- Email: [email protected]
Demonstration of the formation of porous silicon films with superior mechanical properties, morphology and stability
Materials Letters 60 (2006) 1166 – 1169 www.elsevier.com/locate/matlet
Demonstration of the formation of porous silicon films with superior mechanical properties, morphology and stability Shailesh N. Sharma
b
a,⁎
, R.K. Sharma a , G. Bhagavannarayana b , S.B. Samanta c , K.N. Sood b , S.T. Lakshmikumar a
a Electronic Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110 012, India Materials Characterization Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110 012, India c Superconductivity Department, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110 012, India
Received 7 March 2005; accepted 29 October 2005 Available online 28 November 2005
Abstract Highly stable and mechanically strong thick porous silicon (PS) films have been obtained on textured silicon substrates. Porous silicon formed on textured substrates exhibits higher porosity, better mechanical strength, non-fractured surface morphology and lower stress compared to porous silicon formed on polished silicon substrates at the same current density, time of anodization and method of drying. The improved properties are attributed to the formation of localized highly porous macroscopic plastic regions. © 2005 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Mechanical properties; Morphology; Current density; Surfaces; Porosity
1. Introduction PS has been investigated for many optical and opto-electronic applications because of the easiness, simplicity and integratibility with the highly advanced Si technology. The quantum nature of Si nanostructures is the key to the development of the future nano-electronics [1,2]. In PS films, Si nanocrystals remain attached to the Si substrate after partial electrochemical dissolution and are surrounded by large pores. The large surface area and the presence of large number of unpaired dangling bonds alter the surface reactivity and stability of PS [3,4]. The presence of pores also causes lattice expansion and leads to elastic stress and curvature which often leads to fragility and long-term failure [5]. Very few reports are there which has demonstrated the role of the surface morphology of silicon wafer on the properties of PS [6]. For use as an anti-reflection coating, thin porous silicon layers were formed earlier on textured Si solar cells [7]. However, an ⁎ Corresponding author. Tel.: +91 11 25742610 12; fax: +91 11 25726938, 25726952. E-mail address: [email protected] (S.N. Sharma). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.101
intensive study on the properties of PS formed on textured substrates and its effect on the stability of PS appears to be lacking. In this work, we have demonstrated that the texturization of silicon surface is a simple and effective method for the formation of thick films of porous silicon with reduced stress, improved stability and superior mechanical properties. SEM, AFM and XRD measurements have been used to demonstrate the superiority of porous silicon films formed on textured substrates. 2. Experimental PS was formed by electrochemical anodization process on boron doped (100) p-Si wafers (8–10 Ω cm, 400 μm thick). Silicon wafer is used as the anode and Pt as the counter electrode in an acid resistant cell. PS films were made on both polished and textured substrates with current densities between 10 to 50 mA cm− 2 range for 30 min in HF–C2H5OH of 1 : 1 volume ratio. The films were washed in deionized water and dried in nitrogen after anodization. Textured substrates were made using 2% NaOH at 85 °C for 30 min. Formation of a proper texturized surface with pyramidal morphology was
S.N. Sharma et al. / Materials Letters 60 (2006) 1166–1169
1167
confirmed by SEM using a LEO 440 scanning electron microscope. A multicrystal X-ray diffractometer developed at NPL employing a fine focus X-ray generator has been used to determine local radius of curvature of PS films [8]. 3. Results and discussion Visual observation shows that the porous silicon films formed on textured surfaces appear more compact, uniform and strong even at higher current densities while the PS films formed on polished substrates appears to be rough, powdery and had a tendency of peeling off from the substrate particularly at high current densities. It has also been observed that the absolute photoluminescence (PL) intensity is higher and PL decay is lower for the porous silicon formed on textured substrates than that formed on polished substrates, the details of which would be published elsewhere [9]. The porosity and thickness of PS films are estimated from gravimetric measurements [1]. These parameters for both the kinds of films are summarized in Table 1. It is observed that both porosity and thickness of PS films increases with increase in current density, however, the PS films formed on textured substrates exhibits higher porosity as compared to the corresponding films formed on polished substrates. When subjected to ultrasonic cleaning for 60 min as a destructive test of the film strength, porous silicon formed on the polished substrates easily disintegrates while porous silicon formed on textured silicon even at high current densities is not affected. Fig. 1 shows the weight loss of PS films prepared using electrolyte HF : C2H5OH (1 : 1 by volume) at different Id, as a function of time of ultrasonic treatment. There is a substantial weight loss of polished PS samples when subjected to an ultrasonic treatment 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 current density owing to higher porosity and this effect is felt more for PS films prepared on polished substrates (Fig 1). SEM studies were done in order to probe the microstructural variations in PS formed on textured and polished Si-substrates at a typical Id ∼20 mA cm− 2. Fig. 2 [(a) and (b)] shows SEM micrographs of polished silicon substrate without any PS formation and with PS formation at Id ∼20 mA cm− 2. Polished silicon substrate shows a plain surface morphology while a cracked surface morphology is obtained for PS on polished substrate (Fig. 2(b)). For the textured substrate without any PS formation, the surface morphology consists of randomly sized and spaced pyramids homogeneously distributed on the surface (Fig. 2(c)) and for the PS formed on textured substrate, the surface morphology does not essentially differ from the textured silicon substrate (Fig. 2(d)). Here, there is no evidence of any fracture or cracks formation unlike in the case of polished silicon substrate for the same current density. Even for PS films formed on textured substrates Table 1 Values of the thickness (μm) and porosity (%) of PS films formed on textured and polished substrates Substrate
Current density Id (mA cm− 2)
Thickness (μm)
Porosity (%)
Textured Textured Textured Textured Polished Polished Polished Polished
10 20 35 50 10 20 35 50
12 25 41 55 40 55 73 89
52 60 70 76 46 49 65 70
Fig. 1. Weight loss of porous silicon samples prepared at different current densities (Id ); (a) textured substrate, Id = 20 mA cm− 2; (b) polished substrate, Id = 20 mA cm− 2; (c) textured substrate, Id = 35 mA cm− 2; (d) polished substrate, Id = 35 mA cm− 2; (e) textured substrate, Id = 50 mA cm− 2 and (f) polished substrate, Id = 50 mA cm− 2.
at higher current densities, Id ≥ 50 mA cm− 2, the surface morphology remains the same and this indicates lower stress for PS films formed on textured silicon substrates. At higher magnification for PS film formed on textured substrate, it is noticed that the pyramids are sharply separated and the regions between the pyramids show deep etching and possibly higher porosity or larger pore size and macroscopic cracking is not observed (Fig. 2(e)). This result can be confirmed from the cross-sectional view SEM micrograph (Fig. 2(f)) which reveals that pore (diameter between ∼50 and 100 nm) formation occurs in a unidirectional manner from the surface into the bulk, leading to aligned pores and columnar silicon structures. The PS layers also appeared thicker. This surface morphology is not affected by current density. In case of PS formed on polished substrates, the higher Id of 20 mA cm− 2 results in increased porosity and the inability of the silicon nanowires to withstand the stress leads to cracking. At further higher Id ≥ 35 mA cm− 2, a pronounced cracking pattern is observed. Similar observations on the fragility of thick and highly porous films had been noted earlier [2]. Fig. 3 (a) and (b) shows the AFM images of PS films formed on textured and polished substrates, respectively. In Fig. 3(a), the presence of large 100 nm size pores are indicated at some regions. This is not seen in case of PS films formed on polished substrates (Fig. 3(b)). The roughness factor is higher for PS films formed on textured substrates as compared to that on polished substrates indicating higher porosity (Fig. 3). The roughness factor of PS films increases with increase in current density for both textured and polished substrates as also observed by others [10]. The presence of porous region leads to the lattice expansion of the PS film and elastic stress results due to the lattice mismatch between the PS film and the substrate. The bending of porous silicon layer due to stress developed by the pores formed at Id ∼20 mA cm− 2 was determined for both polished and textured (100) silicon specimens. The radius of curvature was determined by the X-ray diffractometry method. The change in the orientation of the diffraction vector g for (400) planes was recorded in the symmetrical Bragg geometry as a function of the linear position of the wafer as it was traversed across the primary beam [11]. The initial (100) silicon blank wafers selected for the specimens were almost plane having radius of curvature of the order of a ∼1000 m. The plots (a) and (b) in Fig. 4 show the curvature plots for both polished and textured specimens, respectively. The positive slope for both the specimens indicate that the initial plane surface of the wafer has been bent into a convex shape (with respect to
1168
S.N. Sharma et al. / Materials Letters 60 (2006) 1166–1169
Fig. 2. Scanning electron micrographs of porous silicon; (a) polished substrate without PS formation; (b) polished substrate, Id = 20 mA cm− 2; (c) textured substrate without PS formation; (d) textured substrate, Id = 20 mA cm− 2 and (e) at higher magnification; textured substrate, Id = 20 mA cm− 2 and (f) cross-sectional view, textured substrate, Id = 20 mA cm− 2.
porous side) after anodization indicating the induced biaxial tensile stress due to pores. The significantly lower value of radius of curvature determined for as-polished sample (R = 35 m) compared to that of the textured sample (R = 192 m) confirms that the stress induced in textured specimens is significantly lesser compared to that of the polished
specimens. Similar values for the radius of curvature were also obtained by Astrova et al. [12] for as-grown PS films formed on (001) Si polished wafers using a similar technique. However, the actual value depends on the initial wafer bending, growth conditions, and particularly the current density, post growth treatment like annealing
Fig. 3. Typical AFM images of porous (100) silicon specimens prepared at Id = 20 mA cm− 2; (a) textured substrate, (b) polished substrate.
S.N. Sharma et al. / Materials Letters 60 (2006) 1166–1169
1169
region capable of absorbing the intrinsic stress in the porous silicon nanowires and aids in the formation of thick films with better mechanical strength and more stable surface bond configurations. The results conclusively demonstrate the viability of surface texturization as a simple recipe for the formation of thick and highly porous silicon films with superior mechanical properties and stability. Acknowledgements
Fig. 4. Typical curvature plots of porous (100) silicon specimens prepared at Id = 20 mA cm− 2; (a) textured substrate, (b) polished substrate.
We thank Director NPL for permission to publish this work supported by CSIR network project on custom tailored special materials. Acknowledgements are also due to Dr. Ram Kishore of the Electron Microscopy Group for help in SEM studies. RKS thanks CSIR for providing a research associateship.
etc. [13]. In this work, the effect of texturization on wafer bending between textured and polished specimens has been compared. Hence the higher radius of curvature can be attributed to the textured surface of the substrate. Earlier attempts to initiate the pore formation at specific locations have not been successful [14]. Pore size can be experimentally varied between several microns to a few nm but the pores are initiated at random locations. On the textured surface, the nucleation of nanopores may be preferentially initiated at the boundaries between the pyramids. This would be assisted by the slower pore growth [15] on the denser b111N faceted surfaces compared to the b100N 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. This is consistent with our surface morphology studies where a deep macroscopic porous region is found between the pyramids and thus high porosity of PS films formed on textured substrates can be understood.
References
4. Conclusions
[13]
From SEM, AFM and XRD results, the improved properties of PS films formed on textured substrates can be attributed to the formation of a material with large (100 nm or more) region between pyramids which permits the formation of a plastic
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[14] [15]
O. Bisi, Stefano Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1. A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909. S.T. Lakshmikumar, P.K. Singh, J. Appl. Phys. 92 (2002) 3413. S.N. Sharma, R. Banerjee, D. Das, S. Chattopadhyay, A.K. Barua, Appl. Surf. Sci. 182 (2001) 333. K. Barla, G. Bomchil, R. Herino, J.C. Pfister, J. Cryst. Growth 68 (1984) 721. S. Bandopadhyay, S.K. Datta, H. Saha, M.K. Mukherjee, Bull. Mater. Sci. 19 (1996) 725. R.J. Martin Palma, L. Vazquez, P. Herrero, J.M. Martinez-Duart, M. Schnell, S. Schaefer, Opt. Mater. 17 (2001) 75. K. Lal, G. Bhagvannarayana, J. Appl. Cryst. 22 (1989) 209. G. Bhagavannarayana, S.N. Sharma, R.K. Sharma and S.T. Lakshmikumar, Communicated). S. Zangooie, R. Jansson, H. Arwin, Appl. Surf. Sci. 136 (1998) 123. K. Lal, S.N.N. Goswami, J. Wurfl, H.L. Hartnagel, J. Appl. Phys. 67 (1990) 4105. E.V. Astrova, V.V. Ratnikov, A.D. Remenyuk, I.L. Shulpina, Semiconductors 36 (2002) 1033. K. Lal, R. Mitra, G. Srinivas, V.D. Vankar, J. Appl. Crystallogr. 29 (1996) 222. H. Foll, M. Christophersen, J. Carstensen, G. Hasse, Mater. Sci. Eng., R Rep. 39 (2002) 93. M. Guendouz, P. Joubert, M. Sarret, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 69-70 (2000) 43.
Demonstration of the formation of porous silicon films with superior mechanical properties, morphology and stability Shailesh N. Sharma
b
a,⁎
, R.K. Sharma a , G. Bhagavannarayana b , S.B. Samanta c , K.N. Sood b , S.T. Lakshmikumar a
a Electronic Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110 012, India Materials Characterization Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110 012, India c Superconductivity Department, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110 012, India
Received 7 March 2005; accepted 29 October 2005 Available online 28 November 2005
Abstract Highly stable and mechanically strong thick porous silicon (PS) films have been obtained on textured silicon substrates. Porous silicon formed on textured substrates exhibits higher porosity, better mechanical strength, non-fractured surface morphology and lower stress compared to porous silicon formed on polished silicon substrates at the same current density, time of anodization and method of drying. The improved properties are attributed to the formation of localized highly porous macroscopic plastic regions. © 2005 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Mechanical properties; Morphology; Current density; Surfaces; Porosity
1. Introduction PS has been investigated for many optical and opto-electronic applications because of the easiness, simplicity and integratibility with the highly advanced Si technology. The quantum nature of Si nanostructures is the key to the development of the future nano-electronics [1,2]. In PS films, Si nanocrystals remain attached to the Si substrate after partial electrochemical dissolution and are surrounded by large pores. The large surface area and the presence of large number of unpaired dangling bonds alter the surface reactivity and stability of PS [3,4]. The presence of pores also causes lattice expansion and leads to elastic stress and curvature which often leads to fragility and long-term failure [5]. Very few reports are there which has demonstrated the role of the surface morphology of silicon wafer on the properties of PS [6]. For use as an anti-reflection coating, thin porous silicon layers were formed earlier on textured Si solar cells [7]. However, an ⁎ Corresponding author. Tel.: +91 11 25742610 12; fax: +91 11 25726938, 25726952. E-mail address: [email protected] (S.N. Sharma). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.101
intensive study on the properties of PS formed on textured substrates and its effect on the stability of PS appears to be lacking. In this work, we have demonstrated that the texturization of silicon surface is a simple and effective method for the formation of thick films of porous silicon with reduced stress, improved stability and superior mechanical properties. SEM, AFM and XRD measurements have been used to demonstrate the superiority of porous silicon films formed on textured substrates. 2. Experimental PS was formed by electrochemical anodization process on boron doped (100) p-Si wafers (8–10 Ω cm, 400 μm thick). Silicon wafer is used as the anode and Pt as the counter electrode in an acid resistant cell. PS films were made on both polished and textured substrates with current densities between 10 to 50 mA cm− 2 range for 30 min in HF–C2H5OH of 1 : 1 volume ratio. The films were washed in deionized water and dried in nitrogen after anodization. Textured substrates were made using 2% NaOH at 85 °C for 30 min. Formation of a proper texturized surface with pyramidal morphology was
S.N. Sharma et al. / Materials Letters 60 (2006) 1166–1169
1167
confirmed by SEM using a LEO 440 scanning electron microscope. A multicrystal X-ray diffractometer developed at NPL employing a fine focus X-ray generator has been used to determine local radius of curvature of PS films [8]. 3. Results and discussion Visual observation shows that the porous silicon films formed on textured surfaces appear more compact, uniform and strong even at higher current densities while the PS films formed on polished substrates appears to be rough, powdery and had a tendency of peeling off from the substrate particularly at high current densities. It has also been observed that the absolute photoluminescence (PL) intensity is higher and PL decay is lower for the porous silicon formed on textured substrates than that formed on polished substrates, the details of which would be published elsewhere [9]. The porosity and thickness of PS films are estimated from gravimetric measurements [1]. These parameters for both the kinds of films are summarized in Table 1. It is observed that both porosity and thickness of PS films increases with increase in current density, however, the PS films formed on textured substrates exhibits higher porosity as compared to the corresponding films formed on polished substrates. When subjected to ultrasonic cleaning for 60 min as a destructive test of the film strength, porous silicon formed on the polished substrates easily disintegrates while porous silicon formed on textured silicon even at high current densities is not affected. Fig. 1 shows the weight loss of PS films prepared using electrolyte HF : C2H5OH (1 : 1 by volume) at different Id, as a function of time of ultrasonic treatment. There is a substantial weight loss of polished PS samples when subjected to an ultrasonic treatment 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 current density owing to higher porosity and this effect is felt more for PS films prepared on polished substrates (Fig 1). SEM studies were done in order to probe the microstructural variations in PS formed on textured and polished Si-substrates at a typical Id ∼20 mA cm− 2. Fig. 2 [(a) and (b)] shows SEM micrographs of polished silicon substrate without any PS formation and with PS formation at Id ∼20 mA cm− 2. Polished silicon substrate shows a plain surface morphology while a cracked surface morphology is obtained for PS on polished substrate (Fig. 2(b)). For the textured substrate without any PS formation, the surface morphology consists of randomly sized and spaced pyramids homogeneously distributed on the surface (Fig. 2(c)) and for the PS formed on textured substrate, the surface morphology does not essentially differ from the textured silicon substrate (Fig. 2(d)). Here, there is no evidence of any fracture or cracks formation unlike in the case of polished silicon substrate for the same current density. Even for PS films formed on textured substrates Table 1 Values of the thickness (μm) and porosity (%) of PS films formed on textured and polished substrates Substrate
Current density Id (mA cm− 2)
Thickness (μm)
Porosity (%)
Textured Textured Textured Textured Polished Polished Polished Polished
10 20 35 50 10 20 35 50
12 25 41 55 40 55 73 89
52 60 70 76 46 49 65 70
Fig. 1. Weight loss of porous silicon samples prepared at different current densities (Id ); (a) textured substrate, Id = 20 mA cm− 2; (b) polished substrate, Id = 20 mA cm− 2; (c) textured substrate, Id = 35 mA cm− 2; (d) polished substrate, Id = 35 mA cm− 2; (e) textured substrate, Id = 50 mA cm− 2 and (f) polished substrate, Id = 50 mA cm− 2.
at higher current densities, Id ≥ 50 mA cm− 2, the surface morphology remains the same and this indicates lower stress for PS films formed on textured silicon substrates. At higher magnification for PS film formed on textured substrate, it is noticed that the pyramids are sharply separated and the regions between the pyramids show deep etching and possibly higher porosity or larger pore size and macroscopic cracking is not observed (Fig. 2(e)). This result can be confirmed from the cross-sectional view SEM micrograph (Fig. 2(f)) which reveals that pore (diameter between ∼50 and 100 nm) formation occurs in a unidirectional manner from the surface into the bulk, leading to aligned pores and columnar silicon structures. The PS layers also appeared thicker. This surface morphology is not affected by current density. In case of PS formed on polished substrates, the higher Id of 20 mA cm− 2 results in increased porosity and the inability of the silicon nanowires to withstand the stress leads to cracking. At further higher Id ≥ 35 mA cm− 2, a pronounced cracking pattern is observed. Similar observations on the fragility of thick and highly porous films had been noted earlier [2]. Fig. 3 (a) and (b) shows the AFM images of PS films formed on textured and polished substrates, respectively. In Fig. 3(a), the presence of large 100 nm size pores are indicated at some regions. This is not seen in case of PS films formed on polished substrates (Fig. 3(b)). The roughness factor is higher for PS films formed on textured substrates as compared to that on polished substrates indicating higher porosity (Fig. 3). The roughness factor of PS films increases with increase in current density for both textured and polished substrates as also observed by others [10]. The presence of porous region leads to the lattice expansion of the PS film and elastic stress results due to the lattice mismatch between the PS film and the substrate. The bending of porous silicon layer due to stress developed by the pores formed at Id ∼20 mA cm− 2 was determined for both polished and textured (100) silicon specimens. The radius of curvature was determined by the X-ray diffractometry method. The change in the orientation of the diffraction vector g for (400) planes was recorded in the symmetrical Bragg geometry as a function of the linear position of the wafer as it was traversed across the primary beam [11]. The initial (100) silicon blank wafers selected for the specimens were almost plane having radius of curvature of the order of a ∼1000 m. The plots (a) and (b) in Fig. 4 show the curvature plots for both polished and textured specimens, respectively. The positive slope for both the specimens indicate that the initial plane surface of the wafer has been bent into a convex shape (with respect to
1168
S.N. Sharma et al. / Materials Letters 60 (2006) 1166–1169
Fig. 2. Scanning electron micrographs of porous silicon; (a) polished substrate without PS formation; (b) polished substrate, Id = 20 mA cm− 2; (c) textured substrate without PS formation; (d) textured substrate, Id = 20 mA cm− 2 and (e) at higher magnification; textured substrate, Id = 20 mA cm− 2 and (f) cross-sectional view, textured substrate, Id = 20 mA cm− 2.
porous side) after anodization indicating the induced biaxial tensile stress due to pores. The significantly lower value of radius of curvature determined for as-polished sample (R = 35 m) compared to that of the textured sample (R = 192 m) confirms that the stress induced in textured specimens is significantly lesser compared to that of the polished
specimens. Similar values for the radius of curvature were also obtained by Astrova et al. [12] for as-grown PS films formed on (001) Si polished wafers using a similar technique. However, the actual value depends on the initial wafer bending, growth conditions, and particularly the current density, post growth treatment like annealing
Fig. 3. Typical AFM images of porous (100) silicon specimens prepared at Id = 20 mA cm− 2; (a) textured substrate, (b) polished substrate.
S.N. Sharma et al. / Materials Letters 60 (2006) 1166–1169
1169
region capable of absorbing the intrinsic stress in the porous silicon nanowires and aids in the formation of thick films with better mechanical strength and more stable surface bond configurations. The results conclusively demonstrate the viability of surface texturization as a simple recipe for the formation of thick and highly porous silicon films with superior mechanical properties and stability. Acknowledgements
Fig. 4. Typical curvature plots of porous (100) silicon specimens prepared at Id = 20 mA cm− 2; (a) textured substrate, (b) polished substrate.
We thank Director NPL for permission to publish this work supported by CSIR network project on custom tailored special materials. Acknowledgements are also due to Dr. Ram Kishore of the Electron Microscopy Group for help in SEM studies. RKS thanks CSIR for providing a research associateship.
etc. [13]. In this work, the effect of texturization on wafer bending between textured and polished specimens has been compared. Hence the higher radius of curvature can be attributed to the textured surface of the substrate. Earlier attempts to initiate the pore formation at specific locations have not been successful [14]. Pore size can be experimentally varied between several microns to a few nm but the pores are initiated at random locations. On the textured surface, the nucleation of nanopores may be preferentially initiated at the boundaries between the pyramids. This would be assisted by the slower pore growth [15] on the denser b111N faceted surfaces compared to the b100N 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. This is consistent with our surface morphology studies where a deep macroscopic porous region is found between the pyramids and thus high porosity of PS films formed on textured substrates can be understood.
References
4. Conclusions
[13]
From SEM, AFM and XRD results, the improved properties of PS films formed on textured substrates can be attributed to the formation of a material with large (100 nm or more) region between pyramids which permits the formation of a plastic
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[14] [15]
O. Bisi, Stefano Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1. A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909. S.T. Lakshmikumar, P.K. Singh, J. Appl. Phys. 92 (2002) 3413. S.N. Sharma, R. Banerjee, D. Das, S. Chattopadhyay, A.K. Barua, Appl. Surf. Sci. 182 (2001) 333. K. Barla, G. Bomchil, R. Herino, J.C. Pfister, J. Cryst. Growth 68 (1984) 721. S. Bandopadhyay, S.K. Datta, H. Saha, M.K. Mukherjee, Bull. Mater. Sci. 19 (1996) 725. R.J. Martin Palma, L. Vazquez, P. Herrero, J.M. Martinez-Duart, M. Schnell, S. Schaefer, Opt. Mater. 17 (2001) 75. K. Lal, G. Bhagvannarayana, J. Appl. Cryst. 22 (1989) 209. G. Bhagavannarayana, S.N. Sharma, R.K. Sharma and S.T. Lakshmikumar, Communicated). S. Zangooie, R. Jansson, H. Arwin, Appl. Surf. Sci. 136 (1998) 123. K. Lal, S.N.N. Goswami, J. Wurfl, H.L. Hartnagel, J. Appl. Phys. 67 (1990) 4105. E.V. Astrova, V.V. Ratnikov, A.D. Remenyuk, I.L. Shulpina, Semiconductors 36 (2002) 1033. K. Lal, R. Mitra, G. Srinivas, V.D. Vankar, J. Appl. Crystallogr. 29 (1996) 222. H. Foll, M. Christophersen, J. Carstensen, G. Hasse, Mater. Sci. Eng., R Rep. 39 (2002) 93. M. Guendouz, P. Joubert, M. Sarret, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 69-70 (2000) 43.