Optical And Electrical Properties Of Porous Silicon

Due to indirect band gap of silicon, the photoluminescence (PL) intensity and the luminescent efficiency of silicon-based devices are very low, which hinders its application in opto-electronic technology. In 1990, the observation of visible light emission from porous silicon (PS), and then realization of PS-based light emitting diodes gave a way to develop the silicon-based photo-electronics devices. But after a decade, there are many important problems those still exist now, such as low efficiencies, strong degradation of light emitting. In addition, up to now, the luminescence of PS is controversial.In this thesis, the recent progress of PS was reviewed. And then the relationship between microstructure, minority carrier lifetime, post-treated process and the luminescent-electrical properties of PS devices was systematically studied. Furthermore, the formation mechanism and the luminescent nature of PS are also investigated.The PL and other properties of n-type PS with different doping concentration were systematically studied by means of TEM, XRD, Raman and luminescent and electrical measurements. The results proved that larger pores could form in n-type PS with lightly doping, while the nano-size pores in the n-type PS with heavily doping. The results also show that PS with larger pores consisted of single crystalline, polycrystalline and amorphous silicon, but nano-size PS is made up of single crystalline and amorphous silicon. Moreover, the diameter of pores in PS has also an influence on the light emitting.By methods of FTIR, Raman, SEM and PL spectra, duo to the annihilation of non-radiation centers on the surface of PS, it is found that the coated nano-boron particles enhanced blue emission efficiency. The blue emission might not come from crystalline quantum effect or from the Si~C band on PS surface, but from the recombination of light-generated carriers and defects in the oxide layer coated on the wall side of PS.The different films prepared by PECVD method were used to stabilize PS luminescence. The results proved that the films cound modify the structures of samples, enhance emission efficiency and improve luminescent stability. When the samples were ageing in different chloride salts, it is found that the enhancement of emission efficiency was related to metal type. When an oxidative metal is used, the metal could play a role to enhance luminescent properties. In addition, the chloride formed on the surface of PS played as emitting centers to improve emission efficiency.Minority carrier lifetime and PL on PS with different microstructures has been also investigated. Depth and pores density of PS effected the minority carrier lifetime. Before anodization, introduction of metals onto silicon-wafer surface could play a key role to effect microstructure, minority carrier lifetime and optical properties of PS. When front illumination was used, the holes cound be annihilated by the metals on front surface, which depended on oxidative degree of metal, leading to decrease etching rate;when the samples were prepared under back illumination, the reductive metal could increase etching rate, and enhance optical properties. This result also improved that holes are joined to form PS.Compared PL spectra and cathodo-luminescence (CL) spectra of PS, the PL might be related to the suboxide defects on the surface of samples, but the CL came from radiative centers may be related to hydrogen atoms. The enhancement of emission efficiency of solid-state light emitting devices is limited by the luminescent stability from PS, the mechanical and thermal properties of PS, and the device design. Thus, the improved ability by injecting carriers efficiently through contacts is necessary and the modification of prepared process also is important. It is indicated that the magnitudes of photoconductivity signal are proportional to doping concentration of PS, interface and incoming photon flux. The interface potential energy may play a key role in the injecting efficiency and the transfers of the excess carriers in the PS devices.