CHANGE OF MINORITY-CARRIER DIFFUSION LENGTH IN POLYCRYSTALLINE SILICON BY ULTRASOUND TREATMENT

We present a new approach for defect engineering by ultrasound treatme nt (UST) in solar-grade polycrystalline silicon wafers, leading to sig nificant enhancement of minority carrier diffusion length (L) in wafer regions with short L. The maximum value of the UST effect is a 2.7 ti mes increase of the diffusion length in regions of the sample with L = 10 to 25 mu m. Using the surface photovoltage (SPV) method for non-co ntact mapping of the diffusion length, we have found both a positive a nd negative variation of L after UST in different wafer regions. The U ST effect depends upon temperature, showing an activation energy of ab out 0.17 eV. A relaxation study of the UST effect shows two different behaviours: (i) stable result versus post-UST holding time, and (ii) p artial or complete recovery of the diffusion length. Process (ii) is l inked to the ultrasound-enhanced dissociation of FeB pairs followed by pairing kinetics.