Effect of silicon nitride layers on the minority carrier diffusion length in c-Si wafers

摘要

Silicon nitride layers prepared from silane and ammonia based gases by microwave assisted plasma enhanced chemical vapor deposition (PECVD) and by low pressure chemical vapor deposition (LPCVD) techniques on p-type c-silicon substrates were studied via the methods of surface photovoltage (SPV), Fourier transform infrared (FTIR), and secondary-ion-mass spectroscopy (SIMS). The effective diffusion length in silicon was evaluated by the SPV method, and it was strongly influenced by the deposited SiN_x layer. The FTIR spectra show the form of chemical bond of hydrogen in the layer. Two absorption bands belonging to Si-H and N-H groups and their modification after temperature treatment were found in the spectra of PECVD samples, while in the spectra of LPCVD samples only N-H bonds were recognized. Transport of H from PECVD silicon nitride into Si subsurface layer during the annealing process is shown by SIMS profiles of hydrogen. Positive influence of the penetrated H manifests in passivation of defects in the subsurface Si layer and, consequently, in better operation of the space charge region below the nitride and in longer effective diffusion length of minority carriers in the Si bulk. The average value of the diffusion length in the Si samples with the LPCVD nitride was shorter and dependent on the location of wafers in the reactor.