Study On Molybdenum-doped Black Silicon Fabricated By Femtosecond Laser And Black Silicon Photodetector

As one of the most mature semiconductor materials,silicon has been widely used in microelectronics and optoelectronics fields such as integrated circuits,photoelectric detection,optical waveguide integration,solar cells and so on.Silicon photodetector is an essential part of the future all-silicon optoelectronic integrated chip.However,due to the limitation of the band gap,crystalline silicon cannot absorb the light whose wavelength is larger than 1100 nm,thus limiting the application of monocrystalline silicon in the field of infrared detection.The appearance of supersaturated sulfur(S)-doped black silicon obtained by femtosecond laser irradiating monocrystalline silicon breaks this limitation and enables the absorption edge of silicon to extend to the infrared region.For example,supersaturated S-doped black silicon has an average absorption rate of more than 90%in the wide band range of 250 nm?2500nm.However,there is a high concentration of background free carriers in the supersaturated S-doped black silicon layer,and the absorption caused by free carriers is not beneficial to infrared detection,but will increase the complexity of infrared absorption in the black silicon layer.In addition,the high background free carrier concentration will produce large current noise and reduce the signal-to-noise ratio of the device when making photoconductive photodetectors.In addition,thermal annealing is needed to repair lattice damage of black silicon layer and improve crystal quality,but the infrared absorption of supersaturated S-doped black silicon decreases greatly after annealing,resulting in poor thermal stability of infrared absorption.In view of the existence of high concentration of background free carriers and unstable infrared absorption of supersaturated S-doped black silicon,this paper uses molybdenum(Mo)transition metal as dopant to prepare supersaturated Mo-doped black silicon material.Mo can introduce deep energy levels into the band gap of silicon,for example,the deep donor level is introduced at the top of valence band of0.34 e V.The super-doped Mo will form an intermediate band around the discrete deep energy levels in the band gap of silicon,and then produce infrared absorption contribution.In addition,the higher ionization energy of deep level impurities can ensure the low concentration of background free carriers in supersaturated Mo-doped black silicon.Using Hall effect,the bulk carrier concentration of supersaturated Mo-doped black silicon at room temperature is less than 10¹⁵ cm^-3.In this paper,Mo film was deposited on the surface of monocrystalline silicon by magnetron sputtering coating process and used as dopant of black silicon.Then supersaturated Mo-doped black silicon was prepared by irradiating the coated silicon substrate with femtosecond pulse laser.The concentration distribution of Mo impurities in the black silicon layer was measured by secondary ion mass spectrometry.The peak concentration of Mo impurities in the black silicon layer was up to 1.59×10¹⁹ cm^-3.The ultraviolet-vision-near infrared absorption spectra of supersaturated Mo-doped black silicon were measured,and it was found that the absorption rate of black silicon in the wide spectrum range of 250 nm?2500 nm was greatly improved.The absorption rate of black silicon increased with the increase of laser fluence,and the absorption rate of 1310 nm light was up to 65%.For example,after annealing at 1223 K for 30 minutes,the absorption rate of 1310 nm of black silicon processed at 0.71 J/cm~2 decreases by only 8%,which proves that the infrared absorption thermal stability of supersaturated Mo-doped black silicon sample is good.Scanning electron microscope(SEM)was used to observe the surface morphology of black silicon.It was found that there were periodic micro-nano structures on the surface of black silicon,and the morphology and size of micro-nano structures changed with the difference of laser fluence.The light trapping effect of the micro-nano structures can reduce the reflection of the sample surface to the light of the test band,so as to increase the light absorption rate of the sample.According to the results of Raman spectroscopy,high laser fluence will introduce high density structural defects in the black silicon layer.In this paper,black silicon detectors with bulk structure and plane structure were fabricated,and the fabrication process of supersaturated Mo-doped black silicon and the influence of device structure on the performance of black silicon photodetectors were investigated.In bulk structure devices,the heterojunction based on black silicon layer and monocrystalline silicon substrate has a higher responsivity,while the Schottky junction based on metal and black silicon layer has a lower responsivity.In view of this,the device is changed from the heteroplanar electrode structure to the metal-oxide-metal(MSM)transverse interdigitated electrode structure,which solves the problem that the photogenerated carriers cannot be effectively separated by the space charge region formed by Schottky junction because the test light source cannot penetrate the gold electrode.The MSM structure significantly improves the responsivity of the black silicon detector.For example,the responsivity of the device to 1310 nm light increases from 0.8 m A/W to 25.1 m A/W under 10 V reverse bias.In addition,the performance of the two devices depends on the laser fluence.The appropriate laser fluence can ensure the infrared absorption of black silicon and reduce the defect density in the doping layer as much as possible.At last,the diffusion of Mo impurities were inhibited by rapid annealing combined with silica passivation,and the defect density on the surface of supersaturated Mo-doped black silicon was reduced and the surface defect recombination was reduced.Finally,the responsivity of the MSM black silicon photodetector prepared by the optimized process reaches 495m A/W to 1310 nm light at-30 V bias.