We have characterized the effectiveness of phosphorus (P) diffusion gettering in silicon wafers for solar cell substrates by the photoluminescence (PL) technique. Gettering with P diffusion at 95OOC for 30 min increases the intensity of the band-edge emission, correlating positively with the minority carrier lifetime determined by the photoconductive decay method. We suggest that the lifetime is retarded by the presence of the nonradiative recombination centers and that the centers are gettered by the P diffusion. The spectral broadening and shift observed in a certain area of the gettered sample are explained by the residual stress of about 20 MPa at maximum. Microscopic PL mapping on a cross section of the substrate reveals that the depth of the denuded zone formed by gettering is about 30 pm. We demonstrate that gettering under the present condition is particularly useful in increasing the production yield of solar cells as a result of the imlprovement in quality of short-lifetime wafers.
INTRODUCTlONAn increase in the minority carrier lifetime is essential for improving the performance of silicon solar cells. The minority carrier lifetime varies from one wafer to another in commercially available wafers and even in the same lot of wafers with the same specifications. Gettering using phosphorus (F') diffusion is known as a useful technique to improve the lifetime in substrate materials. In our experience geittering improves the performance of the solar cells. Therofore, an understanding of the factors which reduce the lifetime and of the ability and the mechanism of gettering is very important.The minority carrier lifetime has generally been measured by the microwave probed contactless photoconductivity decay method, however, the technique has the following problems. First, the lifetime value obtained by the technique includes the effect of the surface recombination. The surface oxidation may get rid of this effect, but it changes ihe thermal history of the samples. Second, the valuo depends on the thickness and the resistivity of the sample, which makes it difficult to compare a variety of samples. Thirdly, the technique is not a spectroscopic method, and cannot provide us a clue for understanding the physical background of the lifetime variation.Photoluminescence (PL) spectroscopy is a powerful technique for quantitative and qualitative analysis of impurities and defects in semiconductor materials [l-31. The method has advantages of high sensitivity, high spatial resolution, and non-contact. Since the PL intensity varies as a function of the minority carrier lifetime, the PL signal can give us useful information on the lifetime, which cannot be obtained by the photoconductive decay met hod.In this study, we have characterized the silicon wafers for solar cell substrates by the PL technique together with other conventional analytical methods. We will demonstrate the capability of the PL method for investigating the effectiveness of P diffusion gettering.
EXPERIMENTMeasured samples were substrates for space so...