Fabrication And Photoelectrical Properties Researches Of Chalcogens-doped Silicon Via Femtosecond Laser Irradiation

Silicon is well known as the most commonly used material in photoelectric field because it is rich in the crust and easy to integrate.However,its high reflectivity at wavelength from 0.25².5?m seriously affects the performance of silicon based photodetectors,and its bandgap limits its application in infrared detection field.Chalcogens-doped microstructural silicon has overcome these shortcomings;its characteristics of wide spectrum and high absorption make it have potential application value in infrared detection field.Hyperdoped silicon with chalcogens can be synthesized by laser irradiation on Si surface in the atmosphere of gas such as SF₆,ion implantation followed by pulsed laser melting,and laser irradiation on Si with chalcogens thin-film,respectively.However,introducing the dopants under the circumstance of gas is only available for the preparation of sulfur-doped silicon.Moreover,the infrared absorptance of sulfur-doped Si changes from an initial value of 90%to about 50%after thermal annealing.Ion implantation is a higher-cost method and it disorders the material leaving it mostly amorphous.Introducing the dopants as a thin film is one technique that provides a low cost and more flexible alternative to fabricate chalcogens-doped silicon.Se-and Te-doped silicon have been prepared by femtosecond-laser irradiation of Si coated with dopant thin films.The preparation and electro-optical characteristics of the silicon supersaturated with chalcogens have been studied.The main contents and results are as follows:1.Density-functional theory based methods are utilized to systematically investigate the properties of chalcogens?S,Se,Te?doped silicon at different doping concentration.The calculated crystal structures indicate that the lattice distortion degree and impurity formation energy of S-and Te-implanted Si increase with increasing the doping concentration,but Se-implanted Si at concentration of 1.56%shows minimum lattice distortion and formation energy.Intermediate band appears in the chalcogens?S,Se,Te?doped silicon and the chalcogen doped Si shows strong optical absorption in the infrared range.The calculations carried out demonstrate that energy band structure and optical absorption coefficient are all associated with the doping concentration.The bandwidth of intermediate band and forbidden energy gap are broadened due to the increase of impurities concentration,but the strongest absorption in infrared and visible region appears at concentration close to 1.04%.2.We fabricate microstructure Se-doped silicon with femtosecond laser irradiation in the presence of Si/Se bilayer films.Improvement of doping concentration and optical-electrical properties caused by sputtering a Si thin film onto the Se film before femtosecond laser irradiation is investigated.Results show that the infrared optical absorption increases by 5%and the sheet carrier density increases slightly.In addition,we report the dependence of surface morphology,optical-electronic properties on laser micro-structuring conditions?scanning speed,laser fluence and pressure of background gas?.The experimental results show that both the aspect ratio of surface microstructures and the absorptance over the visible to infrared wavelength increase with the decrease of scanning speed,the absorption of Se-doped silicon can reach 96%in the visible range and 92%in the infrared range when the scanning speed is 0.5mm/s.However,the electrical properties become worse as the scanning speed decreases.The infrared optical absorption and sheet carrier density for sample prepared at the fluence of about 4.5kJ/m² are greater than those of samples prepared at other fluence.Both the infrared optical absorption and sheet carrier density of the Se-doped silicon increase with the increase of background gas pressure.3.Se-doped silicon is prepared using deposited Si-Se bilayer thin films followed by femtosecond-laser irradiation.n⁺-n photodiodes are fabricated from this material for the first time.The effects of annealing temperature and reverse bias voltage on the near-infrared responsivity are investigated.The photodiode exhibits optimal rectification and photoresponse at annealing temperature of 500?.At 12V bias,a responsivity of 2.41A/W at 1064nm is obtained.The linear increase at bias from 0 to10V and faster-than-linear increase at bias from 10 to 12V for the responsivity are observed with the increase of the bias voltage.The EQE beyond 100%is observed at-5V,and a remarkable gain at-12V corresponds to EQE 280%,indicating internal gain in this device.The results suggest that the gain mechanism should be photoconductive gain.4.We compare the surface morphology,optical properties and infrared photoresponse of Se-and Te-doped silicon prepared by femtosecond-laser irradiation of Si coated with dopant thin films.Both of the two samples show similar microstructures and strong sub-bandgap light absorption.Annealing the doped silicon leads to attenuation of the sub-band gap absorption.However,the attenuation degree of the Se doped silicon is greater in comparison with that of Te doped silicon.To explain the cause of the difference in the attenuation,we fit the attenuation of experimental absorption coefficient using the Arrhenius equation.Thermal activation energy and pre-exponential factor in the equation are considered to be associated with metastability of chalcogen-Si bonds and dopant diffusivity respectively.We extract the thermal activation energy and pre-exponential factor of Se-and Te-doped silicon from the fitted data,and the results suggest it is different dopant diffusivity instead of different chalcogens-Si bond energy that causes difference in the attenuation.Appropriate n⁺-n photodiodes were fabricated from the two materials,with Fourier-transform photoconductivity spectroscopy measurements,so sub-bandgap photocurrent features are observed for Se-and Te-doped diodes.The optical activation energies for Si:Se are in good agreement with the known Se levels Se_c⁰?X₁??116meV?and Se₂⁺?390meV?.The optical activation energies for Si:Te are in good agreement with the known Te levels Te⁺?411meV?,Te_c⁰?X₁??92meV?,Te_c⁺?X₁??364meV?and Te_c⁺?X₂??173meV?.Te doped silicon photodiode exhibits higher photocurrent response,which makes it possible to be more valuable candidate for fabricating Si-based infrared photoelectric detector.