Properties And Applications Of The Femtosecond Laser Formed Nitrogen-Hyperdoped Silicon Material

The thesis explores a femtosecond-laser-hyper-doped technique to dope nitrogen in the surface of ordinary silicon material. The nitrogen concentration in the doped layer formed by this method is about 1020/m3, far exceeds the solid solubility of nitrogen atoms in silicon crystals (about 4.5×1015/cm3) and the doping concentration formed by the traditional methods(1015/cm3). The silicon-based material exhibits unique physical properties due to the hyperdoping of nitrogen. The hyperdoped nitrogen can not only expand the absorption range from visible wavelengths to infrared wavelengths, but also make the material exhibit high crystallinity in the doped layer due to the repairing effect of nitrogen on defects in silicon lattices. Meanwhile, in the femtosecond-laser (fs-laser) doping process, a forest of micrometer sized conical spikes is formed on the silicon surface, which can also reduce the light reflection to some extent. The thesis is divided into three parts to explore the structural features, optical properties, and practical applications of the material in detail.First, the thesis studies the structural features of the nitrogen-hyperdoped silicon material. The samples with different surface microstructures are obtained by irradiating the silicon surfaces with fs-laser in nitrogen trifluoride (NF3) and nitrogen (N2), respectively. The microstructures formed in NF3 atmosphere are very sharp, which is attributed to both laser-assisted chemical etching and laser non-thermal ablation processes. While the microstructures formed in N2 atmosphere have a large and blunt shape, which is mainly attributed to the laser non-thermal ablation effect. In addition, the NF3-prepared structures show high crystallinity, which is due to the dramatic reduction the content of defects in silicon by forming the nitrogen-vacancy and interstitial complexes. The N-prepared structures have a lower crystallinity, which is related to the different atomic structures of nitrogen in silicon crystals. As the laser fluence increases, the etching and ablation actions are more and more violent, and then lead to the microstructures with more large and sharp shape. Compared with the N2-prepared samples, the NF3-prepared structures are smooth and free of nano-sized particles, which is attributed to the cleansing effect of NF3 on the etched silicon surface.Next, the thesis discusses the optical properties of the nitrogen-hyperdoped silicon material. The results show that the absorptance is strong in the mid-infrared wavelength range of 2.5-17 μm, and the absorptance changes little as the structural size increases. The reason is that the hyperdoping of nitrogen can introduce impurity states in the silicon band gap. When the doped nitrogen atoms are present in sufficiently high concentration, the impurity states can be expanded to the impurity band, and then lead to the below-band-gap absorption. Additionally, as for the NF3-prepared silicon, the mid-infrared absorptance remains almost unchanged after annealing at different temperatures (600,800, and 1000 K), which is related to the stable structures of nitrogen in silicon lattices. While for N2-prepared samples, the absorptance improves in the mid-infrared wavelength range of 3-7 μm and 14-16 μm after annealing. In the near infrared wavelength ranges, the two materials exhibit similar optical properties. The absorptance reduces gradually with the increases of wavelength, and stabilized around 30%-40%. After annealing the sample, the absorptance decreases in the wavelength range of 1000-1700 run, and stabilized with annealing temperature finally. The absorption in this wavelength range may be due to the co-effect of nitrogen and defects.Third, the thesis explores the optoelectronic applications of the nitrogen-hyperdoped silicon material formed in NF3. From the results of the Ⅰ-Ⅴ characteristics of the prototype device, we find that the photodiode has a good rectification after 800 K annealing, and generates photocurrent and photovoltage when exposure it to light. However, the FF and photoelectric transformation efficiency are not high. The responsitivity of the material is similar to that of commercial silicon photodiodes in the visible. In the near infrared wavelength range, the material also has responsitivity, which is due to the near infrared absorption of the material.