Endotaxial growth mechanisms of Sn Quantum Dots in Si matrix

Transcription

1 Mt. Res. Soc. Syp. Proc. Vol Mterils Reserch Society I1.7.1 Endotxil growth echniss of Sn Quntu Dots in Si trix P. Möck 1,Y.Lei 2,T.Topuri 2,N.D.Browning 3,2,R.Rgn 4*,K.S.Min 4**, nd H.A. Atwter 4 1 Deprtent of Physics, Portlnd Stte University, P.O. Box 751, Portlnd, OR , poeck@pdx.edu 2 Deprtent of Physics, University of Illinois t Chicgo, 845 W. Tylor Street, Chicgo, IL Deprtent of Cheicl Engineering nd Mterils Science, University of Cliforni t Dvis, One Shields Avenue, Dvis, CA 95616; nd Ntionl Center for Electron Microscopy, MS , Lwrence Berkeley Ntionl Lortory, Berkeley, CA Thos J. Wtson Lortory of Applied Physics, Cliforni Institute of Technology, MS , Psden, CA * now t: Hewlett-Pckrd Lortories M/S 1123, 1501 Pge Mill Rd., Plo Alto, CA 94304, ** now t: Intel Corportion, Cliforni Technology nd Mnufcturing, MS RNB-2-35, 2200 Mission College Blvd., Snt Clr, CA ABSTRACT Two distinct echniss for the endotxil growth of quntu dots in the Sn/Si syste were oserved y ens of nlyticl trnsission electron icrocopy. Both echniss operte siultneously during teperture nd growth rte odulted oleculr e epitxy coined with ex situ therl tretents. One of the echniss involves the cretion of voids in Si, which re susequently filled y Sn, resulting in quntu dots tht consist of pure α-sn. The other echnis involves phse seprtion nd leds to sustitutionl solid solution quntu dots with higher Sn content thn the predecessor quntu well structures possess. In oth cses, the resultnt quntu dots possess the diond structure nd the shpe of tetrkidechedron. (Sn,Si) precipittes tht re severl ties lrger thn the typicl (Sn,Si) quntu dot possess n essentilly octhedrl shpe. INTRODUCTION Self-sseled seiconductor quntu dots (QDs) re expected to led to prdig chnges in seiconductor physics [1]. For seiconductor opto-electronic devices, the QD size ust e of the order of gnitude of the exciton Bohr rdius. The energy nd gp of the QDs ust e sller thn tht of the surrounding seiconductor trix. No structurl defects such s disloctions, which led to non-rditive recointion, re llowed to exist in the QDs [2]. As α-sn is direct, 0.08 ev, nd gp seiconductor nd sustitutionl Sn x Si 1-x solution re predicted to possess direct nd gps for 0.9 < x < 1 [3], QDs in Si trix consisting of pure α-sn or sustitutionl (Sn,Si) solutions with sufficiently high Sn content hve potentil pplictions s direct nd-gp teril for chep nd effective optoelectronic nd thero-photovoltic devices. There re, however, t roo teperture 41.8 % ulk unit cell volue istch etween α-sn nd Si nd n equiliriu solid soluility of Sn in Si of only 0.12 %. This restricts the growth of pseudoorph Sn x Si 1-x lyers on Si y oleculr e epitxy (MBE) [4-7] to Sn content of out 10 % nd thickness of the order of gnitude 10 n. At growth tepertures in the rnge 220 to 295 ºC, pseudoorph Sn x Si 1-x lyers with up to 5 % Sn content hve een grown with fil thicknesses up to 170 n. Therl tretents of these lyers t tepertures ove 500 ºC for 1 hour led to the fortion of α-sn precipittes, β-sn precipittes, precipittes tht consisted of oth α-sn nd β- Sn, nd isfit disloctions [5-7]. While these α-sn precipittes y e considered to constitute QDs in this teril syste (ccording to the requireents given ove [2]), the siultneously present isfit disloctions re clerly undesirle for device pplictions. Alterntively, teperture nd growth rte odulted MBE [8,9] produces Sn x Si 1-x /Si superlttices with essentilly pseudoorph Sn x Si 1-x sustitutionl solutions hving Sn copositions in the rnge of x = 0.02 to 0.05 nd fil thicknesses rnging fro 1 to 2 n. Typicl growth procedures [8,9] re s follows. The growth tepertures of the Sn x Si 1-x lyers rnge fro 140 to 170 ºC nd the growth rte is 0.02 n per second. To suppress segregtion of Sn to the surfce during growth, the Sn x Si 1-x lyers re overgrown with 4 to 6 n of Si t the Sn x Si 1-x growth teperture nd t growth rtes rnging fro 0.01 to 0.03 n per second. The teperture is then rised to 550 ºC nd Si cpping lyer with thickness of the order of gnitude 100 n is grown t rte of 0.05 n per second. By the tie this growth sequence hs een copleted, the Sn x Si 1-x lyer hs experienced n in situ therl tretent t 550 ºC for tie of the order of gnitude 30 inutes. For the growth of Sn x Si 1-x /Si ultilyer structures, the whole growth sequence is repeted severl ties, effectively resulting in n

2 I1.7.2 in situ therl tretent for the first Sn x Si 1-x lyer t 550 ºC for tie on the order of gnitude few hours. In ddition to this in situ therl tretent, ex situ nnels t tepertures etween 550 nd 900 ºC re perfored for 30 inutes. The surily effect of the epitxil growth nd these therl tretents is the crystllogrphiclly oriented precipittion of Sn within the Si trix. Precipittion process such s this re rodly descried s endotxy (fro the Greek, literlly ening rrngeent within). There is n unexplined peculirity in the plots of verge QD precipitte sizes versus ex-situ nneling ties, e.g. Fig. 1, susequent to teperture nd growth rte odulted MBE [9]. Eploying nlyticl trnsission electron icroscopy in oth the prllel illuintion nd scnning proe ode, we will ddress this issue in the in prt of this pper. A inor issue to e ddressed t the end of this pper is the shpe of lrger (Sn,Si) precipittes. Therodynics driven structurl trnsfortions y occur in oth α-sn nd Sn y Si 1-y (y > x) QDs. As one cn esily estite, there is n essentilly hydrosttic pressure of the order of gnitude 10 GP on these entities with corresponding excess Gis free energy tht significntly exceeds the therl energy of the tos in the QDs [10]. This excess Gis free energy y e reduced y structurl prototype trnsfortions fro the diond structure into either the β-sn structure or into toiclly ordered Sn-Si copounds (over long enough tie, possily even t roo teperture [11]). Here we would like to ention only tht we ctully did oserve in teperture nd growth rte odulted MBE grown Sn/Si quntu dot sples β-sn precipittes [10] nd precipittes with showed ½ {110} superlttice spots in their [001] diffrction pttern [12] (esides α-sn nd Sn rich Sn y Si 1-y QD precipittes). Finlly, we would like to rerk tht the equiliriu shpe of void in Si hs een deterined experientlly [13] to e tetrkidechedron, Fig. 1. This shpe is deterined y {111}, i.e. octhedron, nd {100}, i.e. cue, plnes. A = t / is shpe preter, Fig. 1; for A = 0 the shpe is n octhedron nd A = 2 / 3 corresponds to cue. EXPERIMENTAL DETAILS Three sets of pirs of ultilyer sples (one with nd one without n dditionl ex-situ nnel for 30 inutes t 800 ºC) with four Sn x Si 1-x /Si lyers nd sustitutionl Sn contents of noinlly 2 %, 5 %, nd 10 % in ech of the Sn x Si 1-x lyers were grown y teperture nd growth rte odulted MBE [8,9], stored t roo teperture for few yers, nd eventully selected for our trnsission electron icrocopy (TEM) investigtions. Our structurl nlyses eployed nlyticl TEM in oth the prllel illuintion nd scnning proe ode using oth JEOL JEM-2010F Schottky field eission STEM/TEM nd JEOL JEM-3010 TEM t the University of Illinois t Chicgo. Prllel illuintion TEM utilized conventionl diffrction contrst (CTEM), selected re electron diffrction, nd high-resolution phse contrst (HRTEM) iging. Atoic resolution Z-contrst iging in the scnning proe ode (STEM) proved to e especilly useful for our investigtions s the effects of strin fields in nd round QDs nd interference effects such s the fortion of oiré fringe due to doule diffrction re negligile [12]. In ddition, the intensity in Z-contrst iges is roughly proportionl to the squre of the toic nuer (Z) of the scttering tos. For precipitted Sn (Z = 50) in Si (Z = 14), n ige contrst s high s out (50 2 /14 2 ) - 1 is possile, which llows for direct nd intuitive interprettion of such Z-contrst iges in ters of distriution p of Sn. TEM specien preprtion followed stndrd procedures involving echnicl grinding nd ion illing to electron trnsprency. RESULTS AND DISCUSSIONS Erlier pln view TEM investigtions of Sn x Si 1-x lyers nneled t 650 ºC t the Cliforni Institute of Technology reveled n initilly rpid increse of the verge QD volue (<r> 3 ) with tie (t) for the first 2.2 hours of the nnel, Fig. 1, [9]. After this tie, the function <r> 3 (t) showed the typicl liner ehvior tht is expected for precipitte corsening y volue diffusion [14]. Non-linerities of <r> 3 with tie re coonly ttriuted to diffusion shortcuts such s disloctions, stcking fults, grin oundries, nd other coon lttice defects [14]. Our TEM/STEM investigtions of siilr sples in pln view nd cross section, Figs. 2-c, however, showed tht these structures re essentilly free of the coon lttice defects, entioned ove [12]. The lower gnifiction cross section iges of Figs. 2, show tht there re sll precipittes of Sn within the Si trix quite fr wy fro the sptil positions of the Sn x Si 1-x lyers. Such Sn precipittes were found in oth the ex-situ nneled nd the s grown sples, which only hd in situ therl tretents. Higher gnifiction Z-contrst STEM iges such s Fig. 3 show directly tht these Sn precipittes re voids within the Si trix tht re prtilly filled with Sn. In this (Fig. 3) ige one cn directly discern tht Sn (which ppers righter due to its lrger Z) lines the interfce etween the void nd the Si trix. As entioned ove, the equiliriu shpe of void in Si is tetrkidechedron, Fig. 1, [13]. The pplictions of Neunn s nd Curie s syetry principles [15] to the

3 I1.7.3 deterintion of the shpe of sll α-sn nd Sn y Si 1-y precipittes in Si trix shows tht this cn e tetrkidechedron s well [10,16]. Filling n equiliriu shpe void in Si y precipittion of Sn should, therefore, e n energeticlly fvorle process. Additionl evidence for the existence of voids in Si tht re prtly filled with Sn hs een gthered y quntittive electron energy loss spectroscopy (EELS) [17], Fig. 3, nd will e presented elsewhere [18]. The filling up of prtilly Sn filled voids in Si y ore Sn s result of diffusion (into diffusion shortcut) due to therl tretent t oderte preters (300 ºC for pproxitely three hours) ws directly (in situ) oserved under the electron icroscope, Figs. 4,. More or less well fored α-snqds, Fig. 4, resultedsα-sn nd Si oth possess the diond structure. We consider this ltter in situ oservtion s ost direct proof of our void-filling hypothesis, proposed ove. This echnis lso provides strightforwrd explntion for the initilly rpid increse of the verge QD precipitte volue with nneling tie, <r> 3 (t), in Fig. 1. The cretion of voids in Si nd their susequent filling with Sn eerges, therefore, s the first of the two echniss y which quntu dots grow endotxilly in the Sn/Si syste. [001] [010] Figure 1. () Results of erlier ex situ nneling experients t 650 ºC, fter ref. [9]. The plot of the verge QD precipitte size (<r> 3 ) versus the nneling tie (t) shows n initil rpid increse of the <r> 3 (t) function which lter on shows the typicl liner reltionship tht is expected when the precipitte corsening is governed y volue diffusion. () Sketch of tetrkidechedron, odified fter ref. [15] (with tht uthor s perission). [100] c 100 n 50 n Figure 2. Sn x Si 1-x /Si ultilyer structures in [110] cross sections, the sller lck rrows points towrds Sn precipittes tht grew within the Si lyers, the lrger white rrows indicte growth directions; () CTEM overview, x = 0.1, ex-situ nneled; () Z-contrst STEM overview, x = 0.1, ex-situ nneled; (c) HRTEM ige showing perfectly pseudoorph Sn 0.02 Si 0.98 lyer s deposited, power spectru s insert. An interesting question is how these voids y hve risen in the first plce. The nswer to this question y e found in echnis nlogously to tht descried in ref. [19]. According to tht echnis, when freshly prepred Si surfce is exposed to ir t roo teperture, voids of out 10 n dieter nd nuer density of out c -2 for spontneously pproxitely 10 n elow the surfce due to copressive strin tht rises

4 I1.7.4 fro the fortion of Si0 2 on the surfce [19]. Teperture nd growth rte odulted MBE deposited Sn x Si 1-x lyers y lso cuse the fortion of voids during the (low teperture) growth process since they lso copress freshly grown Si surfce. The therl cycling during teperture nd growth rte odulted MBE [8,9] of ultilyer structures ensures tht there re ny vcncies in the structure nd this could llow prefored voids of ny shpe to grow nd rech their equiliriu shpe. clculted epty void experient clculted fully Sn filled void experient 2n Figure 3. () [110] cross section Z-contrst STEM ige of void in Si tht is lined y Sn on its interfce with the Si trix. The rrow represents the growth direction, x = 0.1, ex-situ nneled. () Si L-edge counts nd Sn M-edge counts t series of sptil positions cross void (nd long line perpendiculr to the growth direction) in Si tht is prtly filled with Sn. While the solid circles represent the experientl Si L-edge counts, the tringles represent the experientl Sn M-edge counts. Clculted Si L-edge nd Sn M-edge counts re superiposed s dshed lines for oth ssuptions, tht this void is either epty or fully Sn filled. The close proxiity of the experientl nd clculted sets of Si L-edge counts nd the lrge difference etween the respective sets of Sn M-edge counts oth indicte tht the EELS dt re due to prtilly Sn filled void in Si. The clcultions were perfored for tetrkidechedron with A = 0.26 nd specien thickness of 33 ± 3 n, s deterined experientlly. 5n 5n Figure 4. [110] cross section Z-contrst STEM iges of α-sn QD precipittes, x = 0.05, ex-situ nneled, power spectr s inserts; () Voids in Si tht re prtilly filled with Sn, rked y rrows; () The se voids ut significntly ore filled with Sn s result of oderte dditionl therl tretent (in situ) underthestem. Phse seprtion of Sn fro pseudoorph Sn x Si 1-x predecessor lyer results in QD precipittes s well since the diond structurl prototype cn e conserved. Fig. 5 shows n erly stge of the fortion of such QD t the sptil position of pseudoorph Sn 0.1 Si 0.9 lyer. When fully fored, these QDs re sustitutionl solid solutions of Sn in Si (i.e. Sn y Si 1-y ) with higher Sn content thn the pseudoorph Sn x Si 1-x predecessor lyers (i.e. y > x). We consider this echnis s the second fortion echnis for endotxilly grown QDs in the Sn/Si syste. While we proposed this echnis erlier on theoreticl grounds [9,20], the Z-contrst STEM iges, e.g. Fig. 5, suggest tht this phse seprtion strts with the fortion of {111} Sn-Si interfces nd preferentil sustitutionl

5 I1.7.5 Sn-Si replceents in the res round intersecting {111} plnes. Note tht only for very high Sn content, Fig. 5, is direct nd gp predicted for Sn x Si 1-x lloys [3]. A shpe trnsition with size of Sn rich precipittes tht y e due to n incresing contriution of the elstic istch strin energy to the totl energy of the QDs hs een oserved, Figs. 5,c. While sller α-sn nd Sn rich Sn y Si 1-y QD precipittes possess the typicl tetrkidechedron shpe, Figs. 1, 4, nd 5, which proly results fro the nisotropy of the interfce energy density [16], uch lrger precipitte hd shpe tht reseles ore closely n octhedron, Fig. 5c. The shpe of this lrge precipitte y results fro the nisotropy of the elstic istch strin energy, i.e. the therodynics of isfitting precipittes in other words [15]. c 5n 5n 25 n Figure 5. [110] cross section Z-contrst STEM iges, x = 0.1, ex-situ nneled, the rrows represent the respective growth directions; () Prtilly fored Sn y Si 1-y precipitte in Si with y > x = 0.1, grown endotxilly y phse seprtion fro Sn 0.1 Si 0.9 lyer. Shpe trnsition of Sn rich precipittes with size fro () tetrkidechedron to (c) essentilly n octhedron. Intereditely sized Sn rich QD precipittes possess tetrkidechedron shpes with sller {001} fcets, i.e. sller shpe preters suggesting grdul trnsition to the shpe of n octhedron (A = 0) with incresing size. As the lrge precipitte in Fig. 5c is locted prtly t the predecessor sustitutionl Sn 0.1 Si 0.9 solution lyer nd prtly within the Si spcer lyer, it y hve fored y the siultneous opertion of oth endotxil growth echniss. The upper prt of the QD y, therefore, consist of α-sn nd the lower prt of sustitutionl Sn y Si 1-y solution with high Sn content. This hypothesis is consistent with the contrst in Fig. 5c s there sees to e coprtively less Sn in the center of this precipitte tht round its {111}/(001) pex. Finlly, we would like to suggest tht the eployent of the void cretion nd susequent filling echnis (y with α-sn precipitte QDs grow endotxilly in Si) y offer n opportunity to ke progress in other QD systes such s endotxilly grown (In,As,Si) QDs in Si [21]. With the equiliriu shpe of the voids [13] y proly lso e connected n equiliriu size so tht QDs with rther hoogenous size distriution y e creted y the eployent of the newly proposed echnis. SUMMAY AND CONCLUSIONS Two distinct echniss for the endotxil growth of quntu dots in the Sn/Si syste hve een oserved. The first of these echniss is of novel type nd involves the cretion of voids in the Si trix nd their susequent filling with Sn tos y diffusion. The second echnis results fro phse seprtion, s proposed erlier. While the QDs tht result fro the first echnis consist of pure α-sn, the quntu dots tht result fro the second echnis re sustitutionl Sn y Si 1-y solutions with high Sn content. Both of these echniss result in QDs which possess the diond structure nd the typicl shpe of tetrkidechedron. Sn precipittes tht re severl ties lrger thn the typicl quntu dot possess n essentilly octhedrl shpe. ACKNOWLEDGMENTS This reserch ws supported y oth grnt to PM y the Cpus Reserch Bord of the University of Illinois t Chicgo nd grnt to NDB y the Ntionl Science Foundtion (DMR ). The growth of Sn x Si 1-x /Si structures ws supported y Ntionl Science Foundtion grnt (ECS ) to HAA.

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