Key Technology And Mechanism Of Heteroepitaxial Diamond Nucleation And Growth

Diamond is known as the‘ultimate semiconductor’and‘Mount Everest of electronic materials’,which can be widely applied in power devices,detectors,quantum computing,etc,showing great support for the development of emerging fields such as aerospace,new generation of communication technology,and artificial intelligence.The growth of large-size high-quality single crystal diamond wafers is the prerequisite for its wide applications,and heteroepitxy shows a technology advantage at extending the wafer size.However,heteroepitaxy involves in Ir-multilayer substrate preparation,diamond epitaxial nucleation with a high density and textured growth,so it is very important for tackling key problems of single crystal diamond preparation by heteroepitaxy to overcome the technology bottlenecks in the links above and clarify the mechanism of diamond epitaxial nucleation and growth.Therefore,this thesis mainly studies the key tecknology and mechanism of heteroepitaxial diamond nucleation and growth.In this thesis,Ir-multilayer substrate preparation and diamond nucleation and growth were mainly carried out by combining experimental and computational methods,using electron beam evaporation,magnetron sputtering,infrared rapid annealing,high temperature annealing in air,microwave plasma chemical vapor deposition(MPCVD)combined with bias enhanced nucleation(BEN).Scanning electron microscope,atomic force microscope,transmission electron microscope,X-ray photoelectron spectroscopy,X-ray diffraction and laser confocal Raman spectroscopy were used to characterize the surface morphology,interfacial structure,chemical composition and crystal quality of Ir or diamond epitaxial layer.Meanwhile,the mechanism of diamond nucleation and growth in CVD atmosphere was studied based on the first principle calculation.To meet the demand for diamond wafer with different orientations and clarify the epitaxial mechanism of Ir film,the preparation of Ir-multilayer substrates with(001)and(111)orientations was systematically studied,including Ir(001)/SrTiO3,Ir(001)/Al2O3(1120),and Ir(111)/Al2O3(0001)multilayer substrates,adopting the electron beam evaporation,magnetron sputtering and ion beam etching methods.The in-plane orientation relationship of Ir(001)[001]/SrTiO3(001)[001]and the Ir(002)rocking curve FWHM of 0.21°are confirmed.Al2O3(1120)substrate surface easily adsorbs carbon components in the air and surface reconstruction occurrs,which affects the orientation transformation from[001]to[111]on Al2O3(1120)surface;however,the ion etching can eliminate the surface reconstruction and then realize the epitaxial growth of Ir film.This finding can promote the understanding of the epitaxial growth mechanism.The orientation relationship of Ir(111)film on A12O3(0001)is Ir(111)[110]/Al2O3(0001)[1100],and the mosaicity gradually decreases with the increase of thickness,showing an improvement in the crystal quality.This study provides a research fundamental for the following diamond epitaxial nucleation and growth.To solve the problem of high density epitaxial nucleation and growth at a large scale,diamond nucleation and growth on Ir(001)substrate were studied using MPCVD combined with BEN method.The BEN process window was searched,and the interdependent relationship between the bias voltage,reactor chamber pressure,bias time in the BEN process was clarified.Epitaxially oriented grains are obtained with a nucleation density up to 109-11cm-2 in the BEN window.The greater the reactor chamber pressure,the greater the bias voltage threshold,and the larger the bias voltage,the shorter the required bias time.The phenomenon of nanocrystalline diamond clusters under a high methane concentration was explained.The plasma change during BEN was studied,and the content of species(CH,Hα,etc)increases with the application of the bias voltage.Factors influencing the bias current were studied,and the bias current increases with the decrease in the reactor chamber pressure increase in the bias voltage,and the predeposition of the diamond layer;the bias current first increases and then decreases with the increase of methane concentration.Two nucleation modes of surface nucleation and bulk nucleation were explained.The bulk nucleation undergoes two basic processes:the formation of an amorphous hydrogencarbon layer(a-C:H),and diamond domain nucleation,while the surface nucleation refers to that diamond nucleation never undergoes the domain formation.Diamond film growth on 10×10mm Ir/SrTiO3 substrate was studied,and the orientation relationship,crystal quality and surface morphology were confirmed.For a 180 nm-thick film,the orientation relationship was diamond(001)[100]//Ir(001)[100]and a moasicity(diamond(004)rocking curve FWHM)is 0.39°.When the film was grown to be about 50 μm,diamond can be freestanding and the crystal quality was further improved with the moasciity of 0.35°This work overcomes the technology problem to prepare the freestanding single crystal diamond film,providing a strong support for the subsequent inch-size diamond growth.To study the surface modification on Ir substrates during BEN and its influence on the nucleation,the surface modification inside and outside BEN window were compared.Two microstructures formed on Ir substrates were reported outside the BEN window.They are epitaxially grown Ir3Si species and etched Ir pillars.Both microstructures are formed because of Ir sputtering due to the bombardment of energetic charged ions and the transport of Ir surface atoms at a high temperature.Heteroepitaxial nucleation and growth of thin diamond films on roughened and flat Ir/SrTiO3 substrate surfaces modified under different bias conditions were studied.The surface morphology,crystal quality and internal stress were characterized,and the diamond film on the Ir-roughened surface has a lower roughness,smaller mosaic spread,but larger residual stress than the film deposited on the Ir-flat surface.This work has deepened us the understanding of surface modification during BEN and its influence on nucleation and growth,and provided a technological solution for preparing high-quality diamond films.To answer the question why diamond can be epitaxially nucleation and grown on Ir substrates in the CVD atmosphere,diamond nucleation and growth were simulated based the nucleation and growth atomic scale processes by the first principle,adopting Materials Studio software and VASP package.The adsorption sites on the surface and the stability of carbon atoms at different positions from the surface were confirmed,and the octahedral site was more stable than the tetrahedral site and surface adsorption was more stable compared to the bulk nucleation.The gas nucleation and bulk nucleation were compared,and results showed the nucleation from a-C:H layer was more advantageous than the direct nucleation from the gas on the substrate surface.The influence of ion kinetic energy on the nucleation was studied and the existence of bias voltage window was confirmed.The layer-by-layer and island growth were analyzed,and the island growth was more favorable from the perspective of energy.The epitaxial nucleation mechanism was studied.When the orientation relationship between diamond and substrate was diamond(001)[010]/Ir(001)[010](the angle is 0°),the binding energy of-0.58 eV/atom was the lowest,and the binding energy increased with the angle rising to 45°(the orientation relationship is diamond(001)[110]/Ir(001)[010]).Namely,the lowest binding energy was the decisive factor for epitaxial nucleation.This work not only explains the mechanism behind some experimental phenomena,but also guides the next experimental work.