**3.1 Microstructure of the capping layer**

The applied recrystallization energy density strongly influences the surface morphology and microstructure of the recrystallized silicon film. With the energy increasing, the capping layer becomes smooth and continuous and less and small pinholes form in the silicon film. Excess of recrystallization energy density leads to larger voids in the capping layer, more WSi2/Si eutectic crystallites, a thinner tungsten layer and a thicker tungstendisilicide layer.

Fig.4 gives the top view of the polycrystalline silicon film after the recrystallization. The EB surface treatment leads to recrystallization to obtain poly-Si films with grain sizes in the order of several 10µm in width and 100µm in the scanning direction as shown in Fig.5. The polycrystalline silicon films in Fig.4 are EB remelting with four different EB energy densities. Area A was treated with an energy density of 0.34J/mm2 (the lowest of the four areas) while area D was treated with an energy density of 0.4J/mm2 (highest of the four areas) on the same nanocrystalline silicon layer.

Fig. 4. Top view of the recrystallized silicon film, with increase of applied energy density from the left to the right

Application of Electron Beam Treatment in Polycrystalline Silicon Films Manufacture for Solar Cell 81

Fig.6 and Fig.7 show the morphology and microstructure of the EB treated layers. The nanocrystalline silicon is zone melted and recrystallized (ZMR) completely under all the energy chosen in this experiment. It can be seen that after the EB surface treatment, microsized silicon grains were formed in all the samples treated under different electron beam

The outmost surface was silicon dioxides with some voids and pinholes (bright spots), as shown in Fig.6. Large areas with a rough surface were where the silicon dioxide capping layer (SiO2) existed. The voids (the dark area in Fig.6) in the silicon dioxide capping layer penetrated into the silicon layer with smooth edges. The bright areas were the bottom of the

Influences of the EB energy density on the morphology of deposited films are summarized in Table 1. The energy density influences the surface morphology of the film system strongly. The capping layer exhibited more voids when a lower EB energy density was used, as shown in Fig.6a. The SiO2 capping layer is rougher and appeared as discontinuous droplet morphology in this condition. In addition, large tungstendisilicide pinholes formed due to the lower fluidity and less reaction between the silicon melt and the tungsten interlayer. When the EB energy density was increased, the capping layer becomes smoother and the size of voids was reduced. The number and size of pinholes also became smaller. However, when excess EB energy was applied, the solidification process became unstable and the amount of pinholes increased again. The silicon dioxide capping layer became

Fig. 7. Microstructure of the capping layer and silicon grain under different energy density є

**Silicon Silicon** 

**Cappi**

**Pinhol**

It was suggested that the voids are caused by the volume change of the capping layer and the silicon melt during the recrystallization process. Early work [6] suggested that the silicon dioxide in the capping layer could be considered as a fluid with a relatively high viscosity at the EB treatment temperature. For the same amount of silicon, the volume of the solid VS is about 1.1 times of that of the liquid VL. Therefore, during solidification process of the silicon melt, the volume increases will produce a curved melt surface. This will generates a tensile stress in the capping layer because of he interface enlargement between the viscous capping layer and the molten silicon. Once the critical strain of the capping layer is surpassed, voids will form in the capping layer. Due to the surface tension of the capping layer and its

energy density є.

pinholes in which the WSi2 remained.

discontinuous in this case, as shown in Fig. 6d.

**(a) Cappi (b)** 

(a) є=0.34J/mm2 ; (b) є=0.4J/mm2

(Fu et al., 2007)

**Pinhol**

Fig. 5. Grain microstructure of Ploy-Silicon absorber after recrystallization

(a) є=0.34J/mm2 ; (b) є=0.36J/mm2 ; (c) є=0.38J/mm2; (d) є=0.4J/mm2

Fig. 6. Surface morphology of the recrystallized silicon layer under different energy density є (Fu et al., 2007)

Fig.6 and Fig.7 show the morphology and microstructure of the EB treated layers. The nanocrystalline silicon is zone melted and recrystallized (ZMR) completely under all the energy chosen in this experiment. It can be seen that after the EB surface treatment, microsized silicon grains were formed in all the samples treated under different electron beam energy density є.

The outmost surface was silicon dioxides with some voids and pinholes (bright spots), as shown in Fig.6. Large areas with a rough surface were where the silicon dioxide capping layer (SiO2) existed. The voids (the dark area in Fig.6) in the silicon dioxide capping layer penetrated into the silicon layer with smooth edges. The bright areas were the bottom of the pinholes in which the WSi2 remained.

Influences of the EB energy density on the morphology of deposited films are summarized in Table 1. The energy density influences the surface morphology of the film system strongly. The capping layer exhibited more voids when a lower EB energy density was used, as shown in Fig.6a. The SiO2 capping layer is rougher and appeared as discontinuous droplet morphology in this condition. In addition, large tungstendisilicide pinholes formed due to the lower fluidity and less reaction between the silicon melt and the tungsten interlayer. When the EB energy density was increased, the capping layer becomes smoother and the size of voids was reduced. The number and size of pinholes also became smaller. However, when excess EB energy was applied, the solidification process became unstable and the amount of pinholes increased again. The silicon dioxide capping layer became discontinuous in this case, as shown in Fig. 6d.

(a) є=0.34J/mm2 ; (b) є=0.4J/mm2

80 Solar Cells – Thin-Film Technologies

**50µm** 

**Pinhol**

**Voids** 

**Scanning direction** 

Fig. 5. Grain microstructure of Ploy-Silicon absorber after recrystallization

**(a) (b)**

**(c) (d)**

(a) є=0.34J/mm2 ; (b) є=0.36J/mm2 ; (c) є=0.38J/mm2; (d) є=0.4J/mm2

є (Fu et al., 2007)

Fig. 6. Surface morphology of the recrystallized silicon layer under different energy density

Fig. 7. Microstructure of the capping layer and silicon grain under different energy density є (Fu et al., 2007)

It was suggested that the voids are caused by the volume change of the capping layer and the silicon melt during the recrystallization process. Early work [6] suggested that the silicon dioxide in the capping layer could be considered as a fluid with a relatively high viscosity at the EB treatment temperature. For the same amount of silicon, the volume of the solid VS is about 1.1 times of that of the liquid VL. Therefore, during solidification process of the silicon melt, the volume increases will produce a curved melt surface. This will generates a tensile stress in the capping layer because of he interface enlargement between the viscous capping layer and the molten silicon. Once the critical strain of the capping layer is surpassed, voids will form in the capping layer. Due to the surface tension of the capping layer and its

Application of Electron Beam Treatment in Polycrystalline Silicon Films Manufacture for Solar Cell 83

liquid-solid transformation temperature, and their growth will be not immediately accompanied by the tungstendisilicide crystallite formation. Therefore, the silicon phase forms dendrites, which grow over a range of temperature like ordinary primary crystallites. Below the eutectic reaction temperature, the remaining melt solidifies eutectically as soon as

the melt is undercooled to a critical temperature to allow silicon crystallite growth.

Fig. 8. Phase diagram of the Si-W alloy system in equilibrium (Hansen, 1958)

tungsten enriched silicon melt. This is a nonequilibrium solidification process.

**different recrystallization energy** 

**3.3 Microstructure and distribution of the eutectic crystallites (WSi2/Si) under** 

Tungstendisilicide (WSi2) was formed at the tungsten/silicon interface but also at the grain boundaries of the silicon throughout all the EB energy density range. A top view scanning electron spectroscopy (SEM) and EDX analysis of the surface region showed that eutectic structure (tungstendisilicide precipitates / silicon) were mainly localized at the recrystallized silicon grain boundaries, as is shown in Fig.9. A typical hypoeutectic structure was found in the exposed silicon layer, which consisted of cored primary silicon dendrites (dendritic characteristic was not very evident) surrounded by the eutectic of the silicon and the tungstendisilicide precipitates. In this eutectic, tungstendisilicide (white areas in the lamellar shape) grew until the surrounding silicon melt had fully crystallized. The eutectic statistically distributed at the primary silicon grain boundaries. The formation and distribution of the eutectic depended on the crystallization and the growth dynamic of the

The size and the amount of the tungstendisilicide/silicon eutectic depended on the course of the process: when the higher the energy was used in the recrystallization process of the silicon layer, more and large tungstendisilicide crystals grew in the silicon melt. In addition, the WSi2/Si eutectic became coarser at the primary silicon grain boundaries and spread more widely. This was due to the prolonged solidification period for the tungsten enriched silicon melt in the remaining liquid, primarily at the grain boundary. At these sites, the tungstendisilicide crystallites precipitated in the final solidification areas at lower temperature than in case of equilibrium, due to the high tungsten concentration in the

adhesion to the silicon melt the capping layer also arches upwards and widens the voids. This effect is enhanced by thermal stress and outgassing during the solidification process [5]. As the size, area and viscosity of the SiO2 layer is affected by the EB energy density, the size and the number of the voids in the capping layer are dependant on the EB energy density as well.


Table 1. Influence of the recrystallization energy on the surface morphology of the silicon film system

#### **3.2 Formation of eutectic (WSi2/Si)**

This Chapter gives the details about the formation of Tungstendisilicide (WSi2). The film system consists of a 20μm thick silicon layer on a 1.2μm thick tungsten film. Tungstendisilicide (WSi2) is formed at the interface tungsten/silicon but also at the grain boundaries of the silicon. Because of the fast melting and cooling of the silicon film, the solidification process of the silicon film is a nonequilibrium solidification process.

It was claimed that tungstendisilicides were formed in their tetragonal (Hansen, 1958; Döscher et al., 1994) by the solid/solid state reaction and the solid/liquid state reaction between tungsten and silicon according to equation (1) and (2).

$$\text{V}\,\text{2Si}\_{\text{(s)}} + \text{W}\_{\text{(s)}} \xrightarrow{700^{\circ}\text{C} - 1390^{\circ}\text{C}} \text{V}\,\text{8Si}\_{\text{2(s)}}\tag{1}$$

$$\text{V}\,2\text{Si}\_{\text{(l)}} + \text{W}\_{\text{(s)}} \xrightarrow{\text{>1390°C}} \text{V}\,\text{V}\,\text{Si}\_{\text{2(s)}}\tag{2}$$

Formation of the eutectics can be explained using the phase diagram of the Si-W alloy system, as shown in Fig.8. The reactions should start at temperatures above 700°C. The eutectic crystallites (WSi2/Si) are precipitated from the silicon melt at a eutectic concentration of 0.8 at% W at the eutectic temperature 1390°C in thermal equilibrium. With the temperature increased to above the eutectic temperature (1390°C) for tungsten enriched silicon melt, the WSi2 layer mainly formed through a solid-liquid reaction and the thickness of the silicide layer increased rapidly. Because 100ms (the FWHM of the electron beam related to the scan speed) were sufficient to generate the tungstendisilicide layer. However, in this experiment, the solidification process of the nanocrystalline silicon was completed within 12.5 seconds for a sample of 10cm2 area. Therefore, the solidification process was completed in a nonequilibrium state and the liquid-solid transformation line will divert from equilibrium line shown in Fig.8. At the beginning of the silicon solidification, the formation of tungstendisilicide crystallites will be suppressed by the rapid freezing and followed by the formation of solid silicon. These crystallites start to form just below the

adhesion to the silicon melt the capping layer also arches upwards and widens the voids. This effect is enhanced by thermal stress and outgassing during the solidification process [5]. As the size, area and viscosity of the SiO2 layer is affected by the EB energy density, the size and the number of the voids in the capping layer are dependant on the EB energy

High density, biggest

Low density,

Table 1. Influence of the recrystallization energy on the surface morphology of the silicon

This Chapter gives the details about the formation of Tungstendisilicide (WSi2). The film system consists of a 20μm thick silicon layer on a 1.2μm thick tungsten film. Tungstendisilicide (WSi2) is formed at the interface tungsten/silicon but also at the grain boundaries of the silicon. Because of the fast melting and cooling of the silicon film, the

It was claimed that tungstendisilicides were formed in their tetragonal (Hansen, 1958; Döscher et al., 1994) by the solid/solid state reaction and the solid/liquid state reaction

> 700 1390 () () 2( ) <sup>2</sup> *C C Si W s s WSi <sup>s</sup>*

1390 () () 2( ) <sup>2</sup> *<sup>C</sup> Si W l s WSi <sup>s</sup>*

Formation of the eutectics can be explained using the phase diagram of the Si-W alloy system, as shown in Fig.8. The reactions should start at temperatures above 700°C. The eutectic crystallites (WSi2/Si) are precipitated from the silicon melt at a eutectic concentration of 0.8 at% W at the eutectic temperature 1390°C in thermal equilibrium. With the temperature increased to above the eutectic temperature (1390°C) for tungsten enriched silicon melt, the WSi2 layer mainly formed through a solid-liquid reaction and the thickness of the silicide layer increased rapidly. Because 100ms (the FWHM of the electron beam related to the scan speed) were sufficient to generate the tungstendisilicide layer. However, in this experiment, the solidification process of the nanocrystalline silicon was completed within 12.5 seconds for a sample of 10cm2 area. Therefore, the solidification process was completed in a nonequilibrium state and the liquid-solid transformation line will divert from equilibrium line shown in Fig.8. At the beginning of the silicon solidification, the formation of tungstendisilicide crystallites will be suppressed by the rapid freezing and followed by the formation of solid silicon. These crystallites start to form just below the

solidification process of the silicon film is a nonequilibrium solidification process.

**WSi2 ratio**

(>200µm) 21.7% fine

(<50µm) 13.3% coarse

bigger(<100µm) 10.5% coarser and

(1)

(2)

**WSi2 /Si eutectic** 

widely spread

**Energy level SiO2 capping/ voids pinholes Wremaining/** 

rough, droplet morphology

smooth, discontinuous

between tungsten and silicon according to equation (1) and (2).

(0.36-0.38J/mm2) smooth, continuous sporadic, small size

density as well.

Low (0.34J/mm2)

Middle

High (0.4J/mm2)

**3.2 Formation of eutectic (WSi2/Si)** 

film system

liquid-solid transformation temperature, and their growth will be not immediately accompanied by the tungstendisilicide crystallite formation. Therefore, the silicon phase forms dendrites, which grow over a range of temperature like ordinary primary crystallites. Below the eutectic reaction temperature, the remaining melt solidifies eutectically as soon as the melt is undercooled to a critical temperature to allow silicon crystallite growth.

Fig. 8. Phase diagram of the Si-W alloy system in equilibrium (Hansen, 1958)

#### **3.3 Microstructure and distribution of the eutectic crystallites (WSi2/Si) under different recrystallization energy**

Tungstendisilicide (WSi2) was formed at the tungsten/silicon interface but also at the grain boundaries of the silicon throughout all the EB energy density range. A top view scanning electron spectroscopy (SEM) and EDX analysis of the surface region showed that eutectic structure (tungstendisilicide precipitates / silicon) were mainly localized at the recrystallized silicon grain boundaries, as is shown in Fig.9. A typical hypoeutectic structure was found in the exposed silicon layer, which consisted of cored primary silicon dendrites (dendritic characteristic was not very evident) surrounded by the eutectic of the silicon and the tungstendisilicide precipitates. In this eutectic, tungstendisilicide (white areas in the lamellar shape) grew until the surrounding silicon melt had fully crystallized. The eutectic statistically distributed at the primary silicon grain boundaries. The formation and distribution of the eutectic depended on the crystallization and the growth dynamic of the tungsten enriched silicon melt. This is a nonequilibrium solidification process.

The size and the amount of the tungstendisilicide/silicon eutectic depended on the course of the process: when the higher the energy was used in the recrystallization process of the silicon layer, more and large tungstendisilicide crystals grew in the silicon melt. In addition, the WSi2/Si eutectic became coarser at the primary silicon grain boundaries and spread more widely. This was due to the prolonged solidification period for the tungsten enriched silicon melt in the remaining liquid, primarily at the grain boundary. At these sites, the tungstendisilicide crystallites precipitated in the final solidification areas at lower temperature than in case of equilibrium, due to the high tungsten concentration in the

Application of Electron Beam Treatment in Polycrystalline Silicon Films Manufacture for Solar Cell 85

volume. For high EB energy density there was more time for the precipitation and growth of tungstendisilicide and thus more tungstendisilicide crystallites were precipitated at the silicon grain boundaries. The strong tendency of formation of tungstendisilicide at the primary grain boundaries would reduce the efficiency of the solar absorber. Thus a high

Fig.10 shows the cross section of a typical resolidified silicon film remelted with different EB energy densities. Tungstendisilicides (WSi2) were formed in the region between the tungsten layer and the silicon layer without relationship to the EB energy density range applied in this research. A thick tungstendisilicide of 2.0-2.86μm exhibited in this experiment. The higher the applied EB energy density, the thicker the tungstendisilicide layer between the

0 100 200 300 400 500

Fig. 11. SEM top view of the recrystallized silicon film in the pinhole area and its EDX line

distance (um)

 Si W

1

10

100

relative intensity (a.u.)

profile mapping results

1000

tungsten and the silicon layer, the thinner the remaining tungsten layer will be.

10000 O

energy density is not favorable for the recrystallization process.

(a) є=0.34J/mm2; (b) є=0.36J/mm2; (c) є=0.38J/mm2; (d) є=0.4J/mm2

Fig. 9. SEM results of the eutectic structure under different recrystallization energy density є (Fu et al., 2007)

(a) є=0.34J/mm2 ; (b) є=0.40J/mm2

Fig. 10. Cross section of typical silicon film system under different energy density є (Fu et al., 2007)

(a) є=0.34J/mm2; (b) є=0.36J/mm2; (c) є=0.38J/mm2; (d) є=0.4J/mm2

**(a) (b)**

**(a) (b)** 

**(c) (d)** 

(Fu et al., 2007)

**WSi2 W** 

**S**

(a) є=0.34J/mm2 ; (b) є=0.40J/mm2

al., 2007)

Fig. 9. SEM results of the eutectic structure under different recrystallization energy density є

**W WSi2**

**S**

Fig. 10. Cross section of typical silicon film system under different energy density є (Fu et

volume. For high EB energy density there was more time for the precipitation and growth of tungstendisilicide and thus more tungstendisilicide crystallites were precipitated at the silicon grain boundaries. The strong tendency of formation of tungstendisilicide at the primary grain boundaries would reduce the efficiency of the solar absorber. Thus a high energy density is not favorable for the recrystallization process.

Fig.10 shows the cross section of a typical resolidified silicon film remelted with different EB energy densities. Tungstendisilicides (WSi2) were formed in the region between the tungsten layer and the silicon layer without relationship to the EB energy density range applied in this research. A thick tungstendisilicide of 2.0-2.86μm exhibited in this experiment. The higher the applied EB energy density, the thicker the tungstendisilicide layer between the tungsten and the silicon layer, the thinner the remaining tungsten layer will be.

Fig. 11. SEM top view of the recrystallized silicon film in the pinhole area and its EDX line profile mapping results

Application of Electron Beam Treatment in Polycrystalline Silicon Films Manufacture for Solar Cell 87

(CSC) and the Research Fund of the State Key Laboratory of Solidification Processing

Diehl W., Sittinger V. & Szyszka B. (2005). Thin film solar cell technology in Germany.

Döscher M., Pauli M. and Müller J. (1994). A study on WSi2 thin films, formed by the

Green M. A., Basore P. A., Chang N., Clugston D., Egan R., Evans R. Hogg D., Jarnason

Goesmann F. & Schmid-Fetzer R. (1995). Stability of W as electrical contact on 6H-SiC: phase

Gromball F., Ong K., Groth C., Fu L., Müller J., Strub E., Bohne W. & Röhrich J. (2005).

Hansen M. (1958). Constitution of binary alloys, In: *Metallurgy and Metallurgical Engineering* 

Lee G. H., Rhee C. K. & Lim K. S. (2006). A study on the fabrication of polycrystalline Si

Li B. J., Zhang C. H. & Yang T. (2005). *Journal of Rare Earths*. Vol.23, No. 2, (April 2005),

Linke N., Gromball F., Heemeier J. & Mueller J. (2004). Tungsten silicide as supporting

*Cells*, Vol.84, No. 1-4, (October 2004), pp.71-82, ISSN: 0927-0248

0931690181, ISBN-10: 0931690188, London

2006), pp.220-225, ISSN: 0038-092X

pp.228-230, ISSN: 1002-0721

*Solid Films*, Vol.239, No. 2, (March 1994), pp.251-258, ISSN: 0040-6090 Dutartre D. (1989). Mechanics of the silica cap during zone melting of Si films. *Journal of Apply Physics*, Vol.66, No. 3, (August 1989), pp.1388-1391, ISSN: 0021-8979 Fu L., Gromball F., Groth C., Ong K., Linke N. & Müller J. (2007). Influence of the energy

*Surface and Coatings Technology*, Vol.193, No. 1-3, (April 2005), pp.329-334, ISSN:

reaction of tungsten with solid or liquid silicon by rapid thermal annealing. *Thin* 

density on the structure and morphology of polycrystalline silicon films treated with electron beam. *Materials Science and Engineering B*, Vol.136, No. 1, (January

S., Keevers M., Lasswell P., O'Sullivan J., Schubert U., Turner A., Wenham S. R. & Young T. (2004). Crystalline silicon on glass (CSG) thin-film solar cell modules. *Solar Energy*. Vol.77, No. 6, (December 2004) , pp.857-863, ISSN: 0038-

relations and interface reactions in the ternary system W-Si-C. *Materials Science and Engineering B*, Vol. 34, No. 2-3, (November 1995), pp.224-231, ISSN: 0921-5107 Gromball F., Heemeier J., Linke N., Burchert M. & Müller J. (2004). High rate deposition and

in situ doping of silicon films for solar cells on glass. *Solar Energy Materials* & *Solar* 

Impurities in electron beam recryatallised silicon absorbers on glass, *Proceedings of 20th European Photovoltaic Solar Energy Conference and Exhibition*, Barcelona, Span,

*Series*, Kurt Anderko, pp.100-1324, McGraw-Hill Book Company, ISBN-13: 978-

wafer by direct casting for solar cell substrate. *Solar Energy*, Vol.80, No. 2, (February

layer for electron beam recryatallised silicon solar cells on glass, *Proceedings of 19th European Photovoltaic Solar Energy Conference and Exhibition*, Paris, France,

(NWPU), China (Grant No. 78-QP-2011).

2007), pp.87–91, ISSN: 0921-5107

**6. References** 

0257-8972

092X

July, 2005.

July, 2004.
