research article controlled pore formation on mesoporous single crystalline silicon...

Research ArticleControlled Pore Formation on Mesoporous Single CrystallineSilicon Nanowires Threshold and Mechanisms

Stefan Weidemann1 Maximilian Kockert1 Dirk Wallacher2 Manfred Ramsteiner3

Anna Mogilatenko4 Klaus Rademann5 and Saskia F Fischer1

1Novel Materials Group Humboldt-Universitat zu Berlin 10099 Berlin Germany2Department for Sample Environment Helmholtz-Zentrum Berlin 14109 Berlin Germany3Paul-Drude-Institut fur Festkorperelektronik 10117 Berlin Germany4Ferdinand-Braun-Institut Leibniz-Institut fur Hochstfrequenztechnik 12489 Berlin Germany5Nanostructured Materials Humboldt-Universitat zu Berlin 10099 Berlin Germany

Correspondence should be addressed to Stefan Weidemann weidemannphysikhu-berlinde

Received 19 December 2014 Accepted 16 April 2015

Academic Editor Yu Deng

Copyright copy 2015 Stefan Weidemann et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Silicon nanowires are prepared by the method of the two-step metal-assisted wet chemical etching We analyzed the structure ofsolid rough and porous nanowire surfaces of boron-doped silicon substrates with resistivities of120588 gt 1000Ωcm 120588= 14ndash23Ωcm and120588 lt 001Ωcm by scanning electron microscopy and nitrogen gas adsorption Silicon nanowires prepared from highly doped siliconreveal mesopores on their surface However we found a limit for pore formation Pores were only formed by etching below a criticalH2O2concentration (119888H2O2

lt 03M) Furthermore we determined the pore size distribution dependent on the etching parametersand characterized the morphology of the pores on the nanowire surface The pores are in the regime of small mesopores with amean diameter of 9ndash13 nm Crystal and surface structure of individual mesoporous nanowires were investigated by transmissionelectron microscopy The vibrational properties of nanowire ensembles were investigated by Raman spectroscopy Heavily boron-doped silicon nanowires are highly porous and the remaining single crystalline silicon nanoscale mesh leads to a redshift and astrong asymmetric line broadening for Raman scattering by optical phonons at 520 cmminus1 This redshift 120582Si bulk = 520 cm

minus1rarr

120582Si nanowire = 512 cmminus1 hints to a phonon confinement in mesoporous single crystalline silicon nanowires

1 Introduction

Nanopatterning can improve bulk properties of crystallinematerials [1] Silicon nanowires attract significant interestsin biochemical-sensors catalysis and photocatalysis becauseof their adjustable electrical properties and their enormoussurface-to-volume ratio [2ndash4] Due to their modified elec-trical behavior silicon nanowires have been proposed forhigh performance field effect transistors [5] whereas theirenhanced optical adsorption is advantageous for photovoltaicapplications [6 7] In particular silicon nanowires with roughsurfaces have strongly decreased thermal conductivities andhence have been proposed as promising candidates forthermoelectric devices [1 8]

Silicon nanostructures can be generated by chemicaletching in alkaline and acid solutions [9] In both cases thewafers orientation etchant concentration and the differentetching rates of silicon and silicon dioxide play significantroles for the nanostructure morphology Etching along thecrystallographic axis can be enhanced by applying an externalelectrical potential [10]

There are various ways for the preparation of siliconnanowires such as laser ablation vapor-liquid-solid growth(VLS) and growth by molecular beam epitaxy (MBE) [11]However these methods may lead to imperfections such asdislocations grain boundaries site defects and impurities

The method of metal-assisted chemical etching has theadvantage that it is a low-cost fabrication easily scalable

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 672305 11 pageshttpdxdoiorg1011552015672305

2 Journal of Nanomaterials

to wafer size [12 13] The metal-assisted etching differs inthe number of etching steps [14] In the one-step etchingthe solution containing the catalytic particles propels thenanowire etching whereas in the two-step etching theamount of catalytic particles is limited in a first etching stepand in a second etching solution an oxidizing agent affectsthe nanowire etching The limitation of catalytic particlespromises a better control in silicon nanowire preparation

Zhang et al [15] presented a two-step wet chemicaletching synthesis of silicon nanowires prepared from p-doped and n-doped silicon substrates with (100) and (111)orientations Qu et al [16] and Lin et al [17] investigatedhow etching parameters like etching time and concentrationof etching solutions affect the formation of n-doped siliconnanowires as well as nanowire porosity Hochbaum et al[18] and Yuan et al [12] showed for the one-step etchingprocess that with decreasing wafer resistivity the nanowiressurface roughnessporosity increases Although the existenceof mesopores on etched silicon nanowires from highlyboron-doped silicon is known [14 16ndash18] their shape andthe detailed formation mechanism of rough surfaces andnanoporous structures on silicon nanowires remain unclear

Here we present a systematic analysis of pore evolutionby gas adsorption studies for the two-step etching processof silicon nanowires We have fabricated silicon nanowireensembles from undoped Si (resistivity 120588 gt 1000Ωcm)medium boron-doped Si (120588 = 14ndash23Ωcm) and highly boron-doped Si (120588 lt 001Ωcm) and determined pore diameterdistribution pore volume and sample surface area for highlydoped nanowires The influence of etching time and etchantconcentration illumination and temperature on nanowireformation is discussed and the pore formation in depen-dence of etchant concentration and time is analysed Sur-face morphology roughness and crystallinity of individualsilicon nanowires are investigated by transmission electronmicroscopy (TEM) The vibrational properties of nanowireensembles are investigated by Raman spectroscopy

2 Experimental Section

Silicon nanowires are prepared by the method of metal-assisted chemical etching in a two-step etching approachwhich has been reported previously [14 16 17] The siliconwafers are cleaned with sonication in acetone (10min) iniso-propanol (10min) and in boiling acetone (10min) and inboiling iso-propanol (10min) Organic residues are removedfrom the wafer with a fresh prepared boiling Piranha-solution a mixture of 3 1 H

2SO4(97) an H

2O2(35)

for 10min The generated oxide layer is removed by anHF etching (5 HF for 10min) The sample is rinsed withdeionized water

In the first etching step with a solution of 48M HFand 002MAgNO

3for 60 s the silver (Ag) nanoparticles are

placed on the wafer surface In this step the silicon wafer iscoated by a galvanic replacement of silicon by silver Silvernanoparticles start to sink into the silicon substrate and beginto form silver dendrites

After 60 s the sample is rinsed with deionized water againand in a second etching solution consisting of 48MHF and

01ndash05MH2O2for an etching time of about 1ndash3 h the silicon

nanowires are formed by the HF etching of the silicon in thefollowing way

At first [14] oxidation of silicon takes place as follows

Si + 2H2O + 4h+ 997888rarr SiO

2+ 4H+

Si + 2H2O 997888rarr SiO

2+ 4H+ + 4eminus

(1)

Second etching of silicon dioxide produces

SiO2+ 6HF 997888rarr H

2SiF6+ 2H2O (2)

Etching of SiO2occurs with a higher rate than etching of pure

Si in HF Therefore it is the dominant etching process [9]Additionally the silicon oxidation is promoted by the silver(Ag) and so a higher etching rate of silicon mediated throughthe catalytic activity of the noble metal is achieved [14 19]

Through the charge transfer on the interface of silver andsilicon the silver nanoparticles sink deeper and the siliconnanowires arise as the remainings of unetched silicon Thedriving force is the oxidizing power of the oxidizing agentH2O2

The driving force is given by

H2O2+ 2H+ 997888rarr 2H

2O + 2h+ (3)

H2O2oxidizes the Ag particles which are in turn reduced at

the silicon surface by an electron transfer from the siliconand thereby oxidize the silicon This can be described as acharge transfer process in which the silver injects holes intothe silicon (1)

After etching the sample is rinsed with deionized waterand the Ag particles are removed by a cleaning step in HNO

3

The wafer is washed in deionized water several times andthen dried in a nitrogen flow Normally all etching steps areperformed in the dark and at room temperature

Here we prepared nanowires from three types of siliconwafers

(I) undoped silicon (100) (specific resistivity 120588 gt1000Ωcm doping level 1013 cmminus3)

(II) medium boron-doped silicon (100) (120588 = 14ndash23Ωcm1015 cmminus3) p-type

(III) highly boron-doped silicon (100) (120588 lt 001Ωcm1019 cmminus3) p-type

The as-prepared ensembles of silicon nanowires were charac-terized by scanning electron microscopy (SEM Hitachi TM-1000) and details like themesoporous surface of single wireswere characterized by SEM (Raith E Line plus) The surfacearea of whole silicon nanowire ensembles was measured bythe Brunauer-Emmett-Teller (BET) nitrogen gas adsorptionmethod [20] The automated gas adsorption station Quan-tachrome Autosorb-1 was used The measurement data wereanalyzed by Autosorb-1 software to determine the mean poresize distribution (pore diameters andpore volumes) onhighlydoped nanowires For this analysis the 1 cm times 1 cm waferswere put into the sample chamber and degassed in a vacuum

Journal of Nanomaterials 3

at 140∘C for at least 8 h until the vacuum in the sample cham-ber has been better than 10minus4mbar At 119879 = 77K nitrogenwas dosed in controlled increments into the sample chamberThe pressure equilibrates and the adsorbed nitrogen quantitywas calculated The chamber was filled successively withnitrogen and so the adsorption isotherm was obtained by theadsorbed volume for each relative pressure Afterwards thesample chamber was emptied again by successive evaporatingenabling us to determine the desorption isotherm [21]

Transmission electron microscopy was performed usinga JEOL JEM-2200 FS electron microscope Raman measure-mentswere performed at room temperature in backscatteringgeometry The 4825 nm line of a Coherent Kr+ ion laserwas focused onto the samples by a confocal microscopemagnification 20x numerical aperture (NA) 05 The powerdensity on the sample surface was about 400Wcm2 for10mWand aOD 10The scattered signal was collected by thesame objective and dispersed spectrally by a grating with 600linesmm located in an 80-cm Jobin-Yvon monochromatorThe signal was recorded using a LN

2-cooled CCD with a

spectral resolution of 75 cmminus1

3 Results and Discussion

After the first step (HFAgNO3) of the two-step etching

process we observe the formation of small silver particles onthe surface of the silicon wafer after a few seconds which isin accordance with [22] These particles grow in size withincreasing time and some start to form silver dendritesDuring the first minutes of etching in the second solution(HFH

2O2) the silver particles sink into the substrate the

silver dendrites grow on and the nanowires are formed as theremains of the unetched silicon Longerwires are obtained forincreasingH

2O2-concentration and longer etching timeThis

is in accordance with [14 16 17] However we find that thisprocess is limited and for excessive etchant concentrationsand etching times the nanowires are etched on their tips aswell

Figure 1 shows scanning electron micrographs (SEM) ofsilicon nanowires prepared from different doping concen-trations (a) undoped (120588 gt 1000Ωcm) silicon nanowireswhich are solid straight and therefore rigid Approximately110 120583m long nanowires formed on both sides of the siliconwafer with a remaining width of the wafer of less than 90 120583mare depicted The morphology of nanowires of the mediumboron-doped silicon not shown is similar to undopednanowires Figure 1(b) shows highly boron-doped siliconnanowires (120588 lt 001Ωcm) These wires are thinner andmore flexible than wires from other silicon types This iscaused by the high length-to-diameter ratio The wires formbundles and lean against each other The nanowire tips arebent to the tips of the neighboring nanowires This bendingcould be attributed to a change in the Youngrsquos modulus likedescribed before by Lee and Rudd [23] Hoffmann et al [24]and Sohn et al [25] measured a strongly decreased Youngrsquosmodulus for silicon nanopillars Our qualitative bendingexperiments with an indium tip on single highly dopednanowires confirm the results in [24] and suggest at least

a strong decrease of the Youngrsquos modulus In contrast to[23 24] our highly doped nanowires exhibit a mesoporoussurface A possible influence of the porosity on the Youngrsquosmodulus demands further investigations for example byatomic force microscopy bending experiments and forcemeasuring at the nanowires such as in [24] In Figure 1(c) wedemonstrate that a homogenous distribution of nanowires bythe two-step etching process is feasible on wafer scale

In Figures 2 and 3 the main growth trend is presentedfor nanowires from undoped silicon Figure 2(a) mediumboron-doped silicon Figure 2(b) and highly boron-dopedsilicon Figures 3(a) and 3(b) The measurement uncertaintyin the length is about 3ndash5 which is mainly caused by theinclination and bending of the nanowires In some cases thenanowires on the bottom side of the wafer are shorter than onthe upper side because the wafers were lying on the bottomof the etching vessel

In general we find that longer wires result from higheretchant concentrations and longer etching times similar toresults for n-type silicon nanowires [16 17] However highetchant concentrations affect the etching of the nanowire tipsIf the tip-etching is as fast or even faster than the sinkingof the etching front into the substrate the nanowire lengthremains the same or is decreased This can be seen in thelimiting cases of the chart in Figure 3(a) where the growthtrend is stagnating or even declining so that the maximumwire length corresponds to about 42120583m for highly boron-doped silicon In summary nanowire lengths up to 110 120583m(undoped silicon specific resistivity 120588 gt 1000Ωcm) 90120583m(medium boron-doped silicon 120588 = 14ndash23Ωcm) and about40 120583m (highly boron-doped silicon 120588 lt 001Ωcm) areachieved

Figure 3(b) shows the nanowire growth trend in depen-dence of the bath temperature of the etching solution fortwo different silver nitrate concentrations in the startingsolution (119888AgNO

3

= 001M 119888AgNO3

= 0014M) and twoetching times (60 120min) respectively It is clear that ahigher silver nitrate amount in the first etching solutionleads to longer nanowires Higher etching bath tempera-tures and illumination during the etching procedure (notshown) also increase the nanowire lengths These resultsare in accordance with a recent study on medium boron-doped silicon nanowires (120588 = 10ndash20Ωcm) prepared by theone-step etching process [26] In the case of higher AgNO

3

concentrationwe observe higher etching rates during the firstetching step which can be explained by the presence of ahigher number of catalytic particles promoting the chargetransfers at the silicon-solution interface

Substrate illumination during etching leads to a highernumber of photo-excited charge carriers and a higheretching temperature of the etching solution causes a highermobility of the etching educts and products increasing thecirculation of the etchant components

We confirm that the etching occurs along the [100]-axis into the substrate but also along the other crystal-lographically equivalent ⟨100⟩ axes as has been observedbefore [14 15 19] and depicted in Figure 4(a) Therefore weconclude that the Ag particles are not simply sinking into


50120583m

(a)

30120583m

(b)

03mm

(c)

Figure 1 Scanning electron micrograph of silicon nanowire ensembles from (a) undoped silicon (100) (120588 gt 1000Ωcm) 119888HF = 48M 119888H2O2 =05M etching time 180min There are long solid wires (about 110120583m) on both sides of the thin (lt90 120583m) remaining silicon substrate (b)Highly boron-doped silicon (100) (120588 lt 001Ωcm) 119888HF = 48M 119888H2O2 = 03M etching time 180min The wires form bundles and lie nearagainst each other The nanowire tips are bent to the tips of the neighboring nanowires indicating smaller diameters and pore formation (c)Large scales of uniform silicon nanowire standing on the wafer in cross sectional view

120

20

40

60

80

100

Wire

leng

th (120583

m)

Etching time (min)60 80 100 120 140 160 180

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

c(H2O2) = 04Mc(H2O2) = 05M

(a)

0

Etching time (min)

Wire

leng

th (120583

m)

0 50 100 150 200

20

40

60

80

100

c(H2O2) = 01Mc(H2O2) = 02M

c(H2O2) = 03Mc(H2O2) = 04M

(b)

Figure 2 Length of the silicon nanowires as a function of the etching time for different concentrations of the oxidizing agent H2O2 (a)

Undoped silicon (100) specific resistivity 120588 gt 1000Ωcm (b) Medium boron-doped silicon (100) (120588 = 14ndash23Ωcm) p-type

the substrate but rather that the crystallographic orientationis the dominant factor for the etching direction [14 19] Thiscan be explained by the fact that HF etching of silicon showsa higher etching rate along [100]-crystallographic axis [9 19]

Figure 4(a) shows etched structures at the edges of a sili-con waferThe original wafer surface is (100) oriented Siliconnanowires are standing parallel and perpendicular to thesurfaceThe (100)-directions are indicated with white dashed

lines Figure 4(b) shows scanning electron micrograph of abundle of silicon nanowires which are approximately 35 120583mlongThese nanowires are prepared fromhighly boron-dopedsilicon (120588 lt 001Ωcm) and indicate a porous surfacestructure that can be seen in the magnifications Figures4(c) and 4(d) The magnifications of the bottom regionof some single wires of this bundle show strong intensitycontrasts as typical for mesopores of different size and


10

15

20

25

30

35

40

45W

ire le

ngth

(120583m

)

20 40 60 80 100 120 140 160 180

Etching time (min)

c(AgNO3) = 001M

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

(a)W

ire le

ngth

(120583m

)

0

10

20

30

40

50

0 10 20 30 40 50 60

60min c(AgNO3) = 014M120min c(AgNO3) = 014M60min c(AgNO3) = 01M120min c(AgNO3) = 01M

Temperature of etching solution (∘C)

(b)

Figure 3 Length of the silicon nanowires obtained from highly doped silicon (100) (120588 lt 001Ωcm) as a function of the etching time fordifferent concentrations of the oxidizing agent H

2O2(a) and as a function of etching temperatures for different silver nitrate concentration

(b) There seems to be a saturation value so that the lengths of the wires are limited to about 50 120583m

[100]

[010][001]

50120583m

(a)

(c)(d) 10120583m

(b)

200 nm

(c)

100 nm

(d)

Figure 4 (a) Medium boron-doped Si (120588 = 14ndash23Ωcm) etched for 119905 = 2 h 119888H2O2 = 02M 119888HF = 48M SEM image of a slightly tiltedsilicon wafer after the etching illustrating the etching along the ⟨100⟩-directions perpendicular and parallel to the waferrsquos surface which hasthe (100)-orientation ⟨100⟩-directions are indicated with white dashed lines (b) SEM image of a single bundle of silicon nanowires fromhighly boron-doped silicon (120588 lt 001 Ωcm) marked boxes are magnified in (c) showing the cylindrical porous surface structure and (d)interconnected pores forming meander-like trenches on the porous surface


Carbonfilm

Si NW

[100]

[100]

(a)

(b)

(c) (d)

[100]

400

040

[001]Si

100 nm 2 nm 20 nm

Figure 5 Transmission electron microscopy (TEM) images of highly boron-doped mesoporous silicon nanowires (a) Scanning TEM(STEM)micrograph of a 100 nm thick nanowire Lying on a carbon film the nanowire exhibits a porous surface with a uniform distribution ofpores (b) Selected area electron diffraction (SAED) pattern showing the remaining single crystalline structure of the wire (c) High resolutionTEM of the inner part of silicon nanowire with lattice fringes confirming single-crystalline structure inset Fourier transform proving thesilicon diffraction pattern and the presence of an amorphous amount resulting from the native SiO

2layer around the nanowire (d) TEM

image showing the rough and porous surface structure allowing one to estimate a pore diameter distribution from 5 to 15 nm and a depth ofabout 3 nm

shape In Figures 4(c) and 4(d) small mesoscopic (10 nm)cylindrical wholes and dendritic meander-like channels arevisible on the surface of the nanowires These patterns canbe interpreted as opened (by continued etching) mesoporesclose to the surface The total volume and surface of thismesopores visible and invisible (below the surface) areinvestigated by gas adsorption measurements as describedbelow

For transmission electron microscopy (TEM) analysisthe silicon nanowires were harvested from their substratewashed in a water solution and dropped on a carboncoated copper TEM-grid Scanning transmission electronmicroscopy (STEM) imaging confirms the rough and poroussurface structure of the highly doped nanowire along itswhole length (Figure 5(a)) Selected area electron diffraction(SAED) pattern and high resolution TEM imaging (HRTEM)confirm the remaining single crystalline structure of the wire(Figure 5(b))

HRTEM reveals that the nanowire core is single crys-talline (Figure 5(c)) Silicon lattice planes can be seen inthe inner part of the wire and weakened in the imageby the presence of silicon dioxide and thickness variationratio around the nanowire The inset in Figure 5(c) showsthe Fourier transform confirming the presence of a single-crystalline nanowire with some amorphous amount whichcan be attributed to the SiO

2layer formed at the porous

nanowire surfaceThe TEM-based analysis of the surface morphology

from cross-sectional images for example Figure 5(d) showssurface indents from 5 to 15 nm indicating pores of about 8ndash15 nm in length and 3 nm in depth This is in agreement with

one-step etched wires [18] HRTEM images (not shown) alsoallowed the estimation of the natural silicon dioxide layerthickness ranging from 3 to 6 nm The surface oxide layeris formed by the oxidation of the nanowires in air as wellas in aqueous solutions Darkbright variations in intensityoriginate from the thickness variation of the wire due to therough surface

A detailed analysis of the porous structure of thenanowire surfaces has been investigated by the method ofnitrogen gas adsorption For this analysis the samples aredegassed (at 119879 = 140∘C in vacuum) and the sample chamberis filled with controlled increments of nitrogen starting at arelative pressure of 119901119901

0= 10minus5 at 119879 = 77K where 119901

0is

the saturation vapour pressure of liquid N2at 77 K (119901

0=

105 Pa) For the adsorption isotherm the adsorbed volume

quantity is calculated for each pressure Decrementing theadsorbed nitrogen amount in the sample chamber reveals thedesorption isotherm The low pressure region of a sorptionisotherm corresponds to a mono- and multilayer adsorptionregime of the adsorbate on the substrate From these datathe total surface area of the sample can be derived by themethod of Brunauer Emmet and Teller (BET) [21] Bythe approaches from Barrett Joyner and Halenda (BJH) ordensity functional theory (DFT) we determine the meanpore diameter and the total pore volume BJH considers thatmultilayer adsorption could result in capillary condensationunder the assumption that the pressure for spontaneouscondensationevaporation of the adsorptive in a cylindricalpore is determined by the pore size according to the Kelvinequation [20 21] BJH is recommended for the purpose ofcomparing the pore sizes among the different materials with


(b)

(a)

(c)

00

05

10

15

20

0000

0005

0010

0015

0020

Highly-doped Si

Undoped Si

Undoped Si

Volu

me o

f nitr

ogen

adso

rbed

(STP

cm3)

00 02 04 06 08 10

Relative pressure pp0

Relative pressure pp00000 0003 0006 0009

Volu

me (

STP

cm3)

Figure 6 Example for nitrogen gas adsorptiondesorptionisotherms on an ensemble of nanowires of (a) highly (bluegreentriangles) doped and (b) undoped (redblack) siliconThe isothermsof the highly doped substrate show a hysteresis which is charac-teristic formesoporous surfaces on the nanowires Silicon nanowiresfrom undoped wafers show no pores Inset (c) magnification ofthe low-pressure region of the adsorption isotherm of the undopedsilicon nanowires to illustrate the formation of the first adsorbednitrogen monolayer indicated by the knee at 119901119901

0= 005

the same mesostructures [27] DFT is modeling interactionsand pore geometry by amicroscopic treatment of sorption onamolecular level and thereby gives realistic density profiles asa function of temperature and pressure

Figure 6 depicts two representative gas adsorptionisotherms which show a completely different behavior In thecase of undoped silicon graph (b) the ldquokneerdquo (see inset) at1199011199010lt 005 indicates the formation of a complete monolayer

of nitrogen adsorbed on the sample surface In the relativepressure range of 005 lt 119901119901

0lt 09 a further multilayer

growth of nitrogen on the surface takes place The samplersquossurface area can be calculated by multipoint-BET analysis inthe pressure range 015 lt 119901119901

0lt 035 For higher relative

pressures 1199011199010gt 09 the adsorbed gas amount increases very

sharply This rise announces the filling of the space betweenthe nanowires and finally the rest of the sample chamber byliquid nitrogen This shape corresponds to a type II sorptionisotherm which is typical for macro- or nonporous materialswhere unrestricted multilayer adsorption can occur [21]

Graph (a) in Figure 6 shows the gas adsorption isothermfor silicon nanowires prepared from highly boron-dopedsilicon wafer This is a type IV isotherm with its hysteresisloop which is typical for mesoporous materials [21] Forrelative pressures 119901119901

0lt 005 a more pronounced ldquokneerdquo

from the adsorbed nitrogenmonolayer is visible In the rangeof 005 lt 119901119901

0lt 06multilayer growth appears which will be

evaluated by a multipoint-BET fitIn the regime of 06 lt 119901119901

0lt 09 there is a hysteresis

loop between the adsorption and desorption isotherm whichindicates capillary condensation of nitrogen in mesoporesFrom the detailed shape of the isotherm the pore sizedistribution in the sample can be calculated by the BJH orDFT method The narrower the pore size distribution is

the steeper is the hysteresis loop corresponding to capillarycondensation [21]The top of the hysteresismarks the amountof nitrogen where all mesopores are filled which can beused to calculate the total pore volume of the sample Thishysteresis is similar to a type H1 with a broad pore sizedistribution according to IUPAC classification For relativepressures higher than 119901119901

0gt 09 there is a sharp increase

such as before indicating the filling of the space between thenanowires and of the sample chamber

Table 1 shows the nitrogen gas adsorption results forsilicon nanowire ensembles fromhighly boron-doped siliconThe surface area (multipoint BET) points out the area ofall nanowires and mesopores of the ensemble as well asthe area of the substrate itself which is about 3 cm2 andtherefore negligible Although the nanowires from undopedandmedium-doped silicon are much longer the total surfacearea of the ensembles is smaller than for wires of highlydoped silicon This could be attributed to a higher nanowiredensity and hence smaller nanowire diameters in the caseof highly doped samples Also it could be attributed to anadditional surface area resulting from the pore surface onthe highly doped nanowires The increasing or stagnatingmean pore diameters and their total pore volume are shownfor highly doped silicon nanowire samples prepared withdifferent etching times and H

2O2-concentrations In the case

of c(H2O2) = 01M the total pore volume and the mean

pore diameters grow from one to two hour etching andthe pore volume decreases for longer etching time whilethe mean pore diameter further increases In the case ofc(H2O2) = 02M the total pore volume increases and the

mean pore diameters stagnate with etching timeThe detailedpore diameter distributions are given in Figure 7

For allmeasured isotherms of silicon nanowire ensembleswhich show hysteresis behavior between ad- and desorptionbranches the pore size distribution has been derived withthe BJH method DFT results (not shown) confirm the BJHresults shown in Figure 7The two graphs show the calculatedpore volume distribution for related pore diameters fordifferent etching times (1ndash3 hours) and different H

2O2-

concentrations Figure 7(a) 119888 = 01M Figure 7(b) 119888 =02M respectively As can be seen in Figure 7(a) the porevolume of the sample etched for two hours is increased withrespect to the sample etched for one hour and the averagepore diameter is shifted to higher diameters This indicatesthat existing pores are broadened and deepened andor thatadditional pores are generated with bigger diameters Afterthree hours of etching the peak is much broader and flatterthan before This is a sign of further pore broadening andflattening and the parallel growth of smaller pores For thehigher etching concentration there is a similar situationAfter one-hour etching there are small mesopores on theensemble of silicon nanowires Up to two hours etchingtime the total pore volume grows and after three hours thepores size distribution is broadened again and the total porevolume is increased further Silicon nanowires prepared witha H2O2concentration 119888 = 03M reveal no pores detectable

by nitrogen adsorptionPore formation seems to originate only near the etching

front because continuous pore forming along the wire would


Table 1 Gas adsorption data for nanowire ensembles prepared from highly boron-doped silicon (120588 lt 001Ωcm) For comparison undopednanowire ensemble (120588 gt 1000Ωcm 119888(H2O2) = 05M etching time 119905= 185min wire length = 1111 120583m) reveals aMBET surface area of 0113m2Nanowire ensemble of medium boron-doped silicon (120588 = 14ndash23Ωcm 119888(H2O2) = 04M etching time 119905 = 199min wire length = 1016 120583m)reveals a MBET surface area of 0212m2

119888(H2O2) etching time [min] Wire length [120583m] BET surface area [m2] BJH total pore volume[10minus3 cm3]

BJH mean porediameter [nm]

01M 60 267 0581 164 8501M 122 373 0701 222 9901M 180 301 0622 199 13102M 65 188 0467 158 10502M 122 311 0524 197 9902M 180 385 1082 281 89

10 1000

1

2

3

Diameter d (nm)

dV(d) t = 1h c = 01MdV(d) t = 2h c = 01MdV(d) t = 3h c = 01M

dV(d)

(10minus4

cm3)

(a)

Diameter d (nm)

0

1

2

3

4

10 100


dV(d)

(10minus4

cm3)

(b)

Figure 7 Pore size distribution (BJH) for measured isotherms of silicon nanowire ensembles which show hysteresis between ad- anddesorption branches The calculated pore volume fraction dV is plotted versus the pore diameters d The pore size distribution is broadenedwith longer etching time Silicon nanowires prepared with 119888 = 03M reveal no pores (a) Highly doped silicon preparation parametersetching time t = 1ndash3 h and H

2O2-concentration 119888 = 01M (b) Highly doped silicon preparation parameters etching time t = 1ndash3 h and

H2O2-concentration c = 02M

result in a steady increase of the peaks for pores smallerthan 9 nm For longer etching times the pore diameters arebroadened however the pores are not deepened anymore seeFigure 7(a) For 119888 = 02M the total pore volume increasescontinuously and the pore size distribution is broadenedfor longer etching times For three hours of etching thereare more small pores attributable to slower pore broadeningcaused by an advanced consumption of hydrogen peroxideAs for the case of the higher concentration (119888 = 03M) thebroadening leads to interconnections of the pores leading toa strong surface roughness of the wires For low etchant con-centration the calculations reveal mesopores in the diameterrange of 6ndash18 nm with an average diameter between 9 and13 nm

Our gas adsorption results obtained on two-step preparedsilicon nanowires compare well with these published forsilicon nanowires prepared by the one-step metal assistedetching process [18] Consistently we found mesoporoussurfaces on highly boron-doped nanowires also for thepreparation by the two-step version of the etching and nopores onundoped and additionally onmedium-doped siliconnanowires Hochbaum et al [18] show gas adsorption datafor one nanowire sample whose diameter pore range (2ndash20 nm) and mean pore diameter (97 nm) overlap with ourresults The advantage of the two-step etching method is thatthe amount of silver can be limited during the first etchingstep so that it becomes possible to vary the H

2O2concen-

tration similar to [17] for porous n-type silicon nanowires


Furthermore we have figured out the pore size distributiondepending on etching time and the etchant concentrationWith our results we confirm the finding of the scanning andtransmission electron microscopy studies of Yuan et al [12]There the porosity increases from the nanowire root (wherethe wires are connected to the substrate) to the nanowire tipThis is consistent with our gas adsorption results that the poreformation seems to appear only at a certain distance but nearthe etching front Afterwards the existing pores continue togrow becoming broadened and flattened The pore flatteningcan be explained by the thinning of the nanowire with itsexposure time in the etching solution The pore broadeningexplains the finding in [12] that there are an increasingnumber of interconnected pores in themiddle and upper partof the nanowires

Our results can help to understand the formation processof porous silicon nanowires As depicted in [12] siliconnanowires of different doping concentrations are formedby a vertical etching of the silicon substrate promoted bycatalytic active silver particles and as in our case acceleratedby the oxidizing agent hydrogen peroxide This process isaccompanied by a slight thinning of the nanowires dependingon their exposure time to the etching solution (nanowire tipsare thinner than their roots [12 14 17]) For highly boron-doped silicon nanowires this vertical etching is accompaniedby a local lateral etching into the nanowire resulting in aporous surface Its high dopant concentration leads to surfacestates acting as nucleation sites where the silver ion reductionoccurs randomly spread around the nanowire As shown in[12] for the one-step etching process the pore formation startssome hundreds of nanometers above the etching front Oneexplanation could be that charge injections (holes p+) at theetching front locally increase the carrier concentration andholes with a certain mobility in p-type silicon move along thealready formed nanowire In some distance near the etchingfront these injected charges reach the nanowire surface andpromote the etching at random points So formed pores growdepending on etching concentration and etching time Ourgas adsorption findings confirm our electron microscopyresults about the porous surface structure Because of thepore shape distribution it is valid to compare gas adsorptioncalculations for silicon nanowires among themselves butit is probably doubtful to compare them to other materialsystems

We have investigated the prepared silicon nanowireensembles with respect to their vibrational properties byRaman spectroscopy The laser power used for excitation hasbeen reduced to a level where the influence of local heatingis negligible [28] The Raman spectra shown in Figure 8are dominated by the zone-center optical (O

Γ) phonon line

of silicon at about 520 cmminus1 (equal to 64meV) [29 30]The difference in the signal strength between the differentnanowires and especially to the silicon substrate can beexplained by the different excited optical probe volume Forundoped as well as medium doped nanowires the O

Γphonon

peak exhibits a redshift of 3 cmminus1 and a moderate broadeningas compared to that of the silicon substratesThemodificationof the Raman spectrum becomes pronounced for heavily

0

50

100

150

200

250

Ram

an in

tens

ity (a

u)

All Si substrates times 35

Medium-doped SiNWsUndoped SiNWsHighly-doped SiNWs times 45

420 440 460 480 500 520 540

Stokes shift (cmminus1)

Figure 8 Raman spectra of silicon nanowire ensembles preparedfrom silicon substrates with three different doping levels Thespectrum of a silicon substrate with the zone-center optical phononline at 520 cmminus1 is shown for comparison

doped nanowires with a redshift of 8 cmminus1 and a strong asym-metric broadening (full width at half maximum (FWHM) of15 cmminus1) Inhomogeneous strain can be excluded as the originof the observed spectral changes since the observed redshiftswould require the assumption of an unreasonably largemagnitude of average strain [31 32] In fact the observedRaman spectra can be explained by the spatial confinementof optical phonons in silicon nanostructures which leads toa relaxation of the pseudomomentum conservation [28 33ndash35] Both the observed Raman peak position and FWHM forthe heavily doped nanowires can be explained by a modelassuming nanospheres with diameters between 3 and 4 nmand a phonon confinement function chosen in analogy tothe ground state of an electron in a hard sphere [34] Thephonon confinement can be explained by the formation ofa single crystalline silicon nanomesh in a sub-10 nm rangecreated by pore formation or a pronounced surface roughnesswhich have been observed for all heavily doped nanowiresTherefore these nanowires appear interesting for further full-thermoelectrical investigations on individual nanowires asrecently been demonstrated [36 37]

4 Conclusions

In our work we report on synthesis and morphology char-acteristics of silicon nanowires prepared by the two-stepelectroless etching process We have prepared nanowireensembles from different boron-doped substrates and haveshown their growth trend and surface morphology whichwere investigated by scanning electronmicroscopy and nitro-gen gas adsorption both revealing a mesoporous surfacestructure on highly doped silicon nanowires Transmissionelectron microscopy proves that the structure of mesoporous


nanowires remains single crystalline However there is a for-mation of an oxidized surface layer Mesopores are formednear the etching front and are growing and flattened depen-dent on the etching time and etchant concentration Thisallows for a controlled formation of porous silicon nanowiresConsequences of the nanopatterning to phonon energies andvibrational properties of the nanowires are a redshifted andasymmetric Stokes signal in the Raman spectroscopy for thehighly doped nanowires The contribution of confinementeffects is considered to play the dominant role for thisredshift Effects of the porous surface of the silicon nanowireson their mechanical properties such the Youngrsquos modulusand their electrical and thermal transport properties demandfurther investigations

Conflict of Interests

The authors declare no conflict of interests

Acknowledgments

The authors gratefully acknowledge financial support fromDFG within SPP 1386 and thank Dr Sven S Buchholzformerly at Humboldt-Universitat zu Berlin and RaithGmbH for access to E line Plus and SEM imaging StefanWeidemann wants to thank Jurgen Solle and Ulrike Heidenfor technical support

References

[1] A I Boukai Y Bunimovich J Tahir-Kheli J-K Yu W AGoddard III and J R Heath ldquoSilicon nanowires as efficientthermoelectric materialsrdquoNature vol 451 no 7175 pp 168ndash1712008

[2] JOh TGDeutschH-C Yuan andHMBranz ldquoNanoporousblack silicon photocathode for H

2production by photoelectro-

chemical water splittingrdquo Energy and Environmental Sciencevol 4 no 5 pp 1690ndash1694 2011

[3] K-Q Peng X Wang and S-T Lee ldquoGas sensing propertiesof single crystalline porous silicon nanowiresrdquo Applied PhysicsLetters vol 95 Article ID 243112 2010

[4] X T Zhou J Q Hu C P Li D D D Ma C S Lee and S TLee ldquoSilicon nanowires as chemical sensorsrdquo Chemical PhysicsLetters vol 369 pp 220ndash224 2003

[5] Y Cui Z Zhong D Wang W U Wang and C M LieberldquoHigh performance silicon nanowire field effect transistorsrdquoNano Letters vol 3 no 2 pp 149ndash152 2003

[6] G Jia I Hoger A Gawlik et al ldquoWet chemically preparedsilicon nanowire arrays on low-cost substrates for photovoltaicapplicationsrdquo Physica Status Solidi (A) vol 210 no 4 pp 728ndash731 2013

[7] V Sivakov G Andra A Gawlik et al ldquoSilicon nanowire-based solar cells on glass synthesis optical properties and cellparametersrdquo Nano Letters vol 9 no 4 pp 1549ndash1554 2009

[8] A I Hochbaum R Chen R D Delgado et al ldquoEnhanced ther-moelectric performance of rough silicon nanowiresrdquo Naturevol 451 no 7175 pp 163ndash167 2008

[9] V Lehmann Electrochemistry of Silicon Instrumentation Sci-ence Materials and Applications Wiley-VCH Weinheim Ger-many 3rd edition 2002

[10] V Lehmann and S Ronnebeck ldquoThe physics of macroporeformation in low-doped p-type siliconrdquo Journal of the Electro-chemical Society vol 146 no 8 pp 2968ndash2975 1999

[11] V Schmidt J V Wittemann S Senz and U Gosele ldquoSiliconnanowires a review on aspects of their growth and theirelectrical propertiesrdquoAdvancedMaterials vol 21 no 25-26 pp2681ndash2702 2009

[12] G Yuan R Mitdank A Mogilatenko and S F FischerldquoPorous nanostructures and thermoelectric power measure-ment of electro-less etched black siliconrdquoThe Journal of PhysicalChemistry C vol 116 no 25 pp 13767ndash13773 2012

[13] K Peng J Hu Y Yan et al ldquoFabrication of single-crystallinesilicon nanowires by scratching a silicon surface with catalyticmetal particlesrdquo Advanced Functional Materials vol 16 no 3pp 387ndash394 2006

[14] Z Huang N Geyer P Werner J De Boor and U GoseleldquoMetal-assisted chemical etching of silicon a reviewrdquoAdvancedMaterials vol 23 no 2 pp 285ndash308 2011

[15] M-L Zhang K-Q Peng X Fan et al ldquoPreparation of large-area uniform silicon nanowires arrays through metal-assistedchemical etchingrdquo Journal of Physical Chemistry C vol 112 no12 pp 4444ndash4450 2008

[16] Y Qu L Liao Y Li H Zhang Y Huang and X DuanldquoElectrically conductive and optically active porous siliconnanowiresrdquo Nano Letters vol 9 no 12 pp 4539ndash4543 2009

[17] L Lin S Guo X Sun J Feng and Y Wang ldquoSynthesisand photoluminescence properties of porous silicon nanowirearraysrdquo Nanoscale Research Letters vol 5 no 11 pp 1822ndash18282010

[18] A I Hochbaum D Gargas Y J Hwang and P Yang ldquoSinglecrystalline mesoporous silicon nanowiresrdquo Nano Letters vol 9no 10 pp 3550ndash3554 2009

[19] K Peng A Lu R Zhang and S T Lee ldquoMotility of metalnanoparticles in silicon and induced anisotropic silicon etch-ingrdquo Advanced Functional Materials vol 18 no 19 pp 3026ndash3035 2008

[20] S Brunauer P H Emmett and E Teller ldquoAdsorption of gasesin multimolecular layersrdquo Journal of the American ChemicalSociety vol 60 no 2 pp 309ndash319 1938

[21] S Lowell J E Shields M A Thomas and M ThommesCharacterization of Porous Solids and Powders Surface AreaPore Size and Density Springer Dordrecht The Netherlands3rd edition 2006

[22] W-K To C-H Tsang H-H Li and Z Huang ldquoFabricationof n-type mesoporous silicon nanowires by one-step etchingrdquoNano Letters vol 11 no 12 pp 5252ndash5258 2011

[23] B Lee and R E Rudd ldquoFirst-principles study of the Youngrsquosmodulus of Silt001gtnanowiresrdquoPhysical ReviewBmdashCondensedMatter and Materials Physics vol 75 no 4 Article ID 0413052007

[24] SHoffmann IUtke BMoser et al ldquoMeasurement of the bend-ing strength of vapor-liquid-solid grown silicon nanowiresrdquoNano Letters vol 6 no 4 pp 622ndash625 2006

[25] Y-S Sohn J Park G Yoon et al ldquoMechanical properties ofsilicon nanowiresrdquo Nanoscale Research Letters vol 5 no 1 pp211ndash216 2010

[26] S L Cheng C H Chung and H C Lee ldquoA study of the syn-thesis characterization and kinetics of vertical silicon nanowirearrays on (001)Si substratesrdquo Journal of the ElectrochemicalSociety vol 155 no 11 pp D711ndashD714 2008


[27] D Zhao Y Y Wan and W Zhou Ordered Mesoporous Materi-als Wiley-VCH Weinheim Germany 2013

[28] S Piscanec M Cantoro A C Ferrari et al ldquoRaman spec-troscopy of silicon nanowiresrdquo Physical Review BmdashCondensedMatter and Materials Physics vol 68 no 24 Article ID 2413122003

[29] R Hull Properties of Crystalline Silicon INSPEC The Institu-tion of Electrical Engineers London UK 1999

[30] B Li D Yu and S-L Zhang ldquoRaman spectral study ofsilicon nanowiresrdquo Physical Review BmdashCondensed Matter andMaterials Physics vol 59 no 3 pp 1645ndash1648 1999

[31] C-Y Peng C-F Huang Y-C Fu et al ldquoComprehensive studyof the Raman shifts of strained silicon and germaniumrdquo Journalof Applied Physics vol 105 no 8 Article ID 083537 2009

[32] M J Suess R A Minamisawa R Geiger K K Bourdelle HSigg and R Spolenak ldquoPower-dependent raman analysis ofhighly strained Si nanobridgesrdquo Nano Letters vol 14 no 3 pp1249ndash1254 2014

[33] H Richter Z P Wang and L Ley ldquoThe one phonon Ramanspectrum in microcrystalline siliconrdquo Solid State Communica-tions vol 39 no 5 pp 625ndash629 1981

[34] I H Campbell and P M Fauchet ldquoThe effects of microcrystalsize and shape on the one phonon Raman spectra of crystallinesemiconductorsrdquo Solid State Communications vol 58 no 10 pp739ndash741 1986

[35] R-P Wang G-W Zhou Y-L Liu et al ldquoRaman spectralstudy of silicon nanowires high-order scattering and phononconfinement effectsrdquo Physical Review B vol 61 no 24 pp16827ndash16832 2000

[36] D Kojda R Mitdank M Handwerg et al ldquoTemperature-dependent thermoelectric properties of individual silvernanowiresrdquo Physical Review B vol 91 Article ID 024302 2015

[37] D Kojda R Mitdank A Mogilatenko et al ldquoThe effect of adistinct diameter variation on the thermoelectric properties ofindividual Bi

039Te061

nanowiresrdquo Semiconductor Science andTechnology vol 29 no 12 Article ID 124006 2014

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



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NanoparticlesJournal of



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Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


to wafer size [12 13] The metal-assisted etching differs inthe number of etching steps [14] In the one-step etchingthe solution containing the catalytic particles propels thenanowire etching whereas in the two-step etching theamount of catalytic particles is limited in a first etching stepand in a second etching solution an oxidizing agent affectsthe nanowire etching The limitation of catalytic particlespromises a better control in silicon nanowire preparation

Zhang et al [15] presented a two-step wet chemicaletching synthesis of silicon nanowires prepared from p-doped and n-doped silicon substrates with (100) and (111)orientations Qu et al [16] and Lin et al [17] investigatedhow etching parameters like etching time and concentrationof etching solutions affect the formation of n-doped siliconnanowires as well as nanowire porosity Hochbaum et al[18] and Yuan et al [12] showed for the one-step etchingprocess that with decreasing wafer resistivity the nanowiressurface roughnessporosity increases Although the existenceof mesopores on etched silicon nanowires from highlyboron-doped silicon is known [14 16ndash18] their shape andthe detailed formation mechanism of rough surfaces andnanoporous structures on silicon nanowires remain unclear

Here we present a systematic analysis of pore evolutionby gas adsorption studies for the two-step etching processof silicon nanowires We have fabricated silicon nanowireensembles from undoped Si (resistivity 120588 gt 1000Ωcm)medium boron-doped Si (120588 = 14ndash23Ωcm) and highly boron-doped Si (120588 lt 001Ωcm) and determined pore diameterdistribution pore volume and sample surface area for highlydoped nanowires The influence of etching time and etchantconcentration illumination and temperature on nanowireformation is discussed and the pore formation in depen-dence of etchant concentration and time is analysed Sur-face morphology roughness and crystallinity of individualsilicon nanowires are investigated by transmission electronmicroscopy (TEM) The vibrational properties of nanowireensembles are investigated by Raman spectroscopy

2 Experimental Section

Silicon nanowires are prepared by the method of metal-assisted chemical etching in a two-step etching approachwhich has been reported previously [14 16 17] The siliconwafers are cleaned with sonication in acetone (10min) iniso-propanol (10min) and in boiling acetone (10min) and inboiling iso-propanol (10min) Organic residues are removedfrom the wafer with a fresh prepared boiling Piranha-solution a mixture of 3 1 H

2SO4(97) an H

2O2(35)

for 10min The generated oxide layer is removed by anHF etching (5 HF for 10min) The sample is rinsed withdeionized water

In the first etching step with a solution of 48M HFand 002MAgNO

3for 60 s the silver (Ag) nanoparticles are

placed on the wafer surface In this step the silicon wafer iscoated by a galvanic replacement of silicon by silver Silvernanoparticles start to sink into the silicon substrate and beginto form silver dendrites

After 60 s the sample is rinsed with deionized water againand in a second etching solution consisting of 48MHF and

01ndash05MH2O2for an etching time of about 1ndash3 h the silicon

nanowires are formed by the HF etching of the silicon in thefollowing way

At first [14] oxidation of silicon takes place as follows

Si + 2H2O + 4h+ 997888rarr SiO

2+ 4H+

Si + 2H2O 997888rarr SiO

2+ 4H+ + 4eminus

(1)

Second etching of silicon dioxide produces

SiO2+ 6HF 997888rarr H

2SiF6+ 2H2O (2)

Etching of SiO2occurs with a higher rate than etching of pure

Si in HF Therefore it is the dominant etching process [9]Additionally the silicon oxidation is promoted by the silver(Ag) and so a higher etching rate of silicon mediated throughthe catalytic activity of the noble metal is achieved [14 19]

Through the charge transfer on the interface of silver andsilicon the silver nanoparticles sink deeper and the siliconnanowires arise as the remainings of unetched silicon Thedriving force is the oxidizing power of the oxidizing agentH2O2

The driving force is given by

H2O2+ 2H+ 997888rarr 2H

2O + 2h+ (3)

H2O2oxidizes the Ag particles which are in turn reduced at

the silicon surface by an electron transfer from the siliconand thereby oxidize the silicon This can be described as acharge transfer process in which the silver injects holes intothe silicon (1)

After etching the sample is rinsed with deionized waterand the Ag particles are removed by a cleaning step in HNO

3

The wafer is washed in deionized water several times andthen dried in a nitrogen flow Normally all etching steps areperformed in the dark and at room temperature

Here we prepared nanowires from three types of siliconwafers

(I) undoped silicon (100) (specific resistivity 120588 gt1000Ωcm doping level 1013 cmminus3)

(II) medium boron-doped silicon (100) (120588 = 14ndash23Ωcm1015 cmminus3) p-type

(III) highly boron-doped silicon (100) (120588 lt 001Ωcm1019 cmminus3) p-type

The as-prepared ensembles of silicon nanowires were charac-terized by scanning electron microscopy (SEM Hitachi TM-1000) and details like themesoporous surface of single wireswere characterized by SEM (Raith E Line plus) The surfacearea of whole silicon nanowire ensembles was measured bythe Brunauer-Emmett-Teller (BET) nitrogen gas adsorptionmethod [20] The automated gas adsorption station Quan-tachrome Autosorb-1 was used The measurement data wereanalyzed by Autosorb-1 software to determine the mean poresize distribution (pore diameters andpore volumes) onhighlydoped nanowires For this analysis the 1 cm times 1 cm waferswere put into the sample chamber and degassed in a vacuum




2-cooled CCD with a














3

= 001M 119888AgNO3


3





50120583m

(a)

30120583m

(b)

03mm

(c)


120

20

40

60

80

100

Wire

leng

th (120583

m)

Etching time (min)60 80 100 120 140 160 180

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

c(H2O2) = 04Mc(H2O2) = 05M

(a)

0

Etching time (min)

Wire

leng

th (120583

m)

0 50 100 150 200

20

40

60

80

100

c(H2O2) = 01Mc(H2O2) = 02M

c(H2O2) = 03Mc(H2O2) = 04M

(b)







10

15

20

25

30

35

40

45W

ire le

ngth

(120583m

)

20 40 60 80 100 120 140 160 180

Etching time (min)

c(AgNO3) = 001M

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

(a)W

ire le

ngth

(120583m

)

0

10

20

30

40

50

0 10 20 30 40 50 60



(b)




[100]

[010][001]

50120583m

(a)

(c)(d) 10120583m

(b)

200 nm

(c)

100 nm

(d)



Carbonfilm

Si NW

[100]

[100]

(a)

(b)

(c) (d)

[100]

400

040

[001]Si

100 nm 2 nm 20 nm












0= 10minus5 at 119879 = 77K where 119901

0is


0=




(b)

(a)

(c)

00

05

10

15

20

0000

0005

0010

0015

0020

Highly-doped Si

Undoped Si

Undoped Si

Volu

me o

f nitr

ogen

adso

rbed

(STP

cm3)

00 02 04 06 08 10



Volu

me (

STP

cm3)


0= 005

























2O2-








01M 60 267 0581 164 8501M 122 373 0701 222 9901M 180 301 0622 199 13102M 65 188 0467 158 10502M 122 311 0524 197 9902M 180 385 1082 281 89

10 1000

1

2

3

Diameter d (nm)


dV(d)

(10minus4

cm3)

(a)

Diameter d (nm)

0

1

2

3

4

10 100


dV(d)

(10minus4

cm3)

(b)






2O2concen-






Γ) phonon line


Γphonon


0

50

100

150

200

250

Ram

an in

tens

ity (a

u)



420 440 460 480 500 520 540




4 Conclusions






Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






2-cooled CCD with a














3

= 001M 119888AgNO3


3





50120583m

(a)

30120583m

(b)

03mm

(c)


120

20

40

60

80

100

Wire

leng

th (120583

m)

Etching time (min)60 80 100 120 140 160 180

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

c(H2O2) = 04Mc(H2O2) = 05M

(a)

0

Etching time (min)

Wire

leng

th (120583

m)

0 50 100 150 200

20

40

60

80

100

c(H2O2) = 01Mc(H2O2) = 02M

c(H2O2) = 03Mc(H2O2) = 04M

(b)







10

15

20

25

30

35

40

45W

ire le

ngth

(120583m

)

20 40 60 80 100 120 140 160 180

Etching time (min)

c(AgNO3) = 001M

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

(a)W

ire le

ngth

(120583m

)

0

10

20

30

40

50

0 10 20 30 40 50 60



(b)




[100]

[010][001]

50120583m

(a)

(c)(d) 10120583m

(b)

200 nm

(c)

100 nm

(d)



Carbonfilm

Si NW

[100]

[100]

(a)

(b)

(c) (d)

[100]

400

040

[001]Si

100 nm 2 nm 20 nm












0= 10minus5 at 119879 = 77K where 119901

0is


0=




(b)

(a)

(c)

00

05

10

15

20

0000

0005

0010

0015

0020

Highly-doped Si

Undoped Si

Undoped Si

Volu

me o

f nitr

ogen

adso

rbed

(STP

cm3)

00 02 04 06 08 10



Volu

me (

STP

cm3)


0= 005

























2O2-








01M 60 267 0581 164 8501M 122 373 0701 222 9901M 180 301 0622 199 13102M 65 188 0467 158 10502M 122 311 0524 197 9902M 180 385 1082 281 89

10 1000

1

2

3

Diameter d (nm)


dV(d)

(10minus4

cm3)

(a)

Diameter d (nm)

0

1

2

3

4

10 100


dV(d)

(10minus4

cm3)

(b)






2O2concen-






Γ) phonon line


Γphonon


0

50

100

150

200

250

Ram

an in

tens

ity (a

u)



420 440 460 480 500 520 540




4 Conclusions






Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




50120583m

(a)

30120583m

(b)

03mm

(c)


120

20

40

60

80

100

Wire

leng

th (120583

m)

Etching time (min)60 80 100 120 140 160 180

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

c(H2O2) = 04Mc(H2O2) = 05M

(a)

0

Etching time (min)

Wire

leng

th (120583

m)

0 50 100 150 200

20

40

60

80

100

c(H2O2) = 01Mc(H2O2) = 02M

c(H2O2) = 03Mc(H2O2) = 04M

(b)







10

15

20

25

30

35

40

45W

ire le

ngth

(120583m

)

20 40 60 80 100 120 140 160 180

Etching time (min)

c(AgNO3) = 001M

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

(a)W

ire le

ngth

(120583m

)

0

10

20

30

40

50

0 10 20 30 40 50 60



(b)




[100]

[010][001]

50120583m

(a)

(c)(d) 10120583m

(b)

200 nm

(c)

100 nm

(d)



Carbonfilm

Si NW

[100]

[100]

(a)

(b)

(c) (d)

[100]

400

040

[001]Si

100 nm 2 nm 20 nm












0= 10minus5 at 119879 = 77K where 119901

0is


0=




(b)

(a)

(c)

00

05

10

15

20

0000

0005

0010

0015

0020

Highly-doped Si

Undoped Si

Undoped Si

Volu

me o

f nitr

ogen

adso

rbed

(STP

cm3)

00 02 04 06 08 10



Volu

me (

STP

cm3)


0= 005

























2O2-








01M 60 267 0581 164 8501M 122 373 0701 222 9901M 180 301 0622 199 13102M 65 188 0467 158 10502M 122 311 0524 197 9902M 180 385 1082 281 89

10 1000

1

2

3

Diameter d (nm)


dV(d)

(10minus4

cm3)

(a)

Diameter d (nm)

0

1

2

3

4

10 100


dV(d)

(10minus4

cm3)

(b)






2O2concen-






Γ) phonon line


Γphonon


0

50

100

150

200

250

Ram

an in

tens

ity (a

u)



420 440 460 480 500 520 540




4 Conclusions






Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




10

15

20

25

30

35

40

45W

ire le

ngth

(120583m

)

20 40 60 80 100 120 140 160 180

Etching time (min)

c(AgNO3) = 001M

c(H2O2) = 01Mc(H2O2) = 02Mc(H2O2) = 03M

(a)W

ire le

ngth

(120583m

)

0

10

20

30

40

50

0 10 20 30 40 50 60



(b)




[100]

[010][001]

50120583m

(a)

(c)(d) 10120583m

(b)

200 nm

(c)

100 nm

(d)



Carbonfilm

Si NW

[100]

[100]

(a)

(b)

(c) (d)

[100]

400

040

[001]Si

100 nm 2 nm 20 nm












0= 10minus5 at 119879 = 77K where 119901

0is


0=




(b)

(a)

(c)

00

05

10

15

20

0000

0005

0010

0015

0020

Highly-doped Si

Undoped Si

Undoped Si

Volu

me o

f nitr

ogen

adso

rbed

(STP

cm3)

00 02 04 06 08 10



Volu

me (

STP

cm3)


0= 005

























2O2-








01M 60 267 0581 164 8501M 122 373 0701 222 9901M 180 301 0622 199 13102M 65 188 0467 158 10502M 122 311 0524 197 9902M 180 385 1082 281 89

10 1000

1

2

3

Diameter d (nm)


dV(d)

(10minus4

cm3)

(a)

Diameter d (nm)

0

1

2

3

4

10 100


dV(d)

(10minus4

cm3)

(b)






2O2concen-






Γ) phonon line


Γphonon


0

50

100

150

200

250

Ram

an in

tens

ity (a

u)



420 440 460 480 500 520 540




4 Conclusions






Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Carbonfilm

Si NW

[100]

[100]

(a)

(b)

(c) (d)

[100]

400

040

[001]Si

100 nm 2 nm 20 nm












0= 10minus5 at 119879 = 77K where 119901

0is


0=




(b)

(a)

(c)

00

05

10

15

20

0000

0005

0010

0015

0020

Highly-doped Si

Undoped Si

Undoped Si

Volu

me o

f nitr

ogen

adso

rbed

(STP

cm3)

00 02 04 06 08 10



Volu

me (

STP

cm3)


0= 005

























2O2-








01M 60 267 0581 164 8501M 122 373 0701 222 9901M 180 301 0622 199 13102M 65 188 0467 158 10502M 122 311 0524 197 9902M 180 385 1082 281 89

10 1000

1

2

3

Diameter d (nm)


dV(d)

(10minus4

cm3)

(a)

Diameter d (nm)

0

1

2

3

4

10 100


dV(d)

(10minus4

cm3)

(b)






2O2concen-






Γ) phonon line


Γphonon


0

50

100

150

200

250

Ram

an in

tens

ity (a

u)



420 440 460 480 500 520 540




4 Conclusions






Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(b)

(a)

(c)

00

05

10

15

20

0000

0005

0010

0015

0020

Highly-doped Si

Undoped Si

Undoped Si

Volu

me o

f nitr

ogen

adso

rbed

(STP

cm3)

00 02 04 06 08 10



Volu

me (

STP

cm3)


0= 005

























2O2-








01M 60 267 0581 164 8501M 122 373 0701 222 9901M 180 301 0622 199 13102M 65 188 0467 158 10502M 122 311 0524 197 9902M 180 385 1082 281 89

10 1000

1

2

3

Diameter d (nm)


dV(d)

(10minus4

cm3)

(a)

Diameter d (nm)

0

1

2

3

4

10 100


dV(d)

(10minus4

cm3)

(b)






2O2concen-






Γ) phonon line


Γphonon


0

50

100

150

200

250

Ram

an in

tens

ity (a

u)



420 440 460 480 500 520 540




4 Conclusions






Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







01M 60 267 0581 164 8501M 122 373 0701 222 9901M 180 301 0622 199 13102M 65 188 0467 158 10502M 122 311 0524 197 9902M 180 385 1082 281 89

10 1000

1

2

3

Diameter d (nm)


dV(d)

(10minus4

cm3)

(a)

Diameter d (nm)

0

1

2

3

4

10 100


dV(d)

(10minus4

cm3)

(b)






2O2concen-






Γ) phonon line


Γphonon


0

50

100

150

200

250

Ram

an in

tens

ity (a

u)



420 440 460 480 500 520 540




4 Conclusions






Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







Γ) phonon line


Γphonon


0

50

100

150

200

250

Ram

an in

tens

ity (a

u)



420 440 460 480 500 520 540




4 Conclusions






Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







Acknowledgments


References









































039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials















039Te061









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article controlled pore formation on mesoporous single crystalline silicon...

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