SCANNING-TUNNELING-MICROSCOPY OF POROUS SILICON SURFACES

The scanning tunneling microscope (STM) was used to examine several po rous silicon based surfaces. First are samples of photoluminescent por ous silicon made from single crystal wafers of 13.5 OMEGA cm p- (100) and 0.002 OMEGA cm p+ (100) silicon. These were observed in an ambient He environment. The development of the porous structure in these wafe rs due to different etching stages is investigated with the STM. Measu rements show that the vertical surface roughness increases with etchin g while the lateral dimensions of the structures present remain almost constant in agreement with known etching models. The porous silicon s urface density of states (DOS) was also measured using scanning tunnel ing spectroscopy. DOS measurements of porous silicon formed on both p- and p+ substrates do not show the expected increase in band gap energ y when compared with unetched crystalline silicon. However, these meas urements do show that while the unetched silicon surface remains p typ e, porous silicon surfaces fabricated from p- Si behaves like an n-typ e material whereas those formed from p+ Si remain p type after etching .