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Superlattices and Microstructures 60 (2013) 208–216
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Superlattices and Microstructures
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co m / l o c a t e / s u p e r l a t t i c e s
Effects of annealing treatments on thephotoluminescence decay properties of Si-richoxide/SiO2 multilayer films
0749-6036/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.spmi.2013.04.034
⇑ Corresponding author. Tel./fax: +86 (0312)5079560.E-mail address: [email protected] (W. Yu).
Xinzhan Wang, Wei Yu ⇑, Xiang Yu, Huina Feng, Wanlei Dai, Xi Wang,Wanbing Lu, Guangsheng FuHebei Key Laboratory of Optic-Electronic Information Materials, College of Physics Science and Technology, Hebei University,Baoding 071002, China
a r t i c l e i n f o
Article history:Received 22 April 2013Accepted 26 April 2013Available online 14 May 2013
Keywords:Si-QDs/SiO2 multilayerTime-resolved photoluminescenceQuantum confinement effect
a b s t r a c t
In this work, Si-rich oxide (SRO)/SiO2 multilayer films have beendeposited and the photoluminescence (PL) decay properties ofthe films with different annealing temperatures are studied. ThePL shifts toward low energy with increasing the annealing temper-ature, and intense PL at around 1.4 eV is obtained after annealing at1100 �C. The PL decay curves can be well fitted by a multiexponen-tial PL decay model, and the peak of the PL lifetime distributionband shifts toward longer time with increasing the annealing tem-perature. Two lifetime distribution bands are obtained after theformation of crystallized Si-QDs, and the proportion of slow com-ponent increases from 69.72% to 77.04% after hydrogen passiv-ation. Analyses show that defect states recombination in the SROlayer is the main optical emission mechanism when the annealingtemperature is lower than 900 �C, and interband transition in theSi-QDs due to quantum confinement effect is the main PL mecha-nism when the film is annealing at 1100 �C.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Visible photoluminescence (PL) from porous Si was observed in 1991, and extensive research forunderlying the luminescence mechanism has been done in order to realize efficient Si-based lightemitters [1–3]. Considerable technologies have been used to fabricate nanoscale Si with visible PL
X. Wang et al. / Superlattices and Microstructures 60 (2013) 208–216 209
to overcome the problem of chemical and mechanical instability of porous Si [4,5]. The nanoscale Sishould be incorporated to dielectric matrix due to the small size, and the multilayer structure whichcomprises alternating layers of Si-QDs and SiO2 matrix has been demonstrated as a good candidate forthe excellent confinement effect of the oxide matrix [6,7]. Thermal annealing in tube furnace is oftenemployed to promote the growth of Si-QDs/SiO2 multilayer films, and the optical emission propertiesof Si-QDs can be well controlled by changing the annealing environments [8].
Recent results show that quantum confinement effect (QCE) and defect states recombination aretwo important optical emission processes in the Si-QDs/SiO2 multilayer films, and the QCE relatedPL varies with the size of Si nanocrystals, while the PL caused by defect states recombination is sizeinsensitive [9]. Qin and Li have suggested that both QCE and interface defects contribute to the PLin nanoscale Si/Si oxide system, and the interface defect state recombination is the main PL originwhen the size of Si-QDs is larger than a certain threshold, while QCE is the main optical emissionmechanism when that of Si-QDs is smaller [10]. On the other hand, nonradiative defects such as Pb
centers exist at the interface between the Si-QDs and SiO2 matrix, and they are considered to quenchthe PL of the Si-QDs based systems [11]. Up to now, it is still a very difficult task to separate theinterfacial defects from QCE in the Si-based system due to the lack of effective detection method.Time-resolved PL spectra can be used to investigate the carrier recombination processes in theSi-QDs/SiO2 multilayer films, and stretched-exponential PL decay curve fittings are usually adopted[12]. The PL in Si-based system is sensitive to the annealing temperature, and the evolution of PL decayproperties still needs further investigation. In addition, the carrier recombination in Si-QDs systemcontaining many defects can be achieved by various paths [13]. The PL decay times cannot be ex-pressed as one or several time constants, and PL decay time distribution bands should be adoptedto describe them precisely.
In the present work, Si-rich oxide (SRO)/SiO2 multilayer films have been prepared by plasma en-hanced chemical vapor deposition (PECVD) technique, and Si-QDs/SiO2 multilayer films are obtainedby annealing in N2. Further, the effects of annealing treatments on the PL decay properties of SRO/SiO2
multilayer films are investigated. Analyses show that two decay time distribution bands are obtainedin the crystallized Si-QDs/SiO2 multilayer film, and they correspond to the decay times of Si-QDs anddefect states related optical emission, respectively.
2. Experiment
The SRO/SiO2 multilayer films were deposited on Si and silica substrates by the PECVD technique.SiH4, H2 and N2O were used as precursor gases, and the flow rates of SiH4 and H2 were 1 and 100 sccm,respectively. The flow ratio of N2O and SiH4 were kept at 25 for SiO2 layer, and 0.1 for the SRO layer.Deposition pressure, substrate temperature and RF-power were kept at 120 Pa, 220 �C and 40 W,respectively. Multilayer films consisting of 30 SRO/SiO2 bilayer sequences were fabricated. Belowand on top of the multilayer structure, a 10 nm SiO2 layer was deposited as buffer and capping layer,respectively. After deposition, the samples were annealed from 500 �C to 1100 �C in a step of 200 �C inN2 for 1 h. In the end, hydrogen passivation is performed on the film annealed at 1100 �C, and the pas-sivation temperature is 450 �C for 1 h.
The infrared absorptions were deduced from the transmittance measurement in a Fourier trans-form infrared spectrophotometer (FTIR, Perkin–Elmer2000). The transmission electron microscopy(TEM) was performed on a JEOL J2010F (S)TEM microscope operating at 200 keV. The steady andtime-resolved PL spectra were detected by a FLS920 fluorescence spectrometer (EdinburghInstruments), and the excitation sources are 450 W steady Xe lamp and 100 W pulse Xe lamp, respec-tively. The experimental data in the PL measurement were obtained from a time-correlated singlephoton counting system at room temperature. Distribution analysis of lifetimes was carried out bya program supplied by Edinburgh Analytical Instruments. The fitting function utilized the Fredholmintegral and used up to 200 lifetime values, and no a priori assumption about the distribution shapewas made.
800 2000 3000 4000
1100 oC
900 oC
700 oC
Abs
orpr
ptio
n C
oeff
icie
ntcm
-1
Wavenumber /cm-1
500 oC
Deposited
Fig. 1. FTIR spectra of the multilayer films annealed at different temperatures.
210 X. Wang et al. / Superlattices and Microstructures 60 (2013) 208–216
3. Results and discussion
Fig. 1 shows the FTIR spectra of the SRO/SiO2 multilayer films annealed at different temperatures,and five IR absorption bands can be observed. The main absorption peak locating at around 1050 cm�1
corresponds to the asymmetric stretching vibration of oxygen atom in its twofold coordinated bridg-ing bonding site [14]. The peak position moves from 1037 to 1077 cm�1 with increasing the annealingtemperature, and it indicates that phase separation between Si and SiO2 is obtained in high temper-ature. The absorption band around 2250 cm�1 corresponds to the Si–Hn stretching mode [14]. Theabsorption band around 3400 cm�1 is related to the stretching vibration mode of N–Hn, and theabsorption band about 875 cm�1 is related to the stretching vibration of the Si–N bonds [15]. Theintensity of hydrogen and nitrogen related vibration bands decrease with increasing the annealingtemperature, and it cannot be observed for annealing at 700 �C or higher. For annealing temperatures
320 400 480 560 640
1000 oC
900 oC
700 oC
Ram
an I
nten
sity
(a.
u.)
Wavenumber /cm-1
500 oC
Fig. 2. Raman spectra of the multilayer films annealed at different temperatures.
X. Wang et al. / Superlattices and Microstructures 60 (2013) 208–216 211
higher than 900 �C, an additional band at 810 cm�1 is observed, and it is related to the absorption ofSi–O bending mode [16].
Fig. 2 shows the Raman spectra of the films. A Raman peak shifting from 483 cm�1 to 516 cm�1 isobtained when the annealing temperature increases from 500 �C to 1100 �C. The Raman peak locatingat 483 cm�1 is associated with ultra-small amorphous Si clusters, and the blue shift from 483 to491 cm�1 with increasing the annealing temperature from 500 �C to 900 �C suggests that the size ofamorphous Si clusters increases with increasing the annealing temperature. The Raman peak movingto 516 cm�1 further indicates that crystallized Si-QDs is obtained [17]. The results indicate that crys-tallized Si-QDs in SRO/SiO2 multilayer films can be obtained only when the annealing temperature isas high as 1100 �C.
Fig. 3 shows the TEM images of the SRO/SiO2 multilayer film annealing at 1100 �C. Clear multilayerstructure is shown in Fig. 3a, and the thicknesses of SRO and SiO2 layers are 4 and 5 nm, respectively.The formation of Si-QDs is proved by the HRTEM image in Fig. 3b, while no clear Si-QDs can be seen
Fig. 3. (a) A TEM image for multilayer structure of the film annealed at 1100 �C, (b) a HR-TEM image of the film annealed at1100 �C.
1.0 1.2 1.4 1.6 1.8 2.0
PL I
nten
sity
(a.
u.)
Photon Energy / eV
500 x20
700 x20
900 x101100
Fig. 4. Steady PL spectra of the multilayer films annealed at different temperatures.
212 X. Wang et al. / Superlattices and Microstructures 60 (2013) 208–216
when annealing temperature is low. The mean size of Si-QDs is obtained by averaging the Si-QDs over30 nanoparticles, and it is about 3.6 nm for the film annealed at 1100 �C.
Fig. 4 shows the steady PL spectra of the multilayer films. A broad PL peak at around 1.6 eV exists inthe films when the annealing temperature is below 700 �C, and the PL peak shifts to 1.4 eV withincreasing the annealing temperature to 900 �C. The slight increase of PL intensity indicates that phaseseparation is not finished in the SRO layers below 900 �C [18], and the SRO layers are mainly composedof amorphous Si clusters and SiOx compounds. Little PL peak change is obtained with increasing thetemperature to 1100 �C, while a 40 times PL intensity enhancement is obtained. The remarkable in-crease of PL intensity indicates that crystallized Si-QDs become the main light-emitting centers. Theluminescence of Si-based system is related to defects in the SRO layer, interface defects or interbandtransition in Si-QDs [19]. The band gap of Si cluster is much higher than the energy of PL, and defectstates related optical emission in the SRO layer should be the main PL process when the annealingtemperature is below 900 �C [20]. Due to the structural disorder, large amounts of nonradiativerecombination centers present in the amorphous SRO films, which leads to the weak PL intensity.New radiative recombination centers forms in the crystallized Si-QDs after formation, and interbandtransition becomes the main optical emission process due to QCE. In addition, interface defects exist inthe interface of Si-QDs, and they may also contribute to the intense PL [3].
In order to investigate the carrier recombination processes of the multilayer films, time-resolved PLspectra are measured. Fig. 5a shows the PL decay curve at 860 nm for the film annealing at 1100 �C,and the strong non-single exponential decay behavior suggests that complex carrier recombinationdynamics exists in the PL decay process. A large number of papers have described that stretched-
0 200 400 600 800 1000
103
104
(c)
(b)
10 1000.0
0.3
0.6
0.9
Nor
mal
ized
Int
ensi
ty (
a.u.
)
Time /µs
M uti-expStr-exp
0 200 400 600 800 1000-6
-4
-2
0
2
4
Res
idua
ls
Time /µs
Muti-expStr-exp
Phot
on C
ount
s
Time/µs
Decay CurveMuti-exp FittingStr-exp Fitting
(a)
Fig. 5. (a) Time-resolved PL spectrum of the film annealed at 1100 �C and the fitting results, (b) residuals of the fitting resultsand (c) lifetime distribution and the stretched-exponential fitting result.
X. Wang et al. / Superlattices and Microstructures 60 (2013) 208–216 213
exponential PL decay exists in Si-QDs based systems, and it is usually expressed as:IðtÞ ¼ I0 exp½�ðt=sÞb� [12,21], where I0 is the initial PL intensity, s is the characteristic PL lifetime,and 0 < b < 1 is the dispersion factor. However, as can be seen in Fig. 5b, the residuals in the short timeregion are large, and it indicates that the stretched-exponential model cannot well fit the PL decaycurves in our sample. In order to explore the carrier recombination dynamics precisely, the PL lifetimedistribution is extracted by a fitting procedure based on the maximum entropy method of data anal-ysis. G(s) is defined as the probability of encountering light-emitting center with lifetime, and the totalPL intensity [I(t)] can be expressed as [22]:
Phot
on C
ount
s
Fig. 6.differen
IðtÞ ¼ CZ 1
0s�1GðsÞ expð�t=sÞds
Base on the above formula, the decay curve is fitted by a sum of single exponentials with time con-stant j. In order to calculate the lifetime distribution simply, the integral can be replaced as [11]:
IðtÞ ¼X
j
s�1j Aj expð�t=sjÞ
where Aj is determined by 200 time constants j. The life time distribution is shown in Fig 5c, and twoPL life time distribution bands are obtained. The well fitting result and small residuals in Fig. 5b in allthe detecting times suggest that the PL decay processes can be well expressed by the multiexponentialPL decay model. The peaks of the two PL life time distribution bands at 25 ls and 113 ls suggest thattwo main carrier recombination processes contribute to the PL in our sample. Both the defects at theSi/SiO2 interface and the quantum confinement of excitons in Si-QDs contribute to the PL in Si-QDsbased system. The carrier relaxation from defect states is often faster than that of Si-QDs, and the fastdecay component may be related to the carrier relaxation in interface defect states, while the slow PLband is caused by the interband transition of Si-QDs due to the QCE [3].
Fig. 6a shows the time-resolved PL spectra at 830 nm for the multilayer films, and the PL decay be-comes slow with increasing the annealing temperature. Multiexponential fittings show that only onePL time distribution band can be obtained when the annealing temperature is lower than 900 �C, andthe peak of PL lifetime distribution band shifts toward longer time with increasing the annealing tem-perature. The PL decay rate k (1/s) can be expressed as k ¼ krad þ knon�rad [23], where krad (1/srad) andknon-rad (1/srad) are the radiative and nonradiative decay rates, respectively. As can be seen in Fig 6b, theaverage PL decay rate decreases with increasing the annealing temperature. The observed features canbe explained by the defect states radiative processes in the multilayer films. High structural disorder
0 20 40 60 80 100
101
102
103
(b)
500 600 700 800 900
o
11000.0
0.2
0.4
0.6
0.8
PL D
ecay
Rat
eµ
s-1
Annealing Temperature / CTime/µs
5007009001100
(a)
1000
(a) Time-resolved PL spectra of the films annealed at different temperatures and the fitting results, (b) PL decay rates att temperatures.
10 100
Nor
mal
ized
Int
ensi
ty (
a.u.
)
Time /µs
650750800850950
Fig. 7. Lifetime distributions at different wavelengths for the film annealed at 1100 �C.
214 X. Wang et al. / Superlattices and Microstructures 60 (2013) 208–216
exists in the amorphous SRO films when the annealing temperature is low, and the photon-generatedcarriers are mainly relaxed by nonradiative recombination, which is much faster than the radiativeprocess. The structural disorder decreases with increasing the annealing temperature from 500 �Cto 900 �C, and the proportion of radiative recombination process increases continuously. Due to thebig carrier capture section, the optical emission mainly occurs in or at the interface of Si-QDs whenthe film is annealing at 1100 �C, which generate two PL lifetime distribution bands.
Fig. 7 shows the normalized PL lifetimes at different wavelengths for the sample annealed at1100 �C. The peak of the PL lifetime distribution bands shift toward longer time with the redshift ofdetecting wavelengh, and the proportion of the slow lifetime distribution band increases. Accordingto Wolkin et al. [24], the PL mechanism of oxygen-passivated Si nanocrystals strongly depends onthe size. Excitons recombination due to the QCE is the main PL process when the size of Si-QDs islarger than 3 nm, and both the surface states and QCE contribute to the optcial emission when it issmaller than 3 nm, further, surface states recombination is the main recombination mechanism when
1.0 1.2 1.4 1.6 1.8 2.0
(b)
10 100
Inte
nsity
(a.
u.)
Time /µs
1100 N21100NH
PL I
nten
sity
(a.
u.)
Photon Energy /eV
1100N21100NH
(a)
Fig. 8. (a) PL spectra before and after hydrogen passivation, (b) lifetime distributions for the film before and after hydrogenpassivation.
X. Wang et al. / Superlattices and Microstructures 60 (2013) 208–216 215
the size of Si-QD is smaller than 1.8 nm. The averge size of the Si-QDs in our sample is 3.6 nm, and thesize distribution leads to a broad PL spectrum. The fast PL decay in the high energy region is mainlycaused by small Si-QDs, and it is mainly related to recombination of surface defect states. While theoptial emission in the low energy region is mainly related to large Si-QDs, and the proportion increaseof slow decay component suggests that QCE related PL become the main carrier recombinationmechanism.
In order to explore the PL decay mechanism further, hydrogen passivation at 450 �C is performedon the film annealed at 1100 �C. As shown in Fig 8a, a 1.5 times PL intensity enhancement is obtainedafter passivation, and a redshift about 10 meV is observed. The PL enhancement is generally ascribedto hydrogen passivation of nonradiative Pb centers arising from Si dangling bonds at the Si/SiO2 inter-face [25], while the PL enhancement is not as large as references, which suggests that the main PL orig-inate from the interband transition within the Si-QDs due to QCE [26]. Time-resolved PL spectra at860 nm are measured on the samples before and after hydrogen passivation, and both of them canbe well fitted by the multiexponential PL decay model. As can be seen in Fig. 8b, two PL lifetime dis-tribution bands can be seen in both the curves, and the proportion of the slow component increasesfrom 69.72% to 77.04%, in addition, the average decay time for the slow bands increases from 130 ls to140 ls. The result indicates that nonradiative probability is suppressed by hydrogen passivation, andit leads to the slow PL decay.
4. Conclusions
The SRO/SiO2 multilayer films have been synthesized by the PECVD technique, and the PL decayproperties of the films annealed at different temperatures are studied. The PL shifts toward low energywith increasing the annealing temperature, and intense PL at around 1.4 eV is obtained after the for-mation of Si-QDs. Time-resolved PL spectra show that the PL decay becomes slow with increasing theannealing temperature, and all the decay curves can be well fitted by the multiexponential PL decaymodel. The average PL decay rate decreases with increasing the annealing temperature, and it iscaused by the increase of radiative processes in defect states. Two lifetime distribution bands areobtained after the formation of Si-QDs, and the main PL mechanism is QCE in Si-QDs.
Acknowledgements
This work was supported by the Key Basic Research Project of Hebei province, PRC (Grant No.12963929D) and the Natural Science Foundation of Hebei province, PRC (Grant Nos. F2012201007and F2012201042).
References
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