Opt Quant Electron Vo. 41,No.3,2009

This article was published in the above mentioned Springer issue.The material, including all portions thereof, is protected by copyright;all rights are held exclusively by Springer Science + Business Media.

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ISSN 0306-8919, Volume 41, Number 3

Opt Quant Electron (2009) 41:189–201DOI 10.1007/s11082-009-9341-y

Fabrication and characterization of porous silicon layersfor applications in optoelectronics

R. S. Dubey · D. K. Gautam

Received: 2 June 2009 / Accepted: 14 October 2009 / Published online: 6 November 2009© Springer Science+Business Media, LLC. 2009

Abstract In the present paper, several samples of porous silicon monolayers and multi-layers were prepared at different anodization conditions with fixed HF concentration. Theroom temperature photoluminescence wavelength observed to be increased with increasedetching time and current density respectively. By Raman measurement it has been observedthat as the size of silicon crystallites decreased with increased etching time, the silicon opti-cal phonon line shifted somewhat to lower frequency from 520.5 cm−1 and became broaderasymmetrically. The surface roughness and pyramid like hillocks surface was confirmed byAFM measurement. In SEM images, the porous silicon layers were clearly observed by whiteand black strips. It was also observed that the reflectivity increased as the number of poroussilicon layers was increased.

Keywords Porous silicon · Photoluminescence · Raman peak · Reflectivity · Photoniccrystals

1 Introduction

Porous silicon (PS) offers major potential for integrated optoelectronics technology and hasaccepted the new challenges to fabricate silicon based photonic devices using existing estab-lished silicon technology. Due to shifting of fundamental absorption edge into the shortwavelength and observed photoluminescence in the visible region of the spectrum, poroussilicon has opened the door to a multitude of applications in advanced optoelectronics technol-ogy (Canham 1990; Lang et al. 1993; Hou et al. 1996; Kalem and Yavuzcein 1999; Barillaro

R. S. Dubey (B)IACQER, Advanced Research Laboratory for Nanomaterials and Devices, Swarnandhra Collegeof Engineering and Technology, Seetharampuram, Narsapur, Andhra Pradesh, Indiae-mail: [email protected]

D. K. GautamDepartment of Electronics, North Maharashtra University, Post Box 80, Umavinagar, Jalgaon,Maharashtra, India

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Author's personal copy

190 R. S. Dubey, D. K. Gautam

et al. 2001). Porous silicon is defined as a matrix or a network of nanoscale sized siliconwires and voids which are formed when crystalline silicon wafers are etched electrochemi-cally in hydrofluoric acid based electrolyte solution under constant anodization conditions.Several models have been proposed to explain the observed photoluminescence from poroussilicon, these are hydrogenated amorphous silicon model, surface hydrides model, defectmodel, siloxene model, surface states model and quantum confinement model (Pavesi andGuardini 1996; Vasquez-A et al. 2007; Voos et al. 1992; Weng et al. 1993; Bisi et al. 2000).A multilayer structure of porous silicon exhibits a strong modulation of light hence, canbe replaced other dielectric layers in interference filters or mirrors. Such periodic dielectricmedia is known as one-dimensional (1D) photonic crystals (PCs) and have attracted intenseresearch after the pioneer works of Yablonovitch (1987) and John (1987). The forbiddenfrequencies or wavelengths form a band gap for light waves, which is the core of photoniccrystal operation (Dubey and Gautam 2008). Using porous silicon layers 1D photonic crystalcan be easily prepared by the periodic variation of the electrochemical formation parameterswhich leads to periodic variations of the refractive index of each layers (Agrawal and delRio 2003; Lugo et al. 2002; Setzu et al. 2000). Due to wide use of solar energy, there is needof creation of new technologies and materials which can be reduced the cost of solar cellstherefore, porous silicon is expected to be promising one (Boeringer and Tsu 1994; Lipiskiet al. 2003; Dobrzanski et al. 2006; Smestad et al. 1992; Lin et al. 2008). For solar cells,porous silicon layer acts as an ultra efficient anti-reflection coating, while a graded layer withvarying expanded band gap offers increased absorption in visible spectrum regions. Poroussilicon is also used as smart transducer material for sensing applications for the detection ofvapors, liquids and biochemical molecules (Singh et al. 2008; Moretti et al. 2007; Descroviet al. 2007; Ouyang et al. 2006).

In this paper, the synthesis and characterization of monolayers and multilayers of poroussilicon is presented. In Sect. 2, the experimental details for the preparation of porous siliconlayers have been presented. The optical and structural properties of porous silicon monolayersand multilayers are discussed in Sect. 3. Finally, Sect. 4 concludes the paper.

2 Experimental details

Various samples of porous silicon monolayers and multilayers were prepared by using <100>oriented boron-doped p-type substrates of 10 ohm. cm as starting material. The electrochem-ical cell used has two electrode configurations with a platinum electrode and a silicon waferas anode. Before synthesis, the silicon wafer was rinsed in de-ionized water after heating sep-arately in trichloroethylene, methanol and acetone for 5 min. Then the cleaned silicon waferswere dried in presence of nitrogen. The electrolyte used was HF (48%): H2O:C2H5OH in avolume ratio of 1:1:2. The mixing of ethanol in electrolyte solution is helpful to improve thelateral homogeneity and the uniformity of the porous silicon layer by promoting the hydrogenbubble removal. For the formation of monolayer of porous silicon, a fixed current density ‘J’was applied for a fixed etching time ‘t’. In order to prepare a multilayer structure of poroussilicon, we have first prepared a stack of two layers of different refractive index by applyingcurrent density ‘J1’ & ‘J2’ for the etching time ‘t1’ & ‘t2’ respectively. Further, this processis repeated for desired stacks.

The prepared samples were characterized for reflectance by UV–visible spectrophotome-ter (Shimadzu UV-1601). The photoluminescence was measured by using a monochromator(Jobin Yvon) with an attached charge coupled device. A beam of 488 nm line from argonlaser at 10 mW output power was used for excitation. To link crystallographic perfection of

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Fabrication and characterization of porous silicon layers 191

Table 1 Photoluminescencepeaks at various etching time J = 50 mA/cm2 J = 30 mA/cm2

Etching time( min)

PL peak(nm)

PL peak(eV)

Etchingtime ( min)

PL peak(nm)

PL peak(eV)

2 640 1.93 2 629 1.97

3 670 1.85 3 630 1.96

4 690 1.79 4 650 1.90

the starting material with resulting porous morphology the Raman measurements using Lym-pus EX41 Raman Division (HR800) were done. The surface morphology and roughness ofprepared samples were obtained by atomic force microscopy Nanoscope E (NSE) in contactmode. The scanning electron microscopy using Leica Cambridge 440 Microscope (UK) wasdone to see the porous zone clearly distinguished from the substrate.

3 Results and discussion

Several samples of porous silicon monolayers and multilayers were prepared at differentanodization conditions with fixed HF concentration. The prepared samples show distinctcolor distribution over the entire surface of porous silicon. The prepared porous structure inbulk silicon is strongly responsible for the photoluminescence on its surface which can beconfirmed by shifting in the band gap energy of bulk silicon. The room temperature photo-luminescence measurements of various samples prepared at different anodization conditionsare depicted in Figs. 1, 2, 3. The photoluminescence peak corresponding to the red bandemission at wavelength 640, 670 and 690 nm can be observed in Fig. 1a–c respectively. It isobvious that as the current density increases the silicon crystallite size will decrease whichcauses the variation in the PL intensity. In fact, the porosity of porous silicon layer is adirect function of applied current density and as the pore size varies accordingly the size ofsilicon crystallites will change. Therefore, the PL wavelength shifts to higher wavelengthor lower energy as the etching time is increased. In case of Fig. 1b, a broad range of PL atwavelength 670 nm is observed which is probably due to a wide distribution of the energyband gap caused by different pore sizes. It is noted that the wavelength of red emissionpeak varies between a minimum of 640 nm and maximum of 690 nm. This shows that theband gap can be tuned from 1.93–1.79 eV by adjusting the anodization time from 2–5 minat applied 50 mA/cm2 current density. Similarly, Fig. 2a–c shows the red emission PL ofporous silicon prepared at 30 mA/cm2 under 2, 3 and 4 min anodization time, respectively.In this case, if the current density is kept lower (30 mA/cm2) than previous one, so poresof reduced size is expected. On the basis of this, if we compare the Figs. 1 and 2 we willfind that the PL wavelength is decreased. However, in Fig. 2b a broad peak of red emissionis observable at 650 nm. In this case, the wavelength of red emission peak varies between aminimum of 629 nm and maximum of 650 nm. It indicates that the band gap can be tunedfrom 1.97–1.90 eV by adjusting the anodization time 2–4 min at applied 30 mA/cm2 currentdensity. Red emission band has attracted the most attention because it is the only band to beefficiently electrically excited (Bisi et al. 2000). Table 1 shows the PL peak and band gapenergy observed at various anodization time and current densities.

We have also prepared multilayers of porous silicon to observe the variation in the PLwavelength. Figure 3 shows the PL samples prepared by the tuning of 50 and 30 mA/cm2

current density under 2 min etching time. Figure 3a, b are corresponding to the two and three

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Fig. 1 Photoluminescence ofporous silicon monolayerprepared at J = 50 mA/cm2

under etching time a 2 min,b 3 min, c 5 min

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Fig. 2 Photoluminescence ofporous silicon monolayerprepared at J = 30 mA/cm2

under etching time a 2 min,b 3 min, c 4 min

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Fig. 3 Photoluminescence a two, b three bilayers of porous silicon anodized at J1 = 50 & J2 = 30 mA/cm2

under 2 min etching time

Fig. 4 Raman spectra of poroussilicon monolayers formed undervarious etching time atJ = 30 mA/cm2

500 520 540

Raman Shift (cm-1)

Inte

nsi

ty (

a.u

.)

C-Silicon

t=2min.

t=3min.

t=4min.

bilayers (stacks) of high and low refractive index layer of porous silicon. By comparing thesefigures, we have concluded that as the number of bilayers increases the PL peak shifts to thelower wavelength while remaining in the red band. It is actually expected phenomena since,as the number of pores is increased in silicon wafer it ultimately reduces the size of siliconcrystallite as a consequence there will be a shift in Raman as well as PL peak.

Figures 4 and 5 shows the Raman shift of porous silicon monolayers formed at variousetching time for J = 30 and 50 mA/cm2, respectively. The solid curves in Figs. 4 and 5shows the Raman peak from pure single crystalline silicon observed at 520.5 cm−1 with itsshape is nearly Lorentzian. In crystalline silicon, the optical phonon is observed in the centerof the Brillouin zone with its energy of 520.5 cm−1 and this is due to the conservation ofquasimomentum in crystals. It is known that phonons in small crystallites are localized hence;their quasimomentum is no longer well defined according to the uncertainty principle. Thus,the conservation law of the quasimomenturn is no longer valid. As a result, all the phononsof dispersion relation estimated with a weight function which contributes to the measuredRaman signal. As the size of nanocrystal decreases, the silicon optical phonon line shifts tolower frequency and becomes broader asymmetrically (Yeo et al. 2005; Sui et al. 1992; Choet al. 1998; Kozlowski and Lang 1992). This shifting of peak attributes the reduction in thephonon energy as a result of disturbances in the silicon lattice due to porous structure. Thisattributes the theory of the light emission in porous silicon as due to transition of the carriers

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Fig. 5 Raman spectra of poroussilicon monolayers formed undervarious etching time atJ = 50 mA/cm2

500 520 540

Raman Shift (cm-1)

Inte

nsi

ty (

a.u

.)

t=3min.

t=4min.

t=5min.

C-Silicon

for L1 and L2 valleys of band structure as reported by Gautam et al. (1988) and Khokle(1987). In fact, the band structure of silicon gets modified by making the silicon porous andthe valleys shift towards K = 0. In simple words, the maximum part of applied energy isconverted into emission therefore the band structure of silicon is modified. This suggests thatthe Raman contribution is exclusively from the porous silicon layer not from bulk silicon. Asdepicted in Figs. 4 and 5, as the etching time increases the Raman peaks gradually shifts from520.5 cm−1 with the increment in full width at half maximum. According to the variation inthe anodization parameters, the shifting of red emission can be observed. Photoluminescenceof ‘red’ band originates from centers located at the surface of the silicon branches is logicalwhen one consider that the large surface area of porous silicon would provide numerousrecombination centers. The recombination centres are formed by silicon atoms at the surfaceof the crystallite adjusting their bond lengths and angles to accommodate changes in localconditions. Among different hypothesis presented so far on the photoluminescence fromporous silicon surface, quantum confinement effect is popular one which is due to the chargecarriers in narrow crystalline silicon wall separating the pore walls. As we have shown inFigs. 4 and 5 the shifting of Raman peak is minor (in the range of 0.8) at various anodizationconditions. Usually, as the size of nanocrystals decreases, the silicon optical phonon lineshifts to lower frequency and becomes broader asymmetrically (Yeo et al. 2005; Sui et al.1992; Cho et al. 1998; Kozlowski and Lang 1992). However, our results are consistent withanother hypothesis according to which the porous silicon luminescence is due to the presenceof surface confined molecular emitters i.e., siloxene (Pavesi and Guardini 1996; Vasquez-Aet al. 2007; Voos et al. 1992; Weng et al. 1993; Bisi et al. 2000). The same conclusion is alsoreported by Zhao et al. (2006), Yeo et al. (2005). Hence, the Raman peak observed in Figs. 4and 5 indicates the less possibility of quantum confinement effect.

Figures 6 and 7 shows the AFM images of monolayers of porous silicon anodized atdistinct anodization parameters. Figure 6a shows the AFM image of porous silicon mono-layer prepared at J = 50 A/cm2 under 2 min anodization time. The prepared porous siliconlayer shows the surface roughness and pyramid like hillocks surface. The observed averagediameter of pores and its roughness are 39.06 and 6.83 nm respectively. The surface mor-phology confirms the pore formation with its depth is about 8.926 nm. Figure 6b shows theAFM images of sample prepared at same current density but increased etching time i.e.,3 min. This image also confirms the formation of pores. It is observed that as the anodiza-tion time increases the column length also increases. The average diameter and depth of thepores is 42.05 and 13 nm respectively. In this case, the pore depth is increased due to changeof anodization time 2–3 min, however, the average roughness of porous silicon is 6.33 nm.

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Fig. 6 3D AFM image of porous silicon monolayers prepared at J = 50 mA/cm2 under a t = 2 min, bt = 3 min, c t = 5 min

Fig. 7 3D AFM image of porous silicon monolayers prepared at J = 30 mA/cm2 under a t = 2 min,b t = 3 min, c t = 5 min

In photoluminescence study, we have found the enhanced PL intensity which is due to theincreased size and height of the pitches which is observable in the AFM images of poroussilicon prepared at increased anodization time. Figure 6c shows the surface morphology ofthe sample prepared at increased anodization time of 5 min. At this etching time, the averagediameter and depth of the pore are 54.69 and 14.4 nm, respectively. However, the surfaceroughness is 7.918 nm. From Fig. 6a–c, we have found that as the anodization time increasesthe depth of the pores are increased in the manner of 8.926, 13 and 14.4 nm anodized at2, 3 and 5 min etching time, respectively. Similarly, Fig. 7a–c shows the AFM images ofporous silicon surfaces anodized at 30 mA/cm2 constant current density with varying etchingtime 2, 3 and 4 min respectively. Figure 7a shows the AFM image of the prepared sampleat J = 30 mA/cm2 under 2 min etching time. The average diameter, depth and roughness ofthe pores are 26.43, 8.2 and 3.6 nm, respectively. Figure 7b shows the surface morphologyof the porous silicon sample anodized under 3 min at 30 mA/cm2 current density. The aver-age diameter, depth and roughness of the pores are 35.1, 11 and 4.7 nm, respectively. Thesurface morphology of the porous silicon sample shown in Fig. 7c is anodized under 4 minat 30 mA/cm2 current density. The diameter, depth of pores and the surface roughness are42.07, 12 and 5.133 nm, respectively. AFM images in Figs. 6 and 7 shows different size ofpores formed at different anodization parameters during synthesis. The formation of thesepores is responsible for the shift of Raman peak due to change in crystallite size of silicon.Accordingly, these changes are highly responsible for its photoluminescence in the visiblewavelength range.

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Fig. 8 Plane view SEM image of sample S1 figure (a) and S2 figure (b) anodized under J = 30 and50 mA/cm2, respectively

Fig. 9 Plane view SEM image sample S3 figure (a) and S4 figure (b) anodized by tuning of current J1 & J2for three periods

Figures 8–10 shows the surface morphology of as-grown porous silicon monolayers andmultilayers. Figure 8a,b shows the scanning electron microscopy (SEM) plane view imagesof porous silicon monolayer (sample S1 and S2) etched under current density J = 30 and50 mA/cm2. The etching time of sample S1 and S2 are 2 min respectively. The morphologyof the porous silicon layer shows that the electrochemical etching of silicon occurred uni-formly and made the granular structure in a columnar shape which can be observed in theimage of sample S1 and S2. It is also observable that as the current density increases fromJ = 30–50 mA/cm2, the etching depth is also increased. The array of void spaces (dark)in silicon matrix (bright) can be seen clearly in another top view SEM of porous siliconmultilayer (sample S3 and S4) which is shown in Fig. 9a,b. These samples composed ofthree periods of high and low refractive index layers prepared by tuning of current densityJ1 = 50 & J2 = 30 mA/cm2 for 2 and 3 min etching time, respectively. A large number ofsmall pores that are aligned in random direction are observable in SEM images. The averagepore diameter obtained from sample S3 and S4 are to be 4.5 and 12 µm, respectively.

Figure 10a,b shows the SEM images of multilayered structure of porous silicon (sample S5and S6) in which different formed layers can be seen clearly. For S5, we have found the firstand second layer thicknesses are 1.36 and 0.56 µm for current density of 50 and 30 mA/cm2

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Fig. 10 Cross-sectional SEM image of sample S5 (a) and S6 (b)

under 1 min etching time. However, for S6 we have found that for current density of 50 and30 mA/cm2 the obtained thicknesses are 1.84 and 0.72µm corresponding to the first andsecond layer etched under 2 min etching time. By changing the ratio of silicon and air, awide range of refractive indices can be obtained. The sample S5 and S6 are composed of sixperiods of high and low refractive index layers respectively. Figure 10a,b, shows the layeredstructures of porous silicon by white and black strips of refractive index n1 = 2.14 andn2 = 1.88 of sample S5 and n1 = 1.42 and n2 = 2.65 of sample S6. The SEM micrographof S5 show the non-uniform layers under 2 min. etching time but, as the etching time reducesto the 1 min the structure of S6 becomes more uniform with good homogeneity as depicted inSEM micrograph. The refractive index contrast of S6 is large as compared to the S5 hence;a large photonic band gap can be obtained by tuning the etching time.

The reflectivity of porous silicon layer varies with respect to the anodization parametershence, so it is essential to check the reflectivity of prepared porous silicon layers. Figure 11,shows the reflection spectra of porous silicon monolayers (sample S1 and S2) prepared atcurrent density J = 30 & 50 mA/cm2 respectively. The prepared porous silicon layers are verythin which can be regarded as thin films. Hence, the bumps shown in the reflection spectraare obviously caused by constructive and destructive interference of the beams reflected fromupper and lower interfaces of porous silicon layer. The number of bumps increases as theanodization time is increased. It is observed that the reflection varies sinusoidally on wave-length scale. The doted and solid curves are corresponding to the porous silicon samplesS1 and S2 prepared at 2 min anodization time. It is remarkable that the increase in currentdensity from 30 to 50 mA/cm2 causes to increase the reflection through the surface of poroussilicon layer. Figure 12 shows the typical reflection spectra of the samples S3 and S4 whichare composed of three periods of low and high refractive index layers. The dotted and solidcurves are corresponding to the sample S3 and S4 prepared by optimizing the current den-sity J1 = 50 & J2 = 30 mA/cm2 for 2 and 3 min etching time, respectively. In this figure,the spectral bandwidth of maximum reflectivity is observed as the number of bilayers isincreased. The progressive decrease of the fringes maxima at lower wavelength is due tothe increasing absorption of the porous silicon layers. As the current density is changed therefractive index is also changed which change the porosity of the layer as a result of it, thereflectance is varied. Similarly, Fig. 13 shows the reflectivity of multilayered structure ofporous silicon (i.e., sample S5 and S6) which are composed of six periods of low and highrefractive index layers. The structure is anodized by the tuning of 30 and 50 mA/cm2 current

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Fig. 11 Reflection spectra ofsample S1 and S2 at currentdensities J = 30 & 50 mA/cm2

for 2 min etching time

Fig. 12 Reflection spectra ofsample S3 and S4 prepared bytuning of current density J1 = 50& J2 = 30 mA/cm2 for 2 and3 min etching time, respectively

Fig. 13 Reflection spectra ofsample S5 and S6 prepared by thetuning of current densitiesJ1 = 30 & J2 = 50 mA/cm2 for1 & 2 min etching time,respectively

density for 1 and 2 min etching time. The progressive decrease of the fringes maxima outsidethe stop band indicates the absorption of incidence light on the surface of the porous siliconlayers.

4 Conclusions

Various samples of porous silicon prepared under different anodization conditions were char-acterized and studied. It was observed that as the current density increases, the porosity ofthe porous silicon layer is increased and consequently refractive index is reduced. As theanodization time increases, the size of silicon crystallites reduced which causes the corre-sponding shifting in the Raman peak as well as band gap of porous silicon. From Ramanstudy it was revealed that as the size of silicon crystallites decreased with increased current

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density and etching time the silicon optical phonon line shifted somewhat to lower frequencyfrom 520.5 cm−1 and became broader asymmetrically. This shifting of peak attributed thereduction in the phonon energy as a result of disturbances in the silicon lattice due to porousstructure. As the maximum part of applied energy is converted into emission hence, the bandstructure of silicon is modified. This suggests that the Raman contribution is exclusively fromthe porous silicon layer not from bulk silicon. AFM characterization shows the rough siliconsurface which can be regarded as a condensation point for small skeleton clusters to form. InSEM analysis, the structures of porous silicon layers are clearly observed by white and blackstrips. It is also observed that the reflectivity increases as the number of porous silicon layersis increased which is useful in order to tune the maximum reflection band for 1D photoniccrystals. It is important to note that the small shifting of Raman peaks at different etching andcurrent density attributes the less possibility of quantum confinement in prepared samples.Hence, the red band emission observed in visible spectrum attributed to the hydrides presenceon the surface of porous silicon. This study explores the applicability of electrochemicallyprepared porous silicon layer for its various applications in advanced optoelectronics field.

Acknowledgments The authors wish to express their gratitude to Dr. V. Ganesan, and Dr. V. Sathe for char-acterizing the porous silicon samples at UCG-DAE Consortium for Scientific Research Laboratory, Indore(MP), INDIA.

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Opt Quant Electron Vo. 41,No.3,2009