1491
Fo ma ion o CuxAu1−x phases by cold homogeniza ion
o Au/Cu nanoc ys alline hin ilms
Alona Tynko a1,2, Gabo L. Ka ona2, Gabo A. Lange 2, Se gey I. Sido enko1,
S e lana M. Voloshko1 and Dezso L. Beke*2
Full Resea ch Pape Open Access
Add ess:
1Na ional Technical Uni e si y o Uk aine “Kie Poly echnic Ins i u e”,
37 P ospec Pe emogy, 03056 Kie , Uk aine and 2Depa men o
Solid S a e Physics, Uni e si y o Deb ecen, P.O. Box 2, 4010
Deb ecen, Hunga y
Email:
Dezso L. Beke* - [email p o ec ed]
* Co esponding au ho
Keywo ds:
Cu/Au; g ain bounda y di usion; nano ilms o in e me allic
compounds; seconda y neu al mass spec ome y (SNMS); solid
s a e eac ion
Beils ein J. Nano echnol. 2014, 5, 1491–1500.
doi:10.3762/bjnano.5.162
Recei ed: 10 Ap il 2014
Accep ed: 20 Augus 2014
Published: 10 Sep embe 2014
Associa e Edi o : P. Leide e
© 2014 Tynko a e al; licensee Beils ein-Ins i u .
License and e ms: see end o documen .
Abs ac
I is shown, by using dep h p o iling wi h a seconda y neu al mass spec ome e and s uc u e in es iga ions by XRD and TEM,
ha a low empe a u es, a which he bulk di usion is ozen, a comple e homogeniza ion can ake place in he Cu/Au hin ilm
sys em, which leads o o ma ion o in e me allic phases. Di e en compounds can be o med depending on he ini ial hickness
a io. The p ocess s a s wi h g ain bounda y in e di usion, which is ollowed by a o ma ion o eac ion laye s a he g ain bound-
a ies ha leads o he mo ion o he newly o med in e aces pe pendicula o he g ain bounda y plane. Finally, he homogeniza-
ion inishes when all he pu e componen s ha e been consumed. The p ocess is asymme ic: I is as e in he Au laye . In
Au(25nm)/Cu(50nm) samples he inal s a e is he o de ed AuCu3 phase. Dec ease o he ilm hicknesses, as expec ed, esul s in
he accele a ion o he p ocess. I is also illus a ed ha changing he hickness a io ei he a mix u e o Cu- ich AuCu and AuCu3
phases (in Au(25nm)/Cu(25nm) sample), o a mix u e o diso de ed Cu- as well as Au- ich solid solu ions (in Au(25nm)/Cu(12nm)
sample) can be p oduced. By using a simple model he in e ace eloci y in bo h he Cu and Au laye s we e es ima ed om he
linea inc ease o he a e age composi ion and i s alue is abou wo o de s o magni ude la ge in Au (ca. 10−11 m/s) han in Cu
(ca. 10−13 m/s).
1491
In oduc ion
Solid-s a e eac ions in nanos uc u ed hin ilm sys ems a e
in e es ing and challenging no only om he poin o iew o
pu e undamen al esea ch, bu a e also impo an o echno-
logical applica ions. Examples o he la e a e he me alliza-
ion o in eg a ed ci cui s ( he o ma ion o a nanome ic NiSi
laye on he Si subs a e [1,2]), o he p oduc ion o hin chemi-
cally o de ed FeP ilms o pe pendicula magne ic da a
eco ding [3,4]. Rega ding he basic unde s anding o such eac-
Beils ein J. Nano echnol. 2014, 5, 1491–1500.
1492
ions he ques ions abou he con ibu ions o a as mass ans-
po along di e en g ain bounda ies (GBs, i.e., sho ci cui s)
can be men ioned; hey can ha e an impo an e ec on he
en i e in e mixing p ocess in nanoc ys alline bi- o mul ilaye s.
In addi ion he GB di usion coe icien s can co e a ange o
se e al o de s o magni ude, depending on he ype o GB s uc-
u e (low o high angle GBs [5], iple junc ions [6]). Fu he -
mo e i can be obse ed a e y low empe a u es ha he
mo phology o he o ma ion and he g ow h o he new
phase(s) can be di e en om he usual plana g ow h o a eac-
ion laye [7-9]: The new phase(s) can be o med a g ain
bounda ies, GBs, and can g ow u he by he mo ion o he
new in e aces pe pendicula o he o iginal GB plane [10].
The e a e examples in he li e a u e, in which he so-called
“cold homogeniza ion” was obse ed: Al hough he bulk di u-
sion p ocesses we e p ac ically ozen in bina y nanoc ys alline
couples e en a comple e in e mixing o componen s leading o
ull homogeniza ion was ound [11-15]. Two easons we e
men ioned as possible explana ion o his phenomenon:
i) di usion-induced g ain bounda y mo ion (DIGM) and/o
di usion-induced e-c ys alliza ion (DIR), and ii) g ain
bounda y mo ion du ing usual e-c ys alliza ion [11,12]. In he
la e he alloying is he consequence o he alloyed zones le
behind by e-c ys alliza ion du ing di usion in e mixing and as
a esul o g ain g ow h he g ain size should be inc eased. On
he o he hand du ing DIGM he composi ion behind he
mo ing bounda y can be abou se e al en h o an a omic ac-
ion, i.e., he homogeniza ion o a hin ilm wi h small g ain size
is also possible by his mechanism. Du ing DIR, which is
ano he mani es a ion o he s ess elaxa ion caused by he
ini ial inequali y o he GB di usion luxes o he wo compo-
nen s, new g ains a e o med, wi h a composi ion mo e discon-
inuously di e en om he su ounding g ains as compa ed o
DIGM.
In addi ion, i is di icul o make a dis inc ion be ween DIR and
DIGM expe imen ally [16]. DIR has mainly been in es iga ed
in bina y sys ems wi h a wide mu ual solubili y ange abo e
ei he he miscibili y gap o he c i ical empe a u e o o de ing
(e.g., in Cu/Pd [16], Au/Cu [15], Ag/Pd [7], Ni/Cu [17-19],
NiPd [20]). Less wo ks ha e been de o ed o sys ems wi h eac-
i e di usion [21]. Fo ins ance in he Cu/Pd sys em [8,22] no
eac ion laye s we e de ec ed a he o iginal in e ace (see he
ansmission elec on mic oscopy (TEM) image in Figu e 9b o
[8]) a 200 °C, bu he selec ed-a ea di ac ion pa e ns in TEM
indica ed he p esence o he PdCu phase. A e hea ea men s
a 260 °C, an ex ensi e in e mixing had aken place, accompa-
nied by g ain bounda y mig a ion, g ain g ow h, and o ma ion
o CuPd and Cu3Pd phases. Simila esul s we e ob ained in
Cu/Pd [23] Ni2Si/Si [24] and Fe/P [9] sys ems. In [10] hese
esul s we e summa ized and an in e p e a ion based on he
g ain bounda y di usion induced eac ion laye o ma ion,
GBDIREAC, was o e ed. Thus, i was p o en ha in bina y
sys ems wi h in e me allic laye s no only a homogeniza ion (by
o ma ion o solid solu ions) bu o ma ion o compounds is
also possible. In addi ion he mo phology o he g owing phases
in such hin ilm couples can be di e en om he plana
g ow h mode: Ins ead o nuclea ion and g ow h o he eac ion
laye a he ini ial in e ace, he eac ion akes place in he GBs
and he amoun o he p oduc phase g ows by he mo ion o he
o med new in e aces pe pendicula o he GBs. Thus, he
en i e laye o he pu e pa en ilms can be consumed by in e -
ace di usion d i en in e ace mo ion and a ully homogeneous
p oduc laye can be ob ained. Fu he mo e, in he i s s age o
such a p ocess, assuming ha he in e ace eloci y is cons an ,
he a e age composi ion in he cen e o he ilms should
linea ly inc ease wi h ime (by g adually consuming he ini ial
ma e ial o he g ains) and he slope o his unc ion is p opo -
ional o in e ace eloci y [24-26].
In his s udy, simila p ocesses a e in es iga ed in Au/Cu
sys em a low empe a u es. No e ha in [7] his sys em was
in es iga ed in de ail abo e he o de ing empe a u e and
compa a i e measu emen s we e ca ied ou only a 230 and
350 °C (below he o de ing empe a u e). I was ob ained ha a
highe empe a u es he phase o ma ion kine ics was e y
simila o he one ob ained in Ag/Au sys em, when only diso -
de ed phases we e o med. I was also concluded ha in he
Au/Cu sys em he main d i ing o ce was he chemical in e -
mixing and he d i ing o ce o o de ing ga e only a mino
con ibu ion, i.e., he s uc u al ans o ma ion was simila
below and abo e he o de ing empe a u e. The ini ial sha p
plana in e ace s ill exis ed in he hea - ea ed samples and,
simila ly o he esul s o [8] in Cu/Pd, he ea ly s ages in
Cu/Au could no be unde s ood as a plana laye eac ion. I was
also obse ed ha new g ains we e o med in he eac ion zone
(DIR). While in [7] he main phenomenon was he elaxa ion o
misma ch s ess (accumula ed mainly by bulk di usion abo e
he o de ing empe a u e), in his pape we will concen a e on
p ocesses a low empe a u es, a which he bulk di usion is
comple ely ozen.
Resul s
The concen a ion p o iles o Au(25nm)/Cu(50nm) samples
annealed a di e en empe a u es a e shown in Figu e 1. The
p esence o he smea ed in e ace in he as-deposi ed sample
can be explained by some ini ial su ace oughness, di usion
du ing he sample p epa a ion o ins umen al e ec s o he
spu e dep h p o iling [27,28]. I can be clea ly seen ha ,
du ing hea ea men s he Cu pene a ion in o he Au laye is
mo e in ensi e han he Au pene a ion in o he Cu laye .
Beils ein J. Nano echnol. 2014, 5, 1491–1500.
1493
Figu e 1: Concen a ion p o iles o Au(25nm)/Cu(50nm) sys em a) as deposi ed sample and annealed b) a 160 °C o 1 h and c) 3 h, (d) 180 °C o
5 h, (e) 200 °C o 10 h, ( ) 330 °C o 4 h.
Du ing annealing a 160 °C o 3 h, on he Au-side a signi ican
in e mixing occu s wi h Cu concen a ion up o 20 a om %,
while on he Cu-side he concen a ion o Au a oms eaches
only 7 a om % (Figu e 1c). I can also be seen (see, e.g.,
Figu e 1b and Figu e 1c) ha he e a e no pla eaus on he
composi e p o iles (as indica ions o o ma ion o compound
laye s wi h plana in e aces in dep h p o iles aken, e.g., by
SNMS) a sho e imes, bu he e is a ela i ely high a e age
composi ion o Cu inside he Au laye (Figu e 1b) as well as o
he Au composi ion inside he Cu laye (Figu e 1d). In addi ion
he e is a minimum o he Cu p o ile inside he Au laye . These
a e he consequences o he GB mass anspo along he GBs.
The comple e illing-up o g ain bounda ies, e.g., in Au would
lead o a maximum a e age composi ion o abou 7 a om %
(since, wi h δ = 0.5 nm g ain bounda y hickness, 2δ/d is 0.066),
and abou 10% would be expec ed o his alue in he Cu ilms,
using he es ima ed a e age GB olume ac ion calcula ed
om he g ain sizes (dAu = 15 nm, dCu = 10 nm, see below). In
Beils ein J. Nano echnol. 2014, 5, 1491–1500.
1494
Figu e 2: XRD θ–2θ pa e ns o Au(25nm)/Cu(50nm) samples a) as deposi ed, b) annealed a 180 °C o 5 h, c) o 10 h and d) a 200 °C o 44 h.
Figu e 1e he o e all composi ion o he di using elemen s on
bo h sides is a he high and canno be simply explained by a
illing-up o g ain bounda ies only, because he alues ob ained
a e la ge han he one co esponding o he a e age alue es i-
ma ed om he olume ac ion o he g ain bounda y a ea a
he obse ed g ain sizes.
I is no ewo hy ha he appea ance o some Cu a oms a he
opmos su ace and he de elopmen o a minimum can be
obse ed in he cen e o he Au laye (Figu e 1c). I can be
explained by he seg ega ion o Cu [29] and/o by he coexis-
ence o as and slow di usion bounda ies (bimodal GB
ne wo k) [25]. I was shown in [25] ha in he la e case he
GB di usion s a s along GBs wi h he la ges di usi i ies and
he e is only sho pene a ion along GBs wi h small di usi i y
alues. A longe annealing imes he GB pene a ion leng h is
la ge han he hickness o he ilm and he anspo ed a oms
sp ead ou on he ee su ace o ming a new sou ce o di u-
sion along he s ill no illed GBs wi h smalle di usi i ies. As
a esul a minimum in he a e age composi ion p o ile de elops
inside he ilm, close o he ee su ace. Thus in ou case he
minimum o he Cu composi ion in he Au laye (Figu e 1c) can
also be a consequence o he bimodal GB s uc u e. A comple e
homogeniza ion o he sys em akes place bo h a low empe a-
u es o longe annealing imes (Figu e 1e) and/o a highe
empe a u es (Figu e 1 ).
We would like o emphasize ha we did no obse e a eac ion
laye a he o iginal in e ace in ou dep h p o iles. This indi-
ca es ha ins ead o nuclea ion o he p oduc laye a he o ig-
inal in e ace, he new phase(s) o med in he whole olume o
he ilms. In addi ion he compound phases ha e been o med
wi hou he pa icipa ion o olume di usion (acco ding o bulk
di usion da a he bulk di usion pene a ion leng h is abou
2.8 × 10−11 m in pu e Cu a 250 °C o 30 min [30]). F om
Figu e 1 i is clea ly seen ha he a e age composi ions on
bo h sides g adually le eled o and he esul is he o ma ion
o an almos homogeneous laye wi h abou 75 a om % o Cu
and 25 a om % o Au composi ion.
The XRD pa e ns o as deposi ed and annealed samples a e
shown in Figu e 2. The weak e lec ion a 23.7°, belonging o a
supe la ice s uc u e, indica es ha he AuCu3 phase ha is
o med du ing he homogeniza ion p ocess (Figu e 2d) is
pa ially o de ed. This is in ag eemen wi h SNMS da a, indi-
ca ing he p esence o a sligh ly Cu ich AuCu3 phase.
Beils ein J. Nano echnol. 2014, 5, 1491–1500.
1495
Figu e 3: Concen a ion p o iles o Au(10nm)/Cu(25nm) sys em a) as deposi ed and b) annealed a 180 °C o 5 h samples.
The es ima ed g ain sizes, d ( om he ull wid h a hal
maximum o he (111) peaks o Au and Cu by using he
Debye–Sche e o mula [31,32]), a e 15 nm o Au c ys als
and 10 nm in he case o Cu. A e annealing a 180 °C o 5 h
hey dec ease o d = 9 nm and d = 2 nm o Au and Cu, espec -
i ely (Figu e 2b). The g ain size o he newly o med AuCu3
phase (a e 10 h o hea ea men a 180 °C) was es ima ed
om he (111) peaks and 6 nm was ob ained (Figu e 2c).
Rega ding he eliabili y o he g ain sizes es ima ed i is wo hy
o men ion ha calcula ions based on he Debye–Sche e
o mula p o ide unde es ima ed alues o d [32], because o
he ac ha besides ins umen al e ec s and g ain size o he
ac o s (like inhomogeneous s ain and c ys al la ice impe ec-
ions) also con ibu e o he wid h o a di ac ion peak.
In o de o in es iga e he e ec o he indi idual hicknesses o
he ilms, samples wi h hicknesses o 10 and 25 nm o Au and
Cu, espec i ely, we e also annealed unde he same condi ions.
The concen a ion p o iles a e shown in Figu e 3.
I can be seen ha he p ocess is qui e simila o he samples
wi h ilm hicknesses 25 and 50 nm (compa e Figu e 3b o
Figu e 1d) bu he di usion p ocesses de elop as e . Thus,
al eady a e 5 h o annealing a 180 °C he AuCu3 phase has
been o med. The XRD pa e ns shown in Figu e 4 also illus-
a e he o ma ion o his AuCu3 phase. I is di icul o iden-
i y whe he i is o de ed o diso de ed: The expec ed posi ions
o he supe -la ice e lec ions (100) and (110) a e indica ed oo.
In o de o in es iga e he e ec o he o e all composi ion (i.e.,
he e ec o he hickness a io), measu emen s on samples
Au(25nm)/Cu(25nm) and Au(25nm)/Cu(12nm) ha e also been
ca ied ou . I can be seen in Figu e 5 ha al eady a e 5 h a
180 °C a homogeneous laye (o abou he same hickness as he
hickness o he o iginal Au laye ) has been o med wi h a
Figu e 4: XRD θ–2θ pa e ns o Au(10nm)/Cu(25nm) annealed
samples.
50/50 composi ion in he opmos laye in he place o gold. On
he o he hand, he e is a conside able inc ease (up o abou
10%) o he Au composi ion in he cen e o he Cu laye .

Beils ein J. Nano echnol. 2014, 5, 1491–1500.
1496
Figu e 5: Concen a ion p o iles o Au(25nm)/Cu(25nm) sys em a) as deposi ed sample and b) annealed samples.
Figu e 6 shows he θ–2θ XRD pa e ns o Au(25nm)/Cu(25nm)
annealed samples. I can be seen ha e lec ions o bo h AuCu
and AuCu3 diso de ed phases can be obse ed al eady a e
annealing a 180 °C a e 5 h. These esul s a e in line wi h he
p o iles shown in Figu e 5b: A longe annealing imes he
a e age composi ion o he Au inside he Cu ilm g adually
inc eases and a 200 °C a e 20 h he sys em seems o be a mix-
u e o Cu- ich AuCu and AuCu3 phases.
In addi ion, Figu e 7 illus a es ha wi h a p ope choice o he
ilm a io one can a i e a a mix u e o Cu- and Au- ich AuCu
solid solu ions.
The XRD pa e ns shown in Figu e 8 con i m his: Re lec ions
o he diso de ed AuCu solid solu ions can be iden i ied. The
e ical lines co espond o Au1.5xCux and AuxCu1.5x solid solu-
ions.
Figu e 9 shows b igh ield ( op iew) TEM images and
selec ed a ea elec on di ac ion pa e ns o as deposi ed and
hea ea ed ( o 1 h a 160 °C) Au(10nm)/Cu(15nm) bilaye s,
espec i ely. Fo TEM in es iga ions he specimens we e
p epa ed by subsequen magne on spu e ing on monoc ys-
alline sodium chlo ide subs a es a oom empe a u e. A e
he hea ea men he subs a e was dissol ed and he sel -
suppo ing ilm was used in TEM in es iga ions. I can be seen
ha he e is no de ec able change in he g ain size a e he hea
ea men , which is abou 10 nm. The di ac ion pa e n o as
deposi ed sample shows clea e lec ions om Au (Figu e 9b)
and a e annealing addi ional di ac ion peaks a e ound ha
co espond o he o de ed Au3Cu phase (Figu e 9d). In addi ion,
he a ow in Figu e 9c indica es a egion o he o ma ion o he
eac ed laye a ound a g ain bounda y; he indica ed e lec ions
pe ain o his phase.
Figu e 6: XRD θ–2θ pa e ns o Au(25nm)/Cu(25nm) annealed
samples.
Beils ein J. Nano echnol. 2014, 5, 1491–1500.
1497
Figu e 7: Concen a ion p o iles o Au(25nm)/Cu(12nm) sys em a) as deposi ed sample and b) annealed samples.
Figu e 8: XRD θ–2θ pa e ns o Au(25nm)/Cu(12nm) annealed
samples.
Figu e 9: B igh ield ( op iew) TEM images o Au(10nm)/Cu(15nm)
bilaye a) as deposi ed and c) a e 1 h o hea ea men a 160 °C.
The a ow indica es he a ea o o ma ion o a new phase. Selec ed
a ea elec on di ac ion pa e ns o Au(10nm)/Cu(15nm) bilaye b) as
deposi ed and d) a e 1 h o hea ea men a 160 °C.
Discussion
Ou esul s indica e a special way o nuclea ion and g ow h o
homogeneous eac ion p oduc s in AuCu sys em. In acco dance
wi h he esul s o [7] in he same sys em he obse a ions
canno be unde s ood as a plana laye eac ion: No con inuous
eac ion laye o ma ion was obse ed a he o iginal in e ace.
Ins ead homogeniza ion o he ini ial pu e laye s can be cha ac-
e ized wi h a g adual inc ease o he composi ion in he cen e
o he laye s. Fu he mo e, we can conclude ha all hese
Beils ein J. Nano echnol. 2014, 5, 1491–1500.
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Figu e 10: Dependence o he a e age concen a ion o elemen s on he annealing ime a 150 °C in a) Au(25nm)/Cu(50nm), b) Au(25nm)/Cu(25nm)
and c) Au(25nm)/Cu(12nm) sys ems.
phenomena should be he esul o g ain bounda y anspo as
a as he bulk di usion is negligible a such low empe a u es.
Thus he GB di usion ini ia es he nuclea ion o he eac ion
p oduc and sweeps he GBs pe pendicula o he o iginal
su ace and as a esul , an alloyed zone emains behind he
mo ing GBs. The ob ained g ain sizes a o his in e p e a ion
in con as o he g ain bounda y mo ion du ing usual e-c ys al-
liza ion [11,12]. In he la e case, a g ain g ow h should be
obse ed a he same ime as he homogeniza ion. Bu ou
esul s, bo h ob ained om XRD and TEM in es iga ions, show
ha he a e age g ain size ei he emained he same o e en
dec eased a e he hea ea men s. Thus, he o e all cold
homogeniza ion akes place h ough g ain bounda y di usion
induced g ain bounda y mo ion and a solid s a e eac ion
con olled by he in e ace di usion along he newly o med
in e aces. The XRD and TEM esul s con i med he conclu-
sions d awn om he SNMS dep h p o iles: Indeed he o ma-
ion o eac ion laye s ook place, and a e longe annealing
imes e en he supe la ice e lec ions o he o med AuCu3
phase could be de ec ed, indica ing he o de ing o his phase.
I is no ewo hy ha he abo e p ocesses a e in e ace/g ain
bounda y di usion con olled: A e he o ma ion o he new
phase in he GBs, he p ocess akes place h ough he a omic
anspo along he mo ing GB (like in he classical DIGM,
when a solid solu ion is le behind) o along he in e aces o
he new o de ed phase (in his case he o iginal GB is eplaced
by wo new in e aces). In he la e case i is an in e es ing and
open ques ion whe he bo h in e aces mo e o dominan ly only
one o hese shi s: Di ec in si u TEM in es iga ions can help
o cla i y his poin .
Ne e heless, by using he esul s o [24,26] we can es ima e he
eloci y o he in e aces mo ing pe pendicula o he ini ial
g ain bounda y. Acco ding o his model one can assume ha in
hin ilms he g ain size, d, is usually less han he ilm hick-
ness, H. I is plausible o assume a sphe ical g ain s uc u e
(2R0 = d) wi h a δ/2 hick sphe ical shell a he beginning (a
= 0), which g ows wi h ime by , whe e is he cons an
in e ace eloci y. Then he in e nal shell, in which he ini ial
composi ion is c0, has he adius
Assuming ha he in e ace shi s a s only a e , e.g., Au GBs
ha e been illed up o he equilib ium Cu composi ion o he
g owing phase, ce, hen he a e age concen a ion, c, in he
middle o he ilm can be gi en as
(1)
which, neglec ing e ms (δ + 2 )/d on he hi d powe , has he
o m, wi h ce (in Au) = 0.5, ce (in Cu) = 0.25 and c0 = 0:
(2)
A sho imes, he las e m in he las b acke o Equa ion 2 can
be neglec ed leading o a linea ela ion. Figu e 10 shows he
a e age composi ion inside he gold and coppe laye s as he
unc ion o he annealing ime ob ained a e hea ea men s a
150 °C: The i s pa is linea ( he sa u a ion a longe alues
Beils ein J. Nano echnol. 2014, 5, 1491–1500.
1499
Table 1: Calcula ed alues o he eloci y o mo ing in e aces.
Au(25nm)/Cu(50nm) Au(25nm)/Cu(25nm) Au(25nm)/Cu(12nm)
in he Au laye 7 × 10−12 m/s 1.4 × 10−11 m/s 3 × 10−11 m/s
in he Cu laye 6 × 10−14 m/s 2 × 10−13 m/s 5 × 10−13 m/s
is due o ini e size e ec s). F om he linea ini ial pa he
alues o he in e ace eloci y can be ob ained (Table 1).
I can be seen ha he in e ace eloci y is abou wo o de s o
magni ude highe in Au han in Cu: This is plausible i we ake
in o accoun a simila endency in he g ain bounda y sel -di u-
sion coe icien s. In addi ion, ou esul s o e an explana ion
o he linea g ow h kine ics in his egime: I he on eloci y
is cons an , he amoun o he p oduc phase should g ow
linea ly wi h ime and he ac i a ion ene gy ob ained om his
pa should be close o he ac i a ion ene gy o GB/in e ace
di usion [24,26].
Conclusion
I is shown ha a low empe a u es, a which he bulk di u-
sion is ozen, an almos comple e homogeniza ion can ake
place in he Cu/Au hin ilm sys em, leading o he o ma ion o
in e me allic phases. I is illus a ed ha he p ocess is based on
g ain bounda y di usion induced g ain bounda y mo ion and
eac ion laye o ma ion, he p ocess s a s by g ain bounda y
in e di usion and a e he illing-up o g ain bounda ies he
eac ion s a s he e. A e he o ma ion o he eac ion zone
(solid solu ion o o de ed phase) he a omic anspo along he
o iginal GB, o along he newly o med in e aces pe pendic-
ula o he g ain bounda y plane, esul s in he g ow h o he
eac ed ma e ial. Finally, he homogeniza ion inishes when all
he pu e componen s ha e been consumed.
The ini ial pa , in acco dance wi h he well-known ule o
humb decla ing ha he di usion is as e in he componen
wi h lowe mel ing poin , is asymme ic: The p ocess is as e in
he Au laye .
In Au(25nm)/Cu(50nm) samples, acco ding o Figu e 1 and
Figu e 2d, he inal s a e is he o de ed AuCu3 phase.
Dec easing he ilm hicknesses (see he esul s ob ained in
Au(10nm)/Cu(25nm) and shown in Figu e 3b and Figu e 4)
esul s, as expec ed, in he accele a ion o he p ocess.
I is illus a ed ha by changing he hickness a io ei he a mix-
u e o Cu- ich AuCu diso de ed and AuCu3 phases (see
Figu e 5b and Figu e 6 o Au(25nm)/Cu(25nm) sample), o a
mix u e o diso de ed Cu- as well as Au- ich solid solu ions
(see Figu e 7 and Figu e 8) can be p oduced.
By using a simple model, we we e able o es ima e he in e -
ace eloci y in bo h he Cu and Au laye s om he linea
inc ease o he a e age composi ion and, again in acco dance
wi h he abo e ule o humb, i s alue is abou wo o de s o
magni ude la ge in Au (o he o de o 10−11 m/s) han in Cu
(o he o de o 10−13 m/s).
Expe imen al
Au/Cu nanoc ys alline hin ilms we e p epa ed by DC
magne on spu e ing on o (001)-o ien ed Si wa e s wi h na i e
SiO2 laye . The ollowing bilaye samples we e deposi ed:
Au(25nm)/Cu(50nm), Au(25nm)/Cu(25nm), Au(25nm)/
Cu(12nm), Au(10nm)/Cu(25nm) and Au(10nm)/Cu(15nm).
Du ing he deposi ion o me al laye s he Si subs a e was kep
a oom empe a u e and he A base p essu e was se a 0.5 Pa.
The a es o he deposi ion o Au and Cu laye s we e 0.85 nm/s
and 0.5 nm/s, espec i ely.
The samples we e annealed unde acuum (1 × 10−4 Pa) a
empe a u es anging om 160 o 330 °C. The e olu ion o he
in e mixing p ocess was s udied o e a ime be ween 0.5 and
44 h.
The concen a ion p o iles we e measu ed by using a seconda y
neu al mass spec ome e (SPECS INA-X), ha wo ks wi h
noble gas plasma and he bomba ding ion cu en has an
ex emely high la e al homogenei y. The low bomba ding ene -
gies (o he o de o 100 eV) and he homogeneous plasma
p o ile esul in an ou s anding dep h esolu ion (smalle 2 nm).
De ails o he SNMS de ice and he p o ile e alua ion can be
ound elsewhe e [27,28].
The c ys alline s uc u e was examined by means o X- ay
di ac ion in θ–2θ scanning geome y while using Cu Kα adia-
ion (Rigaku Ul ima IV di ac ome e ) and by ansmission
elec on mic oscopy (TEM, JEOL 2000FX-II).
Acknowledgemen s
The esea ch was suppo ed by he TÁMOP-4.2.2.A-11/1/
KONV-2012-0036 p ojec , implemen ed h ough he New
Hunga y De elopmen Plan co- inanced by he Eu opean Social
Fund, and he Eu opean Regional De elopmen Fund. The
au ho s g a e ully acknowledge he suppo o he Hunga ian
Scien i ic Resea ch Fund (OTKA) h ough G an NF 101329.