Surface Etching and Doping on Monocrystalline Silicon and the Resulted Physical Effects

Author:Zhong Hao

Supervisor:jiang ya dong


Degree Year:2019





Crystalline silicon(C-Si)is widely used in the field of microelectronics and optoelectronics due to its abundant natural resources,low cost and mature technologies.However,because of its high light reflectance and wide bandgap,the traditional photodetectors made from C-Si have many shortages in pratical uses,such as limited response spectrum range,lower device sensitivity and insufficient near-infrared detection ability.Sulfur(S)doped micro/nano-structured silicon is a new type of artificial silicon material,which has the characteristics of lower reflectance but higher absorptance with a wide spectrum range,providing a possibility to break through the performance bottleneck of traditional silicon-based photodetectors.Till now,using effective and controllable methods to realize surface etching and doping on C-Si is still one of the research hotspots in the field of optical engineering.In this dissertation,the S-doped micro/nano-structured silicon material is fabricated by various etching and doping technologies.And the morphological structures,optical and electrical properties,as well as surface defects of the structured material have been intensively studied by use of several modern characterization methods.Meanwhile,based on the optimized S-doped micro/nano-structured silicon material,a novel Si-PIN photodetector with a wider response spectrum range and a higher sensitivity has been successfully realized.The main contents and results can be summarized as follows:1.S-doped micro/nano-structured silicon has been fabricated by a two-step process named as etching followed doping.The etching methods are metal-assisted chemical etching(MCE)and deep reactive ion etching(DRIE),and the doping method is plasma immersion ion implantation(PIII).Our research results show that with the extension of silvering time and etching time in MCE,the aspect ratio of the micro/nano-structured silicon increases but the contact behavior between Ag/NiCr electrode and the structured material becomes worse.When silver plating time is 60 s and etching time is 45 min,the micro/nano-structured silicon exhibits a uniformly distributed nanocone arrays,and the average linght absorptance reaches 80.3%in the range of 4002000 nm.Moreover,both the aspect ratio and light absorptance of the structured silicon material become higher with the increase of mask size and process cycle times in DRIE.A vertical smooth and burr-free cylindrical periodic array can be obtained on the surface of C-Si when the mask diameter is 4μm,center distance is 10μm and process cycle times are100,respectively,with an average light absorptance of 69.1%in the range of 4002000nm.The contact behavior between Ag/NiCr electrode and the structured silicon material,however,becomes worse at this time.After sulfur ion implantation,the peak absorptance of the two previous micro/nano-structured silicon show red shift,and the absorptance improve significantly in the near-infrared wavelength range(11002000nm).2.In the one-step process of femtosecond laser ablation,the effects of laser fluence,scanning speed and processing gas atmosphere on the surface morphologies and photoelectric properties of S-doped micro/nano-structured silicon have been researched,respectively.The results indicate that,only in the atmosphere of SF6 gas can a uniformly distributed tapered micro-structured silicon be formed on the surface of C-Si.In the range of 2.41 kJ/m27.22 kJ/m2,with the increase of laser fluence,the diameter,height and spacing of the cones develop larger.When the laser fluence is 4.81 kJ/m2,the average absorptance reaches 94%from 400 to 2000 nm.However,lower or higher laser fluence will make worse the photoelectric properties of the structured materials.Furthermore,in the range of 4.0 mm/s0.5 mm/s,both the aspect ratio and absorptance of micro/nano-structured silicon increase with the decrease of laser scanning speed,but the contact behavior between Ag/NiCr electrode and the structured material becomes worse.Based on the above results,it is advised that a suitable scanning speed of 1.0mm/s be used in later device applications.3.In the fabrication of S-doped micro/nano-structured silicon,many newly generated defects and impurities will be introduced on the surface of C-Si,whether it is fabricated by two-step or one-step process.Among these processes,femtosecond laser ablation brings the least surface defects,followed by DRIE combined with PIII and MCE combined with PIII.In this disertation,a variety of thin film passivation combinations are considered to modify the surface state defects of the structured silicon.It is shown that double-layer passivation films of SiOx/SiNx or SiOx/Al2O3 have their desired effects,playing the roles of chemical and field effect passivation at the same time,so as to further reduce the surface defect state density of the structured silicon.This double-layer passivation has obvious effects on improving the minority lifetime of S-doped micro/nano-structured silicon.4.Finally,based on the optimized femtosecond laser ablation process in the SF6atmosphere,combined with SiOx/SiNx double-layer passivation technique and a 4-inch wafer platform,we have developed two kinds of novel front-illuminated and back-illuminated Si-PIN photodetectors.The results show that the spectral response range of these two photodetectors is significantly broadened and the peak responsivity is red-shifted.At the wavelength of 1060 nm,the responsivity is as high as 0.53 A/W for the front-illuminated device and 0.57 A/W for the back-illuminated one,respectively,indicating that the near-infrared detection ability of the photodetectors is obviously enhanced.Meanwhile,both of the photodetectors have the characters of low dark current(<1 nA),fast response(<30 ns)and wide operating temperature range(-25℃+60℃),respectively.Our novel Si-PIN photodetectors based on S-doped micro/nano-structured silicon materials perform a much better device behavior,which could be compared to those of so-called infrared enhanced Si-PIN photodetectors made by Hamamatsu Photonics Company.