Effects of annealing ambient on the photoluminescence propertiesof Si-rich oxide/SiO2 multilayer films containing Si-nanocrystals

Xinzhan Wang • Xiang Yu • Wei Yu •

Huina Feng • Jin Wang • Chenchen Yin •

Wanbing Lu • Guangsheng Fu

Received: 10 September 2013 / Accepted: 9 October 2013 / Published online: 22 October 2013

� Springer Science+Business Media New York 2013

Abstract In this study, Si-nanocrystals (Si-NCs) have

been prepared by annealing Si-rich oxide (SRO)/SiO2

multilayer films in Ar and N2, and the effects of annealing

ambient on the photoluminescence (PL) properties are

studied. XPS results show that the chemical compositions

for the SRO and SiO2 layers are SiO1.1 and SiO2, respec-

tively. FTIR results show that phase separation between Si

and SiO2 occurs after annealing treatment, and Si-NCs are

obtained which have been proved by the TEM images.

Large and high density Si-NCs are obtained in the Ar-

annealed film, and high structural disorder exists at the

interface of Si-NCs. Compared with the film annealed in

N2, a 2.4 times PL enhancement is obtained for the Ar-

annealed sample, and the PL peak shifts toward low

energy. Two lifetime distribution bands are obtained by

fitting the time-resolved PL spectra, and the proportion of

slow PL decay component increases from 69.7 to 84.0 %.

The PL intensity for the Ar-annealed film is further

enhanced by hydrogen passivation, and the slow PL decay

component is increased to 87.5 %. Analyses show that both

interface states recombination and quantum confinement

effect (QCE) related optical transition in the Si-NCs exist

in the optical emission process, and intense PL can be

obtained only when the QCE become dominant PL

mechanism.

Introduction

Visible photoluminescence (PL) has been observed in

porous Si since 1991, and extensive research has been done

to realize efficient Si-based light emitters [1, 2]. Various

nanoscale Si structures with visible PL have been prepared,

and the multilayer structure which comprises alternating

layers of Si-rich oxide (SRO) and SiO2 matrix has been

demonstrated as a good candidate for the excellent con-

finement effect of the oxide matrix [3]. The SiOx/SiO2

multilayers have been prepared by electron beam evapo-

ration, cosputtering, and plasma enhanced chemical vapor

deposition (PECVD) technique [4–6]. The PECVD tech-

nique can be easily applied in large areas, which represents

the optimum choice from the economic point. Si-nano-

crystals (Si-NCs) can be obtained in the SRO layer by

annealing treatment. The structural and optical properties

of Si-NCs can be well controlled by adjusting the SiOx

layer thickness, chemical stoichiometry, and annealing

environment [4–7]. Phase separation and crystallization

occur in the annealing process, and the crystallization of Si

clusters can be obtained only when the annealing temper-

ature is no lower than 1100 �C [6].

N2 and Ar are usually used as ambient gas in the

annealing treatment. The PL intensity for the Si-NCs

annealed in N2 is usually larger than that annealed in Ar

and the PL peak energy is higher. Analyses show that

quantum confinement effect (QCE) of Si-NCs is the main

PL mechanism, and the intense and high energy PL is

attributed to the small size of Si-NCs annealed in N2 [8, 9].

However, recent results show that interface defect states

recombination is also responsible for the optical emission

in Si-NCs [10]. Further, Wang et al. [11] have distin-

guished the interfacial defects from QCE in Si-quantum-

dots/SiO2 multilayer by fitting the time-resolved PL spectra

X. Wang � X. Yu � W. Yu (&) � H. Feng � J. Wang � C. Yin �W. Lu � G. Fu (&)

Hebei Key Laboratory of Optic-Electronic Information

Materials, College of Physics Science and Technology, Hebei

University, Baoding 071002, China

e-mail: yuwei_hbu@126.com

G. Fu

e-mail: fugs@hbu.edu.cn

123

J Mater Sci (2014) 49:1353–1358

DOI 10.1007/s10853-013-7819-2

by a multiexponential decay function. Due to the interface

defects related optical emission, different PL properties

may be obtained when the SRO/SiO2 multilayer films are

annealed in different environments.

In the present work, SRO/SiO2 multilayer films con-

taining Si-NCs are obtained by annealing in N2 and Ar at

1100 �C, and the effects of annealing ambient on the PL

properties of Si-NCs are investigated. The PL intensity of

the films annealed in Ar is 2.4 times larger than that

annealed in N2, and it is further enhanced by hydrogen

passivation. Time-resolved PL spectra show that intense

optical emission can occur only when the proportion of the

slow life time band is large enough.

Experiment

The SRO/SiO2 multilayer films were deposited on Si sub-

strate by PECVD technique. SiH4, H2, and N2O were used

as reactant gases, and the flow rates of SiH4 and H2 were

set as 1 and 100 sccm, respectively. The flow ratio of N2O

and SiH4 was 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, 200 �C, and 40 W, respec-

tively. Multilayer films consisting of 30 SRO/SiO2 bilayer

sequences were fabricated. After deposition, the samples

were annealed at 1100 �C in N2 and Ar for 60 min,

respectively. In the end, hydrogen passivation was per-

formed for the film annealed in Ar, and the passivation

temperature was 450 �C for 1 h.

The chemical composition of SRO and SiO2 layers was

measured using X-ray photoelectron spectroscopy (XPS,

EDAX9100). The infrared absorptions were deduced from

the transmittance measurement with a Fourier transform

infrared spectrophotometer (FTIR, Perkin-Elmer2000).

The transmission electron microscopy (TEM) was per-

formed with a JEOL J2010F (S) TEM microscope oper-

ating at 200 keV. The steady and time resolved PL spectra

were detected by a FLS920 fluorescence spectrometer

(Edinburgh Instruments), and the excitation sources are

450 W steady Xe lamp and 100 W pulse Xe lamp,

respectively.

Results and discussion

The XPS curves of the SRO and SiO2 layers are shown in

Fig. 1. According to random bonding (RB) model, the SiOx

is composed of five types of tetrahedrons SiOmSi(4-m)

(where m = 0, 1, 2, 3, and 4). The intensities from SiSi4,

SiSi3O, SiSi2O2, SiSiO3, and SiO4 tetrahedrons are fitted

by Gaussian function. The composition x in SiOx can be

estimated as [12]:

x ¼ 1

2

P4m¼1 mIm

P4m¼0 Im

ð1Þ

where Im is the intensity of SiOmSi(4-m). Gaussian fittings

are performed on both the XPS curves of SRO and SiO2

layers, and calculation results show that the values of x are

1.1 and 2.0, respectively. The calculation results only

suggest that the atom ratios of Si and O are 1.1 and 2.0 for

the SRO and SiO2 layers, and Si-NCs are obtained from the

SRO layers after annealing treatment.

Figure 2 shows the FTIR spectra of the SRO/SiO2

multilayer films before and after annealing treatment. The

main absorption peak for the deposited film locates at

around 1036 cm-1, which corresponds to the asymmetric

stretching vibration of oxygen atom in its two-fold coor-

dinated bridging bonding site [13]. The absorption band

located at about 875 cm-1 is related to the stretching

vibration of the Si–N bond, while the absorption band at

1140 cm-1 is related to the Si=O bond stretching vibration

[6]. The Si–N bond related absorption disappears after

annealing treatment, and the main absorption peak moves

from 1036 to 1076 cm-1. In addition, an additional band at

(a)

(b)

Fig. 1 XPS spectra of the SRO and SiO2 layers

Fig. 2 FTIR spectra of the multilayer films

1354 J Mater Sci (2014) 49:1353–1358

123

810 cm-1 is observed and it is related to the absorption of

Si–O bending mode [6]. The results indicate that phase

separation between Si and SiO2 is obtained, and Si-NCs

may form in the SRO layer. Moreover, the FTIR spectrum

of the film annealed in N2 is narrower than that annealed in

Ar, which suggests that large structural disorder exists in

the interface between Si-NCs and SiO2 matrix due to the

large stress [14].

Figure 3 shows the TEM images of the SRO/SiO2

multilayer films annealed in different ambience. Clear

multilayer structures are shown in Fig. 3a, b, and the

thicknesses of SRO and SiO2 layers are 4 and 5 nm,

respectively. The formation of Si-NCs is proved by the

HRTEM image in Fig. 3c, d, and the inset shows typical

NCs with visible lattice fringes. The mean sizes of Si-NCs

are obtained by averaging the size over 30 NCs, and they

are about 3.6 and 3.7 nm for the films annealed in N2 and

Ar, respectively. Calculations show that the Si-NCs den-

sities are about 5 9 1010 and 1.2 9 1011 cm-2 for the

films annealed in N2 and Ar, which are much smaller than

the references [15]. The small size and low density of N2

annealed sample can be attributed to the suppression dif-

fusion effect of nitrogen during the growth of Si-NCs in the

annealing treatment [16].

Figure 4a shows the steady PL spectra of the multilayer

films. As reported before, PL redshift is found for the

sample annealed in Ar, which suggests large Si-NCs lead to

low energy PL due to QCE. However, the PL integral

intensity of the Ar annealed film is about 2.4 times larger

than that of the N2 annealed film, which is in contrast with

references [8, 9]. The enhanced PL in the Ar-annealed

sample suggests that the PL intensity is proportional to the

number of light-emitting centers in the same excitation

condition [17]. In order to further understand the PL

mechanism of the films, PL excitation (PLE) spectra are

shown in Fig. 4b. The main PLE band locates at around

4.1 eV in both samples, and the PLE integral intensity for

the Ar-annealed sample is 2.2 times larger. The PL inten-

sity is decided by both the optical excitation and carrier

recombination processes. The large carrier absorption

section and high NCs density of the Ar-annealed sample

lead to PLE intensity enhancement, while the weak QCE

for the large Si-NCs leads to weak PL intensity, which

would generate smaller PL intensity enhancement com-

pared with the PLE. The opposite result in our sample

suggests that QCE is not the unique PL mechanism, and

interface state recombination may also exist in the light-

emitting process.

Fig. 3 TEM images for the

multilayer structure of the films

annealed in a N2, b Ar: HR-

TEM images of the film

annealed in, c N2, and d Ar

J Mater Sci (2014) 49:1353–1358 1355

123

Figure 5a shows the time-resolved PL spectra at 1.42 eV

for both samples, and the results show that large PL intensity

corresponds to slow PL decay. Three decay models have

been provided to describe the PL decay processes in the Si-

NCs based system, which are stretched-exponential decay

model, double-exponential decay model, multiexponential

decay model, respectively [18–20]. Our earlier results have

shown that the multiexponential decay model is more suit-

able than the stretched-exponential decay for these samples

[11], and we would further like to compare the multiexpo-

nential decay model with double-exponential decay model.

The fitting results by multiexponential decay model and

double-exponential decay model are also shown in Fig. 5b,

and little difference can be seen. However, as can be seen in

Fig. 5c, the residuals for the double-exponential decay fitted

curve are larger than the multiexponential decay fitted one,

which indicates that the multiexponential PL decay model is

more suitable. The PL intensity [I(t)] in multiexponential

decay model can be expressed as [21]:

IðtÞ ¼X

js�1

j Ajexp(� t=sjÞ

where Aj is determined by 200 time constants j. The life

time distributions determined in this way are shown in

Fig. 5d, and two PL life time distribution bands are

obtained. The peaks of the life time distribution bands at 35

and 130 ls for the N2 annealed sample, while that are 31

and 150 ls for the Ar-annealed sample, and the proportion

of the slow decay component increases from 69.7 to

84.0 %. The results suggest that two carrier recombination

processes contribute to the PL in both samples. The steady

PL spectra have shown that both interface state recombi-

nation and QCE of excitons in Si-NCs contribute to the

light emission. The carrier relaxation from interface states

is often faster than that from Si-NCs, and the fast decay

component should be related to the carrier relaxation in

interface states, while the slow PL decay band is caused by

the interband transition of Si-NCs due to QCE [11]. The

proportion enhancement of slow PL decay suggests that

QCE related optical emission is enhanced due to the high

density of Si-NCs in the Ar-annealed sample. The PL

decay time (s) can be expressed as 1s ¼ 1

sradþ 1

snon�rad[22],

where srad and snon-rad are the radiative and nonradiative

decay times, respectively. High structural disorder at

interface of Si-NCs for the films annealed in Ar leads to

low radiative recombination probability, and the high

nonradiative rate leads to fast PL decay time for the

interface state related optical emission. However, opposite

condition exists inside the Si-NCs. The high density Si-

NCs annealed in Ar leads to high radiative recombination

probability, and the low radiative rate due to indirect

property of Si leads to slow PL decay time. The stretched-

exponential decay model is more suitable for the PL decay

process when the density of Si-NCs is enhanced further,

which has been proved by early results [19, 23]. It suggests

that QCE is the dominant carrier recombination process

only when the density of Si-Ns is high enough. As shown

in the HRTEM images, the densities of Si-NCs are too low

in both samples compared with references, and the low

density leads to large amount of carrier recombination

centers at the interface between Si-NCs and SiO2, which

may lead to the opposite PL intensity and decay processes.

In addition, both the samples are deposited by the PECVD

technique, and N2O is used as the source of oxygen. The

nitridation of Si/SiO2 interface has been finished during

deposition, which may also lead to different PL properties.

In order to modify the structure of the multilayer film,

hydrogen passivation is performed at 450 �C for the Ar-

annealed sample, and this temperature does not influence

the size and nature of Si-NCs [24]. As shown in Fig. 6a, a

1.2 times PL intensity enhancement is obtained after

hydrogen passivation. Hydrogen related bonds have been

removed after annealing treatment at 1100 �C, and a large

number of dangling bonds are produced. The dangling

bonds can act as both radiative and nonradiative centers,

which may enhance the defect states related optical

(a)

(b)

Fig. 4 a PL spectra of the multilayer films. b PL excitation spectra of

the multilayer films

1356 J Mater Sci (2014) 49:1353–1358

123

emission and quench the QCE related PL [25]. Molecular

hydrogen passivation at 450 �C decreases the densities of

dangling bonds and part of other nonradiative centers,

which leads to the PL enhancement due to QCE [26, 27].

Time-resolved PL spectra at 1.42 eV are measured for the

Ar-annealed sample before and after hydrogen passivation,

and both of which are well fitted by the multiexponential

PL decay model. As shown in Fig. 6b, two PL lifetime

distribution bands can be seen in both the curves. The

average PL decay times are 50 and 180 ls, and the

(a) (b)

(d)(c)

Fig. 5 a Time-resolved PL

spectra of the films. b The

fitting results for the N2

annealed sample fitted by

different models. c Residuals of

the fitting results. d Lifetime

distribution of the multilayer

films

(a)

(b)

Fig. 6 a PL spectra of the multilayer film annealed in Ar and the

following hydrogen passivation. b Lifetime distributions for the Ar-

annealed film before and after hydrogen passivation

Fig. 7 Lifetime distributions at different wavelengths for the hydro-

gen passivated film

J Mater Sci (2014) 49:1353–1358 1357

123

proportion of slow component increases from 84.0 to

87.5 %. The proportion enhancement suggests that QCE

related carrier recombination dominates the optical emis-

sion at 1.42 eV, and the slower PL decay in both compo-

nents indicates that nonradiative recombination is

suppressed in both the interface and inner of Si-NCs due to

the reduction of structural disorder caused by hydrogen

passivation.

In order to identify the dimensions where QCE related

carrier recombination prevails on the interface states rela-

ted optical emission. Figure 7 shows the normalized PL

decay times detected at different wavelengths for the

hydrogen passivated sample. The proportion of the slow

component increases with the redshift of detecting wave-

length and it is larger than 50 % when the detecting

wavelength is longer than 750 nm. Exciton recombination

due to QCE is the main PL process when the size of oxi-

dized Si-NCs is larger than 3 nm, and both the surface

states and QCE contribute to the optical emission when it is

smaller than 3 nm [28]. The fast PL decay in the high

energy region is mainly related to small Si-NCs, and it is

mainly caused by the recombination of light-emitting

centers at the interface of Si-NCs. While the low energy PL

is mainly related to large Si-NCs, and the proportion

increase of slow decay component suggests that QCE of Si-

NCs becomes the dominant PL mechanism when the

emission wavelength is longer than 750 nm. Both peaks of

the PL decay time distribution bands shift toward longer

time with the reduction of detecting photon energy, which

suggests that both surface states and QCE related PL

depend on the size of oxygen-passivated Si-NCs [29].

Conclusions

The SRO/SiO2 multilayer films have been synthesized by

the PECVD technique, and the PL decay properties of the

films annealed in N2 and Ar are studied. A 2.4 times PL

enhancement is obtained for the film annealed in Ar. The

time-resolved PL spectra are well fitted by the multiexpo-

nential PL decay model, and two PL lifetime bands are

obtained. The obtained results are opposite with Wilkinson

et al.’s results, and it is related to the high-interface states

due to the low density of Si-NCs in both the samples. The

PL intensity and proportion of slow component are further

enhanced by hydrogen passivation. Analyses show that

both defect states recombination and interband transition in

the Si-NCs due to QCE exist in the optical emission pro-

cess, and intense PL can be obtained only when the QCE

become the main PL mechanism.

Acknowledgements This study 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 No.

F2012201007 and F2012201042).

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