Determination of minority carrier diffusion length from distance dependence of lateral photocurrent for side-on illumination

A method of measurement of diffusion length L in p type c-Si wafers based on the lateral collection of minority carriers is reported. In this method, wafer requires a p–p+ junction on entire back surface and an n+–p interface on a part of the front surface leaving the rest part as bare for illumination. A photo-current Isc is generated when a rectangular area of a part of the bare front surface in the vicinity of the n+–p interface is illuminated with a laser beam. The magnitude of Isc varies with the normal distance d between the electron collecting n+–p interface and the nearest edge of the illuminated region. The slope Φ of the normalized Isc vs. d curve is used to determine a parameter sinh−1θ, which has a linear relation with d. The reciprocal of the slope of sinh−1θ vs. d curve in the linear region gives the diffusion length L. The value of L is less susceptible to error due to the effect of Sf of bare silicon surface if the linear region of sinh−1θ vs. d curve lies in the region of smaller d values. The present method has an advantage over the other methods in that it does not depend on the intensity of the illumination and absorption coefficient of Si. Additionally, method has no limitation in terms of wafer thickness to diffusion length ratio and is applicable to all practical L values.

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► Front surface contains n+–p collection interface and light is incident in the uncovered p-region.
► Illuminated area is a rectangle with its length parallel to the n+–p collection interface.
► Photocurrent Isc varies with distance d between n+-p interface and the illumination rectangle.
► Change in slope of normalized Isc vs. d curve is used for determination of diffusion length.
► Method does not require measurement of illumination intensity.