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The role of hydrogen in the formation of porous structures in silicon
Materials Science and Engineering B58 (1999) 95 – 99
The role of hydrogen in the formation of porous structures in silicon Vitali Parkhutik a,*, Eduardo Andrade Ibarra b b
a Technical Uni6ersity of Valencia, Cami de Vera s/n, 46071 Valencia, Spain Institute of Physics, National Autonomous Uni6ersity of Mexico, Mexico City DF, Mexico
Abstract The role of hydrogen in the formation of porous silicon (PS) structures is assumed to passivate dangling bonds of the surface silicon atoms. The formation of surface hydride complexes SiHx (x =1,2,3) is well documented using infrared absorption spectroscopy and other methods of chemical analysis. In the present work we show by means of infrared spectroscopy of PS films subjected to different post-anodising chemical treatments that hydrogen atoms are incorporated into the corroding silicon wafer through easy paths which are generated in the vicinity of the pore tips as a result of the combined action of the electrolyte solution and dynamic mechanical stress generated during the dissolution reaction. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon structures; Surface hydride complexes; Dynamic mechanical stress
1. Introduction Porous silicon (PS), the material which is produced under electrochemical dissolution of crystalline Si wafers in hydrofluoric acid solutions has attracted much research recently during the perspectives of its implementation in silicon-based LEDs. PS is known for its bright red photoluminescence, although the parent material, monocrystalline silicon, luminesces in infrared and quite inefficiently due to the indirect energy band [1]. Among the various possible mechanisms elaborated until now, the preference is given to the radiative recombination of the electron-hole pairs inside the nanocrystals of silicon embedded into the pore walls [2]. Owing to the extremely small size of nanocrystals their energy gaps are widened as a result of quantum confinement of carriers, so the 1.1 eV gap of monocrystalline silicon grows gradually to 1.6 – 2.1 eV. There are also some concurrent mechanisms speculating about the photoluminescence in terms of hydride covering of the pore walls, oxygen-related surface complexes, etc. [3]. One of the problems to solve on a way of producing PS-based LEDs is a low ambiental stability of the material. Its luminescence, electrical conductivity and other properties are strongly influenced by the time of * Corresponding author.
ageing and type of the ambience [4]. Particularly important is the role of hydrogen atoms in ageing stability of the properties of the PS layers. It is well documented by IR analysis and other analytical techniques that the surface of freshly grown material is passivated by the layer of hydride SiHx (x=1,2,3) which stabilises the surface and even was suggested as the chemical species responsible for the luminescence [5]. Striking evidence of the presence of SiHx passive layer is the triple infrared band at 2100 cm − 1 ascribed to the stretching vibrations of differently co-ordinated silicon hydrides and the line at 910 cm − 1 assigned to bending vibration of SiH2 group. The band at 630 cm − 1 which is ascribed to bending vibration of Si–H is very close to that of Si–Si bond [6]. It is well known that the environmental ageing produces a layer of surface oxide at the pore walls and corresponding increase of the intensity of the IR band at about 1100 cm − 1, a signature of Si–O–Si stretching vibration. Oxidation is a natural process in Si chemistry and it proceeds either due to the incorporation of atomic oxygen into the Si matrix or, in the electrochemical environment, proceeds through the formation of silanole groups and inward migration of negatively charged oxygen ions [6]. The band at 2100 cm − 1 line remains very stable during the oxidation process, only its intensity is re-
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 2 9 7 - 9
96
V. Parkhutik, E. Andrade Ibarra / Materials Science and Engineering B58 (1999) 95–99
duced and a small replica of the line appears at about 2250 cm − 1. This fact is interpreted in terms of oxygen backbonding to Si surface atoms while hydride complexes remain intact. In the present work we have tested the hypothesis of surface-located hydrogen complexes by subjecting the freshly formed PS layers to different post-anodising treatments, such as soaking and boiling in the heavy water, catodic and anodic polarisation of PS in KF dissolved in heavy water, dissolution in alkaline bath.
2. Experimental The majority of the experiments were performed on p + type Si (100) wafers, provided with aluminium contact at backside. The operation of the PS growth was conducted in three electrode cell with both reference and counter-electrodes made of platinum. Polarisation source was PG273A potentiogalvanostat. Anodisation regime was: 50 mA cm − 2 anodic current density and 50–500 s anodising time. Electrolyte for PS formation was conventional HF/H2O/C2H5OH= 1/1/2 solution made of analytically graded chemicals, while heavy water used in post-anodising processing of PS was 99.8% pure. As-formed PS layers possessed low porosity (50–60%) and had smooth surfaces. Infrared analysis was performed using System 2000 spectrofluorimeter (Perkin Elmer — equipped with variable angle specular reflectance accessory). The analysis was performed immediately after the preparation of the samples to avoid ageing (whenever it was necessary). Fig. 1 shows the example of the FT-IR spectrum taken
Fig. 1. Raw FT-IR spectrum of absorption-reflection from the PS layer grown at p + Si. Oscillations of intensity are due to interference of IR beams reflected from the internal and external interfaces of PS layer.
Fig. 2. Change of chemical composition of PS film during its ageing in D2O: 1. IR spectrum of fresh sample. 2. The same after 7 days storage in D2O at room temperature. 3. The same after storage in boiling D2O during 6 h.
from the freshly prepared PS sample. Typical feature is the presence of large interference fringes at the spectrum, coming as a result of the interference of the parts of IR beam reflected from internal and external surface of the PS film. These fringes are useful to obtain the information about the medium thickness of the layer and its refractive index, using existing methods of geometrical optics. Our samples corresponded to 3–20 mm medium thickness of porous layer, depending on the current density and anodisation time used. To obtain the information on the informative IR bands, the spectra were processed using the software ProSpect developed in Technical University of Valencia for this purpose [7]. The spectrum presented in Fig. 1, after processing with ProSpect takes the shape shown in Fig. 2 (curve 1). As is seen in Fig. 2, the dominating IR lines in the IR spectra of PS material are those belonging to SiHx complexes, namely, the triplet at 2100 cm − 1 (stretching vibrations), and narrow line at 910 cm − 1 (bending vibration assigned to SiH2 complex) and other characteristic lines. There is also a variety of bands due to the presence of C–Hx groups (coming either from ethanol additives in electrolyte or from hydrocarbon contaminants). Only minor traces of oxide can be detected. This freshly obtained PS was subjected to different post-anodising treatments. First, we used a storage in a heavy water at room temperature during prolonged
V. Parkhutik, E. Andrade Ibarra / Materials Science and Engineering B58 (1999) 95–99
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Fig. 3. Change of the chemical composition of PS film during its electrochemical treatment in 1M KF dissolved in D2O: 1. IR spectrum of fresh sample. 2. The same after anodic polarisation of PS at 20 mA during 100s. 3. The same after cathodic polarisation at Uc = − 3V during 100 s. The insert shows the parts of the IR spectra corresponding to Si – D complexes.
time. This treatment was undertaken with the intention to observe, if any, isotopic exchange between surface hydrogen and deuterium atoms. The result of this treatment was heavy oxidation of the PS material. Fig. 2 shows a continuous growth of the 1100 cm − 1 band corresponding to Si– O – Si stretching vibration. However the effect of the prolonged contact of the sample in D2O is producing just minor impact onto the line 2100 cm − 1, showing that the hydride layer withstands this treatment efficiently. Boiling in D2O, although changing drastically the appearance of the sample and provoking even more heavy oxidation of PS layer, is still not able to affect the hydride significantly (curve 3 in Fig. 2). Other treatment we used to try to remove hydride layer was cathodic and anodic polarisation of PS in the solution of 1M KF in D2O. The results are shown in Fig. 3. In both processes (cathodic and anodic polarisation at rather high potentials) the result was drastic changes of PS appearance but the band 2100 cm − 1 was only reduced, it didn’t disappear altogether. The presence of Si–D bonds in the composition of treated materials was also detected (insert in Fig. 3) but their density was rather weak as compared with those of silicon dioxide and hydride. The bands correspond to Si vacancy in the crystal lattice with 1, 2 and 3 captured deuterium atoms [8].
Finally, we have processed the PS layer in alkaline solution (5% NaOH) which efficiently dissolves the material. Although, in effect, PS samples were undergoing rather fast chemical dissolution through oxidation of silicon and its removal, the band of hydride at 2100 cm − 1 did not weep off from the IR spectra.
3. Discussion
3.1. Nature of silicon hydride phase in porous silicon Hydrogen is known to incorporate into the growing PS layer and form hydride phases (presumably covering the surface of the pores). On the other hand, it has been known that environmental ageing and, according to the present results, harsh chemical treatments of PS result in its oxidation. The growth of corresponding IR band at 1100 cm − 1 produces little impact onto 2100 cm − 1 one associated with the Si–Hx complexes. This fact quite logically forces a conclusion that the oxygen is incorporated to the Si backbonds without being adsorbed at Si surface. This is a contradiction with the mechanism of the oxide growth on Si which presumes that O atoms (or ions) are to be chemisorbed at Si surface before their incorporation into the oxide [8].
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V. Parkhutik, E. Andrade Ibarra / Materials Science and Engineering B58 (1999) 95–99
Fig. 4. The PS growth according to the mechanical stress-assisted model.
The contradiction can be overcome if one takes into consideration the fact which was not used by the specialists in PS, but is rather familiar to those who deal with the physics of hydrogen in crystalline silicon. Namely, the same 2100 – 2200 cm − 1 IR-bands which are ascribed to surface Si – Hx complexes are typical for Si atoms buried into the crystal volume [8]. It was shown by application of isotopical H lD shifts that the complexes Vacancy – Si – H in monocrystalline defective silicon yield the structure of the IR lines very similar to what is observed in PS [9]. Then it becomes clear that the line at 2100 cm − 1 will survive even very harsh treatments of PS (annealing up to 500–600°C) [10], chemical grafting of the surface [11 – 13], etc. These treatments leave intact Si – H bonds buried into the c –Si volume and into Si wires in the pore walls while all Si–H surface complexes at the pore walls are substituted by silanole groups or oxide molecules. Taking these circumstances in mind we would like to assume that the presence of 2100 cm − 1 band is not necessarily related to the hydride substance at the surface of the pores but can be associated with Si–H bonds buried into the pore walls or even incorporated into the Si crystalline matrix.
3.2. Stress-assisted model of porous silicon growth Based on these results it seems possible to suggest a new mechanism of the pore growth. The mechanism of pore propagation into the Si crystal is thought to involve not only electrochemical factors, but also mechanical stresses and hydrogen-related defects in Si. The role of crystal defects in the formation of PS, either generated in-situ (as the consequence of stresses acting in the PS/Si structure) or existing in Si crystal was never considered adequately.
We assume the existence of the dynamic stress accompanying the pore growth. At the bottom of each pore, dissolution reaction liberates essential amount of hydrogen (one molecule of hydrogen per each dissolved atom of Si). Local evolution of Joule heat would also contribute to the formation of vapour phase [14]. Outward movement of gas bubbles and products of Si dissolution produces an essential hydrodynamic pressure in the PS layer. According to the estimations made using the theory of propulsion of vapour–liquid phase in poroelastic media [15], the pressure can be as high (in the case of the pore sizes corresponding to PS) as 80–100 MPa. High tensile stresses are produced both in PS film (perpendicularly to its surface) and in Si substrates (lateral direction). One could expect the possibility of the formation of horizontal microcracks in PS (and they are seen, indeed in many PS morphologies, see for example [16]). Moreover, tensile stress should provoke the formation of microdefects in Si wafer right in front of each pore which are serving as easy path for further pore growth. The pressure is of dynamic character, it disappears when the anodisation process is cut off and its value depends on the applied electric field. Until now the dominating point of view was that the stress in PS is due to its contact with environment [17]. It was assumed that PS may experience either compressive or tensile stress imposed by surface tension forces appearing as a result of the interaction of the pore wall material with absorbed gases or vapours. According to the stress-assisted model of the PS growth the development of the pore should be assumed somehow similarly to hydrogen embrittlement of the metals [18]. The stages of the process are schematically represented in Fig. 4. Hydrogen atoms, either present in the Si volume or injected from electrolyte, would flow to the vicinity of microcrack (that forms in the vicinity
V. Parkhutik, E. Andrade Ibarra / Materials Science and Engineering B58 (1999) 95–99
of the tip of the real pore due to the presence of tensile stress) to reduce their chemical potential in the tensile stress field (Fig. 4a). This results in the formation of silicon hydride at the tip of the propagating pore (b). The hydride region offers a pathway for pore propagation due to its lower mechanical stability (cleavage) and due to its enhanced chemical activity towards HF (dissolution) (c). As a result, pore is propagating inward the Si volume following not only the direction of current lines, but also the paths of easy defect propagation in the Si crystal (usually aligned with B 100 \ direction) and perpendicularly to it. Recently Allonque et al. [19], have shown that the hydrogen atoms not only passivate the surface of silicon undergoing the reaction of the electrochemical dissolution, but also incorporate into the volume of semiconductor. The mechanism of incorporation is thought to be a random diffusion of hydrogen from the surface to the bulk. According to our mechanism this should be a localised process enhanced by the generation of the defects aligned with the crystallographic directions of the easy defect propagation.
4. Conclusions We have shown that the formation of passive hydride layer in PS may be considered not as a surface but rather as a volume process. The survival of the characteristic band of silicon hydride in the IR spectra of heavily chemically damaged PS shows that hydride complexes are formed not only at the surface of the pore walls, but inside them and in the volume of semiconductor wafer. Both surface and volume hydride phases possess the same IR bands. We offer a new model of the pPS formation according to which the pore growth is assisted by the accumulation of significant stresses both in growing PS layer and underneath, in Si substrate as a result of the dynamic pressure generated inside the pores by evolved gases. The presence of tensile stress in the silicon matrix in the vicinity of the pore tips forces H atoms (either from the surface of the pores or from the volume of semiconductor) to migrate there. The driving force of
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99
this injection is a tensile stress, generated in the structure during the pore growth, hydrogen atoms reduce their energy in the stress field. Thus, the regions with the locally enhanced silicon hydride concentration are formed at the pore tips similarly to the mechanism of hydrogen embrittlment of metals. The local regions of silicon hydride serve as easy paths for further pore propagation, due to either their mechanical properties or enhanced solubility in HF solution. Thus, the pore growth is a self-maintained process, when the growing pore is preparing the path for its further development. The easy paths for the pore propagation are aligned with the crystallographic directions of easy defect propagation. References [1] L. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] P.D.J. Calcott, K.J. Nash, L.T. Canham, M.J. Kane, D. Brumhead, J. Lumin. 57 (1993) 257. [3] A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909. [4] V.P. Parkhutik, Electrochim. Acta 45 (1995) 556. [5] S.M. Prokes, W.E. Carlos, V.M. Bermudez, Appl. Phys. Lett. 61 (1992) 1447. [6] M.H. Brodsky, M. Cardona, J.J. Cuomo, Phys. Rev. B16 (1997) 3556. [7] V.P. Parkhutik, M. San Heronimo Martinez, E. Perez Gomez, J. Porous Materials, 1999, in press. [8] B. Bech Nielsen, L. Hoffmann, M. Budde, Mater. Sci. Eng. B 36 (1996) 259. [9] L.M. Xie, M.W. Qi, J.M. Chen, J. Phys., Condens. Matter 3 (1991) 8519. [10] V. Parkhutik, E.S. Matveeva, F. Namavar, J. Electrochem. Soc. 143 (1996) 3943. [11] M. Warntjes, C. Vieillard, F. Ozanam, J.N. Chazalviel, J. Electrochem. Soc. 142 (1995) 4138. [12] E. Lee, J.S. Ha, M. Saylor, J. Am. Chem. Soc. 117 (1996) 8295. [13] J.M. Buriak, M.J. Allen, J. Amer. Chem. Soc. 120 (1998) 1339. [14] J.P. Sullivan, G.C. Wood, Proc. Roy. Soc. Lond. A317 (1970) 511. [15] S. Lee, N.J. Salamon, R.M. Sullivan, J. Thermophys. Heat Transf. 10 (1996) 672. [16] S.F. Chuang, S.D. Collins, R. Smith, Appl. Phys. Lett. 55 (1989) 675. [17] G. Dolino, D. Bellet, C. Faivre, Phys. Rev. B 54 (1996) 17919. [18] H.K. Birnbaum, in: R. Gibala, R.F. Hehemann (Eds.), Hydrogen Embrittlement and Stress Corrosion Cracking, American Society for Metals, 1984, p. 153. [19] P. Allongue, C. Henry de Villeneuve, L. Pinsard, M.C. Bernard, Appl. Phys. Lett. 67 (1995) 941.
The role of hydrogen in the formation of porous structures in silicon Vitali Parkhutik a,*, Eduardo Andrade Ibarra b b
a Technical Uni6ersity of Valencia, Cami de Vera s/n, 46071 Valencia, Spain Institute of Physics, National Autonomous Uni6ersity of Mexico, Mexico City DF, Mexico
Abstract The role of hydrogen in the formation of porous silicon (PS) structures is assumed to passivate dangling bonds of the surface silicon atoms. The formation of surface hydride complexes SiHx (x =1,2,3) is well documented using infrared absorption spectroscopy and other methods of chemical analysis. In the present work we show by means of infrared spectroscopy of PS films subjected to different post-anodising chemical treatments that hydrogen atoms are incorporated into the corroding silicon wafer through easy paths which are generated in the vicinity of the pore tips as a result of the combined action of the electrolyte solution and dynamic mechanical stress generated during the dissolution reaction. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon structures; Surface hydride complexes; Dynamic mechanical stress
1. Introduction Porous silicon (PS), the material which is produced under electrochemical dissolution of crystalline Si wafers in hydrofluoric acid solutions has attracted much research recently during the perspectives of its implementation in silicon-based LEDs. PS is known for its bright red photoluminescence, although the parent material, monocrystalline silicon, luminesces in infrared and quite inefficiently due to the indirect energy band [1]. Among the various possible mechanisms elaborated until now, the preference is given to the radiative recombination of the electron-hole pairs inside the nanocrystals of silicon embedded into the pore walls [2]. Owing to the extremely small size of nanocrystals their energy gaps are widened as a result of quantum confinement of carriers, so the 1.1 eV gap of monocrystalline silicon grows gradually to 1.6 – 2.1 eV. There are also some concurrent mechanisms speculating about the photoluminescence in terms of hydride covering of the pore walls, oxygen-related surface complexes, etc. [3]. One of the problems to solve on a way of producing PS-based LEDs is a low ambiental stability of the material. Its luminescence, electrical conductivity and other properties are strongly influenced by the time of * Corresponding author.
ageing and type of the ambience [4]. Particularly important is the role of hydrogen atoms in ageing stability of the properties of the PS layers. It is well documented by IR analysis and other analytical techniques that the surface of freshly grown material is passivated by the layer of hydride SiHx (x=1,2,3) which stabilises the surface and even was suggested as the chemical species responsible for the luminescence [5]. Striking evidence of the presence of SiHx passive layer is the triple infrared band at 2100 cm − 1 ascribed to the stretching vibrations of differently co-ordinated silicon hydrides and the line at 910 cm − 1 assigned to bending vibration of SiH2 group. The band at 630 cm − 1 which is ascribed to bending vibration of Si–H is very close to that of Si–Si bond [6]. It is well known that the environmental ageing produces a layer of surface oxide at the pore walls and corresponding increase of the intensity of the IR band at about 1100 cm − 1, a signature of Si–O–Si stretching vibration. Oxidation is a natural process in Si chemistry and it proceeds either due to the incorporation of atomic oxygen into the Si matrix or, in the electrochemical environment, proceeds through the formation of silanole groups and inward migration of negatively charged oxygen ions [6]. The band at 2100 cm − 1 line remains very stable during the oxidation process, only its intensity is re-
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 2 9 7 - 9
96
V. Parkhutik, E. Andrade Ibarra / Materials Science and Engineering B58 (1999) 95–99
duced and a small replica of the line appears at about 2250 cm − 1. This fact is interpreted in terms of oxygen backbonding to Si surface atoms while hydride complexes remain intact. In the present work we have tested the hypothesis of surface-located hydrogen complexes by subjecting the freshly formed PS layers to different post-anodising treatments, such as soaking and boiling in the heavy water, catodic and anodic polarisation of PS in KF dissolved in heavy water, dissolution in alkaline bath.
2. Experimental The majority of the experiments were performed on p + type Si (100) wafers, provided with aluminium contact at backside. The operation of the PS growth was conducted in three electrode cell with both reference and counter-electrodes made of platinum. Polarisation source was PG273A potentiogalvanostat. Anodisation regime was: 50 mA cm − 2 anodic current density and 50–500 s anodising time. Electrolyte for PS formation was conventional HF/H2O/C2H5OH= 1/1/2 solution made of analytically graded chemicals, while heavy water used in post-anodising processing of PS was 99.8% pure. As-formed PS layers possessed low porosity (50–60%) and had smooth surfaces. Infrared analysis was performed using System 2000 spectrofluorimeter (Perkin Elmer — equipped with variable angle specular reflectance accessory). The analysis was performed immediately after the preparation of the samples to avoid ageing (whenever it was necessary). Fig. 1 shows the example of the FT-IR spectrum taken
Fig. 1. Raw FT-IR spectrum of absorption-reflection from the PS layer grown at p + Si. Oscillations of intensity are due to interference of IR beams reflected from the internal and external interfaces of PS layer.
Fig. 2. Change of chemical composition of PS film during its ageing in D2O: 1. IR spectrum of fresh sample. 2. The same after 7 days storage in D2O at room temperature. 3. The same after storage in boiling D2O during 6 h.
from the freshly prepared PS sample. Typical feature is the presence of large interference fringes at the spectrum, coming as a result of the interference of the parts of IR beam reflected from internal and external surface of the PS film. These fringes are useful to obtain the information about the medium thickness of the layer and its refractive index, using existing methods of geometrical optics. Our samples corresponded to 3–20 mm medium thickness of porous layer, depending on the current density and anodisation time used. To obtain the information on the informative IR bands, the spectra were processed using the software ProSpect developed in Technical University of Valencia for this purpose [7]. The spectrum presented in Fig. 1, after processing with ProSpect takes the shape shown in Fig. 2 (curve 1). As is seen in Fig. 2, the dominating IR lines in the IR spectra of PS material are those belonging to SiHx complexes, namely, the triplet at 2100 cm − 1 (stretching vibrations), and narrow line at 910 cm − 1 (bending vibration assigned to SiH2 complex) and other characteristic lines. There is also a variety of bands due to the presence of C–Hx groups (coming either from ethanol additives in electrolyte or from hydrocarbon contaminants). Only minor traces of oxide can be detected. This freshly obtained PS was subjected to different post-anodising treatments. First, we used a storage in a heavy water at room temperature during prolonged
V. Parkhutik, E. Andrade Ibarra / Materials Science and Engineering B58 (1999) 95–99
97
Fig. 3. Change of the chemical composition of PS film during its electrochemical treatment in 1M KF dissolved in D2O: 1. IR spectrum of fresh sample. 2. The same after anodic polarisation of PS at 20 mA during 100s. 3. The same after cathodic polarisation at Uc = − 3V during 100 s. The insert shows the parts of the IR spectra corresponding to Si – D complexes.
time. This treatment was undertaken with the intention to observe, if any, isotopic exchange between surface hydrogen and deuterium atoms. The result of this treatment was heavy oxidation of the PS material. Fig. 2 shows a continuous growth of the 1100 cm − 1 band corresponding to Si– O – Si stretching vibration. However the effect of the prolonged contact of the sample in D2O is producing just minor impact onto the line 2100 cm − 1, showing that the hydride layer withstands this treatment efficiently. Boiling in D2O, although changing drastically the appearance of the sample and provoking even more heavy oxidation of PS layer, is still not able to affect the hydride significantly (curve 3 in Fig. 2). Other treatment we used to try to remove hydride layer was cathodic and anodic polarisation of PS in the solution of 1M KF in D2O. The results are shown in Fig. 3. In both processes (cathodic and anodic polarisation at rather high potentials) the result was drastic changes of PS appearance but the band 2100 cm − 1 was only reduced, it didn’t disappear altogether. The presence of Si–D bonds in the composition of treated materials was also detected (insert in Fig. 3) but their density was rather weak as compared with those of silicon dioxide and hydride. The bands correspond to Si vacancy in the crystal lattice with 1, 2 and 3 captured deuterium atoms [8].
Finally, we have processed the PS layer in alkaline solution (5% NaOH) which efficiently dissolves the material. Although, in effect, PS samples were undergoing rather fast chemical dissolution through oxidation of silicon and its removal, the band of hydride at 2100 cm − 1 did not weep off from the IR spectra.
3. Discussion
3.1. Nature of silicon hydride phase in porous silicon Hydrogen is known to incorporate into the growing PS layer and form hydride phases (presumably covering the surface of the pores). On the other hand, it has been known that environmental ageing and, according to the present results, harsh chemical treatments of PS result in its oxidation. The growth of corresponding IR band at 1100 cm − 1 produces little impact onto 2100 cm − 1 one associated with the Si–Hx complexes. This fact quite logically forces a conclusion that the oxygen is incorporated to the Si backbonds without being adsorbed at Si surface. This is a contradiction with the mechanism of the oxide growth on Si which presumes that O atoms (or ions) are to be chemisorbed at Si surface before their incorporation into the oxide [8].
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V. Parkhutik, E. Andrade Ibarra / Materials Science and Engineering B58 (1999) 95–99
Fig. 4. The PS growth according to the mechanical stress-assisted model.
The contradiction can be overcome if one takes into consideration the fact which was not used by the specialists in PS, but is rather familiar to those who deal with the physics of hydrogen in crystalline silicon. Namely, the same 2100 – 2200 cm − 1 IR-bands which are ascribed to surface Si – Hx complexes are typical for Si atoms buried into the crystal volume [8]. It was shown by application of isotopical H lD shifts that the complexes Vacancy – Si – H in monocrystalline defective silicon yield the structure of the IR lines very similar to what is observed in PS [9]. Then it becomes clear that the line at 2100 cm − 1 will survive even very harsh treatments of PS (annealing up to 500–600°C) [10], chemical grafting of the surface [11 – 13], etc. These treatments leave intact Si – H bonds buried into the c –Si volume and into Si wires in the pore walls while all Si–H surface complexes at the pore walls are substituted by silanole groups or oxide molecules. Taking these circumstances in mind we would like to assume that the presence of 2100 cm − 1 band is not necessarily related to the hydride substance at the surface of the pores but can be associated with Si–H bonds buried into the pore walls or even incorporated into the Si crystalline matrix.
3.2. Stress-assisted model of porous silicon growth Based on these results it seems possible to suggest a new mechanism of the pore growth. The mechanism of pore propagation into the Si crystal is thought to involve not only electrochemical factors, but also mechanical stresses and hydrogen-related defects in Si. The role of crystal defects in the formation of PS, either generated in-situ (as the consequence of stresses acting in the PS/Si structure) or existing in Si crystal was never considered adequately.
We assume the existence of the dynamic stress accompanying the pore growth. At the bottom of each pore, dissolution reaction liberates essential amount of hydrogen (one molecule of hydrogen per each dissolved atom of Si). Local evolution of Joule heat would also contribute to the formation of vapour phase [14]. Outward movement of gas bubbles and products of Si dissolution produces an essential hydrodynamic pressure in the PS layer. According to the estimations made using the theory of propulsion of vapour–liquid phase in poroelastic media [15], the pressure can be as high (in the case of the pore sizes corresponding to PS) as 80–100 MPa. High tensile stresses are produced both in PS film (perpendicularly to its surface) and in Si substrates (lateral direction). One could expect the possibility of the formation of horizontal microcracks in PS (and they are seen, indeed in many PS morphologies, see for example [16]). Moreover, tensile stress should provoke the formation of microdefects in Si wafer right in front of each pore which are serving as easy path for further pore growth. The pressure is of dynamic character, it disappears when the anodisation process is cut off and its value depends on the applied electric field. Until now the dominating point of view was that the stress in PS is due to its contact with environment [17]. It was assumed that PS may experience either compressive or tensile stress imposed by surface tension forces appearing as a result of the interaction of the pore wall material with absorbed gases or vapours. According to the stress-assisted model of the PS growth the development of the pore should be assumed somehow similarly to hydrogen embrittlement of the metals [18]. The stages of the process are schematically represented in Fig. 4. Hydrogen atoms, either present in the Si volume or injected from electrolyte, would flow to the vicinity of microcrack (that forms in the vicinity
V. Parkhutik, E. Andrade Ibarra / Materials Science and Engineering B58 (1999) 95–99
of the tip of the real pore due to the presence of tensile stress) to reduce their chemical potential in the tensile stress field (Fig. 4a). This results in the formation of silicon hydride at the tip of the propagating pore (b). The hydride region offers a pathway for pore propagation due to its lower mechanical stability (cleavage) and due to its enhanced chemical activity towards HF (dissolution) (c). As a result, pore is propagating inward the Si volume following not only the direction of current lines, but also the paths of easy defect propagation in the Si crystal (usually aligned with B 100 \ direction) and perpendicularly to it. Recently Allonque et al. [19], have shown that the hydrogen atoms not only passivate the surface of silicon undergoing the reaction of the electrochemical dissolution, but also incorporate into the volume of semiconductor. The mechanism of incorporation is thought to be a random diffusion of hydrogen from the surface to the bulk. According to our mechanism this should be a localised process enhanced by the generation of the defects aligned with the crystallographic directions of the easy defect propagation.
4. Conclusions We have shown that the formation of passive hydride layer in PS may be considered not as a surface but rather as a volume process. The survival of the characteristic band of silicon hydride in the IR spectra of heavily chemically damaged PS shows that hydride complexes are formed not only at the surface of the pore walls, but inside them and in the volume of semiconductor wafer. Both surface and volume hydride phases possess the same IR bands. We offer a new model of the pPS formation according to which the pore growth is assisted by the accumulation of significant stresses both in growing PS layer and underneath, in Si substrate as a result of the dynamic pressure generated inside the pores by evolved gases. The presence of tensile stress in the silicon matrix in the vicinity of the pore tips forces H atoms (either from the surface of the pores or from the volume of semiconductor) to migrate there. The driving force of
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