Crystalline Structure And Temperature-dependent Optical Properties Of β-(Al_xGa_1-x)₂O₃

In recent years,ultra-wide-bandgap materialβ-Ga₂O₃ is considered as a promising candidate to manufacture high performance high voltage,high power electronic devices,and deep ultraviolet electronic devices.In order to increase the bandgap ofβ-Ga₂O₃ based materials,Al doping is an effective way to modulate the bandgap.However,while increasing the bandgap,the Al-doping also affects the lattice structure,electricity,optics and other physical properties.In this thesis,comprehensive characterization methods,such as X-ray energy spectroscopy（EDS）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,Raman scattering spectroscopy（RSS）,transmission spectroscopy（OT）and ellipsometry（VASE）,were carried out to study the properties of temperature-dependent lattice structure,band gap and optical constants ofβ-（Alx Ga1-x）₂O₃ crystal with different Al components.The major research results are presented as follows:（1）The Al compositions of threeβ-(Al_xGa_1-x)₂O₃ crystal samples measured through EDS were x=0,0.0915,and 0.2069,respectively.According to the analysis of XPS,both Al-O bond and metal vacancy（Ga or Al vacancy）increased with the increase of the Al component.（2）The results of high-resolution XRD indicated that theβ-(Al_xGa_1-x)₂O₃crystals with x=0.0915 and 0.2069 maintained a good monoclinic structure,despite a decline in the interplanar spacings with the increase of Al component.According to high-resolution XRD spectra,theβ-（Alx Ga1-x）₂O₃ with x=0.0915and 0.2069 still maintained a good monoclinic structure,but their interplanar spacings and unit cell volumes decreased with the increase of Al composition.Compared with the un-dopedβ-Ga₂O₃ sample,the crystal plane spacings of the strongest diffraction peak（400）of the two Al-doped samples were reduced by0.02（?）（x=0.0915）and 0.04（?）（x=0.2069）,respectively.As revealed by the Raman scattering spectra,with the increase of Al component,the positions of Raman blueshifted,and the FWHM of Raman was broadened.Besides,Al doping caused a more significant impact on the Raman peaks below 600 cm^-1than Raman peaks above 600 cm^-1,which was attributed to the fact that Al⁺tends to replace Ga⁺in[Ga O₆]octahedron during the Al doping process.The correlation lengths of the strongest phonon modes of threeβ-(Al_xGa_1-x)₂O₃samples with x=0,0.0915,and 0.2069 obtained from the spatial correlation model analysis were 14.8（?）,9.5（?）,and 7.0（?）respectively,indicating that Al doping causes the decrease of crystallinity and long-range order of Al Ga O crystal.In the temperature range from 80 K to 800 K,the Raman frequency shifts of the three samples all exhibited quadratic temperature dependence.However,the characteristics of the FWHM of Raman phonon modes changing with temperature were different:for un-doped sample,the dependence of the FWHM on temperature could be fitted by three-photon and four-phonon coupling equations,while the doped samples had larger deviations and more complex change properties.（3）In the temperature ranges from 11 K to 300 K,the transmission of pureβ-Ga₂O₃enhanced with the increase of temperature,while that of Al-doped samples reduced.As temperature rises,the bandgap values of the three samples decrease,and its changing properties satisfy conforms to the Varshni formula.In the transparent region,the refractive index（n）declined with the increase of the Al component.When the temperature rose from 303 K to 873 K,the refractive index（n）of pureβ-Ga₂O₃ andβ-(Al_0.2Ga_0.8)₂O₃ increased at a slow pace,while that ofβ-(Al_0.1Ga_0.9)₂O₃showed some fluctuations.In the absorption region,the extinction coefficients（k）of all samples were raised with the increase of temperature.Also,in the transparent region,the k of pure Ga₂O₃ was almost unaffected by temperature,while the k ofβ-(Al_0.1Ga_0.9)₂O₃ andβ-(Al_0.2Ga_0.8)₂O₃increased slowly with the increase of temperature.Finally,the relationship between the refractive index,wavelength,and temperature in the transparent region were given.