luth_solid surfaces interfaces&thin films

8/7/2019 luth_solid surfaces interfaces&thin films

1/286

So l id Su . rf a ce s ".Interfaces and. Thin Films

Springer.BerlirJHeidelbergN .e w Y erkBarcd !onaHong KongLotJrJrmMilallPari~Thkyo

.Physics andAstronomyhttp://wVI!'IN.springer,deJphyw

O H U N E L l l I R t i t l Y


2/286

Advan .ced. Texts I n P b ys ic sThis pifOgrnm of advanced ' texts covers" brnad spectrum, of top: ics which are ofc u: rr en l a n d e m e rg jn g i nt ~r e, st i n p hy si cs . B a ch bQ o~ kp r ov id e. s a c om p r eh en s iv e a n d'yet accessible introduction to a.lie]d at the forefront ofmodernreseareh ..As such,these texi8 a re in te nd ed f ur sen kif u n d er g J \1 l du a te a n d graduate students at 'the MSa : nd PhD leve] ;, l :u )weve .r . [~$e ii . l'Ch s c ie l l' t Js l s S JCek i. n gall, ' in t ro d u ct .i o n t o particulataress O 'f physics will also benefi f frum the 'titles ifllh:is oollection.

Hans Liith

S o l id S u r f a c e s ,I n t e r f a c e sa n d T h in F i lm sFourth, Revised andf.xtended EditionWith JB 9 Figures and 13 'lables


3/286

P, ro f~$$o : rDr . Han , sL f r thFQiSih:~~lluum j 'i l li c k GmbJ : II'nstiltlt fii~S c h i e b l l : i 1 . l m d G~n~tI~~h~iI,5"~:4~ JiJlnch~itdRhe~ni~h-'We$t'lillische T:mlt~~die.H.()dmhul~p,061 AlIJ:h~RG~rm~n,y~ 1 T I : a : i 1 1 b,.'II!Jt.htlif~j\!~l:i,dI. ~~

Preface

l,;ihr~ryo r Vo:Ll ,IV 'e$S C.M:i:1R~ ;P~d$ : ' : fukro:Spri ! l l !1~r,~I(M~.ano0d~m~n phy:!iir.s)Wh~5.k:s 81:1dll$tronon1 o : n l i m . . . ! ! lib[u~lJ,. A ! J i l' l. l I .d , T . : 1 . ( 1 1 :1 " 1 "tR'IIs.; g - i 1 r f a o e s ~ p d i ! ' l I ( ~ r [ ~ c ~ of ~id mm l t . " ( i B l sI & S : N 3-540,,!200N

Surface physic~ ill the c i i 1 .S $ i . C ; : f l l 1 S e r . l i S e (jIFulLuihlghwlCuurtl (UHV) based ex-perirl1a'l1tm approaches 110UllclerstaudweU-defined Stll'fru::e:s has nmy become amature beanea ofcond.ensedmat;ter L,e5ttu~h.Meanwh: i l ej ho\vev-er! the theo-retical ooncepts and experiruen.tal ~hniqu()5 developed in this iteM hase alsobe -come the basis for modem interface, thin .filmand nanostr'Uctul'C scleaec.Furthermoee, these research fields are of fundanumtal i !1l .portam::~ fo r mi(l 'eappl]cd braaches of acionce, SUdl as micro. find neneelectronics ~catalysis andc o rt 'o s ;i on r es e ar eh ,I J l1 l' fr u :. ep r ot ec t l. on , . c h cmo - and bil(l8(lJlWl'~,mi:Cl"rey.gte.msa n d n a no s tr u l( :t ured !11a;terialt-l~

Th e physics o f s o li d . s u rf a ce s , intE!il'f~ an d thinfilmsi!'l t.hu:l;an :imporl;a.lltfield whicb IUJ\lds to be taught to "ponding UIIi vEtrni ty euerleula throughou t th e world,

Iutlle pnfflent 4thfditkm oftbls bO( l k ( f { II . , lIH 'U l yntitled ~~Surfucellnd In-' oo rF ; ace so f SQUd Ma: t .edads" ) mQree :mpJ l a s : i s i s p la c ed on t he r el at io n hetwee:11~he iSurfuc.es, iut~r.faccs and thiu H1ms, and Ql1116W ly dlseoveeed phenomena-related to lot", dimeueions. Accordifl ;gly! a few topics of t .h .eeadie-at ed i :LJOUS thatare now only of pel'lpilerru interest ]U1iW been omitted. On thcothcl" hand, nn~}wchapter dealing; \vithcoUecti " ' ! I l l phenomena a:tint~rface$ has b4M1l add():d:Supel 'CQud.1. lct01' '"SemicCll :ldUCLm: lnterfaces an d thin fer,l 'omagne'Lic f i l : n : t < ; :ha:veattrnctcd OO Judd.crablc aUent i ,on ill of late, This is ruainly duete o:u:r imp:l 'owdunderstanding; of these phenomena, but 3.15000hnp'orL$n~ appl i .ca t io :n8S1)ec tswhich ha.ve re cen t l y emerged. FOl"ex.u:nple, gian:t : rnagr:doros : is t< l l1OC" it: typicolthin f]hn phJ~I.l()(lIIenonls of con s id l 1 ir~ 'b l e im . p r. n .' t !l : n. c e far[le~df"ollt d evic es inm a g n et ic i nf or rn a ;t io :n storaf,"e. L: i l rewloo , ferromagnetlsm in low d imens lonsrn~play an i l .11DOl:tant role in f t ! tuJ(e:non-v l ) lMHe!nemO:l 'Y device ,cr.'cuits.The oou:esPQndiugl:iopi.cs ha;~ thus been added to the new edition and thetitle oi l the book has boonmodified slightly to "Solid Surffi008, h'J:wd:a.oesandThin Filmsll.Th18 n~w tlUel;i:et tel' drc:~dbes thfl~videi' range of topics hx:atcdin . the new edition.Fl : t l" thc l ' l l lOl 'C, in 're sp on se to se~ra15ll:ggest;i{lin$ ( ium student8 and cnl , .le;tgues, e!'Wl':;i a:ndinoor.l5istel'lcics in th .c text have boon eliminated O'Vementsmade to clarity, On the topkssupcroonciuctor-5c:mioonci uc tm 'in.terfaces and fe!'rou1~net,i5r.1l lnlm\! dbnel l !! l f;J11S1 1 have be.ne: li ted from dis-

ISSNI435Hi l674ISBN J-540>4~31I Sp' r .i . t tger -Ver lag .Ber li :n l:Iciclelberg New YoikrS) i l ;l l - :lHO. ,S1i7tO-] j thl ld i t iCl ii (}l'jll~'lk:rlBg Be r li n :H ~ d, e lb ~ [g N~W ' 'Y o .r 'k, IM s wm r k 1 5 s il i~ je 'C t 00 . O O i J ? y l1 g 'h t .Al l r i: gh l$ ~ -- er es er ~~ w h ~t 1w r t l : w ; ~ " J w ] 1 ! : ~ 1?~~I~f t~ ,1J l3le - :r iml~8o;)n"~I"!J.cd.S f , I e ~ i j h ; ~ l I y 1!~~Jight~ o r Ir~njJauon, rcp~Ii'ltl 0l:!.reuse of i J _ lu~ lmuom. HI l ; ll li I t lt l _ f! .~roBd.c a $ 1 : i : n . g _ , f e F T ( I : i h . ! ~ ' i o n oil ifllaOfilm ilr in ~ny o t f l . e ' J " w~y,~nd M ? r ~ In d~.rn banks, Dllph~~II~ Q f' t hi s p ' ll b l k31 : i[ J (t ' l o r p a r ts l h er eDfi : s. p 1 l 1' i il l lt . ed O I n 1 y uuder r h o e prO'! ' l~OO\nsQhMGc: rm; l : rL Cr)pyr~[ !.;l,wof ScpremlJ:!n' 9'!.9~6:5.i;!)its eIIf~lIt '!'e~lion.~i!d, p~rn~~(Jn fu~ use iilu~tol!~\\f$ bc,oh ' l l l l l ' l . ed f r o : n ; lSp l ' iI ' I s~ r~Ve -r~ V iUki it tQ I l~U~ l ! 1 1 b : l c f:(lrpKl


4/286

VI Prefaeecussions wi th 'I'bomas Schapers and StefanHIUgel. l~~pccti.Vly, Th~En,glishtext w~sig;llifican:ily lnrproved by An;gel:a.Lahee,.\vho,togetilerwith l{a,tJUl.~r ina, Awheron, a lso CQ ' i l l , T : ' . i o u : t e c l much to the f ina l J , ll :OOQc t ion of the book.

Particula;r thanks are dU G to Chills Aschel'Oll of Spdn~r- VeI'}a,g,who mal l -a g e d the who.lc pub:licaUon precess .

Preface to the SecondEdition

.&.a-chcn and Jlllid!July 2001

Surlace and interface phys.ic:; has in reeenb decades becomean evermoreiIl1po:rtant subdiselpline within the physics ( ) f cendensedraatter, lliIal,y phe-[I0l1li!~na.and opel"imenWllt.echuiqUe5, for exa.J:np1e the Quant urn. Hall effectami phqtomnissj.on spectroscopy for iuv$Ugating e,leetronieb.and .$tl:llc:LU.T~;lijwhich dearly belong to the general field o:fsol i.d,.,s;tal;~p.bysic.s, eallIlOt hebated without a [,ll:Q:{ouud knowledge of surfaceand inteJJ 'fatl l l eJfed;8. This isalso trua in view o r the p re se nt ~ cra l development in SOUd -3 t~ tC : r)ooareh!where the qnon tu l11 ph;ysics of nanostruetures is ber.: :o;rningineretisi ' llgly rele-v an t. T hi s: t tl: 'I Dh ol ds fb r 111()l)e applied. f i~ ld s : li ll ch a s micl'oelet:troniCS1 c~ l>t .aJys isAnd.eeeresion research . The m.m'e one '5tdves to obta .in an atorni~scaJe 1111-de;.stanaing; andthe grea;tel' th0 interest in mlercsteueturcs, the meresurfaceand jn.terface p.hysics becomes an essential p:rerequ~site,

In spite of this situation, there are o:nlya very fewboob O'n the marketwhich b:e~(t the ~ubjE!c . t in. a 'COIllpr.eh.el1siv~ w a y , even Lhough surface ~m dlnterlace physics hasno\iV boon Laugh~or a number O if ye~l"S at many uuiver-slties around Ute woeld. II I I~ V Q!;.\!ntc:aching a n d J ." C '1 I. ca f c bactiv.i.ties lal .wa:y$have the same CXpor.ilClll0t]:when ncw students start d lI~irdiploma or PhDwm'.ki u my g roup 1 can reecmmend to th.em lit number of g o od . l B v.i ew R lLtid~~ .01."advallced mOQogra ' ! _ j i 1$1 but a rea] i11:~l'oduc tory arid oonlpl"fthensiV(~textbook to usher them, intO' this fru;;dnatingneld of modern research hasb ee n ) fJ ,{ ;k iZW ; .I thcrclO!f~\'!IroliC' this hook fur my students 1:.0providJethem with &; tex.tfl 'ornwhicli they can learn the basie morle;ls,l:ogethe:r wi til fundamental ex-(l(;ll'im.eQ.t:.1I1t.echniql.1;&!3 and~he rela:tkmship to applied i1dds such as micra-analysis, catalysis and nri.cl'oelectronics.This textbook OU the physics of sUffaoC~s tmdintarfaces wversboth ex-perimental and Lhoon~l;i,cal.aspGcts of the subject, P8:l,ticnlar atb:mHonis paidto practical cons'ide:ratioI]S in It seri~ of ool f ' - ,ool l ta inoo paneb; which deserlba

UHV tec ]mology1 d~b'()n Op~iC'515lUoface 8Ipec t1 "OSOOPy" aud dectric.al ~ 'U lr l o p -tical illtettface charaeterisatkm t.echni.ques. The main ce:x: tpmvi .c ioo a dearandeol"!. '! .pwlLclllSive descl'lption o f s ur fa c e and l] , toorfaoo pl"C])lllration methods,stmctumlt \yii:lttJ:tioll,aland e1et:twnic propertie$j. and adsorption ~L:lld!ayergro\'l'tb.Because of their essel 't tial role in modern TnicI"OOlectl 'cmiClli , sp,eciaJelllpl: l .


5/286

and hctercetructuree. Empha..~izing se ' l l l lcondtu: : t (u ' mieroelectrcnles as one ofthe major a:ppl ica t iO l l8 of intel'fMCI)hy~ics is fu r t he rmore ju~tifiecl by t ;he factthat be~e the gap between appUc~tiouand bssie reaearch is small, in coutra:ot,for ex."I:mpl~~with catalysis Q f cerroslouand surface protection l:e1':iMroh..The book : is ba sed on i~Ct.Il1XlS grV(m a t t he ro l (~ i: n . l sCh -Wes t f i i li 8 . d l e ,Thcbni . ,sel ie Hochschnle (RWTH) AaChen and Oil student semina:rs Ql'gaJ'Ii .7!Cdwith nl;Yco)le~i~gucsFlew'!. ' Balk, Hl:UlSBcascl, Harald Ibaeh, Jiirgell Kirchner, Claus-Dietel ' I \:ohl and BI1.mQLengeh~l'. I am.grat-eml to ' these c .o llcagu ,es andtoa number of st:tldents palticipatil1g .in thaw 5emil l ; f l ,m fur th~il' contributionsan d fo r the n i~ a:tlllQsphmce durinl l ; : thesiil~OUI'3$, Othl ' l r 'YElIl ,ablBsu~tkms" '~ re 1 11 .0 00 by :some or myroUTI'e~"d.octaral stud.:nts 1 111.particnl.1a:r by AmoForster, MonlkaMattC' f i1- I


6/286

Panel VII: Surface ExLended X-l l,ay A,b:5Ol 'p t ion Fine StructtU'e. . (SEXAFS) . , .. , .. , " . , , , " ".. , 139Pro.blams "' , , ,. ,.., ............... , , , " 145

6. Elech"oQ(nic Surface S'tati:e'S ... _. _, ., , ., .... ' ... " " " ",. " " , .. ' . . 2656. 1 Surface S ta te s f ora SOlni-[nflni;~ Gha : i n

in the Nearly-Free E.lootl:O[l Model _.. . . .. 2660 .2 S tu:fa,oe States ofa 3 D C ry !1


7/286

Contents8,3 Vh't.ualIllducooGap S t a. tc ; s ( V IGS )

at the Semiconductor Heterointerfaee 3978.4 Stl 'uctu.re- and Chem;istry~Depcndent I v . ! o d e . l so f f n te rfa .c:e S taW 8 , , , .. . ., .... ....... , , . . 4 0 IS.5 Some Applications ofMetal -8 mioondnetor Jllut.'tiolll$

and 5emioond uctor Fleterost['llctUtei .. , , 4088,5.1 Schottky Ba.l'riel':!i.......... "' " , . , , , . 4088.5.2 Semiccmdnctol' Heterojunctionsan d Mod.ulation Doping ; 41 08. 5. 3 The High Electron Mobili,ty TI-aosiswJ' (H EM.or) 4.16

8.6 Quantum Effects in 2D Elect ron G~ .at Semieonduetor Interfaces .. " , "........... 419P an e l X I II : : E le ct ri .c al M e as ll .r on u m ts o f S c hQ it tk y- B O ju " l' i~ rHeights

and Band. Ofrf*!tll , , , , _ _ 427Problems , , "" 434

1,0.,Adso~ption on Solid Su:rfa.ees ................. " . , 49910.1 Physiso.l'ption ", , , , .. , 49~10.2 Chem:isul;ption '" ., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50210.3 Work- FuncUon Changes Induced by Adsorbates .. ; . " _. _.. _ .. 501lOA Tw~Dime[lsh:maJPh


8/286

1. Surface and Interface Physics:Its Definition and Importance

A sol id inoorfac:eis defined as a smal lnumber 0. atemie layers that SCI)ar.i.l itctwo solids in intimate contact with ol'leanol:bel' I where the properties di.f-rei ' significal1tly from thosa of the bulk materia l it :>epara too . A metal. :filmdeposited em , i : I , samleendueto crysta ll for examp le is thus separated bJ i 'thesemleonductor-metal interface f rom the bulk of the scmicond udal'".

Tim srrrfaea of a solid is a pa;rticldarly simple 'Lype o f interface, a:Lwhichthe so lid is ill eon taet wi'til t , he sUH"Onud i :n gwol1' ld . , i .e . , U IA ' l{ ! , it I n osphe r~ or , i l l .the idea] Ctl::le1the vacuum. The deve lop:m.e :nt of modem i.n'lit)ifure and thinfilm physics is thusbeslcnlly determl ned by~he tlu . .Ofct ical concepts and the'exper imental tools being developed in ehe f ield of surface ph}'"$ics, i.e., thepbysi.cs fif the simple solld-vaeuum interface- Surface physicsit se-l f has mean-whi le become an important b :J :' lJ ,m;hufl .l licroscopic solid-statephysie-s , eventhough I ts histor:ical roots Jie both ill clas sica:1 bulk solid ... ..ate pbysics, andpitysicalchem.is try, in part lcular the study ur surface reactlons and hete l"oge~nsou s c l 'J . ;t n l :' l1 lS i s. .S o li .t l -: st ! i: tc p b y si cs , is conceptually an atoll:ilC p hy si cs , o f t ,h e o on de :m l B cistate ( l i f mattar , Acco..rdi:llg b:Jthe !i t:I. 'en. ,gthoJ chem i . ca l hO[ldiIlg, the l 'r i;1tev;~Ultenergy scal~ is t ll a: t b Et w- ee n 'irerQ and III couple of electron ' !. ! 'o l t1l. 'The maingoal c on sis ts o f' deriv in g an atotilhl;t: ic cie.(' lCl ' iptlon o :f th e m a cr os co pi e p ro pe r-ties of a sohd, stich a . 'i l el a s ti c ity , spe c if i c heat, e J ec tr ic al oo n du c- ta n ce ~ o p ti ca lresponsa or magnetism. The O O a 1 " a . c w l " i s U c . difference from atomic ph:vsicsstems {l'om the necesstty to deser ibea ~L number of atoms, an assemblyo r about 102!s atoms be:ing contamed In 1 em" of condensed ma.ttcr; 0.1' thellf! aOO l " l 1Shat l ie a lc n g a line o f 1 em In Ii> sol id. In order to m.ake su.eb.a largenumber of atoms acc"eSsihlcto a theoretieal de-scription, Mw concepts had tobe:developed in bulk !;()I .id -sta te physics . The transla tional symmB'~I'y of anideal cl'yst[tIHncoolid leads to the existenee 0:[ pllDillon dispcrs1ol1.brll ,l :1ches orthe eleetronie ba .nd structure and. th~ ~ffccti ' iI 'C mass of au electron, Becau seof thelarge number of atoms involved ~and because o r the difieI'm:tcc betweenthe macroscopleand the atomic l~g;t It seele, most thcoreticalmode"lsi .l l. clas-sic sol id-sta te theory w"e based onthe assum pt101'1 of an . lnfinit.ely extended.solid. Thus , ,in thcserncdels, th e properties of the r eJ a: ti ve l: y s rn a Jl n nm b er (Ifatoms fOl"'plillg th e surface of the ffiilCros.oop1c so l id are negl i l l ic tcd , This sim-plifies the mathematical deserlptlon considerably. The infinite trrulslational


9/286

2 ssym m .c try of the kkmlilXld C l : ' Y S t l l l l . in e ~lida l lO' !vs t , h H l applica:ticm o f a n nnbel' of s~yu:unet-ry o p er at lo m s ;, w hL c h m a le es a bflJldymaLhc :matica . lkea tmeutpossible. ThiiS descri.pti.ou of the solid in tel'm~ of en in:l1ui tely extended ob-joc;t, which lIegl0Ct~ the pl'opcr t]es of the few ciif ferent atomic laycJ's at. t .he~u f fa .C { l ,i :3a g o o d a p l' :l l' o xim ; at io n f o ,r deriving macl:oscopit;l)(tQpetth~s tha t de-pend on the total mnaber ' ofatom:s containooin. this solid. Furthermore, t.hisd e sc :r .i p L iJOuho ld s f o r ~U k i n ds of spectroscopic experiments, whe re the probes(X-I:ay!l~ neutrons, f!l:'i:Lelectrons, ete.] pcneb'a,te deep ll'lW the solid mateeialan d w here the e ffe ct o f ther1l1~tlveb' f ew s ur fa ce ! lL tQ1U:S (R j Ulu~c ;m , - : 2 ) canhe t!.egl,~ted.

Th,1.'!approach of classical soHds ta~. e phy~i,~ lntel'1tJs of an.inl1ni.tclyextended wl i dhe~omes highJy qtlcstionable and. incorrect, however ,wilenpro bes are usedwhich; ~1;Ll'ongl.y" i.n:ooract ' '!,':i.thsclldmattcr an d 'thU5pen.e-tmte only a oouple of An.gst; l 'OtnsinLo thewlid ~~1U;;h !IS Jaw-energy electrons,momic , l ,nd molecular Learns, ere. Hen;:;th,e properties of 8urfaooatoms, bcingdifferent ri'OlU those of bulk atoms, beeome lm,pcII.'tMlt, The samf;;1 s. b 'l it l forspoc trostO]) les where th e padldes detected ou ts td.c Uv; !surface or.iginflte f rom .ex.citation ' p rocesses Closeto the surlaea In photeemiseion c .x.pe l ' i ! :nentsj f O ! 'e:x:ample,e.lootrons from ceeupled eleetroule state. ' jIin. th.: solid.arc excited b o yX-:ra.y~ O~UV light;th:ey escape into the vacuum fhlOUgh the su:rfac.e andare analysed and.detected by fill el.ectmIl SJJlllctl'Qmeter. Due to thevel'Y lim-io o pcnctl'atjon dep~h of these ph.otoelectl'l:ln,..~( : : i - a oA . cle,pending on Umirenergy) t.he ~ff(let of t he t op n lO S ta to m ic la ,y ~! l'1 'J L wl ow t he surfaee Cllll:noLbe neglected. . The photoc lootl"OD spectr01l;c . u 'ry i l 1fo l~m! :J . t ia nspcdHc 1: 0 lhes~oopmo," I t Il.~omi[;.aefs ..Ch ,l lr !; lc t( \r i$ l. i. c p : l' op a rt ie s o f the s urfa ee e n:m t tbethe-o ll "e ti ca l d es cr ip t io n o f a p h ot oe m is si ou e .' {p e l' im c n t ( P. fi .J :I il i' :I .I: C ha p .. 6). ~ E ve nwhen bulk electto)1i'C sta tes are ~ttldied'jLhe l i l . : rl .~lysisof the da,ta is deae w.i th' inthe fia;mowork of models devloped in surface pbY-SlCS.Fur thm'mclI. 'e. in order1 : ; 0 ge t iu form.a tionabout 111tL ' in s i c p t 1 Jl le l "~ i .e s o f thep.'l[rtk~nlar$olid., th.c ex -pel ' i lmo:nt. ha.o; to be performed u nd er U I tr !l 1! I" H ig h V a cn u m (UnV) c:ondi :tions~on a. freshly prepared clean srunple surface, Because oJ the surface sen~itivi1t.y~the slightest OOl l taTI ' l . i na : t io l l on. the su r faae wonkl . modifyth(l results,The coneepta ofsurface and f noo rf : ao e p i 1Y l ll o s are impol'ta:llt in solfd-statephysi .cs not only Jn eonnccneuwith special e;;;per.il[llmbl1 tools, but also fQ r

C o e I ' L a l u p hys ica l s ys te ms . A t hin 5 Qlid l1 1m d ep os it ed o n, a s ub :: it ra te is b ou n de dby a solid-solid .interllu.'eand by its s ur fa ce ( fit m va cu ur ni nt e: ['f ~lC e) . . The]J lCnper t ies (JIf su m ;;L t:bin f i l i I : ! . : m : - e thus basically o e t ( ! I ' 1 l : 1 i n e e l by t h e p r o p' C :l rL i ~ 'lof its t w r o intel 'f:a.c:es.Thin-.fi lm. physic!; cannot be reduced to t he o o n .c e pt sof bulk 801id"sttlte physics, but instead the mod,e is . of :int!!tfacc phys~cs haveto he appHccl ., . SinlU:a. tly! Lh~ pbys, ics of smal l ~t .o :m1c c lm , t e :m , which oftenpossess more 5rn:face than ~'lJulk"atoms, must take into account the l'e$ul tsf rom : ; Jul ' fa c e pbys i :C5 .Surface and interface p . b ,y ~ . C ! J ~ flli .1 ; well -defined sub-discipline oJ geJuer.1Il(,, ' I.11tde:nsod-lnruttcrphysl .cst i s thu s iu t e t l' d a :tOOin . ~loornp l l f ; lK w a : y wi th a n um -

MoterulM~

VOIw:! IT i u l ! l l ! ! ! t CW

SurfQClo:!a n dI n t Q r m c . eP h y s U : i S

(tII'.r~I I I I I ! I I~f~ProtKlj:o

Reel'Su-msThi n F il m sAtoo: l l c[ lustersBe.ctrod1emistryN ' : l n l G ' r e c h n o l o g y _

AUIi(re~,~

:Fig. 1.1. Int.erre],atloll of !lmJace sed !n~"wf!lccphy&i","sall!!.; subdiseipflne ( ) I fctlllde.llseO." ' .lli:El: t , tel 'p.hysi .csw~th other~och 1i ,e ;k1s

bel."of other :rescan::h fields (F:ig. I .JI) . This is particulady true jf oae com~iden'!th~ iIlpU t [tbI'U other domains of p:irys:i.cs and dWl1l.istry and tile output intoiUIPQ:rtanLfi.elds of appJkation such 11$ :;;emkOlnrluctor electrouics and thedevelopment of IHj'!\,1 ~~.poer.jmenla.lequipment and methods, The seileule InFig. 1.1 empha.sises the w . . a y :i.u which surfaee and iut.e(faro p]rysi~ is eIU-b ed de d in , t he g Cllm a l Ad d o f c nn de Qs ed -m ~ tt er p hy s: IC Iii .a s w e ll a s th stl'Qngh u p < . l .C t ofLht ' : m o d e l s o r bulk .wJid-sta : te pbysics (phonon di[olpeJ'si.o.n~lec tronicbands, 1; . ra . I l . s :podmechan isms, etc.] emthe , eoucepts 0 . i n ter faoE! ! pby.s.ocs;OJ , l , ' Lhe other h ~[u :l,w it hin g en era l . .soUd-state physiGS,iuterface physi . i.ca .lpropel . t ,ies and their g r ow t h n l. eoC ih ru ri s: rt 1s . T h e physic$ of r;:1tI~ma: l : . omk clustersa.lsoben.efits: f ro m s l1 rf aJ J c l )hys. i .cs:~ as does the wide field of elect:ro-chcJl1lstrYjwhe re t h e l 'e ;; I. .c : L io n) iF~Q1id. s ur fa ee s w it ha l) f l.m J :i i.e :n td cc :t ro ly te i! i: t he c en t ra ltopic..F' l t rLbm'mOI :e . , the n ew branch o f ua[lotechnDlogy~ i.e, e.ng i l . lee l ' ing 011 0.,nanemeter scale (PaIJd VI: Cb.ap..3)~which has em.erged as aCOI1:sequen.ceof the a.ppl.ic1(tion of scanning tlUlIUlJing I11im'Qf>COPY and rula.t.md tcclU1iql1~.,~ISC}S c on c e: pt s t ha t Itl!l'i"3 la rg ciy been devdup ed in surfaeesciences.


10/286

Modern. 8:urfDlo~antiintel:f!lC@ physks 'WOuldnot have boon possl:blewitll!-ou t the use o r r es u lt s f ro w n r es ea rc h fields ether tha~lbu lk ~ ( ) li . d " ,s l .8 , t ehysiCli i .From the e:x:peril11cnt.ru vhJWl)olllt,.the pr:p.ar~tion of well-defined, clean SUI.' -faces, 0 1 1Whleh ::!nri~lte ~tlu: : l i :es ar e llliually pel ' fol ' Inod, becamep~ bleonlyaf t~L' the deVlo1 lme :n t . of UJlV tcdl :rn iques . Vi,lcuum physics and technologyhad 1 ' 1 . Ilitrol'ij'!;impact on 5lu'faceMd thin film ph;ySlcS. Sl1rf~\.Ce8CIi51t i .ve 5~C-b'O$Qopie!>tlse particles (lov,Hluergy electrons, nOOIJ:lS,molecnles~e tc , .) becaaseof theil' "strong" interactien with m l l t t e r , and U I 1 , l : Sthe developm,ent of p.[lrti-de beam optics, spectrQnl~l~ersand detectQl's is int:hnatd.y relatedto the ad-vent of mQde : r~1 surface p h Y b 1 . C l i . Since M801'p ' t ion pil 'OC'ffiSf1."l OIl solid surfacesare a cent r; ' l i . l topic in surf! ' lce pbysk;5j nntouly th b properties of the solid sub-s tra te 'b ut 011w t h er : l. ly ~ i( :s o f th e 8d:sorhi:ng molecule i s a n l ll :g r .e d ie n t in th ~u[ tdo~"S~n( l i l lg of t h ec o n 'l p h~ . ..l:adso:l"[1tiQn process" Thf;l phy si .( " S and c . hem . i st r yof molecules a ls o p 1a .\ y8 ~,lte8lSentiallJole iuman,. qtlGst io. llS in surface pb)'5i,cs.Last ~but nOlt; letst~ modern surface and inlerfaccphysic5 ,>voulcl n~ver h.av(lreached the p.l 'e>ent level of theoretical undel':.'ltandiug wltbout the pm1sib i l -it;y of .Ia:rgeand oompl~x eomputor c a lc u la ti on s , M a ny c a lc n lf l. it io l1 8 ~u'emuchmore exLcr~iycand tedious than in elassleal bulk wlirllj:t.ate physics since,e'l,l'en f(ll' a cl"jfstallinc solid, a 811rfacti 01: intel'fare breaks the tra:[lslational3ymu'u~ltry ",end thus ct :uls idnrabLy incl 'OOses U1:ell.umbcr of equatiol1sl!p beheated (10$ of ~jlmmetl.Y) .. i : ' O f f l the viewpoint of a.pplications, surface and inteIJ:ac.eph;_vs]cs (:J:n 'becnll$iderod !li;Stho basic science ;fura Ilum ber of etlgin~el.il1g branches and ad-v anced techno.logies. A bettel.u1delst~ndlrlg of cOl~rosion precesses , and thusalso Ure.d f ;Wdop l : u c n t of surface protection methods, can only be ex.pec tec l OIlthe ba9is otsurfaee I' l tudJ.es. IVlodcw8eniioolliludo~ de 'Vi ,c{ l teclnmlogy wO l d dbe quite unlh.i:u,k:;th:le without 1('Sclln~hon semicoad uctor smfaees and intel"faces. \ IV] th anhtcreasing trend towards gl"e.atel"mil1i3tud~l:I.tiQfi O:ar~5Calei nt eg ra ti on ) s ur fa ce s andill~tll'f~ b@!X)(lnc an : incre.al i i i .ngly impo.rtMt fac-LOI' in the J u ru : : :t < ion in , go f a d.ev~ce"F urthe rm o re , the pl'cpal'atlon tedhniquei- . f o r com.plex mn l t .i la , ye r d e v ic e s t n1 .c ; tum . ;. -MolEcul!l1' EcumEpitaxy (MEI.E),metal o : rg 1 l: 1 l1 .cMBE (Cha p . .2) - are large1y derlvedIrom St t rrl) ;c~enceLech~niques, In this field. surface !'Jnienocl'c~al 'Ch ha5 led to the de\lclopmcnt ofnew wdb[lologks fo r sen1i.o !:mclu(f t ,ur- ] : f tYf;W pn:warfl:~ion.

All in.tc.rdcpcl)d~ce betwoon i'iUl'fa.CB p l : L y s . i c s and applied e,ata!ysislUosoor .cbcan slso be observed. Su l of.u : :e5del lor~ h3S ,eont: r i buted :rnuch to.a deeperaliomistic; ul1,d,el.'stan~i.ng of im]?orta;n:t ad so rp tio n an d reactlon medhanlSff i : . ' !o f m o le cu le s on cn:trnyti~an,y oo t :i v e s u rf a c es , even th.d\Jgl ; l praetiealhetero-gcnecus cat~"yl5i!l QO~ U l ' : : lunder Le inpa ra tu re and pressuee ool.lciitions totanydiflian~ut Fn ;ml thos-e on a clean solid surfaco in a UHVvesreLQn th e otherbaud, thclaTgc amount of :knowledg~ derived [roin elassical ca: taJ .ys i s stud-if';!> , ;uu: le1;esswe11.deEined OOllclitlOllS. ha s $lso in flu eu ced su rfa ce scleu ee re -s ea rc h o n w e l l- d .e :f in e rl m o dc l systems. A sir(lilw; 1l1ierdf;1pEmdcnoo C:Xls ls he-t .\v.een sudace physics a ni ! t he g en .e n :u . fi el d o f a p pl ie d l ll iC J . 'oann l y si s . Tb ! l: !de -

1 01 1n d f a r ex t reme ly surface-scnsi t ivc;J l)l 'Ob es in s nr f'r u: e an d i n t e rf f iZ e p i lY : ' ;l e shas had an enermous impaet on the developm.cllt and . impri ll il (H :nen : e of n . ewpartlde spectroscopies, Auger Eloctron Spech"OOoopy (.kE8), Seoolldru:y killM il ss S p ec t.l X! ." it :O p y( SIM S) a n d H i gh - R e se l t it ian .Eloctron Enel:gy L o ss S pe e-troocopy (HH.EELS)a:m good examples, These W.dUliqucs 'we re developedwtthin ~hc Add of ~ u r r: a oo a nd i l ' ! tGr~r~ l . cephysics lLl] ...MMn\.,bik~they have .become standard techniques in many other Helds of pJ:act iCl IlI 'E : l searchl where:rl1iCl'l)perhn:entallllcthodsdescribed in so-called pMJ~ls ._In ~pjt;e o f the dive:t'flit,y of the e; l [pelrn i ! 2 :u ta [metheds and a p pl 'O ~ ;l ,d :l es ,~ p pl ie d 0 0 [ai' in this fie:ldj, there!s one baslo teeh-n iquew hich S0Cm s to be eem m en to aU l'l1od,enl s u rf a ce , i n :t ed o o :: and thinfilm experilnents: unv equipIIle:nt li s required to. { !_ st ub l sh d e an conditionsfor the pl ' e pa r :: ; l; t i, onQf a wel l~d.e:I1nd o o lt d s n rf ~~ e 01 ' th e pe:r(01;.m.anoo of in&l1;;utudies on a fn'!shly pn~pa..red i!lterf~. Jf one enters a l .abQl '! l~ory fursurfaoe orintevface studioq. , large DRV vassels wi th cor.re~ponding :pl lm:pins;sta tio m~ are :a lw ays to b e fo UJ.l.d .SimilarLyj, t .h~ iUlpmtf inc.e of p!l!l't.id.e- beamo p ti cs a n d a [l .a 1y ti ca !L o o ls ~ iup.a.rticulal' fo:rlow-CIli~rgy ~Leet rQ.ns , .deir . ivesfro m the nece$S . l ty t o h a ;v e s u r .f .a c e s r u lS i ti v e pt-o~ available to est.a:blisJl the.c l 'y s t !l L l logn l l: p h ic ] J e rf uc t iQu and deanlinffiS of 8, fresbly propared SlU~ra,(Je.


11/286

Panel IUltrahjgh Vacuum (U'HV) Technology

,1


12/286

iii 1, Surfaee mud IntedacePhysic:s: I ts Definition and 1111PQl"tEince

Fig. 1.4, SCliClJla,t i-c eross sooL~pnh.rOl~lJa :mtm-y mughi:ng pump. During the gas in-I ~t p ha se ,the ini .e twlume c,xpa. l i tds. F'! .lr-dl er mtl'lti ol 1 of 'the ecc-cl1tr ic rotol' C8!l5e5( 'O I 'I l. '[ rr es s: i on o f this ".olmue u:nHl the ouUe tph n ! ! le i s r e a c ll e d

the sorptionprocess wiU saturate sooner ell: later ,the sorption pump cannotbe used continuously.In corn biuation w ith tUI'hOIl).ol :ecubu pumps, one thus U5e! rotary pumps toobta\l!'l ' th e n eCC 'S s a ry l :m . c l dn g pl 'CSSU1:e ( F i g . VI). The I 'o t .a :ry pump fllndlCltlSo n t he b m ,li s.o f changing gas volumes P100l lCCd by the rotation of ~l;lleeeeatriel'OtOl', whieh liastwo blacl~ in a dia_lnetdca I .slot. During Ute gas : inlet phase,the open ' \ : , o }mne nenr the in le t e'xpands, until ; afte r furt her [ 'ota:tiun Ud~volume is separa. ted from the: inle t. Than, during; the compression phase, thegas is compressed and forced through tbo exhaust valve (oU tightened). Iuorder to .avoid. ' t:h~oond asatlou of vapor contaln~d inthe pumped air ! mootpumps are !luppU.d Wit:i l a gas lol'lrl va:lvel~hl'(]lugh which a - certain amount< 1 air, the gas load is added to the compressed gas. Sea]ing between rotaryb1aumJ and inner pump w-al!s is performed by !lll au film,

The pUUlp5 that ar e l~ldar]Y ' used in the Ul:lV ~i:"ue ar e the t-\~I.bo-moleeular pump, the ditrt' ls: ion p U _ l T 1 P , '~heon pump and the Ct'yofmrnp,

The principle of the turiJO'moleeu1a1' pump (or turbopurnp) rests on the ao-Lion 0,[ ,9, h ig h-sp eed ro uo r (1 5 0 0 0 -3 0 00 0 rp m ) w h i'c h " sh uf fl es " g as molseules(nnn the ua v s ide to the backing side, wbertlthoy are pumped !I:\\'ayby a ro-tar!! , pump (Fig . :1 .5).The rotor, whiCh turns through, and is interleaved \ \' it :h,U t e S ! H :a 1 I. ed ,sbdor, ha s "shuffling" blad s, Whi.h a r e i n c li n ed \ \' i' iL h respectto the nytatioll axis (as are the inversely inclined stator blades], This meansthnt the pmmlbility of a. molecule penetrating the rotor from 'the backing sldeto the U'HV side l~ much lower than that of a molecule rnovlng in the revoraedirec::tloL1. This. becomes clear if O ne considers the posil .h1o paths of moleculesHlov ing thrcugh the a!ilScmbly o f ro to r b la .d cs (Fig. I.5b). A . mo l e cu l e hi tti ' J . ! _ ! lthe l'QWl" blade at point A (least favo . l ' ahle case) can, in p:dnc iple , pass fromthe UHV side 'to the backing side, if it impinges at an angle of at UlQSt lJI andleaves wlthi u lit. Fo r a molecule to p ass th l:oug h the ro to r in the OllPosi . t . i :d i . rection it must ilnpil\bi'Cw-itilin all angular range 02 a nd l ea -v ewithin ~ in th eleast favorable case itl whleh i'~ll. rr i.ves at point B. The probabilit;ies of thesetwo p~ths can be estil'lllJted fromthe rat~!;s of angles 61/Oi and Q d P 2 ' Sincef i - ' l j f 32 is cOl1sider.ably smaller than f J J ! f3 . . t.he path f rom the UHV side out-

[1lirh's.~ p . m ptzazttttU?t???t-tt-ib' r- lt .I'bimuI_k~IIl' ~

!ZW{liZ?Z?Z.U'iUtMPi'-UO-i.ti: r o o pJ'I1I

I I I. I . iI Ilr'P R E S S U R E imba! '!

K nift ~"__--C~rn-rn",.---i---"""",,"'Ki-,;o;;>;l ,~t

Fig. 1.3,. erOS:! section through a stainless steel Un~flat A O l l g - whi .ch ill 1 1 S I ; l d . illUHV e qu_ ip n : i! l 'l l It fm sealing

known as f 'O'l:i .Yki"l l f} O'l.~t the. sysicl1n and su ch . pumps oflen go by '~he eorre-spoadi ng name of r T n J . . 9 h i 1 ' ~ g pump.5,A Si'Jrpt lQll PUfll:P contains pulverized rn i, l .Lerial (e.g,! zeolite) with a largeacU ve eurfaee al'oo~the so-called mo le cu la r B i e- ve ,which I!ICt.


13/286

10

o~ ' r o ' I 1---I---+-7'''''--'t-----'Ic:;,:z~ 1Q~J;; ~- "A t8 '~!Ii!:.......J:.J___L--"'il;....LI'-!

o 2 4 6~ R O O T O f M O L W E IG 1 f I ' 'V i i '

R c l w b t o . d e sR m n r ---=.:1 S k J . ! l ) r

Slum; hbdes

tFo re v ar uum f la n gE! Flg. '1.6. Go:rnpress:lonT"at i ,Qof a, t i1l l 'Domo!ooulf l ,r PI lU l ,P as a ftmcti tm of n lO leeu4u ''welg,htM of the molecules pumped (left) . f},ndoCthe m to t...e lo cit y ( rig ht). { After[~lb(jld HeraelJ$ Gl'nbH)R o t a r y axis , UJlll.~.-

l ! b !w . i :\jl\.I!,,\t\~\ \Sii!je

Fig,. 1.5. Schen'u~:Licept~en,t~l tion of a tmbomo.lctuhu fllJm]) (a,) ge:ne~I I ;1a:rrange-~ rn !m : t el f l'ator and f;LFLWr_Rolora:nd s . ta :OO [b l l1 iQe s ( n o t shown. i n, d ,c tl Ii l) a re l ne ll ue dw,i th respect to Q11~OU!ot'hlJr, ( 1 ; Qu~Ht!'ttj\ie"'ie", of the BI,Tanger :ne:n tof therotorbladi!J5,with, ,lwpect to th~ axis ll.f rotntien. The pos:.o:ihiklpn:l:ns of l:nolet-t.l!~ fWLTJ,th,e UliV side 00 the b~crd[lg side and'l"ice ~ are geam~t~'icaUj'determined byt h ' t . 1 l l J 1 gL e - s ! 3 J ,I~h,:lind t J : ~ .a ' ~ ,res~~l "'~Iy

.//V/ -- -I

1 8 2 2 Z 6 )lR O T O R. V E lO t~ T '{ (1 .0 00 rpmt

Flg.I.1011,b. Schematic 'view of an i ( ln-g$t:L~l ' pillnp: (8) The basicmlJlti,c>ll11 at'-l'9.n:g~ment. Ell.l::h cell, co:nsi5'~ "*!s .er lt iaUy qf atube-Iikeanede, The cel l:s,m'esaad-wie lu!d het~n twO ccnnnon ca ;lho


14/286

12 13b) To ~1IllliOOl s y s t e mt

backing sides. Wllen the mean f r - t . " ' I ; path of the gas moleeulesat thrtliutake~!> less thl'!:ltthe deilran(le~the pump acts 11.8an e Jec t.Q l p ump . The gaB is en-'t; l'aJ.11''ld byv1s.oous d l'ag and trn:hll lentmi~il lgj aud is earried dow 11 t he l J iUmpchamber and through all Ql:illoe ncarthe backiog side, InW!llemodc!tll typesof ' l i' m ; p o r p ump$ j combi.nations o f t he diffusion an d ejector p l ' in c i : p loo a re u sed ;these pum ps are called vapor ooader pumps. Di ff us ion P l iIT I .p . 5SUJ r e I'fl"(l(mtwQd:ra.wbacks that limit t he ir f in al pressu re . Back- s t l " e a n1 ing and hack-m. igra : tkmof molecules of the wOl~kj:ng fluid give rise to particle: migratkm ill th.ew:wngd.il)t:1ctioll .The va :po : r p r eg s;u re of t:he workhlgHukl is t hug ' ir!lportant f.orthcfinally obtai:uabl,{J pn;~ooure. The same is true forlnol~c[ll~s oJ the pumpedgas l ; ,Vhich c an also. b3ck:~di:ffuse to th.e high 'i.!'~lo!;;UU;midle. Both e.ffc,ts c a n . bereduced by using O O i R c . , s and c o l d trapsl which obviously Imverthe net pump-lng spood but 3! ' I l : . necessary to reseh DEV conditiQt!s. The ha . rnes eoatalaH q ni d- ni tr u& ; oe n C ( ! (I .I ~ b bl dm . o n w b ic h O l e bnck.streamin,gspecics. condense(Fig. LSa). The cO. l l :S is tenc :y of th~wo:rking ft aid is thus very impodan.t fo r thep el 'f or m an ce o f v .l lP O { pumps , Me l 'c u :r y , w h i c h was 111.~ c lu s iv t l u s e i n . fm 'm . oJ :tl:1tI~~ ha s !lOW been la['g;e :lj~dii5pl.aoed b 'y high-qua l i ty - ulLmhighwl,cuum di : ls ,which enable PiWSSU!'el!! in the 1.0=Ul Th[!: range to be r e ; ,. d l .e dW ' ] lmCO OU l lgtr , aps (j.l'e u sed .

Beeause of th.1i rextremely h.igh purnpi.ng speeds ~'lfJQP'!l,rlRilk~ eze g~i'ningpopularity fu r large UBV systeJ: t1.s .Cryo.genic pum.ping liSbaaed Oil the f . a . c ttha t if 11$'Ul.r~ wi th i n !I. v ac uu m s ;y st em i~ CDolcc l , ll!l:por ( g as m o l ec u le s ]tends to OOII . d sI I s c upon . itb th:us redncing the ambient pressure. .Atypicalru : yo l :mmp i$ $ketcbed in F ig . 1,9. The main pa r t .b,j a l l ) Jc ! t .aHic helb:. whichsal -YCSas the condeIlSsl:surface, If. hl mounted in a c h ambm : that 18dh'e!~tly:fiangedto the UHV ~l to be evacuated. The ccelant, 'I~~mlnyiquid helium,i :S 5 up p li ed f ro m a. dewa : r to th e h elix thl'ough a v a e unm - i n s u la t ed [oed. tube .

~~rgI~f n l _ f - -,

C klH e t -Fig.I.S. Simplified : roprese l I taHQl l . of..a_,Va;por ({IiITiJ~ion)pump (b) toget.hcfwith,1 bttlile Qt' cold tr~[p (a) on U1ii!'high V!)CtlU:n1 side. Baffle ( ,a) and pump (b)ll:r(:!a:r:rangedoneen top of the ,olb,C!l"ina pumping ! ;t .a t:JQif i

increased, A high efficiency of ion f O IT l1 li tl on dOW l lOO p:roSStU'C5 of 10-12 Ton"and less is ospl:l~ l:e cant!!'IJ.!!>1% AI). Thil':)problem can be tackled to some extent by using aux:iliaryta:th-ode s . Sputtet'iollp'lImpg a:1\g l.lv!lil;abI~with a. wide range of pumping speeds,~ baLw~n 1 I t l a and S(]OO(I./s. Th.ep:re~nl'e rang'tl covered :isW-'L to less thanto -12 Tarr; thus a oackingplunp is neededte ~ta:rt an io:n-get~cr pump.r.o.n-gettx~[ pumps should ]lot be usedin Stu:d1e:5 of adsorption. p:l."oc~es andsurface c:hemi!i,~rywith l~r molecnles, ..since crackillg o f tbe baekground gasmolecules might. occur, Lhel'eby incluci:ng addstional unwa.nt{. ld reaetions,

In those cast}S.~P'w vu,rayn; am aconveuiellt altaI'native, The generaltermvapor pnmpinclud.csb.oth ej~ctQr pumpsand dl l l \ l saml pumps, In bath tYJ l~of pump, a. vapur stream 1 5 prcduced bya heatera:t the hase of the pUllI.})(Fig ., I .Sb). The vapor, oi l or mercury . t I' [l ;vcls til) a column ( 0 1 " n omnhinat:ionof several CQIIUlln~)ao d l'earhes an. uUlol'eUa-likc deA! ' l c to l ; " placed at U1e 'oop.There the va.po:rmolecllles coLlid.ewiLh the gas molecules cnte:riIlgthmughth.e in.tak.c p a l' t . .Whcn the mean - f r e e path o f the ga s molec:nlesls gJ;ea tew t h . a . . ~thf ii th]:ua.t wiclt.h,thfi:intoraction b~Lweenga5a l l d vapo.r is based Oil diffu-sion, which i~respouslb la Eardragging the .ga5 molecules tewards the ooc . k i : n gre gio n. T hu s c l iH ' u g k l 1 1 . in iLuc . ! l '8 h e p re :s ." l1 re gi:a d~ cn .t b et we en t he U H V a nd

C a o l o . r d' Y E ! s ! i i e ~ F:ig.; l.9. Sc : hel : ll ; at k d i a g rt ' l1 . u o f a ,c['yopl1mp. (After LeyWlcl FJ&i)f!UBGmbH . )


15/286

1 5

.~~~.r1--==~--~~-'~'----=~~~~g 1 0 ~ 1-r-----1~4-::=~rF:---__:#q-4/.4L1E.. I~10~~ 1-I1_.,..,.,~:::;;;"_.J-_Ib,."...I"""~:;;_ I--l,'11-1--11~ iO -! t-+--+--+-~q._..;f---t---!-IIi!l':W...-J~ 1O"U-+----t--:7F-f--~~__1--...---:::N I.I,'-+-+--I

~

v j5~o: ! ! ' i ty o r a ga ls a. direct :i:ullcti.on of its pressure, tbe measuremem; of thedcca.y of a .mac:I'ooco:pic metionindueed byrllolecular dI.agcan he u s e d . to de-termine the ~re$Sllro. Even llre~l1retii as low c ru ea Pt :l l'o pd at e f ur N ~ . Modern] so-called Bayani-Alpe;r t .9(1.11.9(1S U l 'e o oJ ls tm c l ;e d a a i n Fig. Lnb, wi t h $ eve : ra l 6I amen :i :. s ( ! ' ISspares}1

41. 1 6 ., 8 9 1 0 'T E H f ' E A A T I J R E U Q

F IG . 1 1: .1 0. . : : : lat :u:r1; l . t loo.v dpo r pre : : ;: :> l .[m~o f va~ . io !J! ; coo].a.ntltu.tedals EI5 I I fun.ct io i l lf i f~e : J : npe l< t tu r< ' !

lti!!m.ade to flow through the coil by means of a gas pump at the outlet. eOOof the helb. :. The coolant boils as it passesthrough the eoi I~iHYti :CC' C:iol ingthetube. A throttle valve in the gase:xbaust line ecntrels Ute t t ID:. and thusthecooling rase . .Atemperatul 'l e ~e.!l5Q:ri~ed t(ltb~. ooi l ;)J.lltOltlati.cally eontrclsthe . 'valvt!$tti:Ilg '. Closed-loop j'stems axe. also iu U~; he r e , the . .pUt.np coili~dir~ctlyoonnected to a he-limn liqul fler and a cmo.prcssol". .The heH1J~ngi l'strom the exh~l,ust is[Qd backinto the Hquifi~r,

A . SQ eC ll ~d yp e o f fcryopump is tn.e se-called bath pU1: l~p, whose ooolant 15contai ned in it t an k . w h ic hm n s t be~fiJled foom t i m e to tIme. The ultimatepn98 : r bel low. Th e r ea rl in g il ! a ., m p ]i :f i. edoptically or c : le c :L l 'i c a llY t e . g, by a c.. ' l lP~O-IL~I]m measu r emeu t (capacitance. gaug - e ) . A fu :rt .h er t yp e o f g au ge t ha te auoperate in Ute high~pr~~l;It rn:ngeis the u:mlecuhll'uiscosity fJal~ge.Since the


16/286

~gr idw t h O O eI l1 lQm~t)

([1)l/=NkTILile roqUJ.t 'e:lJl ! lm:t of p~l1"tide OQn:: :e rvat io t l yh.lds

vAv = ~ ( ~~+ ~ ) jwhere A,~, and 11.,.ar e the ir.mer surface area and the voiul tLe Qf~hc vac-uu m ch


17/286

'Pa:neIIIBasics of Particle Optics and Spectroscopy

D,iels,[{ ..J~l, It.: LeyooJd V(;lct!ttm Hoo4JXlOk (Pcr.gt ingoftw"( i ,mctallle grids) wi th app l i e d. , >, o lt a r ,6 e Ui:iJ dt' lfiecLcd. Duetethe electric field e(normal to the c~pRciwr plates) only the normal comporwnt Qfth~velodty ischanged from l J :i . (_l)llo 'Ul (.1) th e parallel t . '011lponel l t . is um: ihangnd l Lc,

(ILIa)

. - - . . . - , - . - - ' - " - - ' } ; I ~ u l------,- _._.-.- -h,)

Fig;. n.1. CI_i.cru electrou trajocto:rytl.U:Qugh!I. p-."t .t!l ll .c l-p1atc cap~cit.o:r . The e!oottic f ie ld he -twe t . . "Jl the tw()t;rm:lQPlu\~nL elec~f(xles {di) i ' lboo l:iru;)s]cha nge s d w, eled~9" veil,;u::ity O O :m p OI l~ t 1 !:1(-i) into~ (l-) bnt!E!fIiVBS the component tJ (II) mu:ha:nged


18/286

2 0 Parmi. n:: Bas ic s of Pfl.:tjel~ OptiC' ! ! and SIJectniSCQfJY 2 1b l c)sin a V;ol UI,.!--~-=-sin{j VI nl

This ' r erO lc t iO :u law is .analogous to the opt icalla:w if one idc:ntifi,cs the velocityratio with ' the ratio of refractive 'indk;cs n'2/nl' Asslll'll.ing that the Incidentbearnwi,th velocity 1}] lS prnduced by au .Roce!era:ting'll 'oltage Uo I and thatenergy is conservoo ~li ' ithi .nU te capacitor, l .e.,

{U.lb)

111>2 :rn2. 'U '-v~=-til +e . ' .:2 ~ 2 . ,one obtains the !aw of refractionsin n . _ t'2 _ .~ - .,;.1 ' +U J T . 7 .-_- - - - - - . !.IQ.51l1fJ t.ll 111

( I1.2)I:- -

'. ' .. .(II.3) Fig. 1l.3a-c. Three examples of e1ect.ronl en lOe!:! ,ormed by tllew.1.Iic ilperinres:(a) oocussillg iltrallgemell't, ('b' defocllliSing an, f l l lge rnent , o.nd (c) symm~tr.ica.1 sin-!ile hm~ w; i th f ucm l si n j! p : ro p e X 't y- beach C(' ISi1 !hnraeterfstlc oquipotential. Hoes areshownl llve:l 'sion of the bias Ua deflects the electron beam away (tom the normal.Describing the capacitor grids ItS equipotential surfaces for the f ie ld t: " Uwgenel 'f!:i dcscl ' :iptio_n is, tha ic the eleetrcu beam if; :re&a:cted to\vards or awa .yf rom tile .1.orlual to tbe equi .po ten1Ji , a l lines depending on th,e gl'rulicnt p:rthepo 'W n: t . : ia l . In plrinciple; ([1. 3 o ) i f j , z.k;o .suff ic ient . to c on st ru ct , s tep 'by ~tCip, th et ra jectory of elect rOilS moving in an inholl],ogenous electric field 6' (1'). ' 'rha:t is,of course, only true in the I irni. t (I f c J . a , . .~lcal pw,'Ude motion, where intel 'femnceeffects dueto thfl wave nature of th pru :Udc . can be neglected (Sed. 4.9).A simple but instructive model fOl' an electron 1("118 might thus appeal" asinF,ig. II.:21 in complete Im~logy to an op t i .c a l . i e n s ,. The !'l1el,aUicgrid itselfis not importantj but rather j :hc cul 'vature of the non-material e(luipotcnthilsnrface!; . Eh'JGtron Ie.D~ can therefoee be (lOusb:l'Ictl'!d in a simpler fa5hiofi,'using metallic 8pel'tm:,es w hich are the:m8'P,.l .ves suffi.der.lt eo cau~ curvatureor Uu:!equipot~n'liall.inC5in U:uai.r'vicinity. Tile eXi l l . lnp]es in Fi.g. II,3ajb a . . c ' ~& l ' > focuS8il lg and dofoCLlssln;f!; lenses beenuse of their cbal ' :fu::te:r istir . potenti fJ . lcant-orns. The : : . h : i Jg l c hms in Flg. ILk consists of t J l l:OO ,ape i:~l1 rcs arrangedsymmetdca.lly ill a region of constant ambient potent ial U \ I _ Although the fieldcHstribution in. this lens is :Qlllpletcly symmetric about the central plana withi , l . sadd le point of tbe potential in heeenter ,the lens is alwavs either fo cu ss i ng

Qfl {-Cu ' :dn:w: . .e ru :~ga 'Ue p o't n 't i: al s a ct t h. e cente r ap ertu re, acts ,tlS an electrenmirror, Wh.el l the pooont,i,al o f tlw middle electrodels 1000 ! l e l ; t.han that of ' L I : 1 t ltwo outer electrodes, UU~speed of the electron decreases as it approaches thesaddle point ofthe potentlal. The electron remainslonger in this spatiall'an,geand tho eentral region of 'the poten t ia l riistriblltioll has a more significanteffect on the movement than do 'the an tel' parts. The central part of thepotential , however, has a focussing ef fect. , as one can See qualit~t:i.v'dy bycompan so n w it h F ig . n .3 .. "L !b .

Oil the. other hand, when the inner eleetred ]3 positive with respect tothe outer e lec trodes, the electrcn velocity is lower in 'Lhe uuter re .gio ;us of thelens, the rliEc.lin~tion '00 the central a,,13 is donlinl;l .l1t, and 'Lilus;,in 'this castoo, the lens has a focussing action.

For calculating the fo c a l length. J of an elcetrcstaslc lens we usethe optkaJaualog (Fig. 11.


19/286

b) oJ

e q u i p o r e l l o o 1 surfl :tcl !wlth ru-vahrer o di us . r 0 0 00 x-I'.! ld~,Fig. U.4I,. Compa.rilIDll or the actio:Lto an electooll lens rmmed byseve1'!ll diff~t-ellt c~ equipotential ~u:r(:,u~E.':S() w!~h. the optiC!lJ.l. anr.lltlg,11 single optic1il Ieus(ref l"S!,Qt, ' ive: i :ndex til ell :d:~ooded in a med~um w itb re f:rM U ve im .lcx 11;0) (n)j a.nd a mul-til9jlCl. lens Q~JI :: ; :i s t 1J ' 1gd f dmetellUy CHr'ved In..ye:rtI with dill'm'eut :~fhM:hh:plaumiou, of the fa.-CUSSitlgactl,QJl ..AneloctrOil ,e.IlWl'ioga "lo:ns" 501:ct'loidadopt.


20/286

2 4U lldflf ~l'tain t o ndi ti ! ;) ! 1l l s u eh a. oyHndrkru Ilnalyzcl' h a s < l J dd i t: i' o ll m l fucuS!S':h~gproperties, whi . ch ! : ;ous : ide r ab ly tm]:la:lIce its t ranami tL$fuce~t ld fI.,t-e thus adVOi!I[ t~~n$ f;o:I:inv08tigatioIlS with low.bea.m intem ;itie!l i . Tb l s cml be~1l b yconsidering M electron path i n ci il ll iJ o fL t .11srn~na!lg ie { ~te the central path atthe entranee {F.ig. IL6}. This path lS described by the dynamic eqUB.,tiOH.

d . ' Jr ~2m-._ = m-c e =1lWW2 - e e .< I t ' . ' ! ~ ' (11.14)whert'! w ~s 'thell(lgul~u' ve]od.ty around th!C center C. Fo l s .mal l deviaeionsfrom the centml path 1'(1 one h'1S

(lL15a)F:lg..U.i6. :Eb~ctf(lSti[ti,c:y~iml~lscctot' ent:xgy e,glectil\g &mall

quaJltitit=5wc havcPractical f:arl l ls ofthe magneticlerrs arc a ome t ime constructed ~y means ofshott i.l'otl.:"shielded :601enoids\y-jthoonlpoot, concentrated Held. c i i s t r iouLlOt lSin tI Le .iuterior ( Fi g, I I. 5 b) .,A,J~olmportantin surface p]1Y5o.It LI6 .The pass energy ],5 thus obtainoo as"'__1...2 _ 1 ~Vi): LIjj - 'jmvo- 21n(bjo.)

W?2 ~ ""'.'1,2-- -u(I"From { [1.1 'J ,- H i) on e obtains thempp,roxjnmte dyna :m i c equationd2r .t


21/286

26 27eondi t ien (11.21) an d leads to 3 sector anglefol:' fOcussing O 'J ], :18 ' (3,Q Pol'judging the performeace of such analyzers the 'E lne : rgyrnsolu tio l~ t J . . E IE , isa :QiInpol 'tImt qIlantl ty. From an appro>: irnute, more general solu tlon for tbee l ec t ro n ' ti ra j re t co r ,i e :s o n e o b t ai n stlE Xl -I - X2 -4 2 a2~= ,+-0 +p ; ,E 10 .3 '

where .'l:1 and X2 are '~hc'\oijdth and length 0;( the l'ectanglllar entrance (andexit) sli ts, and a: and {J are the maximum au g ular deviations of the elec~troll tr,aj ctories at the n t J: ,a n ce "i n . ' th e plane and normal to tbe plane ~fP ,i g . 1 1 .6 . P re s en t -d a y 1 1 1 s : t t ' t .r m ( , J l ts etnployoo'in e l t lc~l 'On. 1>Cattcdn,g eJ{1~Eli~l-ments (llREELS! Panel IX: Chup. ,4) aehieven reso]ut:~o:u~EIE 0\[ 10:3-10-4 Decrea. ' i i i : ! lg the !Slit 'w'idtb to iIJI,pl"ove tbe 1''C'801utlOr[ 15 only possiblewil;hiJ '1 certain llmlts, slnee i.t simul taneously reduces the transmit ted cur-rent. Tbis deter ioration is mainly due to space-charge e1focts. Electl'ons mov-ing 1)(Il'allel to one a.:oothCl' through the anfllyzer interact with .Rel! other v iatI :u iir C ou l.o m b rep uls io n an d th eir m utu ally in du co cl m ag ne tic H eld (d ue tothe current) FOl'high electron cicnsiit,iesthese space-charge ,~ffret5 d istOLt theelectrou trajector ies and limit the resolution. A semi-quantitative cs~i~a:Leof the elfect can be madeusing the cbt5Sicai Icrmnla for space-charge-.l lllutedeurrcntsiu radio tubes:

(II.22)

Fig. U. :r. Schematic plot of a hig'h-l 'esolution elect ron anergy I~ !:ipectl'Ome-h:!r"cousistiag of a ca:UI(ltle sl's'telll ('iihmtcutwith lens system)j a monochromator(cy.llndl:icll.l seetoes), a, similar ro1alYl';(l


22/286

2 n dlens

1stlens

Fig. n.s" Schematic plot.o :r a hemispherical ,electronenergy analyzel" consist.-: l l1g of '~wo erIt:rW'l .ce :~e[l :seswhich Focus ' ti re incom-ing eleetroas onto the' en-'trance aperture, tw 'o hemi-s,plte-Ideal electrode!:! f!l.-cili ta 't:1og eJlm:;gy ~uuuY'"sis Hilda dct.!i!ctor {e.g.se{:'Ollcia;ry-electl'{Ifl ~nul, t;ippUet ' }

e n t , r c ! _ n c e1lI[Jl!riUfe

center line (nQrmal to entrance aper ture) , Lens s~t.ems such . 1 1 $ thll.:t ShOWT.iill Fig. II-8 1U(l also used in combinatIon with this ty pe of analyzer.

A w:idely used a . n a 1 : ~ f c Z e l ' , in part icul ,( l[ for Anger Electron Spectrm;(loPY(AES) , is the so-called Cyiitldrical Min


23/286

31

F in ally , w e mcnUnH br.ie.fly theelootnlnic circuitry co mrn ou to i:h ll:!contraluuits of all these e lec trosta t ica t l f t IY~l8 . It is alway!! u~a[y to scan thepateDt 1a:l VU < l : t th e enzranca and ~l t slits, '\'\\'Mch is identical W t h e p c it e nt lmor t h e c en t nl lp a th within~be anal~zm:. Siroul ta l lool lsl .y the potent . ia l i ' : ] antha twe main el(~ct:rocles cyl inder se!:tors~ hemispheres, etc.) must remain8ymmetr.ically dlspo'Sedwith l ' e sp.ec t to t:hepo:wntial an the central pa;l:hwhen the pas..'lvolLage U~is \ ' a c t " . i o o . Thls i" achieved by means of a cireni tsuch ~$ that shown &:hel1la:ticruiy in. Fig. 11.10.Besides the types of a:ll tl\y~l deserihed 3bO\!c, retar


24/286

2. Preparation of WeU-Defined Surfa.ces,Interfaces and Thin FHn'lS

As is geuel'al1y true in pilysi,cs, in thefielcl .o f ">Ill'face and in~daoostlldiesone warl~ to i uv lj l stLga te m .ode l~ys te :m8w hiCh ate simple ihLhe s en se t ha tt.hey can be charaeterised m,a:thern.aticaUy by I ! L . fbw definite pa.ra.meters thata.re detcrrnined l l ) i I 1 11 experiments. Only fill: such systems: can one hope tofind a, thf.~neLiil1~ldE'l~l ' ipti(]n. whkb~llQ\v~ nne to pl.w.ic:t new pl'opetl;ie~.The uuderstandIng of sueh simple model .systems is a conditlon for a. clee])1"insight into more eomplox and more realistic- ones.The 1' (%1.}SUl"(1I . i 'E ! ofasolid under xtmnsphedc[Xl '~I. l re i$ far l "e , 'Q(lwd f rnIOthe ideal system dsslrableiu iaterfaee physics. A fresh" clean surface 1111101-ll1alIY\'eI':Y react! ve ~O'\?Jatdsthepartlclss, atoms au d mol~cu]es~implngingon it. ThU$ ! i 'e 't.3 ! to :m i c l ay el "S o f the sel id , ' I' ll is CQIJ .Mmina : t ion a.dla.yer, whose c.he:mical c:Q:rn~position and geometrical structure .U'C n ot well deBned, hitlil.el 's the oontrol.ledi 1 t; l so . q1 ti on o f a . S'ingle !lllld pure, ~decLed speeies, A better un.d~tandil 'lg ofadsorption 11~~ requires Um Drepa l 'a . t i .on of a. d ea n ,U l lo o nt m n il lB ; tOO S I lI :m . cebefore < 1 1 . we l l d e f i :u e d a d ." IQ ; !' :b !( teof kllown oOl.l5is~limcy and qnanti.ty i s b rQ 'l . lg h t'iluu contact with t]le surface, Sirnil!:u '\y, interf~ behveen two di:i:1'menLcrys-t al li ne m e . 1 ;. cr 1a lsCM o nly h e p zo du eo d by epitaxy in n wcl.I.~contleQncdW'a:y;.when tllil topruostnctomklayer of fhe S[lbstl 'a:1:.e nla:tel 'ial is clean and Cl 'YS-ta l logr .apJr lca l l :y ordered.

2..1 Why Is Ulb'abigh Vacuum Used?A)l:"'leanj'8m:fuces~ ene might alw ch l nk of th e eleetrede 8.ur : raoe~in M e lee -t rocile;mical cell , or of a semieond uetor surfaeeae ~n el,eva:tedtempe.l'atm:ein a J iu :."" .e ac to r w hc re VfJ . po r Phase Epi ta . "{ ,Y {VPE ) is pe r fo rmed at s t a n -ciatdpreS$ure cor~ditiu!ls, The: possi .hiUty of good ep iLm. . :y ill th~ 'laLter cases ho w s t Ju .t c on ta m in an :L " i- m a,y p l~W a m : i U O l : m Je in 'the p re . .' ;e u c e o f the ehem-i caJreact ion8 tpvol ved. Both SYll:Wn18mentloned, hO\Ve\'er j"l'e F.ai . d :ycomplexand di f f ic :1l1t 'W char~ter i~e. as every oloctrochemist 01' s(nnlcondnc t{)1' toch-nologist admits. The simpl .est in tcrfa.ce one COO l think of is that betwoon acl 'ystill iru~ ~lu:f~lJ;:eUldWlcuum, We sh~Jl se e ill C)la.p.a th!lit eW l n such au


25/286

2. Prepal'atio.lI oJ \ iVe l l -Def ined Sm'fac~, In. telJaCl!S and Thill P i lm s 35in terface someelmea poses severe pmb le J ) : l s when one is se ek ing a theore t ica leleserlptlon, But when the vacuum is suffic ien tly high, Qnecan i l h t least ne,glcct'f;heinfl ucnee of the g'd


26/286

2~. Pl'cpara. tiOll of Well-Defined Surmres. In( .erraoo:l> and '!"hin F.il.ms 37

Fig. 2..2. Scheme or a. fIIU Itrple cleav-age set-up, The wm i c o n d u cw l ' SIll:llplcshaped as II: rod with notches cut intothe upperi:iid Ca n be. shifted Jorwardsto hrlnc ,t fie tlex:t notch ullde r the elea;il-agewOO. j !

Fig. '2. ,3, . Z II :O sampLe (hexag(ma l wurtz- ita I .a tt ic c)prepared forde!l;~ alOIl1!; 'the non-polar prism :fac~

ZnC

n eW t f resh:I . } ' c leaved surfaee i! ; p rep are d b y shifting 'l;h(1 bar fo rwa rd s to bl'ingtbe next, notch under the wedge. .Ifthoro is no need to produce a. sl l . lg le -c rysta l surfaee 'l/vith distinct arlen-t .." l.L io [l;le av ed s ur fa ce s c an b e o bt ain ed m u ch m o re [< lim p .lyby c ru sh in g t hin

slabs o fth m ate r ia l w ith a magn~ticaUy operated hammer in U H V. L arg eto ta l areas oom; : i s t i ng of I lUU ' lYsinaIt parts with diJreiing o rie nt at io ns . a reU l U S o bt ain ed a nd c an b e 'L I.8 00 or n cls or pt lo n ! l < t u d i - e 5 .c ' lea \ 'ngc in lJHV lsa, 5impl~ Bud straightforward w ay to p: l 'epa l 'e a Fl'~b jeleansurface. Such surfaces are in general s tolehlometrlc ( importa . f l . t fer com-pound ma:tedalt; ) but thoY' usually conta jl ] defects such as stepl'! which expooce dg e a to m s that noo in a d : i .E fe n :m turrounding co m pared w ith th os e ill t he H at .~U'e~ . Another h'l ' lPo.rt,ant Iim i ta;tioB appl ies to deavage . .only brittle materl-. a I' Sl ik e a J k < JA ih alld es ( Na Gl, l ; ' : : : C I ~ etc,), o .x id c s ( Z n O , TiO:!, Sn021 e tc . .) a a dserriiccndnctors (Ge! Si~GaAs, ete.] can be studied luthia WIjIY; Furthermore,c le av ag e is o nly p os sib le .a lo ng c ert ain c l'y st allo gr ap hic directions w hicn arcdetermined by the geomeb'y fiud nature of the chemical bond. The nnmherof oQviLlcnt bonds being cut, or the ecmpensatkm of electric fields withinUte cleavage plane in the case nFiunic crysta ls , are de te nu ln i ng facters, Cu-bic a lka l i -ha l ide erystals c leave along the {!OO}faces, which anI nonpolar; i .e,they con tain eq ual numbers of both types of inns such that the fields betweenthe opposite charges can be oo .mpcns . a t e d within the surface. The same ~1'-g ume n t explains why c ' rysta l l : ; with ' t, h cw u r s it e s t ru c t ll .l 'e such a s Z nO cle avewf!ll along the no:n:po ,mr h > l ' i S I I l ) fares {lOW} {Fig. 2.3). G l e a ' v a ge s alang thep ola r (0 00 1) an d ( 0 001 ) s ur fa ce s a re a ls e p 0 58 Ib lc , b ut t he C le ." I.v ag equality ismuch poorer end a. high density of s teps and othcl" defects llil , .f iually foundo n th ese p ola r cle< J~'eS .ElemclltaJ SC'mic.onductoI ' ! ; like G t: an d S c i call oillybe cleaved along {lll} (P ig,. 2.411.) ,A detailed a'~mnistic ealeulatlon whichexplains the occurrence of only this cleavage pl a ne on the bssis of the s:p3chemJ[:al bond is lacking so far. For lII-V compound 8e.ITIicQl1,dutt(u ' s such


27/286

38 2. P : r e pm 'a : t .: i un o f Well-Defined !ihufacf!iil, Interfaees and 1'l1il1 Fi~ lms 3 9Table 2.1.. AngJe,,), by which the normals to the di ff ere ll t {nO} f'aCl;:!S hl f.i. .zincblonde 1.9 'l : t ice are 'tilted < l . ! J


28/286

~I(I

sample at e levatedtemperatures ch:n :ingthe ion bombardment. It 1 1 5 uf p~trt:ic-ulal' impor tance 'to I:tl'ioimi:zc the partial. rU"eS8l1l'e!) of residual gases ( :main lyCO) during tho spntte :d . i1g process since these molecules r nay become ad -sorbedand .i.mpli:Hltcdinto the lattice after ionlMticm in 'the ion gun. Once asu:rf~ has been cl IHIod in UHV hyioll bombardment and annealing, sub- S>E lCJuen tl ea ni .n g p ro ce d u re s for la ter e xp er ir ne :n ts a rc 1..Isuaily u r ue h ea .~ ie r;~'OI.nel;i.i:U~,Oldy H ash in g is suffic.i~lIt_

WhUe noble-gas .i011 spl . lUe! l : : ingw. i th subsequent ann aling is tho: mostversatile 'Ie:ming technique ,[0"[ uleta! surfaces and elemental semicenduceorsI : ik c 8 :i a nd G e, th e method l u n ; severe disadvantages \~'bcll applied to C()J;U-p osite m ate r ia ls lik e co m pou nd . semleonducters, oxid . ( :N '01: t l l l O j < ' 5 . Differingsputtering :rates [0 ,1.' thcvarl()uS compoaeuts gcncrnlJy canse a high degree ofu on -~ ~o .i .C h io lU e tr y o n t he o xi de o r t)'iijimiconductor s urf ac e. T he cempositkmof alloyscnn he strongly changed ncar. ' the s~u:,face afwr such a t reatment ,S ince AES find other analysis techniques are sCllsitivetoa,t best about 1.0-3of' a mono]ayer. the non-ii i toich:io:rnetIJ ' 0:1" t he c h.9 .n g e iFI compositien cannctusually be contl'olled,Fof compound sennecnduetors, therefore; cleavage inUHVis much preferred AS a" Pl'epal"at.ion technique. I t should be mentionedthat in the pru:ticulal' case of 8i and Go(tll) , dea:\ragE! in OHV andion born -h!(rcin :w.ot ,l!I.ucianneal ing lead 110d i f f t ' f f : e ' l l t types of atomic surface stl"l,lct ure..Cleaved Stand Ge,(lU).surfacC:8 ~x:hibit a (2 x 1 .) superstructure in LEED,whereas the annealed 8i(111) mid Ge(l11) surfaeea rochibi.L a (7 ' x 7') and(2 x 8) LmED pattern, rospect 'i ,,~ly. Good clean 8i(111)-(7 x 7) surfaces ea.nw oo b e o bta ln ed by . s imp ly Ua."i:hil1g a H i(1 U ) w afer with an epitaxl,d OVCl"-layer up to :J . 2{lWC; len bombardmen in this case of an epitao:.iaUy gI'rn.v;nSi(lll) $"urfu.ce i!; most lv not necessary.

Ber;y11ium(Be)

4 1ThbJe 2 .2 . EW)}i lfa ,t i;on source ta'o !e . Far I:lmnl1bel' of mateL'ials the In Iting points(M ..P.) are gi",ell as well as di'ffernt po slsible. raeans (inclod,ing mati lda I ) for eva .po-orntiol1 [2.2] (W: Tungste n, Ta: n.i"lta.lum,. Mo: ~k'lybde~'lImt:11 . 0 : .AhLmin .~, ,30;BeryUium oxide, Q: Q ; u . a L " t z ; , P: POlfoe la ; in , C: Cm:bo l 1 , Fe: Iron, Bn: Boron nl,t:r~de).E'!la:por.atlon lvU? Bas- Fil~material r C J lret ment Boat em- .Rcllli.U'ks and techniques;cibleAluminum 000 W \V(AI) w Bn,Weti> and a n o y s madily wlth tung&C tell, stranded wlre :supelrLorW sing-le strand. Aluminum 99.,9%-pu1"n is

n l l C \ C " . l l! ! a x y r n r g o o d o p t iJ e .a .1 f i lm s. ,SputtersveJ:"'j !i lowly.

Alumina(AhO. l ) Reoommend RF sputtering;01 ; ' r e' IK:t i ve s:putl;e.ri.og-2050 WA,n:t iulOny(Sb) 63 0 M o, A o ,T h . . C EVa:POI'a:i:.cs : re . '1d i .1yrom al l OO!11 'OOS.US@:external hea te rwi th c r. lJcib' I~.T I :i "Arsenh: :(As) )\0, Sub l ime s , l i se e:xtel 'l ' la .1 : n , e a l : e t ' " l : i witbC eruclble, ' lux;,Brt l ' i l lm(&) 717 W W

($ob-limes]

'W,Th,Mo

Re~dUy e-vaporates from le Jrec torymetfl.1s w i l l l O l J : . l t a.lIoying, Chipsmust 'be scraped c leau o r oxldes ..Sput tet'smodeL't\tely.La, Bo, 'We t s , r e fl : ac : tm ' v metals. H~tt DoW, C e r t l e i .W e ~"..ith~loc tJ: ' (1U lwmbru:d-Mo mell:t. Sputters mooeratel.},. V A l ' Ytm:::ic.

2.4 EvaporaUon and Molecular Beam. Epi.t.~ (MBE)Bi~Jl].uth(Bi) 211 W W Ta, A.o, Heat cmc;ib~ 'with IlxternalW; C heater. Vapors nrc to!! ic,MoBo,fQ[I ( :8) Cilirboll resistance heated b01'!~fIrstrips.ne of the classicel metbods to prepare 8 : l " e S b , dean surface in UHV i;se\~tpo.ri,ltlonand condensation of thin fi.hns.Polycrysta.lline metal films ofPtj

Pd, Ni , Au] CUi' AI, etc. C1):11 easily be pl;eparcd in this way . P't,Pd f ind Ni.can be evapcm;d.fl .d from a s'uitable e le ct ri c" lI y h e at ed f l. 1, fl l1 le n t~i. . 1J . an d Cn ar eusna11y snhl imated from a tungsten crucible. M.ateriAll '> with n high mel t ingte.1U[)el'ahu'e are Illost Eonvcnicntly evaporated by electron bombardment.Dependtng on the melti ns; point, and on \ - \FeU,ing pl'upe.l"l;:ic:s. of the melt, avar,ict~ o .f cvnpolti\tion m ethod s a n d devices are used. [ 2 .2 , 2 ; .3 ] .A co:mpliatiollfora number of materials is given ill. Tahle 2_2.

The usual ,~ to control the thickness of a. sub limatcd f ilm is by meansof a quarla balance lvhich is mounted close to the sample, I'll order 'w avo,idcontanrination le ve ls in the film, w hleh exceed the A E S detection lim it , L hobackground Pl"CSSU:i'e in the 'O I lV :.ystelll shou ld not he h ig her M um 1 0-9 To n 'd u:rio ,g e va po ra tio n. T his c an be a:c1. l .1evOOby su:lliderrt outgaslng p ro ce d u re si l l . ad val)


29/286

2. Prepai:atiou of W~Il-Dp.;lhu~d.Surfaces, Interfaces and Thin Films 2.4 Evapol'!l .t iQIl and M:Oloolllll l"Beam Epitaxy (MBE)T.lbLe 2.2 (continued)Th.ble 2:,2 (conthltleCl)E'II'JlI;POI,'.!ltio;n, MP Bas-111ate:dal Fc ] kl;lt F'1I.a~ B03 : L Cru- Remarks an d ' te i h : r l i ( I ,1 iU~6men;t cibleE V R ] J o r a t i 1 < I l MI;:1 Bas- Fila-material I ! }o) ket mCtt Boat G'ru- lieinarks .Au t l tc ;c lmiqu .e sclble

W Ao, Cobalt n1loy~ : readily wit.h l'Arrac~ Mol .ybdeu l lm 2622CDb.aI.L 1478 (M.b)( C o O ) Bo 00['.)' metals, t h e : I ' ' i 1 l { o LC evaporant!;ooykl n o l ; e x cr ee c i 3 1 0 % o f w e ;i gh t N icke l 1 ( 1 ) 5 W W A Q "OfOOIU@" Usc embedded heater (N i ) Sowith crucible (H type), Sputrer5well,Copper L O & 3 W W W. Ao Cu eWl I :porate;s w~1l fmll' all :refroo-(Cu) 1'3 . mt'}i 1i l J lne~ . SpuLW:rs w . c i l l ~ Ao Nkhl'onle 1360 W AnMo Co.ated Boa t - (N i /C: r )Gallium 30 I\Q, Reads ~\'iJh rn.etal~ and w in attack(Ga) Q ! crucibles above 1000aC. Heat CfU- Plil ladi.um 1 5 5 5 W W Ao,80 cible wIth eii:terllal h e a t r,A n eoa - (Pd) 1 3 0ted 'boat

1 . h ' Wlatinum 1 7 ' 1 . 4 ,Gej,~motl . ium 9 5 ' 9 W W W . 1 1 . 1 0 . Ge w e ' " W , T' a "ltd Mo, Denslfied (Pt)(Gel Th, c gfiipfl.U,e l :esistan.C'.Coou.rce r e eom-Mu n~end l f l (1 . Sputters {air. Mo,. Ao ,eL~1i i um. 2 1 1Germanium 1115 W W W Evapora,t~ similat' to SiO. U! ' ! ! l (Se) Tn h l J o ,. ,.oxide Th cevered boat or bux OOUI :Ce 'S- : m . . 'fa(CeCh) .Mo sb\in- Q

Gold w e ts , \' ir Rnd Mo. Filament lassGold J( l6S \"l W W. Ao steal(Au) .Mo should be wrapped Ughtly withfine w.ire. Sputters '.. ..ll. A! ) ;(lilted Si l ioOt l 1420 Bo,I:ma1.. (Si) CIndium 1.57 W . \ < \ ( " U 5e l ido fur boot"l>.( 1 1 1 , ) MoInd. oxide PI. Use platinum SOUr \ ' l ' C I l . . U~ Toorium 1827 W W;( lI .~th) Mo, either internal. Ct' cxt mal erucibleTh hooters.

Sputters well, Eva;prn:l 'l tion sourcemate r i a l .Electmp:ia;ted ooating Olihea\ 'Yt l :l J Jg l i lt . e n f i ta : r1191 l t I'lCit 1:.0 e l- 'eedg o a " " of the filamcnt mass. Usedi.m.pLA type boat, Sputters vm -yw~llR ea ct s w i .t hr e! fr ac 't .o :r y e ou re e, U s .! 1!20 % O r l e ss , F r a ct i. o r:m ' t. e s. S:pu lhH'Swell. dcpo~H.sas the a : U . o y ,.Eva;pora:tes. rapidl.y, AUQ ;Y s w i: Lb~rl"{leun'Y lUCtaJS. Spl,ltteI'8 wel l .AU.op '\",ith t'ef:l '!1ctcrry W l l l '- . e sand rnuS'~be tl>rapo:ratied rupidly . .Sput ters well.Wets all sotlr~and maycontaminate VtH: :uum s~tem.Trode.

Use 1)j11y small amounts wi.thc rucible souree. Use exteena lhea te r , DHficultto CYIErpOn' l ' tefre e of Sin. Uso I'lF sputwringtechoi.queUse RF (ll'fflat:h'e Sputteringtechniqlle.

Wi ngAg 'fY1l'e ~j.~htl:yO!~ filo maut,SpuUen ; wen..A o C r.r at oo B o at s.Sput{t' : !. 'I 'Swdl. Evaporationsource material, Getters O2Wets without" aJloyl:ngre:fhwtorymetals, :H:ea:t crl~cib: la exllCl'lm.lly,:Muy couLaJ.ninatc vaeunm system.Toxic_Worts without alloying 'la. Useexterna l hea tc :l ' w itb eruelble.'llixi.c.


30/286

44 2, Preparation ofWelt..DeHlled Surfaces" :ll1terfm:~ and Thin Ftlms 2.4 Evapm'! l' t. .mnand MoleculM" BeRm Epitf l; ;:y (MRE) ~15Th.ble 2.2 (rontinued)

W~t-5 Mo r~dUy~ Spnttcl:s'!'O'tll1,

as CdTc'~Pbs! etc. Furthermore, there isa stmllg technologlcelintarest in thepreparation o f [ ! :p i t~ \X: ia l fl lm$ o f t h es e I lem i o o n cl u ct .i n g materials.MBE allml.l'S it controlled growth of flhns whh sharp d.oping prof 1.t'Sand different ehemleal compoSition changing OVCT a spatial depth of sev-

eral. A.ngstlXllns. J\.Iulti1ayel structures with alternating doping [n-intrlnsie-p-intrinsic: =-i-p-I] (2.7] or . " : I , ' terna t ing hllnd~p (GaAs-GaAIAs:-'GaAs- ...) canbtl grown; a whele new fleld of semioondl l . c 'w[ ' lluJ..teI'ial '~tailoril,lgl! w~m.~.epossible by ' the development ,of MBE: [2.4-2.8].It is therelQrc useful to present some more detmls .aoout IVIBE. This tech-uique is discussed with relerence tu ill-V eompmmd semiconductors beeauseof thelr t o c h I l.Q !Og i r . :! l l . .importallce and since thy also 8e1"Ve !IS an example tO l l '

oth!; \r systems . .Figure 2 .6 showsthe scheme of}l ~YPlCflIUHV chamber wi.thu:u::i1itiesfor !\tIBE. [{nudsen-~}"pe crucibles are used as effusion cells for:theeVltpOl"a.tlo'11. 'Fo'r moot . purposes these cells tt l l 'e tubular crucibles, open at ( l I The .end and made from PYl'olyt i.cBN (bOl:Oll nitdrie) Or highpmi ty graphite. Thecrucibles ll.l"e mounted within spiral Ta beater wlndings which are t ]u~mS!:!lvesenclosed wiUl :in T.....oilradia:timl sh ie lds . A require Ii' leut for the SOUl 'C- . : ! ovenandthOli\rholc uult is flo Yel'y l cwpmdue t i on of unpuritles ill. the mo: i : cuhu '

Evapo.l'atioll MP Bas- _Fila.-material l D C ] :~l,JIUmt Boat Cru- Reirn9:rI,si liLndtechniqueacibb~Tin 232 } , I w , M .o Mo, Ao,(Sn) Ta 'fa CTltanlum 17.27 W1 W W C(Tl) To, Ttl .Tungsten 3 3 82(W)Uraainm 1132 W'(U)Vallooitl l11. 16'97 'W 'W .(V) MeZ in c 'U g '" iN (8ub- W ! Ao,(Zit) H m e : !! ) Tn P

Zi:rconi'lrlIl :2127 \ Vi v " rtZl')

Sputtco; weU. R,eEu::t ll w:ith ' \ ' \ 7 anddeposiss oontain traces,EVapOl'!ltiio:n source matedal.Spu't t!:!l ' t :rw ellForms eedded depooit.


31/286

46 2. Preparation of W!;]II-Ocf:il 'ledS u rr ac e s, . 1!w F a, !' s ami Thin Fihus 2 ..4 Ev.apol"a. t io :n a nd N lo l.o cu la .: r B e am .E p it ax y { ?> .fB E } 47beam, '1'.110'oven set-up is t .hel"cfQl'\~ urrounded by a. liquid-nitrogen cooledcryopauel onwhich shuttel's are mounted which can dose and open one or (heo~lu::lreffu.sion cell (mostly auLonuttk: : : JI l Iy eonerolled], Also the $p.acc betweensources and the sample is shiAi lded by a cooling 5hidclat Hquid ni.trogen.Often amass speetrometerIs mounted at aposition close to the sampl.c~ in,order to oontl'ol !Iud adjust the beam. fluxQS from the souu:.es. The sample( ill the p rese n t c ase a I I i~V wa ff .lr) can be hca,wd t u f .e m p er at l! I'C S o f a t I Je a. "I t700C. Hemnepi taxla l CaAs growth ~ 'Cqui~'1substrate tempeeatures betweenS OO a n d G O O C> C.n o rd er 1 1 0 c bta in a n O X:U 'e Ju ely hemageneous temperatureprof Heover 'thegrowi~ surface, the wai'er is sometimes "glued;1 by means ofl iquid In 01: Ga, to a Mo sfl.1inpleholder 1 which is electrically heated.

A vCI'y n:;e[uI technique to cennel the cr 'stallogi'aphic structure of theepitaxial surface is RHEED (.Panel VII I: Chap, 4). Because O 'f the ]ong geo-metric: distanoes between HHE:ED cl . t lCtlD:O gun, sample surface a;udplmsphn-1"08 screen tbC-L"C is noproblem in. incorporating th is technique into an MB Eehamberas an in 8 ittl met hod to centro! the crystallographic structure of the.g;J~()wirlgSUI"w..ce during the growth prt1l1essi;tself.

An excess of' Ga at' of As on n growing G i[,A~ surface is usually COQ-neeted \VitIl the appearance of pa:l 't iculru' ncn-integrel -order spot . .. be tweenthe n:gnlaI' Bl'am spots in the difl, 'I 't1ction pa;tLer'nin R.IIEED (auperstruo-tllfe! Chap. 3). Ful'tberillOliXi1 durl ng the growth :p,r-ocessitse1f, the lnLells'itjfof ~ i linglc Bl';, lgg ~[Jot on the RHEED :;c: roon can be' monitored hy an opt i-cal device (Fig. 2, 7). Within a certain range of growth eondltlons it showsesclllat lous .with a regular period { .HI - fEED O . i l C U l a H ' 0 1 U i ) {2.9]. Theinter:p:re-ta:tion of these csclllatlens i.s based on the gro\'vt h mechanism, When. a full~ ) tomi c : l aY - I "11 ;( . 'On l p l e ted dUl.in,g gl 'Owth, the :m per iodici ty of thetopmostatomic i a .,veJ 'i s ni i ll ; lJ . : lyideal and the di: ITr! l.ct i( }Jt O n t his rog~daJ ' pariodle an1l'j'o f a toms causes 8. certainrnitXllTIllln spot intensity ( Se .c t . 4 . 3 ). Further gl'ow'~hleads to irregulaely distr ibot .oo atom 01' ]:i 't tleislands on top, o :f this ~Ilmplcte

aOOmicla . -ye r b er :m e th i ne xt fu ll atomi.


32/286

2.4 Evaporation aud Ivlolocillar Beam Epitaxy (MBE) 49! ~ A / ,4/

., M.llhl'l 'iI I./J /.V/. . . . p a o lI ' ! ' Ii'l'~ A i .//1/11 1 /,I v - I ,I 1/ //l~I 1 - 1 1//'- I 1 _ "IV I 1 I II I LI r z I 1 : / III 1I 1 1 II II .II if:-1 i I f H v 1 ! Il A7 A I -# I~ IJ I l t l l l,~/ : J I~I~ I i iI -1fl If 1 1 II II II jv . 1 / I1I t t il II I II !Il

Ga.A:s3 0 0 1 1 (

Fig. 2..8. .EquiJLbdul'n VOlJ)orpr~u['e of il:trpo:rt-at!t com-pound m111kol1dueto : r sourcem%i.ter ials

10" Id S U y S o c r ' 'IIJ! n:l~DOPING C 'ONCEN tR .A ,HON ~cm-l,~ o o 6 0 0 ! l iO O IO C IO '1 50 0 2 00 0TEMPERA.TUIi l 'E (lit)

Table 2.3,. Mclt.ing lempe.n:l.tuf Til l for selected matel'iMS and crucible source(K l l ud sen l ;ype ) te!llpcr.ratm-e Ts nC~ - l 'YID ostabhshan equillbrinm v a I } O I ' p r e s --sure P{Ts ) or 10-: .1 Terr. This pressure is con" .u iefi t to achieve l 'eaO O!l9b l . vapo-r n . ti o n r a .t e s iJ L ' lMDE

metheaol and rinsed jn methanol and water. ARe'I; b aking out the :systenl. f O l "at least ,8 11at 100- 200" 'C, epitaxia l growth is possible after heatingthe sa.m,plcin th,e As ' b e . 8 . 1 l I 'to grQwth eonditlons, L . to about IlOQ-6000C. Sometfmesion bombardment and ,fJnnca:HI. ' i~ cycles am also appl:ied for cleaning before8t~rting ' th e gL " t ri 'f -l h procedure.It is interesti uS to rnenslon the diffcrenoea between GmA s layers which

haw been grow n fl'om one 5ing:le~mr-ce~1i'ith polycrY15talli.ne GaA s , and t . Iwseg l lO ' \YH usi n g s e pa ra te sources for Ga and As. III the :lattCl:ca;seAs e:vapo(l"atelo!as AJ'i4,


33/286

50 51G,row'hc hambe r ,Analys isch iombe,r load lockci1;Qrnber

1 1 1 .~u

Sl '1uU@rIi!'!trot

(I~ 0.2 1),( 1l.~ MI, ~ \oGaA;~ GaPMO L E: F R ACnO N G il P IiiFlg. 2. U. (a.) Olmpositional dqpcf.lden~ of the directand illdh'llct energy bllndgapr u t ' GaA~l_;:;:P ~ Oi t 3O[).K, alia {b) ~hel:llatk ene:t'gy~\lel:5u3 W;f i . \ 'e .voclO l: depaJldeJlc~E( k)o:f 'valence and conduct lou baml ,for arion...alloy oo .mpo61t lo rJI .> . :l,:: 1,2.13]

F'ig., 2.101, Schc:matic~o.p~!lew orer a C O I L 1 b i I l ! \ t i> t m of g l . ' o w t . h [THV ch~mD~r for.MHB (01' MOMBR ) w i t. h a n !l .l ys is a: ll .d I O 't l. d~ ], o( )k chamber. AlllIllV unii . 's l),11fl$.Cip-Rra.to.ly pmnped ( i I J I I l differ!lnt cl'utibles. In oonl l '1)St to other me thod s of ep i .La l l: ,Y (H.qui .d-pha se or oon~tLon81,nornull-pre~l1n! vapor-phase a p it u ." lY ) .MBE tdlows afast change {wit.hin fractions of a . second] of the co:mpQ$i.tion. For grrn.vtbrates of !J.ro/h (i.~" .A/s) one can switch (!Vel' [rOln one componentLo au-o t he .r r lu d u ,( ! ; t h e gl 'U\wth t im e foJ ' a singlclllonolat;,)rer. Thu s I ), 't ;om i c ru l y~hat'pgmwth of pJ.'ofilescM be genel 'uJte,d by .MBE. This fhdHta:tes the fabricationof ,cxh'emely well-defiaed interfaces beL1ll 'een two semioonduc.tors (Chap. 8).

As onewould expect fOJ . ' simpl-e geometric ~'e!;lS(m:;;jhro materials car! growe:pitaxially on each o thm: wifh a high-quality interface, and pllld;icula.f ly ast,b:icker films, jf the crystallogm;phIcmi.s;mntchi:slow. The dia:g;ra:m(Fig. 2,.12)of thel~ttice ooilsta:nt...of rr:t!ijor elemental and compouad semleeudnctorahdpsin cil00sing a,ppropda~e nl.ate:riaLso r certain .~pplicatlolll'l. Because ofVeT;.'s:imU,


34/286

5 2 2. P:rep.ara.tKl inof"VeU-Dcl in~d Surfaces, Inwrfa=; and Thi.n F'i lms 533 .0 r--""-----------------,

ZIiSe direct } __~ im:lifil!ct gap n s f ": :1.

grown cobalt silicides on Si have been used to faoricato a fast metal-basetJ.an:;isw, [2.151, Sillet: GaAs and Fe diff~r in theu'l&t'ice eonstant by a factot'of abou t two, it is possi blo to gr-ow Cl"ji 'St ;'! III iueFe fllmscn both GaA~( 100)and on GaAs( 110) at < 'k substrate temperature of 1800C r2.16] , .Even thoughthe Fe/GaA:s i:nte.lofare :isnot ~h.aI'P and 'wen defined OOCIJ .1l8C of cons:id~J:fiblciuterdiffusion and lnterfaea reaction. the iron films exhib! t cxy~tal.l,ugr!l,phk:structurescon ill, electron diffrac t ion (tEEm, Rll :E:ED, Panel VUI: -Chap"4).

2.5 E:pitaxy by ' Means of Chemical Reacti.o:ns

o 5 . . , 5.6 5.,8 6.0 6.2L A TT IC E C O NS tA N T (1)

6 . 4 & .6Under standard pressure eondltions, epitax,y of or-v c.o.I11ponnd semicoa-dneters bYlneuls of chemical reactionf:! ki well establtshed es all im.pol'tantmethod tn produce ep:i layen:; fO T se:miconduc:oo'l' device technology. In a cold-w, aU flU)i: reae to r G aA s can be grown on a O aA s 5ubstrnoo n t 5 (l (H 1 0 0c C fromgas !It.l''~i!ll'(gS ofarsine (AsH;!) and '&riIuethyl ,gnll iu:m [TMGn =G!i(Cll3 } 3 1:us:ingH2, N il etc" as a carrier gM. T:his technique belOll~~ 00 a more genera lc la. ' i. i l o r plTIC~eS1 which are cal led M' e' la l -O r 9 a n~c Che -t n ic .t d V a v o r Deposinon(MOCVD). This technique of growing GaAs from arsine and TMGa. 01 " TEGa.[Ga(C21f::;M canalso be a .ppU.ed under UHVconditions [2.17]. The method;'"h,i.chiS T o f c o ur se ~i lso l i ,pplic~lbl-= to m an y o ther I lI -V system s ( G aJ 'n A S j l uP ,ete.], th'n combinesthe advantages of a DRV technique like M.BEwith thoseof 3 continuously running preparation method; usefu l ill applied device tech-noLoro'. In the prcsc.nt context it i s t -e .levant th .n t th is method, which I:; canedMetal~Oryanic ,MlJB ( T h . . : I O i V l B E ) . or CbelJl'ic.'l1Beam E p i . t a x y (CBE), can beused in si tutu prepare clean, welI-dd ned eurfaees of ccmpeund semleon-ductm1i 1 :2 .l8, 2.19]. The epitaxiallayers can be transferred from U:1C gtO'i\rthUHV eham her iuto aseparate chamber, where 8:llrfa.ce ()\r interface studiesca rl be p erfo rm ed u nd er U H V O O l)d itiO i;I S, F ig ure 2 .]~0 8 sh o,,"" 'S ;e :hlrnl !1l , t ic~lyItDHV growth chamber Iu which GaAs can he grown e)litaxia]]y by U'lea!lSof 'MOiYIBE. As is the case ill. MBEj the sample and the sources of AsH3and TMGa or '!',EGa, ar surrounded by a hquid nitrogen cooled cry~pim.elto miuimi7ie the OO1ltanlin,.'ltionlcvcl durlng g l 'Ow th . The monnting for theGaAs waFer used as substrate are the sl;iJne asin MB:Ethe wafe!" might beglued by Gam' In t:oa Mo support which isheateel during growtb to tam-peratures between 500 and 6001)C. In mere l ' e


35/286

c

ropiU"7''Y~whel'e the hydt':idcs A."EI~ .P,HJI etc. * 1 : J : e thc~m!JJlyd~cOJ:l l \ l )osed byuleans of a . hC ! ) J , . t ( .' c l mct a l f ihu : nen t (e-g., T ' ; l I ; or W at ~ 1 0 0 0 K) { F i g . 2.13b)-'l'yvieall~' the AsHlI ,.gas is il:l;ject~lintothe UFIV system via, a eontrellablcleak valve~ '\.,hld1 ~Uowsa reproducl Me adj ustmcnt of the beam pressure, andthus of-U18 flux towi~hin a:bout 0.'2%. Duriug it:. p1:l.,'j~!!geUnvngh the qmu't~capilla;I'Y alullg t he hC 'n tc d flt au um t, th( ~! 1;li." beam changes from lam:in!lLJ, '(hy-drodynf lm ic ) f low ( 10 -3 0 0 . Pa .) to molectd~rAmv c en ditlo us (~ ur" Fa). In!;uch H. set-up, Ill,} to 90% decompo.'l: i!:.ianof AsH3 is achieved,

A difl'eL'rn1t Pj'illcipl(] o


36/286

56 57reo

4 0 2 0.

p~S aA s (~O O },

F. ig . 2.15.Room~. tC.m:penl .. ture.Hun I n . ' .! l l . bJ:Htie5 ~ao. o, of .MOMA&-grO '" ' ; l l , i.:Il.t.Il!II~ir.IIl.;f:llIY..p-doped (c'aJ 'bo:n) GaAs la-yet'S [w:ienta,tl0IU ( W fJ }J b y th~ usc of TMGa o:n.l:y{6.)jTIl.:Ca ooly {O), ~UI.d ll1ixtlU'E!'SoJ h~th f1.1kyls( .) ., . Solid Hne: boe8Lmobi~il: .ie$ fl'Oml lt ef al tu re i 2- 22 l

10 ss

The u-t;ypc doping of HIV compound la,.yeJ.'t;;:an also be performoo IJ;Ymeans of gas nne sources. Using SiB,1 (5% in H ;! >()ID"l'iel'g~J fur S i dopi.ng Q fGaA5~electron ooncenlr!l:ticlIll> between IOl!:. and 1019 'Cln-~ can be aehievedat room t~mperat!ll'e. For doping level !; above 10 16 Cl1l,-ll 'Iring the den-si ty (If Lh,e~_utf~lcedefects that incvj ;tably occur during ti legrm'1l th pr~>the MOMBE precess .i~su . pe r iDr to .IVIB.E.La rgc r jBo"ca l i ed wal dejects 113\' tng typical diameters of about 101401 ar~ a~el'lt an MO""IBE-grown sfll'facetii[ 2 . : : ; ! 3 j ,

F ro m th e e xp er.ie :n oo with UI--V c p it a1 ty f ro m gasooU:ii~ : inp . : jI , t t : i culi l l ' .MO8QlU'{!:{; ." 'Sj itig infel 'l :ed that a numbee OIFother semicenductor and metal over-layffi'S might also be deporutedin a UHV systcrn by means of gaseous: Wtll~lJ]aLcda1~. Beside H1etal,-orgaui.cg such as TMG~l TEG.a,~ TE..f.f1jcte., cfI.l 'bonylssuch MNi(CO)a,. Fe(OO)~~etc. , m:ight a lso beiIlmrest ing [2.24].Another interesting m"po c t cm.c:r:g~'\vhen o l 1 . e 'C .Q 'm.paresMDEaudMOiVlBE, A'SOOlllpaJ. 'ed. with MB~ the surf1CC chemical reaetions leading tofilm gn:;rwthil1. ,l\;I.lOMBE are far mOl ' e complex (disoociation of 'TEGa, etc.),This l lU .m'VSa d .e ll n o d , c c nt ro l o r th e l'eaction:;l and U tuS of : thu growthprocooshy ~tf)!tualpararw;lters such 3 . \ < > light irradkl~ionor eleetron bornoordluEl!J:'J:L. By'scanning a focused light beam ofapP'xopriate photon en.el 'gf over the g' l'ow.illg surface, grow'Lh cau be latenJly~nha!100d mld suppressedIn othe'l" areas

F.ig . 2.14 .. l'e


37/286

58 2 . P~i'n:tjO:Il ( 1 ; f W e ll- De li n e d. 8lU'fao~,. I l : I t ie r [ a ,OO 'afind Th i . nF, ihm;which MO' l:lot hit by the light beam. 'l~\~d:inl!;i~siona! structu res can thus bep .I :o d uc ed . d u ri ng t .h cg r1 .1 \, vt h p ro c es s i t.s F ;l lf :,T ec h.n iq ue s ~ U dl as MEE and M OM BE a l o e bes,tapplicd t o ~ u rf ;: lc .e re-searehas in situ m.ethods,i.c. the fuss,hly , g l 'QW nsurface lSiLI , ' a J : l . ! Si le :n :OO unde rOlIV eondi tions from the growth chamber into 510n proc.esl :: the f i n .al s [ ; :.a teof the atom now hascnemore hole and ls, thusmore highlyiHnb~.


38/286

P an el I II : A u ge r E I .e ct l.' O Jl . Sp 11 !C 'Lm r.o py (A ES ) 6 1

M~r ; = F = . . . . .t ~ ~ 1 '" "K - - - . . 4 - . - .

To illustrate 'the 0:1ulation ofthe chal'acteristicnel"gy of an A[I~r iran-8ition~ ~ljeeonaider as an example the I(LtL2 process o r , F ig . In.la,. 111.t !);m-ple, one-electron pic ture the kinetic enc.,rgyof the outgoil!g Al, lge l ' elrec t ;nl l1 ,would be gi ren by a differen~e between the ool.creopollding , cOl~l~eJ energies::EMi) =E ,{ - ELl - E:L~. These energies can be obta.in.ed fWITI X-['ay pl io -toemisslou ISpec:Lroscop:y ('xP8, Sect. 6,3)- By US-lug expe.rimenbll XPS (Ltta"o n e a k eM I .: li ' takes i n to a c (; u u: rr t ul fI ,] :1 ) ,- e l( lc t to n r e la x at io n e f f: e c' ts , (Sect. 6.3),HOWCVG fLhc Augee precess d m e n ; from. p ncto em iss io u by tile fOI ' :ma t i iOl l ofan add itio na l core helc . A Iurthee COI'l"e


39/286

02 63

S W 9 ; ! 1 pS i o P P t yData(l (Q!Jisi '~ion

E l e C ' U ' Q n 1. ( J e 't e : c t 1 Y '

:Ei ': ig.ill.a. Schenurt lc plot O 'f a gtandlU'd e : x: p edm f lf lt ! d s et - up f or AU l l; _e t: ' l e ct ro nSpec t t '~O r ' } I ' - (A:E;S), The J?l". imaryelectron beam i s g e , n e: n l .t c d by a n I i! l. ec L TO tigUlwh:icl1i~integrated 'ou the central 3%i~ oJ n Cy l inddc al : 1 I! \ I: ir r ol .mil~'sel; (C~irA).AIl(I(b:Iitio:nal sputter l .Q\l1 gun provides tho pCll>llibjHty U if depth ; l I ; l ta lys isFi,g.IU.2: , Ptl f1c:ipal . Augf!r Bl.cctoolJ ' l enel'gi~ as ~. fu;ndion Q,r'the ato:mk: ,muu-h er Z _ T hl'l 8U'01Igesttrans1.t.iolllil (If each element ~. Indicated by bold pOIU'tg P U 2 1 icalor Cyl.incir.kalM.incOl' Amtlysl:!l"s"CMA), The electron gun is scmetimesin l : .egl :n tod hito the CMA on i15 central axia (Fig ' . .m.3). Th:is is particu-lar.ly useful 01'depth profiling, where AES is eomblaed with ion sputterillg.Because o r Urn s m a n A!.tger signals AES is usually care ied (lout in the d e l : i W , i . . .t ive mode to SUpPl'e$S. ' the hi,l:gebackground. of true secondary electron."l . Thecliffe-rentiat.ion is performed by !;upednlposi ng a small wten- .a : t ing voltage1J = t'(I S i l H ! . > ' t on the oute l' cyl :indei VQlt~" I@"! if and. synchronously detectingthe in-phase sigual from 'the electron nmll i ipl ie' l ' with a leek-In amplific,r. Inthia mede the detectm ' curranb

leV .. ') .1 dl . t ++ ~'(l8mwt '~ .()+ -1;0 iil1n w~- .. " .d,1}

and tJlC:l electron taking upthe corresponding energy to be emitted fromthe atom 38 an Auger dcc't;I'Oll. This energy transferli, 'om one 'electron tothe other is facilitated ttl,ail.lll' by t h e CO l t lomb i u lm ' , a c ti o n . AIlgeI'~ra:nsith:mprobabilitlcs canthus be apP I "OK ima i ; e l y calculated using COU1 0 l l l b interae-tion potentials of the type tl/irl - ' 1 ' : 2 1 1 ' [ . 'two ~lecb'onsat '7'1 and 1'",. Thetl'finsitioll pro babili ty fur a l{LL process (Fig , [ll.la) is thus obtained as

. ';i! I ) I ~f l < t : . L 0. : I ( . . .ls(tl.}ei~N'~ I I e I ~(rl } ' 2P(" '2) . ". . ' ' r : 1 -'r",;!. (111.3') (1U.4)T.h.e two-e lec tron initial state ~~(l'l ) , Z p ( r 2 ) is described b: : ' I 'he two single -electron wave funetlonszs and 2p, The final5taLe'~I~(1"1) e:\'' 'P(i1;vri!l oollt.a.i:nselectron 1 in its, Is state and electron 2 escapes as a free e1ect;L"ullwioth thewave vector .k (p' lan.ewnve sta te). More complex maliy-cledl 'Ol'l lI'eels ar1;'lnet contained in this l l iU1P Ie description: A detailed caleelaticn Yields~ aaits mHln result, that the A : u g C l ' ~ t : .: l ' ; :1 . l 1 s ; i t io n probability is ,'cltlghly iudependentof' Z,in COl'ltl'm;t to the strong Z dependence of radiative t.['{l;[lSitious.Fur-the!'.lD.om ,Augel ' lJ:oocooses do no t obey the dipole select ion l 'u :les~bat governoptica l ' t :nl . l lsi t io:ns. Th e trll.llsitioIl P1'O ha bH i ty 15 d e w . . m i n e d e s :: re n ti a I ll ' by'I;.heCoulomb int--l 'a. .ctiou and not by a dipole matrix el emen t . For~x'aJnpl,c,t ha p lo rn i n.eot KL1LJ A tt g l." U' j j \118 , ibon is forbidden optically. s ince i : t , doesnot $atis:fy III =l and llj = = 1, O. _Til standard equipment fUI" AES cono; ! ; ists of an e.lectl'o:.n gun, which pro-duces the primary electron beam with a typical cnrgy of 2000 to 5000 nV.The most eonnnnnly used cnar,gy analysers f(: lr A'U~ c l e c t l ' O l lS ".1'0 hem,l~pher-

contains t.l le first derivatlve dlJdv as the: prefnctor of the pl.lase~eniit i\r'elydeliec.ted AC signal with 'the a:ngldar frequency W.

On the husi~ of this a.ctretiOl~ mode. Auger lin energies are usually givenillreference 'wm:1was th position of the minimum of the deriva t : ive spee-trum d N /d E ( _P 'i g. 1 IL 4,). This eue.[gy~ of course, does not coinc ide" wi. th th otuaximum of the Ituger peak: in th.e non-dl ffcecrulatcd spectrum, As an exatu-p le of t h e a p p l ic l iJ . .t io n of A E S, .F ig . lIT.5 ~how:s di.liel,'(.I(It:ia,ood dN/ dE spec trameasured fu,r a : l: Io o il y clean Ga-As surface (b) and for th e same surface COli"1-e


40/286

EF ,i ;g . 1 l: U .4 ,. Q(Ialj.t9ti.V!'1 oom,par:isol) < ' l ' fQ. n(l i1"d:ifJe,tenLiatOO A uger spoctl~umN(E}wUh 'it]S di:!I!~rent;i.ated ~':Ou:n~rl)altdN{E)'jdE (1ml!el" plot). The AFki peakwith ;:I I ll ,aximUIJI ~ .Eo:! l;~lleLat 'Cls n~'I'e30u:ance~.:nke stl'w::tul'eill d.N/dE,whose rn,ostl lO,ga,tIve eX.cu:rS:LOin,at ,8.".co.r, re: : ip'DJ1d~to tll.e' st wp oo L sl op e o r N(E)L E C TR O N E N E R G Y E

o ' 1 0 0 . 2 0 0 3 0 0 4 0 0 S O D 1 ' 0 0 1 2 0 0 1 3 0 0E LE C T RO N E N ER G Y le V '1

:Fig .e lil.5a!b. Dif( 'Ct:l . '11tl t . tOOAugtlI e:lootlUi\ spentrtl. dN{B)/dE measured wl~hl :I pr im m ;y e le ct ro n e :1 1.e rg y or 2000 eV O i l l ll ,GnAs(1 00) surface prepared by Metal-Orgat1.~ M{llecnlar BE:t:1Y I ) l 1 m : . e s s tlres u rf Il :~ \v f. L S c O " -' Ct l" C dlnthc MOkilBE sys tem by' . 1 . 1 1 amorphous arsen lcH lm andtransk'TI'e;:I thmu,gh . air j nto ~he Il :naJys is ebamber f.or 'he AlES anah"Sis . This f:iJ)ec-tt'um CQ,[l'


41/286

>-PanelIV-----~~--~-----Secondary Ion. Mass Spectroscopy (SIMS)

In Se t o IH : lO 'U 'Y 1011M~""; :g[H~t:trost:Opy ( S rr v IS ) , [ IV . I, IV .2] n p l' im~ y ie n beamCQlls is t ing. ~.g., of AI.'+io[ls with a t y p i . C E l I . ene rgy bet.\JroCll. 1 . and 10 ke V ),5inc: idenL o n , a f il U :.f ac c . Duck! the tran~fen'ecl iml:t.o'lCt e l .l e :t ; g y n e u tftll.atQ015,m elec ules an d jons - so-cal led secondary ions - a.re ~m,itted. : f : rom tlresnr-face. they are flnaly~l !!;ndd .e tecood by a. mass: spect rometer. The measuredIM&S 8;poctnun then yields infol 'Il1,artion about tlle chemicaleompositjen or the:;,\ulCfacc.This t yp e o f 5 tf l, t; ic SIMS isuseel .in surface phYl'J ic5 to s tudy the com: -p O& it ion of' the t o: pm . o st a to m i c l a . v r n : , in d u diI lg ; t he na~l!~(l.ndp:rnpertiHs ufadsorbed laye~'$"Ina l't"CQudtype, the uyurunic SIMS: higher primary-beameurrenrs ar c used, 'T h us, a much bighc~:rI'ate of emlss ion of . ' IDO!Jnc im'J f ions re-sults, a:lld the spu~tc:l'iug proeess fe111QVeS 0!Y11siden1hl{~ ti,anti;ti.~, afl:natedal ..Dm:jrlg ch i . . ]JJ'(l(le$$ one mo.ni tors. the secondary-ion 1nRS spectra~ whi('.hyieldinfol 'nta : t ion al'mut the chemical dements contained in the removed matedal.This kiu ( l o J l'tIe


42/286

68 2. Prepru:nlkm of Wen-De l ine

F:ig. IV.2. Typic:.a1. ~ r ft . iJ : s l1 l is s io u rM io o f a quadrupole mass filter as & rl;mctioll ofmass number (nnm) inthe .range 4c-Z50. lmpmtWJt arsenle mass ~. are i.ndic.atedby 91'm~\I'I! 1 V ,~lJ .

. T a r ge t A k tr n rs

Fig. IV .3. &he!Uati.cl'epi'ese:jit~tign of ' the ~pltU:,edng preeess. The cascade of~i:ngle : p~II ' t i cl e co l l is ions iuvoL'o'CSthe impact or Ei , pr : im. ru :y ion. fUl~ l I . Io : l : : i on .f defects,lmpl!;l.ntation of ions, and l : emoV'" .: t Jo f a snl'f!U:]~rom (substrate. 0[ adsorbate) all 1: \neutral or u 5CConcl~ry ion .

SIMS, t h e i o ni z: a ti o l1 chamher in f i :o l1 t of the quad,mpole rodscauees padi:'ll'CT


43/286

70 r . o 0~ ... , ~ .

blI"Fig. IV.43o-c. Three- roglmes of :;pU'ttel"ingby eJosti r .oll i :sions. (a) The singleknock-en t 'ep;ime; rceoil atom~ from ion-'Larget ooJ l ig iom!! reeelve sufficie:ntly highCl1C1'!lV to be sputtererl (b) The 11fle!'L['r-=c.ij,dc :regime; recoil a:tO115 iI'om lou-targetco11il ;io:l ' :I~"eOOt",esl l f f ic1e:ndy h ig h e no rg y tog ' e lU : " I ,' a te l .' c ooH c a sc a d es . ( c ) Th e i:ip:ike:regirl~e;the (Jensity of :recoil.atems is so high. '~h8t the J]l,llJOIl'ity , o ; f < team: ; withi n ,b \~Grtu:ill velumeaeo 'in motion

as.neutrals or as secondary ions w hichalJ'e detected in tho QMS. Removal ofa surface atom ln, thlilsputtering process requires tbllt~the elastically trans-fc-rrcd energyeA-oeeds the ' b inding tlllClm'. Con'espondingly, tli:l'eeregim(.'S ofsputtcl':ing by clastic ool l : i siOlu!can be dis.Lingulshed (Fig, IVA). In the stngle-knm:k-uu Tf:.gi.flW~atoms recoillng froJtl the ion-ta['t~t oollisiol11'eceive suffide:nt' ene l"gj ' to be sp u ttered out of the s.a.mple, bu t nut enough t o ,g e1 1r at e r ec oilcascades (Fl.g. :rVA~1).Th:IS regime applles mainly to pdmal"Y ion cuergtesbelow 1 keV . III t :he , l inoo:r-c~UiOOdc ' f 'e} ime ( :p i r inuu 'y ,am:l:lY 1 keV-l MeV )thcrccoll atoms theJIu~elVlBS C!U'l;y ,rtougbc 'nm1lJ" to p ro du ce { in ot he r recoils.A cascadeis gcnaratoo ~but the density of recoil atoms islo~..euol.l:gh thatknock-on COJ l iSiOHS domiuate all d wUlsions between moving a:toms are Infra-quent (Fig. IVAh). For heavy primary l(liilrS with high energy th . so-calledspik.e re9 i t} te is ; attained, wbe' re the dCl1siti,Y of l )I;c011 llItom:;, is so l1igh that'within a certain ,'oltlUle(5pik~,'olnme), tl:li~rnajor.ity.of atoms am inmot.ion.,Thc bssieparameter fora qnantik'lUvedescription of the ~puttering :pro-cessis the sputter y:idd 'y (number of sputtered surface particles per lneid ntparticle). Wit.h a. pl'ilu;aIY ion current density of jPI =CV (

luth_solid surfaces interfaces&thin films

Documents