from nnosecond excimer KrF lser, providing rdition energy density of mj cm per pulse. The ltion threshold ws slightly less thn mj cm. The mximum grting depth rechle in single pulse mounted pproximtely to / of the grting period t moderte energy density F» mj cm. The relief contrst decreses with incresing F. At F = mj cm, nnoscle formtions, i.e. quntum dots with typicl size of few tens of nnometers nd density of cm, re oserved on the crystl surfce. The mximum grting depth ws chieved under irrdition using series of pulses with n energy density F» mj cm, which corresponds to slight excess of the ltion threshold nd mounted to.7 of the grting period. A further increse in the grting depth is proly ssocited with the prolem of removl of ltion products from deep grooves. A possiility of formtion of twodimensionl grtings with periods of nd. mm is demonstrted. The proposed method cn e employed in the cse of mid-ir lsers to form prtilly ntireflection microrelief on the ctive elements of CdSe nd ZnSe doped y the trnsition metls of Cr nd Fe. More reserch is needed to ensure n increse of ntireflectivity y the formtion of microrelief with greter contrst nd optimised profile. Acknowledgements. 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