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doi:10.1016/j.ph

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Physica E 33 (2006) 268–272

www.elsevier.com/locate/physe

Surface photovoltage studies of Si nanocrystallites prepared byelectrochemical etching

B.K. Patel{, S. Rath, S.N. Sahu�

Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India

Received 3 February 2006; received in revised form 13 March 2006; accepted 13 March 2006

Available online 16 May 2006

Abstract

Nanocrystalline Si has been prepared by anodic etching of Si in an electrolyte consisting of ethanol and HF. The structure and surface

morphology have been studied using transmission electron microscopy which reveal the cubic structure and porous morphology of Si

nanocrystals (NCs). Electrochemical etching has resulted in surface oxidation of Si NCs as confirmed from X-ray photoelectron

spectroscopic measurements. The average size of the Si NCs has been estimated from the line broadening analysis of the Raman

scattering. Unique optical transitions associated with porous Si=SiO2 quantum well (QW) like structure has been investigated by surface

photovoltage (SPV) measurements.

r 2006 Elsevier B.V. All rights reserved.

PACS: 78.20.�e; 81.60.�Cp; 71.55.Ht; 73.60.Gx

Keywords: Surface photovoltage; Quantum well; Si nanocrystals; Raman scattering

1. Introduction

Silicon is the main constituent of the microelectronicdevices, which has s2p2 valence electron configuration.During sp3 hybridization, all the three p-orbitals (px; py

and pz) are hybridized with s-orbital. As a result, four sp3

orbitals are created and its structure matches with that ofthe diamond in cubic phase. The main disadvantage of thiscubic Si is that the lowest conduction band minimum lies atX ð2p=a ð1; 0; 0ÞÞ along Dð1; 0; 0Þ axis of the Brillouin zoneand has an indirect band gap, Eg ¼ X 1 � G25 ¼ 1:17 eV,which is responsible for the non-radiative recombinationprocess. Therefore, it is not a suitable material for thedisplay devices like light emitting diode (LED). However,the properties of the Si can be modified when it is in theform of low-dimensional structures (crystallites are in thenanometer range) [1]. For example, the band gap of the Siwidens as the Si crystallite dimensions are comparable tothat of its Bohr exciton radius and becomes a pseudo-direct

e front matter r 2006 Elsevier B.V. All rights reserved.

yse.2006.03.153

ing author. Fax: +91 674 581142.

ess: sahu@iopb.res.in (S.N. Sahu).

band gap semiconductor with quantum efficiency compar-able to that of direct band gap semiconductor like GaAs[2]. The Si nanocrystals (NCs) can be prepared by anelectrochemical anodic etching technique in HF electrolyte[1–15]. The technique yields a porous morphology, ingeneral one-dimensional (1D) rod like structures [1,6],having Si crystalline size in the nanometer range and,hence, known as porous Si (PS). However, the technique issuch that mostly an oxide (SiO2) layer surrounding thenano-Si core is always formed. Thus, a heterostructureresulted between nano-Si (core) and nano-SiO2 (envelope).Such system can be viewed as a quantum well (QW)structure [16] like the Si/SiO2 as in case of GaAs/GaAlAs.The SiO2 is expected to generate interface traps [17] andinfluence the QW properties. Profound consequence ofquantum confinement in a QW is the splitting of bandstructure into a series of 0D or 1D sub-bands with electronsin the conduction band, EC, and holes in the valence band,EV, are confined in the Si NCs [2]. Surface photovoltage(SPV) measurements were carried out for free standing PSsamples to study the band tailing effect controlling theluminescence properties of PS by the groups, Burstein et al.[18] and Fefer et al. [19]. However, the present study

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97 99 101 103 105 107Binding Energy (eV)

0

2000

4000

6000

Inte

nsity

(a.

u.)

Si 2p

SiO

370 420 470 520Raman shift (cm-1)

0

10

20

30

40

50

Inte

nsity

(a.

u.)

s1

(s3) Size of Si crystallite=3.7 nm

J =30 & 50 mAcm-2

Time =30 min

(s2) Size of Si crystallite=4.6 nm

s2

s3

4.0 eV

P orous S i

Pore

Silicon Wire

J = 50 mAcm-2

Electric Field Direction

111

111 111

220

111

220

P orous S i

80 n

m

(a)

(b)

(c)

Fig. 1. (a) Typical transmission electron micrograph (TEM) of PS

prepared at 50mAcm�2. Pores propagate parallel to the electric field

direction as shown by an arrow. The 1D Si crystallite (wire) with low

dimension is shown. Inset shows the corresponding selected area

diffraction (SAD) pattern revealing the monocrystalline nature. (b) XPS

spectrum of the Si 2p region of the PS etched for 30mAcm�2. (c) Raman

spectrum of single crystal bulk-Si (s1), PS prepared at 30mAcm�2 (s2) and

50mAcm�2 (s3). The average crystalline sizes are estimated as 4.6 and

3.7 nm using spatial correlation model for samples s2 and s3, respectively.

B.K. Patel et al. / Physica E 33 (2006) 268–272 269

assumes the formation of a QW structure in the config-uration, bulk-Si/porous Si/SiO2 and attempts have beenmade to identify the different electronic transitions of theQW that may arise from band edge and surface traps usingSPV measurements by Kelvin probe method.

2. Experimental

Two PS samples were synthesized using p-Si (1 0 0) withresistivity r ¼ 0:01Ocm, anodically etched at 30 (s2) and 50ðs3ÞmAcm�2, respectively, for 30min duration each. Theanodization was carried out at 22 �C using an electrolyteconsisted of ethanol and HF (49%) mixture (1:1). Detailsof synthesis procedure have been described in the literature[17]. The structure, surface, impurity and optical propertiesof the PS have been studied using transmission electronmicroscopy (TEM), X-ray photoelectron spectroscopy(XPS), Raman scattering and SPV measurements. Forthe SPV measurements, the non-contact Kelvin probetechnique [20] was used.

3. Results and discussion

For TEM analysis, the PS layer was peeled off by a sharpblade and are lifted on a Cu grid. The bright field (BF)image of a typical sample (s3) shown in Fig. 1(a) reveals anisotropic porous morphology. The pores appear topropagate parallel to the electric field direction during thesynthesis. The corresponding selected area diffraction(SAD) pattern is shown in Fig. 1(a) inset. The diffractionpattern reveals a well defined Bragg diffraction spot similarto the single crystal Si. Different crystal planes, (1 1 1) and(2 2 0), are clearly identified. An amorphous phase sup-posed to give a broad diffuse ring [13] is not detected in thepresent case which suggests that the PS retains itsmonocrystalline character of bulk cubic phase. The SADpattern further shows the streaking spots and diffusedscattering which suggests that the Si-crystallites in theskeleton of PS are small and is consistent with theobservation of Cullis and Canham [3].

XPS measurements were carried out to identify the Sioxide and its electronic structure. A detailed scan of the Si2p region of the typical oxidized PS (s2) is shown inFig. 1(b). The spectrum is fitted to two Gaussian lines, oneat 100.1 eV and the other at 104.1 eV. The peak at 100.1 eVcan be ascribed to the binding energy, Eb of Si 2p [15]. AsSiO2 can provide Si4þ state whose binding energy is inbetween 3.4 and 4.9 eV higher than that of the Si 2p, thepresent work with binding energy Eb ¼ 104:1 eV (4.0 eVhigher than the Si 2p) clearly suggests that the Si crystallitein the skeleton of PS layer is covered with SiO2 [15].

Fig. 1(c) shows the Raman spectra of the PS samples, s2,s3 and crystalline Si, c-Si (s1) as the reference sample. ForRaman scattering measurements, 488 nm Ar ion laser isused as an excitation radiation. The crystalline Si Ramanmode that appears around 520 cm�1 has a full width halfmaxima (FWHM) as 3:0 cm�1 and the Raman intensity is

weak compared to that of PS (s2 and s3). The higherintensity in case of PS could be related the resonance effector due to surface contribution enhancement [9]. The blueshifting of the Raman peak of PS from c-Si could berelated to the confinement of the optical phonons. Afinite size of Si crystallite in PS skeleton corresponds toa momentum which is inversely proportional to the

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crystalline size. For large crystal (c-Si), wave vector q ¼ 0,hence, the zone center phonons are allowed for Ramanscattering. The qa0 (relaxation of selection rule) whichresults in the shifting of Raman peak in case of PS [9–12].In addition, we did not observe a broad peak at 480 cm�1

which indicates that there is no substantial contribution tothe Raman intensity from possible amorphous Si (a-Si)phase which might arise in PS synthesis [17]. This againconfirms the results obtained from the TEM studydepicting crystalline character. From the line width, thecrystalline sizes have been estimated and found to be 3.7and 4.6 nm, respectively, for samples s2 and s3.

Fig. 2 shows two different QW structures (idealized)corresponding to two different well widths. The lowerwidth corresponds to the QW structure formed at higheranodic etching current and vice versa. In practice, PSsynthesis mostly results in size distributions. However,shown in Fig. 2 is the average sizes of the crystallites. Foroptical transition, all the combination of the conductionband (CB) and valence band (VB) sub-levels are notallowed. The symmetry properties of the wave functionsallow only the transitions between the sub-levels with samevalue of n. Although Burstein et al. [18] have used SPV

Fig. 2. Energy quantization in QW structure of two different well widths.

The two PS samples, s2 and s3, are analogous to two different quantum

wells of width 4.6 and 3.7 nm, respectively. Concept of heavy and light

holes are ignored here. m is the reduced mass of electron and hole and d is

the well width.

technique (Kelvin probe method) to understand the originof the luminescence properties of PS in the aspect of theinvolvement of the type of majority carriers, band edgetailing and oxygen mediated defect states yielding non-radiative recombination centers, in order to study the QWproperties and to identify the splitting of the energy bands,we have used SPV techniques using Kelvin probe whichhave been successfully applied to the PS structure earlier byseveral groups [14,20]. In SPV, upon illumination, thegenerated electron–hole pairs change the surface potentialbarrier of the semiconductor and Kelvin probe can detectthe signal as surface photovoltage signal is a function ofwavelength. The photovoltage configuration used in ourexperiment was Au/PS/c-Si/Al which actually includes theAu–PS, PS2c�Si and c�Si2Al interfaces. The intensity ofthe incident light is so chosen that its penetration depth isless than the thickness of the porous silicon layer. In fact,the PS layer thickness �3:2mm as observed from Ruther-ford backscattering spectroscopy (RBS) technique [21] isfairly large. Thus, light only reaches to the Au2PSinterface and is far from the PS2c�Si interface in thisregion. Depending upon the light absorption by the PSlayer, the band bending/built-in potential of Au–PS inter-face is forward biased, hence, the signal detection byKelvin probe essentially depends upon the quality of theinterface. While recording the SPV spectrum, the incidentphoton energy distribution is being considered and thespectra were corrected by their relative contributions. Withthese considerations, the photovoltage signal is expected tocontribute mainly from the absorption of PS and Au2PSjunction.Fig. 3 shows the SPV spectra of non-etched silicon,

c-Si (s1), and the two PS samples, s2 and s3. Firstly, itcan be noted that the SPV spectra of PS (s2 and s3) arevery different from that of the c-Si (s1) and some peaksin the high energy sides are shifted to shorter wave-length compared to c-Si (1.12 eV). Secondly, for thesample s2, two peaks are observed, one at 1.32 eV and theother at 2.05 eV. Assuming the system to be a QWstructure the widening of the band gap due to quantumsize effect (QSE) is En � Eg ¼ 0:20 eV (first transitionfrom n ¼ 1 to CB to n ¼ 1VB). Since the first transitionin the SPV spectrum is shifted by 0.2 eV from the c-Siband gap, we should expect the second transition (n ¼ 2of CB to n ¼ 2VB) should be shifted four times withrespect to the first transition (i.e. E1 � Eg ¼ 4ðE1 � Eg)).The present SPV spectrum of sample, s2, shows thesecond peak to occurs at 2.05 eV consistent with ourmodel as depicted in Fig. 2. Similar explanation can bemade for the sample s3 where the well width is smaller incomparison to sample s2. Since n ¼ 2 transition for thesample s3 is out of the detection limit of the instrumentwhich is supposed to occur around 4.3 eV according to therelation

En ¼ Eg þn2_2p2

2d2

1

m%

e

þ1

m%

h

� �, (1)

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0.0 2.0 4.0 6.0 8.0 10.0QW width,d (nm)

1.1

2.1

3.1

4.1

5.1

Ene

rgy,

En

(eV

)

0.50 1.00 1.50 2.00 2.50Photon energy (eV)

0

10

20

30

Pho

tovo

ltage

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u.)

∆ π

(a)

(b)

Fig. 3. (a) SPV spectra of PS samples, s1, s2 and s3. Inset shows some

transitions (around 1.59 and 1.68 eV) arising from defect states in Si/SiO2

interface in case of sample s2. (b) Sub-band transition energies, En

ðn ¼ 1; 2; . . .Þ, of the Si 1D QW as a function of well width. Filled circle

and square are the experimental data points. Solid and dashed lines are

theoretical plots obtained for 0D and 1D QW structure, respectively. The

experimental well width, d (PS size), and the energy, En, are estimated

from the Raman scattering and SPV measurements, respectively. Large

error between experimental and theoretical plots are reduced when shifted

by 2 nm for both s2 and s3 samples. Inset shows the energy band scheme

for bulk-Si and the increase of the confined gap as a function of etching

current density.

B.K. Patel et al. / Physica E 33 (2006) 268–272 271

only one SPV peak is observed at 1.95 eV for s3. The SPVspectra are extremely sensitive to anodization conditions.

The SPV bands are shifted to the higher energy side withincreasing current density of etching, which is consistentwith Raman data (Fig. 1(c)). Our Raman data show thecrystallite dimension to decrease with increasing anodicetching current. The average size estimated from the TEMmicrograph (3 nm) (Fig. 1(a)) nearly agree with thatobtained from Raman measurements (3.7 and 4.6 nm).The Bohr exciton radius [1], aB, of c-Si is 5 nm and our sizesare much smaller than this. Thus, based on the results ofthe SPV, Raman and TEM, our PS samples are obeyingthe QCE as proposed by Canham [1].

As is well known, the SPV reveals an excitation processesby taking the photovoltage as a signal. The photovoltagebands imply that there exists a distribution of the excitation

energies and different processes in porous silicon. Besidesthe broad main peak (s2) at 1.32 and 2.05 eV, one observesa weak double shoulder with two onsets near 1.59 and1.68 eV, respectively, as shown in Fig. 3(a) (inset). Thisshoulder must be due to electron excited either from VB toempty surface states or de-excited from CB to filled surfacestates. The surface state is expected to arise due to Si/SiO2

interface. Similarly, in case of sample s3, one observes aweak shoulder with an onset near 1.52 eV, which again canbe ascribed to the presence of surface states.The sub-band energies of the QW structure for two

transitions (n ¼ 1 and 2) corresponding to 0D and 1D aretheoretically plotted using Eq. (1) and shown in Fig. 3(b).The effective mass for electron ðm%

e Þ and hole ðm%

h Þ aretaken as 1:08m0 and 0:8m0, respectively, where m0 is thefree electron mass. The square and circles are theexperimental points corresponding to samples s2 and s3.Note that the experimental points both for n ¼ 1 and 2states do not agree with the theoretical plots which needexplanation. The well width/size has been estimated fromthe Raman measurements assuming the crystallites are tobe spherical which in reality is not true. The confinementenergies were calculated using effective mass approxima-tion (EMA) which does not hold good for very small sizes.Further, the experimental X-ray diffraction (XRD) resultsfor Si nanocrystalline thin films for Richter et al. [22] showa consistent shift 2 nm from size estimated by Ramanmeasurements again assuming the spherical nature of thecrystallites. Hence, a mismatch of experimental data ofabout 2 nm from the theoretical plot has been observed inthe present study. Further study in this regard is necessaryto find out a plausible explanation. The ideal energy banddiagram for c-Si and the shifting of the CB edge EC and VBEV for sample s2 and s3 are shown in Fig. 3(b) inset. Thedifference of EC from vacuum level for c-Si is 4.01 eV(electron affinity [23]). The shifting of the EC and EV arecalculated using the relation DEV ¼ _2p2=2m%

h d2 andDEC ¼ _2p2=2m%

e d2, respectively (from Eq. (1)), and theband edge positions are shown as in Fig. 3(b) inset.

4. Conclusion

Nanocrystalline Si has been prepared by an anodicetching of crystalline Si in HF electrolyte. The XPS studiesidentify the presence of SiO2 in the PS samples. Ramanscattering measurements show the average crystalline sizein the range of 3.0–5.0 nm. The TEM micrograph anddiffraction patterns reveal an elongated pore morphologyand monocrystallinity. The SPV studies identify differenttransitions from sub-band states in conformity with theproposed QW structure.

Acknowledgments

Thanks are due to Prof. S.N. Behera and Dr. R.K.Chaudhary for their helpful discussion and constantencouragement. Prof. A. Subrahmanyam, IIT, Madras,

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India, is gratefully acknowledged for helping in surfacephotovoltage measurements.

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