JAIST Repository: Raman Scattering from Trimethylaluminum (TMA) :Free and Adsorbed Molecules on GaAs(100)

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(1)JAIST Repository https://dspace.jaist.ac.jp/. Title. Raman Scattering from Trimethylaluminum (TMA) :Free and Adsorbed Molecules on GaAs(100). Author(s). Sano, H.; Sakai, J.; Mizutani, G.; Ushioda, S.. Citation. Journal of Electron Spectroscopy and Related Phenomena, 64-65: 865-870. Issue Date. 1993. Type. Journal Article. Text version. author. URL. http://hdl.handle.net/10119/4407. Rights. NOTICE: This is the author's version of a work accepted for publication by Elsevier. H. Sano, J. Sakai, G. Mizutani, and S. Ushioda, Journal of Electron Spectroscopy and Related Phenomena, 6465, 1993, 865-870, http://dx.doi.org/10.1016/0368-2048(93)80161-E. Description. Japan Advanced Institute of Science and Technology.

(2) ( TM A)‥宇 eeand Ads or bed. S cattering from Tr im ethylaluminum Mol e c u l esonGaA s()00) Ram an. I.Sa n o, I.Sak ai, GIMi z utan iJ andS.Us h i oda Res ea r c hln s t i t u t eofEt e c t r i c a . ICo mmu n i c a t i o n,To h o k uUniv e r s i t y , 2 ･ TLKata h i r a , Ao ba ･ k u, S end ai9 8 0, J a pan W e ha Ne mea Sur ed t he Ra man sca t t e r ing fr om an ads orbedl a y e roftrimet h y t a t u min um (TM A)on Ga A s()00)a saf unct o in ofc o l e r agedo wnto L7mon ot ayer sITh ea b s o l utescatt e rin gc r o s s ･section o ft he. CH3･ S y mm etricstret c hingmod eofTMA wasfoundt obe8 . 0 ×) 0-2 9c m2]dm iers rl i ne. Th ec o r res pond･. i ngc nt hegaseouss r o s s ･section i t ateis2 . 6x L 0 ･29c m2[d i me rs rl i n e .Th eobserved enha nc e me nto fthe c ross ･ s e c t i o ni nt h emultila y e risd u e to thee ffe c to ft h em otecuta rfield･Th es c a tte r i n gc r o SS･SeC t i onof t h eCH3 ･ S ymme t ricstre t c hing m .deo fg aseoust r i mt e hylgaltium (TMG)isL3 .xt 0･29c m2 J m. 1 e c ules r l i ne.Ba s e d on thesedataon thea b s otuteRa ma nc r o s s ･ s e c t i ons,wee xpectt hatt h emo n o l a ･ y e ro fthese. molecu t e so n Ga As wo u t d be eltreme ty dif B c u l ttoobs e r l eby Ram n. en く i H Onヨe t. . )N TRODUCTrON. an scat t e rin g ln a C r y S t a lg rowlng. h a ve d e t e rmined the abs o l ut ec r o s s . s e c t . i onofa multita y e r of TM A mote c ut e sa d s o r be d onthe Ga As ( ) 0 0 )surfa c e 1. I. Trime t h y l a t u mi n u m (TM A)a ndt r i me t hylgal･ u r c emo l e c u l esused t i u m( TMG)a r ei mpo r t antso i nt h egrowtho fGa AsandAt Asc r y s t alsb ymet･ 2. X PER rM EN T alo r g an ic che mi Cal 1aPOr depost i i o n( MOCVD). Al tho u gh b o t ht he stan dardCVD an d atom ic Th es u b strate was a Sido ped n･type Gad s s l nge lc r y s ta･ t wi th a carrie rconcentratio n of xy( ALE)m｡th.ds ar｡ .ft｡n us y e r . ｡pita ｡ di n l a Asu slng the s eg a s e s ,the g r o wl n gGadsa ndAI 1 . 9xL O 1 9cm .3 1Z t Smi r r Or･tike( )00)sur f a c ewa s eaction.processesin t a i l softhe atom icr v o t v ed de c t ean e di ni t l i al yl nairbythefol l o wl n gPr OC e dure. i nt hegr owt h me c ha ni s m a r en ot wellun der･ Thefir stst ep ofcl Sd one byme o l eanl ngWa t han fo n ec a nobserve the lb s t o o d. I irat i o n a tspe c ･ n da c e t o ne , an t h en itwasetc he di nami xtur e a t r aofthese mo tec u l eswh e n the ya r e adsorbed 0( 5‥) ‥ ))a 02=H2 t6 0 . C ofH2SO4=n2 ona g r o wlng S u r f ac eofGaAso r Al As cry stalS , e m ol ln gt heetc h ings f or6 0SI Afterr o tution by a t etheatomi ca ndmolecutard y I onecaninvestig i t hp urifie dwater,thes ubstra t ewasi m･ r i nsl ngW na miCsofcrystal g r owthi n situ. Tomakesuch mer sed i n as o l u t ion of(NH4 )2Sq wit hH >)･By st ud iespossibl e ,t he Ra ma ns l g nalf r o ms u r f a c e t h i sprocedur eat hinoxi d ela y ero nt h eGaAssur･ d sorbed m o t e c u l e smu s tbesufBcient t ylar g ef o r a f aceisr em oveda ndal ayerofs ulfurisd e posited t heobser v a t iono fa mono l a y e r.Wi t ht h i sapp l i･ [ 2 ] .Thes u l f url ayerpr ot urface against e c t sthes c a t i o no fRa man spe c t r o s c o p y lnmind,Weha v e oxi dat i on i nai r. b e e nn ivest i g a t i ngt heab s o lute Ram a n scatter･ t e rpla c l ng theGaA ss u bstlat ei nanu l t r a ･ Af thi n n g cross･sectioロS Of T氾 A aロd TM G bo h igh 1aC rwithabase pr u u m (UZIV)chambe e s ･ thef r e eg a s e o u sstatean dthe adsorbeds t a t eo n o r r )t h esa m pl ewa sheat e d s u r eo f3 x 10-10t ) 0 0 ) I GaAs ( t or e mo v eth gerspe esul furl a y er.TheAu c t rum I no u rprevo iusw ork wedeter min ed thea bs o ･ oft he ct eaned s ur f ac ei ndica. t est . hat .t . her esi dual CtionsofTMAa l u. t eR Am an Cr OSSISe nd TMGi n ed etec oxygen i sbet ow t h t i o n lim t i andthat s e o u sst a t e[ ) ] .I n thepresentwork we t hega .e s e n t add r e s s ‥Ja panAdyancedZ n s t i t u t e o f S ce i n c e andTe c lm ologylTas t t n o k uc h i ･ maci h , i s h i hwa･ken9 2 3 ･ ) 2 . Japan .. E. d. P r.

(3) C ar bon and s ul f ura r ebar el y de t e c t a bl e. Thi s l e ve lofc ar bon a nd s ul f urdoe snot .a ffec t .t he pr e s ente xpe rime nt .Thes ubs e que ntads or pt i on o fTMA and Raman s c a t t e r i ng me a sur e me nt s we r edonei nt . I L i sUZ I Ve nvi r onme nt h The TMA mol e c ul e s we r ea ds or be d on t . he Ga Ass ur f ac ebyc ool i ngt hes ubs t r a t ebe l ow 1 58 K andbac k･ r T t t i ngt hec hambe rwi t ht hevaporof TMA atapr e s s ur eoflI4×1 0-6t or r ･Cool i ng tt hat o ft hes ubs t r at ewa snec e s s i t at e dbyt hefc nomor et hanas L nglemonol a ye rofTMA ads o r b o nt heGa Ass ur f ac eatr oom t e mpe r at ur e.The sc ali t ) r at e di ns i t us i ･ fil mt hi c kne s so fTMA wa mul t ane ous l y wi t ht heRaman me a sur eme ntby me a s ur l ngt he e fle c t i vi t yoft heAl ma saf un? t i onoft hee lPOS ur eandbyAt t i ngt heda t awi t h at . he or e t . i cal c ur ve.Thet , he or e t , i c al c ur vewa sc alc ul at e df orat hr e e ･ l a ye r e ds t ruc t ur et hatc ons i s t s T l m,andvac uum [ 3 ] .I twa sc al･ . fGa As,TMA r 0 c ul at ed fort hes ･ pol ariz e di nc i de ntl i gh tat4881 nm wi t ht hei nc i dentangl eof240.The nt hefit ･ t i ngpar a me t e r sar et hefil mt hi c kne s siandt he , mE . di e l e c t , ( ･ i cc ons t ant .oft . hefil. a. r. Wene e dedt ode t e r mi net heEm t hi c kne s sby t hi sr e nec t i vi t yme t hodr at he rt han bys i mpl yc al･ i br at i ngt het hi c kne s sa ga i ns te xpos ur e〉bec a us t hepr opor t i onali t yf ac t orbe t we ent hefil mt hi c k･ mi nedc ne s sa nde xpos ur ec oul dnotbedet e r on･ s i s t e nt l yf ordi f f e r e ntruns .I nc onne c t i on wi t h t hi spr obl e m,Wewe r es ur pr i s edbyt hee xt r a Dr di ･ nar i l yhi gh l e ve lofe xpos ur er e qui r edt ode pos i t as u諦c i e l L t .numbe rofTMA mol ec ul e st obede . e. t eC t edby Ra mans c at t e ring･ ( Se et hee lPOS ur e s calei nFi g･2･ )Tos e ewl L atm. 1 e c ul e sar eac t u･ a t t ypr e s e nti nt hec hamb erwhentheTMA gasi s akedi nt oi t ,weanalyz e dt hega Si nt hec ha m･ berwi t haquadrupol ema s sanalyz e r .Ther e s ul t s howst hati mme di a t e l ya f t e rt heTMA c ont a i ne r i si ni t i yope ne dt ot hec hambe rmuc h oft he all ga st hate xi s t si nt hec hambe rc ons i s t sof CH3 andCZ I 4.Evi de nt t yal ar gef r a c t i onoft heTM A mol e c ul e sdec ompos e sa st he ys t rik et hewal l sof t hec hambe r .Thust hepa r t i al pr e s s ur eofTMA i sonl yas mallf r ac t i onoft het otal press ur er e ad byt hei ongauge.Thi sf ac te xpl ai nswhyanun･ us ual l yl ar geamountofappar e nte xpos ur ei sr e ･ qui r e d be f .r et heRa mans l gnalf r om TMA be ･ c ome sde . e c t . a bl e.Wef oundt . het , hi c kne s sde t e r I t mi nedf r om t her e fle c t i vi t yl Sr e l i abl eandr epr o ･ te. ZロS ¶ Fi gur eA:E IPe rime nt al S et uP f or Raman s c at ･ or be dl a ye ri nUZrv ･ND:ne u･ t e ringf r om anads al de pi kefil t er ;AJ2:halfwa ve t r ns i t yfil t er ;SF:s pl a t e;P:pol a r i z er ;CL:c yl i ndriC al l ens ;L:l ens ; M:mi r r or ;W :po we rme t e r ;PMT:phot omul t i ･ pi e r . duc i be. TheRam anmea sur e me nt swe r epe r f or medby t hee xpe riment al s e t upi l l us t r at e di nFi g.i .The i nc i de ntl i gh twa st he488. Onm l i neo fan ar gon i on l a s e ra tapowe rl e ve lof50mW .Thei nc i ･ de ntanglewa s240f r om t hes ur f ac enor mal a nd t hepol a riza t i onwa spe r pendi c ul art ot hepl ane ofi nc i de nc e( S ･ pol ariz e d) , Thes cat t e r ed l i gh t wa sc ol l e c t edi nt , hedi r e c t . i onoft . hes ur f ac enor mal wi t houtapol ariz at i onanalyze r .A bl ac ke ned e c t i onopt i c st o c onewa spl ac edin f r ontoft hec ol l e dt i r om t r e j ec tdi ffus es c a t ter gh tf hes ampl es ur ･ f a c eandt hewi ndow.Thes ot i dan gl eofc ol l ec t i on wa sA〇日 2. 84×4 0-2s r . The s pe c t r ome t e r wa s a doubl e gr at i ng Cz e r ny･ nr ne rt ype ( Jobi n･ Yvon)U･1 000) ,and ue s e ns i t i ve t ube t he phot omul t i pl i e r wa s a bl ow ( ( I ama ma t S u Phot oni c s)R649)wi t h av er t s da r kc ountr a t e.Theabs ol ut es ens i t i vi t yo hi s pe c t r ome t e rs ys t e m wa sc al i br at ed ear l i e ra sa f unc t i ono fwa ve l e ngt ha ndpol a r i zat i onbyus l ng r d ha l .ge nl amp [ 4] . Thust hephot .n an da as t c ount i ngr a t ec anbedi r e c t l yc onver t e dt oabs o･ ut ei nt e ns i t y. The a bs ol ut e Ra man i nt e ns i t y mea sur e me nt f ort . hec r os s . S e c t i onde t , e r mi nat i onwa sdonef or t he s ymme t r i cs t r e t c hi ng mode oft he me t hygr oupwhos ef r e que nc yi s〜2900c m-1･Fr om t he e nt i r es pe c t rum oft hea ds or bedTMA la ye r s,we. y f t.

(4) Thice n ( A. confirmed t hatt heads orbed t ayer sc onsi stoft he. k. ss. ). di mesoご hi smoecue[ 上. W e al s o carr i ed out a Ram an exper i ment t o det . er mi ne t . he t ower l i mi t ,oft , he numberofadI sor bed l ayerst l t atcan be det ect ed by ours pec ･ t r o m et ersyst em .Forthi smeas ur ementt l t eexci t ･. i ng beam wasp･ pol ar i zed and i nci dentalong t he om thesurface norm al.Tl di r ect i on 60.f r t ein cT dentpowerwa sl i mi t ed t o L60m W t o avoi d des ･ Orpt ○ロ OfTM A by heat i ロg.Thescat t er ed i ght wi t h p･ pol ari zat i on wa scol l ect ed i nt hedi r ect i on 60.f r om t hesurfacenorm al .Thi sscat t er L ngcon･ figur at i on wast heor eti call yf ound t obeopt i mum f orfil mt hi cknes s esbel ow 300A,becauset l t eel ec ･ 5] 1 ayerbecom esm axi mum [ t r i cfiet di nt heTM A l 3.. RESULTS. hows t Fe dependence oft he Ram an Fi g. 2 s i nt ensi t y oft l t e 2900 cl n-1L i neofTM A and t he m bot l tasaf unct i onofexp? r enect i vL t yoft hefil s ur e.( As wehavenot ed ear l i er,t l t eexp.sur ede･ ti es sur em ea sur em en s t er mi ned from t hetotal pr mucl lgr eat ert han t heact ual exposur eofTM A. ) Thereflect i vi t yi sshowni nt heupperpanetwher e t heopen ci rcl esaret hedat apol nt Sand t hes ol i d c ur vei st . l i et . heoret . i cal fit . .Theosci L l at i on i nt . he. Fi gur e 2:Reflect i vi t y and Ram an int ens i t y ofa mul t i l ayerofTM A on GaAs( )00)as a f unct i on ofexposur eandl ayert hi c knes s.Theopen ci r cl es ar et he dat a poi ロt Saヮd t hes oi d cur くeS ar et he h te or et i cal Calcul at . i onsI See t he t . ext about t . he unusuauy hi gh exposue.. r eflect i vi t yi scaused by t hei nt er f er enceeffecti n m . The di el ect . T i c const . ant .ofGaAs was t he fit. obs er ved･The Ram an peak at2890 cm -1coul d be observed above t he exposur e of650 LIThe peak i nt e nsi t y att hi sexposur e wa s 〜4 cps and. o be E( 488. Onm )1 20+ i 1 [ 6] . By fit ･ 39 t aken t e r･ t l ngt hedat awi t hthet heor eti cal curye,wedet mi ned t hefil m t hi c knes st hatcor r espondst ot he. t he i nt egr at ed i nt ens i t y wa s 〜6 cps. Assumi ng or t hi s separat e t hat t he t hi c kness cali brat i on f r un i s appr oxi mat el yt he same a sf ort he r es ul t. he di el ectr i c constantoft he fil exposur e and t m. he t hi c knesst hatcor r esponds s l t own i n Fi g. 2,t t o 650 L i s65ILtOA or12-2)m onol ayer s.Thi s i st he fir stt i me t hatthe Ram an spec tra ofad･ s or bed TM A was obser ved.Ther e ar e exam pt es. ( EI 2. 4).The t t 1 i cknes s cali br at i on i ss hown at ftheupperpaner t het opo The Ram an i nt ensi t y as a f unct i on ofcoveT agei spl ott ed by open ci r cl esi nt hel owerpanet . The s ol i d curve i sa t . heor et i cal fit . .W e wi l ldi s .. 7 fi nfrar ed obs er vat i on ofTM A on GaAs [ , 8] .. o. cus st he met hod aロd theparamet er sus ed i n c巴･. 4.. cut at i ng tl l i scurve i nt he nextsect i on.W enot e t hattheRam anint ensi t ydoesnoti ncrea semono･. 41).. t oni cl y wi t hi ncr ea sL ng exposur e;t l t er ei sat ocal e･ mi ni ヨuヨ ar Ouロd -叫00 A.Thi s mi ロi mum cor r sponds t ot Fet ocal m axi mum ofther enect i vi t y. TheRal nan i nt ensi t y decr ea sesher e,becauset he. ロCideロ〓 ight into the TM A penet-at i oロ ○ニ hei l ayerhasal ocal m i ni mum atthi sthi c knes s. he Ram an spect r af orthe l ow Fi g. 3 Shows t end oft hecover agef orwhi c ht hesi gnal coul d be. D tSCU SStON , i on ofad･ A bsot ut , e R am an cross･sect sorbed TPAA. W e fir st descr i be t he t heor et i cal m ethod by whi ch t he fitt i ng oft he RAm an and r eflect i vt i y dat ai n Fi g1 2 wa s done and how t he absol ut e Ram an scat t er i ng cr os s ･ sect i on wa s obt ned by ai eri ng L n･ t hefitt i ng pr ocedur e.TheRam an scat t t ens i t yf r om an ads or bed l ayeron a substratei s.

(5) 01 ⊂). _ ト ⊂). RamanI ne tnsi t y( cps). 0) ⊂). 1 0 2 1dim ersJc m3 .Th e np , (i )- ( 3 . 1 4x1 0 2 1 )×i d i mersJcm 2 1A O a ndq , y,a ･ r et h e so l i dangle ofcot t e c t i o na n dt h ee f R c i e ncy ofthes pectrom ･ ee trs y s t e m,r e s pectivety . Thes ystem ef nce i n c y l u d i n g thep h o tom ultiplier efi s9 ･ 3 3×1 0 -3 inc i gh t fici en cy at t hewav e l e n g t ho ft h es ca･ tt ered l . 4nm ). ( 568 The sol i dt h e o r e t i calcurve in tltel o we rpanet ofFig. 2wa so bt a i n ed by Eq, ()).Wes ee that thc fits ie xcel l c n tand t h a t thistheory property expl ainSt h n ･ eu n usualvaria t ion ofthescatte redi k ness. Th t ens t iy a s a func t i o n ofth e film thic e b s o l ute Ram an s l n g l et t r a m eteri ingp･ a s tile a cross ･ secti on,nam e t yt h es caleofthev e r t i c ala xisI Asa re s ul to ft h i snttingp r o c e d ur eweo bt ained t l t ev alu eo fthe a. bsol ut ecr o s s.sect, i on:. 30 2900. st ok es Sh芳. 3 000 m･l ). fi. 3100. 2 800. (c. c t r ao fl nul ti l ayersofTMA Fi gure3:Ram an spe on Ga As for differe n te x po sures. The specr t a. (網. sh ow on-∫the retevantpor to o rt h e sym m e inf t ･ st r etcttingmo d eoftheme thy tgro u p. ri c. ) a d-. 8･. × 1 . 12 9C -2 Jdi- ersrlin e. (2. ). ). f. ･. n i to t ng cross･Sect inofthe ads o r baten it heu o. where (dc o l ute Ra man s c a t t e r r JdO ) a di s the abs. cm 2Jd im ers rlin e . TFis isthe para m eter tob e. (網. )TM. ". A,. -. (網. -. T. ). G, ". -. (〕 ). 3×) 0 -2 9. ).. c. cm 2[m o le u le. L i ne. sr. (4 ). o mparing Eq.( 2 )a nd( 3 ) we see that the Byc. cros s ･ S e c tion forthemul t i lay e rn tm ofT M A is3 .t. t i me sg r e a tert ha nthato ft h eTMA gas.W eb e ･ l i e v etha d i顎ere n c e t .t h s id i f f e r e ncears iesf ro mt , he bet w e en h te m ac r o s c o picne l da ndt he m o t e c ular Ge l d.Thisef f e c twiub ed i s c u s s e dint h ef o u o wl ng s e c t i on.. c o. 2 . 6×1 0 -2 9. c m2 Jdm ie rsrline. d e t e rm ined byfitting the data ofFi g .2 .NLi s t h en u mbe ro f incidentph o t o n sp erseco n d】 which i st 1 2×) 0 1 7p hotons J s( 4 8 8 1 0nm, 5 0m W )int h e pe t p r e s entex r i me n, 7( ) ii sthef a c t . It ha ta c c ounts fort he e f F e c t t r a も eo nt h ei nc i dent andscatt e r e d of the subs I Thisf act oris calculated by us･ electric fields l ng th ee t e ctro ma g n e t i cGr e e n' su fn c t i o nfort h e three -ayereds truct u r ethat consis t sofG aAs】 TM Afil m ,and vacuum a bo v e . Th ec a lcu t a tion t l t o dha sbeend e s c r i be dp rev i o u s t yb yus[ 5]. me i e l e c t T i c nI Fort h , i sc alc ul a t i o nweused h ted , t s oft he m e di, a E( Gads ,ン. ･ n) - 20+i 3 .9, s ant ds ,ンa c) I 6 1+ 2 i･ l)a ndE ( TMA)- 2･ 4･ EG (a Th ev a l u eo fT( i )v a r i eswi th thefil m t h i c k n e s st. mp l e itsv a l u ei st . 2a = =1 0 0n m. Forexa p,i ( ) is t he surfa c ed e n s i t yo f the TM A mo l e culesI This p a r a me t e r ws a cal cul at ed f rom t h e2m thick nessib ya s s u m ln gt ha tt h edenst i y ua l t ot ha tofthelq iu i dTM A 】wl t i ch i s3. 1 4× i seq. 1. (. S. )Aぎ. T (i ) p s (i. i). ) a d N L. No b s ( -( 銅. gl Ven by:. l e cular fiel d. 4. 2. T heeffe c to ft , hem o. b t a i ned is o s s ･ S e c t i on we have justo The cr de fined wt i hr e s pe ct o t t he m a . crosc o pI Ce teCt ric. Tobe pe rc i s e ,t h i s cr o s s ･ s e c t i oni st hec o mp o ･ n e n t par a l l e tt o tltes u r f a c e } (d c r xxJd O)a n d ( d c ryyJdO)wh e r eX a ndY a r e the di ons r ecti ral e .Z f the TMA mo l e c u l e s pa t elto t he surf ac ar eorien t e d ran d o ml y and the nlm isi s o t r o p l C , ( dc z rzJd O)als oha st h es am ev a l u e . (n our pr ev i o u sw.rk[ l ]we o btained the ab ･ r oss･S ectionsoft h egaseo usTMA s olute Ram anc c t i o nf o rt h es y mm etr i c a n dTMG.Th ec r o ss･se gmo d eoft h e s emolec ulesare: st ret chi n.

(6). Basedont he abo st ut eco rs s Ie SC t i o n so b t ained b o ve,weest im atet h eRa ma ns c a t t e r l n gI ntent a. exami m an n et h eposi s bilt i yofobsemng theRa spe c t r an i si tui nac r ys t alg r o wLng environm ent･. (5 ). W eu . ()) Witha ppr o p riate rea l i stic ex･ s eEq. s tf weu ft h ed i e l e c t r ic o c nsa tn to f et l te vatueo. e. th. i. between d and ht o be dJh I 1 .9 ,we canr e ･ n c e me ntf a cto rof3.) . p r o d ucethem e a B u r ed enha Sn c et hei ncide n tt i ghtwa BS ･ pO l a r i z e d, e t e? fc ti r cfie l d ispa r e to tt hesura e .Th u st lleabo v e alt r e s u t tmay bei nterpr e t ed tom ipl yt hath te p t a na r TMA din e r sa r e stackedpa r a l lelto h tes u r f ace.. i am eter and theh e i g h t are the d. i s k,r espectively. tf we take ther a t i o of the d. (6 ). h. ) E. where da n°. %. 舘. ･. (1. h e. ar fie the m ol ecul ti d s1 . 4 7 tim es t m acro s c o p I C f ield.Si nce theRa ma nint e n s i t yLs propo r t i o na l fu h powe ro fths if a c t o r ,wee xpec tthe t ot h eo r t e nhanc e me ロ tO ftheRa m ann it e n sity due to this e ffe ct to be 4. 6 tim es.Thisfa c t o ri sc om p a r a b le b serv e df a c t oro f3 . ) . to theo l e c uteisa plana rmo l e c ul ewi t h TlteT M A m o t h r e e m ethy-g r o u p sy -ngn apFn e.Th u si t m ay be mo s i d erth emo t e c ･ r ea p propriate to con ul ar fieldina caviy t w i t hafla td i s ks h ape.Th e n l e cularfietd paa letto thenatf a c eso ft h e the mo rl disk c an bec a-culated‥. For A 〇 and T7y 3JW e it hi swok r, namely u s et h es a mevalues used n A〇 - 2 ･ 8 × 10-2 sr andqJy J-9 ･ 0 -3 ･ 3 3 ×1 o wcris assum ed to b e 5 0 0 mW a t Th ei n c i d entp 4 8 8O m whih ･n cc orrespo nd st oN L- 1 ･ 2 x L018 e s u l t[ ) 2 ] , p hotonJ ss .Ac c o r di n oal Mb l i s hedr gt ed i s t i n c ta d s o rpi t on sitesfo rTMG t h e r e arethre o n G aAs,a n dt hetotatsura f ce d ensiy t ofthefir st T onolaleris3 ･4x l O 1 4m oleculesJcm2 ･Th i svalu e ot h eestm ia t e ob t a i n e d byt a k i n g L sv e r y closet t h e2[3po we ro fthe densityo ft he liqu i dTMG ･ Thee r are n op ubi l s he d da t aon the surface den･ s i t y ofT M A onGaAs.Forh ti sval ue we wi l s e lu stim atef romt h el iq uid T M A density whc i h t hee i s r7x LO14 dm ie r sJc m2 . T herei s soヲ ea mb i g u i t y ine s t i ma t i n gt he v巴 u eo f｣. b e cause wed onotkロ O Wt hed i e e c t ric consa tn to fth e m onolay e ra Ed .Thuswe wi l lt ake t h et woe xte rmeca s es fa dI 1 for zeros c r e e n ･･f 1 ng and fad - 24 o rf u l ls c r e enln gb y theadl sorb e b tain the m ax i mum and dmo n o l a y er.W eo s t imates r r e s ponding tot mi ni mu me ,co h e s t val･ o fE a d .Th ec a l c ula t i o nwa sc a r r i e do utf or p ･ ues pol ari Bedi n c i d e n c e at6 0 . an dt l te scattering an ･. tp a r a me t e r s. perim enta. the T M A fil )w , e fi nd th at ･4 in Eq･ (5 m ,E I 2. I. P ossibii l ty for observ in g a - onola yer fT M A on G aA s o. si t from am onoy ayer o【 T M A o nd nGa As〉a E. 片鴇. whereE i st he diele c t r i cc o n s t a l t tOfthe mc d i u m.. i-. E- 1. 4, 3.. Thefie t df e ltby th emo lecu l ei nag a s eou ss t a t eis eq ua lt ot hema c r o s co pi cfi eld,becausethen e l gh ･ bori ngmolecu t es a r ef a r apart. T huswhenwe h rt ec r o ss･Sections ofthe salne m olecule c om pae i theg n a s eou ssta te a ndi na film,we m u s ttake i t t t o accou n tthed i f f e r ence be t we en thema c r o I l s c o pI Ca nd m otecutarfie l d s . The m olecua rfield mi si ng e n e r a lg r e a t e r thant l t ema croscopLc E fi nf el d.ThustF ec ros. s s e c t . i o orthem ole c u l ei na fi t m wi l la p pearo eg r e a tert hant h a ti nag as. tb Thestreng tFo f the m o l e c ul arn e l dd ep e nd s ont h .e detai t s of the envr i o n me n t inwl t i c ht he t nol ecue li a ced. The u spl sual wa ytoe s t i t e ma i ti st oa s sumet ha tthe lnO l e c utel iesi nam icroS c opc Lc a v i ty ofthes p he r i c a l s h a pec a r v e do u to f ema ii nwFic hi ti splaced.Un derF ti s a p ･ th teral m ati on t proxi lecularfie t d isgLv e ni n terms hemo the ma co r scopi fie l dEb y: of c. o wever-We m ustbec a ut i ous in accp et i ngt h i s H o n t a i ns o cn c lusion,becauseths it i n eofreasonin gc t i o n s . ma n ya s sump T he i ncr eas e in the appa r entc r oss･secto i na s amo o es fo l e c u l eg Ou r m tl l e gB ae SS tate tOai l q ･ u i ds e no b sev re dn i oth e re xam ples t a te hasbe [ 10 ,L] LLnt h e s ec a s e sals ot hee f f e c to ft h em ole c ･ ft h ei n ･ u l arfied lw a sc o nsidereda st hec a u seo crease i n the c sa di n t e n s i ty.W e believe th a t t t e r e iti sr e a s o na b l et oa s s u met h a ttheintrinsicmo l e c ･ u tar co rs s ･ s e c t i o ni su nc ha n g e dfo rt hea d s o r b e d TM A e rl a tiv re T M A , because TM A is et othefe e res il i t t l e chan ge I ) h ysisorbed onGaAsIT husth i n the m ol e c utar orbia tl sofTM A u po na dsorp･ t i o n.. el tb fie ld. Ho wever,the electr iC fiel d that i sf y t he moe l cute in a fTlm is tlte m oe ar fiel d[ 9 ] . lcul.

(7) 31K .Kur o s a.wa,R .M .Pi e r c e ,S. Us hi oda, and ∫.C.He nm ger ,Ph y s .Re v. B33( 1 9 86). o t hf r o mt hesur gt eof60.b f a c en orm al .TheRa ･ bove a ss u ma n ,i nt e n s i t i e sc a l cutated b ythe a. mp ･ t i o n sa r e ‥. in. 789. 4.KISa a mo t o,Ph. D.Th e s i s ,To hokuUni ･ . k. ( Z… : …p,… f ::: : :: ;:M. e r s ity( v ) 9 9 1 ) . 5.H .San Ushi a nI , a ndS. oda,Phys. o, G. Mizut. (7 ). Th ema x i mum i nt e n st i yl SeXPe C t ed whe nt h e r i snos c r e eni ngO ft he ee l c t ricf i e t d s du et ot h ads o rbed tayert i s e l f ,i ･e.wh e n ead- i .Emp i i r･ y t henuc t ua t i o ni n theba c k g r o u ndintens it cal l, f o rt heincidentpo ia boutLcps. werof500mW s Th u st heRam an s pe ct rumi sobservabe tw h e nthe io ft he est i ma te( 28 . cp s )is applica･ u ppe rlim t bt e,bu ti twi ttbed i mcut ttoo bserveiftheo tw e r t ake n. Even ifa s i i m)armotecutarfie l d L i mi tis. me ntflCtOr tO the pr e s e n tr e s ult(-3) .) e nhance e x i s t s , i twi t tr e qu L r ea nim pro v e m entofth es pe ? t r o me t e rs y s te a c t or4toobs e r v e me f Bc i e n cyofaf nOt t hemO a y e r . Ho we v e r ,its id i ncultt o imag･ i ne t h atthe mole c ut a rfie l de f r ecti ss t r o n gf o ra mo no a y e r.. Rev ･B( i mpress) I 6.E.D.PaL , andboo ko Opi l CalCons iani s o f Sol i ds ( Ac ademi cPr e s,Or t a ndo,1 985) ) pp1429･ 443 . 7.A .Tripat hi ,D.Ma e r , z 冒r e S C ,W .C.Conn a ndA . Jones,J.Ee lc tr o n.Ma tt er.t8 ()98 9 ). e. i kH. e. 4t he d.(PergamonPress ,Oxford,)9 70 ) ･〇 .H. W. SchratterandH. W.K6 tC kne r 】Rama n Speci ro d s , e d.A . W? s c opyo fa sesandliqui ber( Sp r i nge r ･ Vet a g】Ber t i n,)9 7 9) )Ch･4･〓. . T .Fujiya ma , ). Ra man Spec t r o sc･t2 ()982) . 19( ) ) 2 .J･R･Creigh t o n)Surf･Sc )287･ i ･23 4( )990. m an. scat r. t erLng cross･se c t i o no fa m ultl i a y e r TM A ad s orbedonGa As Lti sapparently enhancedr e l a･. t ivet othecross･s e c t ion oftFegaseou sTM AI We t ha nc e m ent is due t ot he effectof b e l i e v ethee l them olecularfietd.Theestim atef o rthe Rama n i nte ns i t y ofTMA o ロ Ga As indic a tes tha ti 〓S e xtrem ely dim cuttto ob s erveamo n o t a y e ru nd er reat i st i c con di t i o nsi r o wl n ge n V i r o n ･ nac rystatg me nt.. ･. Wt ,ED G EM EN T S. ACKNO. nk Fu j i t suLaboa rt o r i es,Ltd.forpr o ･ Wet ha vi di ngfin anc ial 1 ndma t e r il a ss uppo r t ･Thi swo r k wassup port e di npartb ya Gran t ･ i n･Aid forSci･ ent i i fc Re searcl tf ro mt heMi ni s t r y ofEdu cato in , t ur e . Sc i e コC e 】aロdCu. R EFER ENCES. o, G.M izNta L.H .San n l a ndS. Us hi oda. ,Chem . Phys.L e t t e rst94( ) 9 9 2 )3 9 8. 2.Y . Na nni c hi ,J. F an ,H O .i g a wa, a ndA.Ko ma , y s .27( 1 988 )L2 367. Jpn.I.Appl.Ph. 9.M .Born and E.Wo t f,PT･ m Ci pl es o fO pi Z . C S ,. g. 5. R a. . ∫ . P hys. CondenS. M atter. (UK)3 ()99)) S1 5 7 .. . CONCZ,USIONS of. L I B. 81D .S Buhaen t e , k o,H.Patel,and M .E.Pemb. r. W ehavedet e r mi ne dt . heabs o t ut e. f. s. y. bS. ･o I.

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