Role of an electrolyte and substrate on the stability of porous silicon

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Physica E 28 (2005) 264–272 Role of an electrolyte and substrate on the stability of porous silicon Shailesh N. Sharma à , R.K. Sharma, S.T. Lakshmikumar Materials Division, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi-110012, India Received 14 March 2005; accepted 21 March 2005 Available online 6 June 2005 Abstract Porous silicon (PS) layers were prepared by anodization on polished and textured substrates of (1 0 0) Si for a fixed anodization time at different current densities in different HF-based electrolytes. Highly stable, mechanically strong, hydrogen-passivated surface and thick porous silicon films have been obtained using HF:ethanol-based electrolyte on textured silicon substrates. Porous silicon formed using HF:ethanol as an electrolyte exhibits superior properties compared to porous silicon formed using HF:H 2 O 2 -based electrolyte at the same current density, time of anodization and type of substrate. Porous silicon films formed on textured substrates exhibits higher porosity and photoluminescence efficiency, negligible PL decay, better mechanical strength, adherence to the substrate, non-fractured surface morphology and lower stress compared to porous silicon formed on polished silicon substrates at the same current density for both ethanol and H 2 O 2 -based electrolytes, respectively. Use of textured silicon substrate and ethanol-based electrolyte is a key parameter for the formation of tailored-made porous silicon films for device applications. r 2005 Elsevier B.V. All rights reserved. PACS: 61.43.Gt; 81.05.Rm; 82.45.Gj Keywords: Porous silicon layers; HF-electrolytes: Si substrates 1. Introduction Porous silicon (PS) exhibits visible photolumi- nescence and electroluminescence which has gen- erated considerable interest [1]. The potential of porous silicon for various technological applica- tions such as chemical sensors [2], optoelectronic devices [3], displays [4] and photodetectors [5] has been extensively investigated. Recent emphasis has been on the utilization of the large surface area of the porous layers for chemical and biological applications [6]. It is possible to control the degree of porosity of the porous layers formed by electro- chemical etching in HF-containing electrolytes (ethanol, hydrogen peroxide, etc.). However, the ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.03.020 à Corresponding author. Tel.: 91 11 25742609 14x2409; fax: 91 11 25726938, 25726952. E-mail address: shailesh@mail.nplindia.ernet.in (S.N. Sharma). nanoscale structure of PS leads to an enormous increase in surface area and the presence of large number of unpaired bonds at the surface which alter the surface recombination rates and conse- quently the PL efficiency, surface reactivity and stability [7]. Several approaches have been tried for preparing uniformly bonded stable surfaces. The formation of a high-quality oxide surface layer is now accepted as a good solution to the formation of a stable surface and improved luminescent properties [8]. Embedding the nanocrystalline silicon particles in an optically transparent med- ium is another way of isolating the surface from the ambient and providing a stable luminescence [9]. Recently, the use of alkyl-terminated mono- layers as a mean of stabilizing the PS surface has received attention where Si–H bonds at the surface during PS formation are replaced by a hydrophilic alkyl termination [10]. The electrolyte composition is one of the most important fabrication parameter for well-defined porous layers. The pore dimensions and porosity change with different ratios of electrolytes. Var- ious electrolytes have been used for the fabrication of porous silicon viz, HF, ethanol, H 2 O 2 and HNO 3 [1,11,12]. HF is mainly used for the dissolution of silicon, ethanol is basically used to reduce the surface tension of the electrolytic mixture since surface wetting is important for good pore uniformity. Recently thrust has been given on H 2 O 2 -based electrolytes preferably as an oxidizing agent [12]. The photochemical etching method with H 2 O 2 solution does not generate a toxic material unlike in the case of HNO 3 [11]. Moreover, the addition of H 2 O 2 to the etching mixture raises the pH of the solution and produces ideal Si surfaces terminated with Si–H bonds thus resulting in a homogeneous PS surface with low defect density [12]. Recently, we have demonstrated by means of high-resolution XRD studies that texturization of silicon surface is an effective method for the formation of stable and thick porous silicon films [13]. In this paper, using PL decay as a probe, we are evaluating the degradation of stability of PS on electrolyte (HF–C 2 H 5 OH and HF–H 2 O 2 ) and current density formed on textured and polished Si substrates, respectively. The emphasis is mainly on the development of PS with high and stable PL, control of pore size distribution and therefore a better control on the formation process. 2. Experimental Boron-doped p-type Si wafers of (1 0 0) orienta- tion, 8–10 ohmcm resistivity and 400 mm thickness were used for preparing PS. The wafers were polished in 40% NaOH for 2 min. These wafers were textured using 2% NaOH at 85 1C for 30 min. For forming the back contact, Ag–Al paste was screen printed on the wafer and dried at 250 1C. The wafer was then heated to 750 1C for 2 min in an IR furnace. PS was formed by the standard anodization process using Si as the anode and Pt as the counter electrode in an acid resistant container. The anodization was carried out at 20–50 mAcm À2 for 30 min, in two different electrolytes. The first is a mixture of HF and C 2 H 5 OH (1:1 by volume) which is almost universally used [1] and would be abbreviated as electrolyte A. The second is a mixture of HF and H 2 O 2 (1:1 by volume) which was extensively used by Nafeh et al. [12] and would be abbreviated as electrolyte B. After the anodiza- tion, the films were washed in deionized water and ethanol and dried in nitrogen. The samples were subjected to continuous agitation in an ultrasonic cleaner to evaluate the speed with which the sample is destroyed. The weight of the sample is con- tinuously monitored. The PL was measured using a home assembled system consisting of a two-stage monochromator, a photomultiplier tube (PMT) with a lock-in amplifier for PL detection, and an Ar + ion laser operating at 488 nm and 5 mW (corresponding to 0.125 W cm À2 ) for excitation in all the measurements. Decay of PL intensity has been used as a measure of the stability of the surface bond configurations [7]. For PL decay studies, the sample was continuously exposed to the laser radiation and PL measurements were carried out at regular intervals. 3. Results and discussion Good porous silicon films exhibiting high photoluminescence intensity could be formed on ARTICLE IN PRESS S.N. Sharma et al. / Physica E 28 (2005) 264–272 265 both textured and polished substrates at various current densities corresponding to both electro- lytes A and B, respectively. The porosity (45–80%) and thickness (12–96 mm) of PS films were estimated from gravimetric measurements [14]. Fig. 1 shows porosity values as a function of I d for PS films formed on textured and polished substrates corresponding to both electrolytes A and B, respectively. As shown in Fig. 1, porosity of PS films increases with increase in current density. As evident from Fig. 1, PS films corresponding to electrolyte B exhibits higher porosity as compared to the corresponding films of electrolyte A for both textured and polished substrates. Fig. 2(A) shows the weight loss of PS films prepared using electrolyte A at different I d ,asa function of time of ultrasonic treatment. There is a substantial weight loss of PS samples on polished substrates when subjected to an ultrasonic treat- ment for an hour by which time the entire porous layer has been removed and the loss of weight saturates. However, for textured PS films, the weight loss is marginal. The rate of weight loss increases with increase in I d and this effect is felt more on PS films prepared on polished substrates. Results of weight loss for PS films prepared using electrolyte B are shown in Fig. 2(B). In this case some loss is observed for textured samples also. However, the rate of weight loss increases with I d and is much higher for the untextured samples (Fig. 2(B)). Typical PL curves for PS films formed at different current densities I d ($20, 35 and 50 mA cm À2 ) on textured and polished substrates corresponding to electrolytes A and B are shown in Figs. 3(A) and (B). As evident from Figs. 3(A) and (B), the absolute PL intensity is higher for the porous silicon formed on textured substrates and for PS films corresponding to electrolyte B owing ARTICLE IN PRESS 10 20 30 40 50 40 50 60 70 80 (d) (c) (b) (a) Porosity (%) Current Density I d (mA cm -2 ) Fig. 1. Porosity of PS as a function of current density (I d ); (a) textured substrate, electrolyte B; (b) polished substrate, electrolyte B; (c) textured substrate, electrolyte A; (d) polished substrate, electrolyte A. 0204060 0.3450 0.3455 0.3460 0.3465 0.3470 0.3475 0.3480 0.3485 (c) (e) (f) (b) (d) (a) Weight Loss (gms) Time of Ultrasonic treatment (mins.) 0 20 40 60 0.3450 0.3455 0.3460 0.3465 0.3470 0.3475 0.3480 0.3485 0.3490 (b) (d) (e) (f) (a) (c) Weight Loss (gms) Time of ultrasonic treatment ( mins. ) (A) ( B ) Fig. 2. Weight loss of porous silicon samples prepared at different current densities (I d ) for (A) electrolyte A and (B) electrolyte B; (a) textured substrate, I d ¼ 20 mA cm À2 ; (b) polished substrate, I d ¼ 20 mA cm À2 ; (c) textured substrate, I d ¼ 35 mA cm À2 ; (d) polished substrate, I d ¼ 35 mA cm À2 ; (e) textured substrate, I d ¼ 50 mA cm À2 and (f) polished substrate, I d ¼ 50 mA cm À2 . S.N. Sharma et al. / Physica E 28 (2005) 264–272266 to its higher porosity. Fig. 3(A) shows that with increase in I d from 20 to 50 mA cm À2 for electro- lyte A, the PL peak position shifts towards low-l side for PS films formed on both textured and polished substrates. Similarly, for PS samples corresponding to electrolyte B, the blue-shift of the PL peak position is more prominent with the PL peak being at $650 nm as compared to 610 nm for PS films prepared on textured sub- strates corresponding to electrolyte A at higher I d $50 mA cm À2 (Fig. 3(B)). This trend is quite prominent for PS films formed on textured substrates as compared to the corresponding films formed on polished substrates for both electrolytes A and B, respectively (Figs. 3(A) and (B)). These results are in accordance with quantum confine- ment effects [1]. It is known that the peak position of the PL intensity is blue shifted when HF-H 2 O 2 is used as the electrolyte [15]. A marginal shift in PL peak position towards low l side is also observed upon texturization (Figs. 3(A) and (B)). Visual observation shows that the porous silicon films corresponding to electrolyte A formed on textured surfaces appear more uniform and strong as compared to the corresponding films prepared using electrolyte B. The PS films at higher current densities (I d X35 mA cm À2 ) on polished substrates shows a break off in PL curves as these films are powdery in nature and hence unstable corre- sponding to both electrolytes A and B. PS films prepared using B are more powdery in nature and shows peeling-off tendency particularly for films prepared on polished substrates. This is even more obvious for films formed at higher I d (X50 mA cm À2 ). Decay of PL intensity is a good indication of the stability of porous silicon particularly of the surface bond configurations [3,16].InFig. 4(A), decay of the PL intensity at the peak wave- length due to exposure to the laser radiation for porous silicon films formed at different I d ¼ ð20250 mA cm À2 Þ on textured and polished silicon substrates for electrolyte A are compared. Simi- larly, the corresponding PL-decay curves for electrolyte B are shown in Fig. 4(B). The PL peak position was recorded for different times corre- sponding to a fixed wavelength. As shown in Figs. 4(A) and (B), significant decay of the PL intensity is observed for PS films formed on polished substrate and the rate of decay increases with increase in I d . This is observed for both A and B- based electrolytes with the rate of PL decay being higher for electrolyte B as compared to electrolyte A at all current densities. However, for PS films formed on textured silicon, no PL decay was observed when ethanol was used as an electrolyte and a very marginal decay was noted when H 2 O 2 - based electrolyte is used (Figs. 4(A) and (B)). To ARTICLE IN PRESS 0 1 2 3 4 5 6 7 (a) (f) (e) (d) (c) (b) PL Intensity (a.u.) 500 550 600 650 700 750 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 (d) (c) (e) (f) (a) (b) PL Peak Intensity (a.u.) Wavelength (nm) 500 550 600 650 700 750 800 Wavelength (nm) (A) (B) Fig. 3. PL spectra of porous silicon samples prepared at different current densities (I d ) for (A) electrolyte A and (B) electrolyte B; (a) textured substrate, I d ¼ 20 mA cm À2 ; (b) polished substrate, I d ¼ 20 mA cm À2 ; (c) textured substrate, I d ¼ 35 mA cm À2 ; (d) Polished substrate, I d ¼ 35 mA cm À2 (e) textured substrate, I d ¼ 50 mA cm À2 and (f) polished substrate, I d ¼ 50 mA cm À2 . S.N. Sharma et al. / Physica E 28 (2005) 264–272 267 ensure the reproducibility of this PL decay, measurements were done repeatedly and for several hours and the PL decay trend was found to be the same. This is a direct evidence for the formation of stable surface and correlates with the superior mechanical stability of porous silicon formed on textured substrates. SEM was used to identify the surface morphol- ogy of the porous silicon formed on textured and polished Si-substrates at different current den- sities for electrolytes A and B, respectively. Silicon nanowires are not visible at these magnifica- tions. Figs. 5 (A) and (B) show the surface of porous silicon formed on polished silicon at I d $10 mA cm À2 corresponding to electrolytes A and B, respectively. A plain featureless surface morphology is observed at I d $10 mA cm À2 for electrolyte A while a cracked surface morphology is obtained for electrolyte B for the same current density. Similar observations on the fragility of thick and highly porous films had been noted earlier [8,17]. For electrolyte A-based samples at lower I d , lack of cracking indicates lower stress while the corresponding electrolyte B-based sam- ple exhibits higher stress. At I d ¼ 35 mA cm À2 , distinct cracking and disintegration is observed for PS films formed on polished substrates for both electrolytes A and B with the cracking being more pronounced for the latter than for the former (Figs. 5(C) and (D)). The higher current density results in increased porosity and the inability of the silicon nanowires to withstand the stress leads to cracking. The surface morphology of PS films formed on textured substrates is significantly different as compared to polished substrates. Figs. 6(A) and (B) shows the surface morphology of porous silicon formed on textured substrates at I d ¼ 35 mA cm À2 corresponding to electrolytes A and B, respectively. Here, the smooth surface morphol- ogy consists of randomly sized and spaced pyramids homogeneously distributed on the sur- face. The pyramids appear to be more sharply separated but no macroscopic cracking is observed even for electrolyte B-based sample unlike in the case of PS film formed polished silicon substrate for the same current density (Figs. 6(A) and (B)). This surface morphology does not essentially differ from the textured silicon substrate (not shown) and is not affected by current density. On polished silicon substrates, PS layers showed a tendency to have a mechanically weak structure at higher current densities (I d $50 mA cm À2 ) owing to its higher porosity resulting in many cracks or peeling off the film from the substrate. This effect is more prominent for electrolyte B-based samples than for electrolyte A-based samples. However, ARTICLE IN PRESS 0204060 0.0 0.8 1.6 2.4 3.2 4.0 (d) (c) (b) (a) PL Peak Intensity (a.u.) Time (mins) 0102030405060 0 1 2 3 4 5 6 (f) (b) (e) (d) (c) (a) PL Intensity (a.u.) Time ( mins. ) (A) (B) Fig. 4. PL decay of porous silicon samples prepared at different current densities (I d ) as a function of time of laser exposure for (A) electrolyte A and (B) electrolyte B; (a) textured substrate, I d ¼ 20 mA cm À2 ; (b) polished substrate, I d ¼ 20 mA cm À2 ; (c) textured substrate, I d ¼ 35 mA cm À2 ; (d) polished substrate, I d ¼ 35 mA cm À2 ; (e) textured substrate, I d ¼ 50 mA cm À2 ; and (f) polished substrate, I d ¼ 50 mA cm À2 . S.N. Sharma et al. / Physica E 28 (2005) 264–272268 this is not so in the case of textured substrates. The cracks observed for PS films formed on polished substrates for both electrolytes A and B indicates higher stress and as a consequence, higher PL decay is observed. Whereas PS samples formed on textured substrates are marked by smooth surface morphology, lower stress and consequently, neg- ligible PL decay. In order to identify the chemical composition of our samples, we have investigated the Fourier transform infrared (FTIR) absorption spectra. From our FTIR data (Fig. 7) obtained for freshly ARTICLE IN PRESS Fig. 5. Scanning electron micrographs of porous silicon prepared on polished substrates at different current densities (I d ); (A) I d ¼ 10 mA cm À2 , electrolyte A; (B) I d ¼ 10 mA cm À2 , electrolyte B; (C) I d ¼ 35 mA cm À2 , electrolyte A; (D) I d ¼ 35 mA cm À2 , electrolyte B. Fig. 6. Scanning electron micrographs of porous silicon prepared on textured substrates at I d ¼ 35 mA cm À2 ; (A) electrolyte A; (B) electrolyte B. S.N. Sharma et al. / Physica E 28 (2005) 264–272 269 prepared samples, it is clear that there are a number of distinct peaks with different intensities. Figs. 7(a) and (b) shows FTIR absorption spectra for PS samples prepared using electrolyte A at I d ¼ 20 mA cm À2 on textured and polished sub- strates, respectively. PS films prepared on textured substrates exhibit mainly Si–H related modes at $2105 cm À1 due to Si–H stretching mode [18], 910 cm À1 due to Si–H 2 scissors or Si–H 3 symmetric or antisymmetric deformation [18,19], 817 and 660 cm À1 due to Si–H 2 and Si–H wagging [19,20] while for Si–O related modes are marked by a broad hump at $1110 cm À1 due to a bulk interstitial Si–O–Si asymmetric stretching mode [18]. However, PS films prepared on polished substrates exhibits mainly Si–O-related peaks with a doublet showing peaks at $2256 cm À1 which is attributed to Si–H stretching modes when the silicon is backbonded to oxygen atoms [21] and at $2117 cm À1 due to Si–H stretching mode, broad peak at $1192 cm À1 and a satellite peak at $1010 cm À1 due to Si–O–Si stretching mode and a weak contribution at $879 cm À1 due to non- stretching Si–H modes [20] and no signal of Si–H wagging modes between 600 and 700 cm À1 was observed. It is worthwhile to note that there is no signature of any O atoms backbonded to Si–H related mode at $2250 cm À1 for PS films prepared on textured substrates (Fig. 7(a)). Another inter- esting difference noted in the FTIR spectra of PS films using electrolyte A prepared on textured and polished substrates is the shift of Si–O related mode from 1110 to 1192 cm À1 which indicates increase in the oxidation state (x) of the SiO x species [22]. For H 2 O 2 -based (B) samples formed on textured substrates, the FTIR spectra (Fig. 7(c)) shows characteristic peaks of both Si–H and Si–O-related modes with a doublet comprising of peak at $2256 cm À1 (O backbonded to Si in SiH stretching mode) and at $2117 cm À1 (SiH stretching mode), a distinct broad peak at $1215 cm À1 (Si–O–Si) stretching mode, a broad peak doublet comprising of peaks at $940 and 840 cm À1 associated with SiH 2 wagging and bending modes and a shoulder at $650 cm À1 due to Si–H wagging modes, respectively. However, for the corresponding PS sample formed on polished substrate, the FTIR spectrum (Fig. 7(d)) exhibits mainly Si–O-related modes at 2250 cm À1 (O backbonded to SiH mode), a broad peak comprising of peaks at $1161 and 1018 cm À1 (Si–O–Si stretching mode) with weak contribu- tions at $880 and 805 cm À1 (Si–H-related bending and wagging modes). Here in Fig. 7(d), the notable feature is the absence of Si–H stretching at $2100 cm À1 and Si–H wagging at $630 cm À1 . Thus, silicon–hydrogen-related modes are stronger for PS samples prepared on textured substrates while silicon–oxygen-related modes are stronger for the corresponding films prepared on polished substrates for the same current density and electrolyte. The effect of oxidation is felt more for H 2 O 2 -based PS films particularly formed on polished substrates as compared to ethanol-based PS films. From the above results, it can be conjectured that there is a change in the surface passivation from hydrogen to oxygen-like species as we go from textured to polished substrate for PS films formed at same current density (I d $20 mA cm À2 ) for both the electrolytes A and B, respectively. In case of H 2 O 2 -based PS films (B), a significant blue shift in PL spectra as compared to the corresponding ethanol based films could be due to the enhanced oxidation of surface of nanocrystalline Si resulting in an increase of SiO x thickness surrounding the Si-core. Oxidation of ARTICLE IN PRESS 2500 2250 2000 1750 1500 1250 1000 750 500 1.0 1.5 2.0 2.5 3.0 3.5 (d) (c) (b) (a) Absorbance (a.u.) Wavenumber (cm -1 ) Fig. 7. FTIR absorption spectra of porous silicon prepared at current density I d ¼ 20 mA cm À2 ; (a) textured substrate, electrolyte A; (b) polished substrate, electrolyte A; (c) textured substrate, electrolyte B; (d) polished substrate, electrolyte B. S.N. Sharma et al. / Physica E 28 (2005) 264–272270 nanocrystalline Si causes shrinkage of the Si-core due to the breaking of Si–Si bonds resulting in a blue-shift in PL spectra [11]. However, apart from interpretation in terms of quantum confinement in silicon clusters that decrease in size upon oxida- tion, the PL blue shift can also be related to Si–O species or due to defects and the silica networks on which OH groups are absorbed as suggested by others [23]. These results are in accordance with our PL and SEM studies where a significant PL decay and cracked surface morphology was observed for PS films formed on polished sub- strates which underlines the importance of tex- tured substrates and ethanol-based PS films which exhibits stable PL, smooth surface morphology and H-passivated surfaces. Previous measurements showed that using H 2 O 2 in a HF-based electrolytic mixture results in the termination of Si surfaces mainly with silicon- monohydrides leading to the formation of stable and low defect density PS films [12]. However, contrary to other studies, we have found that ethanol-based PS films formed on textured sub- strates are relatively more mechanically strong, stable, stress-free and highly passivated with hydrogen than the corresponding H 2 O 2 -based PS films as elucidated by our weight loss measure- ments, PL, SEM and FTIR studies. It seems that the improved luminescent properties of our PS films is more an artifact of the substrate (textured one) rather than that of the electrolyte alone. On the textured surface, the nucleation of nanopores is preferentially initiated at the boundaries be- tween the pyramids. This would be assisted by the slower pore growth [23] on the denser /111S faceted surfaces compared to the /100S surface exposed at the boundaries. This may lead to partial merging of nanopores and the formation of a high porosity region which can deform and release the stress at dimensions small enough to prevent macroscopic crack formation and fragility. Thus high porosity of PS films formed on textured substrates can be explained. However, in case of PS films formed on polished substrates, the etching is not preferential but random thus resulting in lower porosity of PS layers. However, the proper choice of both the substrate (textured) and the electrolyte (ethanol-based) in conjunction can have a profound effect in improving the luminescent properties and stability of porous silicon films. 4. Conclusions The visual observation of mechanically strong, stable surface bond configuration, smooth surface morphology and hydrogen-passivated PS surfaces essentially conforms the viability of textured substrates and ethanol-based electrolyte as a requisite condition for the formation of highly luminescent, thick and stable porous silicon films. Porous silicon using ethanol-based electrolyte is superior to porous silicon formed using H 2 O 2 - based electrolyte at the same current density on both textured and polished substrates, respec- tively. A proper choice of a substrate and an electrolyte are essential for the formation of highly porous silicon films with lower fragility, superior stability and long-term usability. Acknowledgements We thank Director NPL for permission to publish this work supported by CSIR network project on custom tailored special materials. RKS thanks CSIR for providing a research associate- ship. We acknowledge the help of Dr. Ramkishore and Shri. K.N. 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Ruckschloss, T. Wirschem, S. Veprek, Appl. Phys. Lett. 65 (1994) 1537. ARTICLE IN PRESS S.N. Sharma et al. / Physica E 28 (2005) 264–272272 . of our PS films is more an artifact of the substrate (textured one) rather than that of the electrolyte alone. On the textured surface, the nucleation of. improving the luminescent properties and stability of porous silicon films. 4. Conclusions The visual observation of mechanically strong, stable surface bond configuration,