Research on Electric Actived Defects of Semiconductor Thin Film Material by Spectroscopy

Author:Wang Chong

Supervisor:li wei


Degree Year:2019





All semiconductors are not perfect lattice.They all contain defects.They may be foreign atoms(impurities)or crystalline defects in the semiconductor materials.Defects are intentionally introduced as dopant atoms(shallow-level impurities),recombination centers(deep-level impurities)to reduce the device lifetime,or deep-level impurities to increase the substrate resistivity.More often,defects are also unintentionally incorporated during crystal growth and device processing.These defects will become generationrecombination center to reduce the carrier lifetime in devices.In worse conditions,these defects could even disable the device performance.In order to give impartial evaluation on the electrical properties of semiconductor materials in devices,it is of great significance to carry out systematic study about the electrtic actived defects.To have an elaborate knowledge about the defects in semiconductor material could help researchers to develop the device performance and find out the reason about device degeneration.Metallic impurities affect various device parameters.Metals also degrade devices if located at high stress points and in junction space-charge regions.For single crystal semiconductors like silicon,germanium,and gallium arsenide,deep level impurities are usually metallic impurities,but they can be crystal imperfections,such as dislocations,stacking faults,precipitates,vacancies,or interstitials.Usually they are undesirable,but occasionally they are introduced to alter a device characteristic on purpose,e.g.,the switching time of bipolar devices.In some semiconductors like Ga As and In P,deep-level impurities raise the substrate resistivity,creating semi-insulating substrates.For amorphous semiconductors,defects are mainly due to structural imperfections.There are multiple aspects about the electrtic actived defect study,such as,the origin of defects,the location of defects in devices,the type of defects,the electrical properties of defects(trap level,capture cross section,trap density et al.).Four typical semiconductor materials(In Ga As,Al N,c-Si,a-Si1-x Rux)are used to study the electrtic actived defects in this paper.The details are as follow:1.The existence of grow-in defects in In Ga As during MBE processing is proved by the study of as-desposited sample.The trap level locates at 0.37 e V-0.42 e V to the valance band in band gap.Electron filling kinetics result gives a point defect behavior.DLTS indicate the type of this grow-in defect is arsenic(As)anti-site.15 minutes forming gas annealing at 370℃ reduced the surface states obviously.The capture cress section also got a two magnitudes reduction.2.Al N is used to be the insulation layer in MIS to carry out the DLTS measurement.Al atoms diffused into silicon substrate during the Al N CVD deposition.The defects close to Al N/Si interface in P type silicon substreate got the trap level ET=0.37 e V and capture cress section σp=10-16cm2.The trap dencity gave a diffusion profile from the interface to silicon bulk.After annealing,Al atoms diffused deeper into silicon substrate and the trap level ET evaluated from 0.372 e V to 0.421 e V.Based on DLTS research,these defects could be attributed to Al-O complexes point defects.In addiction,after annealing the point defects had a trend to gater into extended defects.3.A dry etching method by hydrogen plasma is developed to replace the HF treatment in the removing of native oxide on silicon substrate.Optimal H2 plasma processing conditions(500sccm,60s)result in the best effective minority carrier lifetime of up to 2.5ms.A higher H2 flow rate may lead to insufficient etching of the silicon oxide layer,and a longer etching time will over etch the silicon substrate.There processings will introduce more defects in to silicon,which will result to a worse device performance.TEM results proved that there is a 1nm Si O2 residual layer between a-Si:H/c-Si.This thin Si O2 layer could prevent the contacting between a-Si:H/c-Si directly,which could prevent the epitaxial growth of Si into a-Si during the subsequent thermal annealing and finally result in an excellent c-Si surface passivation.DLTS measurement results showed that the hydrogen plasma not only removed the native oxide at the silicon substrate surface,but also introduced some structure defects(vacancy related)into silicon bulk.The depth of these defects could be up to 1?m.A deposition of a-Si:H layer with subsequent annealing treatment could passivate the defects efficiently.4.The Ru atoms are introduced into the amorphous silicon thin film.Ru atoms could displace the Si in the network when slightly doping.However,when over solute condition,the substutional Ru atoms become saturate and precipitate into Ru2 Si nanocrystals.The doping of Ru could adjust the micro-structure and electrical property of a-Si1-x Rux thin film material.When Ru doping at x=0.01,the resistivity could get decrease obviously,TCR could still be kept around 2% and the 1/f noisce could also be modified.A subsequent annealing could improve the order of structure and reduce 1/f noise further,which could make this material a potential candidate for application in near-infrared devices.