Influence of Surface Modification on the Conductivity of Ultrathin Silicon Membranes

In a typical bulk sample of silicon, any surface modification, while often interesting in its own right, generally has little influence on the bulk properties of the sample. However, in the case of ultra-thin Si, the sample is virtually all surface, providing a unique opportunity to both probe surface effects in the absence of an extended bulk, and to use surface effects to modify the membranes properties.  Electronic transport in SiNMs is strongly influenced by the nature of both the top and bottom surfaces.  In the case of very thin SOI(001), the density of interface traps is greater than the areal dopant density, implying that it is depleted of mobile charge carriers, and should therefore behave like intrinsic Si, i.e. very resistive. Indeed this is the case with an oxide layer on the top surface (see diagram below).  However, when we remove the top oxide (the bottom traps still remain, and the membrane should still be depleted) with hydrofluoric acid (HF), the conductivity jumps to orders of magnitude higher than can be accounted for by a good surface passivation. Hall measurements reveal that the HF treatment produces an inversion from a nominally p-type to an n-type membrane, with the Fermi level positioned near the conduction band minimum. Ultra-thin SOI offers the opportunity to isolate the surface from the extended bulk, with the consequence that surface effects induce a significant modification of the position of the Fermi level throughout the system, and completely dominate the electronic transport properties. The extreme sensitivity of SiNMs to their surface environments makes them ideal candidates for sensors, and conversely, for applications where the conductivity can be controlled by via regulation of the local environment. 

 

 

Author: Shelly Scott

References:
S. A. Scott, W. Peng, A. M. Kiefer, H. Jiang, I. Knezevic, D. E. Savage, M. A. Eriksson and M. G. Lagally “Influence of Surface Chemical Modification on Charge Transport Properties in Ultrathin Silicon Membranes” ACS Nano, 2009, 3 (7), pp 1683–1692

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