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Fluorine and hydrogen effects on the growth and transport properties of microcrystalline silicon from SiF4 precursor
Journal of Non-Crystalline Solids 299–302 (2002) 113–117 www.elsevier.com/locate/jnoncrysol
Fluorine and hydrogen effects on the growth and transport properties of microcrystalline silicon from SiF4 precursor S. Kasouit
a,* , b
S. Kumar a, R. Vanderhaghen a, P. Roca i Cabarrocas a, I. French
b
a LPICM (UMR 7647-CNRS), Ecole Polytechnique, 91128 Palaiseau Cedex, France Philips Research Laboratories, Cross Oak Lane, Redhill, England, Surrey RH 5HA, UK
Abstract Structural and electrical properties of microcrystalline silicon (lc-Si:H) films grown at 200 °C from SiF4 precursor have been studied. A particular growth mechanism in which the material is completely crystallised from the initial stages of deposition is revealed by in situ ellipsometry measurements. Time resolved microwave conductivity (TRMC) measurements were performed on samples deposited at various conditions and an optimum in the total pressure and the hydrogen dilution which maximise the crystalline fraction and the transport properties was found. A mobility as high as 7 cm2 =V s was measured for a 0.14 lm thick sample. The nature of the substrate (a-SiN:H and SiO2 ) and its deposition temperature were found to affect the composition of the lc-Si:H deposited on it. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.Ac; 81.15.Gh
1. Introduction Microcrystalline silicon (lc-Si:H) is a promising material for device applications such as solar cells and thin film transistors. For the last application, a crystalline fraction higher than 95% and a mobility of a few cm2 =V s are required for a film thickness of less than 50 nm [1]. Thus, besides silane [2], several precursors such as SiH2 Cl2 [3] and SiF4 [4] have been studied. Because of their higher etching efficiency with respect to hydrogen, these precursors are expected to produce thin films with high
*
Corresponding author. E-mail address: [email protected] (S. Kasouit).
crystalline fraction and good transport properties. We have focused our study on the use of SiF4 which allows to produce films comparable to polycrystalline silicon [5]. To obtain a fully crystallised film as thin as possible, we have used in situ spectroscopic ellipsometry to study the thickness dependence of the film composition. Interestingly enough, we have observed a particular growth mechanism on glass substrates, resulting in the direct formation of a porous microcrystalline phase without any amorphous fraction. Moreover, with the aim of using this material as the active layer in bottom gate thin film transistors, it is important to control the effect of the dielectric substrate on the lc-Si:H properties. Thus we have applied the same plasma conditions to the deposition of lc-Si:H
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 8 7 - 5
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S. Kasouit et al. / Journal of Non-Crystalline Solids 299–302 (2002) 113–117
films on silicon nitride and silicon oxide layers produced at different temperatures. Indeed, it has been reported that the deposition conditions of the a-SiN:H have a strong influence on the properties of the subsequently deposited lc-Si:H films [6]. Finally, time resolved microwave conductivity (TRMC) measurements have been used to optimise the transport properties of the lc-Si:H films.
2. Experiments Microcrystalline silicon films were deposited on 1737 Corning glass substrates at 200 °C in a 13.56 MHz rf PECVD reactor by the dissociation of a mixture of 1 sccm of SiF4 , 25 sccm of Ar and hydrogen (1–15 sccm) in the pressure range 0.8–1.2 Torr and under an rf power of 20 W. The structural properties of the deposited material were deduced from in situ UV-visible spectroscopic ellipsometry measurements in the range from 1.5 to 5 eV. During the acquisition of the spectra the rf power was switched off. We checked that turning on and off the discharge does not change the final structure of the material. The ellipsometry data were modelled through the use of a Bruggeman Effective Medium Approximation theory (BEMA) [7]. The optical model used to describe the films consists of two layers, a bulk and a surface roughness, each layer consisting of three components: small grain polycrystalline silicon with a crystalline fraction Fc , amorphous silicon with a fraction Fa , and voids with a fraction Fv . Time resolved microwave conductivity is a powerful contactless technique for studying transport properties of lc-Si:H. TRMC measurements are based on the change of the reflectivity of the material in the microwave range, due to carriers photogenerated by a laser. The change R of the reflectivity at very small concentration of generated carriers is given by
samples of known mobility; in particular crystalline silicon with a mobility of 1000 cm2 =V s and standard hydrogenated amorphous silicon film with a mobility of 1 cm2 =V s. Carriers were photogenerated on the sample by a 5 ns pulsed Nd:YAG laser at 532 nm [8], while the microwaves are generated by a gunn diode at 28 GHz. It is important to note that, the TRMC measures the mobility inside crystallites, taking into account trapping but not grain boundaries barriers [9].
3. Results 3.1. Growth of lc-Si:H from SiF4 precursor Fig. 1 shows the spectra of the imaginary part of the dielectric function ðei Þ obtained from spectroscopic ellipsometry measurements. The spectra were recorded at different stages of the growth of a film produced from the decomposition of the SiF4 – Ar mixture with 10 sccm of hydrogen under a total pressure of 1 Torr. One can see that the amplitude of ei increases and the interference fringes shift to lower energy, indicating the densification of the material and the increase of thickness, respectively. The evolution of the film composition deduced from BEMA modelling of the spectra presented in Fig. 1 is shown in Fig. 2. One can see on this figure
R ¼ K Dn l; where Dn is the excess photogenerated pairs concentration, l is the global mobility in the sample, and K is a sensitivity coefficient which depends on the substrate and the experimental set-up. It is calibrated when measurements are performed on
Fig. 1. Imaginary part of the dielectric function of lc-Si:H films deduced from in situ ellipsometry measurements at different stages of the growth.
S. Kasouit et al. / Journal of Non-Crystalline Solids 299–302 (2002) 113–117
Fig. 2. Evolution of the composition of microcrystalline silicon films deduced from BEMA analysis of the spectra shown in Fig. 1 (Fc : crystalline fraction; Fa : amorphous fraction; Fv : void fraction). The inset shows the evolution of the film thickness for the thickness and less than with time. The errors are 30 A 5% for the composition.
), that in the initial stages of the growth (up to 450 A the material consists of a mixture of crystallites and voids without any amorphous phase. To our knowledge this is the first time such type of growth has been observed. At a thickness of about 45 nm we observe a sharp transition for which the void fraction strongly decreases and the crystalline fraction increases, while the total thickness remains almost constant; i.e., the material becomes dense. Moreover the development of a small amount of amorphous phase (10%) takes place during this phase. After this phase of densification, the thickness increases with a constant deposition rate (see inset in Fig. 2), the crystalline fraction increases slowly and the amorphous and void fractions decrease. This would correspond to the growth regime observed in the silane–hydrogen system [2]. Finally the composition of the material becomes constant and it is only the thickness which increases. Note that this fully crystallised steady state can be obtained for a film thickness smaller than 100 nm.
115
Fig. 3. Evolution of composition with thickness for lc-Si:H films deposited at the same plasma conditions, on three silicon oxides.
lc-Si:H film is of crucial importance. When trying to deposit films on silicon nitride or oxide at the same plasma conditions as for films deposited on glass, we have observed that the composition of the film depends on the deposition temperature of the substrate. Fig. 3 shows the evolution of the crystalline fraction during the deposition on SiO2 films produced at 240, 280 and 330 °C under the same plasma conditions (1 sccm of SiF4 , 1 sccm H2 , and 25 sccm Ar, a total pressure of 1 Torr and an rf power of 20 W). Note that the hydrogen flow rate is smaller than for the films in Fig. 1. We can see the same evolution as a function of the film thickness, however it is interesting to note that, for a given lc-Si:H layer thickness, the crystalline fraction is higher on the dielectric deposited at higher temperature. Table 1 shows the values of Fc ; Fa , and Fv at steady state for deposition on SiO2 and a-SiN:H. In both cases the crystalline fraction increases with the deposition temperature of the dielectric. Further experiments under more reactive plasma conditions (hydrogen flux of 10 sccm instead of 1 sccm) have shown almost no effect of the dielectric on the microcrystalline film composition.
3.2. Effect of the dielectric deposition temperature on lc-Si:H growth
3.3. TRMC measurements
In bottom gate thin film transistors, the interface between the dielectric layer and the deposited
In order to optimise the transport properties of thin microrystalline silicon films we have used
116
S. Kasouit et al. / Journal of Non-Crystalline Solids 299–302 (2002) 113–117
Table 1 Final composition of 130 nm films deposited at identical conditions on three silicon oxides and nitrides, which differs by there deposition temperature Ts (°C)
Fc (%)
Fa (%)
Fv (%)
a-SiN
240 280 330
87 89 92
9 7 6
4 4 2
SiO2
240 280 330
87 88 91
9 8 5
4 4 4
TRMC measurements. The thickness of the films (shown in the brackets in the figure) was in the range 0.15–0.37 lm. As shown in Fig. 4, the total pressure was varied between 0.8 and 1.2 Torr, and the hydrogen partial pressure between 10 and 80 mTorr. Two clear trends are observed: (i) the increase of the total pressure results in a monotonous increase of the effective mobility, and (ii) the increase in the hydrogen partial pressure results in an optimum for a partial pressure in the range 30– 60 mTorr. These results were obtained for fixed plasma conditions. However, higher mobility values could be obtained by gradually changing the plasma conditions during lc-Si:H growth. In particular, the use of a total pressure of 700 mTorr in the first 60 min of deposition allowed to reach a high crystalline fraction at the interface with the glass
Fig. 4. The effect of total pressure (upper axis) and hydrogen partial pressure (lower axis) on TRMC mobility. The thicknesses, in nm, of the measured films are given between brackets. Errors on l are less than 20%.
substrate, while the increase of the pressure at the end of the deposition allowed to achieve a less disordered subsurface layer. Using this two step process we have achieved a fully crystallised 140 nm thin film with a mobility of 7 cm2 =V s.
4. Discussion The growth mechanism shown in Fig. 1, resulting in a fast crystallisation with absence of an amorphous phase and the densification of the film at constant thickness, is a particular feature of the growth from fluorine precursor. Indeed, our in situ studies of lc-Si:H growth from silane–hydrogen discharges either in the standard hydrogen dilution or the layer-by-layer method [10] show that the nucleation of crystallites takes place in a highly porous and hydrogen-rich amorphous silicon layer. Thus, the particular behaviour must be related to the higher etching efficiency of fluorine with respect to hydrogen, which efficiently prevents any amorphous phase to develop in the nucleation phase of the material. As the film grows and the layer becomes more dense, fluorine can hardly diffuse through the layer. At later stages of the growth (about 50 nm in Fig. 1), because of its high diffusion coefficient, hydrogen becomes the key element responsible for the further evolution of the film properties with thickness, as observed in the silane case. Moreover, the formation of an amorphous phase, probably related to grain boundaries, may result from the hydrogen-induced disorder of the film. Indeed it has been shown that hydrogen exposure of crystalline silicon results in the formation of an amorphous layer [11]. This is further supported by the optimum hydrogen partial pressure shown in Fig. 4. Indeed, this optimum can be understood as a balance between a too low hydrogen flux resulting in a incomplete crystallisation of the film and an over-exposure to atomic hydrogen (at high partial pressures) which can damage the crystallites, as it does on a crystalline silicon substrate [11], and thus deteriorate the transport properties. The optimum of the mobility as a function of the hydrogen partial pressure contrasts with its steady increase as a function of the total pressure, which as in the case of the films
S. Kasouit et al. / Journal of Non-Crystalline Solids 299–302 (2002) 113–117
produced by the standard hydrogen dilution of silane can be related to a decrease in the energy of the ions [12]. The effect of the deposition temperature of the dielectric on the evolution of the crystalline fraction shown in Fig. 3 can be understood as an effect of the hydrogen diffusion. Indeed, the increase of the substrate temperature results in the formation of a denser a-SiN:H and SiO2 layers and thus will reduce the diffusion of hydrogen. Now, as the nucleation of crystallites requires the formation of a hydrogen-rich porous layer [10], this hydrogenrich layer will hardly form on a porous dielectric because hydrogen will diffuse deeper. Similar effects have been observed when growing lc-Si:H films on dense and porous amorphous silicon [13].
5. Conclusion In this section, we have shown that the use of SiF4 allows to achieve a particular growth mechanism in which fully crystallised films are obtained from the initial stages of the growth. This has been discussed in terms of efficient etching by fluorine atoms. Moreover we have shown that there is an optimum in the hydrogen partial pressure with respect to the mobility of the electrons. Besides, the effects of the dielectric deposition temperature on the crystalline fraction of the films has been correlated to the diffusion of hydrogen through the
117
dielectric layer. Interestingly enough the optimisation of the plasma conditions has resulted in fully crystallised thin films deposited at 200 °C with mobility values as high as 7 cm2 =V s.
References [1] P. Roca i Cabarrocas, S. Kumar, V. Tripathi, P. Bulkin, R. Brenot, B. Drevillon, R. Vanderhaghen, I. French, Solid State Phenom. 80&81 (2001) 361. [2] S. Hamma, P. Roca i Cabarrocas, J. Appl. Phys. 81 (1997) 7282. [3] M. Mulato, Y. Chen, S. Wagner, A.R. Zanatta, J. NonCryst. Solids 266–269 (2000) 1260. [4] M. Losurdo, R. Rizzoli, C. Summonte, G. Cicala, P. Capezzuto, G. Bruno, J. Appl. Phys. 88 (2000) 2408. [5] S. Kumar, R. Brenot, B. Kalache, V. Tripathi, R. Vanderhaghen, B. Drevillon, P. Roca i Cabarrocas, Solid State Phenom. 80&81 (2001) 237. [6] Y.B. Park, S.W. Rhee, J Appl. Phys. 90 (2001) 217. [7] D.A.G. Bruggeman, Ann. Phys. (Leipzig) 24 (1935) 636. [8] R. Brenot, R. Vanderhaghen, B. Drevillon, I. French, P. Roca i Cabarrocas, Thin Solid Films 296 (1997) 94. [9] R. Brenot, R. Vanderhaghen, B. Drevillon, P. Roca i Cabarrocas, R. Rogel, T. Mohammed-Brahim, Thin Solid Films 383 (2001) 53. [10] A. Fontcuberta, J. Bertomeu, P. Roca i Cabarrocas, Mater. Sci. Eng. B 69 (1999) 559. [11] H.C. Neitzert, N. Layadi, P. Roca i Cabarrocas, R. Vanderhaghen, M. Kunst, J. Appl. Phys. 78 (1995) 1438. [12] E.A.G. Hamers, A. Fontcuberta i Morral, C. Niikura, R. Brenot, P. Roca i Cabarrocas, J. Appl. Phys. 88 (2000) 3674. [13] P. Roca i cabarrocas, S. Hamma, Thin Solid Films 337 (1999) 23.
Fluorine and hydrogen effects on the growth and transport properties of microcrystalline silicon from SiF4 precursor S. Kasouit
a,* , b
S. Kumar a, R. Vanderhaghen a, P. Roca i Cabarrocas a, I. French
b
a LPICM (UMR 7647-CNRS), Ecole Polytechnique, 91128 Palaiseau Cedex, France Philips Research Laboratories, Cross Oak Lane, Redhill, England, Surrey RH 5HA, UK
Abstract Structural and electrical properties of microcrystalline silicon (lc-Si:H) films grown at 200 °C from SiF4 precursor have been studied. A particular growth mechanism in which the material is completely crystallised from the initial stages of deposition is revealed by in situ ellipsometry measurements. Time resolved microwave conductivity (TRMC) measurements were performed on samples deposited at various conditions and an optimum in the total pressure and the hydrogen dilution which maximise the crystalline fraction and the transport properties was found. A mobility as high as 7 cm2 =V s was measured for a 0.14 lm thick sample. The nature of the substrate (a-SiN:H and SiO2 ) and its deposition temperature were found to affect the composition of the lc-Si:H deposited on it. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.Ac; 81.15.Gh
1. Introduction Microcrystalline silicon (lc-Si:H) is a promising material for device applications such as solar cells and thin film transistors. For the last application, a crystalline fraction higher than 95% and a mobility of a few cm2 =V s are required for a film thickness of less than 50 nm [1]. Thus, besides silane [2], several precursors such as SiH2 Cl2 [3] and SiF4 [4] have been studied. Because of their higher etching efficiency with respect to hydrogen, these precursors are expected to produce thin films with high
*
Corresponding author. E-mail address: [email protected] (S. Kasouit).
crystalline fraction and good transport properties. We have focused our study on the use of SiF4 which allows to produce films comparable to polycrystalline silicon [5]. To obtain a fully crystallised film as thin as possible, we have used in situ spectroscopic ellipsometry to study the thickness dependence of the film composition. Interestingly enough, we have observed a particular growth mechanism on glass substrates, resulting in the direct formation of a porous microcrystalline phase without any amorphous fraction. Moreover, with the aim of using this material as the active layer in bottom gate thin film transistors, it is important to control the effect of the dielectric substrate on the lc-Si:H properties. Thus we have applied the same plasma conditions to the deposition of lc-Si:H
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 8 7 - 5
114
S. Kasouit et al. / Journal of Non-Crystalline Solids 299–302 (2002) 113–117
films on silicon nitride and silicon oxide layers produced at different temperatures. Indeed, it has been reported that the deposition conditions of the a-SiN:H have a strong influence on the properties of the subsequently deposited lc-Si:H films [6]. Finally, time resolved microwave conductivity (TRMC) measurements have been used to optimise the transport properties of the lc-Si:H films.
2. Experiments Microcrystalline silicon films were deposited on 1737 Corning glass substrates at 200 °C in a 13.56 MHz rf PECVD reactor by the dissociation of a mixture of 1 sccm of SiF4 , 25 sccm of Ar and hydrogen (1–15 sccm) in the pressure range 0.8–1.2 Torr and under an rf power of 20 W. The structural properties of the deposited material were deduced from in situ UV-visible spectroscopic ellipsometry measurements in the range from 1.5 to 5 eV. During the acquisition of the spectra the rf power was switched off. We checked that turning on and off the discharge does not change the final structure of the material. The ellipsometry data were modelled through the use of a Bruggeman Effective Medium Approximation theory (BEMA) [7]. The optical model used to describe the films consists of two layers, a bulk and a surface roughness, each layer consisting of three components: small grain polycrystalline silicon with a crystalline fraction Fc , amorphous silicon with a fraction Fa , and voids with a fraction Fv . Time resolved microwave conductivity is a powerful contactless technique for studying transport properties of lc-Si:H. TRMC measurements are based on the change of the reflectivity of the material in the microwave range, due to carriers photogenerated by a laser. The change R of the reflectivity at very small concentration of generated carriers is given by
samples of known mobility; in particular crystalline silicon with a mobility of 1000 cm2 =V s and standard hydrogenated amorphous silicon film with a mobility of 1 cm2 =V s. Carriers were photogenerated on the sample by a 5 ns pulsed Nd:YAG laser at 532 nm [8], while the microwaves are generated by a gunn diode at 28 GHz. It is important to note that, the TRMC measures the mobility inside crystallites, taking into account trapping but not grain boundaries barriers [9].
3. Results 3.1. Growth of lc-Si:H from SiF4 precursor Fig. 1 shows the spectra of the imaginary part of the dielectric function ðei Þ obtained from spectroscopic ellipsometry measurements. The spectra were recorded at different stages of the growth of a film produced from the decomposition of the SiF4 – Ar mixture with 10 sccm of hydrogen under a total pressure of 1 Torr. One can see that the amplitude of ei increases and the interference fringes shift to lower energy, indicating the densification of the material and the increase of thickness, respectively. The evolution of the film composition deduced from BEMA modelling of the spectra presented in Fig. 1 is shown in Fig. 2. One can see on this figure
R ¼ K Dn l; where Dn is the excess photogenerated pairs concentration, l is the global mobility in the sample, and K is a sensitivity coefficient which depends on the substrate and the experimental set-up. It is calibrated when measurements are performed on
Fig. 1. Imaginary part of the dielectric function of lc-Si:H films deduced from in situ ellipsometry measurements at different stages of the growth.
S. Kasouit et al. / Journal of Non-Crystalline Solids 299–302 (2002) 113–117
Fig. 2. Evolution of the composition of microcrystalline silicon films deduced from BEMA analysis of the spectra shown in Fig. 1 (Fc : crystalline fraction; Fa : amorphous fraction; Fv : void fraction). The inset shows the evolution of the film thickness for the thickness and less than with time. The errors are 30 A 5% for the composition.
), that in the initial stages of the growth (up to 450 A the material consists of a mixture of crystallites and voids without any amorphous phase. To our knowledge this is the first time such type of growth has been observed. At a thickness of about 45 nm we observe a sharp transition for which the void fraction strongly decreases and the crystalline fraction increases, while the total thickness remains almost constant; i.e., the material becomes dense. Moreover the development of a small amount of amorphous phase (10%) takes place during this phase. After this phase of densification, the thickness increases with a constant deposition rate (see inset in Fig. 2), the crystalline fraction increases slowly and the amorphous and void fractions decrease. This would correspond to the growth regime observed in the silane–hydrogen system [2]. Finally the composition of the material becomes constant and it is only the thickness which increases. Note that this fully crystallised steady state can be obtained for a film thickness smaller than 100 nm.
115
Fig. 3. Evolution of composition with thickness for lc-Si:H films deposited at the same plasma conditions, on three silicon oxides.
lc-Si:H film is of crucial importance. When trying to deposit films on silicon nitride or oxide at the same plasma conditions as for films deposited on glass, we have observed that the composition of the film depends on the deposition temperature of the substrate. Fig. 3 shows the evolution of the crystalline fraction during the deposition on SiO2 films produced at 240, 280 and 330 °C under the same plasma conditions (1 sccm of SiF4 , 1 sccm H2 , and 25 sccm Ar, a total pressure of 1 Torr and an rf power of 20 W). Note that the hydrogen flow rate is smaller than for the films in Fig. 1. We can see the same evolution as a function of the film thickness, however it is interesting to note that, for a given lc-Si:H layer thickness, the crystalline fraction is higher on the dielectric deposited at higher temperature. Table 1 shows the values of Fc ; Fa , and Fv at steady state for deposition on SiO2 and a-SiN:H. In both cases the crystalline fraction increases with the deposition temperature of the dielectric. Further experiments under more reactive plasma conditions (hydrogen flux of 10 sccm instead of 1 sccm) have shown almost no effect of the dielectric on the microcrystalline film composition.
3.2. Effect of the dielectric deposition temperature on lc-Si:H growth
3.3. TRMC measurements
In bottom gate thin film transistors, the interface between the dielectric layer and the deposited
In order to optimise the transport properties of thin microrystalline silicon films we have used
116
S. Kasouit et al. / Journal of Non-Crystalline Solids 299–302 (2002) 113–117
Table 1 Final composition of 130 nm films deposited at identical conditions on three silicon oxides and nitrides, which differs by there deposition temperature Ts (°C)
Fc (%)
Fa (%)
Fv (%)
a-SiN
240 280 330
87 89 92
9 7 6
4 4 2
SiO2
240 280 330
87 88 91
9 8 5
4 4 4
TRMC measurements. The thickness of the films (shown in the brackets in the figure) was in the range 0.15–0.37 lm. As shown in Fig. 4, the total pressure was varied between 0.8 and 1.2 Torr, and the hydrogen partial pressure between 10 and 80 mTorr. Two clear trends are observed: (i) the increase of the total pressure results in a monotonous increase of the effective mobility, and (ii) the increase in the hydrogen partial pressure results in an optimum for a partial pressure in the range 30– 60 mTorr. These results were obtained for fixed plasma conditions. However, higher mobility values could be obtained by gradually changing the plasma conditions during lc-Si:H growth. In particular, the use of a total pressure of 700 mTorr in the first 60 min of deposition allowed to reach a high crystalline fraction at the interface with the glass
Fig. 4. The effect of total pressure (upper axis) and hydrogen partial pressure (lower axis) on TRMC mobility. The thicknesses, in nm, of the measured films are given between brackets. Errors on l are less than 20%.
substrate, while the increase of the pressure at the end of the deposition allowed to achieve a less disordered subsurface layer. Using this two step process we have achieved a fully crystallised 140 nm thin film with a mobility of 7 cm2 =V s.
4. Discussion The growth mechanism shown in Fig. 1, resulting in a fast crystallisation with absence of an amorphous phase and the densification of the film at constant thickness, is a particular feature of the growth from fluorine precursor. Indeed, our in situ studies of lc-Si:H growth from silane–hydrogen discharges either in the standard hydrogen dilution or the layer-by-layer method [10] show that the nucleation of crystallites takes place in a highly porous and hydrogen-rich amorphous silicon layer. Thus, the particular behaviour must be related to the higher etching efficiency of fluorine with respect to hydrogen, which efficiently prevents any amorphous phase to develop in the nucleation phase of the material. As the film grows and the layer becomes more dense, fluorine can hardly diffuse through the layer. At later stages of the growth (about 50 nm in Fig. 1), because of its high diffusion coefficient, hydrogen becomes the key element responsible for the further evolution of the film properties with thickness, as observed in the silane case. Moreover, the formation of an amorphous phase, probably related to grain boundaries, may result from the hydrogen-induced disorder of the film. Indeed it has been shown that hydrogen exposure of crystalline silicon results in the formation of an amorphous layer [11]. This is further supported by the optimum hydrogen partial pressure shown in Fig. 4. Indeed, this optimum can be understood as a balance between a too low hydrogen flux resulting in a incomplete crystallisation of the film and an over-exposure to atomic hydrogen (at high partial pressures) which can damage the crystallites, as it does on a crystalline silicon substrate [11], and thus deteriorate the transport properties. The optimum of the mobility as a function of the hydrogen partial pressure contrasts with its steady increase as a function of the total pressure, which as in the case of the films
S. Kasouit et al. / Journal of Non-Crystalline Solids 299–302 (2002) 113–117
produced by the standard hydrogen dilution of silane can be related to a decrease in the energy of the ions [12]. The effect of the deposition temperature of the dielectric on the evolution of the crystalline fraction shown in Fig. 3 can be understood as an effect of the hydrogen diffusion. Indeed, the increase of the substrate temperature results in the formation of a denser a-SiN:H and SiO2 layers and thus will reduce the diffusion of hydrogen. Now, as the nucleation of crystallites requires the formation of a hydrogen-rich porous layer [10], this hydrogenrich layer will hardly form on a porous dielectric because hydrogen will diffuse deeper. Similar effects have been observed when growing lc-Si:H films on dense and porous amorphous silicon [13].
5. Conclusion In this section, we have shown that the use of SiF4 allows to achieve a particular growth mechanism in which fully crystallised films are obtained from the initial stages of the growth. This has been discussed in terms of efficient etching by fluorine atoms. Moreover we have shown that there is an optimum in the hydrogen partial pressure with respect to the mobility of the electrons. Besides, the effects of the dielectric deposition temperature on the crystalline fraction of the films has been correlated to the diffusion of hydrogen through the
117
dielectric layer. Interestingly enough the optimisation of the plasma conditions has resulted in fully crystallised thin films deposited at 200 °C with mobility values as high as 7 cm2 =V s.
References [1] P. Roca i Cabarrocas, S. Kumar, V. Tripathi, P. Bulkin, R. Brenot, B. Drevillon, R. Vanderhaghen, I. French, Solid State Phenom. 80&81 (2001) 361. [2] S. Hamma, P. Roca i Cabarrocas, J. Appl. Phys. 81 (1997) 7282. [3] M. Mulato, Y. Chen, S. Wagner, A.R. Zanatta, J. NonCryst. Solids 266–269 (2000) 1260. [4] M. Losurdo, R. Rizzoli, C. Summonte, G. Cicala, P. Capezzuto, G. Bruno, J. Appl. Phys. 88 (2000) 2408. [5] S. Kumar, R. Brenot, B. Kalache, V. Tripathi, R. Vanderhaghen, B. Drevillon, P. Roca i Cabarrocas, Solid State Phenom. 80&81 (2001) 237. [6] Y.B. Park, S.W. Rhee, J Appl. Phys. 90 (2001) 217. [7] D.A.G. Bruggeman, Ann. Phys. (Leipzig) 24 (1935) 636. [8] R. Brenot, R. Vanderhaghen, B. Drevillon, I. French, P. Roca i Cabarrocas, Thin Solid Films 296 (1997) 94. [9] R. Brenot, R. Vanderhaghen, B. Drevillon, P. Roca i Cabarrocas, R. Rogel, T. Mohammed-Brahim, Thin Solid Films 383 (2001) 53. [10] A. Fontcuberta, J. Bertomeu, P. Roca i Cabarrocas, Mater. Sci. Eng. B 69 (1999) 559. [11] H.C. Neitzert, N. Layadi, P. Roca i Cabarrocas, R. Vanderhaghen, M. Kunst, J. Appl. Phys. 78 (1995) 1438. [12] E.A.G. Hamers, A. Fontcuberta i Morral, C. Niikura, R. Brenot, P. Roca i Cabarrocas, J. Appl. Phys. 88 (2000) 3674. [13] P. Roca i cabarrocas, S. Hamma, Thin Solid Films 337 (1999) 23.