The Multi-spectrum Vision Technique Studies On Cubic SiC Thin Films

With the rapid development of the electronics industry and technology,Si as the representative of semiconductor materials in the first generation and GaAs,GaP,InP as the representative of compound semiconductor material in the second generation,have been unable to meet the more demanding needs of the military and the high power segments of our industries.Therefore,silicon carbide(SiC)as the representative of compound semiconductor material in the third generation is developing rapidly.Silicon carbide(SiC)is a wide(2.3?3.3 eV)semiconductor material.At the temperature of 930?,it still maintains the low intrinsic carrier concentration,high break down electric field,high saturated electron drift velocity,high thermal conductivity and other features.So silicon carbide(SiC)material is an ideal material to replace Si for high temperature,high frequency,and high power applications.To promote the SiC industrialization,the physical properties of SiC materials such as optical constants,the carrier concentration,strain and lattice phonon vibration related to the growth process and crystal quality are necessary to study penetratingly.In this paper,3C-SiC films on Si(100)substrate were prepared by chemical vapor deposition(CVD).We used the multi-spectrum vision technique to analyze four samples.Various techniques are discussed in great detail,such as spectroscopic ellipsometry(SE)and Raman scattering.The main research contents are as follows:(1)The properties of these films were investigated by variable angle and temperature dependent spectroscopic ellipsometry(SE).The change in the SE polarization state was measured in reflection from the surface and interface of sample.Experimental SE data were obtained at three different incident angles in the spectral range from 270 to 1700 nm(0.73 to 4.6 eV)at ambient temperatures from 25? to 500?.The complex dielectric function ? of 3C-SiC is modeled using one Cody-Lorentz oscillator,which were sufficient to obtain a satisfactory fit over the entire spectral range.The optical constants(n,k)of the 3C-SiC/Si films with the different thicknesses were obtained at different temperatures.(2)A series of cubic 3C-SiC/Si samples with different thicknesses were studied by Raman spectroscopy using laser excitation with different wavelengths plus spectral line shape analysis via two theoretical methods.Through comparative UV and visible excitation Raman measurements and theoretical analysis,Raman spectrum intensity was mainly affected by laser penetration depth and crystalline quality.Using spatial correlation model,we analyzed the transverse optical(TO)mode excited with 532 nm and 325 nm laser light.By comparing the intensities of the TO mode excited using these two lasers,it can be concluded that the TO intensity is mainly affected by the sample crystalline quality in the different laser penetration depth.A theoretical model was used to fit the LO-phonon and plasma coupling(LOPC)mode in order to obtain the free carrier concentration n.To get better fits,the difference spectrum was utilized to remove the second order Raman signals from the Si substrate.According to the fitting results,the value of n obtained from Raman scattering using 325 nm excitation is larger than that of 532 nm excitation,which is the top layer near surface has big difference in electrical and optical properties to the film inside layer.We also investigated how the different values of the photon damping constant 77 and the plasma damping constant y affect LO Raman mode spectral line shape.(3)The lattice misfit between 3C-SiC and Si being as large as 20%produces a high density of dislocations and these interfacial misfit dislocations greatly release the interfacial strains.However,there must be residual strains in the 3C-SiC epilayers.In order to analyze the strain distribution in the 3C-SiC film grown on Si,the Cross-section Raman spectroscopy has been used to determine residual strain of 3C-SiC-C4.Raman shifts covs in-plane strain ?a?/a0 for TO and LO phonon modes were demonstrated by the Porf.Stefan Rohmfeld's the formula.The relationship is linear over the strain regime investigated and a linear regression yields.The LO Raman frequency could not be measured reliably for thin samples due to interference from the intense second-order Raman signal of silicon from the substrate and the form of plasmon-LO-phonon coupling.According to the formula described above,the residual strain ?0 was calculated from the measured TO Raman frequency.As the lattice misfit between 3C-SiC and Si,the maximum stress at the interface was found.(4)Temperature dependence of Raman scattering and analysis were performed on 3C-SiC films with different carrier concentrations and the thicknesses.The Raman peaks of TO mode from two 3C-SiC samples were downshift when the temperature increases.By means of the simulation on the experimental result,the downshift of the TO Raman frequency with the increase in temperature was mainly due to the anharmonic coupling to four phonons.For the LOPC modes in two 3C-SiC samples with the different carrier concentrations,it is disagree with the phenomena of 3C-SiC-C3 with heavy doping concentration,which is first upshift,below downshift.It is the abnormal phenomena of the LOPC modes of 3C-SiC-C3,due to the effects of incomplete ionization of impurities in lower temperatures,and thermal expansion,lattice mismatch-induced strain,anharmonic coupling to other phonons in the higher temperatures.