3C/4H-SiC Hetero-epitaxy Via CVD: Characterization And Evolution Of Defects

Semiconductor heterostructures composed of two different materials show unique electronic properties compared with those of the homostructures.As exemplified by the formation of a two-dimensional electron gas?2DEG?which is perpendicular to the heterostructures face and strong confined in the energy well of conduction band act as carriers,as a result,it has a high sheet density and enhanced mobility.Recently,a new type of heterostructure composed of different polytypes of the same material was proposed.Such a heterostructure has the advantage of avoiding some knotty issues.For example,the interface states can be neglected for most cubic?111?planes and hexagonal?0001?planes of the same material because they have negligible lattice mismatch,and mutual diffusion between the different polytypes should be absent because their chemical components are the same.Among the candidate semiconductor materials for single-material heterostructures,SiC is considered to be the best because its different polytypes have large band-gap differences;for example,3C-SiC has a cubic structure and a band gap of 2.3 eV,whereas 4H-SiC is hexagonal with a 3.2eV band gap.Other semiconductor materials,such as ZnS,CdS and GaN,have much smaller band-gap differences between their different polytypes?less than 0.1 eV?.This property of SiC highlights its potential in heterojunction structures for novel high-electron-mobility transistor.This dissertation describes the research about the preparation of 3C/4H-SiC heterojunction structures based on CVD technology.Among the dissertation,the issues about the characterization and evolution of defects in 3C-SiC films have been focused discuss.The major achievements have been listed as following:1.The evolution of 3C-SiC films epitaxial on Si face of 4H-SiC substrate has been studied with a step-by-step experiment way.The results show there is a cause-effect relationship between DPB defect and macro-steps,which results in the DPB density could be reduced by suppressing the emergence of macro-steps.This means several methods could achieve the propose,for instance,shorting the etch time,using semi-insulation wafer as substrate or adding protection gases likes C₃H₈ or SiH₄ during etching.2.High quality 3C-SiC films were prepared successfully.The quality of 3C-SiC films has been improved by optimizing the epitaxial process.Finally,the average area of DPB-free in films reaches as square-millimeter-class.The experiment process is as follow:i),etching from room temperature to 1600?in H₂+SiH₄?4 sccm?.ii),the SiH₄ flow was stopped for 3 min to remove silicon atoms in the chamber.iii),the precursor C₃H₈ was injected into the reactor for 30s at a flow rate of 4.2 sccm.iv),homoepitaxial growth was conducted for 3 min at 1600?.v),the 3C-SiC epitaxial growth was carried out for 3 h with the temperature ramped down from 1600 to 1550?.During growth?which included both homoepitaxial and heteroepitaxial growth phases?,the flow rates of SiH₄ and C₃H₈were 21 and 7 sccm,respectively,and the sources were diluted in H₂ with a flow rate of 80slm.The growth pressure was kept at 100 mbar at all times.3.3C-SiC film defects grown on C face of 4H-SiC were researched.These major defects have been characterized and classified into three categories:?basins‘,SVSSs and polycrystalline complexes arising from SiC nucleation.The forming model of SVSSs has been built,in which the adatoms'migration has a major effect on the model.Anti-step-flow has been proposed.Both the effect of step-flow and anti-step-flow on the absorption near the step was discussed.4.The difference defects of 3C-SiC films between on C-face and on Si-face were compared.The quality deterioration of 3C-SiC layer grown on C face was caused by polycrystalline nucleation.The emergence of this defect"spread"the chaotic crystal to normal crystal region,which results in a series of segment along[111]direction insert into the origin normal crystal.Besides,there are lots of abnormal defect in polycrystalline nucleation region,such as 6H-SiC-like segment,4H-SiC segment,voids and inclusion complex etc.Contrarily,the defects of 3C-SiC grown on Si-face 4H-SiC are confined in a V-shaped fourfold twinning region.5.The adatoms migration on the surface of fourfold twinning was researched.The DPB defects are classified into two categories.The atoms arrangement model of a special fourfold DPB defect was built based on the TEM image.The twinnings inside the V-shape are two twinning symmetric with?-111?and?5-1-1?plane of two different type 3C-SiC.High density of twin band was observed in the twin-region.Combining the profile of AFM image,it shows that there are three different growth rate regions,DPB region,"twin"region and"hill"region,around DPB.DPB region has a low growth rate because there is high surface energy."Twin"region has the highest growth rate due to the"close-packed plane"growth model."Hill"region has a high growth rate for adatoms gather and nucleate around DPB because of"anti-step-flow"growth model.