- Email: [email protected]
From amorphous to polycrystalline thin films: dependence on annealing time of structural and electronic properties
Journal of Non-Crystalline Solids 227–230 Ž1998. 962–966
From amorphous to polycrystalline thin films: dependence on annealing time of structural and electronic properties T. Mohammed-Brahim a,) , K. Kis-Sion a , D. Briand a , M. Sarret a , O. Bonnaud a , J.P. Kleider b, C. Longeaud b, B. Lambert c b
a GMV, UPRESA-CNRS 6076, UniÕersite´ de Rennes I, Campus de Beaulieu, Bat. ˆ 11B, 35042 Rennes Cedex, France LGEP URA-CNRS 127, Ecole Superieure d’Electricite, ´ ´ UniÕersites ´ de Paris VI et Paris XI, 91192 Gif-sur-YÕette Cedex, France c LPS, INSA, 35043 Rennes, France
Abstract Some new results about the amorphous to polycrystalline transition of silicon thin films obtained by low pressure chemical vapor deposition ŽLPCVD. at 5508C are presented. From in situ Žmonitored during the crystallization annealing. conductance, electron spin resonance, photoluminescence and modulated photocurrent experiments, the density of states is shown to increase in the so-called nucleation regime and reach a maximum just before crystal grain growth starts. These results indicate that crystallization needs the creation of a defected material with large dangling bond densities and wide band-tails. It appears that the electronic quality of the final polycrystalline material is linked to the maximum density of states reached just at the end of the nucleation phase. This level may be associated to a viscous structure. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous; Polycrystalline; Annealing time; Electronic properties
1. Introduction Polycrystalline silicon Žpolysilicon. has been extensively studied as an electronic active material for applications in active matrix–liquid–crystal displays w1x and photovoltaic conversion of solar energy w2x. These major applications both need a low-temperature deposition process Ž- 6008C. due to the use of glass substrates and a high-quality polysilicon. It was shown that high-quality polysilicon can only be obtained from the crystallization of amorphous deposited films w3x. )
Corresponding author. Fax: q33-2 9928 1674; e-mail: [email protected].
An attractive method consists in increasing the grain size by annealing the amorphous film. Indeed, low-temperature annealing Ž- 6008C. of films deposited by low pressure chemical vapor deposition ŽLPCVD. leads to large grains and smooth interfaces w4,5x. Recently w6x, we have shown that high quality polysilicon is obtained by controlling the quality of the amorphous film and the quality of the nucleation time. While several studies w7,8x of high temperature annealing of amorphous silicon have been previously done, only little is known about the structural changes which occur before the first stable seeds appear. The purpose of the present work is to determine
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 3 4 6 - 9
T. Mohammed-Brahim et al.r Journal of Non-Crystalline Solids 227–230 (1998) 962–966
the structural changes during a crystallization annealing of amorphous silicon films at 6008C, and then discuss the link between the starting amorphous material and the final crystalline film.
963
details about the experiment and its analysis can be found elsewhere w9x.
3. Results 2. Experiment
3.1. Kinetics of the SPC at 6008C
Silicon films, typically 1 m m thick, were deposited in a conventional horizontal hot-wall LPCVD reactor using pure silane as a source gas. The substrates used were 5 = 5 cm2 glass ŽCorning 7059. or fused quartz platelets. The deposition conditions were fixed at a silane flow rate of 50 sccm, a temperature of 5508C, and a pressure of 30 or 90 Pa. Under these conditions the deposited silicon films are amorphous. The solid-phase crystallization ŽSPC. of these asdeposited amorphous films during isothermal annealing at 6008C under vacuum was by in situ measurements of the film conductance. Then, 9 = 15 mm2 area samples, originating from the same amorphous platelet, were annealed at 6008C under the same vacuum conditions for different times, which were selected from the conductance behaviour during the SPC. The hydrogen content of these samples was determined using secondary ion mass spectroscopy ŽSIMS. analysis. Their neutral dangling bond density, NS , corresponding an electron spin resonance component with g s 2.0055, was obtained from electron spin resonance ŽESR. measurements. Photoluminescence experiments were carried out using an argon laser Žat a wavelength l s 514 nm, and an intensity of 100 mWrcm2 . as the excitation source, a grating monochromator with a resolution of 3 meV and a liquid nitrogen cooled germanium photodiode. Modulated photocurrent ŽMPC. measurements were performed using a red light Ž l s 640 nm. with a photon flux F s 10 15 cmy2 sy1 , which was sinusoidally modulated in a large frequency range Ž12 Hz–40 kHz.. The modulus of the modulated photocurrent, < Iac <, and its phase shift, F , referred to the excitation light were recorded at several temperatures ranging from 123 K to 273 K. By changing the frequency andror the temperature, it is possible to probe gap states located at different energies. More
Changes of the conductance as a function of annealing time, at 6008C, for samples deposited at a pressure of 30 and 90 Pa are shown in Fig. 1. These changes show a typical crystallization behaviour w6x. At the beginning the conductivity is nearly constant. Then, it increases to a maximum. If we consider as in Ref. w6x that crystallization starts at the substratefilm interface, the slope of the increase of the conductance gives an indication of the crystal growth rate and the time before this increase corresponds to the nucleation and the percolation of crystals at the interface. Fig. 1 shows that the nucleation time is then shorter for the films deposited at 30 Pa than for those prepared at 90 Pa. The slope of the increase of the conductance is however less for the 30 Pa films, indicating that the crystal grain growth rate is less in these films, as previously observed w6x. 3.2. Photoluminescence measurements Samples, originating from the same film deposited at 90 Pa, were annealed at 6008C during 1, 2,
Fig. 1. Variation of the conductance, normalized to its saturation value, of films deposited at 30 and 90 Pa during the crystallization annealing at 6008C.
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T. Mohammed-Brahim et al.r Journal of Non-Crystalline Solids 227–230 (1998) 962–966
shown in Fig. 3. For the fully crystallized film Žafter 22 h annealing., this peak is not detected. Since the 1.44 eV is directly linked to the width of the of the band-tails, these results shows that this width is maximum after the 3 h anneal. Additional measurements have also shown that the position of this peak does not vary with the measurement temperature. We observed the usual intensity decrease when the measurement temperature increases, but the decreasing rate was less than usually encountered. Fig. 2. Photoluminescence spectra, measured at 2 K, of as-deposited Ž0., 3 h and 22 h-annealed samples originating from the same film deposited at 90 Pa.
3, 4, 5 and 22 h. Characteristic spectra of as-deposited, 3 h-annealed and 22 h-annealed samples are shown in Fig. 2. As-deposited and 3 h-annealed samples present 3 peaks at 0.95 eV, 1.44 eV and 1.65 eV. They are characteristic of amorphous silicon w10x. The first two bands are attributed to dangling bonds and the band-tails, respectively. The 0.95 eV band is however too large here and its energy of the maximum varies with the annealing time. Discussion about this variation is not the purpose of the present contribution and this band will not be further considered. The position of the 1.44 eV peak does not vary with the annealing time. Its intensity increases with annealing from 0 to 3 h, and then decreases as it is
Fig. 3. 1.44 eV luminescence band of 1, 3, 5 and 22 h annealed samples originating from the same film deposited at 90 Pa. The intensity is normalized to that of the 3 h sample.
3.3. ESR measurements The variation of the spin density, NS , with the annealing time is shown in Fig. 4 for films deposited at 30 Pa and 90 Pa. Some observations can be made. Ø There are two regimes: NS increases linearly with annealing time in the first regime, and then decreases in the second regime. The duration of the first regime is less for the film deposited at 30 Pa than for that deposited at 90 Pa Ž50 min and 3 h, respectively., Ø The maximum value of NS is 3 = 10 19 cmy3 for 30 Pa films and 3.5 = 10 19 cmy3 for 90 Pa films. The relative error is ; 10% on the ESR measurements, thus the densities of the films are the same. Ø The density of spin in fully crystallized material is larger for 30 Pa films Ž7.4 = 10 17 cmy3 . than for 90 Pa films Ž3.2 = 10 17 cmy3 ..
Fig. 4. Density of spin NS in 30 Pa and 90 Pa deposited films vs. the annealing time. The behaviors are characterized by a maximum of NS . ŽThe lines are drawn as guides for the eyes..
T. Mohammed-Brahim et al.r Journal of Non-Crystalline Solids 227–230 (1998) 962–966
Fig. 5. MPC-DOS in 30 Pa and 90 Pa deposited films at different measurement temperatures.
3.4. Modulated photocurrent experiments To compare the films in terms of density of states ŽDOS. values, we assumed equal capture cross sections Ž4 = 10y1 5 cm2 . and equal mobilities Ž20 cm2 Vy1 sy1 w6x., in both films. From the treatment of the MPC data given in Ref. w9x we deduced, what will be called in the following, the MPC-DOS. Fig. 5 compares the MPC-DOS, obtained after full crystallization, for the two films deposited at 30 Pa and 90 Pa. We observe that the MPC-DOS deduced at low temperatures, corresponding to energies close to the conduction band, are the same for both films. On the contrary, the values at higher temperatures, corresponding to deeper energies are much larger for the film deposited at 30 Pa than for that deposited at 90 Pa. A bump is clearly seen for the film deposited at 30 Pa, which reflects the presence of a larger deep defect density. In addition, the efficiency–mobility-lifetime product deduced from conductivity measurements performed under the same photon flux as for the MPC measurements Ž F s 10 15 cmy2 sy1 . is less for the film deposited at 30 Pa than for that deposited at 90 Pa Ž6 = 10y8 and 3 = 10y7 cm2rV, respectively..
965
nescence band, I LUM , the hydrogen content, C H , and the in situ monitored conductance of the 90 Pa film. We observe a correlation between these quantities in the two regimes already defined in the conductance and spin density variations. In the first regime Ž tAN N - 3 h. the decrease of C H due to the hydrogen effusion correlates with the increase of both NS and I LUM . This simultaneous increase of NS and I LUM may be surprising, since it is known that the luminescence is quenched in a-Si:H by an increase of the dangling bond density w10x. Two explanations are be proposed. Either the increase of the disorder in the material is so large that the simultaneous increase of NS cannot quench the luminescence band at 1.44 eV, or the recombination process in these disordered materials differ from that usually given in a-Si:H w10x. We further observe in Fig. 6 that the onset of the in situ measured conductance corresponds to the maximum of NS and I LUM . This correspondence seems to indicate that the crystal growth, which needs the formation of stable seeds, can only begin if a sufficient level of disorder has been reached in the material. In the second regime of Fig. 6, both NS and I LUM decrease, whereas the in situ conductance increases due to the crystal growth.
4. Discussion Fig. 6 compares the variations with the annealing time, tAN N , of the density of spin, NS , deduced from ESR experiments, the intensity of the 1.44 eV lumi-
Fig. 6. Variations of the density of spin Ž NS ., the intensity of the 1.44 eV luminescence band I LU M normalized to its maximum value, the hydrogen content C H , and the in situ monitored conductance of the 90 Pa film Ž s ., with the crystallization annealing time at 6008C. ŽThe lines are drawn as guides for the eyes..
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T. Mohammed-Brahim et al.r Journal of Non-Crystalline Solids 227–230 (1998) 962–966
To tentatively explain these variations, we must observe that they are not due to the use of an annealing temperature ŽTA s 6008C. higher than the deposition temperature ŽTD s 5508C.. A similar evolution is observed even when TA s TD . The evolution of the structure towards disorder, before the first seeds appear during the crystallization annealing, is a characteristic of the high temperature annealing. The structure deposited at 5508C is not in equilibrium at the end of the deposition. It is possible to imagine that a viscous glass behaviour exists for amorphous silicon. Even though amorphous silicon is not a glass, many experimental data w7,11x have shown glass properties. Then, we propose that in the first regime, due to the hydrogen effusion and to the structural change, the structure tends to glass properties characterized by a disorder and a viscosity, during the annealing. This glass state is however never reached. The trend is limited by the formation of stable seeds due to the heterogeneous nucleation occurring from the atomic impurities and from the natural sites of nucleation as the interface. Considering the present hypothesis, the difference between the two kinds of films, studied here, may be explained. The difference may be summarized below. Ø A smaller crystal growth rate in 30 Pa films. These different rates were also previously detected w6x from experiments such as X-ray diffraction. Ø Improved electronic properties of the final polycrystalline material originating from 90 Pa films, which correlates with a smaller dangling bond density. In addition, thin film transistors produced from this final polycrystalline material are improved w12x. The better electronic quality of the final polycrystalline material originating from 90 Pa films may be explained from the disorder reached just before the beginning of the crystal growth. It was observed that the maximum NS is larger for 90 Pa films than for 30 Pa films. This difference is related to the greater nucleation time in 90 Pa films. The nucleation is delayed by the effusion of hydrogen in these films where C H is larger. This delayed heterogeneous nucleation in 90 Pa films leads to a more viscous
structure, to a greater crystal growth rate, and to less defected crystal grain.
5. Conclusion The electronic quality of the final polycrystalline material, obtained from a solid-phase crystallization of amorphous silicon, is directly dependent on the state of the structure just before the crystal grain growth phase. At this time, the dangling bond density and the band-tails extent are maximum, and the viscosity of the structure probably high. Producing high quality polycrystalline silicon needs to delay the heterogeneous nucleation and to obtain a close glass behaviour structure before the crystal growth.
References w1x A. Mimura, N. Konishi, K. Ono, Y. Hosokawa, Y.A. Ono, T. Suzuki, K. Miyata, H. Kawakami, IEEE Electron Device 50 Ž1989. 351. w2x T. Matsuyama, M. Tanaka, S. Tsuda, S. Nakano, Y. Kuwano, Jpn. J. Appl. Phys. 32 Ž1993. 3720. w3x G. Harbeke, L. Krausbauer, E.F. Steigmeier, A.E. Widmer, H.F. Kappert, G. Neugebauer, Appl. Phys. Lett. 42 Ž1983. 249. w4x M.K. Hatalis, D. Greve, J. Appl. Phys. 63 Ž1988. 2260. w5x L. Haji, P. Joubert, M. Gendouz, N. Duhamel, B. Loisel, Mat. Res. Symp. Proc. 230 Ž1992. 177. w6x T. Mohammed-Brahim, M. Sarret, D. Briand, K. Kis-Sion, L. Haji, O. Bonnaud, D. Louer, ¨ A. Hadjadj, Phil. Mag. B, August, 1997. w7x S. Roorda, W.C. Sinke, J.M. Poate, D.C. Jacobson, S. Dierker, B.S. Dennis, D.J. Eaglesham, F. Spaepen, P. Fuoss, Phys. Rev. B 44 Ž1991. 3702. w8x K. Zellama, L. Chahed, P. Sladek, M.L. Theye, J.H. von Bardeleben, P. Roca i Cabarrocas, Phys. Rev. B 53 Ž1996. 3804. w9x J.P. Kleider, C. Longeaud, Solid State Phenomena, Scitec Publications, 44–46 Ž1995. 597–646. w10x R.A. Street, Adv. Phys. 30 Ž1981. 593. w11x R.A. Street, J. Kakalios, T.M. Hayes, Phys. Rev. B 34 Ž1986. 3030. w12x L. Pichon, F. Raoult, K. Mourgues, K. Kis-Sion, T. Mohammed-Brahim, O. Bonnaud, Thin Solid Films 296 Ž1997. 133.
From amorphous to polycrystalline thin films: dependence on annealing time of structural and electronic properties T. Mohammed-Brahim a,) , K. Kis-Sion a , D. Briand a , M. Sarret a , O. Bonnaud a , J.P. Kleider b, C. Longeaud b, B. Lambert c b
a GMV, UPRESA-CNRS 6076, UniÕersite´ de Rennes I, Campus de Beaulieu, Bat. ˆ 11B, 35042 Rennes Cedex, France LGEP URA-CNRS 127, Ecole Superieure d’Electricite, ´ ´ UniÕersites ´ de Paris VI et Paris XI, 91192 Gif-sur-YÕette Cedex, France c LPS, INSA, 35043 Rennes, France
Abstract Some new results about the amorphous to polycrystalline transition of silicon thin films obtained by low pressure chemical vapor deposition ŽLPCVD. at 5508C are presented. From in situ Žmonitored during the crystallization annealing. conductance, electron spin resonance, photoluminescence and modulated photocurrent experiments, the density of states is shown to increase in the so-called nucleation regime and reach a maximum just before crystal grain growth starts. These results indicate that crystallization needs the creation of a defected material with large dangling bond densities and wide band-tails. It appears that the electronic quality of the final polycrystalline material is linked to the maximum density of states reached just at the end of the nucleation phase. This level may be associated to a viscous structure. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous; Polycrystalline; Annealing time; Electronic properties
1. Introduction Polycrystalline silicon Žpolysilicon. has been extensively studied as an electronic active material for applications in active matrix–liquid–crystal displays w1x and photovoltaic conversion of solar energy w2x. These major applications both need a low-temperature deposition process Ž- 6008C. due to the use of glass substrates and a high-quality polysilicon. It was shown that high-quality polysilicon can only be obtained from the crystallization of amorphous deposited films w3x. )
Corresponding author. Fax: q33-2 9928 1674; e-mail: [email protected].
An attractive method consists in increasing the grain size by annealing the amorphous film. Indeed, low-temperature annealing Ž- 6008C. of films deposited by low pressure chemical vapor deposition ŽLPCVD. leads to large grains and smooth interfaces w4,5x. Recently w6x, we have shown that high quality polysilicon is obtained by controlling the quality of the amorphous film and the quality of the nucleation time. While several studies w7,8x of high temperature annealing of amorphous silicon have been previously done, only little is known about the structural changes which occur before the first stable seeds appear. The purpose of the present work is to determine
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 3 4 6 - 9
T. Mohammed-Brahim et al.r Journal of Non-Crystalline Solids 227–230 (1998) 962–966
the structural changes during a crystallization annealing of amorphous silicon films at 6008C, and then discuss the link between the starting amorphous material and the final crystalline film.
963
details about the experiment and its analysis can be found elsewhere w9x.
3. Results 2. Experiment
3.1. Kinetics of the SPC at 6008C
Silicon films, typically 1 m m thick, were deposited in a conventional horizontal hot-wall LPCVD reactor using pure silane as a source gas. The substrates used were 5 = 5 cm2 glass ŽCorning 7059. or fused quartz platelets. The deposition conditions were fixed at a silane flow rate of 50 sccm, a temperature of 5508C, and a pressure of 30 or 90 Pa. Under these conditions the deposited silicon films are amorphous. The solid-phase crystallization ŽSPC. of these asdeposited amorphous films during isothermal annealing at 6008C under vacuum was by in situ measurements of the film conductance. Then, 9 = 15 mm2 area samples, originating from the same amorphous platelet, were annealed at 6008C under the same vacuum conditions for different times, which were selected from the conductance behaviour during the SPC. The hydrogen content of these samples was determined using secondary ion mass spectroscopy ŽSIMS. analysis. Their neutral dangling bond density, NS , corresponding an electron spin resonance component with g s 2.0055, was obtained from electron spin resonance ŽESR. measurements. Photoluminescence experiments were carried out using an argon laser Žat a wavelength l s 514 nm, and an intensity of 100 mWrcm2 . as the excitation source, a grating monochromator with a resolution of 3 meV and a liquid nitrogen cooled germanium photodiode. Modulated photocurrent ŽMPC. measurements were performed using a red light Ž l s 640 nm. with a photon flux F s 10 15 cmy2 sy1 , which was sinusoidally modulated in a large frequency range Ž12 Hz–40 kHz.. The modulus of the modulated photocurrent, < Iac <, and its phase shift, F , referred to the excitation light were recorded at several temperatures ranging from 123 K to 273 K. By changing the frequency andror the temperature, it is possible to probe gap states located at different energies. More
Changes of the conductance as a function of annealing time, at 6008C, for samples deposited at a pressure of 30 and 90 Pa are shown in Fig. 1. These changes show a typical crystallization behaviour w6x. At the beginning the conductivity is nearly constant. Then, it increases to a maximum. If we consider as in Ref. w6x that crystallization starts at the substratefilm interface, the slope of the increase of the conductance gives an indication of the crystal growth rate and the time before this increase corresponds to the nucleation and the percolation of crystals at the interface. Fig. 1 shows that the nucleation time is then shorter for the films deposited at 30 Pa than for those prepared at 90 Pa. The slope of the increase of the conductance is however less for the 30 Pa films, indicating that the crystal grain growth rate is less in these films, as previously observed w6x. 3.2. Photoluminescence measurements Samples, originating from the same film deposited at 90 Pa, were annealed at 6008C during 1, 2,
Fig. 1. Variation of the conductance, normalized to its saturation value, of films deposited at 30 and 90 Pa during the crystallization annealing at 6008C.
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T. Mohammed-Brahim et al.r Journal of Non-Crystalline Solids 227–230 (1998) 962–966
shown in Fig. 3. For the fully crystallized film Žafter 22 h annealing., this peak is not detected. Since the 1.44 eV is directly linked to the width of the of the band-tails, these results shows that this width is maximum after the 3 h anneal. Additional measurements have also shown that the position of this peak does not vary with the measurement temperature. We observed the usual intensity decrease when the measurement temperature increases, but the decreasing rate was less than usually encountered. Fig. 2. Photoluminescence spectra, measured at 2 K, of as-deposited Ž0., 3 h and 22 h-annealed samples originating from the same film deposited at 90 Pa.
3, 4, 5 and 22 h. Characteristic spectra of as-deposited, 3 h-annealed and 22 h-annealed samples are shown in Fig. 2. As-deposited and 3 h-annealed samples present 3 peaks at 0.95 eV, 1.44 eV and 1.65 eV. They are characteristic of amorphous silicon w10x. The first two bands are attributed to dangling bonds and the band-tails, respectively. The 0.95 eV band is however too large here and its energy of the maximum varies with the annealing time. Discussion about this variation is not the purpose of the present contribution and this band will not be further considered. The position of the 1.44 eV peak does not vary with the annealing time. Its intensity increases with annealing from 0 to 3 h, and then decreases as it is
Fig. 3. 1.44 eV luminescence band of 1, 3, 5 and 22 h annealed samples originating from the same film deposited at 90 Pa. The intensity is normalized to that of the 3 h sample.
3.3. ESR measurements The variation of the spin density, NS , with the annealing time is shown in Fig. 4 for films deposited at 30 Pa and 90 Pa. Some observations can be made. Ø There are two regimes: NS increases linearly with annealing time in the first regime, and then decreases in the second regime. The duration of the first regime is less for the film deposited at 30 Pa than for that deposited at 90 Pa Ž50 min and 3 h, respectively., Ø The maximum value of NS is 3 = 10 19 cmy3 for 30 Pa films and 3.5 = 10 19 cmy3 for 90 Pa films. The relative error is ; 10% on the ESR measurements, thus the densities of the films are the same. Ø The density of spin in fully crystallized material is larger for 30 Pa films Ž7.4 = 10 17 cmy3 . than for 90 Pa films Ž3.2 = 10 17 cmy3 ..
Fig. 4. Density of spin NS in 30 Pa and 90 Pa deposited films vs. the annealing time. The behaviors are characterized by a maximum of NS . ŽThe lines are drawn as guides for the eyes..
T. Mohammed-Brahim et al.r Journal of Non-Crystalline Solids 227–230 (1998) 962–966
Fig. 5. MPC-DOS in 30 Pa and 90 Pa deposited films at different measurement temperatures.
3.4. Modulated photocurrent experiments To compare the films in terms of density of states ŽDOS. values, we assumed equal capture cross sections Ž4 = 10y1 5 cm2 . and equal mobilities Ž20 cm2 Vy1 sy1 w6x., in both films. From the treatment of the MPC data given in Ref. w9x we deduced, what will be called in the following, the MPC-DOS. Fig. 5 compares the MPC-DOS, obtained after full crystallization, for the two films deposited at 30 Pa and 90 Pa. We observe that the MPC-DOS deduced at low temperatures, corresponding to energies close to the conduction band, are the same for both films. On the contrary, the values at higher temperatures, corresponding to deeper energies are much larger for the film deposited at 30 Pa than for that deposited at 90 Pa. A bump is clearly seen for the film deposited at 30 Pa, which reflects the presence of a larger deep defect density. In addition, the efficiency–mobility-lifetime product deduced from conductivity measurements performed under the same photon flux as for the MPC measurements Ž F s 10 15 cmy2 sy1 . is less for the film deposited at 30 Pa than for that deposited at 90 Pa Ž6 = 10y8 and 3 = 10y7 cm2rV, respectively..
965
nescence band, I LUM , the hydrogen content, C H , and the in situ monitored conductance of the 90 Pa film. We observe a correlation between these quantities in the two regimes already defined in the conductance and spin density variations. In the first regime Ž tAN N - 3 h. the decrease of C H due to the hydrogen effusion correlates with the increase of both NS and I LUM . This simultaneous increase of NS and I LUM may be surprising, since it is known that the luminescence is quenched in a-Si:H by an increase of the dangling bond density w10x. Two explanations are be proposed. Either the increase of the disorder in the material is so large that the simultaneous increase of NS cannot quench the luminescence band at 1.44 eV, or the recombination process in these disordered materials differ from that usually given in a-Si:H w10x. We further observe in Fig. 6 that the onset of the in situ measured conductance corresponds to the maximum of NS and I LUM . This correspondence seems to indicate that the crystal growth, which needs the formation of stable seeds, can only begin if a sufficient level of disorder has been reached in the material. In the second regime of Fig. 6, both NS and I LUM decrease, whereas the in situ conductance increases due to the crystal growth.
4. Discussion Fig. 6 compares the variations with the annealing time, tAN N , of the density of spin, NS , deduced from ESR experiments, the intensity of the 1.44 eV lumi-
Fig. 6. Variations of the density of spin Ž NS ., the intensity of the 1.44 eV luminescence band I LU M normalized to its maximum value, the hydrogen content C H , and the in situ monitored conductance of the 90 Pa film Ž s ., with the crystallization annealing time at 6008C. ŽThe lines are drawn as guides for the eyes..
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T. Mohammed-Brahim et al.r Journal of Non-Crystalline Solids 227–230 (1998) 962–966
To tentatively explain these variations, we must observe that they are not due to the use of an annealing temperature ŽTA s 6008C. higher than the deposition temperature ŽTD s 5508C.. A similar evolution is observed even when TA s TD . The evolution of the structure towards disorder, before the first seeds appear during the crystallization annealing, is a characteristic of the high temperature annealing. The structure deposited at 5508C is not in equilibrium at the end of the deposition. It is possible to imagine that a viscous glass behaviour exists for amorphous silicon. Even though amorphous silicon is not a glass, many experimental data w7,11x have shown glass properties. Then, we propose that in the first regime, due to the hydrogen effusion and to the structural change, the structure tends to glass properties characterized by a disorder and a viscosity, during the annealing. This glass state is however never reached. The trend is limited by the formation of stable seeds due to the heterogeneous nucleation occurring from the atomic impurities and from the natural sites of nucleation as the interface. Considering the present hypothesis, the difference between the two kinds of films, studied here, may be explained. The difference may be summarized below. Ø A smaller crystal growth rate in 30 Pa films. These different rates were also previously detected w6x from experiments such as X-ray diffraction. Ø Improved electronic properties of the final polycrystalline material originating from 90 Pa films, which correlates with a smaller dangling bond density. In addition, thin film transistors produced from this final polycrystalline material are improved w12x. The better electronic quality of the final polycrystalline material originating from 90 Pa films may be explained from the disorder reached just before the beginning of the crystal growth. It was observed that the maximum NS is larger for 90 Pa films than for 30 Pa films. This difference is related to the greater nucleation time in 90 Pa films. The nucleation is delayed by the effusion of hydrogen in these films where C H is larger. This delayed heterogeneous nucleation in 90 Pa films leads to a more viscous
structure, to a greater crystal growth rate, and to less defected crystal grain.
5. Conclusion The electronic quality of the final polycrystalline material, obtained from a solid-phase crystallization of amorphous silicon, is directly dependent on the state of the structure just before the crystal grain growth phase. At this time, the dangling bond density and the band-tails extent are maximum, and the viscosity of the structure probably high. Producing high quality polycrystalline silicon needs to delay the heterogeneous nucleation and to obtain a close glass behaviour structure before the crystal growth.
References w1x A. Mimura, N. Konishi, K. Ono, Y. Hosokawa, Y.A. Ono, T. Suzuki, K. Miyata, H. Kawakami, IEEE Electron Device 50 Ž1989. 351. w2x T. Matsuyama, M. Tanaka, S. Tsuda, S. Nakano, Y. Kuwano, Jpn. J. Appl. Phys. 32 Ž1993. 3720. w3x G. Harbeke, L. Krausbauer, E.F. Steigmeier, A.E. Widmer, H.F. Kappert, G. Neugebauer, Appl. Phys. Lett. 42 Ž1983. 249. w4x M.K. Hatalis, D. Greve, J. Appl. Phys. 63 Ž1988. 2260. w5x L. Haji, P. Joubert, M. Gendouz, N. Duhamel, B. Loisel, Mat. Res. Symp. Proc. 230 Ž1992. 177. w6x T. Mohammed-Brahim, M. Sarret, D. Briand, K. Kis-Sion, L. Haji, O. Bonnaud, D. Louer, ¨ A. Hadjadj, Phil. Mag. B, August, 1997. w7x S. Roorda, W.C. Sinke, J.M. Poate, D.C. Jacobson, S. Dierker, B.S. Dennis, D.J. Eaglesham, F. Spaepen, P. Fuoss, Phys. Rev. B 44 Ž1991. 3702. w8x K. Zellama, L. Chahed, P. Sladek, M.L. Theye, J.H. von Bardeleben, P. Roca i Cabarrocas, Phys. Rev. B 53 Ž1996. 3804. w9x J.P. Kleider, C. Longeaud, Solid State Phenomena, Scitec Publications, 44–46 Ž1995. 597–646. w10x R.A. Street, Adv. Phys. 30 Ž1981. 593. w11x R.A. Street, J. Kakalios, T.M. Hayes, Phys. Rev. B 34 Ž1986. 3030. w12x L. Pichon, F. Raoult, K. Mourgues, K. Kis-Sion, T. Mohammed-Brahim, O. Bonnaud, Thin Solid Films 296 Ž1997. 133.