Applications of Porous Silicon Cells and Biosensors

Priyanka Singh, Shailesh N. Sharma, and NoM.. Ravindra

An QVeTlliew of the applications ofporous silicon (PS) thin films, as QII­

tireflection coatings (ARC) in siliconsolar cells and transducers in biosen­sors, is presented. The rejlectlJnce gpec­tra of PS films have been comparedwith other conventional ARCs (such asSiN~ nO,jMgF2 and ZnS), and optimalPS ARC with minimum rejlectlJnce hasbeen obtained. The implementationof PS into an industritJIly compatiblescreen-printed (SP) solar cell by boththe electrochemical etching (ECE) andchemical etching (CE) methotb arereviewed. Porous silicon films, formedvia ECE for short anodization times,on textured n+ emitter ofc-Si solar cellhaving SPfront and back contacts, leadto improvements in the performance ofsolar cells and demonstrate their vi­ability in industrial applications.

INTRODUCTION

Porous silicon (PS) is a sponge-likestructure, composed of silicon skeletonpenneated by a network: of pores. Po­rous silicon can be obtained by anod­ization or etching of silicon in aqueoushydrotluoric (HF) solutions and wasfirst discovered in 1956 by Uhlir atthe Bell Laboratories.1 Etching of sili­con substrate generates a thin layer ofporous silicon on the silicon substratewithpores as large as -200 DID in diam­eter (as shown in Figure 1A), providinga large surface area for light trapping(in solar cell applications) and molecu­lar interaction (in sensor applications)inside the porous layer. A scanningelectron micrograph of such a PS filmwhich has been formed on crystallinesilicon (c-Si) substrate is shown in Fig­ure 1A. The right side images illustratethe magnified top and cross-sectionalview ofthe PS layerand clearly indicatethe formed pores of diameter -200 DID

in the PS :film.'1 The poten1i.al. of poroussilicon for various technological appli­cations such as optoelectronic devices,3

displays,4 photodetectors,s and bi08en­sors6 has been extensively investigated.Such devices can be fabricated by ei­ther varying the porosity of the PS :filmitselfor by depositing other :6.Ims on thePS :film such as metals, semiconducting

How would you••....describe tile Im!nIllllignlflr.n....of this peper?

An cwerview oftIte applic:atioM ofporoll.'1 silicon (PS) thinfilms fU

antirejkction coating. (ARC) insilicon lOW cells atId tnIIuducer.sin bioaen.ron ispreleNetL 'I'MimplementDtion ofPS into QIl

intlII.Jt1ially compiJlible SCND!­

printd .olarceU by both tIte.lsctroclwnicul etching llIldcIrnnical etching met1wds arenNiewed.

•••daaibe tIdI work to aIIIlIterial& IclIIlre II1II1 mgiDeerIDgprofesI1IoDIIl with DO aperieDl:e InJUIIl' tech·.) spedaIly?

TIte use ofporrnu silicon ill IOUJrcelLs llIld bio.retlSOP'& is described./n partlclllm; the mdy focuu!J ontile utility ofporoll.'1 silicon tu anti­~ COQIRsgI in solar cells.

•••daaibe tIdI work to a1aypenoD?

/n recePlt yeaT', tltere 1ws hemII growing Were.rt ill rmtrWQ/Jleenergy IOrlT'CU. Solar energyR!p1f!!Je1II& a non-pollMting, rtatwallyavtJilDble lOW'Ce ofeMrgy. Siliconis abundmltly avtJilDble in I/Qtu12 illthe form of.lUca. SlUcoll IOUJ' celLJR!prele1lt tile largut compmumt t1uJtfacjlikJte, die com>eniolt ofnutlightto electricity. However. ailicollreftect.s IIftuctioll of the iIlcidentlight (-33%) which theft gmWQ8tm Anti-rejl«tion coating. helpto minimize refkction kmu. In themIJ1IlUCript, we describe tile uu of

t.:: poroll.'1 silicon tu "" anti-reft«tiOIlCODMg.

oxides, or polymer films depending onthe applications.' The large surface-to­volume ratio of porous silicon gives itthe ability to react with biochemicalspecies and sense them readily. Recent­ly, PS has attracted more attention dueto its sensing properties and has beenutilized in chemical, biological2,6,l-IOand gasu-17 sensing applications. Theadvantages of PS sensors are their lowcost, ability to operate at room tempera­ture, and compatibility with traditionalsilicon device fabrication technology.Furthennore, interest in porous siliconin the solarcell industry has grown overthe last decade.1&4.'l

See the sidebar for a discussion ofthe formation and morphology of p<>­rous silicon.

APPLICATION OF POROUSSILICON AS ARC IN SILICON

SOLAR CELLS

AB shown in Figure 2A, bare sili­con surface reflects more than 30% ofthe incident sunlight. These reflectionlosses can be reduced by texturization(it is an important step during solarcell fabrication. which makes siliconsurface textured) and by depositing asuitable ARC on the top surface of thesolar cell, reflection losses are reduced.This is shown in Figure 2A. The ARCeffect relies on destructive interferenceofwaves reflected at the top and bottomof the ARC, as shown in Figure 2B. Itcan be seen from Figure 2B that PSIandPS1 are two ARCs formed on c-Si solarcell via ECE formation results in a verylow refI.ectance in a certain wavelengthrange, which for photovoltaic applica­tions must lie between 650 and 700DID.lI

For example, in Figure 2A. PS1

shows -5% reflectance in the wave­length range of 650 and 700 DID. For an

Reflectance of Porous SiliconFilms

tions I and 2, an ARC thickness (d) of83 om (90 om) is required to producea zero net reflectance at 650 om (700om).

Table I summarizes the availableARC materials and their refractive indi­ces.29,30 In order to cover a broad rangeof the solar spectrum, double layer antireflection coatings (DLARC) have beeninvestigated by several authors.31-33

Higher refractive index materials (from2.2 and 2.6) are preferred as bottomlayer films and lower refractive indexmaterials (from. 1.3 to 1.6) are generallyused as top layer films in DLAR coat­ings. MgFfZnS, MgFfIi02' and SiOISiN, have been reported as DLAR coat­ing materials.:M Canham et aI.24 haveestablished that a PS film used as anARC should be 60% porous to exhibitan optimal nARC value. The PS refractiveindex varies from 1.25 to 3;24.35 there­fore, the additional capability of PSfilms as single and double layer ARC isevident.

The comparative analysis of the re­flectance spectra of PS films with otherconventional ARCs will shed morelight on this aspect of PS films. Fig­ure 3 shows the re:8ectance of PS filmsalong with other ARCs. such as doublelayer TIO.jMgF2' SiN

land ZnS.20,3S-37

In Figure 3A--e. PS films are formedon nolo emitter of multicrysta1line silicon(mc-Si), SP solar cell. In Figure 3D,PS films are formed on n+emitter of SPc-Si solar cell. In Figure 3A, PS filmsare formed via chemical etching3S andin Figure 3B--D, PS films are formedvia electrochemical anodization usinga two-e1ectrode arrangement.2ll,36,37 Thedetailed description of these studies hasbeen presented in the litrature.2O,3S-a7

Figure 3A shows the reflectancecharacteristics of a me-Si solar cellwith (a) PS and (b) double layer (riOIMgF2) ARC.X\,3S This study has beenperformed by a group at Fraunhofer­ISE (ISB) on commercial. cells fromASE GmbH.20 The lower reflectance ofPS ARC than that of the double layer(riO.jMgF2) ARC, in the wavelengthrange of -600-700 DIn, clearly exhibitsits better anti-reflecting properties forapplications in solar cells.3S

In a recent study by Kwon et aI.,~ the

where, ns; is the refractive index of c­Si which depends on wavelength (3.84at 650 om, 3.76 at 700 om), and noiristhe refractive index ofair. According toEquation I, the refractive index (n) ofan optimized ARC material on siliconshould be equal to 1.96 at 650 DIn and1.94 at 700 om.2.1 Hence, from Equa-

FigureA Topviewscanningeleclronmicrographsof three PS fillTl$ fabricated in etching solutionswith different HF concentrations: <8> 12.5%,(b) 16.7%, and (e) 25%. Anodlzatlon current II10 rnA crrr'!'

FORMATION AND MORPHOLOGY OF POROUS SILICON

Formation

The most common method for fabricating porous silicon (PS) is the electrochemi.caetching (ECE) of silicon in HF based electrolyte. The electrolyte contents used for PSformalion may vary substantially. However, the electrolyte is geDe1'8lly a mixtureofaque­OWl HF and eIbanol (cu,~OH). Ethanol is used u a surfactant to facilitate extnu:tionof hydrogen bubbles formed dwing etching.2O,2A Electrochemical anodization is usuallyperformed in an especially designed Teflon cell, using a two-electrode arraDgetDeDt asshown in Figure lB. The back of the silicon wafer is held strongly in ccmtact with amdallic copper plate which acts u anode. The PS formation process is performed at aconstant current density. The platinum electrode acts as cathode which is positioned inthe electrolyte above the silicon surface that is to be etched. The PS film fabricated usingthis method is generally homogenous in both porosity and thickness.:14

An alternative method for processing PS is stain or chemical etching (CE) which re­quires the dipping of silicon substrate in a solution ofBP, nitric acid (RNO,), and water~O). No external bias is used in this proalSS.:14 Nevertheless, the PS formed using thismethod is inhomogeneous in both porosity and thiclmcss due to the fact that hydrogen

gas evolved during formation remains on thesurface of the wafer.2S Howevel'. PS formationby stain-etcl1ing is particularly attractive be­cause of its simplicity and is useful to producevery thin PS films.- .An:hcr' has revealed thatit is possible to create stain films u thin 18 2SAthrough stain etching with HF-HN0

3solution.

MOl1)hology

Porous silicon morphology (i.e.• pore size,porosity, and pore depth) depends on the type

'----_--J1 of silicon wafer, its resistivity, method of PS• 10 IJ.m formation (i.e.• ECE or CE) and on process

conditions such IlII current density, HF ccm­cenIration, HF: CAOH ratio in ECE andHF:HNO, ratio in CE.2UI F'lgUle A shows themorphology of three PS films pn::pIIRld for HFc~of(a)12.5%,{b)16.7%,and(c)25%, in which PS films are prepared by elec­trochemical etching on P type ~Si wafer. Thescanning electron miaograph of the surfila:of PS film (a), in Figure A, reveals ita highlyporous nature; film (b) exhibits lower porosity.

L--_---'I The top view of the film (c) surface reveals DO

b 10 11m porous structure even at the largest magnifica-tion; on the other band. many large cracks havebeen observed. PS films prepared at a cmTemtdensity of 10 rnA cm~ and HF oonc.entratiOlUlhigbcr than 25% showed surfila: morphologysimilar to sample (C).27

c

RAlIC =..Jns;R... (1)

op1imal. ARC on a silicon substrate, therequired refractive index aDd thicknessis given by:

Table I. RefractIve Indices (n) forvarious Antireflection CoIling MatertalsColT88POl1dlng toWavelength of 650 to

7OOnm....

re:8ectance of (a) PS ARC is found tobe quite comparable to the reflectanceof (e) conventional SiNx ARC on a (b)textured mc-Si in the wavelength rangeof -600-700 DID (Figure 3B). It can beseen that PS ARC shows a minimum re­flectance of -3.1% at 570 DID and SiNzARC shows a minimum reflectance of-2.0% at 730 nm. As a result, PS ARCdemonstrates optical performance thatis superior to vacuum-deposited SiNxARC film. In addition, PS is formeduniformly on the entire area of the mc­Si wafer.36

The reflectance of a solar cell withan optimized (a) PS ARC and (b) witha SiN ARC is shown in Figure 3C. Inthis work. PS has beenfmmed on mc-Sisolar cells with a very large area (IOD­164 cm2). It is clearly seen that similarrefl.ectance is obtained for PS and forthe conventional SiN" ARC.20 This in­dicates the advantages ofPS as ARC tomake solar cells cost effective.

The reflectance spectra of the c-Sisolar cells with ARCs (a) PSI' (b) PS2,

(c) Zns, and (d) without ARC, in thewavelength range of 360-600 nm areshown in Figure 3D.37 PSI and PS2 aretwo porous silicon films deposited onn+ emitter of c-Si solar cells at differ­ent charge densities QI = 0.06 C1cm2

and az= 0.9 C1cm2, respectively. It can

be seen that, in the entire investigatedrange of wavelengths, the re:flectance ofthe samples with (a) PSI' (b) PS2 ARCis less, compared to (c) Zns ARC and(d) without ARC. It is clear that, in thewavelength range of 550-600 DID, bothPSI and PS2 show significantly less Ie­

flectance3'l and, hence. PS exhibits good

CurrentSource

by varying current density and time.These results clearly indicate that PS

can be implemented as ARC in indus­lrial solar cells and can possibly replaceother conventional single and doublelayer ARCs.

Application of Porous Siliconin Industrial Solar cells

Pt-electrode

Industrial solar cells are generallyfabricated on large area (-100 cm2) Czc-Si or mc-Si substrates using low~ost

processing techniques to obtain a rela­tively good efficiency at reduced cost.'JYpical efficiency ofcommercially pr0­

duced c-Si solar cells lies in the rangeof -13-17%. The efficiency of solarcells depends on processing techniquesand influence the production cost at allproduction stages. Therefore, substan­tial effort is directed toward efficiencyimprovement. The induslrial solar cellprocessing steps involve texturizaDon,phosphorous diffusion (to make p-njIDlCtion), realization of front and back

Cu-plate

FIgure 1. (A) SEM Image of PS111m formed on silicon substrBIe,1he right imeg., showing 1hemagnified top and cross--sec·110n81 vtew of PS 111m, dear1yshows lt1e pores.· (8) The act1e­mlllic diagram of the cell holderfor PS formation. The Si sampleIs placed on a Cu-p1lrt8. whichIs positively biased, an o-ring(HF·resistant) is placed on 1Dpof the sample. The 0811 materialIs Tellon. The elec1rolyte Is1111edInto 1he reeervolr and a Pt-elec­1rode is placed in the electro·Iyte. The PI·e1ectrode is nega·11\/8ly biased. A amant sourcecontrol81he a.lrren1lblas.

A

B

anti-reflecting properties in solar cellapplications.

In this work, we have formed PSfilms via ECE technique in a mixture ofHF and CAOH (1:1 by volume) usingSi as the anode and Pt as the counterelectrode as shown in Figure lB. PShas been formed on n+ textured emit­ter of c-Si SP solar cell (area-10 cm2)at varying cunent density (1-10-40 mAcm~ and time (t-20 s to 60 s). Figure2A shows the reflectance spectra of twosuch PS films; PSI and PS2 along withbare and textured silicon surfaces asa function of wavelength in the range400-1,100 DID. PSI conesponds to ClU'­

rent density (1) of 10 mAcm-2 and time(t) of 30 s whereas PS

2corresponds

to J =20 mAcm-.2 and t =20 s. It canbe seen from Figure 2A that PS2 ARCshows lower reflectivity values in themost useful part of the solar spectrum(AmID= 650-700 DID) as compared to

PSI' Therefore, it is important to notehere that an optimal PS can be obtained

1.952.30-2.401.80-1.901.5(H.701.30-1.40

1.741.90

1.80-1.901.482.30

2.10-2.302.33

RtIrIcIIn IndII:(n)

ceoceozfld.0•GlassMgF.MgO

~4SiCSiC2

TiO!TSzO.znS

400 500 600 700 800 900 1000 1100

A Wavelength(nm) B

Figure 2. (A) The measured reflectance of the ARCs; PS1 (J = 10 mA cm-2, t = 30 s) andPS2 (J = 20 rnA crTr", t = 20 s) formed on c-Si solar cells along with bare and textured siliconsurface; (B) principle of anti-reflection coating_

versity Microelectronics Center, Bel­gium (IMEC) and Centre National dela Recherche Scientifique--Laboratoirede Physique des Solides de Bellevue(CNRS-LPSB).20 The efficiency poten­tial of a PS ARC has been investigatedby the Interuniversity MicroelectronicsCenter, Belgium on high-quality FZ Sisubstrates (2x2 cm2).20 Centre Nationalde la Recherche Scientifique-Labora­toire de Physique des Solides de Bel­levue focuses on the formation of PSARC for commercial mc-Si solar cells(5x5 cm2) and the PS film is formed at aconstant current density of 50 mAcm-2

for 3.5 s. The details of solar cell pro­cessing and PS formation are describedin the following publications.20,43 Fromour recent studies, it is found that theproperties of PS formed using screen­printed silver and aluminum as the backcontacts are superior as compared to thecorresponding films with evaporatedback contacts.- The PS films formedwith screen-printed silver and alumi­num-back contacts show better crystal­line perfection, higher stability, higherPhotoluminescence (PL) efficiency andnegligible PL decay compared to thatformed with evaporated silver and alu­minum as the back contact for the samecurrent density and time of anodization.The results conclusively demonstratethe viability of screen-printing contacttechnology for the possible applicationofPS films in Si-solar cells.-

Table IT summarizes the measuredphotovoltaic performance parameters;current density (Jsc)' open circuit voltage(Voc)' fill factor (FF) and efficiency (T\)under global AM1.5 illumination forchemical and electrochemical etchingon mc-Si solar cells before and after PSformation. In chemical etching, the effi­ciency of ISE solar cells increases afterPS formation; however, the fill factordegraded which may be attributed to theincrease in R., mainly due to the inter­action of HF acid with glass-containingsilver paste in SP contacts.35 In the RM3cells, the metallization is protected bya polymeric film which explains thesmaller degradation of R., and hence theachieved higher FF and efficiency.43 Inelectrochemical etching, it can be seenfrom Table IT that efficiency of 14.6%and 13.2% has been obtained on thesolar cells of IMEC and CNRS-LPSB,respectively. After PS formation, fill

nair

SIconWafer

during PS formation and therefore de­grade the quality of the SP contacts. Inorder to avoid this degradation problem,the following two approaches can beadopted. In the first approach, ECE canbe performed for a short time and thisseems to be more convenient for large­scale implementation of PS in finishedSP solar cells.20 In the second approachPS formation can be done on the heav­ily doped n+ emitter surface prior to thecontact metallization step.20 However, itis very difficult to obtain a stable ohmiccontact on PS layer because of the largesurface state density.40 Therefore, ECEon finished solar cells, for short timesseems to be a better option to obtain ahomogenous PS ARC. Moreover, inECE by varying the current density andtime, it is possible to obtain PS films withvarying refractive indices and thickness­es.35 Both the methods have been appliedto SP silicon solar cellS.20,35,36,41,42 The re-

sults of PS ARC implementation usingchemical and electrochemical methodsin industrial mc-Si solar cell processingare presented in these studies.20,35,36,41-43We have formed PS ARC for short an­odizaion times via ECE on finished so­lar cells. Solar cells have been fabricatedon c-Si (p-type) wafer using industrialprocessing steps as described above andhave silver fingers on the front side andAg-Al contact on back side.

A research group at the UniversitARoma Tre (RM3)43 and a group fromFraunhofer-ISE (ISE)35 implementedstain-etched PS ARC on screen printedmc-Si solar cell that is currently usedin the PV industry. RM3 used the solarcells from Eurosolare S.p.A.43 where­as ISE used commercial cells fromASE GmbH.35 The electrochemicalmethod has been applied by Interuni-

Bare Silicon (c-Si)35

B 25 TexturedC

~ 20

~ :: -""",/\ /5, ___5 -••. -- -------------- "- P5,

- 30~!1....

contacts using screenprinting techniquesfollowed by deposition of antireflectioncoating. The schematic representation ofone such c-Si solar cell by Sun Power isshown in Figure 4,38 where the expectedsolar cell efficiency is -29%. However,various losses such as reflection, recom­bination and resistive losses reduce thecell efficiency to 14.7%. It can be seenfrom Figure 4 that the total losses ac­count for 14.3%, wherein 1.8% of thereflection losses are due to front metallicgrid structure and 0.4% reflection lossesare after texturization and ARC depo­sition. The main attraction of poroussilicon in the photovoltaic (PV) industryis to use it as a cost effective ARC forlarge-scale applications. The main ad­vantage of PS as an ARC (as discussedin the preceding section) is well known;it reduces the reflection losses resultingfrom the silicon surface. Furthermore,PS has a large active area and is capableof light down-shifting in energy or pho­ton energy down-converter (ultravioletto visible). The use of PS films in solarcells has led to a reduction in the surfacerecombination velocity, enhancement ofthe spectral response in the short-wave­length region, and increase in the pho­togeneration velocity of charge carri­ers.1S-

24 Porous silicon can also serve asa wide bandgap absorber in a multiple­junction cell structure, with c-Si as thesubstrate.24 Attempts have been reportedto use two PS films with two differentbandgaps in a three-bandgap solar cellon a silicon wafer.39

Porous silicon ARC formed via ECEor CE on finished solar cells has a se­rious problem of fill factor degradationdue to the increase in series resistance(R.). Screen printed metalized contactsmake direct contact with HF solution

40

2.6%

\Aluminum

paste

29%-14.3%

14.7%

three-dimensional (3-D) picture of PSlayer, which consists of irregular uprightsurface features with typical size distri­bution of -150--500 nm or even smaller,where the maximum pore size is -500nm long and -50 nm wide whereas theminimum pore size is -150 nm long and-30 nm wide. Such types of structuresare generally observed in PS films.24

Fourier transform infrared (FTIR)analysis is widely used to character­ize the bonding properties in PS films.Here, we present FTIR and photolumi­nescence (PL) spectra on PS films ofvarious thicknesses, formed on p typec-Si wafer via ECE. In this work, PSfilms of various thicknesses are formedat optimized current density (1) of 20rnA cm-2 with time (t) variation from1 to 20 minutes, respectively.45 As cur­rent density increases, the thickness ofPS film increases.44 The PS films ofvarious thicknesses, (a)-{e) in Figure6A and B, are formed at a constant Jof 20 rnA cm-2 and at time t = 1 min, 2min., 5 min., 20 min., and 30 min., re­spectively.45 The thickness of PS films,(a) and (b), are -150 nm and -200 nm,respectively, as measured by an ellip­someter; for films (c)-(e), the thicknessvaries from 5 to 20 f.1IIl as estimatedfrom gravimetric measurements. IS Cor­responding FTIR and PL spectra arerecorded for these PS films (Figure 6 Aand B).

Figure 6A shows the FfIR spectra ofPS films of various thicknesses (a)-{e).During PS formation, a large number

Silver paste pad

Recombinationlosses "

2.0CY~

1000

Texture + ARC Resistive loss

12R loss

3.8%

Limit cell effiCiency

Total losses

Generic cell efficiency

(b)

,! _ .

~.4% Back light\ absorption

/Back

contact

Reflection loss

1.8%

Fron~/contact

Figure 4. Multiple losses (such as reflection, recombination and resistive) in a singlecrystal silicon solar cell.38 (Courtesy of SunPower)

60 - (a) Reflectance of the cell with PS ARC- - - - - (b) Reflectance of the textured mc..si wafer

. (c) Reflectance of the cell with SINx ARC

(a)

10 I.......... "o+-~,___._____,_~-,-~_,_-.--,___,r__r-_I300 400 500 600 700 800 900

Wavelength (nm)

shows surface morphology, as seen bySEM and AFM for film PS2, which hasbeen formed on n+ emitter of textured c­Si solar cell. As can be seen from FigureSA, the textured surface shows uniformlarge pyramids of sizes varying from 1to 10 J..lID. approximately. The SEM im­age indicates that the front screen print­ed silver finger has not been damagedafter PS formation. Figure 5B shows

5D

~41 40UcJ!I 30UCIl ,.,.~ 20:;./

B1200400

/(a)0-1--,--_,.-_-,.-_-,--_ _�

600 800 1000

Wavelength (nm)

80 _ (a) Reflectance of the cell with PS ARC70 . -_.. (b) Reflectance of the cell with SiN ARC

70 _ (a) Reflectance of the cell with PS ARC- - - - - (b) Reflectance of the cell with OLARe

60 (T10,lMgF)

10

A

1.2 ; -.- (a) Reflectance of the cell with PS, ARC

:-0- (b) Reflectance of the cell with PS~ ARC

- 1 0 i- e - (e) Reflectance of the the cell with ZoS ARC- :i . .....t ...- (d) Reflectance of the Siwafer~ 60 ni!if 50 41 0.8 \i: g ht)-e 40 J!lCIlU 0.6 't\\CIl (b) •

~:: \'.\. ~ 0.4 110.:......< (d) ~••••••••\ (b) "' A -

10 0.2 /\.. ~ #

/(a) _~~~ .:.:-.o 0.0+-----,----.--~---,----.--~_I400 600 800 1000 1200 350 400 450 500 550 600

C Wavelength (nm) D Wavelength (nm)

Figure 3. (A) Reflectance characteristics of cells with (a) PS ARC and (b) double film (TiOjMgF2) ARC;20·36 (B) reflectance characteristics of (a) PS ARC (b) alkaline-textured mc-Siwafer, and (c) single-film SiNx ARC;36 (C) reflectance characteristics of mono-Si cells with(a) PS ARC and (b) with conventional SiN ARC;20 (0) reflectance spectra of samples withARCs (a) PS1 ' (b) PS2 ' (c) ZnS and (d) without ARC.37

factor does not change in IMEC solarcells, whereas fill factor increases forCNRS-LPSB. The characteristics of thecell (5x5 cm2 Polix mc-Si) with the PSfilm (11-13.2%) are similar to commer­cial cells.20 Table IT also lists the resultsobtained for optimal PS ARC (PS

2-Fig­

ure 2A), implemented on a screen print­ed c-Si solar cell. It has led to -20.8%relative improvement in Jsc' a significantgain of -15mV in Vee and a relative in­crement of about -.3% in FF. This yieldsa -26% increase in efficiency. In a studyby Kwon et al.,36 the optimization of aPS selective emitter in a screen printedmc-Si (2x2 cm2) solar cell results in a13.2% efficiency with electroplating.36

Thus, PS has immense potential asARC in commercial solar cell appli­cation. Simplicity and low cost of theECE technique as well as its adaptationto silicon solar cell manufacturing pro­vides a very promising technology in anindustrial process.

Characterization of PorousSilicon Films

The surface morphology of PS filmshas been studied by scanning electronmicroscope (SEM) and atomic forcemicroscope (AFM). Figure SA and B

Table II. Ml8lul'8d PhOlOWlltaic Pllramll8l11j Current Density (J.), Open CircuitYabge(YJ, Fill FacIOr (FF),ud Efficiency (1'\l under Global AM1.5llluminatian on Solar cells

using Chlmicalud EI8cIroc:hImicll PS FormItionl'

(~v. FF "PS PI'DC8M Cella From ARC Ar8I (mAcnr-t') (mY) (%)

-

Chemical ASE(lSE) No ARC 100 22.3 575 0.774 9.9ASE(ISE) PSARC 100 28.9 574 0.740 12.3

(Eurosolare)RM3 No ARC 164 22.4 588 0.685 9.0RM3 PSARC 164 27.2 592 0.753 12.1

(ASEGmbH)

Electrochemical IMEC PSARC 4 31M 603 0.78 14.3(ncn-1exIured)

IMEC PS (taxtured) 4 31.3 801 0.78 14.6CNRStPSB No ARC 25 31.8 595 0.73 13.6CNRS-lPSB PSARC 25 3004 583 0.74 132

Present 'Mlrk No ARC 10 19.7 545 0.87 7.15Present 'Mlrk PSARC 10 23.8 580 0.88 9.01

of Si-hydride bonds as well all Si-hy­droxide bonds are formed and pOllsiblypassivate the SilPS interface.47 It can beseen in Figure 6A that PS films exhibitmainly Si-H related modes at -2101em-I due to Si-H stretching mode, -910em-I due to Si-~ scissors or Si-~ sym­metric or antisymmetric deformation.doublet -666 em-I and 627 em-I due to

Si-~ and Si-H wagging while Si-Q re­lated modes are marked by very weaksignatures at -1,035 em-I due to a bulk:interstitial Si-D-Si allymmetric stretch­ing model.1l,2O,4S

AB the thickness of PS films increase(i.e., FTIR spectra from (a)-{e», a clearincrease in both line-width and inten­sity of these modes can be seen. Thisindicates the relatively higher hydrogencontent in the PS films as tbi.ckness in­creases. However, for very thin PS:films,(a), and (b), the FI1R. spectra showsonly weak signatures of these bands.The distinct presence of Si-H stretch­ing mode at -2,101 em-I representsthe quality of passivation for PS films(d) and (e). Small peak: at -2,308 em-Ican be related to 0 back-bonded to Si inSi-H stretching mode.4I Similar FTm.results on PS films are also discussed inthe Reference 20.

FIgUre 6B shows the correspondingroom temperature PL spectra of thesePS films (a)-{e). The thin porous siliconfilm (a) does not exhibit any PL proper­ties which may be attributed to the weaksignals of Si-H bands in the FI'IR spec­tra. However, a weak PL can be seen

Figure5.(A)SEM micrograph,(8) mlcrostn.Jcture using AFMfor optimal ARC 111m PSI (J '"20 mA c:rrrI, t =20 s), formedon textunKl emitter n+ of n+ P B801arcell.

for PS films (b) and (c) which may beattributed to the increase in intensityof Si-H and related bands in the FTm.spectra. The absence ofPL properties inPS film (a), and weak PL for films (b)and (c) are apparently due to the siliconskeletons not being narrow enough forquantum confinement. The PL intensityincreases as PS film thickness increases(from (a}-(e» as shown in Figure 6Band a sharp peak at -688 nm. has beenobtained for the PS films (d) and (e)corresponding to 10 and 30 minutes ofPS formation. The increase in the corre-

sponding normalized PL peak intensityis correlated with the increase in the Si­H and Si-Hzbonds observed in the FI1R.spectra. Therefore, it is worthwhile tonotehere that SEM, AFM, PL and FTlRstudies are important characterizationtechniques to investigate the structuraland passivating properties ofPS films.

APPLICATIONS OF POROUSSILICON IN BIOSENSORS

BiOllen80I'S have emerged as highlyprmnising for rapid diagnosis of bac­teria in foods. The general functionof a biosensor is to convert a biologi­cal recognition event into an electricalor optical signal.49,sll Signal tramIduc­tion has been accomplished with elec­troohemica1,51 field-effect transistor,52

optical absorption. fluorescence andinterferometric devices.53 As discussedpreviously,2,6,lI-I7 a number of theoreti-cal and experimental works, concern­ing the noteworthy properties of nanD­

structured porous silicon in chemicaland biological sensing, have been re­ported, showing that. due to its morpoo­logical and physical properties, PS is avery versatile sensing platform..:w.-17

In our recent studies, we observedthat PS :films formed on textured sub­strates show higher PL intensity andhigher lifetime values as compared tothose forIDed on polished substrates forthe same current density.S4 Porous sill-

800750600 650 700

Wavelength (nm)

surface stability of PS, there is a needto functionalize the surface of PS by asuitable precursor. In one of our recentworks, nanostructured PS surface wasbiofunctionalized by thermally depos­iting thin biocompatible films with alarge density of amine groups, using 3­arninopropyltriethoxysilane (APTS) onto its surface.63 The aim of the study wasto demonstrate the covalent bonding be­tween organic molecules (immunoglob­ulin) and modified inorganic surface(nanostructure PS) which can be usedfor the detection of protein signals. Inthis study, PS films prepared at an op­timized J-50 rnA cm-\ having high PLintensity, stable surface bond configu­rations, mechanically strong structureand hydrogen-passivated surfaces wereused for APTS treatment.63 The pres­ence of reactive amino groups on the PSsurface along with glutaraldehyde as alinker aids in the covalent binding of theantibody (Human IgG) onto the PS sur­face.63 Different antigen concentrationscan be detected with a good reproduc­ibility with this technique which opensa possibility of using this biofunctional­ized material for future biosensors.

550

J=20 mAcm-21.6

1.4

5 min (elOA ,

0.2t§§~~;~~1~m;:in3(a~I::;~~~500

B

-; 1.2

~ 1.0~'iii 0.8c::.! 0.6c::

- 30 min".. (e)

~ / /C'a- 2351 2101

~ 20 min ".. (d)II)c: 5min (e)Sc:

2min ".. (b)

1min ".. (a)

4000 3500 3000 2500 2000 1500 1000 500A Wavenumber (1Iem)

1.8,----------------,30 min (el

\

Figure 6. (A) FTIR spectraand (8) PL spectra of freshPS formed at current density(J) of 20 rnA cm-2 for timevariation from 1 min to 30min., curves (a-e).

J=20 mAcm·2

structures due to over-oxidation of thesilicon columns. It can be inferred thatthe surface passivation either by hydro­gen or oxygen is one of the requisiteconditions for obtaining strong PL ef­ficiency in PS.61 It seems that more thanone emission mechanisms are responsi­ble to explain the luminescence proper­ties of PS.60

Furthermore, PS biosensor technolo­gy has shown great capability in detect­ing biological molecules with high se­lectivity, using specific linker agents andprobe molecules.61 For the biomedicalapplications of PS, biomolecules haveto be first immobilized on its surfacethrough functional groups deposited onit. The common approach is to create acovalent bond between the PS surfaceand the biomolecules which specificallyrecognize the target analytes.61 The reli­ability of a biosensor strongly dependson the functionalization process as wellas its rapidity, simplicity, homogene­ity, and repeatability.62 It is well knownthat, after anodization, the fresh siliconsurface is predominately hydride-termi­nated which is quite reactive and sensi­tive to oxidation.62 Thus, to increase the

con layers and their interfaces have beencharacterized by recording diffractioncurves and measuring lattice mismatch!strain and the radius of curvature due toinduced biaxial stress caused by the lat­tice expansion of PS film due to pores.55

The higher range of strain values exhib­ited by PS films on textured specimenscorresponding to a wide range of bandgaps compared to that formed on pol­ished specimens indicates that stable andhigher porosity PS films can be formedon textured substrates at higher currentdensities. The larger surface area avail­able for PS films formed on texturedsubstrates enables the even distributionof strain, thus, leading to enhanced sta­bility as compared to the correspondingfilms formed on polished specimens.55

This factor is responsible for PS filmsformed on textured substrates to with­stand higher strain without any elasticrelaxation at high current densities andcan be used for gas-sensing measure­ments.56 In the case of PS films on pol­ished specimens, the relaxation of theinduced strain starts at relatively lowercurrent densities as compared with PSfilms on textured substrates and thusis not suitable for gas-sensing applica­tions.

The surface of porous silicon needsto be stabilized for biosensing applica­tions and is achieved by means of oxida­tion, silanization, or hydrosilylation.57

•58

Even without resorting to the abovetechniques, PS surface can be stabilizedusing an appropriate post-anodizationtreatment. At an optimum current densi­ty, passivation of defects by a novel na­scent-H treatment resulted in the signifi­cant enhancement in the PL efficiency.59

The degraded PL intensity in the treatedsamples upon prolonged oxidation forseveral months was higher as comparedto that for the as anodized samples. In­frared vibrational studies indicated thatthe enhancement in PL was due to theH-passivation of defects in the Si-poreinterface as also elucidated from capaci­tance-voltage studies.59 Furthermore, ithas been found that HF-treated PS sur­faces are relatively stable against oxida­tion as compared to untreated PS films.60

Upon oxidation of the HF treated PSfilms, the PL intensity initially increasesas a result of reduction in crystallite sizeto exhibit quantum size effects and thendecreases owing to loss of luminescing

Tml(IIC.)

5.780....-....,....---,--....,....---,---.----,--,----,--,--.-----r--.---....... 1 B

binding of molecules induces changesin the refracti'Yeindex of the porous sili­con films.Ci1be biosensor has been dem­onstrated for small organic molecules(biotin and digoxi.genin), 16-nucleotideDNA oligomers, and proteins (strepta­vidinandantibodies) atpico- and femto.molar analyte concentrations. The sen­sor is also highly effective for detectingsingle and multi molecular assemblies.6

Another example is a porous siliconbiosensor chip, fabricated by Mathewet al.,ID which detects Escherichia coli(E. coli) bacterium and is based on thelight emitting properties of porous sili­con. In this work, PS films are made viaelectrochemical etching of P type c-Siwafer and the biosensor adapts single­tube chemiluminescence-based assayfor detection of E. coli.ID The reactionof P-galaetosidase enzyme from E. coliwith the dioxetane substrate gener­ates light at 530 nm. Figure 8B showsthe light emission of a control poroussilicon biosensor chip (blank with noE. coli culture) as well as that of p0­

rous- and planar-silicon biosensor chipstested with an overnight E. coli pureculture. As shown in Figure 8B. the p0­

rous silicon biosensor chip has higherlight emissions compared to the planarsilicon biosensor chip due to the highersurface area of the porous chip.1.O Sensi­tivity of 1be porous silicon biosensor isdetermined to be 101-111 colony form­ing units (CPU) of E. coli.

A potentiometric biosensor for thedetection of tryglycerides for metabolo­micshasbeen developedby Setzu et a1..f;]

In this study, PS layers are formed on n+type c-Si wafer by electrochemical etch­ing. ThiB biosensor immobilizes lipaseenzyme in the PS layer and it induceshydrolysis of a triglyceride, which intum results in a decrease in the pH andhence shifts the open circuit potentiaL1bis approach of immobilization of theenzyme within PS layer considerablyincreases the working life of the sensoras compared to other lipase-based 1ri­glyceride-detection methods.

A label-free optical biosensor for thedetectionofrabbitIgG(rabbitpolyclonalantibody) in whole blood using a PS mi­crocavity has been reported by Bonannoet al.61 Porous silicon microcavities areformed via ECE into highly doped n­type silicon. Shift in wavelengthenablesthe sensor to detect antibodies in either

Figure 7. (A) Nanoporousbiosensor principle. (B)5chemallc representa­tion of interferogram 01 atypical nanoporous siliconbiosensor experiment"(Source: Silicon KInetics)

changes in refracti.'Ye index in the PSlayer, when light reflected from the topofthe porous region interferes with lightreflected from. the bottom of the porousregion and creates optical interferencepatterns. The optical path differencesignal (OPD signal) is derived fromthe interferogram as in Figure 7B andrises proportionately with the amount ofbound biomolecules. AB can be seen inFigure 7B, the first step is to immobilizethe first biomolecule of interest i.e., thetarget on sensor surface (with the de­sired surface chemistry) and, then, thesolution with the second biomolecule ofinterest i.e., the analyte is introduced;any net binding of the analyte to thetarget changes the effective index of re­fraction in the porous region as biomo­locules displace buffer solutions withlower indices of refraction.6l'i

The simplest example of optical in­terfermnetric PS biosensor is shown inFigure SA6 which monitors changes inthe refractive index that occur in PS sin­gle- or double-layer :films, wherein PS:films are made via electrochemical etch­ing of P type c-5i wafer. In this sensor,

,.

_5,840E..s~°5,820

5,800

5,860

5,880

In recent developments, the processof filling the PS pores with metallic,dielectric or semi.coIlducting oxide, en­zyme. or molecular receptor films, hasresulted in PS sensors that are capableof detecting penicillin, alkali metalions, humidity, and hydrocarbons.64 Ina paper by Jane et al.,fi5 numerous PSbiosenso:rs are discussed. One categoryof biosensor belongs to interfeIeDCe ef­fects in PS single and double layers; thesecond category discusses the photolu­minescence-based transduction; thirdcategory is based on PS microcavitiesand fourth category discusses the elec­trochemical transduction with poroussilicon. The PS biosenso:rs of differentcategories are compared on the basis oftheir performance, such as: concentra­tion range, sensitivity for analyte andtheir detection limit.(is

Silicon Kinetics has introduced nanoPS biosensor and chips to monitor bio­molecular interactions,6l'i where a nano­porous region has been formed on ac-Si wafer via eleclrochemical etching.Figure 7A shows the principle of nano­porous biosensor, which is based on the

implemented as ARC into an industri­ally screen-printed solar cell by both theelectrochemical and chemical etching.PS films, formed. via ECE for short an­odization times are applied on textmedn+ emitter of c-Si having SP from andback. contacts. Implementation of opti­mal PS ARC has led to -20.8% relativeimprovement in J"", a significant gainof -15 mV in V0; and a :relative incre­ment of about -1.3% in FF. This yieldsa -26% increase in efficiency. PS filmsformed on textured c-Si substrates ex­hibit higher porosity and PI.. efficiency,negligible PL decay, better mechanicalstrength, adherence to the substrate,non-fractured surface morphology andlower stress compared to porous sili­con formed on polished c-Si substratesat the same current density and demon­strate the viability of possible applica­tion ofPS films in Si-solar cells. PS sm­face morphology on textured emittersin solar cells, as seen by SEM, showsthat PS formation does not degrade theunprotected from metallic grid pattern.AFM measurements ofPS ARC on tex­tured emitters in solar cells :reveal thatthe mayjrnnm pore size is -500 nm.longand -50 om wide whereas the minimum

Ugh! Souree .. Figure B. (A) SChematic of PS-bued optIcaIlnterleromstrlc blo-I18n80r. Reftection ofwhile lightatthe top and bottom of the PS filmresut18 in an interference pallern.Interactions of molecular speclll8on PS surface induce a changein 1'8fractiV8 indexof nanocryslal-line semiconductor, giving rise toWIMllengttl shifts In fr1nge pal-

...... 1em thai can be easily detectedby charge-coupleddevice (CCD)camera.' (B) Comparison of lightsmlselon of E. coil pure cufture Inporous 8IlIcon and planar siliconbiosensors agailllll porous sili-con control.1o

A7

~ : c c c c c c

.3 4cD

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010 20 30 40 50 60

TIIIIII (min.)

B • Blank D PorcusSi P1anerSi

detection of target molecule by FfR­IPS, where trypsin and its inhibitor areused as the model probe-target system..The FfRIFS biosensor has been com­bined with a UV detector for screeningthe target molecule from. complex com­ponent mixtures separated by a LC (liq­uid chromatography) column, as shownin Figure 9. The outerPS layer, attachedwith trypsin by amino-silane and glutar­aldehyde, can specifically bind with thetrypsin inhibitor and acts as a samplechannel. while the bottom layer servesas a reference signal channel. The bind­ing event between trypsin and trypsininhibitor is simultaneously detected bythe FI'RIFS biosensor in real-time bymonitoring the change in optical thick­ness of the porous silicon layer. Opticalsignals have a linear relationship withthe concentration of trypsin inhibitor inthe range of 10-200 ng mL-1,71

CONCLUSIONS

Porous silicon has a large potentialfor applications in photovol.taics. Re­flectance ofPS film is quite comparableto the :reflectance ofconventional ARCs(such as SiN", double layer TIO/MgF2and ZnS). The PS film can be easily

undiluted serum or whole blood.68 Im­mobilization of antibody on PS surfaceis done by using biotin-streptavidin. Thebiosensor has exhibited linear detectionrange of2--10mgImL.

Another example of PS microcavitysensor has been reported by Ouyang etaI.,li9 in which macroporous silicon mi­crocavities were electrochemically syn­thesized from n-type c-Si wafers. Thissensor is operated by analyzing the in­duced red-shift in the absorbance peakwhich is caused by the binding of ananalyte to an immobilized probe mole­cule. The device detects an extracellulardomain of intimin (intimin-ECB), a pro­tein associated with the pathogenicity ofenteropathogenic Escherichia coli. Im­mobilizition has been carried out by at­taching probe molecules: Th-ffin, theb'anslocated intim:in receptor-intiminbinding domain, covalently to the PSsurface. Intimin-ECB canbe detected ina concentrationof4 j.LMafteroptimizingthe concentration ofprobe molecules.

Rossi et aI.70 have developed a newmethod for improving the sensitivityfor detection of the bacteriophage vi­rus MS2 using thin films of nanoporoussilicon. The PS films are prepared usingECE technique on polished p+- c-Si wa­fer. They have shown that a 100 omthickPS layer with a covalently immobilizedantibody has a sensitivity and dynamicrange similar to that of the Lum:inexliquid array-based assay while outper­forming protein micro-array methods.70

A porous silicon optical biosensorhas been developed for monitoring pro­tein-protein binding, specificallyproteinA with IgG.71 Thin film. of (5 J1Dl) PShas been used for immobilization andtransducing matrix and the sensor oper­ates by measurement ofthe Fabry-Perot:fringes in the white light reflection spec­trum from the PS layer. Analyte bindingcauses change in effective optical thick­ness of functionalized porous siliconFabry-Perot film and thus transductionis achieved. Sensor demonstrates its sta­bility, reversibility and insensitivity toI1OII8pCCific inte:ractions.

Recently, a Fourier transformed re­ftectometri.c interference spectroscopy(FfRIFS) PS biosensor has beenreport­ed by Shang et aL71 In this setup, doublelayers of porous silicon films have beenprepared by ECE on p-type c-Si wafersand used. as a sensing element for the

uv Detec1Dr

Figure 9. The Le-lJV-FTRIFS system, which Includee a chromatographic columnhyphenated with a UV detector and FTRIFS biosensor in series. A tungsten light sourceis focused on a porous silicon surface through optical fiber probe, and the interl8rometricre'IIectance spectra of ponlus silicon are collected UBlng a CCO spectlometer coupled 10 ablfun:ated fiber optic cable.71

Sample Injection

pore size is -150 nmlong and -30 nmwide. Fourier transform infrared andPL spectra are utilized to character­ize the PS films of various thicknesses.Fourier ttansfonn infrared results showpassivating capabilities ofPS films and.therefore, allow fabricating solar cellswithout an additional passivation coat­ing. The study of the optical propertiesby PL spectra show that the increase inthe PL intensity may be attributed to thepresence of Si-H and Si-~ bonds withincrease in PS film thickness.

PoroUll silicon biosensors based onoptical interferomelry and light emit­ting properties are discussed. For anoptimized ClD'J.'eIl1: density, the func­tionalization of the PS surface has beenachieved by silanization method usingAPTS as a precursor. The presence ofreactive amino groups on the PS smfacealong with glutaraldehyde as the linkeraids in the covalent binding of the anti­body (Human IgG) onto the PS smfaceleading to detection of different antigenconcentrations with a good reproduc­ibility.

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