Heteroepitaxial Growth And Device Studies Of Gallium Oxide

Gallium oxide?Ga₂O₃?is a new oxide compound semiconductor.There are five commonly identified polymorphs of Ga₂O₃,labeled as?,?,?,?and?.Among these different phases of Ga₂O₃,the monoclinic?-gallia structure??-phase or?-Ga₂O₃?is the most stable crystal structure and has attracted most of the recent attention.Compared with third-generation semiconductor materials such as GaN and SiC,?-Ga₂O₃ has a larger bandgap,a larger expected Baliga's figure of merit?FOM?value,higher breakdown voltage(V_br)and efficiency,ultraviolet sunblind response characteristics,gas sensing characteristics,and good excellent properties such as thermal stability and chemical stability.Because of its excellent material properties and suitability,which is attracting interest for power electronic devices,as well as solar-blind UV photodetectors,photocatalysts,gas sensors,solar cells,phosphors,and transparent conducting films for electrodes on a variety of optoelectronic devices.The characteristics of the device are closely related to the quality of the material.The prerequisite for achieving these applications is to obtain high quality?-Ga₂O₃ films.Considering the cost and size of single crystal?-Ga₂O₃ wafers,the heteroepitaxial growth of?-Ga₂O₃ on the extrinsic substrate has important significance.For the epitaxial growth of Ga₂O₃ thin films,there have been many reports such as molecular beam epitaxy?MBE?metal–organic chemical vapor deposition?MOCVD?.Different growth techniques have their own advantages.In addition,different preparation techniques and different process parameters have a great influence on the structural and optical properties of the film.In general,the heteroepitaxial growth of?-Ga₂O₃ is still immature and cannot meet the requirements of device application.Therefore,it is important to study the epitaxial growth of?-Ga₂O₃ and improve its quality.Based on the above background,the heteroepitaxial growth of?-Ga₂O₃ and its devices were studied.The main content is divided into the following parts:1.Ga₂O₃ thin films deposited on sapphire substrates by PEALD and their thermal annealing were studied.Metastable?-Ga₂O₃ thin films were obtained.The Ga₂O₃ thin films deposited were annealed at high temperature.With the increase of annealing time,metastable?-Ga₂O₃ thin films were annealed at high temperature and transformed into?-Ga₂O₃ thin films.In addition,with the increase of annealing time,the preferred orientation of the films was enhanced,and?2?01?preferred orientation of the films was obtained.The surface of the films was gradually flat and the roughness was reduced.The films annealed at high temperature for 2 h showed the best crystal quality,surface morphology and optical properties.2.The effect of growth parameters?temperature,pressure,oxygen flow rate?on Ga₂O₃ film was studied by MOCVD deposition of Ga₂O₃ film on sapphire substrate.It is found that the critical temperature window of?-Ga₂O₃ and?-Ga₂O₃ is about 550°C,and the temperature of?-Ga₂O₃ is higher than that of?-Ga₂O₃.The reaction chamber pressure study shows that the growth rate changes linearly under oxygen-rich conditions.Under gallium-rich conditions,the growth rate deviates from the mass transport curve,and there is a parasitic reaction during the growth of the film.The results of counter-research of chamber pressure in the range of 20-101Kpa show that when the pressure in the reaction chamber is lower than 35Kpa,the film with Ga₂O₃tends to?phase and?and?coexisting phase continues to increase the pressure of the reaction chamber,which can inhibit?-Ga₂O₃.The formation of phase facilitates the formation of a?-phase Ga₂O₃ film.Therefore,increasing the temperature and the pressure in the reaction chamber can inhibit the formation of the?-Ga₂O₃ phase,and the crystal has a strict requirement on the oxygen flow rate,which may be related to the selection of the preferred surface for different growth conditions.3.The effect of different film thickness on the properties of?-Ga₂O₃ film was studied.With the increase of thickness,the FWHM of the film decreased,and the minimum 0.38°of FWHM was obtained when the thickness was 1330nm.According to the evolution of the surface morphology of?-Ga₂O₃ films with different thicknesses,we believe that the growth mode of?-Ga₂O₃ is“multilayer growth”mode at 500°C.The transmission spectrum of the?-Ga₂O₃ film shows that the film has a high transmittance in the visible light region,and a very steep light absorption edge can be observed at about 250 nm,and the corresponding optical band gap is about 4.92 eV.There are six different out-of-plane directions of?-Ga₂O₃ in the?-Ga₂O₃ film,which have high dislocation density and exhibit high-resistance characteristics and three different CL bands.4.Heteroepitaxial growth of?-Ga₂O₃ thin films on off-axis?0001?c-sapphire substrates by MOCVD was studied.The results show that the use of the off-axis substrate is a method that can effectively improve the in-plane symmetric rotation and at the same time promote the conversion of the growth mode.5.The?-Ga₂O₃ thin films were used to construct the metal/semiconductor/metal?MSM?structure UV detectors.The detector exhibits good I-V characteristics.At 5V bias,the maximum response is 2.253A/W and the UV-visible suppression ratio exceeds6×10³.