Interface formation and microstructural evolution in a-Si:H/a-SiNx:H heterostructures

I n situ ellipsometry measurements have been used to determine the optical structure of interfaces during the growth of a-Si:H/a-SiNx:H heterostructures. We have observed significant deviations of the ellipsometry data from calculations which assume both uniform growth and abrupt interfaces. For the growth of a-SiNx:H on a-Si:H such deviations are attributed to a 40 Å layer, less absorbing than bulk a-SiNx:H, which is left behind at the interface. This layer appears to result from a higher void density or enhanced N incorporation in the initial 20 s of a-SiNx:H growth. It is also possible, however, that a thin layer on the a-Si:H surface is converted to a transparent nitride. In support of the latter possibility, the a-Si:H surface is found to be highly reactive in a N2 plasma: a 150 Å nitride layer is formed during a 10-min exposure time at the same plasma power used for a-SiNx:H growth. For the growth of a-Si:H on a-SiNx:H, we have found that the interface structure is sensitive to the nitride preparation technique. The evolution of the surface roughness on a-Si:H alternate layers is obtained in real time during the preparation of an a-Si:H/a-SiNx:H multilayer structure. The implications of these observations for recent interpretations of optical measurements on multilayer structures is treated in detail.