In this communication we report the results of a systematic study on the formation of chemically etched porous silicon. Samples have been fabricated in HF/HNO3-based solutions for various etching times, using different molar concentrations of nitric acid. The growth of the porous layer has been studied by correlating film porosity and thickness to the etching time. Pore formation is shown to be more related to the surface morphology than to the etch characteristics. Preliminary results on photoluminescence measurements show that substrate morphology also play a major role in the luminescent behavior of chemically etched porous silicon.
In this paper we report on the frequency dependence of the AC conductivity of porous silicon in the range 10 Hz-100 kHz. Two types of testing devices have been fabricated on three different series of samples formed electrochemically using as a starting material p-type, n(-)-type and n(+)-type silicon substrates. For frequencies less than 20-40 kHz the conductivity is found to follow a sublinear frequency dependence. This behaviour is typical of a carrier transport mechanism determined by an anomalous diffusion process. At higher frequencies me find that sm face states influence the transport mechanism. This suggests a double-channel transport mechanism: one related to porous-silicon ''volume'' properties and the other more connected to the ''surface'', itself
Porous silicon layers 7 μm thick with a porosity of 80% have been prepared for 1 Ω cm, ⟨100⟩ oriented, polished silicon wafers. As the measurement temperature decreases from 300 to 10 K the width of the PL spectra remains almost constant, while the lifetime of the 2 eV PL band increases from 25 to 3000 μs and the lifetime of the 1.5 eV emission changes from 100 to 200 μs. At all the temperatures, the broad PL bands show distinct peaks. We have interpreted these results in terms of the confined quantum wire model recalling for the first time the presence of classes of wires of different size, multiple of the minimum etchable dimension a/4, being a the silicon lattice parameter.
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