Scanning Tunneling Microscopy of Porous Silicon-Based Surfaces
The ability of porous silicon to emit visible light of different wavelengths has been the subject of intense study. While many groups have reported visible light emission from this material, no consistent theory has emerged to explain the effect. Many agree though that understanding the surface structure of this material is an important step in determining the physical processes responsible for luminescence, particularly for the quantum confinement models. Therefore, the scanning tunneling microscope (STM) and the atomic force microscope (AFM) are used to characterize the surface morphology of several porous silicon-based surfaces. A 7 nm gold coating was initially used to get an accurate estimate of vertical dimensions in the porous silicon layer. While a second coating of a 150 nm indium tin oxide on porous silicon was too thick to gain any structural data, changes in the tunnel current (I) and bias voltage (V) curves with respect to ambient light intensity showed that a photovoltaic effect was present in this heterojunction device. Finally, STM and AFM scans (in an inert He atmosphere) of bare photoluminescent porous silicon formed on p− and p+ Si wafers showed that these surfaces are composed of a random network of nanometer-scale features. However, no features of the size and geometry consistent with the quantum wire model for luminescence were observed in all the scans taken of these surfaces.
KeywordsAtomic Force Microscopy Porous Silicon Scanning Tunneling Microscopy Quantum Wire Tunnel Current
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