Carbon

Carbon is often found as an impurity in Si crystals used for solar cells, in amounts depending on the growth technique and on the feedstock used. Also, a vast literature exists on carbon considering its close partnership with oxygen in heterogeneous processes involving the oxygen/carbon precipitation (see below) and the detrimental effects of SiC precipitates on solar-cell behaviors. In modern Cz-Si, the carbon is, however, generally below the detection limit of FTIR measurement. It is usually of the order of 1016 cm-3, taking its lowest value at the seed-end part of crystal and then increasing along the axial direction of the crystal, thanks to its segregation coefficient that is about 7 x 10-2, while the equilibrium solubility of carbon in liquid Si is 4 x 1018 cm-3 and that in Si crystal is 3.5 x 1017 cm-3 at the melting point. The situation is very different in mc-Si, where the average carbon concentration is in the order of 1017 cm-3, due to a significant gas-phase (CO) carbon transport from the hot graphite parts of the growth furnace to the liquid Si and the general absence of gas shields in the growth furnace that would allow a better control of the dissolved carbon in the crystal. The distribution of carbon in the mc-Si blocks follows that in Cz-Si, with the lowest concentration at the bottom of block and the highest at the block’s top, where SiC might precipitate from a supersaturated solution. The concentration of carbon in polycrystalline sheet and ribbon Si is even higher, in the range of 5 x 1017-2.5 x 1018 cm-3. Usually, carbon atoms in

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Figure 3.16 Substitutional carbon and interstitial oxygen concentrations of crystal silicon before and after 1260° C annealing.

crystalline Si sit in substitutional sites (Cs). Since the covalent carbon radius is much smaller than that of Si, near to substitutional carbon sites the lattice is under local strain. Although the substitutional carbon atoms have a low diffusion rate, they can easily jump to interstitial sites after capture a fast diffusing Si self-interstitial (ISi)

Cs + ISi ^ C + Si (3.22)

Carbon easily interacts with oxygen to form C-O complexes. It has been experimen­tally proven [79] that these complexes behave as the heterogeneous nucleation sites for oxygen precipitates during both crystal growth and following heat treatments. Figure 3.16 shows the substitutional carbon and interstitial oxygen concentrations of crystal Si before and after a 1260°C annealing. It can be seen that the carbon and oxygen concentrations simultaneously increase after the annealing, due to the dissolution of oxygen precipitates and decomposition of C-O complexes.

Carbon in supersaturated conditions should segregate in the form of SiC. Because of the large volume decrease associated with carbon precipitation and the relatively low diffusivity of Cs, carbon precipitation is difficult to proceed without the assistance of Si self-interstitials. However, as oxygen precipitation is associated with a volume increase and its strain relief occurs by punching out dislocation loops, or more probably, by emitting Si self-interstitials, the opposite volume change associated with oxygen and carbon precipitation favors their coprecipitation.

The substitutional carbon is electrically neutral, but the interstitial carbon maybe present in a negative, neutral or positive charge state, depending upon the position of the Fermi level. These carbon centers are highly mobile above room temperature and form complexes with the remaining substitutional carbon, oxygen, boron and various other impurity atoms. It has been reported [80] that wafers with higher concentration of Cs have a reduced minority-carrier lifetime degradation associated with the B-O complex formation respect to sample with similar B and O concentrations but low Cs as shown in Figure 3.17. The dissolved carbon might, therefore, have a strong influence on the pho­tovoltaic properties of crystalline Si. It is found that the minority-carrier diffusion length,

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Interstital Oxygen concentration [О,] [1017 cm 3]

Figure 3.17 Degraded lifetime as a function ofthe interstitial oxygen concentration [90]. The solid line represents the degraded lifetime derived from the empirical equation as described by Bothe et al. [71]. The impact of carbon on the degraded lifetime is shown. Reproduced from [90].

the deep trap density and the solar-cell efficiency are, in fact, correlated with the carbon contents in Si [91]. Also the presence of SiC precipitates might cause severe drawbacks in solar-cell performance, associated with local ohmic shunts and short-circuit in the n+-emitter and in the back surface field [92], due to the high electronic conductivity of SiC. Four-probe measurements show, in addition, that SiC precipitates are often n-type and cause the setup of a local heterojunction at the Si/SiC interface, which behaves as a potential barrier for the majority carrier transport and a recombination center for minority carriers.