Study On Surface Properties And Optoelectronic Devices Fabrication Of Silicon By Femtosecond Lasers

With coming of femtosecond laser many experiments for the reaction between laser and mater can be researched. In this paper the properties of Si surface and Si1-xGex alloys have been investigated by second-order nonlinear effect generated by process of femtosecond laser. Also the properties of Si surface can be inflected by intensive femtosecond laser pulses and the period microstructured black silicon surface with excellent optical properties have been fabricated, and using it the photodetector of high performance can be prepared.In order to understand that effect of strain on crystal structure, the strain-induced second-harmonic generation (SHG) in silicon surface was investigated by femtosecond laser. The uniaxial tensile strain device was designed and fabricated and using it cylindrical strain was formed in silicon stripe. The dependence of SHG intensity on azimuthal angle of sample determined the structure property of crystal Si. Based on the knowledge we find the strained silicon (111) has C3v point group symmetry instead of original Oh symmetry. Additionally, compared the strain-induced SHG with the counterpoint before applying of strain we can know that the SHG intensity generated from silicon surface was increased by uniaxial cylindrical strain, which means a strain-induced second-order nonlinear susceptibility was appeared. Next, the direct relation between SHG intensity of two polarizations combination (s-in/p-out and s-in/s-out) and external cylindrical strain were studied, respectively. The dependent of two components (χzyy(2),strain andχyyy(2).strain) of strain-induced second-order nonlinear susceptibility on strain were quantitatively determined to linear relation. For the s-in/s-out polarization SH intensity is experimentally enhanced by 13% atε0=2.93x10-4, which is comparable with the theoretical enhancement of 13.7%. And then the Fourier transfonuation of SH intensity as a function of azimuthal angle of silicon sample show that the strain measured by SHG method can be compared with other method measuring of strain such as laser beam deflection. Therefore the surface SHG method is an effective and excellent diagnostic tool for solid surface strain.Following the above research, in order to systematically study on strain-induced SHG the biaxial compressive and biaxial tensile strain device were designed and fabricated, respectively. Using the two strain device the SHG from native oxided silicon sarface under compressive and tensile strain state was measured, respectively. From the dependence of strain-induced SH intensity on azimuthal angle of sample, the C3v. crystal symmetry of strained silicon (111) was further identified. In the same way, the SH intensity as function of external compressive and tensile strain were measured, respectively. Experimental results show that the SH intensity from native oxide silicon surface increased with the biaxial tensile strain, but decreased with biaxial compressive strain firstly and then increased. Although there is a difference between two strain states in variation tendency of curves, the similar conclusion that the strain build-in native oxide silicon surface is tensile can be obtained. The strain value was determined to 3.07×10-4. This value corresponds to the strain when SH intensity is minimum in the varied curve of SH intensity with compressive strain.Similarly, SHG technique can apply to measure the properties of Si1-xGex alloy films. Fully strained and strain-relaxed Si1-xGex alloy films were prepared by ultra-high-vacuum chemical vapor deposition (UHV/CVD). The strain state and crystal quality of two type-alloy films were determined by double-crystal x-ray diffraction (DCXRD). From the dependence of SH intensity on rotational angle of sample we can know the crystal structure of two alloy samples. For the fully strained Si1-xGex alloy film, strain-induced appearance of second-order nonlinear bulk effect and formation of C2 crystal symmetry, and then the strain-induced second-order nonlinear susceptibility was calculated. For the strain-relaxed Si1-xGex alloy film, the structure symmetry of crystal is not changed because the strain is relaxed, but there is a remaining strain in Si1-xGex alloy film due to the strain is not fully relaxed. This conclusion can be obtained from both the dependence of p-in/s-out polarization SH on rotational angle of the samples and the measurement for DCXRD.Femtosecond laser can not only characterize silicon surface without damage, but also can change the properties of silicon surface with stronger intensity. Another important study in this paper is that the fabrication of femtosecond laser microstructuring black silicon photodetector. The microstructuring black silicon surface was prepared by amplified femtosecond laser with 800-nm wavelength. Firstly the relation between femtosecond laser-induced periodicity microstructure and laser parameters at SF6 gaseous background was studied on. And the periodicity stripe structure and conical structure were obtained under different conditions of laser parameters, respectively. The formed conical structure is attractive and the height of structure is dependent of the laser power and pulses. Further study on optical properties of microstructured black silicon surface shows that the absorbance of the black silicon fabricated under SF6 background from ultraviolet to near-infrared wavelength is very high. Especially, the absorption for light that energy lowers than the band gap of silicon breaks restriction of absorption for the light traditional silicon. The high absorbed property of black silicon is caused by the introduction of high concentration of S-impurity levels to silicon band gap. In fact microstructured silicon surface was heavily doped with high concentration of S impurity, so the n+-n junction was naturally formed between microstructured surface layer and undisturbed silicon substrate below. Finally the black silicon photodetector was fabricated based on the high absorption properties and formed junction of femtosecond microstruciured silicon. The detector with Mesa structure was designed and fabricated by typical microelectronic process. Their current-voltage characterization and photoresponse were also investigated.