Scanning probe microscopy and scanning tunneling spectroscopy of porous silicon

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Abstract
Atomic force microscope and scanning tunneling microscope images of photoluminescent anodically etched porous silicon are presented and the development of the porous structure due to different etching stages is investigated. These measurements show that the vertical surface roughness increases with etching while the lateral dimensions remain almost constant, in agreement with known etching models. In addition, the results give strong evidence for an amorphous surface layer that partially covers the porous structure. Tunneling spectroscopy of porous silicon and unetched silicon are also presented. The surface density of states (DOS) measurements of porous silicon are compared with its photoluminescence spectrum. The DOS measurements do not show a peak corresponding to the 1.8 eV photoluminescent peak energy nor do they show an increase in the band gap energy of porous silicon compared with crystalline silicon. However, these measurements do show that while the unetched silicon surface remains p type, the porous silicon surface behaves like an n-type material.