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Physica E 41 (2009) 1002–1005

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Two-photon absorption in Si-nanocrystals deposited by plasma-enhancedchemical-vapor deposition

A. Martınez a, S. Hernandez a,�, Y. Lebour a, P. Pellegrino a, E. Jordana b, J.M. Fedeli b, B. Garrido a

a EME, Departament d’Electronica, IN2UB, Universitat de Barcelona, Martı i Franques 1, 08028 Barcelona, Spainb CEA-LETI, MINATEC, 17 rue des Martyrs, 38054 Grenoble, Cedex 9, France

a r t i c l e i n f o

Available online 22 August 2008

PACS:

78.67.Bf

42.65.�k

78.66.�w

78.20.Nv

Keywords:

Si-nanocrystals

Two-photon absorption

z-Scan

Nanosecond excitation

77/$ - see front matter & 2008 Elsevier B.V. A

016/j.physe.2008.08.023

esponding author. Tel.: +34 93 4039176; fax:

ail address: shernandez@el.ub.es (S. Hernand

a b s t r a c t

We present a systematic z-scan study of the nonlinear absorption coefficient b of Si-nanocrystals

embedded in SiO2 (Si-nc/SiO2) excited in the nanosecond regime. Two different wavelengths of a

Nd:YAG laser have been used, l ¼ 1064 and 532 nm. The samples under study were deposited on silica

substrates by plasma-enhanced chemical-vapor deposition technique and subsequently annealed up to

1250 �C. We have observed an increase in b as the Si content in the Si-nc/SiO2 films rises, indicating that

the nonlinear behavior strongly depends on the amount of Si. Typical values of b around 1 cm/MW were

obtained at 1064 nm, while they are one order of magnitude higher exciting at 532 nm, following the

same trend as bulk Si with respect to the excitation energy. Large Si-nc present an energy of high-

symmetry transitions (Eog, E2) that approaches the system to a resonant condition with the excitation

energy. On the other hand, we observed an increase of b with the annealing temperature that can

be associated to an additional absorption from free carriers, as the crystalline degree scales with the

thermal budget.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Si-nanocrystals (Si-nc) embedded in SiO2 display outstandingoptical and electrical properties, suitable for many applications inintegrated photonics (LEDs, amplifier medium with Er-doping,storing elements, gas sensors, etc.) [1,2]. The strong dependence oftheir third-order nonlinear optical properties with high opticalfluencies could be exploited in high speed photonic applicationssuch as all-optical switches or modulators [1–3]. Some authorshave studied different aspects of the nonlinear optical propertiesin Si-nc/SiO2 films [4–8]. For instance, Vijaya Prakash et al. foundvalues of nonlinear refractive index around � 10�11 cm2=W andnonlinear absorption coefficients of about � 10�7 cm=W [4], on aset of Si-nc/SiO2 films with different Si excesses by exciting atl ¼ 813 nm with femtosecond pulses. On the other hand, in aprevious work we found nonlinear refractive indexes eitherpositive or negative under excitation with femtosecond(l ¼ 833 nm) or nanosecond pulses (l ¼ 1550 nm), respectively[5]. We also found small values of the nonlinear absorptioncoefficient for both excitation conditions ðo10�8 cm=WÞ.

Although high index variation can be achieved in Si-nc/SiO2

films excited at high intensity, nonlinear absorption could beinduced at the same time, producing an increase in the total

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+34 93 4021148.

ez).

absorption. A few works have focused their studies on thenonlinear absorption under specific excitation conditions, forwavelengths in the visible or near-infrared, obtaining alsonegative values at short wavelengths (saturation of the absorptionprocess) [4–6]. Vijayalakshmi et al. [6] studied the behavior ofboth the nonlinear refractive index and nonlinear absorption of Si-nc/SiO2 films with two different Si excesses, exciting over a widerange of wavelengths, with different peak intensities and pulsedurations (nano-, pico- and femtoseconds). However, their non-linear results presented large scattered data.

Here we present a systematic z-scan study of the nonlinearabsorption exciting with nanosecond pulses on Si-nc/SiO2 filmswith Si excess up to 24 at%. The films were excited by a Nd:YAGlaser using two different wavelengths, l ¼ 1064 and 532 nm,which correspond to an energy well below the bandgap of Si-ncand close to it, respectively.

2. Experimental

SiOx films were deposited on silica substrates by plasma-enhanced chemical-vapor deposition (PECVD) technique. A N2Oflow rate was kept at 2000 standard cubic centimeters per minute(sccm) and different flow rates of SiH4 from 100 to 500 sccm wereused in order to change the SiOx composition. X-ray photoelectronscattering (XPS) measurements were performed in the as-

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Table 1Structural and optical parameters of the Si-nc/SiO2 films

Sample Composition Si excess dmean Thickness k k

code SiOxNy (at%) (nm) (nm) (l ¼ 532 nm) (l ¼ 1064 nm)

A x ¼ 0:84, y ¼ 0:14 24 4:8 432 0.064 0.026

B x ¼ 0:90, y ¼ 0:17 20 3:9 475 0.034 0.021

C x ¼ 1:05, y ¼ 0:17 16 3:6 461 0.024 0.017

D x ¼ 1:1, y ¼ 0:20 12 2:9 533 0.012 0.011

E x ¼ 1:36, y ¼ 0:15 8 o1 545 o10�4 o10�4

F x � 2, y � 0 o5 – 646 o10�4 o10�4

The mean diameter of the precipitated Si-nc (dmean) has been determined in samples annealed to 1250 �C.

A. Martınez et al. / Physica E 41 (2009) 1002–1005 1003

deposited films to determine the Si and O concentrations ([Si] and[O], respectively). All the films presented a sizeable incorporationof nitrogen impurities (� 8 at%) into the layers from one of theprecursors ðN2OÞ. More details on the XPS results are givenelsewhere [5]. After deposition, annealing at 800, 1100 or 1250 �Cwere carried out in a conventional furnace for 1 h, in order toprecipitate the nanostructures. From the relative concentration of[Si], [O] and [N] we determined the Si excess for each sample byusing the relation ½Si�exc ¼ ð1� 0:75y� 0:5xÞ=ð1þ xþ yÞ (being x

and y the ratio [O]/[Si] and [N]/[Si], respectively). The obtainedvalues (x, y and Si excess) are given in Table 1, where the samplesare labeled from A to F as the Si excess decreases from 24 at%to � 0 at%.

Energy-filtered transmission electron microscopy (EFTEM)measurements were performed in the annealed samples to imagethe Si-precipitates. For the highest annealing temperature wefound a Gaussian distribution of sizes with averages values of4:8� 1:5, 3:9� 0:8, 3:6� 0:7 and 2:9� 0:6 nm for samples A, B, Cand D, respectively, (see Table 1). On the other hand, no Siaggregates have been observed in samples E and F. In fact, sampleF has revealed a stoichiometry quite close to the one of SiO2, whilesample E (8 at% Si excess) has displayed an optical emission [5],which suggests a very low Si-nc concentration, with sizes belowthe detection limit of EFTEM measurements (� 1 nm). In order toevaluate the phase segregation and the crystalline state of theSi-aggregates, Raman scattering measurements were performedon the whole set of samples. We have observed the presence ofamorphous clusters at low annealing temperatures (o1000 �C)and Si-aggregates with high crystalline degree when the SiOx

films are treated to high temperatures [9]. Therefore, the filmsannealed to 1250 �C present a high density of Si-aggregates withsizes up to 4.8 nm and high crystalline order. The extinctioncoefficient and thickness of the films were obtained by means of aspectrophotometric ellipsometer and corroborated by threedifferent laboratories. The experimental values of thickness andextinction coefficient are reported in Table 1.

In order to evaluate the nonlinear optical absorption of theSi-nc/SiO2 films, z-scan measurements were performed in theopen aperture configuration. The excitation was provided by 5-nspulses of a Nd:YAG laser, with 10 Hz repetition rate, and using thefundamental and the second-harmonic modes (l ¼ 1064 and532 nm). Different peak intensities were used, ranging from I0 ¼

109 to 1010 W=cm2 for l ¼ 1064 nm, and from I0 ¼ 108 to109 W=cm2 for l ¼ 532 nm. As verified by optical microscopy, noapparent change in morphology has been produced in the samplesafter exciting them with such peak intensities, being all the resultsreproducible. The transmission of the Gaussian laser beamthrough the sample was recorded in the near-field configuration(open aperture) by a photodiode while a continuous motor wasmoving the sample along the optical axis (z-axis). The laser beamwas focused by a 150-mm focal-length lens to a beam waist of� 15 (l ¼ 532 nm) or � 25mm (l ¼ 1064 nm). A small portionof the input beam through a beam-splitter was monitored by

another photodiode, and the ratio of the two signals was recordedas a function of the sample position z. The experimental setup canalso be used to measure the nonlinear refractive index byrecording the transmission in the far field configuration (closeaperture). A more detailed explanation of the z-scan setup andprocedure can be found elsewhere [5].

3. Method of analysis

The analysis of the experimental data was performed withinthe frame of the model developed by Sheik-Bahae et al. [10],which consists in a decomposition of the complex electric field atthe exit plane of the sample into a summation of Gaussian beams.Taking into account some assumptions (Gaussian beam, thinsample, instantaneous nonlinearity, and near-field condition), thenormalized transmittance TðzÞ of the measured sample is given by

TopenðzÞ ¼ 1�bI0Leff

2ffiffiffi

2p½1þ ðz=z0Þ

2�, (1)

where z is the longitudinal scan position along the optical z-axisfrom the focal point of the incident Gaussian beam. The peakintensity at the focus position is represented by I0, while z0 is theconfocal beam parameter (for the used experimental conditions,we have found z0 to be � 1:0 mm and � 1:8 mm for l ¼ 532 and1064 nm, respectively); Leff ¼ a�1ð1� e�aLÞ is the effective opticallength, a is the linear absorption coefficient (a ¼ 4pk=l) and L isthe thickness of the films [11,12]. By fitting Eq. (1) to theexperimental data, it is possible to extract information about thenonlinear absorption coefficient b of the SiOx films.

4. Results and discussions

Nonlinear optical absorption have been systematically studiedin the Si-nc/SiO2 films. In Fig. 1 we present the z-scantransmittance in the near-field geometry of samples A and Bannealed to 1250 �C (24 and 20 at% Si excess, respectively) andexcited at l ¼ 1064 nm [Fig. 1(a)] and l ¼ 532 nm [Fig. 1(b)]. Thetransmittance of the other Si-nc/SiO2 films has not been includedin Fig. 1, as they were measured using higher peak intensities toobtain a similar nonlinear response.

For l ¼ 1064 nm and using a peak intensity I0 ¼ 3:1�109 W=cm2, we found that the transmittance of the Si-nc/SiO2

films presents a dip at the focus position, with a maximumvariation of about 10% and 15% for the samples A and B,respectively [see Fig. 1(a)]. This is a clear indication that nonlinearabsorption occurs in the Si-nc/SiO2 films excited at this wave-length. A similar result was observed in the other Si-nc/SiO2 films,displaying a reduction in the nonlinear absorption for lowerSi excesses.

The transmittance of the Si-nc/SiO2 films excited at l ¼ 532 nmrevealed also a dip around the focal point but with a more

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0.9

1.0

-10

0.8

1.0

Tran

smitt

ance

24 at.%

20 at.%

I0 = 3.1 x 109 W/cm2λ = 1064 nm

Tran

smitt

ance

24 at.%

20 at.%

λ = 532 nmI0 = 2.8 x 108 W/cm2

-5 0 5 10z (mm)

Fig. 1. z-Scan traces of the samples A (24 at%) and B (20 at%) annealed to 1250 �C in

the open aperture configuration, exciting at two different wavelengths: l ¼1064 nm (a) and l ¼ 532 nm (b). The solid lines denote the best fits to the

experimental data.

0 4 8 12 16 20 24

0

2

4

6

8

10

0

2

4

6

8

10

12

14 λ = 1064 nm

λ = 532 nm

β ( ×

10-6

cm

/W)

Silicon excess (at. %)

β (×

10-5

cm

/W)

Fig. 2. Nonlinear absorption coefficient b of the Si-nc/SiO2 films annealed to

1250 �C as a function of the Si excess for l ¼ 1064 nm (left y-axis) and 532 nm

(right y-axis).

200 400 600 800 1000 1200

1.5

3.0

4.5

6.0

7.5

1.5

3.0

4.5

6.0

7.5 λ = 1064 nm

λ = 532 nm

β (

x 10

-5 c

m/W

)

β ( x

10-6

cm

/W)

Tann (°C)

Fig. 3. Nonlinear absorption coefficient b of SiOx films with 20 at% Si excess as a

function of the annealing temperature up to 1250 �C.

A. Martınez et al. / Physica E 41 (2009) 1002–10051004

pronounced minimum, of about 25% and 40% for the samples Aand B annealed to 1250 �C, respectively [see Fig. 1(b)]. It is worthnoting that, in order not to damage the Si-nc/SiO2 films, theemployed peak intensity has been reduced for this wavelength(I0 ¼ 2:8� 108 W=cm2). Taking into account this reduction,the results from Fig. 1 indicate that the nonlinear absorptionof the Si-nc/SiO2 films is much stronger at l ¼ 532 nm than atl ¼ 1064 nm.

In order to extract the nonlinear absorption coefficient fromthe z-scan traces, we have fitted the experimental data usingEq. (1), keeping as a free parameter only the b coefficient.Excellent fits of the z-scan traces were obtained on the whole setof samples for both wavelengths, as can be seen in Fig. 1(a) and(b). In Fig. 2 we show the evolution of the absorption coefficient bas a function of the Si excess for the layers annealed to 1250 �C. Asthe Si content in the Si-nc/SiO2 films rises up to 24 at%, one canobserve that the absorption coefficient increases from 2:1� 10�6

to 9:2� 10�6 cm=W and from 0:28� 10�5 to 11:5� 10�5 cm=Wfor l ¼ 1064 and 532 nm, respectively. This is a clear indicationthat nonlinear absorption strongly depends on the amount of Si,which has been associated to two-photon absorption process(however, other effects can also occur, for instance, absorptionfrom free carriers [12]).

In the case of l ¼ 1064 nm, we observed that the strongestnonlinear absorption (once normalized to the number of Si–Sibonds) occurs for the Si-nc/SiO2 film with the highest Si excess(sample A). In fact, the Si-nc in this sample present an opticalbandgap Eog close to the resonant condition 2Ephoton ¼ Eog [13,14],which produces an enhancement of the two-photon absorptionprocess. By lowering the Si excess in the Si-nc/SiO2 films, the

average size of Si-nc decreases (see Table 1). This induces anincrease in the optical bandgap of Si-nc that shifts away from theresonant condition and, therefore, reduces the two-photonabsorption of Si–Si bonds. For l ¼ 532 nm, the behavior is verysimilar although, as the excitation energy is close to the bandgapenergy of the Si-nc, the new resonant condition would be reachedfor 2Ephoton ¼ E2 (ESi-nc

2 � 4:424:8 eV) [13,14]. Actually, this E2

energy gets smaller by increasing the Si-nc size down to 4.6 eV forlarge Si-nc. Therefore, near resonant conditions for a two-photonabsorption mechanism occurs for both excitation wavelengthsas the Si excess gets to its maximum, displaying an increase in thenonlinear absorption.

Once established the general behavior of the nonlinearabsorption in Si-nc/SiO2 films annealed to 1250 �C, we have alsostudied the nonlinear absorption of the SiOx films for a givencomposition (20 at% Si excess) after treatment at differenttemperatures (i.e., different aggregation states and/or crystallinedegree [9]). In Fig. 3 we present the evolution of the nonlinearrefractive index of sample B as a function of the annealingtemperature exciting at l ¼ 1064 or 532 nm. The nonlinearabsorption coefficient b shows a similar trend for both excitationwavelengths, displaying a progressive increase as the annealingtemperature was varied from the as-deposited state up to 1250 �C.By analyzing the evolution with the annealing temperatureof b, two different regimes can be singled out, related to themorphology of the Si precipitates: while for low annealing

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A. Martınez et al. / Physica E 41 (2009) 1002–1005 1005

temperatures (o1000 �C) the main effect is the precipitation ofamorphous Si clusters, at the highest temperature the crystal-lization of the nanoparticles dominates [9]. Therefore, theobserved steep rise of b above 1000 �C correlates with thecrystalline transformation, suggesting that long range order inthe Si-nanoparticles highly affects the nonlinear response. Underns-excitation, the excitation of free carriers has to be taken intoaccount in Si-aggregates with a high crystalline order. Therefore,the observed increase in the nonlinear absorption with theannealing temperature could be related to the excitation of freecarriers by the absorption of two photons, in a first step, whichpromote the absorption of single incident photons, in secondterm, producing an increase of the total nonlinear absorption[12]. Consequently, it is important to consider both two-photonabsorption and free carrier absorption, when exciting highlycrystalline Si-nanoparticles under ns-pulses. On the other hand, asthe experimental results show in Fig. 3 it would be possibleto tune the nonlinear absorption of the system by controllingthe crystalline degree of Si-nanoparticles (i.e., the excitationof free carriers by long pulses).

5. Conclusions

z-Scan measurements were performed in Si-nc/SiO2 filmsdeposited by PECVD with Si excesses up to 24 at%, exciting by5-ns pulses of an Nd:YAG laser working at l ¼ 1064 and 532 nm.The traces in the open aperture configuration have shown in allthe samples a dip at the focus position, indicating the existence ofa nonlinear absorption process. The analysis of the experimentaldata has revealed a nonlinear absorption coefficient from 2:1�10�6 to 9:2� 10�6 cm=W and from 2:8� 10�6 to 1:1� 10�4 cm=W,for l ¼ 1064 and 532 nm, respectively. The nonlinear absorptioncoefficient increases as the Si content in the Si-nc/SiO2 films rises.The nonlinear process is ruled by two-photon absorption, reach-

ing resonant conditions for large Si-nc sizes. On the other hand, aclear influence of the thermal treatment on b has been observedwhich could be related to absorption from free carriers.

Acknowledgments

We acknowledge financial support from the EuropeanCommission under the Sixth Framework (Phologic, ContractHP6-017158) and from the Spanish Ministry of Science andTechnology (TEC2006-13907-C04-02/MIC). One of us (A.M.)acknowledges support from Departament d’Universitats i Recercade la Generalitat de Catalunya and the European Social Fund.

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