Study On Crystal Properties And Electronic Band Structure Of Nb Doped β-Ga₂O₃

In the past two decades,β-Ga₂O₃ has attracted great attention as an ultra-wide bandgap semiconductor material.It is a potential material for the preparation of next-generation high-power electronic and deep-ultraviolet optoelectronic devices.The bandgap ofβ-Ga₂O₃ is about 4.5～4.9 e V.Andβ-Ga₂O₃ has many advantages,such as a high breakdown field of 8 MV/cm,high voltage resistance,high melting point,and easy to grow.For the development ofβ-Ga₂O₃semiconductor devices,a doping material with excellent performance has always been a main research issue.Among kinds of doping elements Nb with high valence state（+5 valence）and atomic radius close to Ga atom,is considered as a new doping element that can realize high carrier concentration inβ-Ga₂O₃crystal,which has attracted wide interest of people.Doping will not only change the electrical properties of semiconductor materials but also affect their crystal quality,energy band structure,and optical properties.Therefore,it is a crucial subject to study the doping concentration,defect luminescence,photon lifetime,and electron band structure of Nb-dopedβ-Ga₂O₃ samples.The main contents of this thesis are as follows:1.The crystal quality and carrier concentration properties ofβ-Ga₂O₃ samples with four Nb-doped concentrations of 0.0001 mol%,0.01 mol%,0.1 mol%,and0.5 mol%were studied by X-Ray Diffraction（XRD）spectroscopy and Raman scattering spectroscopy.The XRD measurement results show that the four samples areβ-Ga₂O₃ with a good monoclinic phase.In the Raman scattering spectra of four samples,the Raman phonon mode intensity of the high-frequency component ofβ-Ga₂O₃ crystal with Nb doping concentration of0.5 mol%decreased significantly,which indicated that the lattice integrity of tetrahedron in the crystal cell was reduced by Nb doping.After fitting of A_g¹⁰ Raman peak according to the spatial correlation mode,the coherence lengths of the phonon modes of four samples were obtained:15?,14?,9?,8?.As the Nb doping concentration increases,the coherence length of the phonon mode gradually decreases,indicating that the symmetry of the crystal microstructure of the Nb-doped samples was worse than that of the undoped sample.And the lattice integrity and crystal uniformity are affected to a certain extent.It is found that as the Nb doping concentration increases from 0.0001 mol%to 0.1 mol%,the Raman peak position shift and electron concentration of the sample gradually increase by the analysis of Raman phonon modes（A_g³）and A_g¹⁰).The Nb-dopedβ-Ga₂O₃ concentration of 0.1 mol%has the most significant carrier concentration:1.024ⅹ10¹⁸ cm^-3,revealing that the effective doping component of Nb is gradually increasing.However,the Raman peak position of the Nb-dopedβ-Ga₂O₃ concentrations of 0.5 mol%shifts to low frequencies and has a smaller carrier concentration,showing that the effective doping may be relatively low.2.The bandgaps of the four samples were 4.73 e V,4.71 e V,4.77 e V,and 4.71e V,respectively.According to the Varshni formula,the temperature-band gap curve of the sample with a Nb doping concentration of 0.0001 mol was fitted.The four samples have different photoluminescence（PL）properties,indicating that they have different defect concentrations and defect energies.For the sample with a Nb doping concentration of 0.01 mol%,as the temperature increases,it was found that the blue and red light emission peaks have different trends.Besides,the exciton activation energies of the two luminescence centers were calculated as 0.7667 e V and 0.8835 e V,respectively.The radiative composite photon lifetime and the non-radiative composite photon lifetime of the sample with Nb doping concentration of 0.01 mol%were about 7-16 ns and about 100-160 ns,respectively.3.The ratios of Ga and O elements in the four samples were 2/2.831,2/2.83,2/2.81,and 2/2.73,respectively.It was revealed that there were more O vacancies in the sample with Nb doping concentration of 0.5 mol%.The measurement results of Photoluminescence Excitation Spectrum（PLE）and Deep Ultraviolet Photoelectron Spectroscopy（UPS）showed that the E_f（Fermi level）of the four samples are 4.05 e V,4.35 e V,4.44 e V,and 4.29 e V,respectively.E_vacuum did not change significantly with the increase of Nb doping content.Also,the differences between the bottom of the conduction band and the Fermi level of the four samples were calculated to be 0.68 e V,0.36 e V,0.33e V and 0.42 e V,respectively.Finally,the electron band structure diagrams of the four samples are given.