Single-Step Plasma Process Producing Anti-Reflective and Photovoltaic Behavior on Crystalline Silicon

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Single-Step Plasma Process ProducingAnti-Reflective and Photovoltaic Behavioron Crystalline Silicon

Rosa Di Mundo,* Marianna Ambrico, Paolo Francesco Ambrico,Riccardo d’Agostino, Francesca Italiano, Fabio Palumbo

Reactive ion etching (RIE) plasma processes fed with CF4 have been investigated as single-stepmaskless method for nanotexturing the surface of crystalline silicon. Variation of surfacetopography under different plasma conditions has been evaluated with scanning electronmicroscopy and correlated with total, diffuse, andspecular reflectance. Chemical features have beenevaluated by X-ray photoelectron spectroscopyand current–voltage characteristics have beenmeasured under dark and illuminated conditions.Results indicate that a widely tunable nanoscaletexture can be generated onto silicon surfaceleading to a reduced total reflectance. A signifi-cant uptake of carbon and fluorine is detectedonto treated silicon with fluorine mainly in ionicform. Further, the plasma modification is per secapable, without further doping procedures, togenerate a photovoltaic behavior onto treatedsilicon, with higher short circuit current in lessreflective samples.

Introduction

Micro- and nanotexturing of silicon is attracting a

significant interest in the last years as powerful strategy

R. Di Mundo, R. d’Agostino, F. ItalianoDepartment of Chemistry, University of Bari, via Orabona 4,70126 Bari, ItalyE-mail: [email protected]. Ambrico, P. F. Ambrico, F. PalumboInstitute for Inorganic Methodologies and Plasmas (IMIP) – CNRvia Orabona 4, 70126 Bari, ItalyR. d’AgostinoPlasma Solution Srl, Spin-Off of the University of Bari, Bari, Italy

Plasma Process. Polym. 2011, 8, 239–245

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlin

to reduce reflectivity in the route to high-efficiency

solar cells, photodetectors, and other optoelectronic

devices.[1]

It iswell knownthat the featureofoptical surfacesgiving

rise tounwantedFresnel reflections is thesuddentransition

or impedance mismatch, from a medium to another. If the

transition can be made more gradual, i.e., the refractive

index,n, canbevariedmoregraduallyand the reflectioncan

be significantly reduced. As an alternative to the conven-

tional antireflective technology, based on destructive

interference upon tailored stack of layers, it is possible to

design antireflective surfaces by focusing on the material

itself and its topography, that is mixing the material with

air on a subwavelength scale, thus using porous or textured

elibrary.com DOI: 10.1002/ppap.201000148 239

240

R. Di Mundo, M. Ambrico, P. F. Ambrico, R. d’Agostino, F. Italiano, F. Palumbo

surfaces. Such an effect was argued by Bernhard to occur

over the eye of the night flying moth, covered with a fine

regular hexagonal array of protuberances which have a

period of about 200nm, a similar depth and a cross-section

that is approximately sinusoidal: the natural corrugation

provides a mixed solid/air layer with graded refractive

index.[2] This phoenomenon canbe interpretedbyusing the

theories of the effective medium approximation and/or

the gradient refractive index. The former considers the

layer with a certain rough or porous topography like an

homogeneous medium with an effective refractive index

betweenthesubstrateandtheair.[3] The latter, anextension

of the former, postulates a variation of topography along

the depth such that a continuous increase of refractive

index occurs fromair to the bulkmaterial.[4] This effectwas

first reproduced by Clapham and Hutley[5] by recording

finely spaced optical interference fringes in two orthogonal

directions of a photoresist, reaching results similar to

multilayer interference coating over the visible and the

near infrared.

The problemof surface reflection is particularly acute for

semiconductors which have very high values of refractive

index in the visible and the near-infrared spectral regions.

For crystalline silicon n is 3.8, and the reflection losses at

normal incidence are about 40%, representing a drawback

for solar cell applications and other optoelectronic devices.

Many texturing methods have been proposed in this

framework, generally based on various photolithographic

approaches,[6] electron beam lithography,[7] colloids aided

procedures,[8,9] wet etchingwith acids or bases, sometimes

assisted by metal or oxides catalysts.[10,11]

Needle-like, pyramid-like, or porous silicon surfaces

structures are obtained in most cases. In order to further

reduce reflectance, even complicated approaches to obtain

hierarchical structuring (micropyramids and nanostruc-

tures) have been tested.[12] These and other examples of

fabrication of micro/nanotextured (NT) silicon, both for

bottom–up and top–down procedures, are finely reviewed

in reference[13]

Among the top–down approaches, much attention is

nowadays given to plasma etching, for the intrinsic

possibility of easy scaling up to large substrates with good

dimensional control and also under maskless conditions.

In some cases these processes are called self-masked

since the discharge itself produces both nanomasks and the

etchant active species. This, for instance, is assumed to

occur inprocesses composedof consecutive stagesofCF4/O2

etching, SF6 etching, C4F8 passivation (commonly knownas

Bosch process or time-multiplexed etching process), result-

ing in highly antireflective silicon nanotips (SiNTs).[14]

Similar SiNTs have been produced with ECR plasmas fed

withSiH4, CH4,Ar, andH2where etching isbelieved tooccur

simultaneously to the formation of SiC nanomask.[15]

Further, SF6/O2 fed RIE processes have been shown to


� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

generate micron-sized needle-like and cylinder-like struc-

tures, probably as a consequence of non-homogeneous

masking by silicon oxide[16] or ‘‘inverted pyramids’’.[17]

H2 based plasmas with substrate biasing[18,19] or

heating[20] have been also shown to develop a nanotexture

onto silicon.

In several cases among those cited a remarkable

reduction of total reflectance is reported although an

optoelectronic characterization of the modified silicon is

rarely performed.[16]

The possibility of producing nanostructures uniformly,

though randomly, distributed onto the surfaces of various

polymers (polystyrene, polymethylmethacrilate, and

others) with fluorine or oxygen containing plasmas in

maskless single-step processes has been widely shown in

recent years and exploited for wetting and optical

applications of plastics.[21–23]

In particular with fluorocarbon plasmas reflectance has

been reduced without hindering polymer transparency.[22]

This nanotexturing method can be finely tuned with

varying plasma parameters, and, though themechanism is

not yet completely understood, it seems to be reasonably

originated by sputtered metals from electrodes, acting as

nanomask.[24] In thisworkaRIE radiofrequency (RF)process

fed with pure CF4 has been utilized to address nanoscale

surface texturing of crystalline silicon. The variation of

nanofeature shapes and sizes under different plasma

conditions has been evaluated and correlated with total,

diffuse, and specular reflectance. Chemical modification of

silicon surface has been assessed with X-ray photoelectron

Spectroscopy. Current–voltage characteristics under dark

and illuminated conditionshavealsobeenmeasuredon the

as obtained specimens in order to investigate the optoelec-

tronic variations that the plasma process itself can induce

on this material.

Experimental Part

P-type (100) crystalline silicon (c-Si) slabs (polished wafers, with

nominal resistivity 1–50V cm, thickness 0.7mm, from MEMC St.

Peters) were utilized as silicon substrates. Plasma treatment

experiments were run in a RF capacitively coupled parallel plate

plasma reactor. It consists of a cylindrical stainless steel vacuum

chamber (internal diameter of 30 cmandheight of 13 cm), pumped

by a turbomolecular-rotary pumping system. The pressure is

monitored by a capacitive membrane transducer (MKS instru-

ments, Baratron). The gas flow rates are controlled by electronic

mass flow controllers (Brooks Instruments). The plasma is ignited

with a 13.56MHz power supply (Caesar Dressler) through an

impedance matching unit. The upper stainless steel electrode

(20 cm diameter) is grounded while the bottom electrode (15 cm

diameter, spaced 5 cmapart), onwhich the substrates are placed, is

RF powered. A shower head inserted in the upper electrode allows

gas admission.

DOI: 10.1002/ppap.201000148

Single-Step Plasma Process Producing Anti-Reflective and Photovoltaic . . .

Discharges were fed with CF4 at a flow rate of 50 sccm and

constant pressure of 45mTorr. The effects of input power (100–

300W) and of treatment duration (20–60min) have been

investigated.

Samplesmorphologywasobservedat tilt angleof0and608with

a Zeiss EVO 40 VP scanning electronmicroscope (SEM) at a voltage

of 20 kV. Image J softwarewas used to process the acquired images

and measure the topographycal features.

Diffuse and total reflectance at 08 angle of incidence was

measured in the 200–1000nm range with unpolarized light by

means of a Varian Cary 5000 spectrophotometer equipped with a

diffuse reflectance accessory (DRA), whose integrated sphere can

separate the diffuse component from the total reflectance (the

specular component has been calculated by subtraction).

XPS analyseswere carried outwith Thermo

Electron Corporation Theta Probe Spectro-

meter using a monochromatic Al Ka X-ray

source (1 486.6 eV) at a spot size of 400mm

corresponding to a power of 100W at a take-

off angle of 538 from surface normal. Survey

(0–1200 eV) and high-resolution spectra were

recorded at a pass energy of 200 and 150eV,

respectively. The C1s signal for C–C(H) bonds,

with a binding energy of 284.7 eV, has been

used as an internal standard for the correction

of the samples charging.

The at.-% reported have been calculated by

using the sensitivity factors of the instrument

from the high-resolution peaks.

Best fitting of the XPS line shapes has been

performed by means of the instrument soft-

ware (Avantage). The full-width at half max-

imumofeach lineshapewasallowedtovary in

the range 1.8–2.4 eV.

The optoelectronic electrical characteriza-

tion has been performed on the as treated

silicon slabs (in this frame called NT-Si/(p-

type)c-Si junctions, where NT stands for

nanotextured), without any further doping

procedure.Auwasdepositedbyevaporationas

front side contact (0.2� 0.2 cm2) and an InGa

pastewaspaintedontheuntreatedbacksideof

theslabs for theohmiccontact formation.Dark

current and photocurrent density versus

voltage characteristics have been measured

byusingaKeithleyK617electrometerandaHP

DAC power supply. The white light of a

halogen lamp simulating solar spectrum

(AM 1.5G, standard CEI/IEC 904-3) with

irradiance 100mW/cm2 was used as light

irradiation source at room temperature in air.

Figure 1. Top (left) and 608 tilted view (right) SEM images of crystalline silicon samplestreated in CF4 plasma at 100 (a), 200 (b), 300 W (c) for a treatment time of 20 min, and at100 W for 60 min (d).

Results and Discussion

Top (left) and 608 tilted view (right) SEM

images of silicon samples treated in CF4plasma at 100W (a), 200W (b), 300W (c)



for a treatment time of 20min, and at 100W for 60min (d)

are reported in Figure 1.Generally it canbeappreciated that

a deep variation of both size and shape of the structures is

achieved as a function of the working conditions. The 100

and 200W samples after 20min treatment present similar

nanosharpened shape but at a different dimensional scale

(at 200Wstructures are taller, larger, andmore spaced). The

300W/20min sample displays paddle-like structures

(round, flat, and with a thinner base), while the sample

obtained at 100W for longer treatment time shows slender

stem-like structures. Features mean height increases with

input power (from 100 to around 400nm) and with

treatment duration: 60min stem-like structures are the

www.plasma-polymers.org 241

Table 1. Topographical features, evaluated by SEM images pro-cessing, of the plasma treated crystalline silicon samples.

Sample Mean height

(nm)

Mean distance

(nm)

100W/20min 100� 30 70� 30

200W/20min 200� 50 130� 50

300W/20min 380� 100 250� 100

100W/60min 550� 50 280� 60

Figure 2. Total (a), diffuse (b), and specular (c) reflectance spectrain the range 200–1 000 nm of c-silicon samples treated in CF4plasma at 100, 200, and 300 W for a treatment time of 20 min andat 100 W for 60 min.

242


tallest (550nm). Conversely, the features are less dense as

both power and time increase. Mean height and distances

among structures are reported in Table 1.

Total (a), diffuse (b), and specular (c) reflectance spectraat

normal incidence in the range200–1 000nmare reported in

Figure 2. Untreated silicon shows the typical high-

reflectance spectrum with the three maxima caused by

the direct interband transitions.[25]

The increaseof inputpowerat20min leads toareduction

of total reflectance which passes from (averaging the 400–

800nm range) 38.0% of untreated Si to 16.8% at 300W. The

sample processed at lower power but for longer time show

the lowest reflectance, 14.1%. As well known diffraction

effects take place when the wavelength of incident light is

comparable to periodicity or width of structures. The

100W/20min surface, with mean distance below 200nm,

does not give diffraction effects (diffuse reflectance almost

absent) in the explored range. On the other hand these

effects are quite significant in the spectral range 200–

500nm for all other surfaces, characterized by longermean

distances among structures, (anyway diffuse reflectance

does not exceed 20%). In spite of such a difference in diffuse

component, specular reflectance basically follows the trend

of total reflectance showing only a slight reduction for the

100W/20min surface while it is more steeply lowered for

all theothers. The100W/60minsample (stem-like), though

appearing the most diffusive surface, with a broad diffuse

reflectance in the range 200–600nm, responsible for its

gray–bluish color, displays the lowest total reflectance.

Reductionof specular reflectanceappearsmorepronounced

below500nm, justasaconsequenceof thediffractive losses

before described.

The trend of total reflectance confirms the predicted

dependence of reflectivity within the effective/gradient

index theory on the thickness of the structured layer (mean

heightof structures in this case), i.e., reflectance reaches0as

the h/l exceeds 0.4.[5] At this regard, the 300W/20min and

100W/60min samples appear of suitable thickness (for

l¼ 600nm, lowest reflectance for h> 240nm), however,

reflectivity does not fall below 10%. This is likely a

consequence of the not tapered profile of these structures.

Getting structures with at least 300nm of height and very



thin apexes could contribute to a reduction of diffuse

contribution (not only thedistance but also structurewidth

can produce diffraction) as well as to an abatement of the

specular one by making still more gradual the refractive

index transition from air to the bulk material (gradient

index).

In Table 2 the XPS atomic concentrations are reported for

native and for all treated silicon surfaces. Besides a

conspicuous amount of oxygen (also present in native

DOI: 10.1002/ppap.201000148

Table 2. XPS at.- % of untreated and plasma treated crystalline silicon.

Sample Si

(%)

F

(%)

Fion(%)

C

(%)

O

(%)

Tot. Metals

(%)

Untreated silicon 49.4 0 (0) 9.6 41 0

100W/20min 16.5 13.3 (7.8) 28.3 36.4 5.5

200W/20min 8.1 24.5 (21.8) 25 35.8 6.6

300W/20min 8.1 27.9 (27.4) 25.2 27.8 11

100W/60min 12.2 34.8 (16.0) 23.4 23.2 6.4


silicon), a high-surface enrichment of both fluorine and

carbon is detected, with fluorine gettingmore abundant at

higher power and longer time. Presence of metals (mostly

Fe, with small traces of Cr), is found onto all the treated

surfaces, and is believed to play a role (as nanomask) in

initiating the non-homogeneous ablation.[24] The F/Si and

C/Si ratios, which can be considered as parameters of the

extent of chemical modification, are plotted in Figure 3a.

Figure 3. XPS F-to-Si and C-to-Si atomic ratios (a) and high-resolution F1s signals (b) of the CF4 plasma modified siliconsurfaces.



Although the trendare similar, it canbe seen that theF-to-Si

ratio shows a steeper increase as a function of power and

time.

High-resolution C1s spectra, not shown, are mainly

composed of a C–C(H) component at 284.7 eV, and smaller

contributions of C–CF(O) (286.6 eV) of C–F (288.7 eV) and C–

Si (283.3 eV). Moreover no significant variation has been

observed in the quite low Si2p signal.

Figure 3b reports the XPS F1s high-resolution spectra of

all the treated surfaces. A component at 684.7 eV is present

inall thesamplesand isascribed to ionicfluorine[26] andone

or two other components, depending on the sample, are

includedat higher binding energy (from686.5 to 688.7). The

latter two, though not univocally discriminated, can surely

beascribed to covalentbinding states offluorineatoms, like

F–C and/or F–Si bonds. As it can be appreciated relative

ionic-to-covalent contribution increases as the input power

rises (at 300W the F signal is almost uniquely composed by

ionic F), while with increasing treatment duration the

covalent componentbecomesmore significant. Thecontent

of ionic F, calculated as absolute percentage, is reported in

Table 2.

In Figure 4a dark current density versus voltage

characteristics (semilog plots) of the full set of NT-Si/(p-

type)c-Si junctions are reported along with that of the

untreated (p-type)c-Si. It can be observed that while for

untreated silicon forward and reverse currents are almost

symmetric, for the NT-Si/(p-type)c-Si junctions, beside a

reduction of the overall current levels, asymmetric currents

are found. This is a rectification behavior similar to that

typically observed in various type of metal semiconductor

or metal insulator semiconductor based junctions.[27] In

particular, the rectification ratio, which is the ratio of the

maximum value of the forward current to the maximum

value of the reverse current, increases from 12 in untreated

silicon to amaximum of 1 500 in paddle-like structure. The

reduction of reverse bias current observed in treated silicon

suggests theonset of aminority (electron) carrier controlled

transport mechanism.[28] It could be derived that plasma

induced surface modification develops a kind of p–n

junction (NT-Si/(p-type)c-Si junction) where the dominant

carriers at the silicon surface are opposite to those in the


-1.0 -0.5 0.0 0.5 1.010-5

10-3

10-1

10

a)

b)

1

103

Dar

k cu

rren

t den

sity

(mA

/cm

2 )

Voltage (V)

untreated pSi 100W 20 min 200W 20 min 300W 20 min 100W 60 min

-1.0 -0.5 0.0 0.5 1.010-2

100

102

Voltage (V)

Phot

ocur

rent

den

sity

(mA

/cm

2 )

Figure 4. Semilog plots of dark current density (a) and photo-current density (b) versus voltage characteristics of the full set ofNT-Si/(p-type)c-Si junctions, i.e., the plasma treated siliconsamples, along with that of the untreated (p-type)c-Si.

Table 3. Short circuit current (Jsc), open circuit voltages (Voc), andtotal reflectance averaged in the spectral range 400–800 nm (Rt)of untreated and CF4 plasma treated crystalline silicon.

Sample Jsc(mA/cm2)

Voc

(V)

Rt

(%)

Untreated silicon – – 38

100W/20min 0.285 0.230 35.1

200W/20min 1.06 0.177 19.9

300W/20min 4.25 0.153 16.8

100W/60min 2.70 0.200 14.2

244


silicon bulk (in this case electrons instead of holes). The

presence of the electrical transport driven by minority

carriers also explains the appearance of a photovoltaic

behavior, appreciable in Figure 4b, where the photocurrent

density versus voltage is reported. NT-Si/(p-type)c-Si

junctions, in fact, show an open circuit voltage (Voc) as

well as a short circuit current density (Jsc) not observed on

the untreated (p-type)c-Si. It is worth remarking that this

occurs without any further doping procedure of the

textured silicon, instead utilized elsewhere.[16] The trend

of the short circuit current, which increases for samples

obtainedathigherpowerand time, ismostlya consequence

of the previously described reduction of reflectance, thus

the increaseof light trappingandoptical absorption (values

reported in Table 3, along with average total reflectance in

the spectral range 400–800nm). The open circuit voltage

(Voc) is a parameter inversely related to carrier recombina-

tion velocity, thus more dependent on surface chemical



modification effects: it displays the highest value for the

100W/20min surface, that, as previously observed, shows

the poorest chemical and morphological modification.

Moreover this parameter has a perfect inverse linear

correlation with the surface content of ionic fluorine

determined with XPS (Table 2). Such an effect could be

indicative of a role of this species in increasing carrier

recombination velocity, and therefore, it will be handled in

detail, among others, within future investigations. It is

worth remarking that the behavior of photocurrent density

versus voltage between 0V and Voc is greatly affected by

shunt and series resistance effects, since the procedure of

junction building up has not been fully optimized. Anyway

under the present experimental conditions, the highest

photovoltaic efficiency h, i.e., the ratio of the maximum

outputpower tothe incident lightpower (100mW/cm2)has

been found in the junctions with the paddle-like and stem-

like structured surfaces (0.28 and 0.17); the best fill factor

(FF), defined as the ratio of the maximum power to the

product Jsc�Voc, has been found in the junction with

paddle-like structures (0.4). These results, though not

optimized, are similar to those reported for junctions

embedding silicon nanowire arrays grown onto silicon and

modified with elaborate doping procedures.[29,30] Deeper

investigations areneeded to improveperformances inview

of the simple, single-step process utilized.

Conclusion

Thisworkprovides an investigationof the capability of CF4-

fed RIE plasmas of generating a nanoscale texture onto

crystalline silicon and modulating light trapping and

optoelectronic behavior. Results indicate that this process

is highly efficient in developing silicon nanostructures

which can be widely tuned in both shape and scale with

playingwith input power and treatment time. Total optical

reflectance in the range 200–1 000nm is remarkably

lowered, especially at high power and time, in spite of

some diffraction losses in the short wavelength range. This

DOI: 10.1002/ppap.201000148


kind of plasma also deeply affects surface chemistry of

crystalline silicon, since a significant uptake of carbon and

fluorine is detected by XPS. Fluorine is present mostly as

ionic species, in particular at high power. These samples,

without any further doping or other procedure, show an

enhancement of the rectification features and under

illuminated conditions the appearance of a photovoltaic

behaviorwithhigher short circuit currents for less reflective

samples. Hence, the process here described represents a

single-step fastmethod for producing an antireflective p–n

junction with crystalline silicon.

Further investigations are necessary for tailoring the

structures profile (more tapered shape) to get still lower

reflectivity as well as for further improving performances

by better correlating chemical and optoelectronic effects.

Acknowledgements: The financial support of the regionalfunding-cod. 51-Public Laboratory of Apulian Industrial Researchof Plasmas ‘‘LIPP’’ is gratefully acknowledged.

Received: October 19, 2010; Revised: December 6, 2010; Accepted:December 9, 2010; DOI: 10.1002/ppap.201000148

Keywords: antireflective; crystalline silicon; nanotexturing/structuring; photovoltaic; plasma etching

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Single-Step Plasma Process Producing Anti-Reflective and Photovoltaic Behavior on Crystalline Silicon

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