Study On The Characterization Of Minority Carrier Lifetime For Polysilicon

In recent years,the rise of AMOLED has injected new vitality to flat panel display industry.Because of the higher mobility,lower leak current and more stable device electronic properties,low temperature poly-silicon(LTPS)thin film transistors(TFTs)are regarded to replace traditional amorphous silicon TFT as driving circuits in the application of AMOLED.However,the technology of low temperature polysilicon is more complicated than that of traditional amorphous silicon,especially in the control of device electronic properties and relevant uniformity during the TFT process.In particular,the quality of the polysilicon film as the active layer has a significant effect on TFT properties,but there is no effective on-line monitoring method for it in the industrial production.Therefore,by using the microwave photoconductive reflection method for minority lifetime characterization,we discussed in this thesis the influence of laser injection power and the various quality parameters of polysilicon on the measurement results.The study in this thesis has practical significance.Firstly,in this thesis,we adjusted the laser injection power of minority carrier lifetime measurement system to test the microwave reflection signal of polysilicon samples under different surface condition,and analyzed the influence of laser injection power on test signal.Then we tested polysilicon samples with different quality for verification.The experimental results indicate that when the laser injection power is within the low injection range and stable,the microwave reflection signal is basically proportional to the minority carrier lifetime.But when the laser injection power exceeds the low injection level,there will be no linear relation between microwave reflection signal and conductivity of the sample.In this case,the minority carrier lifetime measurement method is no longer valid.The surface passivation can increase the microwave reflection signal,but it will make the laser injection power easier to exceed the low injection level.As a result,laser injection power should be chosen carefully to make sure it is within the low injection level for measuring minority carrier lifetime of polysilicon samples with different surface conditions.Secondly,we prepared polysilicon samples with different quality under different film thickness,and measured their grain size,surface roughness,dopant concentration,then analyzed their minority carrier lifetime through utilizing microwave reflection method.When the grain size is less than 270 nm,the effective minority carrier lifetime increases with decreasing film thickness,the opposite trend was obtained when the grain size is larger than 270 nm.These results were assumed to be due to different dominant mechanisms for different grain sizes.The bulk lifetime,which decreases with increasing the film thickness,dominates for the LTPS films with smaller grain sizes,whereas the surface lifetime,which is propositional to the film thickness,becomes dominant for those with larger grain sizes.In addition,when the boron concentration is relatively low,the conductivity of polysilicon film deceases with the increase of boron concentration.Thus the microwave reflection signal also deceases with the increase of boron concentration.The experimental and analysis results are supposed to provide important insights for monitoring of the polysilicon quality in production.