Fabrication of luminescent a-Si:SiO2 structures by direct irradiation of high power laser on silicon...

A

Fh

PD

a

ARRAA

KLaPOR

I

navrocwnpfaMTmllaba

h0

ARTICLE IN PRESSG ModelPSUSC-27569; No. of Pages 9

Applied Surface Science xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

abrication of luminescent a-Si:SiO2 structures by direct irradiation ofigh power laser on silicon surface

artha P. Dey, Alika Khare ∗

epartment of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

r t i c l e i n f o

rticle history:eceived 3 September 2013eceived in revised form 22 March 2014ccepted 25 March 2014vailable online xxx

a b s t r a c t

In this paper, the structural and compositional modification of polished silicon (Si) wafers by irradiationof second harmonic of Q switched high power Nd: YAG laser in air is reported. The surface morphology,recorded by scanning electron microscope (SEM), shows micro cluster formation. Raman spectra revealthe presence of amorphous silicon embedded in silicon dioxide (SiO2) matrix in these structures which isfurther confirmed by energy dispersive X-ray (EDX) and Fourier transform infrared (FTIR) spectroscopic

eywords:aser irradiation-Si:SiO2 nanostructureshotoluminescencexygen defect centersaman spectroscopy

studies. These nanostructures of amorphous Si embedded in SiO2 matrix (a-Si:SiO2) showed luminescencein far red region. The effect of laser fluence on the photoluminescence properties and its possible originwere discussed.

© 2014 Elsevier B.V. All rights reserved.

ntroduction

The nanostructured silicon, viz.; nano-crystalline (nc) Si,anostructured amorphous Si (a-Si), nc Si or a-Si embedded inmorphous SiO2 matrix, porous silicon (po-Si) etc. shows intenseisible and near infrared photoluminescence [1–3]. Radiativeecombination rate for amorphous Si nano particles is two to threerders of magnitude higher than that for crystalline Si nano parti-les. Visible photoluminescence (PL) from nanostructured a-Si, asell as its oxides and nitrides grown by various deposition tech-iques were reported in the literature [4–6]. Photoluminescenceroperties make nanostructured Si potential candidate as emittersor field-emission based devices such as high-definition displayss well as other vacuum microelectronics devices and systems [7],EMS-based memory devices [8] and light emitting devices [9].

he thin nanostructured film can be grown onto a substrate byagnetron sputtering [10], chemical vapor deposition [11], pulsed

aser ablation [4,12] etc. In some cases, it is important to produceocal nanostructured area directly on a silicon-based device or on

Please cite this article in press as: P.P. Dey, A. Khare, Fabrication of lumlaser on silicon surface, Appl. Surf. Sci. (2014), http://dx.doi.org/10.10

ssembled integrated chip. The local etching of a Si based film cane performed by nano second pulsed radiation from a laser with

high precision. In the present paper, the generation of a-Si:SiO2

∗ Corresponding author. Tel.: +91 03612582705; fax: +91 3612582749.E-mail address: [email protected] (A. Khare).

ttp://dx.doi.org/10.1016/j.apsusc.2014.03.168169-4332/© 2014 Elsevier B.V. All rights reserved.

nanostructures via laser ablation and its room temperature PLproperties are discussed.

Experimental setup

The polished silicon (1 0 0) wafers were irradiated by looselyfocusing a second harmonic (532 nm) of a Q switched high powerNd: YAG laser (pulse duration of 8 ns and repetition rate of 10 Hz)in air at room temperature as shown in Fig. 1. The laser beam wassteered by a prism and irradiated on the target at an incident angle∼30◦ to avoid back reflection. The focal spot of laser beam ontothe Si target was elliptical with major and minor axes of ∼1.9 mmand 1.6 mm, respectively. The focusing of high power laser ontoSi target in air results in the breakdown of the material and theatmospheric air in the neighborhood of focal region. This producesSi and oxygen ions which reacts and get deposited onto the tar-get around the periphery of the focal spot to yield a-Si:SiO2. Thelaser fluence was varied approximately from 0.35 to 2.67 J cm−2.The Raman spectra of a-Si:SiO2 were recorded at room temperature(RT) using micro-Raman setup (Lab-Ram HR 800) in back scatter-ing geometry. The 488 nm line of Ar ion laser was used as excitationsource. The same laser was used for PL studies of the samples at RT.

inescent a-Si:SiO2 structures by direct irradiation of high power16/j.apsusc.2014.03.168

The surface morphology was recorded by SEM and the composi-tional analysis was performed by EDX. FTIR transmission spectraof all the samples were recorded to get the confirmation of thepresence of SiO2. Transmission electron microscopy (TEM) images

dx.doi.org/10.1016/j.apsusc.2014.03.168


http://www.sciencedirect.com/science/journal/01694332

http://www.elsevier.com/locate/apsusc

mailto:[email protected]


ARTICLE IN PRESSG ModelAPSUSC-27569; No. of Pages 9

2 P.P. Dey, A. Khare / Applied Surface Science xxx (2014) xxx–xxx

F

wcr

R

tAcis2sat4iTSt

ig. 1. Schematic diagram of experimental setup for laser irradiation of silicon.

ere recorded for analyzing particle size distribution of the whitishlusters. Selected area electron diffraction (SAED) pattern was alsoecorded for all the samples.

esult and discussion

Fig. 2 shows SEM image of the laser irradiated spot onto Siarget at a laser fluence of 1.46 J cm−2 after 6000 shots of laser.

central dark crater-like region surrounded by white clusters islearly visible in the SEM image. The magnified view of white regions shown in Fig. 3(a). It shows the formation of cauliflower liketructure with non-uniform size distribution varying from 10 to0 �m. The Raman spectrum (200–700 cm−1) of these structures ishown in Fig. 3(b). It displays a prominent band (400–550 cm−1)nd a shoulder toward the low energy tail (350–400 cm−1) ofhis band. The 400–550 cm−1 band with broad peak at around83 cm−1 (FWHM ∼ 105.6 cm−1) is attributed to first-order scatter-


ng of vibrational TO phonon modes of amorphous silicon [13,14].he inset in Fig. 3(b) shows the Raman spectrum of unexposedi target displaying a sharp intense peak at 521 cm−1, a charac-eristics of bulk crystalline Si. It arises from the first-order Raman

Fig. 3. (a) SEM image, (b) Raman spectrum and (c) RT Photoluminescence spectrum

Fig. 4. (a) SEM image, (b) Raman spectrum and (c) RT Photoluminescence spectrum

Fig. 2. SEM image of focal spot of laser irradiated Si target at laser fluence∼1.46 J cm−2 after 6000 shots.

scattering of the longitudinal optical (LO) and the transverse opti-cal (TO) phonon modes which are degenerate at the Brillouin zonecenter in crystalline Si [13,14]. Fig. 3(c) shows the RT PL spec-trum of above sample. It displays a broad asymmetric PL ranging


from 1.6 to 2.2 eV, having a band tail in blue region, with a peakaround 1.82 eV [3,13]. Fig. 4(a)–(c) shows magnified SEM image,Raman spectrum and PL spectrum of central dark region of theabove sample, respectively. The SEM image displays formation of

of whitish clusters formed at laser fluence of 1.46 J cm−2 (6000 laser shots).

of dark central region formed at laser fluence of 1.46 J cm−2 (6000 laser shots).


ARTICLE ING ModelAPSUSC-27569; No. of Pages 9

P.P. Dey, A. Khare / Applied Surface

F02

mshicsnptrtmiattacar

ao2p

ig. 5. EDX spectra of whitish area of sample fabricated at laser fluences (a).35 J cm−2, (b) 0.67 J cm−2 and (c) 1.46 J cm−2 with 6000 laser shots and (c).67 J cm−2 with 3000 laser shots.

icron sized rippled structures termed as laser-induced periodicurface structures (LIPSS) [15]. Its Raman spectrum shows a sharpigh intense peak at 521 cm−1 similar to that of a bare target shown

n the inset of Fig. 3(b). No detectable PL was observed in the darkentral regions as evident from Fig. 4(c). The central dark regionurrounded by whitish region is formed by the re-deposition anducleation of material onto the target from the expanding plasmalume. The laser beam spot being Gaussian has an intense cen-ral region with relatively less energy at periphery. The centralegion receives larger energy density and hence reaches higheremperatures compared to peripheral regions. Due to larger ther-

al diffusion length in the central regions the depth of molten masss comparatively more. Therefore, central region has much largermount of melt volume at relatively higher temperature comparedo regions at periphery. In central regions the material has enoughime to re-crystallize from the melt due to increased recalescencend higher temperature, hence providing sufficiently long period ofooling. The peripheral region, having low temperature and smallermount of melt volume, cools much earlier to allow hardly anye-crystallization and hence forms amorphous phase [16,17].

Fig. 5(a)–(d) shows the EDX spectra of whitish region formed


round the periphery of the samples prepared at laser fluencesf 0.35, 0.67 and 1.46 J cm−2 after irradiation with 6000 shots and.67 J cm−2 after 3000 shots, respectively. These spectra show theresence of silicon and oxygen in the nanostructures indicating the

PRESS Science xxx (2014) xxx–xxx 3

SiO2 matrix formation [5]. The atomic proportion of O2 is foundto have increased gradually from 44.9% to 70.9% with increasinglaser fluence. The reaction of ions of Si and oxygen in laser pro-duced plasma results in subsequent SiO2 formation during coolingof plasma followed by the re-solidification onto the target surface.

To study the effect of laser fluence on the formation of a-Siembedded in SiO2 matrix, the Raman spectra of all the samplesprepared at laser fluence of 0.35, 0.67 J cm−2 and 1.46 J cm−2 afterirradiation with 6000 shots and 2.67 J cm−2 after 3000 shots wererecorded. At 2.67 J cm−2 after 6000 shots a hole was drilled on thetarget, so it was not considered. Fig. 6 shows the Raman spectraof the periphery of the samples (white region). The de-convulatedspectra fitted to multiple peaks of lorentzian lineshape is shown inthe figure [18]. After de-convulation five distinguishable peaks havebeen observed in Raman spectra of white region formed at laser flu-ence of 0.35 J cm−2 as shown in Fig. 6(a). The five peaks observed at615 cm−1, 512 cm−1, 478 cm−1, 433 cm−1 and 333 cm−1 have beendesignated as peak 1, peak 2, peak 3, peak 4 and peak 5, respectively.Peak 1 is the least intense which corresponds to defect peak D2 ofSiO2 matrix. Peak 2 observed at 512 cm−1 could be due to pres-ence of small fraction of nc-Si. Peak 3 has the maximum intensitywith broad FWHM ∼ 67 cm−1 peaking around 478 cm−1 which isattributed to first-order scattering of vibrational TO phonon modesof a-Si [19,20]. Peak 4 observed at around 433 cm−1 has maximumFWHM of 116 cm−1 with peak intensity nearly half of that of peak3. This peak is attributed to the six-membered rings (Si2O4) andpresence of five-, seven- and higher member rings is responsiblefor broadening of this band [21]. Moreover, a blue shift of this peakis related to decrease of Si O Si bridging angle between differ-ent tetrahedral units forming the SiO2 matrix. Finally, a broad peak5 observed at 333 cm−1 is attributed to combination of LA and LOlike modes of amorphous silicon [19,20]. Similar Raman peaks wereobserved for the samples prepared at laser fluence of 0.67, 1.46 and2.67 J cm−2. In Fig. 6(b)–(d), D2 peak was absent. In Fig. 6(d), addi-tional peak at 491 cm−1 assigned as peak 2 is observed. This peakcorresponds to D1 peak of SiO2 which though present in other spec-tra could not be deconvuloted due to close proximity and broadnessof vibrational TO phonon modes of a-Si present around 480 cm−1.D1 and D2 peaks are associated with in-phase breathing motions ofoxygen atoms in three- and four-member rings, respectively [21].Thus Raman spectra reveal the presence of both a-Si and disorderedSiO2 in the whitish micro-clusters. The data of different decon-vulated peaks and respective FWHM of Raman spectra of whitishregion formed around the periphery of the irradiated Si target atdifferent laser fluences are listed in Table 1. The last column of thetable displays the assignments of peaks to different Raman activevibrational modes.

Fig. 7 shows the FTIR transmission spectra of laser irradiatedregion in c-Si wafer at different laser fluences. The baseline cor-rection is being done w.r.t the FTIR transmission spectrum ofunirradiated intrinsic c-Si target. The intensity of absorption band(a) located around 458 cm−1 which is attributed to rocking vibra-tion of Si O, increases with increasing laser fluence. At higherlaser fluences of 1.46 and 2.67 J cm−2, absorption bands markedas (b) and (c) evolved at 652 cm−1 and 789 cm−1, respectively.The band at 652 cm−1 is attributed to Si Si stretching vibrationin a-Si. The band at 789 cm−1 is due to in plane Si O Si bend-ing vibration in SiO2 matrix. The absorption band ranging from1000 cm−1 to 1300 cm−1 is the most prominent absorption fea-ture observed in all these samples. This spectral region consistsof the most intense absorption band designated as (d) with a broadshoulder leveled as (e). The absorption band (d) located around


1074 cm−1 for samples prepared at 0.38 J cm−2 shows a gradualblue shift with increasing laser fluence. It shifts to 1081 cm−1 atmaximum laser fluence of 2.67 J cm−2. The absorption shoulderfound to be slightly blue-shifted with increasing laser fluence, from




f whit

1aeaSveaTaS

TP

Fig. 6. Curve-fitted and deconvulated RT Raman spectra o

189 cm−1 to 1192 cm−1. Both of these bands showed an increase inbsorption intensity with increasing laser fluence and at higher flu-nces the shoulder becomes more prominent. The absorption bandt (d) is attributed to the in phase bond-stretching vibrations alongi O bond and that of (e) are due to out-of-phase bond-stretchingibrations along Si O bond. [22]. The FTIR spectra for laser flu-nce of 2.67 J cm−2 shows an additional absorption band marked


s ‘*’ around 930 cm−1, attributed to Si OH stretching vibration.he absorption bands associated with the rocking (a), bending (c)nd stretching (d and e) vibration modes of the Si O Si bonds iniO2 reconfirms the formation of SiO2 matrix in the Si target. The

able 1eak centers and FWHMs of deconvulated RT Raman spectra of whitish clusters of a-Si:Si

Laser fluences(J cm−2)

Peak no. Raman peaks(cm−1)

FWHM (cm

0.27 1 615 30

2 512 44

3 478 67

4 433 116

5 339 203

0.5 1 519 32

2 483 47

3 445 77

4 360 119

5 301 43

1 1 515 45

2 482 65

3 443 68

4 380 37

2 1 518 30

2 491 41

3 471 47

4 441 74

5 384 50

ish clusters of a-Si:SiO2 formed at different laser fluences.

increase in absorption dip with the laser fluence further indicatesthe increase in proportion of SiO2 in confirmation with the EDXresults (Fig. 5).

The SEM images of whitish region formed around the periph-ery of the samples prepared at laser fluences of 0.35, 0.67 and1.46 J cm−2 after irradiation with 6000 shots and 2.67 J cm−2 after3000 shots are shown in Fig. 8(a)–(d), respectively. The images


show micron sized cauliflower-like clusters with a nanostructuredenvelop in all the samples. Fig. 9(a)–(d) shows TEM micrographsand Fig. 9(e) and (f) shows the corresponding SAED pattern ofwhitish clusters formed at different laser fluences. From these

O2 formed at different laser fluences with appropriate peak assignments.

−1) Peak assignments

D2 localized 3-membered siloxine ringRed shifted LO and TO phonon modes which are degenerated atthe Brillouin zone center in nc-siliconTO phonon modes of a-Si6-Membered ring in silica networkCombination of LA and LO modes of a-SiRed shifted LO and TO phonon modes of c-Si (signature of nc-Si)TO phonon modes of a-Si6-Membered ring in Silica networkCombination of LA and LO modes of a-SiLA phonon modes of a-SiRed shifted LO and TO phonon modes of c-Si (signature of nc-Si)TO phonon modes of a-Si6-Membered ring in Silica networkLO modes of a-SiRed shifted LO and TO phonon modes of c-Si (signature of nc-Si)D1 localized 4-membered siloxine ringTO phonon modes of a- Si6-Membered ring in Silica networkLO modes of a-Si



P.P. Dey, A. Khare / Applied Surface

Ff

inflTploFoassot

Fl

ig. 7. RT FTIR transmission spectra of whitish clusters of a-Si:SiO2 formed at dif-erent laser fluences.

mages, it can be seen that at lowest laser fluence densely packedanosized clusters of variable sized were formed while at high laseruences the particle are relatively less dense and more distinct.he inset of Fig. 9(c) and (d) shows the histograms depicting thearticle size distribution. It can be observed that with increased

aser fluences from 1.46 J cm−2 to 2.67 J cm−2, the average sizef spherical nanoclusters decreases from nearly 12 nm to 5 nm.rom the corresponding SAED pattern shown in Fig. 9(e)–(h), it isbserved that diffraction pattern of these nanostructures shows

characteristic of the amorphous material. The inset of Fig. 9(c)


hows the HRTEM of white clusters formed at 1.46 J cm−2. It can beeen from the image that the clusters composed of nanospheresf nearly 15 nm diameter is amorphous in nature as no crys-al planes is visible. Hence there is no formation of significant

ig. 8. SEM images of whitish clusters formed at different laser fluences (a) 0.35 J cm−2, (baser shots.


nanocrystallites in all these samples. These results are consistentwith the results of Raman studies which claimed that whitishclusters are basically composed of a-Si: SiO2.

The PL spectra of the whitish region of samples fabricated atdifferent laser fluence are shown in Fig. 10. The room temperaturePL for all the samples shows an asymmetric broad luminescenceband with peak at around 1.82 eV. The multiple peak fitting of PLspectra using Gaussian lineshape function are shown in Fig. 11.After deconvulation, two peaks can be clearly distinguished on eachcurves as shown in Fig. 11, one is low energy peak (peak 1) rangingfrom 1.6 to 1.8 eV and other is high energy peak (peak 2) rangingfrom 1.6 to 2.4 eV. The low energy peaks are less intense than highenergy peaks in each spectrum. From Fig. 12, plot of PL peak energyversus laser fluence, it is observed that the low energy PL peaksshows a slight blue shift from 1.66 to 1.75 eV with increasing laserfluence while that for high energy peaks were relatively constantat around 1.83 eV for all fluence except for white clusters formedat 2.67 J cm−2 after 3000 shots whose PL peak 2 was observed at1.95 eV.

The origin of PL in the present case could be two fold. The bulk a-Si gives a near-infrared PL around 1.3–1.4 eV at room temperaturedue to structural disorder and can get blue shifted to 2.7 eV when innanostructure form [23,24]. In present case, peak 1 in the PL spec-tra of samples fabricated at different laser fluences clearly showsa blue-shift from PL peak of bulk a-Si. This blue shift is attributedto the nanostructured a-Si having particle size less than 20 nm asobserved in TEM results. Unlike nc-Si, visible PL and related blueshift with decrease in size could not be explained by quantumconfinement model in a-Si. This blue-shift in PL peak energy canbe attributed to nanostructured a-Si clusters on the basis of spa-


tial confinement model [24]. The blue shift observed in PL fromIR region in bulk a-Si to red region in the nanostructured samplescould be explained entirely on the basis of the statistical distribu-tion of available bandtail states [3,25]. With the reduction of the

) 0.67 J cm−2 and (c) 1.46 J cm−2 with 6000 laser shots and (c) 2.67 J cm−2 with 3000


Please cite this article in press as: P.P. Dey, A. Khare, Fabrication of luminescent a-Si:SiO2 structures by direct irradiation of high powerlaser on silicon surface, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.168



Fig. 9. TEM micrographs (a)–(d) and corresponding SAED pattern (e)–(h) of whitish clusters formed at different laser fluences 0.35 J cm−2, 0.67 J cm−2 and 1.46 J cm−2 with6000 laser shots and 2.67 J cm−2 with 3000 laser shots, respectively. Histograms showing particle size distributions in whitish clusters formed at laser fluences of 1.46 J cm−2

and 2.67 J cm−2 are given in insets of 9 (c) and 9 (d), respectively. Inset of 9 (c) also shows the HRTEM of nanostructures formed at laser fluence of 1.46 J cm−2.



P.P. Dey, A. Khare / Applied Surface Science xxx (2014) xxx–xxx 7

Ff(

sdHdCttdttinfr

F0

ig. 10. RT Photoluminescence spectra of whitish clusters of a-Si:SiOx formed at dif-erent laser fluences ∼0.35 J cm−2 (6000 shots), 0.67 J cm−2 (6000 shots), 1.46 J cm−2

6000 shots), and 2.67 J cm−2 (3000 shots).

tructure size, electron–hole (e–h) pairs get confined in a smallimension resulting in decrease of the available band-tail states.ence, probability of e–h recombination is higher between theeeper states in proximity to mobility edges of transition bands.onsequently, the lowest energy accessible for recombination ofhe e-h pairs is greater than that of bulk a-Si. The broad line width ofhe PL spectra, which were observed for all samples, could be mostlyue to the statistical distribution of states in a-Si:SiO2, though a dis-ribution of structure sizes may also contribute further to broadenhe peak [23]. Peak 1 also showed small but gradual blue shift with


ncrease in laser fluence. This is perhaps due to formation of smalleranoparticles with increasing laser fluence which is confirmed

rom TEM results [26]. A PL peak shift from 1.5 eV to 1.75 eV iseported for a-Si spherical nanostructures of diameter from nearly

ig. 11. Curve-fitted and deconvulated RT Photoluminescence spectra of whitish cluste.67 J cm−2 (6000 shots), (c) 1.46 J cm−2 (6000 shots), and 2.67 J cm−2 (3000 shots) showin

Fig. 12. Plot showing variation of PL peak energy of peak 1 (at low energy) and peak2 (at high energy) versus increasing irradiating laser fluence.

12 nm to 4 nm [24]. But in present case, PL peak is observed to beblue shifted from 1.71 eV to 1.75 eV for a-Si spherical nanostruc-tures of average size of 12 nm and 5 nm formed in whitish clusters atlaser fluence of 1.46 J cm−2 and 2.67 J cm−2, respectively. The expla-nation for this reduction in particle size with increasing laser flu-ence is as follows. The laser produced plasma plume was filled withless Si atoms at low laser fluence while the plasma temperature andpressure were also low. So, the nucleation sites are very few. Thesenuclei continue to grow until nearby silicon clusters are completelyconsumed. The small number of nuclei formed accommodates allthe surrounding Si clusters, forming large sized particles. Withincrease in laser fluence, plasma plume attains relatively high tem-


perature and particle density resulting formation of large number ofnucleation sites. Therefore, though there is an increment in Si atomsat high laser fluence the nucleation sites are too numerous, leading

rs of a-Si:SiO2 formed at different laser fluences (a) 0.35 J cm−2 (6000 shots),(b)g two peak centers (in eV).



8 P.P. Dey, A. Khare / Applied Surface

Fi

tsiaSSR[gmt1Tete2ii

gdtwPooe

FO

ig. 13. Plot of atomic percentage of O2/Si (from EDX) and peak 2 PL intensity versusncreasing irradiating laser fluence.

o formation of small sized nanostructures [27]. This decrease inize with laser fluence can also be explained in context of increasen O2 content with laser energy. With the increase in laser fluencend subsequent oxidation of Si, O2 content increases and morei are consumed reducing the size of a-Si clusters embedded iniO2 matrix. As the presence of SiO2 is confirmed by EDX, FTIR andaman spectra, the peak 2 in PL spectra could be due to O2 defects5]. In Fig. 13, the variation of ratio of the atomic percentages of oxy-en to silicon (O2/Si) from EDX and PL intensity of peak 2 of whiteicro-clusters with increasing laser fluence are shown. The ratio of

he atomic percentages of oxygen to silicon increased from nearly.52 to 2.4 with increasing laser fluence from 0.35 to 2.67 J cm−2.he increase in O2 content with increasing laser fluence could bexplained by the fact that with increasing laser fluence from 0.35o 2.67 J cm−2 the plasmas of Si and air get more ionized and densernhancing the probability of oxidation of Si. The intensity of peak

is plotted against O2/Si from EDX in Fig. 14. It is seen that PLntensity of peak 2 varies non-monotonically, first increases withncreasing O2 content in the a-Si:SiO2 matrix and then decreases.

Basically, excess O2 and O2 deficiency are major variety of oxy-en defects which contribute to PL. The PL originating from oxygeneficient defect like ODC(I), ODC(II), E’ center etc. were reportedo have peak within 2.2–4.8 eV while excess O2 related defect PLere reported to be observed between 1.8 and 2.0 eV [28,29]. The


L spectra of white clusters for sample irradiated at laser fluencef 2.67 J cm−2 shows high energy peaks (Peak 2) at 1.95 eV and thatf other samples around 1.83 eV. Hence, the PL could be due toxcess O2 related defect. The potential candidate for such PL are

ig. 14. Plot showing variation in PL intensity of peak 2 with increasing atomic % of2/Si (from EDX).

PRESS Science xxx (2014) xxx–xxx

basically the defect states formed by non-bridging oxygen holecenter (NBOHC), interstitial O2

−, O3− and their respective neu-

trals. NBOHC has absorption bands peaking at 1.97 and 4.8 eV whiledefect states due interstitial O, O2 and O3 has that around 4.8 eV.So, as the excitation photon energy used in PL studies is 2.54 eV(488 nm) only, the probable cause for PL emission peaking around1.83 eV could be due to absorption of 2.54 eV photon and electronicexcitation to energy states formed by NBOHC defect correspondingat 1.97 eV absorption band and subsequent de-exitation [28]. Thenon monotonous behavior of PL intensity of Peak 2 with increase inlaser fluence can be explained as follows. With increased fluence,deposited energy increases which may strain the undisturbed Si2O7network ( Si O Si ) finally breaking it to give Si� (E’ center) and�O Si (NBOHC). The increased oxidation in a-Si:SiO2 matrix withincreasing laser fluence oxidizes Si� to convert them into �O Sicreating additional NBOHC defects. This enhances the PL intensitywith increasing laser fluence till 1.46 J cm−2 [29–31]. At 2.67 J cm−2

the oxidation of white clusters was too large that it might have pas-sivated the defects reducing the PL intensity. Hence the origin of PLcorresponding to peak 2 is attributed to electronic transition fromstates formed by NBOHC defects.

Conclusions

Micron sized amorphous silicon clusters embedded within SiO2matrix were fabricated by direct laser irradiation on crystalline Sitarget in air. The amorphous nature of the clusters was confirmedby Raman spectra as well as SAED patterns. Further both Ramanand FTIR spectra showed the presence of a-Si and amorphous SiO2.The EDX results showed increase of oxidation of Si with increasinglaser fluence which is also confirmed by gradual increase in infraredabsorption corresponding to Si O Si stretching vibration. Thesenanostructures exhibited an intense room temperature broad bandPL ranging from 1.6 eV to 2.2 eV with two peaks around red region.The origin of luminescence in these structures is attributed to nano-structures of a-Si and predominantly to NBOHC defects in SiO2matrix. This broad band luminescent property of a-Si: SiO2 withinvisible range via such a simple fabrication technique may find appli-cations in optoelectronic devices.

Acknowledgments

This work is partially supported by DRDO, New Delhi, India,project no. ERIP/ER/07003/30/M/01/1138. SEM, TEM and Lasermicro Raman facilities of CIF, IIT Guwahati, India, is also acknowl-edged.

References

[1] A. Serpengüzel, A. Kurt, I. Inanc , J.E. Cary, E. Mazur, Luminescence of blacksilicon, J. Nanophotonics 2 (021770) (2008) 1–9.

[2] C. Wu, C.H. Crouch, L. Zhao, E. Mazur, Visible luminescence from silicon surfacesmicrostructured in air, Appl. Phys. Lett. 81 (11) (2002) 1999–2001.

[3] T.K. Chini, D.P. Datta, S. Facsko, A. Mücklich, Room temperature photolumine-scence from the amorphous Si structure generated under keV Ar-ion-inducedsurface rippling condition, Appl. Phys. Lett. 92 (101919) (2008) 1–3.

[4] A.V. Kasbashin, M. Meunier, Fabrication of photoluminescent Si-based layersby air optical breakdown near the silicon surface, Appl. Surf. Sci. 186 (2002)578–582.

[5] G. Kurumurthy, K.S. Alee, D.N. Rao, Photoluminescence studies of Si/SiOnanoparticles synthesized with different laser irradiation wavelength ofnanosecond pulse duration, Opt. Commun. 282 (2009) 3509–3512.

[6] Y.Q. Wang, Y.G. Wang, L. Cao, Z.X. Cao, High-efficiency visible photoluminesce-nce from amorphous silicon nanoparticles embedded in silicon nitride, Appl.Phys. Lett. 83 (17) (2003) 3474–3476.


[7] B. Zeng, G. Xiong, S. Chen, S.H. Jo, W.Z. Wang, D.Z. Wang, Z.F. Ren, Field emissionof silicon nanowires, Appl. Phys. Lett. 88 (213108) (2006) 1–3.

[8] K. Lilienthal, M. Fischer, M. Stubenrauch, A. Schober, Self-organized nanostruc-tures in silicon and glass for MEMS, MOEMS and BioMEMS, Mater. Sci. Eng., B169 (2010) 78–84.


http://refhub.elsevier.com/S0169-4332(14)00719-3/sbref0005






























































































































































































































ING ModelA

urface

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

ARTICLEPSUSC-27569; No. of Pages 9

P.P. Dey, A. Khare / Applied S

[9] V.T. Vu, D.C. Nguyen, H.D. Pham, A.T. Chu, T.H. Pham, Fabrication of a sili-con nanostructure-based light emitting device advances in natural sciences,Nanosci. Nanotechnol. 1 (2010) 1–4 (025006).

10] F. Gourbilleau, X. Portier, C. Ternon, P. Voivenel, R. Madelon, R. Rizk, Si-rich-SiO2 nanostructured multilayers by reactive magnetron sputtering, Appl. Phys.Lett. 78 (20) (2001) 3058–3060.

11] P. Photopoulos, A.G. Nassiopoulou, Photoluminescence from SiO2/Si/SiO2 struc-tures, J. Phys.: Condens. Matter 15 (2003) 3641–3650.

12] X.Y. Chen, Y.F. Lu, Y.H. Wu, B.J. Cho, W.D. Song, D.Y. Dai, Optical propertiesof SiOx nanostructured films by pulsed-laser deposition at different substratetemperatures, J. Appl. Phys. 96 (6) (2004) 3180–3186.

13] M. Benyoucef, M. Kuball, J.M. Sun, G.Z. Zhong, X.W. Fan, Raman scattering andphotoluminescence studies on Si–SiO2 superlattices, J. Appl. Phycol. 89 (12)(2001) 7903–7907.

14] A.S. Mahmood, K. Venkatakrishnan, B. Tan, M. Alubiady, Effect of laser parame-ters and assist gas on spectral response of silicon fibrous nanostructure, J. Appl.Phys. 108 (094327) (2010) 1–6.

15] B. Tan, K. Venkatakrishnan, A femtosecond laser-induced periodical surfacestructure on crystalline silicon, J. Micromech. Microeng. 16 (2006) 1–6.

16] T. Sameshima, S. Usui, Mechanism of pulsed laser induced amorphization ofsilicon films, Appl. Phys. Lett. 9 (21) (1991) 2724–2726.

17] J. Siegel, J. Solis, C.N. Afonso, C. García, Bulk solidification and recalescencephenomena in amorphous Ge films upon picosecond pulsed laser irradiation,J. Appl. Phys. 80 (12) (1996) 6677–6682.

18] J.M. Perez, J. Villalobos, P. McNeill, J. Prasad, F.L. Cheek, J. Kelber, J.P. Estrera, P.D.Stevens, R. Glosser, Direct evidence for the amorphous silicon phase in visiblephotoluminescent porous silicon, J. Appl. Phys. 61 (5) (1992) 563–565.

19] C. Smit, R.A.C.M.M. van Swaaij, H. Donker, A.M.H.N. Petit, W.M.M. Kessels,


M.C.M. van de Sanden, Determining the material structure of micro-crystalline silicon from Raman spectra, J. Appl. Phys. 94 (5) (2003)3582–3588.

20] Dong-Jin Won, Mariola O. Ramirez, Hoonsoo Kang, Venkatraman Gopalan, NeilF. Baril, Jacob Calkins, John V. Badding, Pier J.A. Sazio, All-optical modulation

[

[


of laser light in amorphous silicon-filled microstructured optical fibres, J. Appl.Phys. 91 (161112) (2007) 1–3.

21] Yuri Borodko, Joel W. Ager III, G. Edward MartiF, H. Song, Krisztian Niesz, GaborA. Somorjai, Structure sensitivity of vibrational spectra of mesoporous silicaSBA-15 and Pt/SBA-1, J. Phys. Chem. B 109 (37) (2005) 17386–17390.

22] J.A. Luna-López, G. García-Salgado, T. Díaz-Becerril, J. Carrillo López, D.E.Vázquez-Valerdi, H. Juárez-Santiesteban, E. Rosendo-Andrés, A. Coyopol, FTIR,AFM and PL properties of thin SiOx films deposited by HFCVD, Mater. Sci. Eng.,B 174 (2010) 88–92.

23] C. Tsang, R.A. Street, Recombination in plasma-deposited amorphous Si:H.Luminescence decay, Phys. Rev. B: Condens. Matter 19 (6) (1979) 3027–3040.

24] M.J. Estes, G. Moddel, A model of size-dependent photoluminescence in amor-phous silicon nanostructures—comparison with observations of porous silicon,Appl. Phys. Lett. 68 (13) (1996) 1814–1816.

25] R.B. Wehrspohn, J.N. Chazalviel, F. Ozanam, I. Solomon, Spatial versus quan-tum confinement in porous amorphous silicon nanostructures, Eur. Phys. J. B 8(1999) 179–193.

26] C.X. Wang, P. Liu, H. Cui, G.W. Yang, Nucleation and growth kinetics of nanocrys-tals formed upon pulsed-laser ablation in liquid, Appl. Phys. Lett. 87 (201913)(2005) 1–3.

27] P. Chewchinda, T. Tsuge, H. Funakubo, O. Odawara, H. Wada, Laser, wavelengtheffect on size and morphology of silicon nanoparticles prepared by laser abla-tion in liquid, Jpn. J. Appl. Phys. 52 (2013) 1–4 (025001).

28] R. Salh, Defect related luminescence in silicon dioxide network: a review, in:S. Basu (Ed.), Crystalline Silicon—Properties and Uses, InTech, Rijeka, 2011, pp.135–172.

29] L. Skuja, Optically active oxygen-deficiency-related centers in amorphous sili-con dioxide, J. Non-Cryst. Solids 239 (1998) 16–48.


30] L. Skuja, Time-resolved low temperature luminescence of non-bridging oxygenhole centers in silica glass, Solid State Commun. 84 (6) (1992) 613–616.

31] G.G. Siu, X.L. Wu, Y. Gu, X.M. Bao, Enhanced and stable photoluminescence frompartially oxidized porous Si coated with Si thin films, J. Appl. Phys. 88 (6) (2000)3781–3783.



















































































































































































































































































































































































































































































































































































































































































































Date post:	23-Dec-2016
Category:	Documents
Upload:	alika
View:	220 times
Download:	4 times

Fabrication of luminescent a-Si:SiO2 structures by direct irradiation of high power laser on silicon...

Documents