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.
Sa´nchez hanks he Science Ci y Resea ch Alliance and he
HEFCE S a egic De elopmen Fund o unding suppo .
1
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FIG. 3. (a) In and Sb p o iles (solid lines) including In seg ega ion a he
GaAs-on-WL in e ace yielding he R
002
sim
cu e bes ep oducing he ex-
pe imen al in ensi y a io, R
002
exp
(b). In seg ega ion e iciency amoun s o
R
In
¼0.84.
011601-4 Luna e al. Appl. Phys. Le . 101, 011601 (2012)
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