Research On Key Techniques For Ultra-wide Bandgap Semiconductor Ga₂O₃ Power Diodes

As the core component of efficient power conversion,power devices are the key to improve the utilization rate of electric energy,which have been widely used in our life and industry,such as industrial manufacturing,photovoltaic,new energy vehicle and so on.The ultra wide band gap semiconductor gallium oxide（Ga₂O₃）has the advantages of high critical breakdown field strength,low on resistance,and low cost.Therefore It is one of the ideal material systems for power devices,which is recognized as one of the most competitive power electronics technologies in the next generation.The gallium oxide has been listed as the strategic development direction of power electronics technology in the United States.In recent years,with the strong support of various countries,gallium oxide power devices have made considerable progress,the performance of which doubles every year.However,there are also lots of serious problems existing in gallium oxide power devices,such as the difficulty in terminal structures design and low breakdown voltage caused by the lack of P-type gallium oxide doping technology,the difficulty of slicing processing caused by the extremely easy fracture of（100）cleavage surface,the difficulty of heat dissipation caused by the low thermal conductivity of the gallium oxide substrate,and so on.In view of the above problems,the targeted researches have been carried out in this paper.Lots of novel terminal structures and technologies have been proposed to overcome these problems.The main research contents and conclusions are as follows.（1）The characterization of the gallium oxide epitaxial material and the models of gallium oxide power device have been investigated.The qualities of（001）β-Ga₂O₃epitaxial wafer have been characterized by X-ray diffraction（XRD）,Raman,atomic force microscope（AFM）and surface defect tester.The（001）full-width at the half of the maximum（FWHM）is smaller than 35arcsec,measured by X-ray diffraction.Meanwhile,the surface roughness Ra is less than 0.22nm,measured by atomic force microscope.Based on the results measured by surface defect tester,it is clear that lots of defects exist in the surface of the gallium oxide epitaxial material.It is difficult to achieve high breakdown voltage for gallium oxide power devices,especially for the large size gallium oxide power devices.In a word,the quality of gallium oxide epitaxial material needs to be further improved to realize high breakdown voltage power devices.In addition,the physical simulation model of gallium oxide diode is established,laying a solid foundation for further study of breakdown mechanism and novel terminal structures design.（2）There is a common problem for the Schottky barrier diode（SBD）,the leakage current becomes higher with the decrease of the Schottky barrier height caused by the high electric field around anode.Reducing the peak electric field around the anode becomes the key technology to reduce the the leakage current and improve the breakdown voltage of SBD.The thermal oxidation terminal technology was introduced in the gallium oxide SBD for the first time,decreasing the peak electric filed and high leakage current around anode in the gallium oxide SBD.The electron concentration of gallium oxide epitaxial layer was reduced by thermal oxidation,may be caused by the decrease of oxygen vacancies.Then,the electronic concentration gradient is formed,regulating the peak electric field around the anode.Therefore The breakdown voltage of gallium oxide SBD is improved.Moreover,the air bridge like field plate is introduced for further improving the breakdown voltage.Beside,heterogeneous p-Ni O junction termination extension,positive grinding angle terminal technology and other terminal structures were also investigated.Among these terminal structures,the inverted trapezoid terminal technology has the most significant effect on improving the breakdown voltage,which was increased as high as 253%.While the oxidation terminal technology is an undamaged terminal technology with the simplest process,which has been used in the high power gallium oxide SBD.（3）In order to balance the breakdown voltage and on-resistance,the p-Ni O/Ga₂O₃ heterojunction diode is introduced to realize higher breakdown voltage.Based on the ultra-low mobility of p-Ni O and the controllable thickness of Ni O film,the novel self isolating thin layer p-Ni O junction termination extension was proposed for the first time,improving the breakdown voltage significantly.Moreover,the self isolating thin layer p-Ni O junction termination extension has little effect on forward current and leakage current between devices,due to the super-large lateral spread resistance.Compared with the conventional extended junction termination extension,the peak electric field at the edge of the terminal is eliminated in the self isolating thin layer junction termination extension,and a stronger regulation effect on the peak electric field around the anode is realized.Besides the self isolating thin layer p-Ni O termination design and fabrication is very easy.Moreover,we propose stepped junction terminal expansion and small-angle slop field plate composite terminals to further improve breakdown voltage.A 4450V high voltage p-Ni O/Ga₂O₃heterojunction diode with average electric field strength of 4.45MV/cm for 10μm epitaxial layer and Baliga’s figure-of-merit(BV²/R_on,sp)of 4.7GW/cm²has been realized by the novel composite terminals.This is the first time that the performance ofβ-Ga₂O₃power devices exceeded the theoretical limit of Si C materials.The thin infinitely extended p-Ni O junction termination extension is one of the major innovations of this paper,which has shown significant effect in improving the breakdown voltage of the power devices.It is expected to promote gallium oxide power diodes to the ultra-high voltage field.（4）In order to promote the application of gallium oxide diodes,high-power gallium oxide SBD devices has been investigated in this paper.The novel surface treatment technology with low damage shallow etching process and high-temperature thermal oxidation is introduced to solve the problem of the influence of surface and interface states on device characteristics.The Schottky junction with ideal factor of1.03 is realized.Combined with anode edge groove technology,the high-power gallium oxide SBD device with anode area of 2mm×2mm is fabricated.The forward current,turn-on voltage,breakdown voltage,reverse recovery time,rectification frequency and thermal resistance of the device are 3.9A@2V,0.8V,-1400V,12ns,1MHz,and 2.5K/W,separately.In addition,the influence of high temperature on the forward conduction characteristics and switching characteristics of the device is investigated.With the room temperature rises to 175℃,the forward current under 2V and breakdown voltage only decrease by 31%and 20%,respectively.While the reverse recovery time remains almost unchanged at 12ns.It shows that the ultra wide band gap gallium oxide power devices have certain application potential in high temperature environment.