Fabrication and characterisation of porous silicon.

A systematic study has been made of the electrical conduction processes through electrically etched porous silicon (PS) films sandwiched between two metal electrodes. The PS layers were formed by anodisation of p-type silicon wafers in a hydrofluoric (HF) acid solution. The effect of fabrication conditions on the structural and electrical properties of PS have been investigated. The thickness of PS layers was found to depend on the anodisation time, whereas porosity was regarded to be controlled by the current density and HF acid concentration.The dark current-voltage I(V) characteristics at fixed temperature and the variation of current as a function of temperature have been established. The characteristics for all devices, regardless the metal contact, show a rectifying behaviour with ideality factor close to unity. It was found that PS films fabricated from p-type silicon substrates behave like n-type silicon due to the depletion of electronic holes. The results suggest that a pn heterojunction between PS and p-Si is responsible for the rectifying behaviour. A value of 0.7 eV was obtained for the barrier height at the interface between PS and p-Si at room temperature. The barrier height was found to increase with rising temperature. Recombination conduction process was found to be dominant at low temperatures as the activation energy did not exceed 0.22 eV. At high temperatures, thermionic emission diffusion process was found to be responsible for the current transport in the PS structures. A band model was proposed for metal/PS/p-Si/metal structures in order to explain the observed characteristics. A.c. dark current measurements revealed that the a.c. conductivity varies as ws where w is the angular frequency and s' is an index which depends on temperature and having a value less than unity. A.c. activation energy was interpreted in terms of hopping conduction at low temperatures (less than 200 K) and diffusion transport of charge carriers through PS layers at higher temperatures. Measurements of capacitance as a function of frequency and temperature showed a decrease with increasing frequency and increase with increasing temperature. The photoconduction behaviour of PS was characterised by high dark resistivity, a clear photosensitivity for visible light, and a bias voltage dependence of the spectral response.