Research On The Mechanism And Reliability Of Enhancement-Mode AlGaN/GaN HEMTs With F Ion Implantation

AlGaN /GaN HEMT has advantages of high frequency,high voltage,large output power and high efficiency,which leads to a promising candidate for wireless communication,military and aerospace applications.Due to the strong polarization effect of GaN materials and the high density two-dimensional electron gas at the heterojunction interface,the fabricated AlGaN /GaN HEMT is a depletion device.However,owing to the power depletion,the complexity of the circuit design and the security of the power system,the enhanced device is indispensable in the practical application.Therefore,based on the technology of fluoride ion implanted GaN enhanced devices,this paper explores influence mechanism of F ions on GaN and HEMT devices.Then,the enhancement-mode of AlGaN /GaN HEMT devices are fabricated.Moreover,the reliability and traps effect of devices are analyzed.The related findings are of great significance in promoting F ions implanted AlGaN /GaN HEMT devices.The first-principles simulation model of F-interstitial doping GaN was established,and the influence mechanism of F-interstitial doping on material GaN on crystal structure band characteristics and optical properties was systematically studied.The un-doped GaN,F-substituted doped(GaN_0.9722F_0.0278,GaN_0.9375F_0.0625),F-interstitial doped(Ga₃₆N₃₆F,Ga₁₆N₁₆F)and other various unit cells are constructed.Then the band gap,electronic energy density of state and absorption spectrum are studied.In the case of substitution doping,with the increase of the doping concentration of F,the difficulty of doping,the formation energy of defects and the instability all increase.At the same time,the gradual substitution of Ga-4s state by F-2p state determines the density of states at the bottom of the conduction,which results in narrower band gap,lower effective mass of electrons and red shift of absorption spectrum.When F exists in the form of interstitial doping,the formation energy of unit cells and difficulty of doping decreases with the increase of F doping content.And the more stable of the structure of unit cell.Meanwhile,the Ga-4s state at the bottom of the conduction band moves toward the higher energy level,which leads to the band gap of materials widens,blue shift of the absorption spectrum,the effective mass of the holes of the top valence band and the ionization energy decrease,and the Bohr radius increases.The conclusions prove that F ions exist mainly in substitutions and interstitial sites in GaN and it is more stable in interstitial sites.The wider band gap caused by interstitial sites F can improve the breakdown characteristics of GaN based devices.The theory and technology development of enhanced GaN-based HEMT devices are developed by fluoride ion implanted.And this paper introduces the fabrication process of enhancement AlGaN /GaN HEMT devices and proposes a low damage remote plasma surface pretreatment process based on NH₃/N₂which can avoid RF damage under the condition of high power（200 W）RF plasma and reduce the degradation of DC and dynamic characteristics of devices caused by etching damage.NH₃/N₂ plasma treatment changes the height of the surface barrier and the donor distribution,resulting in the increase of the two-dimensional electron gas density.NH₃ can partially suppress the reduction of N vacancy,which reduces the influence of plasma on ohmic contact resistance.The technology is used for pretreatment before passivation,effectively reducing the series resistance and improving the current collapse.This technology is used for the pretreatment under the gate,which can reduce the surface state under the gate,suppress the gate leakage and improve the device performance.The frequency-dependent measurement of conductance and the light assisted C-V measurement are introduced in this paper,and the effect of fluoride ion implantation on the distribution of interface defects in GaN HEMT devices was investigated.First,the intrinsic interface trap of devices is studied based on the structure of Al₂O₃/AlGaN /GaN MOS HEMT through the frequency-dependent measurement of conductance and the light assisted C-V measurement.The trap energy levels and the density of states in AlGaN /GaN interface are obtained by C-V measurement and conductance analysis.In order to detect deeper interface states at the interface of Al₂O₃/AlGaN ,the deep level interface states with energy levels from2.53 eV to 3.01 eV below the conduction band edge are detected by using light assisted C-V analysis.Although the density of deeper interface states is much lower than that of shallower interface states at the interface of Al₂O₃/AlGaN ,it can cause obvious threshold voltage instability.Based on the conductance method and intrinsic interface states of HEMT device,the influence of fluorine implantation process on the interface traps of Schottky junction and heterojunction in metal gate/AlGaN /GaN Schottky diode structure is studied,which shows that the deep trap may be produced at the interface of Schottky junction and heterojunction by fluorine ion implantation.We develop the enhancement-mode AlGaN /GaN HEMT device based on TiN-based source contact ledge and two-step fluorine treatment,and further analyzes the stability and reliability of F ions.This paper also explores the effect of the fluorine injection process on the characteristics of the device.With the increase of injection time,the concentration of increased fluorine ions in the material increases,which can enhance the depletion of channel,and shift the threshold voltage positively to realize the enhancement devices.The stability of low-power fluorine ions in AlGaN /GaN heterostructures is studied.The migration model of fluorine ions introduced by low power injection under the off-state stress is proposed and the physical mechanism of the effect of the migration on the device characteristics is analyzed.Then,the high-performance enhancement AlGaN /GaN HEMT based on TiN-based source contact ledge and two-step fluorine treatment are fabricated,which decreases source parasitic resistance and leakage current of GaN buffer layer effectively and increase by 20%of output current of devices.And the frequency and breakdown characteristics of the device are enhanced and its value of BV_DS×f_max achieves 23.8 THz·V.This shows that the enhancement type device is very suitable for high power and high frequency applications.Finally,the electrical stress experiments of devices are carried out in this thesis to explore the degradation law and physical mechanism of degradation.