Table Of ContentArad-VoskandSa'arNanoscaleResearchLetters2014,9:47
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NANO EXPRESS Open Access
Radiative and nonradiative relaxation phenomena
in hydrogen- and oxygen-terminated porous
silicon
Neta Arad-Vosk and Amir Sa'ar*
Abstract
Using time-resolved photoluminescence spectroscopy over a wide range of temperatures, we were able to
probe both radiative and nonradiative relaxation processes in luminescent porous silicon. By comparing the
photoluminescencedecaytimesfromfreshlypreparedandoxidizedporoussilicon,weshowthatradiativeprocesses
shouldbelinkedwithquantumconfinementinsmallSinanocrystallitesandarenotaffectedbyoxidation.Incontrast,
nonradiativerelaxationprocessesareassociatedwiththestateofoxidationwhereslowerrelaxationtimescharacterize
hydrogen-terminatedporoussilicon.Theseresultsareinagoodagreementwiththeextendedvibronmodelforsmall
Si nanocrystallites.
Keywords: Porous silicon; Photoluminescence; Quantum confinement; Surface chemistry; Nonradiative processes;
The vibron model
PACS: 78.55.Mb; 78.67.Rb; 78.47.jd
Background nanocrystallites located in the PSi matrix [24]. Experi-
The efficient room-temperature visible photolumines- mental evidencessupporting thismodel includea shiftof
cence (PL) from porous silicon (PSi) has attracted much the energy bandgap with size [1-3,25,26], resonant PL at
attention in recent years, mainly due to open questions lowtemperatures[27-29],andPLdecaytimespectroscopy
and controversies concerning the mechanism responsible [1,2,27].However,theQCmodelcannotaccountforother
forthePLemission[1-7].Inaddition,numerousPSi-based experimental observations, mainly the dependence of the
deviceshavingpotentialapplicationsindiversefieldssuch PL on surface treatments [30-34]. Several reports pro-
as photonics, optoelectronics, and photovoltaics, were posed a more complex picture of QC combined with
proposed and investigated [8-15]. In particular, PSi has localizationofchargedcarriersatthesurfaceofthenano-
beenconsidered as anattractive candidate forsensingap- crystals[35-38],particularlytheworkofWolkinetal.[36]
plications [16-21] where its large surface area can be whodemonstratedastrongdependenceofthePLonsur-
exploited for enhancing the sensitivity to surface interac- face chemistry. This group has shown that while in fresh
tions. In such a sensor, the PL emitted from PSi can be PSi the PL peak energy depends on the size of the nano-
used as a transducer that converts the chemical inter- crystals(i.e.,followstheQCmodel),theQCmodelcannot
action into a measurable optical signal. For example, PL accountforthelimitedPLshiftobservedforoxidizedPSi.
quenching due to surface interactions withvarious chem- By introducing surface traps into the model, the behavior
ical species has been utilized for developing various bio- ofthePLpeakenergyforoxidizedPSicouldbeexplained
photonicsensors[16,22,23]. [36]. Otherreports have shown thatbothQC and surface
Originally, the efficient PL from PSi was attributed chemistry shape the PL characteristics [37,38]. The ex-
to quantum confinement (QC) of charged carriers in Si tended vibron (EV) model provides a simple explanation
to the mutual role of surface chemistry and QC [39-41].
According to this model, QC affects radiative processes
*Correspondence:[email protected]
RacahInstituteofPhysicsandtheHarveyM.KrugerFamilyCenterfor that are less sensitive to the state of the surface, while
NanoscienceandNanotechnology,TheHebrewUniversityofJerusalem, nonradiativerelaxationprocessesaremostlyinfluencedby
Jerusalem91904,Israel
©2014Arad-VoskandSa'ar;licenseeSpringer.ThisisanopenaccessarticledistributedunderthetermsoftheCreative
CommonsAttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,and
reproductioninanymedium,providedtheoriginalworkisproperlycited.
Arad-VoskandSa'arNanoscaleResearchLetters2014,9:47 Page2of6
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the surface chemistry. However, both QC and surface bonds [45]). For continuous wave (cw) PL and TR-PL
chemistrycontributetotheefficientPLfromPSi. measurements, the samples were excited by Ar+ ion laser
In this work, we investigate the role of surface chemis- operating at 488 nm while the PL signal was dispersed by
try,particularly therelationshipbetween thestateofoxi- a 1/4-m monochromator and detected by a photomulti-
dation and the PL characteristics of luminescent PSi plier tube. For time-resolved measurements, the laser
samples. We examine the contribution of radiative and beam was modulated by an acousto-optical modulator
nonradiative decay processes to the overall PL lifetime driven by a fast pulse generator, while the PL signal has
and the sensitivity of these processes to surface treat- beenanalyzedbyagatedphotoncountingsystem.During
ments. Furthermore, we examine the EV model by com- PL measurements, the samples were kept under vacuum,
paring radiative and nonradiative decay times of freshly inacontinuous-flowliquidheliumopticalcryostatthatal-
prepared hydrogen-terminated PSi (H–PSi), with those lows temperature control from approximately 6 K up to
of oxidized PSi (O–PSi). This allows to experimentally roomtemperature.
test the hypothesis that radiative processes are not sensi-
tive to surface treatments while nonradiative processes Results
are. Utilizing temperature-dependent, time-resolved PL IR absorbance spectra of H–PSi (red line) and O–PSi
(TR-PL) spectroscopy [42], we extend our previous work (black line) are presented in Figure 1.Si-OH and Si-O-Si
on silicon nanocrystals embedded in SiO matrices and vibrational bands at 875 cm−1 and 1,065 to 1,150 cm−1
2
silicon nanowires [37,41,43,44] to PSi, as this system respectively [46-48], which indicate the presence of oxy-
allows a modification of the surface chemistry by sim- gen in the films, clearly increase after 6 days of exposure
ple means and tracing quite accurately the state of the to ambient atmosphere. The Si-hydrogen vibrations
surface. [46,47] at 906 cm−1 and 2,112 cm−1 did not fade away
after 6 days of exposure, and typically disappear only
Methods after several weeks of exposure to air [49]. Hence, these
PSi samples were prepared by electrochemical etching of IR absorbance spectra confirm the modification of the
p-type (10 to 30 Ω⋅cm) silicon wafers under standard PSi'ssurfaceduringtheexposuretoair.
dark anodization conditions [25,26]. A 1:1 mixed solu- The cw-PL spectrum ofH-PSi,measured atroomtemp-
tion of aqueous hydrofluoric (HF) acid (49%) and etha- erature with a PL maximum at approximately 1.80 eV
nol was used as the electrolyte at a current density of (about690nm)andafullwidthathalfmaximum(FWHM)
70 mA cm−2 for 200 s to yield a PSi layer of approxi- of about 0.4 eV, is presented at the inset to Figure 2. A
mately 9.5 μm (measured by scanning electron micro- similar spectrum with a slight blue shift of the PL max-
scope) with average pore size of a few nanometers [25]. imumto1.85eV(approximately670nm)hasbeenmea-
The freshly prepared PSi is terminated by Si-hydrogen sured for O-PSi, in agreement with results obtained in
bonds that are known to be quite unstable under ambi- references [50-52]. In order to probe both radiative and
ent conditions. These bonds are subsequently replaced nonradiative relaxation processes, the PL decay curves
by the more stable Si-oxygen bonds upon exposure to
air. Hence, in order to investigate the optical properties
of H–PSi, we introduced the freshly prepared samples 10
into a vacuum optical cryostat and kept them under Si-OH Si-O-Si H-PSi
O-PSi
vacuum conditions for the entire experiment. Oxygen-
8
terminated O–PSi was obtained after taking the same PSi Si-H2
sampleoutofthevacuumcryostatandlettingitageunder u.) Si-H
ambientconditionsfor6days.ThestateofthePSisurface e (a. 6
(havingeitherSi-OorSi-Hbonds)wasmonitoredbyFou- nc
a
rier transform infrared (FTIR) spectroscopy. To eliminate orb 4
interference phenomena, thinner PSi samples were pre- s
b
A
pared for these measurements (10 s of anodization under
2
the same conditions, resulting in approximately 450 nm OSi-H
n
thick PSi film). Bruker's Vertex-V70 vacuum FTIR spec-
trometer (Bruker Optik GmbH, Ettlingen, Germany), 0
1000 1500 2000 2500
equipped with a mercury-cadmium-telluride (MCT) pho-
Wavenumber (cm-1)
tovoltaic detector, has been exploited for these experi-
ments. Measurements were performed in the grazing Figure1FTIRspectra.InfraredabsorptionspectraofH-PSi(freshly
preparedPSi)andO-PSi(thesamesampleafteraging).MainSi-H,
angle reflection mode, at an incidence angle of 65° and
Si-OH,andSi-Ovibrationmodesaremarked.
under p-polarization (to enhance the sensitivity to surface
Arad-VoskandSa'arNanoscaleResearchLetters2014,9:47 Page3of6
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1
d)
e
z
ali
m
or
n
y ( 0.1
sit
n
e
nt
L i
P
0.01
0 1 2 3
Time (ms)
Figure2PLdecaycurves.ThePLdecaycurvesofH-PSimeasured
ataphotonenergyof2.03eV(610nm)andatvarioustemperatures.
Thesolidlinespresentthebestfittoastretchedexponential
function(Equation1).InsetshowsthePLspectrumofH-PSi
measuredatroomtemperature.
were measured at several photon energies and at tem-
peraturesranging from6Kuptoroomtemperature.As
will be discussed and explained later on, at room tem-
perature radiative processes dominate over nonradiative
processesandtherefore,forthestudyofnonradiativepro-
Figure3PLlifetimeandintegratedPL.(a)Arrheniusplotofthe
cesses,itisnecessarytomeasurethePLdecayatlowtem-
PLlifetime(onasemi-logarithmicscale)asafunctionof1/T,ata
peratures.TypicalPLdecaycurves,measuredforH-PSiat photonenergyof2.03eV(610nm)forH-PSi(redcircles)andO-PSi
a photon energy of 2.03 eV (610 nm) and at various tem- (blacksquares).Thesolidlinesrepresentthebestfittothesinglet-
peratures, are presented in Figure 2. A pronounced de- tripletmodelofEq.2.(b)ArrheniusplotoftheintegratedPL.Inset
showstheschematicsoftheexcitonictwo-levelmodelwiththe
pendence of the PL decay on temperature can clearly be
upperexcitonicsinglet-tripletstateandtheground(noexciton)
seen,similartotheresultsofothergroups[1,2,53].Asthe
state.Thearrowsrepresenttheallowed(fromthesinglet)andthe
temperaturedecreases,thePLdecaytimebecomessignifi- forbidden(fromthetriplet)opticaltransitions.
cantly longer (by two ordersof magnitude over the entire
range of measured temperatures). Notice that the tem-
poral behavior of the PL cannot be described by a simple decay times at low temperatures. This unique behavior
exponentialdecayfunction(seethesemi-logarithmicscale of the PL decay has been attributed to a splitting of the
ofFigure 2)and is typicallyfitted toa stretched exponen- excitonic ground state (i.e., the photo-excited electron–
tial decay function [54,55]. This nonexponential decay is hole pair) due to the Coulomb exchange interaction,
commontodisorderedsystemsandhasbeenattributedto giving rise to a lower triplet level (S = 1) and an upper
a dispersive diffusion of the photo-excited carriers [54]. singlet level (S = 0) [53] (see inset to Figure 3b). The
Thesolidlines inFigure 2representthe bestfit of the PL splitting is further enhanced by confinement of the
decaycurvestoastretchedexponentialfunction,givenby charged carries in small nanocrystals, giving rise to a
h i larger excitonic overlap. Optical transitions from the
IðtÞ¼I exp −ðt=τÞβ ð1Þ lower triplet and the upper singlet states are forbidden
0
and allowed respectively, due to spin selection rules
where τ is the PL lifetime, and β is the dispersion expo- [1,2,39,40,53]. However, the lifetime of the triplet state
nent that was found to vary in between 0.4 to 0.8 and becomes weakly allowed due to spin-orbit interaction
will not be discussed here (see [37] for more details). [39,40,53]. Hence, the triplet lifetime is expected to be
Arrhenius plot (semi-logarithmic scale versus the in- considerably longer than the singlet lifetime. At low
verse temperature) ofthe measured PL lifetime for both temperatures (where kT<< Δ, and Δ is the singlet-
H- and O-PSi (at a photon energy of 2.03 eV) is shown triplet splitting energy; see inset to Figure 3b), only the
in Figure 3a, presenting exponentially fast decays at triplet level is populated and therefore, the PL decay
hightemperaturesandapproximatelylongand constant timeisdominatedbythetripletlifetimeandisrelatively
Arad-VoskandSa'arNanoscaleResearchLetters2014,9:47 Page4of6
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long(thelow-temperatureplateauregionsinFigure3a).
As the temperature increases (above approximately (a)
1E-5
30 K), the upper singlet level becomes thermally popu-
lated and the overall lifetime shortens according to the
followingexpression:
s)
1 ¼τgLþτ1U expð−Δ=kTÞ ð2Þ τ( U 1E-6
τ gþ expð−Δ=kTÞ
R
H-PSi
whereτ standsfortheradiativedecaytimeandτ andτ
R L U O-PSi
arethelowertripletandtheuppersingletlifetimesrespect-
ively (g = 3 is the levels degeneracy ratio) [1,39,40,53]. At 1E-7
(b)
high temperatures, the decay time is dominated by the
muchfasteruppersingletlifetime.
20
From Figure 3a we found that within the experimental
errors, the upper singlet decay times of H- and O- PSi V)
(at photon energy of 2.03 eV) are essentially the same e
m
(1.0±0.2 μs and 1.3±0.2 μs for H-PSi and O-PSi, re- Δ ( 10
spectively). However, at low temperatures the H-PSi
decay time is faster than that of the O-PSi (200±50 μs
relative to 480±50 μs, respectively). To further explore
the differences between H- and O- PSi decay times, the
0
singlet and the triplet lifetimes as well as the energy 6E-4 (c)
splitting were extracted over the measurement's range
4E-4
of photon energies and are plotted in Figure 4. As ex-
pected, the upper singlet lifetime (τ ) is significantly
U
shorter (by about one to two orders of magnitude) than s)
the lower triplet lifetime (τL) over this range of photon τ( L 2E-4
energies. The energy splitting, ΔE (see Figure 4b), was
found to vary between 7 and 25 meV, as in [53] and in
1E-4
accordance with the calculated values in [56-58]. Com-
8E-5
paring H- and O- PSi, we note that the upper singlet
6E-5
lifetimes and the excitonic energy splitting of both H- 1.6 1.8 2.0 2.2 2.4
PSi and O-PSi remarkably coincide over the entire range Energy (eV)
of measured photon energies (see Figure 4a,b), while Figure4Tripletandsingletlifetimesandenergysplitting.(a)
the lower triplet lifetime of H-PSi is shorter than that theuppersingletlifetime;(b)theexcitonicenergysplitting;(c)the
of O-PSi over the same range of energies (Figure 4c). lowertripletlifetime(extractedfromthefittothesinglet-triplet
model;seeFigure3)asafunctionofthephotonenergy.
This result is the basis for our conclusion (to be dis-
cussed hereafter) that oxidation of (freshly prepared)
H-PSi gives rise to slower nonradiative lifetimes, leaving
decay rate. The integrated PL (i.e., the area below the PL
radiativelifetimesunaffected.
spectrumshownattheinsettoFigure1)isproportionalto
thequantumyieldthatisgivenbytheratiooftheradiative
Discussion to the total decay rate, η¼τ−1=τ−1 ¼τ=τ . The variation
As explained above, the main finding of this work is that R R
oftheintegratedPLwithtemperatureisshowninFigure3b
the oxidation of freshly prepared luminescent PSi gives
ona semi-logarithmicscale,similartothatofFigure3afor
rise to slower triplet lifetimes, keeping the upper singlet
the PL lifetime. Notice that while the PL lifetime varies by
lifetimes unaffected. Before discussing the implications
approximately two orders of magnitude over the 30 to
of this result, let us denote that the measured decay rate
300 K temperature range, the integrated PL varies by less
is the sum of two competing relaxation processes given
than 3. Hence, one concludes that at this temperature
by range, τ < < τ , leading to, τ≈τ (Equation 3), and η≈
R NR R
τ−1 ¼τ−1þτ−1 ð3Þ constant(asinreference[37]).Thus,attemperaturesabove
R NR
30to40Kthemeasuredlifetimeisdominatedbyradiative
whereτ−1istheradiativetransitionrate(givenbyEquation2), transitions.Inaddition,thestrongdependenceoftheupper
R
τ−1 is the nonradiative relaxation rate, and τ−1 is the total singletlifetimeonphotonenergy(adecreasefrom6to7μs
NR
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at 1.6 eV down to 200 to 300 ns at 2.3 eV; see Figure 4a), the Si nanocrystallites and therefore, are not affected by
suggests again that this lifetime should be associated with oxidation.Ontheotherhand,nonradiativerelaxationpro-
radiative transitions (where τU~τ U < < τ U). In this cesses are affected by oxidation and by the state of the
R NR
case, the fast radiative lifetime is due to the influence of nanocrystallites surface. These results are consistent with
confinement on the spontaneous emission rates in small the extended vibron model that assigns radiative relax-
Sinanocrystals[39,40].Ontheotherhand,thelowertrip- ation to QC, while nonradiative processes are assigned to
let lifetime that is dominant at low temperatures is ap- surfacechemistry.
proximately constant (varies by less than factor of 2 over
the same range of energies) and roughly independent of Abbreviations
EV:Extendedvibron;H–PSi:Hydrogen-terminatedporoussilicon;
thephotonenergythatprobesagivensizeofnanocrystals.
O-PSi:Oxidizedporoussilicon;PL:Photoluminescence;PSi:Poroussilicon;
This suggests that the low-temperaturelifetime should be QC:Quantumconfinement;TR-PLspectroscopy:Time-resolved
associated with a nonradiative relaxation time (of the photoluminescencespectroscopy.
whole system) that dominates over the (forbidden) ra-
Competinginterests
diativetripletlifetime,inagreementwith[37,39,40].
Theauthorsdeclarethattheyhavenocompetinginterests.
Turning back to our main findings, we conclude that
oxidation (in ambient conditions) has a minor impact Authors’contributions
on the size of the nanocrystals (giving rise to about 3% NAVcarriedouttheexperiments,contributedtotheinterpretationofthe
dataanddraftedthemanuscript.AScontributedtotheinterpretationofthe
blue shift of the PL spectrum) and no noticeable effect
dataandrevisionofthemanuscript.Bothauthorsreadandapprovedthe
on the radiative lifetime and the excitonic energy split- finalmanuscript.
ting (via their dependence on photon energy). On the
other hand, nonradiative relaxation times, which are as- Acknowledgements
ThisworkhasbeenpartiallysupportedbytheIsraelScienceFoundation(ISF),
sociatedwiththestateofthesurface,areexpected tobe
grantno.425/09.NAVacknowledgesthesupportofDr.IlanaLevitan
sensitive to oxidation and to a modification of surface fellowshipforwomeninphysics.
bonds as experimentally observed (see Figure 4c). This
Received:2September2013Accepted:7January2014
result can be explained by the EV model [39,40], which
Published:28January2014
assigns the slow nonradiative relaxation times to reson-
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