Research Of The Energy Level In Si Induced By Femtosecond Laser And The Performance Of Enhanced Near Infrared Detector

The hyper-doped micro-nano-structured silicon,also known as black silicon,is a new kind of silicon material,which is fabricated by the surface treatment on crystalline silicon.The appearance of black silicon is going to solve the problem that the absorption of crystalline silicon in visible to near infrared is lower than we want.The ultimate goal is to fabricate the photodetector based on silicon substrate,which can detect the visible to near infrared light.In this paper,the preparation of black silicon material by femtosecond laser irradiation has the advantage of easy to operation,high instrument integration and strong reproducibility.The disadvantage is that the cone surface set higher demands of the device fabrication process and the preparation speed can not fit the mass production.The high-energy femtosecond laser irradiates the surface of crystalline silicon in different background atmosphere to achieve the hyper-doping of the dopants.Our experiments set six groups of mixed doping control groups,respectively,in different concentrations of SF6 and N2 atmosphere.From the SEM characterization of the structure,the morphological volume of the cone is related to the laser energy and the ratio of S,F and N in the dopant.With the higher laser energy and N atom content,the morphology of the cone is larger and blunter.The analysis of the absorption characteristics of SF6 and N2 mixed black silicon demonstrate the big change of the absorption in the near infrared band(1100-2000nm).The results prove the diffusion of sulfur atoms in silicon substrate is very strong and the thermal stability is very poor.In contrast,the thermal diffusion of nitrogen is very weak and the thermal stability is very strong.The First-principles calculation is used to analyze the band structure and the state density of black silicon.The effect of S,N and F atoms on the substitutional doping of super-cell silicon structure was studied.The simulation results show the non-intrinsic semiconductor material formed by hyper-doping is transformed from the original indirect bandgap semiconductor to the direct bandgap semiconductor.An impurity energy level has been found between the material's forbidden band,and the band structure has been changed to the lower energy.From the state density structure,we find that the electronic structure of the dopant has certainly influenced the electronicstructure of silicon.Compared the Fermi level of intrinsic silicon with the hyper-doped silicon,it can be concluded that the S,F atom hyper-doped structures show some certain degree of metal properties.Our device uses a PIN-type structure,the sulfur hyper-doped silicon layer as a photosensitive layer to enhance the absorption of visible and near infrared area.The near-infrared light responsivity and light-dark current of sulfur hyper-doped PIN photodiode were measured by Fourier transform infrared spectroscopy and 1064 nm laser light source.The results show that compared with ordinary silicon PIN photodiode,sulfur hyper-doped PIN photodiode has a significant high light responsivity and the photocurrent is two to three orders of magnitude higher than the dark current in the reverse bias operating state of 3V and 5V.Silvaco TCAD software is applied to simulate the hyper-doped PIN photodiode.The simulation results are basically consistent with the experimental results.All of the data proved that working under 5V reverse bias,the response peak can reach 5.9A/W,which is far batter than the commercial silicon-based photodetectors.