The 5th International Symposium on Advanced Science and Technology of Silicon Materials (JSPS Si Symposium), Nov. 10-14, 2008, Kona, Hawaii, USA
Enhancement of Ga doping in Czochralski-grown Si crystal by B- codoping
Satoshi Uda1*, Xinming Huang2, M. Arivanandhan1, Raira Gotoh1
1 2
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980- 8577, Japan JA Solar USA Inc., 860 Hillview Court, Suite#100, Milpitas, CA, 95035, USA email: uda@imr.tohoku.ac.jp, huangxm@jasolar.com, arivu@imr.tohoku.ac.jp, raira-510@imr.tohoku.ac.jp Abstract
Codoping Boron (B) with Gallium (Ga) in a Czochralski-silicon (CZ-Si) crystal has been proposed to be an effective way to enhance the Ga doping into Si crystal by increasing the small equilibrium segregation coefficient of Ga , k0, from 0.008 to 0.01. It was observed that the axial resistivity variation in a Ga- and B-codoped Si crystal was smaller than that in simply Ga-doped Si crystals. We have modified the solute segregation theory developed by Thurmond and Struthers and calculated by Weiser. The modification includes the segregation behavior of Ga and B codoping in CZ-Si crystal growth by taking into account the change of the average bonding energy of -Si-Ga- by being partially replaced with –Si-Ga-B-Si- , and the relaxation of the strain energy by reducing the radius difference from Si by combining B with Ga. As a result, the effective segregation coefficient of Ga was almost kept constant until the B concentration increased up to 1019 atoms/cm3, however, it increased when the B concentration was over 1019 atoms/cm3. On the other hand, the effective segregation coefficient of B was almost constant when the B concentration was lower than 1019 atoms /cm3 while it decreased at higher B concentrations. We should also note that it has been analytically demonstrated that there is strong interaction between Ga and B in a Si crystal, which leads to an increase in the segregation coefficient of Ga in Si crystal growth. Introduction Gallium (Ga)-doped Czochralski-silicon (CZ-Si) crystal is a promising substrate material for Si-based solar cells, with high minority carrier lifetimes and no light-induced degradation [1]. However, the resistivity of a simply Gadoped CZ-Si crystal varies widely over the length of the crystal due to the very small equilibrium segregation coefficient of Ga (k0=0.008) [2]. A small segregation coefficient of dopant in crystal growth results in a low production yield of Si crystal due to the stringent specification of limited resistivity required by device manufacturers. This problem becomes more serious for crystal growth using a dopant with a very small segregation coefficient such as Ga. Even though the resistivity variation can be controlled by doping two different types of impurities (p-type and n-type), based on the compensation of different kinds of carriers [3], an increase in the absolute dopant (impurity) concentration in a Si crystal will result in a decrease in the carrier mobility, and finally result in lower conversion coefficient of solar cells. Consequently, doping a Si crystal using the methods which are reported in ref. [3] cannot be used for solar cell applications. The segregation phenomenon of dopants in CZ crystal growth explains the variation of resistivity in the grown crystal fairly well. Thurmond and Struthers [4] established a theory for the partitioning of impurities in a dilute solution to crystal based on thermodynamics. By following the theory of Thurmond and Struthers, Weiser [5] calculated the segregation coefficients of Group III, IV, and V impurities in Si and Ge crystals. Among the group III elements, Ga and B are well-known p-type dopants for Si crystal. B has a much larger equilibrium segregation coefficient (k0 =0.8 or 0.7) [2, 10] compared to Ga, and both have different covalent radii in comparison with Si, i.e., the covalent radius of B (rB = 0.88) is smaller than that of Si (rSi = 1.17 ), whereas the covalent radius of Ga (rGa = 1.26 ) is larger. Hence, Ga and B codoping in CZ-Si crystal growth has been proposed [6] to solve the problem of rapid resistivity variation by compensation of the difference in the radius of these elements from the radius of Si. The segregation phenomenon of codoped Ga and B in CZ-Si crystal growth has been investigated by comparison with simply Ga-doped and simply B-doped Si crystal growth. The segregation behavior of Ga and B was analyzed based on Thurmond and Struthers [4] and Weiser's [5] work. Experimental procedure Ga- and B-codoped oriented Si single crystals were grown using the CZ method with 11N purity Si source material in a high purity argon atmospheric pressure of 20 Torr. A series of crystal growth experiments were conducted by varying the Ga and B concentrations partitioned into Si crystals in the range between 1014- 1019

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