The Study Of Electrical And Optical Properties Of GeSn Thin Films And Their Thin Film Transistors

As a group IV element,GeSn alloy compatible with the CMOS process,is a quasi-direct band material with high carrier mobility and response wave band covering near-infrared and even mid-infrared.It is an ideal material for silicon-based high-performance optoelectronic devices,especially one of the best candidates for silicon-based high-efficiency light sources.This thesis focuses on the process and rule of GeSn thin films crystallization by thermal annealing and pulse laser annealing.GeSn films with large grains on insulating substrates(GSOI)and the single crystals GeSn films on Ge substrates were fabricated.A new method for preparing GeSn quantum dots with high Sn fraction by Sn diffusion induced crystallization was proposed.The direct band gap luminescence and quantum confinement effect of GeSn quantum dots were observed.Polycrystalline GeSn junctionless thin film transistor with high effective mobility was developed.The main research content and innovation points are listed as follows1.The crystallization mechanism and properties of amorphous GeSn films with various Sn chemical contents using rapid thermal annealing were studied.The results indicate that the Sn-Sn and Ge-Ge amorphous peaks of Raman spectra redshifted with the increase of Sn chemical content.And the Sn chemical content of amorphous GeSn is proportional with the integral strength ratio of two Raman peaks.The crystallization temperature decreases with the increase of Sn chemical content.When the Sn chemical content is less than 7%,the crystallization temperature is more than 500?.As the chemical content of Sn increases to more than 20%,the crystallization temperature decreases below 400?.With the annealing temperature increased,the surface roughness increases caused by Sn segregation,and the Sn content decreases,but with the better the crystallinity2.The single crystalline GeSn films with Sn fraction of 1.8-4.7%and almost completely relaxed on Ge substrates were fabricated by rapid thermal annealing.During rapid thermal annealing,there is a competitive process between Ge substrate induced crystallization and nucleus induced crystallization.When the annealing temperature is lower,the rate of single crystallization is slow,and the thickness of single crystalline film is relatively thin.When the annealing temperature is higher,the rate of single crystallization is faster,and the thickness of single crystalline film is thicker.However,the Sn fraction would be decrease with the annealing temperature increased.3.The mechanism and properties of GeSn films crystallization by pulse laser annealing were studied.The polycrystalline GeSn films with uniform grain size and Sn fraction(up to 18%)were prepared on Ge substrates by pulsed laser annealing with appropriate energy density ranges of 250-550 mJ/cm2.It is proposed to extract and compare the disorders of polycrystalline GeSn films for the first time.The poly crystalline GeSn films with Sn contents higher than 10%were prepared on quartz substrate with poor thermal conductivity by low energy density and multipulse laser annealing.The GeSn films with grain size up to 8 ?m were prepared on insulating substrates using the ?-Czochralski technique with the grain position controlled4.GeSn junctionless thin film transistors were designed and prepared sub-300?.The effects of geometrical parameters,crystal defect states and interface states on the performance of polycrystalline GeSn TFTs were studied.Both theoretical and experimental results show that when the GeSn thickness decreased(from 60 nm to 10 nm),the switching ratio of the device gradually increased,at the same time,the effective mobility decreases sharply due to the influence of interface scattering.For device with channel thickness of 10 nm,the effective mobility is increased to 54 cm2/V·s due to the interface passivation by oxygen plasma.It is expected that the junctionless TFT based on polycrystalline GeSn materials has a great potential to be the next generation high performance TFT or to the application of 3D and flexible circuits.5.A new method of preparing ultra-high density GeSn quantum dots by Sn diffusion induced crystallization at low temperature was proposed by combining the principle of metal diffusion induced crystallization with the principle of stress limiting effect.The Sn fraction and size of GeSn quantum dots can be well controlled.Two groups of directband GeSn quantum dots,with the Sn fraction up to 44.2%and the density of 2.1×1012 cm-2 for one group,and with the Sn fraction of 13.6%and the density of 6×1011 cm-2 for the other were developed through annealing temperature control.The photoluminescence spectra of the quantum dots locate at 0.787eV and 0.800 eV,confirming the direct band properties and quantum confinement effect Therefore,the GeSn quantum dots prepared in this work may pave the way of Si-based photoelectronic integrated circuits(PICs)in the communication area.