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Applied Surface Science 285P (2013) 564– 571

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h omepa g e: www.elsev ier .com/ locate /apsusc

ole of nanocrystalline ZnO coating on the stability of porous siliconormed on textured (1 0 0) Si

aisy Vermaa, Shailesh N. Sharmaa,∗, Aneeta Kharkwala, G. Bhagavannarayanaa,ahesh Kumara, Shiv Nath Singha, Parakram Kumar Singha, Syed Sazad Mehdibb,ushahid Husainb

National Physical Laboratory (CSIR), Dr. K.S. Krishnan Marg, New Delhi 110012, IndiaDepartment of Physics, Jamia Millia Islamia, Jamia Nagar, New Delhi 110025, India

r t i c l e i n f o

rticle history:eceived 22 August 2011eceived in revised form 12 June 2013ccepted 21 August 2013vailable online 28 August 2013

eywords:olloidsnO

a b s t r a c t

In this study, a colloid of nanocrystalline ZnO particles prepared by chemical route is sprayed on poroussilicon layers. Porosity and thickness of PS layers were estimated by gravimetric analysis. Upon adsorp-tion of ZnO colloids on PS films, oxidation of nanocrystalline Si causes shrinkage of the Si-core due to thebreaking of Si Si bonds resulting in a blue-shift in PL spectra. The PL blue-shift can also be related to Si Ospecies or due to defects and the silica networks on which OH groups are absorbed due to ZnO incorpo-ration as also supported by our Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) studies,respectively. From high resolution X-ray diffraction (HRXRD) studies, a better crystalline perfection andconsiderable reduction in stress/strain values were observed for PS/ZnO layers as compared to virgin PS

orous silicontabilityLPS and FTIR

layers. The changes in the chemical composition at the surface of PS upon adsorption of ZnO colloids aselucidated by FTIR and XPS studies could be responsible for different PL emission and lattice-mismatchcharacteristics. The improved stability properties of PS are attributed to the strong absorption/adsorptionof ZnO into the highly porous vertical layers separating macroscopic domains of nanoporous silicon andthe mechanism of light emission from PS/ZnO layers is discussed on the basis of proposed energy bandgap diagram.

. Introduction

Silicon nanocomposites have emerged as a very strong field ofesearch due to their potential applications. The porous silicon (PS)as first discovered by Uhlir [1] in 1956. It has characteristics ofigh resistance, large internal surface area and strong absorbability2]. With the demonstration of room temperature strong photolu-

inescence (PL) from this material by Canham [3], a great dealf research into PS has focused on its potential applications inhe development of silicon based optoelectronics devices. Various

ethods have been developed to explain the strong PL property ofS in which quantum confinement effect can explain the PL photonnergy in the visible spectrum region and has been widely rec-gnized. In addition, different surface chemical species have been

uggested to be the light emitting centers for the visible PL suchs siloxene, hydride species or polysilanes or hydrogenated amor-hous silicon [4–6]. Till today exact origin PL mechanism of PS is

∗ Corresponding author. Tel.: +91 11 45608566; fax: +91 11 45609310.E-mail addresses: shailesh@nplindia.org, shailesh@mail.nplindia.ernet.in

S.N. Sharma).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.08.094

© 2013 Elsevier B.V. All rights reserved.

still not conclusively accepted. Recently surface defect passivationplays a dominant role in order to protect the dangling bonds forma-tion at atmospheric conditions and to avoid the chemical instabilityof the PS surface. This surface defect passivation can be achievedfrom various chemical adsorbates by several techniques. Thus, asurface defect passivation mechanism by encapsulation of PS layerby a wide band gap layer (ZnO) would play a predominant role inavoiding the chemical instability of the PS surface and in improv-ing its PL stability [7]. Furthermore, ZnO is a direct and wide bandgap (∼3.37 eV) material and owing to its superior optoelectronicproperties is widely used as the functional material for transparentconductors, gas sensors, solar cell windows and UV-light emittingdevices, etc. [7,8]. It is believed that the vast surface area and strongabsorbability of PS in conjunction with wide band gap ZnO layerwould lead to a new-type potential functional material for appli-cation in optoelectronics.

2. Experimental

The starting material was Cz grown single crystal silicon wafersof size 50 mm diameter and 300 �m thickness. Wafers were ofp-type (B-doped) conductivity, 1 0 0 orientation and 1–5 � cm

ace Sc

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where S is the area of the PS formed and � is the density of Si wafer.From Fig. 1(a) and (b), it is evident that both porosity and thick-

ness of PS films increases with increase in etching time. However,

5 10 15 20 25 30

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D. Verma et al. / Applied Surf

esistivity. They were mechanically lapped on both sides and thenextured in a solution of NaOH:IPA:H2O at 85 ◦C for 35 min. Anl metal layer was deposited by thermal evaporation in vacuumnd was sintered at 450 ◦C in hydrogen ambient for a good ohmicontact on the back side. Porous silicon was formed on the front sur-aces of different silicon wafers for different etching durations (te)arying from 5–30 min and at constant anodization current densityf 20 mA cm−2

. The electrolyte consisted of a mixture of 48% HF andthanol (C2H5OH) in a ratio of 1:2 by volume.

The as formed PS layers and ZnO-coated PS layers were char-cterized for photoluminescence (PL), Fourier transform infraredpectroscopy (FTIR), powder X-ray diffraction (PXRD) and scanninglectron microscope (SEM). For PL measurement, the excitationavelength for ZnO NPs was 300 nm and the PL emission spectraere measured in the wavelength range of 300–800 nm with theelp of Perkin Elmer spectrophotometer. However, the PL emissionpectra of PS layers and ZnO-PS layers in the wavelength range of50–850 nm were measured using a home assembled PL systemonsisting of a two stage monochromator, a photomultiplier tubePMT) with a lock-in amplifier for PL detection, and a diode contin-ous laser operating at 405 nm and power ∼5 mW with beam areaeing ∼1 mm2 for excitation in all the measurements. During theourse of PL measurements of bare PS layers and ZnO-coated PS lay-rs, care was taken to ensure that all the parameters which are anrtifact of the PL system are the same viz. excitation power density0.125 W/cm2; angle of incidence of the laser ∼45◦; the sample toetector distance ∼8 cm, collection angle of the PL emission ∼90◦.

To know the crystalline perfection of the PS film and to deter-ine the expected tensile strain due to formation of the pores in

he PS film, these were characterized by HRXRD by employing aulticrystal X-ray diffractometer developed at NPL [9]. The well-

ollimated and monochromated MoK�1 beam obtained from thehree monochromator Si crystals set in dispersive (+,−,−) config-ration has been used as the exploring X-ray beam. The specimenrystal is aligned in the (+,−,−,+) configuration. Due to dispersiveonfiguration, though the lattice constant of the monochromatorrystal(s) and the specimen are different, the unwanted dispersionroadening in the diffraction curve (DC) of the specimen crys-al is insignificant. This arrangement improves the spectral purity��/� � 10−5) of the MoK˛1 beam. The divergence of the exploringeam in the horizontal plane (plane of diffraction) was estimated toe �3′′. The specimen can be rotated about the vertical axis, which

s perpendicular to the plane of diffraction, with minimum angu-ar interval of 0.4′′. The rocking or diffraction curves were recordedy changing the glancing angle (angle between the incident X-rayeam and the surface of the specimen) around the Bragg diffrac-ion peak position �B (taken as zero for the sake of convenience)tarting from a suitable arbitrary glancing angle and ending at alancing angle after the peak so that all the meaningful scatteredntensities on both sides of the peak include in the diffraction curve.he DC was recorded by the so-called ω scan wherein the detectoras kept at the same angular position 2�B with moderate open-

ng for its slit [10]. This arrangement is very appropriate to recordhe short range order scattering caused by the disorder individ-al pores if any and the Bragg diffracted beam from the PS filmhich is expected to be very close to the silicon substrate peak

11].

.1. Preparation of ZnO colloids

ZnO nanoparticles (size < 10 nm) were formed via precipitationn ethanolic solution as earlier reported Spanhel et al. [12]. An

mount of 0.1 M zinc acetate dihydrate (2.19 g) was added to00 mL of ethanol and refluxed for 2 h followed by gradual coolingo room temperature with a drying tube attached at the end (inrder to prevent exposure to ambient moisture). The precursor

ience 285P (2013) 564– 571 565

solution was (0.1 M) stored in the desiccator and remained stablefor weeks. When required for experimentation, small batchesof colloidal suspension were prepared by hydrolysis using LiOHpowder. This simplified procedure enabled us to maintain repro-ducibility of sample preparation. In a typical procedure, lithiumhydroxide (22 mg) was added directly to 10 mL of the precursorsolution, diluted to 50 mL with ethanol, and sonicated at 273 Kfor 45 min. The ZnO suspension (0.02 M) prepared in this mannerwas transparent and stable. These ZnO nanoparticles were thencoated onto PS films by spraying method. For PS films anodizedat different current densities, the concentration of ZnO used forcoating was the same, i.e. ∼20.0 mM to ensure uniform thickness.

3. Results and discussion

3.1. Porosity and thickness

The porosity (45–75%) and thickness (4–15 �m) of PS films wereestimated from gravimetric measurements [13]. According to thegravimetric method the mass (m1) of the bare Si wafer and the mass(m2) of the Si wafer after PS formation and finally the mass (m3) ofthe Si wafer after PS removal in 1% KOH solution.

Porosity (%) = (m1 − m2)(m1 − m3)

Thickness (mm) = (m1 − m3)S · �

Etching Ti me (min s.)

Fig. 1. (a) Porosity and (b) thickness of PS layers formed for different etching timedurations at constant current density of 20 mA cm−2.

566 D. Verma et al. / Applied Surface Science 285P (2013) 564– 571

Fig. 2. SEM images of ZnO layer, PS layer and PS/ZnO layer: (a and b) as prepared ZnO and after 5 days ZnO nanospheres; (c and d) textured Si-substrate and PS layer; (e andf layer.

bipWssalreosdfi

) top view of PS and PS/ZnO layer; (g and h) cross-sectional view of PS and PS/ZnO

eyond an optimum current density Id ∼ 20 mA cm−2 and etch-ng time te ∼ 20 min, PS films becomes quite powdery, fragile witheeling-off tendency and great care has to be taken to handle them.hen subjected to ultrasonic cleaning for 60 min, as a destructive

trength of the film test, porous silicon formed on Si-textured sub-trates corresponding to Id ∼ 20 mA cm−2 (te = 20 min) remains firmnd weight-loss is marginal. However, there is a substantial weightoss of PS samples prepared at etching duration te > 20 min. Theate of weight loss increases with increase in current density andtching time owing to higher porosity. It has been experimentally

bserved that beyond an optimum etching time and current den-ity, both porosity and thickness decreases (not shown in Fig. 1)ue to excessive etching and higher peeling-off tendency of the PSlms.

3.2. SEM studies

SEM was used to identify the surface morphology of the ZnOlayer, porous silicon layer prepared as such and after ZnO treat-ment. Figs. 2(a) and (b) correspond to the surface morphology ofZnO nanoparticles after five days of synthesis at different magni-fications. As shown in Fig. 2(a), the SEM image of pure ZnO afterfive days from synthesis show distinguished spherical morphologywith a self-aligned nanoparticles at low magnification range. Butat high magnification range it is observed that there is a aggre-

gation in ZnO nanospheres after a time period of 5 days whichremains stable thereafter as shown in Fig. 2(b). Fig. 2(d) showsthe surface morphology porous silicon formed on textured sub-strates at Id ∼ 20 mA cm−2 corresponding to te = 20 min. Here, the

D. Verma et al. / Applied Surface Science 285P (2013) 564– 571 567

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Fig. 3. XRD pattern of PS/ZnO layers.

mooth surface morphology consists of randomly sized and spacedyramids homogeneously distributed on the surface. The pyramidsppear to be more sharply separated but no macroscopic cracking isbserved. This surface morphology does not essentially differ fromhe textured silicon substrate (Fig. 2(c)). Figs. 2((e and f) and (gnd h)) show the top-view and cross-sectional view SEM imagesf PS and PS/ZnO films, respectively. The top-view image of asrepared PS and PS/ZnO reveals a uniform distribution of poreshich has been duly filled with ZnO. Their corresponding cross-

ectional views show the presence of sharp columnar structuresith pores inter-connected with each other. Due to capillary con-ensation in pores, the physical adsorption by a porous surfaceas increased thus leading to a fairly adsorption of spherical ZnOano-crystallites immersed in a network of pores within the Si-anocrystalline skeleton (quantum sponge) which could improvehe structural stability of the PS layer. The SEM images of PS/ZnOeveals that ZnO coating was adequate into the nano-pores of PSnd the nano-grains of the two materials connected with each otherightly.

.3. XRD studies

Fig. 3 shows the grazing incidence X-ray diffraction (GIXRD)attern of PS/ZnO films corresponding to Id ∼ 20 mA cm−2 and

e = 20 min by Brucker AXS D8 advance X-ray diffractometer. TheIXRD can be used to record the diffraction patterns of thin filmsith minimum contribution from the substrate. This is a non-estructive surface sensitive technique. The measurement waserformed at low angle of incidence (1◦). The observation of strongeak at 2� ∼ 55.48◦ corresponds to (3 1 1) plane of the c-Si crystal.he presence of peaks at 2� ∼ 31.33◦, 33.91◦ and 35.707◦ corre-ponds to (1 0 0), (0 0 2) and (1 0 1) planes of pure ZnO (ASTM Cardo. 4-831). The pattern of ZnO shows hexagonal wurtzite struc-

ure. This could be attributed to the incorporation of Zn/ZnO in theifferent pores of PS indicating the changes in the lattice constant.his could be attributed to the increased interaction at the interfacef PS and ZnO.

.4. High-resolution X-ray diffractometry and straineasurements

To elucidate further, the changes in the lattice expansionnd strain for as-prepared PS and PS/ZnO films on textured Si-ubstrates, high resolution XRD studies were performed. Fig. 4hows a typical diffraction/rocking curves (DC/RCs) recorded under

Fig. 4. HR-XRD of (a) textured Si substrate; (b and c) PS and PS/ZnO layers.

identical experimental conditions like the power of the X-ray tubeand the size of the exploring beam for (4 0 0) diffracting planes usingmulticrystal X-ray diffractometer in symmetrical Bragg geometry.The curves (a), (b) and (c) in Fig. 4 are, respectively, for pure Si(1 0 0) crystal after texturization, (b) same textured Si sample butafter formation of PS film at Id = 20 mA cm−2 for te = 20 min and (c)for the same PS specimen but after dispersion of ZnO solution ontothe PS film. As seen in Fig. 4, the DC of curve (a) contains a sharppeak with full width at half maximum (FWHM) of 6.5′′ which isvery close to that theoretically expected value for a perfect crystalfrom plane wave dynamical theory of X-ray diffraction and hencechosen for the present studies. As seen in curve (b), the solid linewhich fits well with the experimental curve is the convoluted curve

of two peaks obtained by the standard Lorentzian fit. The peak at thelower diffraction angle is due to crystalline PS film confirmed by thediminished peak observed at the same position when the RC was

5 face Science 285P (2013) 564– 571

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(C) Zn2p3/2

68 D. Verma et al. / Applied Sur

ecorded from the backside of the substrate [13,14]. A recent studyy double crystal XRD by Lamedica et al. [15] also revealed that theeak observed at lower diffraction (or incidence) angle is due to PSlm. This feature indicates that the lattice parameter of the PS filmormal to the surface is greater than that of the Si substrate due to

attice expansion of the film because of the pores. It is well knownhat due to pores on the surface of Si crystal, the tensile stress devel-ps due to expansion of the crystalline lattice surrounded by poresnd dangling bonds and lead to a slight increase in the lattice con-tant of the PS film. Due to change in lattice constant of PS film,nother peak is expected at a lower diffraction angle with respecto the substrate peak. From the angular separation �� betweenlm and substrate peaks, the lattice mismatch or the stain (�a/a)ormal to the interface was calculated using of Bragg equationa/a = −cot�B�� in its differential form (�B being the Bragg diffrac-

ion angle). The low diffraction angle of film peak shows that thelm has a tensile stress and its magnitude is 1.08 × 10−4. Since theC is recorded with the high-resolution multicrystal X-ray diffrac-ometer whose wavelength spread, i.e. ��/� is much less than 10−5,he instrumental error due to wavelength spread in the value of

a/a is insignificant. However, the uncertainty in measuring thengular separation �� between film and substrate peaks dependsn the least count of the angular movement of the turntable of thepecimen stage of the diffractometer which is 0.4′′ with which thencertainty in the value of �a/a is obtained as ±0.07 × 10−4. Curvec) is also having the same features as that of curve (b) with twoeaks due to PS film and Si substrate. But the peak separation iseduced to 5′′ from 6′′ which shows that the strain is reduced from.08 × 10−4 to 0.90 × 10−4 with an uncertainty of ±0.07 × 10−4. Thisould be well understood in the following way. Due to disper-ion of ZnO nanocrystallites onto PS film, ZnO nano particles areormed inside the pores of PS film and thereby the tensile stressn the film is reduced. Thus both SEM and HR-XRD studies con-rm the incorporation of ZnO nano particles within the pores ofS and consequently the increased stability of PS/ZnO films. Thearger surface area available for PS films formed on textured sub-trates enables the even distribution of strain upon adsorption ofnO nanocrystallites within the PS-columnar morphology.

.5. XPS studies

The composition and chemical states of the as-prepared PS andS/ZnO films were further investigated by X-ray photoelectronpectroscopy (XPS) analysis. Figs. 5(A and B) show the high-esolution XPS spectra for the Si 2p (a) and O 1s (b) for as-preparedS and PS/ZnO films and Fig. 5(C) shows Zn 2p (c) regions for PS/ZnOlms. As-prepared PS on textured substrates displays the Si2p bind-

ng energy of the main peak at ∼103 eV, and a sub-peak at ∼99.6 eVFig. 5A(a)) corresponding to oxidized and pure (unoxidized) silicon16]. On the other hand, PS/ZnO exhibits a feature which is char-cteristic for SiO2 at ∼106 eV [17] as can be seen from Fig. 5A(b).rom Fig. 5A(b), it is interesting to note that PS upon ZnO adsorp-ion shows mainly the peak at ∼106 eV corresponding to Si bondedo oxygen. Thus SiO/Si for PS/ZnO films is greater than SiO/Si fororresponding virgin PS films indicating higher degree of oxidationor the former as compared to the latter. The higher FWHM of Si 2por PS/ZnO as compared to virgin PS is also a testimonial to this. The

1s peak for PS/ZnO films is very broad as compared to the cor-esponding virgin PS films on textured substrate (Fig. 5(B), curves

and b) with a major peak at ∼535 eV indicating the presence ofore than one chemical (oxidation state) [18]. From Fig. 5(C), for

S/ZnO film, the XPS binding energies of Zn 2p3/2 and Zn 2p1/2 are

024 eV and 1047 eV, respectively [19], and the with the O 1s peakentered at 535 eV, which agrees well with the reported values fornO since elemental or free zinc has XPS signature of Zn2p3/2 andn2p1/2 at ∼1020 eV and 1043.7 eV, respectively, and shift of Zn 2p

Fig. 5. XPS spectra for the Si 2p (A), O 1s (B) and Zn 2p (C) regions of (a) PS and (b)PS/ZnO layers.

XPS peaks toward higher binding energy clearly shows that Zn isoxidized as ZnO. Thus the comparison of the XPS spectra for PS filmsand PS/ZnO films shows the presence of higher sub-oxide groupsfor PS films upon ZnO adsorption which commensurates well withthe PL blue-shift.

3.6. Fourier transform infrared studies

The FTIR spectra of the virgin PS and PS/ZnO films correspond-ing to Id ∼ 20 mA cm−2 (t = 20 min.) are shown in Fig. 6. For virgin PS

films (Fig. 6(a)), the presence of SiHx modes are evident at ∼2092and 2194 cm−1(stretching), ∼905 cm−1 (bending) and ∼661 cm−1

(wagging), respectively [20]. Upon ZnO adsorption on PS films,the intensity of SiHx stretching modes decreases considerably and

D. Verma et al. / Applied Surface Science 285P (2013) 564– 571 569

4000 3500 30 00 250 0 2000 15 00 1000 5000

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ity(a

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Wave leng th (nm)

(b)

(B)

Fig. 6. FTIR spectra of (a) PS and (b) PS/ZnO layers.

mergence of new peak at ∼2244 cm−1 corresponding to O SiHtretching mode is observed (Fig. 6(b)) [21]. The Si Si stretchingode is centered on 624 cm−1 [22]. Another interesting difference

oted in the FTIR spectra of PS films upon ZnO adsorption is thehift of Si O related mode from 1128 to 1229 cm−1 which indicatesncrease in the oxidation state (x) of the SiOx species [23].

.7. Photoluminescence (PL) and PL decay studies

Typical PL curves of ZnO prepared as such and after 5 days andS and PS/ZnO films corresponding to various anodization etchinguration (te) ranging from 5 to 30 min at constant current density of0 mA cm−2 are shown in Figs. 7(A) and (B). Fig. 7(A)(a) shows theL spectra of ZnO colloids corresponding to excitation wavelength300 nm. The broad visible photoluminescence band as shown inig. 7(A)(a) has actually been reported earlier in ZnO quantum dots24,25]. The photoluminescence spectra in Fig. 7(A)(b) of ZnO col-oids shows there is aggregation of ZnO particles with time so afterve days ZnO shows red shift. It has been reported that the pho-oluminescence (PL) spectrum of ZnO usually consists of two maineaks, near band-edge (∼380 nm) and green (530 nm) emissions.he origin of the green emission is still debatable [26]; some authorsttribute it to oxygen vacancies [27] while others attribute it to zincnterstitials [28]. UV emission is the characteristic emission of ZnOhat originates due to the band edge emission or the exciton transi-ion and shows the presence of surface states. Both UV and visiblemissions from ZnO depend highly on the shape and size of theabricated nanostructures. In our case, the freshly prepared ZnOanoparticles show only green emission at 500 nm, however, afterging for four to five days, the minor UV emission peak between50 and 400 nm developed and the visible emission peak shiftedoward the red region (PL peak position ∼520 nm) as a consequencef surface state formation due to oxidation and aggregation of ZnOanoparticles, respectively. Such PL changes for ZnO films after first

ew days of synthesis have also been observed by other researcherss well [24–26].

As seen in Fig. 7, the PL intensity at � ∼ 680 nm at particulartching duration ∼20 min (Fig. 7(B)(b)) is higher for virgin PS asompared to PS films prepared at lower (te ∼ 5 min) (Fig. 7(B)(a))nd higher (te ∼ 30 min) (Fig. 7B(e)) etching durations, respectively.n lowering the etching duration to 5 min (porosity ∼45%), a rel-tively weak PL with a peak at 675 nm was obtained (Fig. 7(B)(a)).his is in accordance with quantum confinement theory since at

his lower porosity, the Si-wires are not narrow enough to exhibituantum size effects. At higher etching duration (te) of 30 min,xcessive etching of Si columns (porosity ∼75%) leads to the col-apse of PS nanostructures thus leading to a concomitant decrease

Fig. 7. (A)(a) PL spectra of ZnO and (A)(b) ZnO after 5 days, PS and PS/ZnO layersformed for different etching time at constant current density of 20 mA cm−2; (B)(a)and (B)(b) te ∼ 5 min; (B)(c) and (B)(d) te ∼ 20 min; (B)(e) and (B)(f) te ∼ 30 min.

in PL intensity (�PL ∼ 680 nm) (Fig. 7(B)(e)). Upon ZnO adsorp-tion, a significant blue-shift in PL spectra (from 680 to 630 nm)(Fig. 7(B)(f)) as compared with virgin PS films (etching durationte ∼ 20 min)(Fig. 7(B)(c)) is observed. A similar blue-shift in PL spec-tra upon ZnO adsorption was observed for other PS films preparedat lower and higher etching duration te ∼ 5 and 30 min as well(Fig. 7(B)(b and f)). However, the blue-shift is appreciable partic-ularly for PS corresponding to the etching duration (te) of 20 min(Fig. 7(B)(d)). This blue-shift in PL peak position for PS/ZnO filmscan be attributed to the enhanced oxidation of nanocrystalline Siresulting in an increase of SiOx thickness surrounding the Si-core.A similar blue shift of PS layers has also been reported after depo-sition of ZnO nanoparticles by Elhouichet et al. [29]. A blue shiftof PS layers has also been reported for oxidized PS layers in airambient [30,31]. Upon adsorption of ZnO colloids on PS films, oxi-dation of nanocrystalline Si causes shrinkage of the Si-core due tothe breaking of Si Si bonds resulting in a blue-shift in PL spec-tra. The PL blue-shift can also be related to Si O species or due todefects and the silica networks on which OH groups are absorbeddue to ZnO incorporation. In the present investigations, formationof oxygen-related defects could be responsible for the enhance-ment and blue-shift of PL emission for PS films upon ZnO adsorptionas also supported by FTIR and XPS studies. This could be attributedto intrinsic defects, Si O related species and luminescent centerslocated at the interface between the Si-nanocrystallites and the

SiO2 matrix.

Decay of PL intensity is a good indication of the stability ofporous silicon particularly of the surface bond configurations. ThePL peak intensity was recorded for different times corresponding to

570 D. Verma et al. / Applied Surface Sc

0 20 40 60 80 10 0 12 0

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Pea

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wtrtetwdffssa

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Laser Expos ure Time (mins. )

Fig. 8. PL decay of (a) PS and (b) PS/ZnO layers.

avelength of the peak of PL spectrum. In Fig. 8(a and b), decay ofhe PL intensity at the peak wavelength due to exposure to the laseradiation for PS and PS/ZnO films formed at constant etching dura-ion (te) of 20 min on textured silicon substrates are compared. Asvident from Fig. 8(a), a PL decay is observed only for virgin PS withhe rate of decay being ∼0.08. However for PS/ZnO films, PL decayas negligible (�PL/PLi ∼ 0.02) (Fig. 8(b)). To ensure the repro-ucibility of this PL decay, measurements were done repeatedlyor several hours and the PL decay trend was found to be the sameor both virgin PS and PS/ZnO films. This suggests the formation oftable surface bonds in correlation with the superior mechanicaltability of porous silicon formed on textured substrates upon ZnOdsorption.

For a better understanding of the carrier dynamics associatedith the light emission of ZnO/PS nanocomposites, a model has

een proposed to explain the PL spectra with the band energy dia-ram as shown in Fig. 9. ZnO nanoparticles that are introduced intohe PS layers improve the PL intensity and the photo stability ofhe PS layers. The mechanism of PL emission of ZnO/PS layers haseen understood by an oxygen-bonding model in PS layers and aative defects model in ZnO. The observed blue shift is correlatedo the deep level defects in ZnO. The nature of the visible emis-ion was first thought to be associated with divalent impuritiesut later intrinsic defects such as interstitial zinc ions or oxygenacancies were assumed to be the recombination centers. Since,nO nanoparticles are sprayed on the PS layers then ZnO/PS com-

osite is formed, where PS plays an important role in the emissionrocess from the ZnO/PS composite. From Fig. 9, it is clear that there

s an interface between PS and ZnO and the process of tunnelingccurs at this interface. We denote the electron affinity of silicon

Fig. 9. Energy level diagram of ZnO/PS nanocomposite.

ience 285P (2013) 564– 571

and ZnO by �si = 4.01 eV and �ZnO = 4.35 eV. Due to the quantumconfinement model the band gap of PS layers broadens in compar-ison to the band gap of crystalline silicon (1.12 eV). When 480 nmwavelength of Ar ion laser is incident on the ZnO/PS layer, only thePS layers are excited. The photons are absorbed on the surface ofthe PS layer surface and red emission is obtained and this emissionis modified by the ZnO coated PS layers. From both FTIR and XPSstudies, it is clear that the SiHx species are present on the bare PSlayer but when the ZnO nanoparticles are coated on the PS layers,SiH bond would break and Si O bonds form. Wu et al. [32] haveshown that oxidation of surface Si atoms will lead to the blue shiftof the red emission and this shifting of PL peak is basically due to theoxidation of the PS surface due to the coating of ZnO nanoparticles.

From the above results, it can be conjectured that there is achange in the surface passivation from hydrogen to oxygen-likespecies upon adsorption of ZnO onto PS films. This surface oxidelayer for PS/ZnO films acts as a good passivating agent as it pro-vides surface stability to the fragile PS internal network structureresulting in the improved optical and structural properties.

4. Conclusions

ZnO/PS layers exhibiting broadband luminescence across mostof the visible region of spectrum have been fabricating using asimple and low cost process. In case of PS/ZnO films, a significantblue-shift in PL spectra (from 680 to 630 nm) as compared withvirgin PS films was observed which could be due to the enhancedoxidation of nanocrystalline Si resulting in an increase of SiOx thick-ness surrounding the Si-core. Upon adsorption of ZnO colloids onPS films, oxidation of nanocrystalline Si causes shrinkage of the Si-core due to the breaking of Si Si bonds resulting in a blue-shiftin PL spectra. The PL blue-shift can also be related to Si O speciesor due to defects and the silica networks on which OH groups areabsorbed due to ZnO incorporation as also supported by our Fouriertransform infrared (FTIR) and X-ray photoelectron (XPS) studies,respectively. From high resolution X-ray diffraction (HRXRD) stud-ies, a better crystalline perfection and considerable reduction instress/strain values were observed for PS/ZnO layers as comparedto virgin PS layers. The changes in the chemical composition at thesurface of PS upon adsorption of ZnO colloids as elucidated by FTIRand XPS studies could be responsible for different PL emission andlattice-mismatch characteristics. The improved PS stability proper-ties are attributed to the strong absorption/adsorption of ZnO intothe highly porous vertical layers separating macroscopic domainsof nanoporous silicon and the mechanism of light emission fromPS/ZnO layers is discussed on the basis of proposed model. It is con-jectured that the vast surface area and strong absorbability of PS inconjunction with wide band gap ZnO layer would lead to sensors.

Acknowledgments

We thank Director NPL for the encouragement to perform thiswork. The author D.V. greatly acknowledges the financial assistancefrom CSIR (Council of Scientific & Industrial Research), New Delhi.Dr. K.N. Sood (NPL, Delhi) is acknowledged for his help in ScanningElectron Microscopy (SEM) measurements.

References

[1] A. Uhlir, Bell Syst. Tech. J. 35 (1956) 333.[2] O. Bisis, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1.[3] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046.

[4] L.T. Canham, Properties of Porous Silicon, IEE INSPEC The Institution of Electrical

Engineers, London, 1997.[5] H.L. Matthias, Crit. Rev. Solid State Mater. Sci. 21 (1996) 265.[6] Cullis AG, canham LT, Calcutt PDJ (1997) Appl. Phys, Lett. 82:909.[7] W.H. Zhang, J.L. Shi, L.J. Wang, D.S. Yan, Chem. Mater. 12 (2000) 1408.

ace Sc

[[

[[

[

[

[

[[

[

[

[

[

[[[[

[

D. Verma et al. / Applied Surf

[8] Y. Li, G.W. Meng, L.D. Zhang, F. Philipp, Appl. Phys. Lett. 76 (2000) 2011.[9] G. KrishanLal, Bhagavannarayana, J. Appl. Crystallogr. 22 (1989) 209–215.10] G. Bhagavannarayana, S.K. Kushwaha, J. Appl. Crystallogr. 43 (2010) 154–162.11] G. Bhagavannarayana, S.N. Sharma, R.K. Sharma, S.T. Lakshmikumar, Mater.

Chem. Phys. 97 (2006) 442.12] L. Spanhel, M.A. Anderson, J. Am. Chem. Soc. 113 (1991) 2826.13] S.N. Sharma, G. Bhagvannarayan, R.K. Sharma, S.T. Lakshmikumar, Mater. Sci.

Eng. B 127 (2006) 255.14] S.D. Milani, R.S. Dariani, A. Mortezaali, V. Daadmehr, K. Robbie, J. Optoelectron.

Adv. Mater. 8 (2006) 1216.15] G. Lamedica, M. Balucani, A. Ferrari, V. Bondarinko, L. Dolgyi, Mater. Sci. Eng. B

91 (2002) 445.16] R.G. Lemus, J.D. Moreno, R.J. Martin-Palma, F. Ben-Hander, J.M. Martinez-Duart,

J.L.G. Fierro, P. Gomez-Garrido, Thin Solid Films 354 (1999) 34.

17] J. George, S. Shylesh, A.P. Singh, Appl. Catal. A 290 (2005) 148.18] J.C. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Phys. Chem. Phys. Chem. 2

(2000) 1315.19] L. Fernendez, N. Garro, J.E. Haskouri, M.P. Cabero, J.A. Rodriguez, J. Latorre, C.

Guile, A. Beltran, P. Amoros, Nanotechnology 19 (2008) 1.

[[[[[

ience 285P (2013) 564– 571 571

20] S.N. Sharma, G. Bhagavannarayana, U. Kumar, R. Debnath, S.C. Mohan, PhysicaE 36 (2007) 65.

21] R. Prabakaran, T. Monteiro, M. Peres, A.S. Viana, A.F.D. Cunha, H. Aguas, A.Goncalves, E. Fortunato, R. Martins, I. Ferreira, Thin Solid Films 515 (2007)8664.

22] G. Li, X. Hou, S. Yuan, H. Chen, F. Zhang, H. Fan, X. Wang, J. Appl. Phys. 80 (1996)5967.

23] S.N. Sharma, R.K. Sharma, S.T. Lakshmikumar, Physica E 28 (2005) 264.24] V. Subramanian, E.E. Wolf, P.V. Kamat, J. Phys. Chem. B 107 (2003) 7479.25] G. Ramakrishna, H.N. Ghosh, Langmuir 19 (2003) 3006.26] L. Shoutian, J. Stuart Silver, M. EL-Shell Samy, Mater. Res. Soc. Symp. Proc. 42

(1997) 389.27] J. Fallert, R. Hauschild, F. Stelzl, A. Urban, M. Wissinger, H. Zhou, C. Klingshirn,

H. Kalt, J. Appl. Phys. 101 (2007) 073506.

28] N.S. Norberg, D.R. Gamelin, J. Phys. Chem. B 109 (2005) 20810.29] H. Elhouichet, M. Oueslati, Mater. Sci. Eng. B 79 (2001) 27.30] T. Maruyama, S. Ohtani, Appl. Phys. Lett. 65 (1994) 1346.31] Y.H. Zhang, X. Jian, Q.W. Chen, Phys. Rev. Lett. 81 (1998) 1710.32] X.L. Wu, X.J. Xiong, D.L. Fan, Y. Gu, X.M. Bao, Phys. Rev. B 62 (2000) R7759.