Preparation And Characterization Of β-Ga₂O₃ Films On GaN Substrates

Gallium oxide(Ga2O3)has five different crystalline phases,including α-,β-,γ-,γ-and E-Ga2O3.Among these phases,the monoclinic structured P-Ga2O3 is the most stable.Compared to the typical GaN material of the third generation semiconductor(Forbidden band width:3.4 eV,Breakdown electric field:3.5 MV/cm),beta gallium oxide(β-Ga2O3)of the monocline system presents wider’band gap width(4.9 eV),higher breakdown electric field(8 MV/cm)and better chemical and thermal stability.Therefore,β-Ga2O3 has a very broad application prospect in the fields of light-emitting diode,thin-film solar cell,semiconductor laser,solar blind area ultraviolet detector,gas detector,high-power device(field effect transistor,schottky barrier diode)and so on.At present,β-Ga2O3 thin films grown by traditional processes such as magnetron sputtering and electron beam evaporation have poor crystallization quality and most of them are amorphous,microcrystalline or polycrystalline.Amorphous and polycrystalline thin films have higher defect density,which makes them have lower luminous efficiency.、doping efficiency and voltage resistance.It is difficult to be used in the manufacturing of high-efficiency semiconductor photoelectric devices.Therefore,how to grow high quality β-Ga2O3 single crystal thin films has become a hot frontier research field.This thesis was divided into three parts.In the first part,the Si-doped β-Ga2O3 epitaxial thin films on GaN substrates were grown via a pulsed laser deposition(PLD)process.By controlling the growth temperature,the preparation process,growth mechanism,structure and optical propert:ies of the film were studied.In the second part,the ideal growth temperature of the thin film was selected to change the Si doping concentration of the target material.The effects of silicon content on the structure and electrical properties of the films were systematically studied.In the third part,nanoporous GaN distributed Bragg reflector(DBR)was prepared by electrochemical etching,which was used as the substrate to heterogeneous epitaxialβ-Ga2O3 thin films by PLD.1.Preparation and properties of β-Ga2O3 single crystal thin filmsHigh quality β-Ga2O3 monocrystalline thin films were grown by heteroepitaxy on(0002)GaN epitaxial films using PLD process.Si-doped Ga2O3 was used as the target material(Si doping concentration was 0.4%).Growth temperature is 600℃、650 ℃ and 700 ℃ and the atmosphere was nitrogen.The structural and optical properties of the films grown at different temperatures were systematically studied.By testing the related structures of β-Ga2O3 thin film samples,we found that the crystallization quality of β-GaeO3 film grown at 700℃is best.The β-Ga2O3 film was grown along the single orientation of[-2 0 1].The epitaxial relationships between β-Ga2O3 thin film and GaN isβ-Ga2O3[0l0]‖GaN[1210]and β-Ga2O3[102]||GaN[1 010].The lattice mismatch[(β-Ga2O3 edge length-GaN edge length)/GaN edge length]along the direction ofβ-Ga2O3[010]is 4.61%.The lattice mismatch along β-Ga2O3[102]direction is 11.30%.Two cycles of beta-Ga2O3[102]grow in three cycles GaN[1 010]direction.2.Effect of doping concentration on the structure and properties ofβ-Ga2O3thin filmsOn the basis of the above research work,the targets were Ga2O3 bulks with different fractions of Si:0.2,0.4,and 0.6 wt%.Growth temperature of 700° leaded to the best crystal quality of β-Ga2O3.X-ray diffraction(XRD)indicated that all films are homoepitaxial along the direction of[-4 0 1].Moreover,the crystal quality of the thin film is the best when the Si doping concentration is 0.4%.The electrical properties of different doped Ga2O3 thin films were tested.The results show that all the thin film samples can conduct electricity.The square resistance of them is 2x105MΩ,4.5x104 MΩ and 450 MΩ,respectively.Because of its high resistance,it is difficult for hall tester to measure its electron mobility.3.Preparation of nanoporous GaN distributed Bragg reflector and re-growth of gallium oxide thin filmUnintentionally doped GaN/Si doped n-type GaN periodic films grown on c-surface sapphire substrates were grown via metal organic chemical vapor deposition.GaN/nanoporous-GaN(NP-GaN)distributed Bragg reflector(DBR)was prepared by electrochemical etching of GaN epitaxial wafer.The etching conditions were as follows:etching liquid is 0.3 mol/L nitric acid solution,etching voltage is 8v and 12 v,Si-doping concentration of～1 × 1019cm-3.The reflector prepared under this experimental condition presents high reflectivity(>90%),wide stop band(>50 um),and low cycle number of GaN/nanoporous GaN.In order to study the possible application of nanoporous.GaN DBR structure,β-Ga2O3 thin films were grown on DBR mirror under optimal conditions,and the structure and optical properties of the thin film were studied in detail.