Quantitative study of the interfacial intermixing and segregation effects across the wetting layer of Ga(As,Sb)-capped InAs quantum dots

Quan i a i e s udy o he in e acial in e mixing and seg ega ion e ec s ac oss he we ing laye o Ga(As,Sb)-capped InAs quan um do s Espe anza Luna, 1,a) Ana M. Bel a´n, 2,b) Ana M. Sa´nchez, 3 and Se gio I. Molina 2 1 Paul-D ude-Ins i u u¨ Fes ko¨ pe elek onik, Haus og eipla z 5-7, D-10117, Be lin, Ge many 2 Depa amen o de Ciencia de los Ma e iales e I.M y Q.I., Facul ad de Ciencias, Uni e sidad de Ca´diz, Campus Rio San Ped o, 11510 Pue o Real, Ca´diz, Spain 3 Physics Depa men , Uni e si y o Wa wick, Co en y CV4 7AL, Uni ed Kingdom (Recei ed 7 Ma ch 2012; accep ed 13 June 2012; published online 2 July 2012) Quan i a i e chemical in o ma ion om semiconduc o nanos uc u es is o p ima y impo ance, in pa icula a in e aces. Using a combina ion o analy ical ansmission elec on mic oscopy echniques, we a e able o quan i y he in e acial in e mixing and su ace seg ega ion ac oss he in ica e non-common-a om we ing laye (WL) o Ga(As,Sb)-capped InAs quan um do s. We ind: (i) he WL-on-GaAs(bu e ) in e ace is ab up and pe ec ly de ined by sigmoidal unc ions, in analogy wi h wo-dimensional epi axial laye s, sugges ing ha he in e ace o ma ion p ocess is simila in bo h cases; (ii) indium seg ega ion is he p e ailing mechanism (e.g., o e an imony seg ega ion), which e en ually de e mines he composi ion p o ile ac oss he GaAs(cap)-on-WL in e ace. V C2012 Ame ican Ins i u e o Physics.[h p://dx.doi.o g/10.1063/1.4731790] Expanding he usable wa eleng h o op o-elec onic de ices owa ds 1.3 and 1.55 lm is a subjec o cu en ech- nological ele ance. 1 P esen e o s ocus on he de elop- men o new GaAs-based unc ional uni s, whe e nanos uc u es based on quan um do s (QDs) emain as one o he mos p omising op ions. 2–4 The inco po a ion o an i- mony in o (In,Ga)As QDs has been ound o be an e ec i e solu ion o edshi he emission, due o he possibili y o a s agge ed ype-II band alignmen o an imony con en s abo e 14%. 5 Indeed, oom empe a u e emission a 1.6 lm has al eady been epo ed wi h Ga(As,Sb)-capped (In,Ga)As QDs. 6,7 The complexi y o he g ow h o InAs-GaAs-GaSb he e os uc u es is well-known, in pa icula , he di icul y o ab ica ing ab up he e oin e aces, since bo h ca ion and anion seg ega ion may occu . 8,9 Seg ega ion may no only esul in he deg ada ion o he in e ace bu also in he unin- en ional o ma ion o a e na y and/o qua e na y alloy om he nominally deposi ed bina y compounds. In addi ion o he inhe en di icul ies p esen in he chemical analysis o any qua e na y compound, In and Sb seg ega ion may simul- aneously occu , which hampe s he de ec ion and e alua ion o he indi idual con ibu ions. 9 The impac o seg ega ion on he s uc u al p ope ies o Ga(As,Sb)-capped InAs/GaAs do -in-well s uc u es has been s udied on he a omic scale by c oss-sec ional scanning unneling mic oscopy. 9,10 While mos o he s udies ocus on he s uc u al and chemical p ope ies o QDs, quan i a i e analysis o he chemical p o- ile and composi ional sha pness o we ing laye s (WLs) a e sca cely epo ed. Mo eo e , ecen ly, in iguing pho olumi- nescence (PL) emission om ecombina ion a he WL in e - ace o In(As,Sb) QDs has been epo ed. 11 In hei wo k, he au ho s explici ly demand he need o speci ic in es iga ions in o he s uc u e and composi ion o he WL in e ace o u - he unde s and he WL- ela ed PL ea u e. Based on he analysis o he in ensi y con as o g 002 da k- ield ansmis- sion elec on mic ocopy (DFTEM) mic og aphs, Luna and co-wo ke s ha e ecen ly p oposed a me hod o he quan i- a i e e alua ion o he chemical in e mixing a he in e a- ces o InAs/GaSb supe la ices, which includes seg ega ion e ec s. 12 The p ocedu e, howe e , has no ye been applied o he analysis o he chemical in e ace in nanos uc u es, e.g., ac oss QDs pe se o ac oss WLs. In his wo k, we apply ou quan i a i e DFTEM (qDFTEM) me hod o he chemical cha ac e iza ion o he in e ace in a Sb-based non-common- a om (NCA) WL. In pa icula , we a e able o de e mine he chemical composi ion, including he iden i ica ion and quan- i ica ion o III- and V-elemen seg ega ion e ec s, ac oss he in e aces o he in ica e (In,Ga)(As,Sb) WL o Ga(As,Sb)-capped InAs QDs g own on GaAs(001). The s udied sample was g own by molecula beam epi- axy and consis s o 2.2 monolaye s (ML) InAs QDs, capped wi h 6 ML GaAs and 3 ML GaSb, upon which a 100 nm GaAs cap laye was deposi ed. The nominal s uc u e is shown in Figu e 1(b). G ow h condi ions can be ound else- whe e. 13 The mo phological and chemical cha ac e iza ion has been conduc ed by con en ional TEM (CTEM) in c oss- sec ional iew in a JEOL-JEM 1200EX ope a ed a 120 kV. 14 TEM specimens ha e been p epa ed ollowing s anda d p o- cedu es. In addi ion o CTEM, analy ical TEM echniques such as elec on ene gy loss spec oscopy (EELS) has been pe o med. 14 Because we deal wi h NCA in e aces be ween qua e na y and/o e na y alloys, he unambiguous de e mina- ion o he WL composi ion equi es he combined analysis o EELS, high-angle annula da k- ield scanning ansmission elec on mic oscopy (HAADF-STEM), and DFTEM da a. Figu e 1(a) shows a c oss-sec ional g 002 DFTEM image o he WL. As al eady epo ed in Re . 12, qDFTEM analysis o he chemical in e ace in NCA he e os uc u es elies on he analysis o wo-beam DFTEM images ob ained wi h he di ac ion ec o g¼002, which is sensi i e o he chemical composi ion o semiconduc o s wi h zincblende s uc u e. a) Au ho o whom co espondence should be add essed. Elec onic mail: luna@pdi-be lin.de. Telephone: þ49 30 20377 281. Fax: þ49 30 20377 515. b) P esen add ess: n-Ma G oup, CEMES-CNRS. 29 Rue Jeanne Ma ig, BP 94347, 31055 Toulouse, CEDEX 4, F ance. 0003-6951/2012/101(1)/011601/4/$30.00 V C2012 Ame ican Ins i u e o Physics101, 011601-1 APPLIED PHYSICS LETTERS 101, 011601 (2012) 17 Oc obe 2023 12:03:32 In sho , he me hod is based on he p oposal o a dis ibu ion p o ile o he di e en cons i uen elemen s ha we pu in o he calcula ion o he co esponding di ac ed in ensi y unde kinema ic condi ions (I 002 ). We compa e he simula ed in ensi y (R 002 sim ), no malized o ha o a e e ence a ea o known composi ion, R 002 ¼I 002 laye /I 002 e e ence , wi h he ex- pe imen al one (R 002 exp ) and look o he composi ion p o- iles bes i ing he expe imen al con as . The eby, he p ocedu e would esemble he analysis o x- ay di ac ion da a whe e he laye in o ma ion (composi ion, hickness, s ain, e c.) is ex ac ed a e compa ing he expe imen al and simula ed cu es. Since he iden i y o ca ion and anion bo h change ac oss he he e oin e ace, we ha e o inpu he con ibu ions o he III- and V-subla ices sepa a ely, which, in p inciple, allows he independen de e mina ion o he chemical wid h and su ace seg ega ion in each subla ice. Al hough he me hod is based on an ape u e-limi ed imag- ing mode (i.e., DFTEM, wi h a esolu ion o abou 0.5 nm), he p ocedu e allows he de ec ion o a ia ions in he in e - ace wid h and laye hickness as small as 0.1 ML. 12 The key issue is he iden i ica ion o he elemen dis ibu ion p o ile. P e iously, om he analysis o di ec ly de e mined expe i- men al composi ion p o iles in se e al III-V semiconduc o he e os uc u es, we ha e demons a ed ha he smoo h a i- a ion o he elemen concen a ion x(z) wi h he posi ion z ac oss he in e ace ollows a sigmoidal unc ion: x(z) ¼x 0 / [1 þexp(z/L)], whe e he in e ace wid h Lis he main i - ing pa ame e and x 0 deno es he nominal mole ac ion. 15,16 Fo laye s and/o quan um wells (QWs) cen e ed a z¼0, he exp ession eads 15 x¼xðlÞ 0 1þe zþN 2 ðÞ Llowe o z<0ðlowe in e aceÞ; x¼xðuÞ 0xðuÞ 0 1þe zN 2 ðÞ Luppe o z>0ðuppe in e aceÞ: (1) x 0 (l) and x 0 (u) deno e he nominal mole ac ion co espond- ing o he lowe (l) and uppe (u) in e ace, espec i ely. Nis he wid h o he laye , and L lowe and L uppe a e he in e ace wid h a he lowe and uppe in e aces, espec i ely. Fo he analysis o Sb-based NCA in e aces, he ealis ic dis ibu- ions o he di e en elemen s, In and Sb ( hose o Ga and As a e ob ained a e mass conse a ion: [In] þ[Ga] ¼100% and [As] þ[Sb] ¼100%) a e ob ained assuming ha he change in composi ion ac oss he in e aces ollows he sig- moidal unc ion in Eq. (1). In o de o es ima e he p esence o seg ega ion e ec s, we conside inpu dis ibu ion p o iles ha a e ob ained om he combina ion o a seg ega ed p o- ile de i ed a e Mu aki’s phenomenological model 17 and he sigmoidal unc ion o he desc ip ion o he in e - ace. 15,18 This inno a i e p ocedu e has p o en e y success- ul in he analysis o InAs/GaSb sho -pe iod-supe la ices and Sb-based he e os uc u es, e en i ex emely hin laye s (<3 ML) o (un)in en ionally inse ed in e acial laye s a e conside ed. 12,19 Figu e 1(c) shows he p o ile o he expe imen al di - ac ed in ensi y no malized o ha o GaAs in he bu e laye ( e e ence), R 002 ¼I 002 laye /I 002 GaAs . The da a a e ex ac ed om line scans in he a ea ma ked in Fig. 1(a), a away om any QD. As obse ed, he in ensi y p o ile e eals a p onounced asymme y, which esembles seg ega ion ea- u es. A p e ious mo phological and quali a i e chemical cha ac e iza ion o he sample has al eady e ealed ha cap- ping he InAs QDs wi h 6 ML GaAs/3 ML GaSb gene a es a complex WL consis ing o a (In,Ga)(As,Sb) co e wi h Sb- deple ed (In,Ga)As in e aces. 14,20 The composi ion p o iles o Figs. 2(a) and 2(c) (open symbols), which a e ex ac ed om Re . 14 and a e de e mined using EELS da a (no e ha EELS in es iga ion o his sample only gi es quali a i e in- o ma ion 14 ) indica e ha he In con en ac oss he WL is no homogeneous, bu he e a e wo indium- ich egions close o he in e aces, which a e sepa a ed by a hin Sb-con aining laye wi h a educed In con en , i.e., he (In,Ga)(As,Sb) co e. FIG. 1. (a) g 002 DFTEM mic og aph (sensi i e o composi ion) o he QD s uc u e (b) (no scaled). Also displayed is he a ea ac oss he WL om whe e he in ensi y p o ile (R 002 ) is ex ac ed (c). The labels (1) and (2) in (c) e e o he baseline and o he ail edge a he GaAs(cap)-on-WL in e - ace, espec i ely. FIG. 2. (a) and (c) Two se s o p oposed In and Sb sigmoidal p o iles o qDFTEM (solid lines) yielding a simila R 002 sim p o ile ep oducing he ex- pe imen al in ensi y a io, R 002 exp [(b) and (d), espec i ely]. Seg ega ion e ec s a e no included ye . The g ow h di ec ion is om le o igh (indi- ca ed by an a ow). 011601-2 Luna e al. Appl. Phys. Le . 101, 011601 (2012) 17 Oc obe 2023 12:03:32 Despi e he comp ehensi e in es iga ion in Re . 14, quan i a- i e in o ma ion on he chemical p o ile and composi ional sha pness o he WL is s ill missing. In o de o quan i y he WL in e ace, we apply he qDFTEM p ocedu e desc ibed abo e, whe e he i s s ep is he p oposal o he inpu ele- men p o iles. Ini ial i a emp s using elemen dis ibu ions, which a e based on he nominal s uc u e (i.e., 2.2 ML InAs/ 6 ML GaAs/3 ML GaSb) ailed, as we ound ha R 002 sim o ally disag eed wi h R 002 exp (no shown). Nex a emp s using composi ion p o iles deduced om he EELS da a 14 we e mo e success ul, as shown in Fig. 2(d), which displays he expe imen al and simula ed in ensi y p o ile, oge he wi h he co esponding elemen dis ibu ion o qDFTEM along he g ow h di ec ion (solid lines in Fig. 2(c)). Fo sim- pli ica ion, seg ega ion e ec s a e no included ye . The In dis ibu ion consis s o wo o e lapped peaks (he ea e deno ed as In1-peak and In2-peak, espec i ely), while he e is a single Sb p o ile. The ollowing alues o he composi- ion, laye hickness, and in e ace wid h a e deduced om he i : 9.9% In, N¼5.8 ML and L lowe ¼1.1 ML, L uppe ¼0.85 ML o he In1-peak; 11.5% In, N¼7 ML and L lowe ¼0.8 ML, L uppe ¼3.6 ML o he In2-peak; and 3% Sb, N¼1.6 ML and L lowe ¼L uppe ¼1.5 ML o he Sb peak. The elemen al concen a ions a e simila o hose es i- ma ed by EELS. 14 We ind, howe e , ha con a y o ou p e ious wo k on qDFTEM o bina y and/o e na y alloys wi h NCA in e aces, 12,19 whe e he elemen p o iles could be unambiguously de e mined, he e he se o dis ibu ion p o iles gi ing he bes i is no unique, since di e en com- bina ions o In and Sb dis ibu ions can lead o simila esul s. This a ises om he ac ha di e en combina ions o In and Sb con en s yield simila alues o he s uc u e ac o and, hus, a simila in ensi y con as . Indeed, his is one o he main inhe en di icul ies associa ed wi h he chemical analysis o in e aces be ween qua e na y and/o e na y alloys. We ind, o ins ance, ha he se o elemen p o iles in Fig. 2(a), which consis s o a single In peak (which could be conside ed as he en elope o he wo In peaks) and a single Sb peak, yields likewise a good i [excep a he uppe GaAs(cap)-on-WL in e ace, Fig. 2(b)] and is e y simila o he i in Fig. 2(d). We deno e he p o- iles in Fig. 2(a) con igu a ion 1 (con . 1), since he e is a sin- gle In peak. In a simila way, he p o iles shown in Fig. 2(c), consis ing o wo In peaks, a e deno ed con igu a ion 2 (con . 2). The pa ame e s o he i o con . 1 a e: 12%, N¼16.2 ML and L lowe ¼1.1 ML, L uppe ¼3.6 ML o he In; and 10.9%, N¼2.8 ML and L lowe ¼L uppe ¼1.5 ML o he Sb. O he con igu a ions (combina ions o In and Sb p o- iles) can esul in simila i s as well. These, howe e , ei he in ol e un ealis ic In and/o Sb p o iles o a e in clea dis- ag eemen wi h he HAADF-STEM and EELS measu e- men s 14,20 and, hence, a e no conside ed. Compa ison o he p o iles cons i u ing con . 1 and con . 2, espec i ely, p o- ides he ollowing in o ma ion: (i) al hough seg ega ion e ec s a e no ye explici ly included, o bo h con igu a- ions, he la ge b oadening a he GaAs-on-WL in e ace in he In p o ile (L uppe ¼3.6 ML) e lec s he exis ence o In seg ega ion a leas ; (ii) while he elemen concen a ions in con . 1 a e in close ag eemen wi h he nominal alues (no e howe e ha only a small amoun o Sb inco po a es, wi h [Sb] <11% o , 0.4 ML i compu ed as he equi alen hickness o GaSb deposi ed 21 ), he In and Sb dis ibu ions disag ee wi h he quali a i e chemical in o ma ion ex ac ed om HAADF-STEM and EELS; 14,20 (iii) he composi ion p o iles o con . 2 ag ee ema kably well wi h he p o iles ex ac ed om EELS and may explain he peculia HAADF- STEM con as a he WL o his sample, as epo ed in Re s. 14 and 20. The es ima e equi alen hickness o InAs a he WL (a ea unde he [In] cu e) is close o 1.7 ML, in good ag eemen wi h he amoun o In deposi ed be o e he wo- (2D) o h ee-dimensional (3D) ansi ion occu s. 21 Hence, in he ollowing, we ocus on con . 2. No ice ha , due o he complexi y o he WL in his speci ic case, he combined analysis o HAADF-STEM, EELS, and qDFTEM is c ucial o he unambiguous de e mina ion o he composi ion ac oss he in e ace. The in ica e WL elemen dis ibu ion may p obably a ise om a pa ial capping o he QDs, whe e he In om he s ill exposed egions o he QDs will mig a e away o o m a “new” WL on op o he “nominal” one. 22,23 Seg ega ion e ec s can be easily inco po a ed in o he analysis h ough he inpu dis ibu ion p o iles, 12 whe e he p o iles a e ob ained om a combina ion o Mu aki’s model and he sigmoidal unc ion o he in e ace desc ip ion. 15 Again, because o he many pa ame e s in ol ed, he i ing u ns ou ambiguous and cumbe some in compa ison wi h ha on NCA in e aces be ween bina y and/o e na y alloys. The main di icul ies a ise om: (a) as al eady men ioned, di e en combina ions o [In] and [Sb] yield simila alues o R 002 ; and (b) he small amoun o Sb in o he la ice (<5% o con . 2) makes de ec ion o Sb seg ega ion challenging. Acco ding o (a), we ind ha he sligh baseline in R 002 exp [ma ked as (1) in Fig. 1(c)] could be explained by bo h In and Sb seg ega ion, whe eas i is ha d o dis inguish each indi idual con ibu ion, since hey o e lap. On he con a y, he e a e some o he speci ic seg ega ion ea u es, like he ail edge a he uppe GaAs-on-WL in e ace [ma ked as (2) in Fig. 1(c)], which can only be explained assuming In seg ega ion: Sb seg ega ion mainly a ec s he baseline on R 002 bu is no accoun able o he s ong asymme y a he GaAs-on-WL in e ace. Al hough i is di icul o iden i y he con ibu ion om Sb seg ega ion, es ima e o he Sb seg- ega ion e iciency, R Sb , indica es a subs an ial Sb seg ega- ion wi h R Sb >0.8. This would explain he small amoun o Sb, which is inco po a ed in o he laye , despi e he nominal composi ion is GaSb. Coexis ence o Sb and In seg ega ion is likely. 9 Con a y o he Sb case, he iden i ica ion o In seg ega ion is easible. Indeed, he impac o In seg ega ion on he main ea u es o R 002 is signi ican , as i mainly e lec s a he ail edge, hus allowing an accu a e de e mi- na ion o he In seg ega ion e iciency, R In . In his espec , Fig. 3(a) ep esen s he In and Sb p o iles based on he ele- men dis ibu ion o con . 2, bu now including In seg ega- ion, yielding he bes R 002 i [Fig. 3(b)]. Fo simpli ica ion, Sb seg ega ion is no explici ly included. As obse ed, he ag eemen be ween he expe imen al and simula ed R 002 is excellen . The es ima ed R In ¼0.84 60.01 is in good ag ee- men wi h epo ed R In alues o In seg ega ion in (In,Ga)As WLs g own a simila condi ions. 24,25 F om he p o iles in Fig. 3(a), wo main conclusions can be d awn: (1) as a conse- quence o In seg ega ion, he GaAs-on-WL in e ace is e y 011601-3 Luna e al. Appl. Phys. Le . 101, 011601 (2012) 17 Oc obe 2023 12:03:32 b oad and ex ends o e mo e han 15 ML (4.5 nm) along he g ow h di ec ion. Thus, In seg ega ion is ema kable and is he p e ailing mechanism (e.g., o e Sb seg ega ion), which e en ually de e mines he composi ion p o ile ac oss he GaAs-on-WL in e ace. (2) The WL-on-GaAs in e ace is ab up and pe ec ly de ined by sigmoidal unc ions, in anal- ogy wi h 2D epi axial laye s. 12,15,16 Fo laye s g own in he 2D mode, he sigmoidal esponse a ises om a coope a i e inco po a ion o he species du ing he in e ace o ma ion. 26 Hence, we can assume ha he p ocesses p e ailing in he ini ial s ages o he WL o ma ion a e simila o hose go e ning he in e ace o ma ion in F ank an de Me we 2D laye s. How hese unexpec ed esul s econcile wi h he S anski-K as anow g ow h mode is p esen ly unde in es iga ion. In summa y, using a combina ion o TEM analy ical echniques wi h qDFTEM, we ha e quan i ied he chemical composi ion and in e acial in e mixing (including he iden- i ica ion and quan i ica ion o III- and V-elemen seg ega- ion e ec s) ac oss he complex WL o Ga(As,Sb)-capped InAs QDs. The quan i a i e analysis o he composi ional ab up ness in a WL may p o ide an insigh in o he basic mechanisms occu ing du ing i s o ma ion, u he allowing a be e unde s anding and con ol o he ab ica ion o unc- ional uni s based on nanos uc u es. Mo eo e , co ela ions be ween he op ical and chemical p ope ies o WL in e a- ces can also be es ablished. We acknowledge J. M. Ripalda and A. G. Taboada om IMM-CSIC (Spain) o he sample g ow h and M. H. Gass om Supe STEM Labo a o y (U.K.) o he EELS and HAADF da a acquisi ion. DFTEM measu emen s we e ca ied ou a DME-SCCYT, UCA (Spain). We acknowledge X. Kong and R. Ga gallo-Caballe o (PDI) o a c i ical ead- ing o he manusc ip . This wo k was suppo ed by he Span- ish MCI (P ojec s TEC2008-06756-C03-02/TEC and TEC2011-29120-C05-03) and he Jun a de Andalucı´a (PAI esea ch g oup TEP120; P ojec P08-TEP-03516). Co- inancing om UE-FEDER is also acknowledged. A. M. 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