Positron Annihilation Study Of Microstructure And Photoluminescence Properties Of Porous Silicon

The photoluminescence (PL) of porous silicon (PS) in the visible range and at room temperature has attracted much attention. Many research efforts have been invested to realize an optical device with luminescent PS, but the inefficiency and instability of optical characteristic in PS still remains. The ageing PS induces a change of chemical composing of the porous surface and consequently induces a blue-shift of PL peak position and a reduction of the PL intensity. To stabilise and increase the PL intensity of PS, in this dissertation, we modify the surface structure of porous silicon through different passivation method and attempt to investigate the defects in PS using positron annihilation spectroscopy. The main contents and results are as follows:1) We prepared n and p-type PS samples which thickness over 100μm using electrochemical anodization in HF. The microstructure of PS and the annihilation behavior of positroniums in PS were systematically studied by means of positron annihilation lifetime spectroscopy (PALS) and age-momentum correlation (AMOC) measurements. Results show that the lifetime of positroniums is fluctuated in different ambience.2) Different current densities were applied in eroding progress to obtain different silicon grain and pore size. It is found that UV irradiation accelerates the erode process on both p and n-type silicon wafer. Since positronium's lifetime depends on the pore size, we utilize this to investigate the relationship between PL properties and pore size of PS. Results show that a peak PL intensity exists in n-type PS samples with increasing current density, and their PL intensity are fall short of p-type PS samples.3) The inside PS pore layer modification is performed by different solution dipping. It is found that water and ethanol have different modification effects on pore surface and result in different PL peak position and instensities; Irons are impregnated in PS sample using different concentration and passivation time, results show that irons deposite in p-type PS increase the PL intensity, while decrease the PL intensity in n-type PS samples. We also attempt to use other metal ions to modify the PS surface. They all decrease the PL intensity.4) In order to identify the influence of both pore size and the chemical composing of pore surface, we use rapid thermal annealing, water vapor annealing and vacuum annealing method to passivate n-type PS samples and discuss the defect type in PS before and after annealing. Results show that defects that dominate light source are changed after annealing and cause a drastic change in PL property.Besides, we increase the counting rate of AMOC apparatus by using internal trigger mode and the resolution of apparatus is kept at the level of 220ps simultaneity. It shorten the sampling time and improve the credibility of the results.