Optial Studies of the Correlation Between Interfae
Disorder and the Photoluminesene Line Shape
in GaAs/InGaP Quantum Wells
E. Laureto,E. A. Meneses,
InstitutodeFsia\GlebWataghin", UniversidadeEstadualde Campinas,13083-970Campinas-SP,Brazil
W. Carvalho Jr.,A. A. Bernussi, E. Ribeiro,
Laboratorio Naionalde LuzSnrotron,13088-902 Campinas-SP, Brazil
E. C. F. da Silva,
Institutode FsiadaUniversidadede S~aoPaulo,05508-900, S~ao Paulo-SP,Brazil
and J. B. B. de Oliveira
Departamento deFsia,UniversidadeEstadual Paulista,
Av. Eng. LuisEdmundoC.Coube,s/n,17033-360,Bauru -SP,Brazil
Reeivedon23April,2001
Photoluminesene(PL)andexitationPLmeasurementshavebeenperformedatdierent
temper-atures inanumberoflattie-mathedGaAs/In0:49Ga0:51Pquantumwells,where theutuations
ofthepotentialenergyareomparablewiththethermalenergyofthephotoreatedarriers. Two
samples with dierent wellwidths allow to observe aseries of anomalouseets in their optial
response. Theobservedeets arerelated tothe disorder inthe interfae,haraterizing
utua-tionsintheonnementpotentialenergy. Itisproposedthatthearrierrelaxationproessesour
eitherattheloalminimaorattheabsoluteminimumoftheonnementpotential,dependingon
theratioofthethermalenergyandthemagnitudeofthepotentialutuations.
I Introdution
Semiondutor heterostrutures like quantum wells
(QW), quantum wires and quantum dots, have been
extensively investigated in the last deade, sine they
are extremely important for the high performane of
the eletroni and optoeletroni devies [1℄. In these
systemsthe exitoni properties are strengthened due
to thearriersonnementpotential[2,3℄. IntheQW
ase, variations in the quantum well width and alloy
hemial ompositions ause loalized utuations in
this potential. Suh utuations modulate the loal
proleofthevaleneandondutionbands,then
aet-ingtheoptialpropertiesofsemiondutor
heterostru-tures. Optialspetrosopysuhasphotoluminesene
(PL) and exitation PL (PLE) are powerful tools for
investigatingtheseproperties. Theexitonsreatedby
optial absorption relax to lowerenergy states before
radiative reombination. Therefore PL and PLE
to-getherangiveinformationabouttheonnement
po-tential utuationsthroughthepeakposition andthe
lineshapeofexitonispetra.
Inthis work westudy theinuene of the
utua-erties of the exitons in GaInP/GaAs quantum wells.
Similar results were observed by Grassi Alessi et al.
[3℄forInGaAs/GaAsquantumwells. Wehaveusedtwo
single quantum well samples with the same growing
onditions,but dierentnominal QWwidths. PLand
PLEspetraweretakenat2Kandasafuntionof
tem-peratureupto100K.Weshowthattheshapeandthe
temperature dependene of the PLlines are governed
by the exiton energy relaxation. A qualitative
u-tuation potential model is proposed where the arrier
relaxationproessesoureitherattheloalminimaor
attheabsoluteminimumoftheonnementpotential,
depending onthe ratioof the thermalenergy and the
magnitudeofthepotentialutuations.
II Experimental Results
The samples used in this study were grown on GaAs
(100)-orientedsubstratesbymetal-organihemial
va-pordeposition(MOCVD)at580 o
C.Thesamples
on-sist of a GaAs buer, a 500
A GaInP barrier, a
GaAs layerof width L
QW
and another500
GaAs/GaInPinterfaeaGaPlayerwasgrownduring1
se. Thesamplesarenominally undopedandthe
mea-sured lattie mismath is lessthan 10 3
. The optial
measurementsweredoneinaHeimmersionryostat
us-ingaTi-sapphirelaserasexitationsoure(withenergy
xed in 1.71 eVfor PLmeasurements)and exitation
intensityof0.5W/m 2
forallspetra. The
lumines-enewasdetetedandanalyzedbyastandardlok-in
tehnique. ThePLEspetraweredeteted in the low
energysideofPLurveinordertooptimizethee1-hh1
transition line shape. InTableI welistrelevant
infor-mationaboutthetwosamples,fromnowonreferredto
as sampleAandsampleB.
Sample L QW ( A) FWHM PLE FWHM PL SS
A 80 3.8 3.6 7.4
B 30 11.5 8.4 19.4
TABLEI:QWwidth(L
QW
),FWHMforPLEandPL,
and themeasuredStokesShift(SS)forsamplesAand
B.ExperimentalvaluesaregiveninmeV(unertainties
ofabout5%)andweretakenat2K.
0.00 0.03 0.06 0.09 0.12
PL
PLE
e1-lh1
e1-hh1
(a)
P
L
In
te
n
s
ity
(
a
. u
.)
Relative energy (eV)
0.00 0.03 0.06 0.09 0.12
e1-lh1
e1-hh1
(b)
Figure1. PL(dottedline)andPLE(ontinuousline)
spe-trafor samples(a)Aand (b)B.Measurementsweredone
at2K.Theenergysaleisredenedsothatthezero
orre-spondstothePLpeakenergyforeahsample.
In Fig. 1 we show the PL (dotted line) and PLE
(ontinuous line) spetra at 2 K for samples A [Fig.
1(a)℄and B[Fig. 1(b)℄. Inthis guretheenergysale
wasadjustedsothatthezeroenergyorrespondstothe
positionoftheintensitymaximumofthePLline. The
objetiveofthishoieistoemphasizethedierene
be-tweenthepositionsof theenergymaximumofPLand
PLEspetra(Stokesshift,SS)andthefullwidthathalf
maximumofPLandPLEspetra(FWHM).ThePLE
spetra show the absorption resonanes of the heavy
hole(e1-hh1)and light hole(e1-lh1)exitons. Wean
peaksis largerin sampleBthanin sampleA,in
qual-itative agreement with theenergy levels asafuntion
ofL
QW
, alulatedbyaneetive-massmodel(E '
1/L
QW
)[4℄. Weanalsoobservethattheratiobetween
thee1-lh1 ande1-hh1 peakintensitiesin PLE spetra
is inverted in Fig. 1(a) and 1(b). Similar eet was
observedbyJahnet al[5℄. Theyassoiatedthe
redu-tion of PLE resonaneintensity relativeto the heavy
hole(e1-hh1)to strongloalization ofreombining
ex-itons in the potentialutuations. The FWHM
PLE ,
FWHM
PL
and the SSvalues obtainedfrom Fig.1 are
presentedinTableI.WeanseeinthisTablethat
sam-ple B presents FWHM
PLE
, FWHM
PL
and SS values
greaterthantheorrespondingonesforsampleA.
0
20
40
60
80
1.530
1.533
1.536
1.539
1.542
1.545
1.548
0
20
40
60
80
1.508
1.510
1.512
1.514
GaAs bandgap
E
ner
gy
(
eV)
Temperature (K)
(a)
PLE
PL
0
20
40
60
80
1.600
1.604
1.608
1.612
1.616
1.620
1.624
1.628
Peak Energy
(eV)
SS
(b)
Temperature (K)
Figure2. PLE(squares)andPL(irles)peakenergyasa
funtionof the temperaturefor samples (a)A and (b)B.
Continuouslinesareonlyaguidetotheeyes. Insetpresents
theGaAs bandgapdependenewiththetemperature.
In Fig. 2weshow the variationwith the
temper-atureof the energypeak position for PL(irles) and
PLE (squares) spetra for sample A [Fig. 2(a)℄ and
sample B [Fig. 2(b)℄. This provides the temperature
dependene of the Stokes shift (SS) as indiated at
10 K by the double-headed arrowin Fig. 2(b). The
PLE peak energy dereases with inreasing
tempera-tureroughlyfollowingtheGaAs energy band gap
[in-setin Fig. 2(a)℄. Onthe ontrary,thedependene on
TofPLpeakenergyexhibitsananomalousbehaviour,
morepronouned in sample Bwhere thebehaviour is
sigmoid,nonmonotoni.
InFig. 3weplotthe dependene ofFWHM
PL on
temperatureforsamplesAandB.Weobservethatfor
sampleAtheFWHM
PL
inreasesfastlywith
tempera-tureinrease. Dierently, forsample Bthe FWHM
0
10
20
30
40
3
4
5
6
7
8
FW
HM
PL
(m
eV)
(a)
0
10
20
30
40
8
9
10
11
12
13
(b)
Temperature (K)
Figure3. PLFWHM asafuntionofthetemperaturefor
samples(a)Aand(b)B.
III Disussion
We disuss the experimental results presented in the
previoussetionbasedonaqualitativemodelthattakes
intoaounttheinterationofthephotogenerated
ar-rierswiththeutuationsoftheonnementpotential.
TheexitonenergyleveldependsonL
QW
aswellason
theQWdepth,parametersthatdenetheonnement
potential in the growth diretion (z axis). However,
variationsin L
QW
and in thehemialomposition of
the barrier material along the plane perpendiular to
thegrowthdiretion (xyplane)leadtoutuations of
the onnementpotential along this plane. Sine the
variations in the L
QW
and in the hemial
omposi-tionarerandom, theutuationmagnitudeis
statisti-ally distributed. Suharandompotentialutuation
isskethedinFig. 4. Theexitonenergylevelisloally
dependentonthefullonnementpotential. Sinethe
PLEspetrumreetstheabsorptionproess, thereis
adiretrelationshipbetweenthelinewidthofthePLE
resonanesandtheintensityoftheonnement
poten-tialutuations[3℄.
Before reombining, the photogenerated exitons
loose their exessenergy and in this proess theyfeel
the variationof the onnementpotential. Aording
totheenergyexessandthemagnitudeofthepotential
variation,theexitonswillrelaxeithertoloalminima
orto theabsoluteminimumoftheonnement
poten-tial via phonon emission before radiative
reombina-tion. Thus, the FWHM
PL
of PL spetra reets the
distributionof exitonsin theenergy statesgenerated
bythepotentialutuations. Theenergydierene
be-tweenPLEand PLpeaks, theStokesshift, represents
then the average of the dierene between theenergy
ofphotoreatedand minimumloalizedexitons. The
highertheSS,thehighertheonnementpotential
u-tuationsmagnitude.
Theexitondynamisisdeterminedbytheamount
al potential. With the inrease of kT the exiton in
a given potential minimum anjump to adeeper one
or eventuallyreombine from the higher energy level.
The relaxationto deeper minima auses the
displae-mentofthePLpeaktowardslowerenergyvalues. Suh
adisplaementisnotobservedinsampleA[Fig. 2(a)℄,
butisevidentinsampleB[Fig. 2(b)℄,whihshowsthat
theexitonrelaxationisinompleteinsampleBforlow
temperatures. Infat,thepotentialutuation
magni-tude should be higher in sample B than in sample A
duetothethinnerL
QW
. AlsoinFig. 2weanseethat
theSSdereasesinthehighertemperatureregion. This
isexpeted sinethetwo-dimensionaldensityofstates
ishigherthantheloalizeddensityofstatesgenerated
bythepotentialutuations. Inthesameway,wean
see inFig. 3thattheFWHM
PL
presentsdierent
be-haviour for samples A and B. In the seond one the
urvegoesthroughashallowminimuminthelow
tem-peratureregion(T.20K) beforeitstartsto inrease
for inreasing temperature, as observedfor sample A.
The omparison of the FWHM
PL
behaviour then
re-infores the previous onlusion that in samples with
smaller L
QW
exitons are more sensitive to the
inter-fae disorder. Finally,theredutionoftheintensityof
the heavy hole (e1-hh1) PLE resonanefor sample B
relativeto sample A, asshownin Fig. 1(b), is also a
diret onsequeneof thelarger L
QW
of sample A, in
agreementwithJahnetal. interpretation[5℄.
Figure4. SkethoftheonnementpotentialV
onf along
the xyplane of the QW. Openirles represent free
exi-tons. Arrowsindiatetherelaxationproessestotheloal
orabsolutepotentialminima. Fullirlesrepresentexitons
stillloalized attheseminima.
IV Summary
In this work we report a omparative study of
op-tial properties of two InGaP/GaAs quantum wells
with same growth onditionsbut dierent L
QW . The
peak position of PL and PLE, and the FWHM
PLE
and FWHM
PL
asa funtion of temperature are
pre-sented. The dierent behaviour of these parameters
for the two samplesan be desribed by aqualitative
photogener-related tothe interfaedisorder. The onnement
po-tentialutuations present minima that apture
exi-tons, and preventtheomplete relaxationof the
exi-toni population. Fromtheglobalanalysisof thedata
we see that anomalous temperature dependenies are
more pronounedin the samplewith smaller L
QW , in
whih exitonsare more sensitiveto the interfae
dis-order.
Aknowledgements:
The authors gratefully aknowledge nanial
sup-portfrom FAPESP,CAPESandCNPq.
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