Doping and Electrical Properties of Epitaxial Layers

Note that mobility is slightly decreased when higher cooling rate is applied. For lower cooling rate (0.25°C min-1), we did not notice any improvement. Kopecek [12] also showed that the increase of the growth rate resulted in a higher resistivity and a lower quality of the LPE layer for In based melt. 

Diffusion length (or lifetime) is a key parameter for the performance of solar cells. It is usually admitted that diffusion length of minority carriers has to be four times the thickness of the film to assure good photovoltaic efficiencies. At 1,050°C, appropriated values of 136 and 120 pm were obtained with In and Sn, respectively. The lower performance of epilayer grown from Sn melt can be explained by its high solid solubility (5 x 1019 cm 3 at 1,050°C). Incorporation of Sn atoms within the Si crystal could create a large stress and affect carrier transport. It is clearly related to the defects density of epitaxial film (measured by SECCO etching). 

A comparison of Sn and In as solvents (and Ga and A1 as dopants) has also been made at UNSW using single-crystal substrates [14]. They also obtained better results with In than with Sn and with Ga than with Al. It was found that layers grown with Sn solutions or doped with Al exhibit reduced mobility and lifetime. Presence of Al leads to the formation of an oxide on the surface of the melt and to the contamination of the epilayer, which exhibits higher density of shallow pits. 

We summarise some experimental results concerning electrical properties of epitaxial layers presented by different authors in the following table (Table 9.3). 

Boron is another possible p-type dopant. The incorporation of boron into silicon epitaxial layers grown from a tin melt has been studied by Baliga [15] and McCann [16]. Boron is provided via the silicon source wafer and its incorporation is a function of both time and temperature. Its segregation coefficient from liquid tin into solid silicon is temperature dependent and increases with temperature. Therefore, the content of boron into the layer will be maximum at the interface and minimum at the final surface. This doping gradient can be used to create a drift field in the base layer of the 

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