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Solid phase epitaxy of silicon thin films by diode laser irradiation for photovoltaic applications
Thin Solid Films 520 (2012) 7087–7092
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Solid phase epitaxy of silicon thin films by diode laser irradiation for photovoltaic applications T. Schmidt ⁎, I. Höger, A. Gawlik, G. Andrä, F. Falk Institute of Photonic Technology, Albert-Einstein-Straße 9, 07743 Jena, Germany
a r t i c l e
i n f o
Article history: Received 14 December 2011 Received in revised form 1 August 2012 Accepted 1 August 2012 Available online 11 August 2012 Keywords: Seed crystals IR laser Solid phase epitaxy Solar cells Silicon Time resolved reflectivity measurements Temperature simulations Electron beam evaporation
a b s t r a c t High temperature solid phase epitaxial crystallization of amorphous silicon layers prepared by electron beam evaporation is investigated. By using a continuous wave diode laser for heating the films rapidly (in milliseconds to seconds) this method is suitable on glass substrates with low temperature resistance. Therefore, the method is an economically advantageous technique of producing absorber layers for thin film solar cells. For the experiments 500 nm of amorphous silicon was deposited on two different configurations of substrates. In the first one monocrystalline wafers of three different crystallographic orientations were used. In the second one a polycrystalline seed layer prepared on borosilicate glass served as substrate. The crystallization process was monitored in situ by time resolved reflectivity measurements. Depending on the crystal orientation 2 s to 3 s was needed for complete solid phase epitaxial crystallization of the amorphous films. The evolution of temperature during crystallization was simulated numerically. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Polycrystalline silicon thin films on foreign substrates, such as lowcost glass, are of increasing interest in photovoltaics [1]. In contrast to wafer technology, thin film techniques offer the advantage to produce solar cells in an in-line process for a more reasonable price with much lower material consumption. The efficiency of conventional a-Si thin film solar cells is mainly limited by the low electronic quality of the absorber [2], e.g. by short lifetimes of the generated charge carriers. As an alternative, cells based on polycrystalline silicon films are promising since they combine the advantages of thin film technologies with the crystalline properties of the photovoltaically active material. The preparation of thin polycrystalline silicon layers is mostly based on first depositing amorphous silicon (a-Si) on the substrate followed by some crystallization process. The virgin a-Si films used in this work were prepared by electron beam evaporation (EBE), which allows appreciably higher deposition rates than conventional plasma enhanced chemical vapor deposition. For the subsequent crystallization of the films, different methods have been investigated previously, e.g. aluminum induced crystallization [3], solid phase crystallization [4], or crystallization by excimer laser irradiation [5]. To minimize bulk recombination, the resulting grains should be as large as possible and should contain a low concentration of defects. It ⁎ Corresponding author. Tel.: +49 3641206401; fax: +49 3641206499. E-mail address: [email protected] (T. Schmidt). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.08.004
has been shown that melting of thin silicon films on glass substrates by diode laser irradiation with appropriate laser power and scanning speed leads to grains about 100 μm in length and 40 μm in width [6] with a few defects. Due to the short heating times of several milliseconds, this process can be used on glass substrates even if the melting temperature of crystalline silicon (1685 K) is reached. The method is most useful for producing rather thin films (100 nm to 400 nm), not thick enough for the absorber in thin film solar cells consisting of crystalline silicon (c-Si). Therefore, a two-step process has been proposed in which first a thin seed layer is produced in this way, followed by epitaxial thickening. There exist several methods for the epitaxial crystallization of silicon on polycrystalline seed layer, e.g. excimer laser melting [7]. However, for producing solar cell layers it is useful to avoid melting in order to preserve an already existing dopant profile in the virgin a-Si layers. Solid phase epitaxy of silicon has been investigated by using heating plates [8], by halogen lamp annealing [9], and by furnace heating [10]. Epitaxial growth on polycrystalline seed layers on glass was carried out only by furnace annealing [11]. Due to the relatively low temperatures allowed by the glass substrate (b 923 K) this process takes several hours [12]. In this work results of solid phase epitaxial crystallization of a-Si by diode laser irradiation are presented. On the one hand, diode lasers are available in the required power range with almost any desired beam shaping [13]. On the other hand, glass can be used as a substrate due to the local and rapid heating of the films. Contrary to excimer laser irradiation, the relative low absorbance coefficient of the infrared
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radiation in silicon combined with milliseconds of heating time allows to heat films of some microns thickness uniformly. Due to the much higher temperatures used, processing times are much faster than in case of furnace annealing. 2. Experimental details 2.1. Preparation of virgin layers Both, monocrystalline wafers and polycrystalline seed layers were used as a basic layer for epitaxial thickening. Fig. 1 shows these two configurations schematically together with the relevant dimensions. Quadratic samples with edge length of 2.5 cm were used. In the first configuration (Fig. 1a) the surfaces of the wafers were cleaned wet-chemically by the standard cleaning process of the Radio Corporation of America (see [14] for details). The resulting oxide layer was removed with 2% HF. The crystallographic orientations of the wafers were b100>, b110>, and b111 > with respect to the surface normal. For the second configuration (Fig. 1b) polycrystalline seed layers have to be prepared at first. Borosilicate glass substrates were cleaned with acetone and isopropanol. Then 200 nm of a-Si was deposited onto the glass by EBE. For crystallizing the seed layer the line focus beam (13 mm × 0.1 mm) of a diode laser (808 nm wavelength) was scanned at a rate of 3 cm/s so that the a-Si layer was melted. The melt solidifies to crystal grains of about 100 μm in length [6]. A resulting oxide layer on top of the polycrystalline silicon was removed with 2% HF. As the first step of epitaxial thickening a-Si was deposited on top of the oxide free seed layer or wafer by EBE at a rate of 100 to 220 nm/min. The substrate temperature during deposition was 473 K, and the back ground pressure was in the range of 10−5 Pa to 10 −4 Pa. 2.2. Setup for epitaxial crystallization A schematic layout of the experimental setup for solid phase epitaxy is shown in Fig. 2. For irradiation, a diode laser (Rofin-Sinar) with wavelength of 808 nm was used. The focus of the laser was adjusted onto the surface of the sample so that an elliptical gaussian profile resulted with
Fig. 2. Schematic layout of the setup for solid phase epitaxy by diode laser irradiation with time resolved reflectivity diagnostics.
dimensions 0.4 mm× 1.2 mm (1/e-width). By using a 2d positioning stage, different points on the sample could be irradiated. During the irradiation the reflectivity of the layer system was measured by a helium-neon laser, which was focused on the center of the irradiated area. The diameter of the diagnostic laser beam at the sample surface was 10 μm, so that an almost homogeneous temperature distribution within the measured area can be assumed. After crystallization the samples were analyzed by optical microscopy and by electron backscatter diffraction (EBSD). The EBSD analysis has been performed in a Tescan Lyra XMU dual beam microscope equipped with an EDAX/TSL XM4 camera. Data acquisition and treatment have been done with the software package TSL OIM 5. For the measurements cathode voltages of 20 kV to 30 kV were used. So the penetration depth of the electrons in silicon was less than 50 nm which allowed a surface sensitive mapping of the grain orientations. The lateral resolution was better than 5 μm. 2.3. Simulation of temperature evolution In case of wafers as a substrate, the time dependent temperature profiles were determined by numerical simulations. A schematic sketch of the simulated geometry with the used coordinate system and numbered boundaries is shown in Fig. 3. The thermal and optical properties used in the simulations are listed in Table 1 and were taken from [15–18]. The 3d heat equation with temperature dependent coefficients ρðT Þcp ðT Þ
∂T ¼ ∇⋅½κ ðT Þ∇T þ Q ðT Þ ∂t
ð1Þ
was solved over the whole domain by the finite element method. Here ρ is the density, cp the specific heat, and κ the thermal conductivity. The volumetric heating term (in W/m³) is given by Q ¼ ð1−RÞαI:
ð2Þ
For the intensity distribution (in W/m²) of the laser beam a Gaussian profile was assumed according to I¼
Fig. 1. Schemes of the two configurations of samples used in this work. a) monocrystalline wafer and b) polycrystalline seed layer as substrate for epitaxy.
! P x2 y2 exp − 2 − 2 −αz ; πσ x σ y σx σy
ð3Þ
with P (in W) the laser power, σx =201 μm and σy =627 μm the widths of the gaussian profile in x- and y-directions, α(T)=4πk(T)/λ the absorption coefficient, λ the wavelength (808 nm) and k the coefficient
T. Schmidt et al. / Thin Solid Films 520 (2012) 7087–7092
Fig. 3. Schematic sketch of the simulated geometry with the used of coordinate system and numbered boundaries (1: symmetric side walls, 2: outer side walls, 3: upper and lower boundaries). The wireframes show how the simulated geometry is continued by the boundary conditions.
of extinction. The reflectivity R was calculated from the optical constants of silicon. As initial condition T=Troom was used on the whole domain. For the boundaries 2 (outer sides) and 3 (top and bottom of the sample) the condition −n⋅(−κ∇T)=0 was chosen, which means that there is no heat flux over the boundary. Former simulations showed that thermal radiation has no significant effect on the temperature so that this assumption is suitable. For the boundaries 1 a symmetry condition was assumed, which is in fact the same as on the boundaries 2 and 3. By positioning the maximum of the intensity distribution at x=y=0 it is only necessary to simulate a fourth of the whole geometry. The melting and solidification processes of the silicon wafer were included in the simulations by adding a temperature dependent term to the heat capacity cp in the heat conduction Eq. (1) 2 2 Δcp ðT Þ ¼ C exp −ðT−T melt Þ =θ
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In the simulations the a-Si film on the wafer can be neglected, since the absorption in this thin film is negligible and the lateral heat conduction is very small as compared to that of the wafer. Detailed simulations of the setup with and without regarding the a-Si layer showed deviations in the temperature which were much smaller than those caused by the uncertainty in the optical constants. Moreover, even the thermal effect of crystallizing the thin a-Si layer is negligible. Referring to the data from [20] for b 100>‐oriented silicon, one can assume that the speed of the phase front during epitaxial solid phase crystallization of a-Si, vac, will not exceed 10 μm/s even at temperatures near the melting point of a-Si. Taking account the latent heat hac = 11.95 kJ/mol of crystallization of a-Si [16], the power density set free during the crystallization process is given byvac ⋅ hac ⋅ ρ/ M b 104 W/m² where M= 28.1 g/mol is the molar mass of silicon. Compared to the used laser intensities above 10 7 W/m² this contribution can be neglected. In contrast to solid phase crystallization, the phase front between molten and crystalline silicon reaches velocities up to several meters per seconds [5]. This leads to released power densities of > 10 9 W/m² which must be taken into account in the simulations. As a consequence, the thermal effect of the a-Si layer on top of the wafer is negligible and has not to be included in the simulation. Since the experimental verification of the calculated temperature curves is very difficult in the time scale of milliseconds, the results of the calculations have been checked by comparing the times at which the surface begins to melt. This characteristic point is well determined in the experiment by a sudden rise in the measured reflectivity. By doing this comparison for different laser powers and time scales, a sufficient good agreement between theory and experiment has been found.
ð4Þ
taking account of the latent heat hcl of the phase transition [19]. The width of the Gaussian function was taken asθ = 2 K. Therefore, the material parameters changed continuously over a range of Tmelt ± 2 K. The factor C was chosen so that the energy balance ∞
hcl ¼ ∫0 Δcp ðT ÞdT
ð5Þ
is fulfilled. Table 1 Thermal and optical properties used for the temperature simulations. The optical constants are given for λ = 808 nm.T in K. Parameter Heat conductivity c-Si (W/(m K), [15]) Heat conductivity l-Si (W/(m K), [15]) Specific heat c-Si (J/(kg K), [15]) Specific heat l-Si (J/(kg K), [15]) Density c-Si (kg/m³, [15]) Density l-Si (kg/m³, [15]) Latent heat melting c-Si (kJ/mol, [16]) Index of refraction c-Si ([17]) Index of refraction l-Si ([18]) Coefficient of extinction c-Si ([17]) Coefficient of extinction l-Si ([18])
Value −1:226 ; Tb1200 K κ ¼ 152399:52⋅T 900:162⋅T −0:502 ; T > 1200 K κ = 50.3 + 2.93 ⋅ 10−2(T − 1683) cp = 695.009 exp(2.375 ⋅ 10−4T) cp = 1050 ρ = 2320 ρ = 2520 hcl = 50.5 n = 3.6 + 3.5 ⋅ 10−4(T − 298.15) n = 4.5 k = 2 ⋅ 10−2 exp((T − 273.15)/498) k=6
Fig. 4. a) Measured reflectivity and calculated surface temperature for the center of an irradiated area, b) corresponding EBSD map. 500 nm a-Si on a b100>‐wafer was irradiated with 25 kW/cm² for 1 s. The inset shows the color coding of the EBSD map.
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3. Results and discussion 3.1. Experiments on wafers Fig. 4 shows the results of irradiating an a-Si layer on a wafer (1 s at 25 kW/cm²). During the irradiation the reflectivity first (up to about 50 ms) rises due to the heating of a-Si. Then (at about 50 ms) the reflectivity shows an instantaneous decrease. The temperature simulations show, that the melting point of silicon is not reached at this instant. Further experiments with thin films on glass substrates revealed that at this reflectivity step the a-Si layer crystallizes through a solid phase process. This coincides with the observations of Kokorowski et al., which showed that under microsecond to millisecond irradiation no separate melting of a-Si is detectable [21]. So this step in the reflectivity curve indicates the solid phase transition to c-Si, which leads to a reduction of the refractive index. Later on (at about 150 ms) the reflectivity rises again indicating the melting of the surface of the silicon. After melting the simulations indicate that the temperature in the center of the irradiated area stays almost constant at Tmelt, which is due to the fact that the latent heat needed for melting further silicon consumes nearly all of the power input of the laser beam. The related EBSD image shows a uniform red color in a central region which demonstrates that the underlying structure of the monocrystalline wafer was epitaxially transferred to the
Fig. 5. a) Measured reflectivity and calculated surface temperature for the center of irradiated areas on wafers of different orientations (red b100>, green b110>, blue b111>), b) corresponding EBSD maps. 500 nm a-Si on wafers was irradiated with 12.5 kW/cm² for 3.5 s.
surface. From the measured reflectivity and the temperature simulations we conclude that the center area solidified epitaxially from the melt. The surrounding ring of monocrystalline material was not formed via melting and liquid phase epitaxy. According to the simulations, the temperatures in this outer region did not reach the melting point of silicon. Consequently, solid phase epitaxy occurred there. Between the monocrystalline center region and the monocrystalline ring shaped region there is a region in which small grained material (diameter b 1 μm) was formed. There the surface of the a-Si layer may have been melted but not down to the wafer interface, so that the melt crystallized rather by nucleation and growth to fine grained silicon. The samples on b 100>, b110>, and b 111 > oriented wafers shown in Fig. 5 were irradiated for 3.5 s by a notably lower intensity (12.5 kW/cm²). The simulations of the temperature evolution and the measured reflectivities (no subsequent rise after stepdown) indicate that there was no melting involved. The EBSD maps clearly demonstrate the epitaxial crystallization of the films in the center of the irradiated area. The drop in the reflectivity in Fig. 5 indicating solid phase crystallization occurs at different times even if the irradiation power and the film thickness was the same in all the cases. The only difference is the orientation of the underlying silicon wafer. This implies that the crystal orientation influences the time needed for epitaxial growth. This is most easily understood if the epitaxial crystallization speed depends on orientation. As has been shown before, b 100>‐oriented grains exhibit the highest growth rates while b 111>‐oriented crystals grow slowest. This behavior, for instance, was explained by Csepregi et al. [10]. Since epitaxial crystallization finished before a stationary temperature distribution was reached, a declaration of a fixed growth rate is not meaningful in our case. The time needed for completing epitaxial growth according to Fig. 5a was 2.1 s, 2.7 s, and 2.9 s on (100), (110), and (111) wafers, respectively. By contrast, the outer regions shown in Fig. 4b were grown within 1 s or less, since due to the strong laser intensity higher temperatures were reached earlier. Nevertheless, there is an upper limit for the laser intensity at which a full epitaxial crystallization is possible for a given film thickness. At very high heating rates, the temperature rises very fast so that the nucleation rate in a-Si also increases very fast [2]. As a consequence, the amorphous layers then crystallize by random nucleation and growth resulting in fine grained material. This case is shown in Fig. 6 where the reflectivity evolves similar to that of Fig. 5, where epitaxial growth is achieved. Due to the high laser intensity of 25 kW/cm² the decrease in reflectivity occurs already at 40 ms. But it is also obvious that this occurs at the same
Fig. 6. Measured reflectivity and calculated surface temperature for the center of two irradiated areas. 500 nm a-Si on wafers of different orientations was irradiated with 25 kW/cm² for 60 ms.
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time independent of the orientation of the underlying wafer. The related EBSD maps show only fine grained material without epitaxy, which was suppressed by the strong nucleation. If published values for the activation energy and prefactor in an Arrhenius-like behavior of the growth rate are assumed (see e.g. [20,22]), it follows that below 1000 K epitaxial growth is negligible in the relevant time ranges. With respect to the simulated temperature evolution it is obvious, that roughly half of the processing time is needed only for heating up the films until a noticeable growth starts. By time-dependent laser intensities in case of locally fixed irradiation or an appropriate beam shaping in case of scanning processes, it should be possible to rise the temperature faster and hold it on a suitable high level. In this way the processing times could be shortened even more in further experiments.
3.2. Experiments on polycrystalline seed layers Epitaxial growth on polycrystalline seed layers was also realized within a few seconds. Fig. 7 shows a sample which was irradiated with 1.5 kW/cm² for 10 s. The heat conductivity of glass is much smaller than that of a silicon wafer, so that notably lower intensities for crystallization are required. The simulated temperatures and the measured reflectivities, which are not presented here, ruled out the existence of a melt. The EBSD map clearly shows that the polycrystalline structure of the seed layer was transferred to the surface of the sample. The elongated shape of the grains is just as it is in the diode laser crystallized seed layers. Since the growth rate depends on the orientation, the time needed for completing epitaxy cannot be simply derived from time resolved reflectivity measurements. Depending on which grains were hit by the helium-neon diagnostics laser beam, different values of reflectivity can be obtained. In Fig. 8 this inhomogeneous crystallization is shown as an example. The laser heating was interrupted after 500 ms. Areas of varying transmittance can be observed. Within the brightest regions the crystallization was greatly advanced, whereas the darker areas show mainly amorphous material. The sharp boundaries between regions of different brightness are correlated with grain boundaries appearing as black lines. This observation implies that the growth rates depend on orientation. In general, the kinetics of nucleation and growth strongly depend on the type of the a-Si and its preparation conditions. Deposition parameters such as substrate temperature, deposition rate, or the presence of electrical active and inactive dopants, have a strong influence on the rates of growth and nucleation [8]. Further experiments of our group revealed that an appropriate cleaning of the interface between
Fig. 8. Optical micrograph of partially crystallized 1 μm thick layer on a polycrystalline seed layer on a glass substrate. The sample was irradiated with 0.75 kW/cm² for 500 ms.
a-Si and the pre-crystallized seed layer can improve the epitaxial growth, too. 4. Conclusion In this paper results on epitaxial crystallization of a-Si by diode laser irradiation are presented. Time resolved reflectivity measurements are a well suited in-situ tool to distinguish between liquid and solid phase crystallization processes. On monocrystalline wafers the time needed for epitaxial growth has been determined. The temperature simulations are in good agreement with the observed phenomena. Fast solid phase epitaxy was observed on monocrystalline wafers as well as on polycrystalline seed layers. In the experiments films of about 500 nm thicknesses were crystallized epitaxially. Depending on orientation, processing times of less than 3 s were needed. Higher power irradiation, which leads to higher growth rate, does not lead to epitaxy because nucleation rates in the solid state increase even more, so that fine grained silicon is produced. By manipulating the temporal heating profile, modifying the deposition parameters, and improving the cleanliness of the seed layer surface, epitaxial growth can be speeded up. For this, further investigations are necessary. Acknowledgment We gratefully acknowledge the funding from the German state of Thuringia under contract 2008 FE 9160 (SolLUX) cofinanced by the Federal Government as well as the European Community (EFRE). References
Fig. 7. EBSD map of epitaxially grown silicon. 500 nm a-Si on a polycrystalline seed layer on a glass substrate was irradiated with 1.5 kW/cm² for 10 s.
[1] G. Andrä, F. Falk, Phys. Status Solidi C 5 (2008) 3221. [2] G. Mannino, C. Spinella, R. Ruggeri, A. La Magna, G. Fisicaro, E. Fazio, F. Neri, V. Privitera, Appl. Phys. Lett. 97 (2010) 22107_1. [3] O. Nast, T. Puzzer, L. Koschier, A. Sproul, S. Wenham, Appl. Phys. Lett. 73 (1998) 3214. [4] M. Green, P. Basore, N. Chang, D. Clugston, R. Egan, R. Evans, D. Hogg, S. Jarnason, M. Keevers, P. Lasswell, J. O'Sullivan, U. Schubert, A. Turner, S. Wenham, T. Young, Sol. Energy 77 (2004) 857. [5] C.-C. Kuo, Opt. Lasers Eng. 49 (2011) 804. [6] G. Andrä, J. Bergmann, F. Falk, Thin Solid Films 487 (2005) 77. [7] G. Ivlev, E. Gatskevich, Tech. Phys. Lett. 28 (2002) 988. [8] G. Olson, J. Roth, Mater. Sci. Rep. 3 (1988) 1. [9] O. Tuezuen, A. Slaoui, S. Roques, A. Focsa, F. Jomard, D. Ballutaud, Thin Solid Films 517 (2009) 6358. [10] L. Csepregi, E. Kennedy, J. Mayer, T. Sigmon, J. Appl. Phys. 49 (1978) 3906. [11] X. Maeder, C. Niederberger, S. Christiansen, A. Bochmann, G. Andrä, A. Gawlik, F. Falk, J. Michler, Thin Solid Films 519 (2010) 58.
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[12] G. Andrä, T. Gimpel, A. Gawlik, E. Ose, S. Bochmann, A. Christiansen, G. Sáfrán, J.L. Lábár, F. Falk, in: 23rd Eur. Photovolt. Sol. Energy Conf, 2008, p. 2194. [13] N. Lichtenstein, R. Baettig, R. Brunner, J. Müller, B. Valk, A. Gawlik, J. Bergmann, F. Falk, Phys. Procedia 5 (2010) 109. [14] W. Kern, J. Electrochem. Soc. 137 (1990) 1887. [15] S. de Unamuno, E. Fogarassy, Appl. Surf. Sci. 36 (1989) 1. [16] E. Donovan, F. Spaepen, D. Turnbull, J. Poate, D. Jacobson, J. Appl. Phys. 57 (1985) 1795.
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Y. Sun, X. Zhang, C. Grigoropoulos, Int. J. Heat Mass Transf. 40 (1997) 1591. M. Fuchs, J. Phys. Condens. Matter 12 (2000) 4341. C.-H. Chang, L.-S. Chao, Int. Commun. Heat Mass 35 (2008) 571. J. Roth, G. Olson, D. Jacobson, J. Poate, Appl. Phys. Lett. 57 (1990) 1340. S.A. Kokorowski, G.L. Olson, J.A. Roth, L.D. Hess, Phys. Rev. Lett. 48 (1982) 498. U. Köster, Phys. Status Solidi A 48 (1987) 313.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Solid phase epitaxy of silicon thin films by diode laser irradiation for photovoltaic applications T. Schmidt ⁎, I. Höger, A. Gawlik, G. Andrä, F. Falk Institute of Photonic Technology, Albert-Einstein-Straße 9, 07743 Jena, Germany
a r t i c l e
i n f o
Article history: Received 14 December 2011 Received in revised form 1 August 2012 Accepted 1 August 2012 Available online 11 August 2012 Keywords: Seed crystals IR laser Solid phase epitaxy Solar cells Silicon Time resolved reflectivity measurements Temperature simulations Electron beam evaporation
a b s t r a c t High temperature solid phase epitaxial crystallization of amorphous silicon layers prepared by electron beam evaporation is investigated. By using a continuous wave diode laser for heating the films rapidly (in milliseconds to seconds) this method is suitable on glass substrates with low temperature resistance. Therefore, the method is an economically advantageous technique of producing absorber layers for thin film solar cells. For the experiments 500 nm of amorphous silicon was deposited on two different configurations of substrates. In the first one monocrystalline wafers of three different crystallographic orientations were used. In the second one a polycrystalline seed layer prepared on borosilicate glass served as substrate. The crystallization process was monitored in situ by time resolved reflectivity measurements. Depending on the crystal orientation 2 s to 3 s was needed for complete solid phase epitaxial crystallization of the amorphous films. The evolution of temperature during crystallization was simulated numerically. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Polycrystalline silicon thin films on foreign substrates, such as lowcost glass, are of increasing interest in photovoltaics [1]. In contrast to wafer technology, thin film techniques offer the advantage to produce solar cells in an in-line process for a more reasonable price with much lower material consumption. The efficiency of conventional a-Si thin film solar cells is mainly limited by the low electronic quality of the absorber [2], e.g. by short lifetimes of the generated charge carriers. As an alternative, cells based on polycrystalline silicon films are promising since they combine the advantages of thin film technologies with the crystalline properties of the photovoltaically active material. The preparation of thin polycrystalline silicon layers is mostly based on first depositing amorphous silicon (a-Si) on the substrate followed by some crystallization process. The virgin a-Si films used in this work were prepared by electron beam evaporation (EBE), which allows appreciably higher deposition rates than conventional plasma enhanced chemical vapor deposition. For the subsequent crystallization of the films, different methods have been investigated previously, e.g. aluminum induced crystallization [3], solid phase crystallization [4], or crystallization by excimer laser irradiation [5]. To minimize bulk recombination, the resulting grains should be as large as possible and should contain a low concentration of defects. It ⁎ Corresponding author. Tel.: +49 3641206401; fax: +49 3641206499. E-mail address: [email protected] (T. Schmidt). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.08.004
has been shown that melting of thin silicon films on glass substrates by diode laser irradiation with appropriate laser power and scanning speed leads to grains about 100 μm in length and 40 μm in width [6] with a few defects. Due to the short heating times of several milliseconds, this process can be used on glass substrates even if the melting temperature of crystalline silicon (1685 K) is reached. The method is most useful for producing rather thin films (100 nm to 400 nm), not thick enough for the absorber in thin film solar cells consisting of crystalline silicon (c-Si). Therefore, a two-step process has been proposed in which first a thin seed layer is produced in this way, followed by epitaxial thickening. There exist several methods for the epitaxial crystallization of silicon on polycrystalline seed layer, e.g. excimer laser melting [7]. However, for producing solar cell layers it is useful to avoid melting in order to preserve an already existing dopant profile in the virgin a-Si layers. Solid phase epitaxy of silicon has been investigated by using heating plates [8], by halogen lamp annealing [9], and by furnace heating [10]. Epitaxial growth on polycrystalline seed layers on glass was carried out only by furnace annealing [11]. Due to the relatively low temperatures allowed by the glass substrate (b 923 K) this process takes several hours [12]. In this work results of solid phase epitaxial crystallization of a-Si by diode laser irradiation are presented. On the one hand, diode lasers are available in the required power range with almost any desired beam shaping [13]. On the other hand, glass can be used as a substrate due to the local and rapid heating of the films. Contrary to excimer laser irradiation, the relative low absorbance coefficient of the infrared
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radiation in silicon combined with milliseconds of heating time allows to heat films of some microns thickness uniformly. Due to the much higher temperatures used, processing times are much faster than in case of furnace annealing. 2. Experimental details 2.1. Preparation of virgin layers Both, monocrystalline wafers and polycrystalline seed layers were used as a basic layer for epitaxial thickening. Fig. 1 shows these two configurations schematically together with the relevant dimensions. Quadratic samples with edge length of 2.5 cm were used. In the first configuration (Fig. 1a) the surfaces of the wafers were cleaned wet-chemically by the standard cleaning process of the Radio Corporation of America (see [14] for details). The resulting oxide layer was removed with 2% HF. The crystallographic orientations of the wafers were b100>, b110>, and b111 > with respect to the surface normal. For the second configuration (Fig. 1b) polycrystalline seed layers have to be prepared at first. Borosilicate glass substrates were cleaned with acetone and isopropanol. Then 200 nm of a-Si was deposited onto the glass by EBE. For crystallizing the seed layer the line focus beam (13 mm × 0.1 mm) of a diode laser (808 nm wavelength) was scanned at a rate of 3 cm/s so that the a-Si layer was melted. The melt solidifies to crystal grains of about 100 μm in length [6]. A resulting oxide layer on top of the polycrystalline silicon was removed with 2% HF. As the first step of epitaxial thickening a-Si was deposited on top of the oxide free seed layer or wafer by EBE at a rate of 100 to 220 nm/min. The substrate temperature during deposition was 473 K, and the back ground pressure was in the range of 10−5 Pa to 10 −4 Pa. 2.2. Setup for epitaxial crystallization A schematic layout of the experimental setup for solid phase epitaxy is shown in Fig. 2. For irradiation, a diode laser (Rofin-Sinar) with wavelength of 808 nm was used. The focus of the laser was adjusted onto the surface of the sample so that an elliptical gaussian profile resulted with
Fig. 2. Schematic layout of the setup for solid phase epitaxy by diode laser irradiation with time resolved reflectivity diagnostics.
dimensions 0.4 mm× 1.2 mm (1/e-width). By using a 2d positioning stage, different points on the sample could be irradiated. During the irradiation the reflectivity of the layer system was measured by a helium-neon laser, which was focused on the center of the irradiated area. The diameter of the diagnostic laser beam at the sample surface was 10 μm, so that an almost homogeneous temperature distribution within the measured area can be assumed. After crystallization the samples were analyzed by optical microscopy and by electron backscatter diffraction (EBSD). The EBSD analysis has been performed in a Tescan Lyra XMU dual beam microscope equipped with an EDAX/TSL XM4 camera. Data acquisition and treatment have been done with the software package TSL OIM 5. For the measurements cathode voltages of 20 kV to 30 kV were used. So the penetration depth of the electrons in silicon was less than 50 nm which allowed a surface sensitive mapping of the grain orientations. The lateral resolution was better than 5 μm. 2.3. Simulation of temperature evolution In case of wafers as a substrate, the time dependent temperature profiles were determined by numerical simulations. A schematic sketch of the simulated geometry with the used coordinate system and numbered boundaries is shown in Fig. 3. The thermal and optical properties used in the simulations are listed in Table 1 and were taken from [15–18]. The 3d heat equation with temperature dependent coefficients ρðT Þcp ðT Þ
∂T ¼ ∇⋅½κ ðT Þ∇T þ Q ðT Þ ∂t
ð1Þ
was solved over the whole domain by the finite element method. Here ρ is the density, cp the specific heat, and κ the thermal conductivity. The volumetric heating term (in W/m³) is given by Q ¼ ð1−RÞαI:
ð2Þ
For the intensity distribution (in W/m²) of the laser beam a Gaussian profile was assumed according to I¼
Fig. 1. Schemes of the two configurations of samples used in this work. a) monocrystalline wafer and b) polycrystalline seed layer as substrate for epitaxy.
! P x2 y2 exp − 2 − 2 −αz ; πσ x σ y σx σy
ð3Þ
with P (in W) the laser power, σx =201 μm and σy =627 μm the widths of the gaussian profile in x- and y-directions, α(T)=4πk(T)/λ the absorption coefficient, λ the wavelength (808 nm) and k the coefficient
T. Schmidt et al. / Thin Solid Films 520 (2012) 7087–7092
Fig. 3. Schematic sketch of the simulated geometry with the used of coordinate system and numbered boundaries (1: symmetric side walls, 2: outer side walls, 3: upper and lower boundaries). The wireframes show how the simulated geometry is continued by the boundary conditions.
of extinction. The reflectivity R was calculated from the optical constants of silicon. As initial condition T=Troom was used on the whole domain. For the boundaries 2 (outer sides) and 3 (top and bottom of the sample) the condition −n⋅(−κ∇T)=0 was chosen, which means that there is no heat flux over the boundary. Former simulations showed that thermal radiation has no significant effect on the temperature so that this assumption is suitable. For the boundaries 1 a symmetry condition was assumed, which is in fact the same as on the boundaries 2 and 3. By positioning the maximum of the intensity distribution at x=y=0 it is only necessary to simulate a fourth of the whole geometry. The melting and solidification processes of the silicon wafer were included in the simulations by adding a temperature dependent term to the heat capacity cp in the heat conduction Eq. (1) 2 2 Δcp ðT Þ ¼ C exp −ðT−T melt Þ =θ
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In the simulations the a-Si film on the wafer can be neglected, since the absorption in this thin film is negligible and the lateral heat conduction is very small as compared to that of the wafer. Detailed simulations of the setup with and without regarding the a-Si layer showed deviations in the temperature which were much smaller than those caused by the uncertainty in the optical constants. Moreover, even the thermal effect of crystallizing the thin a-Si layer is negligible. Referring to the data from [20] for b 100>‐oriented silicon, one can assume that the speed of the phase front during epitaxial solid phase crystallization of a-Si, vac, will not exceed 10 μm/s even at temperatures near the melting point of a-Si. Taking account the latent heat hac = 11.95 kJ/mol of crystallization of a-Si [16], the power density set free during the crystallization process is given byvac ⋅ hac ⋅ ρ/ M b 104 W/m² where M= 28.1 g/mol is the molar mass of silicon. Compared to the used laser intensities above 10 7 W/m² this contribution can be neglected. In contrast to solid phase crystallization, the phase front between molten and crystalline silicon reaches velocities up to several meters per seconds [5]. This leads to released power densities of > 10 9 W/m² which must be taken into account in the simulations. As a consequence, the thermal effect of the a-Si layer on top of the wafer is negligible and has not to be included in the simulation. Since the experimental verification of the calculated temperature curves is very difficult in the time scale of milliseconds, the results of the calculations have been checked by comparing the times at which the surface begins to melt. This characteristic point is well determined in the experiment by a sudden rise in the measured reflectivity. By doing this comparison for different laser powers and time scales, a sufficient good agreement between theory and experiment has been found.
ð4Þ
taking account of the latent heat hcl of the phase transition [19]. The width of the Gaussian function was taken asθ = 2 K. Therefore, the material parameters changed continuously over a range of Tmelt ± 2 K. The factor C was chosen so that the energy balance ∞
hcl ¼ ∫0 Δcp ðT ÞdT
ð5Þ
is fulfilled. Table 1 Thermal and optical properties used for the temperature simulations. The optical constants are given for λ = 808 nm.T in K. Parameter Heat conductivity c-Si (W/(m K), [15]) Heat conductivity l-Si (W/(m K), [15]) Specific heat c-Si (J/(kg K), [15]) Specific heat l-Si (J/(kg K), [15]) Density c-Si (kg/m³, [15]) Density l-Si (kg/m³, [15]) Latent heat melting c-Si (kJ/mol, [16]) Index of refraction c-Si ([17]) Index of refraction l-Si ([18]) Coefficient of extinction c-Si ([17]) Coefficient of extinction l-Si ([18])
Value −1:226 ; Tb1200 K κ ¼ 152399:52⋅T 900:162⋅T −0:502 ; T > 1200 K κ = 50.3 + 2.93 ⋅ 10−2(T − 1683) cp = 695.009 exp(2.375 ⋅ 10−4T) cp = 1050 ρ = 2320 ρ = 2520 hcl = 50.5 n = 3.6 + 3.5 ⋅ 10−4(T − 298.15) n = 4.5 k = 2 ⋅ 10−2 exp((T − 273.15)/498) k=6
Fig. 4. a) Measured reflectivity and calculated surface temperature for the center of an irradiated area, b) corresponding EBSD map. 500 nm a-Si on a b100>‐wafer was irradiated with 25 kW/cm² for 1 s. The inset shows the color coding of the EBSD map.
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3. Results and discussion 3.1. Experiments on wafers Fig. 4 shows the results of irradiating an a-Si layer on a wafer (1 s at 25 kW/cm²). During the irradiation the reflectivity first (up to about 50 ms) rises due to the heating of a-Si. Then (at about 50 ms) the reflectivity shows an instantaneous decrease. The temperature simulations show, that the melting point of silicon is not reached at this instant. Further experiments with thin films on glass substrates revealed that at this reflectivity step the a-Si layer crystallizes through a solid phase process. This coincides with the observations of Kokorowski et al., which showed that under microsecond to millisecond irradiation no separate melting of a-Si is detectable [21]. So this step in the reflectivity curve indicates the solid phase transition to c-Si, which leads to a reduction of the refractive index. Later on (at about 150 ms) the reflectivity rises again indicating the melting of the surface of the silicon. After melting the simulations indicate that the temperature in the center of the irradiated area stays almost constant at Tmelt, which is due to the fact that the latent heat needed for melting further silicon consumes nearly all of the power input of the laser beam. The related EBSD image shows a uniform red color in a central region which demonstrates that the underlying structure of the monocrystalline wafer was epitaxially transferred to the
Fig. 5. a) Measured reflectivity and calculated surface temperature for the center of irradiated areas on wafers of different orientations (red b100>, green b110>, blue b111>), b) corresponding EBSD maps. 500 nm a-Si on wafers was irradiated with 12.5 kW/cm² for 3.5 s.
surface. From the measured reflectivity and the temperature simulations we conclude that the center area solidified epitaxially from the melt. The surrounding ring of monocrystalline material was not formed via melting and liquid phase epitaxy. According to the simulations, the temperatures in this outer region did not reach the melting point of silicon. Consequently, solid phase epitaxy occurred there. Between the monocrystalline center region and the monocrystalline ring shaped region there is a region in which small grained material (diameter b 1 μm) was formed. There the surface of the a-Si layer may have been melted but not down to the wafer interface, so that the melt crystallized rather by nucleation and growth to fine grained silicon. The samples on b 100>, b110>, and b 111 > oriented wafers shown in Fig. 5 were irradiated for 3.5 s by a notably lower intensity (12.5 kW/cm²). The simulations of the temperature evolution and the measured reflectivities (no subsequent rise after stepdown) indicate that there was no melting involved. The EBSD maps clearly demonstrate the epitaxial crystallization of the films in the center of the irradiated area. The drop in the reflectivity in Fig. 5 indicating solid phase crystallization occurs at different times even if the irradiation power and the film thickness was the same in all the cases. The only difference is the orientation of the underlying silicon wafer. This implies that the crystal orientation influences the time needed for epitaxial growth. This is most easily understood if the epitaxial crystallization speed depends on orientation. As has been shown before, b 100>‐oriented grains exhibit the highest growth rates while b 111>‐oriented crystals grow slowest. This behavior, for instance, was explained by Csepregi et al. [10]. Since epitaxial crystallization finished before a stationary temperature distribution was reached, a declaration of a fixed growth rate is not meaningful in our case. The time needed for completing epitaxial growth according to Fig. 5a was 2.1 s, 2.7 s, and 2.9 s on (100), (110), and (111) wafers, respectively. By contrast, the outer regions shown in Fig. 4b were grown within 1 s or less, since due to the strong laser intensity higher temperatures were reached earlier. Nevertheless, there is an upper limit for the laser intensity at which a full epitaxial crystallization is possible for a given film thickness. At very high heating rates, the temperature rises very fast so that the nucleation rate in a-Si also increases very fast [2]. As a consequence, the amorphous layers then crystallize by random nucleation and growth resulting in fine grained material. This case is shown in Fig. 6 where the reflectivity evolves similar to that of Fig. 5, where epitaxial growth is achieved. Due to the high laser intensity of 25 kW/cm² the decrease in reflectivity occurs already at 40 ms. But it is also obvious that this occurs at the same
Fig. 6. Measured reflectivity and calculated surface temperature for the center of two irradiated areas. 500 nm a-Si on wafers of different orientations was irradiated with 25 kW/cm² for 60 ms.
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time independent of the orientation of the underlying wafer. The related EBSD maps show only fine grained material without epitaxy, which was suppressed by the strong nucleation. If published values for the activation energy and prefactor in an Arrhenius-like behavior of the growth rate are assumed (see e.g. [20,22]), it follows that below 1000 K epitaxial growth is negligible in the relevant time ranges. With respect to the simulated temperature evolution it is obvious, that roughly half of the processing time is needed only for heating up the films until a noticeable growth starts. By time-dependent laser intensities in case of locally fixed irradiation or an appropriate beam shaping in case of scanning processes, it should be possible to rise the temperature faster and hold it on a suitable high level. In this way the processing times could be shortened even more in further experiments.
3.2. Experiments on polycrystalline seed layers Epitaxial growth on polycrystalline seed layers was also realized within a few seconds. Fig. 7 shows a sample which was irradiated with 1.5 kW/cm² for 10 s. The heat conductivity of glass is much smaller than that of a silicon wafer, so that notably lower intensities for crystallization are required. The simulated temperatures and the measured reflectivities, which are not presented here, ruled out the existence of a melt. The EBSD map clearly shows that the polycrystalline structure of the seed layer was transferred to the surface of the sample. The elongated shape of the grains is just as it is in the diode laser crystallized seed layers. Since the growth rate depends on the orientation, the time needed for completing epitaxy cannot be simply derived from time resolved reflectivity measurements. Depending on which grains were hit by the helium-neon diagnostics laser beam, different values of reflectivity can be obtained. In Fig. 8 this inhomogeneous crystallization is shown as an example. The laser heating was interrupted after 500 ms. Areas of varying transmittance can be observed. Within the brightest regions the crystallization was greatly advanced, whereas the darker areas show mainly amorphous material. The sharp boundaries between regions of different brightness are correlated with grain boundaries appearing as black lines. This observation implies that the growth rates depend on orientation. In general, the kinetics of nucleation and growth strongly depend on the type of the a-Si and its preparation conditions. Deposition parameters such as substrate temperature, deposition rate, or the presence of electrical active and inactive dopants, have a strong influence on the rates of growth and nucleation [8]. Further experiments of our group revealed that an appropriate cleaning of the interface between
Fig. 8. Optical micrograph of partially crystallized 1 μm thick layer on a polycrystalline seed layer on a glass substrate. The sample was irradiated with 0.75 kW/cm² for 500 ms.
a-Si and the pre-crystallized seed layer can improve the epitaxial growth, too. 4. Conclusion In this paper results on epitaxial crystallization of a-Si by diode laser irradiation are presented. Time resolved reflectivity measurements are a well suited in-situ tool to distinguish between liquid and solid phase crystallization processes. On monocrystalline wafers the time needed for epitaxial growth has been determined. The temperature simulations are in good agreement with the observed phenomena. Fast solid phase epitaxy was observed on monocrystalline wafers as well as on polycrystalline seed layers. In the experiments films of about 500 nm thicknesses were crystallized epitaxially. Depending on orientation, processing times of less than 3 s were needed. Higher power irradiation, which leads to higher growth rate, does not lead to epitaxy because nucleation rates in the solid state increase even more, so that fine grained silicon is produced. By manipulating the temporal heating profile, modifying the deposition parameters, and improving the cleanliness of the seed layer surface, epitaxial growth can be speeded up. For this, further investigations are necessary. Acknowledgment We gratefully acknowledge the funding from the German state of Thuringia under contract 2008 FE 9160 (SolLUX) cofinanced by the Federal Government as well as the European Community (EFRE). References
Fig. 7. EBSD map of epitaxially grown silicon. 500 nm a-Si on a polycrystalline seed layer on a glass substrate was irradiated with 1.5 kW/cm² for 10 s.
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