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Phys. Status Solidi C 8, No. 3, 969–973 (2011) / DOI 10.1002/pssc.201000420

Comparative study of the nonlinearoptical properties of Si nanocrystalsfabricated by e-beam evaporation,PECVD or LPCVDA. Martınez*,1, S. Hernandez1, P. Pellegrino1, O. Jambois1, P. Miska2, M. Grun2, H. Rinnert2, M. Vergnat2,V. Izquierdo-Roca3, J. M. Fedeli4, and B. Garrido1

1 MIND-IN2UB, Departament d’Electronica, Universitat de Barcelona, Martı i Franques 1, 08028 Barcelona, Spain2 Institut Jean Lamour, UMR CNRS 7198, Nancy Universite, UPV Metz, France3 M-2E/XaRMAE/IN2UB, Departament d’Electronica, Universitat de Barcelona, Martı i Franques 1, 08028 Barcelona, Spain4 CEA, LETI, Minatec, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France

Received 8 June 2010, revised 25 October 2010, accepted 27 October 2010Published online 3 February 2011

Keywords Si, nanocrystal, nonlinear, absorption, photoluminescence, Raman scattering, e-beam evaporation, CVD

∗ Corresponding author: e-mail amartinez@el.ub.es, Phone: +34-934039175, Fax: +34-934021148

The nonlinear optical properties of silicon nanocrystals(Si-nc) embedded in oxide matrices have been studiedin samples obtained by three different technological pro-cess: e-beam evaporation, plasma enhanced or low pres-sure chemical vapor deposition (PECVD and LPCVD,respectively). Z-scan measurements were performed inall the samples at 1064 nm by using ns-pulses of aNd:YAG laser. Similar nonlinear refractive index wasfound in the three systems, while differences in the non-linear absorption were found as a function of the deposi-tion method.

The nonlinear results have been evaluated in terms of thecrystalline and amorphous fractions from Raman scatter-ing and photoluminescence (PL) measurements. WhilePL measurements show a emission spectrum similar forall the samples, Raman spectra reveal a sizeably differentSi crystalline/amorphous ratio, depending on the deposi-tion method. Therefore, the interface between the SiO2

matrix and Si-nc plays a crucial role in determining thenonlinear optical response of the Si-nc rich layers.

1 Introduction Silicon nanocrystals (Si-nc) embed-ded in oxide matrices have been proposed for nonlinearphotonic applications as their nonlinear optical propertieswere found to be larger than the ones of silica or bulkSi [1,2]. The nonlinear optical properties of Si-nc havebeen extensively studied in recent years, in particular tofacilitate applications such as light modulation and am-plification [3–5]. For their fabrication, several approacheshave been widely employed, such as laser ablation [6],ion implantation [7], cosputtering [8], evaporation [9,10], plasma enhanced or low pressure chemical vapordeposition (PECVD and LPCVD, respectively) [11–13].However, the different technological steps strongly affect

the precipitation of Si-aggregates and their surroundingmedium, modifying their structural, and both linear andnonlinear optical properties, leading to a sizeable degreeof uncertainty in interpreting its effects on the nonlinearoptical properties.

In this work we present a comparison of the nonlin-ear optical properties of Si-nc under ns excitation pulsesfor materials produced by three different techniques. Si-rich oxides were deposited onto silica substrates either bye-beam thermal evaporation, PECVD or LPCVD, and sub-sequently annealed at high temperatures in order to precip-itate the nanostructures. Z-scan analysis in the nanosec-ond range at 1064 nm was used to make a comparison

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970 A. Martínez et al.: Comparative study of the NLO properties of Si-nc fabricated by evaporation

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of the nonlinear optical properties of Si-nc for materialsproduced by this different techniques. Photoluminescenceand Raman scattering analysis enable to explain the dif-ferent nonlinear behavior observed in the Si precipitatescontained in the different matrices.

2 Experimental Substoichiometric SiOx films weredeposited on silica substrates using three alternative meth-ods: electron beam evaporation (sample N430B), PECVD(sample P10D) or LPCVD (sample K654A). The firstsample was deposited in a ultrahigh vacuum chamberby successive thermal evaporation of SiO powder andelectron beam evaporation of SiO2 powder, obtaining aSiO2/SiO/SiO2 multilayered structure containing 33 peri-ods of 5 nm SiO stacked between two 5 nm SiO2 layers.During the growth, the SiO2 substrate was kept at 100 ◦Cand the deposition rate was controlled by quartz microbal-ances (≈ 0.1 nm/s). For the PECVD deposition, a N2Oand SiH4 flow rates were kept at 2000 standard cubic cen-timeters per minute (sccm) and 500 sccm, respectively, andthe substrate at 400 ◦C. For LPCVD deposition, N2O andSiH4 flow rates were kept at 160 sccm for the former and40 sccm for the latter. During the deposition, the substratewas under a constant temperature of 600 ◦C. After theirrespective deposition processes, the three samples weresubmitted to thermal treatments under controlled N2 flowto promote Si-nc precipitation. The sample deposited byevaporation was annealed at 1100 ◦C for 10 minutes in arapid thermal furnace, while samples deposited by PECVDor LPCVD were annealed for 1 hour in a conventional fur-nace at 1250 ◦C or 1100 ◦C, respectively. A resume ofthe properties of the three samples are shown in Table 1(for more details of the samples see Ref. [10], Ref. [12] orRef. [14]).

The nonlinear optical response of Si-nc was evalu-ated by Z -scan measurements in open and closed apertureconfigurations using a pulsed Nd:YAG laser as excitationsource, working at λ = 1064 nm with a 10 Hz repeti-tion rate. Measurements were performed using pulses ofdifferent duration from 4.7 ns to 7.8 ns, by delaying theQ-switch of the lamp. Peak intensities around 109 W/cm2

were used in all cases. A more detailed explanation of thez-scan setup and procedure can be found elsewhere [15,16].

Table 1 Structural and optical parameters of the samples de-posited by evaporation, PECVD and LPCVD: thickness (d), lin-ear refractive index (n), crystalline fraction (fc) and volumetricfraction (fv).

Deposition d Si excess n fc fv

Technique (nm) (%)Evaporation 165 25 1.51 0.28 0.20

PECVD 432 24 1.97 0.30 0.19LPCVD 161 21 1.87 0.16 0.16

Figure 1 (Color online) Z-scan traces in the open aperture (opensquares) and closed aperture (full circles) configurations of thesample deposited by evaporation. The solid lines are the best fitsto the experimental data.

Room temperature photoluminescence (PL) was per-formed using the 488 nm line of a Ar+ laser as excitationsource. In order to obtain the whole emission spectrumfrom Si-nc, the spectrum was acquired by combining thePL signal detected by a GaAs photomultiplier tube and aCCD camera, sensible in the range of 400-850 nm and 790-1050 nm, respectively. On the other hand, Raman measure-ments have been performed in backscattering configurationby exciting the samples with the 514.5 nm line of an Ar+laser. The spectra were acquired at room temperature incross-section configuration by using an optical microscopewith an objective ×100 and a motor controller stage (min-imum motor step of 20 nm). This configuration allows toprobe similar volumes from the Si-nc region and SiO2 sub-strate, which strongly reduces the Raman signal comingfrom SiO2 substrate. All the Raman spectra were analyzedthrough a Jobin Yvon T64000 spectrometer equipped witha low-noise liquid nitrogen-cooled charge-coupled devicedetector.

3 Results and discussion Z-scan measurementsusing different pulse durations from tp = 4.7 ns totp = 7.8 ns were performed in all samples in closed andopen configurations (see for instance Fig. 1). In the openaperture configuration a dip around the focal position wasfound for evaporation and PECVD grown samples (about15 % and 10 % respectively), while a small transmittancerise about 5 % is observed for LPCVD grown sample.On the other hand, in the closed aperture configuration,the three samples has shown similar traces with peak-to-valley lineshape. The traces in this configuration presented

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Figure 2 (Color online) Dependence of (a) nonlinear absorptioncoefficient β and (b) nonlinear refraction index n2 as a functionof the excitation pulse duration tp for samples deposited by evap-oration (circles), PECVD (squares) and LPCVD (triangles). Thesolid lines are the best linear fits to experimental data.

an asymmetric shape, which perfectly correlates with thenon-negligible contribution from nonlinear absorption.

In order to extract quantitative information, the experi-mental Z -scan traces were evaluated using the model devel-oped by Sheik-Bahae et al. [15]. By fitting simultaneouslythe experimental traces for open and closed configurations,it is possible to accurately determine the nonlinear coeffi-cients, β and n2 (see solid lines in Fig. 1) [16]. In Fig. 2 wehave plotted the nonlinear response of the Si-nc/SiO2 sys-tem as a function of the pulse duration for all three sam-ples. The experimental results display a linear increasingtrend of the nonlinear response (in absolute values), with ax-axis intersection at tp ≈ 4.0 ns for all the curves. Thislinear time-dependence indicates that other effects differ-ent from bound electrons or two-photon absorption, thatdisplay a typical response in the fs range, are contribut-ing to the nonlinear optical response. In fact, contributionsfrom free carrier excitation or induced sample heating inthe nonlinear optical response show a linear dependenceversus the pulse duration [16,17]. Actually, for ns-pulsesthe excitation of free carriers produces larger changes inthe optical response than induced sample heating and con-

sequently the observed nonlinearities are mainly due to theformer with a small contribution from the latter, as dis-cussed in a recent work [16]. Moreover, markedly differentnonlinear behavior can be obtained from sample to sam-ple for both nonlinear absorption coefficient and nonlinearrefractive index, that could be related to a different localordering of the Si-aggregates and their surrounding media.

In order to clarify the observed nonlinear optical be-havior, we have studied their luminescent emission andtheir crystalline ordering by using PL and Raman measure-ments, respectively. In Fig. 3 we present the PL spectraof the three samples. The luminescence emission for allthe samples has its maximum in the range 860-890 nm.This observation indicates that the three samples containSi-nc of similar sizes. On the other hand, the distributedemission energy is quite different from sample to sam-ple: samples N430B and K654A present a emission bandwith a full width at half maximum (FWHM) of ≈ 150nm, while sample P10D displays a broader energy distri-bution, around 230 nm. Therefore, one can conclude thatthe PECVD grown sample contain Si-nc with a larger sizedistribution than the others two, while sample deposited byLPCVD is the narrowest size distribution. Jambois et al.estimated that the observed luminescence emission can beassociated to Si-nc with average sizes around 5 nm, with asize distribution around ±0.5 nm or ±1.0 nm for the ob-served emission broadening, 150 and 230 nm, respectively[18].

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Figure 4 (Color online) Raman spectra of samples deposited byevaporation, PECVD and LPCVD. The frequency of the bulk SiLO-TO phonon is also shown for comparison.

Raman spectroscopy of evaporation, PECVD andLPCVD grown samples is presented in Fig. 4. All theRaman spectra show a sharp peak around 518 cm−1,with a FWHM approximately similar for all of them(≈ 10.5 cm−1). This Raman feature corresponds to theoptical mode of crystalline Si-nanoaggregates [14].

By using a phonon confinement model, we have es-timated the size of the Si-crystalline aggregates (in ourcase, Si-nc). A detailed description of the employed modelcan be found in Ref. [14]. We found Si-nc sizes of 2.75nm, 2.80 and 2.85 nm for samples deposited by evapora-tion, PECVD and LPCVD, respectively. The size evolu-tion of the crystalline size of Si-aggregates perfectly corre-lates with the PL spectra and both techniques are suggest-ing that the three samples contain Si-nc of similar sizes.Nevertheless, there is a large difference in the determinedsizes using both techniques. Despite that phonons of crys-talline Si are confined in region with high local orderingand estimation by Raman provides information about thesize of this region, the PL energy depends on both Si-ncsize and the quality of the confinement matrix surround-ing the Si-nc [3]. Consequently, the exciton created insidea Si-nc surrounded by a poor quality silicon oxide is moredelocalized and its energy shifts to lower values, providingan overestimation of the total Si-nc size determined by PL.

Moreover, broad structures at 150 cm−1 and in therange between 400 and 500 cm−1 can be observed in theRaman spectra that are related to a reminiscent contribu-

tion from disorder-activated modes, either acoustic or opti-cal [14]. The crystalline fraction for each sample has beenestimated from the Raman spectra. The Raman intensitycorresponding to optical modes has been deconvoluted intobands and their relative intensities converted into volumeof the respective phase. Thus, the crystalline fraction hasbeen calculated by fc = Ic/(Ic + Ia), being Ic and Ia

the crystalline and amorphous Raman integrated intensities[14]. The crystalline fraction results in fc = 28 % for sam-ples deposited by evaporation and PECVD and fc = 17 %the one deposited by LPCVD. Thus, sample deposited byLPCVD is the one which shows much lower crystallineprecipitation than the rest.

In a previous work [14], we reported that this amor-phous phase is surrounding the crystalline aggregates, ingood agreement with observations by Iacona et al. usingtransmission electron microscopy [19]. This Si amorphousnetwork is indeed the one of the suboxide transition regionthat always connects the crystalline nanoparticle to thepure SiO2 matrix. The presence of this amorphous phasecan significantly affect the nonlinear optical response ofthe layers, both the refractive and the absorptive part. Atthis stage, a well developed theoretical model to explainthis behavior is not available, due to the difficulty to getthe detailed structural information about the nanostructureof the different phases. Nevertheless we were able to veri-fied that it is not possible to reproduce the optical nonlin-earities of the three samples by means of a simple combi-nation of the response of amorphous and crystalline phase,weighted by their relative volumes. This implies that it isthe interface between Si-nc and SiO2 matrix that rules thenonlinear optical response of this complex system.

4 Conclusions Z-scan measurements were per-formed in SiOx films deposited by electron beam evap-oration, PECVD or LPCVD, with similar Si excess in thesamples, exciting by ns pulses of a Nd:YAG laser work-ing at λ = 1064 nm. Nonlinear optical measurementsperformed by z-scan technique has revealed a markedlydifferent nonlinear response of LPCVD grown sample,compared with the others two, displaying a negative non-linear absorption coefficient. PL and Raman measurementshave revealed the existence of Si-nc with similar sizes inall the films. On the other hand, the observed amorphousRaman features point out to a different crystalline degreeof sample deposited by LPCVD. This particular samplepresents a much larger amorphous phase that maybe theresponsible of the observed nonlinear absorption coeffi-cient. Consequently, the different fabrication proceduresstrongly affect the precipitation of Si-aggregates and theirsurrounding medium, modifying their structure and, on itsturn, both their linear and nonlinear optical properties.

Acknowledgements We acknowledge financial supportfrom the Spanish Ministry of Science and Innovation (TEC2009-08359 and HF2007-0030). One of the authors (A.M.) acknowl-edges support from ’Comissionat per a Universitats i Recerca del

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DIUE de la Generalitat de Catalunya’ and the European SocialFund.

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