Study Of Low-temperature Formation Of GeSn Nanocrystallite Thin Films By Sn Induced Ge Crystallization On Insulating Substrate

Germanium Tin alloys,being comprised of Group-IV elements,have recently received much attention due to the fact that their directband-transition behavior is predicted when Sn content is greater than 6.3%.Moreover,the bandgap of GeSn alloys can also be readily adjusted from 0 to 0.66 eV by controlling the value of x from 0 to 1.This gives rise to the potential to extend the infrared application of Si-based materials.Meanwhile,GeSn materials can be applied to high-speed TFT for their higher carrier mobility than Si or Ge.Polycystalline GeSn alloys on insulating or flexible substrates deposited by magnetron sputtering have potential applications in low cost solar cells,or high performance flexible thin film transistors(TFTs),etc.However,toward the target,several challenges have to be faced.Firstly,the low solid solubility of Sn in Ge(<1.1%)hamper the formation of high-Sn alloys.Due to comparatively lower surface energy of Sn with respect to Ge,Sn tends to segregation to surface.Secondly,the high lattice mismatch(14.7%)between Ge and Sn enhances the difficulty of fabricating high quality GeSn alloys.In addition,the processing temperature should be lower than the soft temperature of plastic substrate.Aiming at aforementioned issues,this thesis focuses on the study of low-temperature formation of GeSn nanocrystallite thin films(NCTFs)by Sn induced Ge crystallization on insulating substrate.The main works and results are as follows:1.We proposed a method to form GeSn NCTFs with high Sn composition by sputtering Ge on self-assembled Sn nanodots at low temperature.During the sputtering process,Ge atoms diffuse into Sn nanodots and then nanocrystalline GeSn freezes out as temperature is above 150 ?.GeSnNCTFs with high Sn composition of 27.3%are achieved,which decreases with increasing of annealing temperature.The Hall mobility of 14.0 cm2/(V·s)is achieved with the process temperature of 275 0C(suitable for flexible electronics).2.Thick GeSnNCTFs were prepared by depositing Sn/Ge layers in cycles.The grain size of GeSn thin flim increases with the number of cycles.The grain size of GeSn NCTFs with Sn content of 12.2%formed after stacking 6 periods of Sn/Ge layers reaches about 150 nm.The hole mobility of the GeSn NCTFs up to 24.9 cm2/(V·s)is achieved with the process temperature of less than 450 ?.It is found that the increase of thickness ratio between Sn and Ge layers can lower the crystallization temperature and enlarge grain size of GeSn nanocrystals.3.GeSnMSM photodetectors were fabricated on insulating substrate by standard photolithography.330 nm thick GeSn NCTFs with 15%Sn content and about 1 ?m surface grain size was obtained by staking 20 periods of Sn/Ge layers.After annealing at 300 ? for 30 min,the Sn content of GeSnNCTFs decreses to 6.7%with improved crystal quality.The dark current density of GeSn MSM photodetector is as low as 0.28 mA/cm2 at-1 V bias.A redshift of wavelength cutoff to-2000 nm in the response spectrum was observed with respect to Ge photodector.