On the Morphology of Films Grown by
Droplet-Assisted Moleular Beam Epitaxy
T. E.Lamas and A. A. Quivy
Institutode FsiadaUniversidade deS~aoPaulo,Laboratorio deNovosMateriais Semiondutores
C.P.66318, 05315-970S~aoPaulo,SP,Brazil
Reeivedon23April,2001
AtomiForemirosopywasusedtoinvestigatethemorphologyofGaAs(001)layersdepositedby
moleularbeamepitaxyusinganononventionalgrowthmethodthatinvolvesthesupplyofafew
monolayersofgallium (thearseniellisshuttered)followedbytheannealingofthesurfaeunder
anarseniux(thegalliumellisshuttered).Whenthesilionshutterisopenedandlosedtogether
withthegalliumone,thispartiulargrowthmodeallows thesilionatomstobeinorporatedinto
thearsenisitesandp-typeGaAs(001)layersanbeobtained. Severalsetsofsamplesweregrown
and analyzed in order to better understand the mirosopi growth mehanisms of this kind of
layers and minimizethe numberand size of the strutural defets that are harateristi of this
peuliartehnique.
I Introdution
Sine the advent of moleular beam epitaxy (MBE),
beryllium (Be) has been ommonly used to produe
p-type GaAs layers. However, this element has
sev-eral drawbaks[1℄ (large diusion oeÆient,
segrega-tion at high onentration, large memory eet, low
purity, high toxiity) that stimulated the ommunity
to substitute it. Solid arbon hasproven its
suitabil-ity,usuallyprovidingdopinglevelsupto10 20
m 3
and
enablingabruptdopingproles. Anotherattrative
op-tionistoexploittheamphoteriharateristiofsilion
(Si), with the hugeadvantagethat bothn and p-type
GaAs layers ould be obtained in the same struture
byusing asingledopant. Si istheusual dopantfor
n-typeGaAs(001)layers,butithasalreadybeen
suess-fullyusedtoproduep-typeGaAslayersonhigh-index
GaAs(N11) substrates(N=1,2,3) where,depending on
the growthonditions (mainlythe substrate
tempera-ture andV/III ux ratio), theSi atoms anenter the
gallium (Ga)or arseni (As)sites, yielding a n- or
p-type GaAs layer, respetively.[2, 3℄ In a reent work,
Quivy et al. [4℄ reported a new growth method that
produes p-type GaAs layers using Si as the dopant
on aGaAs(001)substrate whih is ommonlyused to
produemiro- andopto-eletronidevies. This new
dopingtehniqueisverypromisingbeausep-type
lay-ersanbeobtainedundernormaluxonditionsjustby
usingadierentsequeneoftheGa,AsandSishutters.
The main dierene with usual growth onditions is
withoutanyAs(theAsshutteriskeptlosed)andthen
thesurfaeisannealedunderanAsux(theGaandSi
shutters are losed) to build the GaAs:Silayers. This
sequeneanberepeated hundredsof timesto obtain
thikGaAs:Si layerswith ap harater. Forinstane,
a0.5m-thikGaAslayergrownat520 Æ
Cusing180
se-quenesof10MLsofGa annealedduring15s underan
Asux showedaholedensityof1.210 18
m 3
fora
nominalSi onentration of4.0 10 18
m 3
.
Atomi-fore mirosopy (AFM) measurements revealed that
largestrutural defets were present at the surfae of
the lm and were most probably at the origin of the
lowarriermobility.
Inthepresentwork,wereportonaompleteAFM
investigation of thesurfaeof suh samples asa
fun-tion of the relevant growth parameters. This kind of
studyis fundamental to understand howthese defets
are formed, in order to be able to derease their size
and onentration and to improve the strutural and
eletrialpropertiesofthelayers.
II Experimental details
Inorderto betterunderstand thestrutural evolution
ofthesampledesribedabove,asetofsixsampleswas
growntoinvestigatethemorphologyofthelmat
dif-ferentmomentsofthegrowthandunderdierent
on-ditions. All the samples were grown in a Mod. Gen
II MBE systemfrom Varian on topof semi-insulating
GaAs(001)substratesfromSumitomo.Intherst
any supplyofAs toobservehowthe initialdeposition
of Ga atoms proeeds in the absene of As. The
se-ond sample was idential to therst one, exept that
As was supplied during 30 s after the Ga deposition
in order to analyze the surfae after a single growth
yle. The third sample onsisted of a 0.25m-thik
GaAs buergrownat 515 Æ
Cobtainedbyrepeating89
timesthesequeneofsample 2. Samples4and5were
idential to samples 1 and 3, respetively, exept by
the fat that they were grown at 300 Æ
C. Finally, the
sixth sample was a 0.25m-thik GaAs buer grown
at 515 Æ
C using 890 sequenes onsisting of 1 ML of
Ga atoms exposed during 4 s to the As ux. Before
the growthof anyof these strutures, a 0.20m-thik
GaAslayerwasdepositedat 570 Æ
Cusingonventional
growthonditionstoprovidethesamesmoothsurfae
toallthesamples. Aswewereonlyonernedwiththe
morphologialharateristisofthesamples,thelayers
were undoped (the absene of Si has no inuene on
the growth mehanisms, as will be heked later,
be-ause the dopant typially representsless than 0.01%
of the total numberof surfae atoms). The reetion
high-energy eletrondiration (RHEED)systemwas
usedtomonitorthegrowthinsituandtodeterminethe
ux of Gaand As atomsthat were of theorder of5.9
and 11.7 10 14
atomsm 2
s 1
, respetively, for all
thesamples. Thesurfaeof thelmswasanalyzedat
ambientonditions with a Nanosope IIIa AFM from
DigitalInstrumentsoperatingin ontat mode with a
sharpenedsilion-nitridetip.
III Results and disussion
Fig. 1 shows the topography of eah sample. In Fig.
1a, one an see that the deposition of 10 MLs of Ga
atoms withoutanyinident As proeeds with the
for-mationof slightly-oval Ga dropletsoriented alongthe
[1-10℄diretion. RHEEDstudiesatuallydemonstrated
thattherstGalayerequallyoversthewholesurfae,
leadingtotheknownfour-foldpatterntypialofa
Ga-rih surfae,whereastherestoftheGa materialis
a-umulatedin dropletsspreadalloverthesurfae.[5, 6℄
For a given amount of material, the size and density
of these dropletsare a funtion of theadatom
mobil-ity whih mainly depends, in our partiular ase, on
the substrate temperature, as will be onrmed later.
Whenthe As ux issupplied on the Ga droplets,the
speular-beam intensityof the RHEEDpattern shows
strong osillations that are related to the formation
of two-dimensional (2D) GaAs layers. Indeed, when
theAsmoleulesimpinge ontheGa-rihsurfae,their
stikingoeÆientis maximum(0.5for As
4
)andthus
2DGaAsislandsarenuleatedonthesurfaeandmerge
plete,theGadropletsprovideafreshlayerofGaatoms
thatwillreatfurtherwiththenewinomingAsatoms.
Eahnew2DGaAslayergivesrisetoanintensity
osil-lationof thespeularbeamthatkeepsosillatinguntil
all the Ga material from thedropletshasreated.
However,weanseefrom Fig. 1b that thenewGaAs
layers are not ompletely smooth, and strutural
fea-tures,alsoorientedalongthe[1-10℄diretion,arenow
Figure 1. 2x2m 2
AFM images of the sixsamples of the
set. a) 10MLsofGagrownat 515 Æ
Cwithoutanyinident
Asux;b)10MLsofGagrownat515 Æ
Cwithoutany
ini-dent Asand then exposed toAs; )a0.25m-thik GaAs
layergrownusing89timesthesequenedesribedinb);d)
samesampleasa)butgrownat300 Æ
C;e)samesampleas
)butgrownat300 Æ
C;f)a0.25m-thikGaAslayergrown
at 515 Æ
Cby repeating890times thedepositionof 1MLof
GawithoutanyinidentAsmoleulesfollowedbythe
expo-sureofthesurfaetotheAsux. Thetotalgreysaleand
thetypialheightofthefeaturesobservedineahimageare
respetively120and29nmina),50and13nminb),80and
17nmin),50and13nmind),150and45nmine)and20
and2nminf).
presentwheretheGadropletswerepreviouslyloated.
These defetsresultfrom theassoiationoftwo
dier-enteets: rst,thereisareationbetweenthe
metal-li Ga of the droplets and the As moleules that fall
diretly on them. Sine the growth rate of the GaAs
havetodiuserstthroughthewhole thiknessofthe
Ga droplets), there existsan aumulationof metalli
Ga in theentralregionofthedropletswherethe
se-ond phenomenon, the melt-bak ething of the GaAs
buer layer, takesplae. This kind of ething ours
whenevermetalli Ga isdeposited onaGaAs surfae,
anditseÆieny(i.e. ethingrate)mainlydependson
thesubstrate temperature.[7℄Bothphenomena, ating
togetherduringtheannealingoftheGadropletsunder
the As ux, are responsible for the peuliar shape of
thedefets. Whena0.25m-thikGaAs layerisgrown
using 89 suh sequenes, thesurfaeof thelm shows
large ovaldefets with small raters inside (Fig. 1).
Thiskindofdefets istypialofGa-rihgrowth
ondi-tions that areknownto generateovaldefets oriented
alongthe[1-10℄diretion,whih isonsistentwithour
growthmethod andAFMresults. Thesmall apparent
ratersaremostprobablyduetothemelt-bakething
of the underneath GaAs layersby the Ga dropletsof
the last ylesof thegrowth. Themorphology ofthis
thikGaAslayerisverysimilartotheoneofthethik
p-typeSi-doped GaAs(001) layergrownin ref. [4℄ by
thesamemethod, onrmingthattheSiatomsdonot
inuene the growthmehanisms andthe morphology
ofthelm.
Sinetheroughness ofthesampleismainly dueto
thepreseneof Gadroplets,aredutionofthesurfae
orrugationheightwouldbeexpetedwhensmaller(or
no)Gadropletsarepresentonthesurfae. Thatanbe
ahieved bydepositing asmaller numberof Ga layers
peryleorbyadoptingalowersubstratetemperature
during Ga deposition. Atrst glane, alowergrowth
temperature has several advantages: the Ga droplets
are more homogeneous, smaller and more numerous
(Fig. 1d),duetothesmallerdiusionlengthoftheGa
adatoms. Inaddition,themelt-bakethingisless
eÆ-ientatlowertemperature,dereasingonsiderablythe
depthoftheraters. Unfortunately,whentheAsuxis
allowedontheGadropletsatsuhalowtemperature,
the growth of the GaAs layers is veryrough and the
total orrugation height is larger than at higher
tem-perature(Fig. 1e). Asaonsequenethe best wayto
redue theroughness ofthelmisusingalarger
num-berof ylesinvolvingthedeposition (at intermediate
temperature)ofasmallernumberofGalayerswithout
any inident As. The growth at 515 Æ
C of a
0.25m-thik GaAs layer onsisting of 445 yles of 2MLs of
Ga exposed to As during 8s showed that the size of
the oval defets and raters were muh smaller. The
limitofthis growthmethod islearlyahievedwhena
single Ga layer is deposited and then annealed in the
presene of the As ux. In this extreme ase, no Ga
dropletsare formed beausethe rst(and unique)Ga
layer is homogeneously spread over the whole sample
and reats with the As-rih surfae, as onrmed by
alear four-fold pattern typial of at Ga-rih
ondi-tions. Sinetherearenodropletsonthesurfaeatany
stepofthegrowthyle,thereare noovaldefets and
no melt-bak ething responsible for the formation of
the raters. In these onditions the roughness of the
surfaeisminimum,asanbeseenin Fig. 1f.
IV Conlusion
WepresentedAFMresultsaboutthesurfae
morphol-ogy of samples obtained by droplet-assisted MBE, a
growth tehnique that was reently employed to
pro-due p-typeGaAs(001) layersusing Si as thedopant.
Attheinitialstage,thedepositionofseveralMLsofGa
withoutanyinidentAsuxproeedswiththe
forma-tionof Ga dropletson thesurfae. WhentheAs ux
is admitted, the Ga droplets reat with the inoming
Asmoleulesandform2DGaAslayers,leavingbehind
them strutural defets and small raters due to the
melt-bak ething of the underneath GaAs layers by
the Ga droplets. Thik GaAs layers an be obtained
by repeatingthis sequene, but the nal roughness of
thelmwasminimized whenasingleGalayerwas
de-positedatatimeandthenannealedundertheAsux.
The optimization of the strutural properties of suh
layers is very important to the devie industry in
or-derto beableto obtainbothnandp-typeGaAs(001)
layerswithasingleelement(Si)usingthesamegrowth
onditionsbut dierentshutter sequenes.
Aknowledgments
Wewouldliketo thankFAPESP fornanial
sup-port (grants99/10510-7, 99/01225-7and 98/14489-0)
andthe\LaboratoriodelmesnosdoIFUSP"forthe
SPMfaility(FAPESPpro. 95/05651-0).
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