Study On Magnesium Ion Implantation And Activation In Gallium Nitride And Device Applications

Power electronic devices are the foundation and core of power electronic technology.As silicon(Si)based power devices have approached their theoretical limits,their speed of improvement has been unable to meet the demands of modern power electronic technology for greater power density,higher frequency,and lower efficiency loss.Gallium nitride(GaN)has proven to be a promising candidate for the next generation of high-performance power devices,due to its advantages of large bandgap,high electron mobility and high breakdown electrical field.However,affected by the quality of epitaxial growth and device manufacturing process,the industrialization of GaN-based power devices is facing many scientific and technological problems.Especially,selective area p-type doping is still not mature for GaN.So far,controlling conduction type of GaN through ion implantation is still a significant challenge due to the inevitable crystalline damage caused by implantation process and the lack of effective methods to improve magnesium(Mg)activation ratio at room temperature,which has hindered the fabrication of high-performance termination structures in GaNbased high-voltage devices.In this thesis,we focus on Mg ion implantation and activation in GaN and device applications.The main results are highlighted as below:1.The realization of p-type GaN by a combination of optimized Mg ion implantation and post-annealing activation processes is demonstrated.By comparing the recovery and activation effects of various samples under different experimental conditions,the effect of Al N capping layer during the ion implantation process and Si O2 layer during activation process are analyzed.The optimized implantation and activation conditions obtained in this work are: annealing at 1230 ? for 30 minutes in N2,while using Al N and Si O2 as the protective capping layers during ion implantation and high temperature annealing process,respectively.The Al N protective layer can effectively prevent the amorphization of surface caused by the bombardment of high energy particles during the Mg ion implantation.The XRD rocking curves indicate that the full-width-at-half-maximum(FWHM)of the annealed Mg-implanted sample is comparable to that of the unimplanted sample,which suggests that the lattice damage caused by ion implantation was effectively healed through the post-implantation annealing process conducted at 1230 ? for 30 min.The Fermi level approaching VBM indicates an effective p-type doping through the optimized implantation and thermal activation processes with proper capping layers.The Seebeck coefficient extracted from thermopower measurements is positive,unambiguously confirming the existence of mobile holes in Mgimplanted GaN.2.To further verify the p-type conductivity,we fabricated a GaN p-i-n diode,which shows excellent rectifying characteristics with a forward turn on voltage of 3V,in consistent with the bandgap of GaN.Electroluminescent(EL)at forward bias and electron beam induced current(EBIC)results also validate the realization of p-type GaN by ion implantation.As revealed in the ultraviolet(UV)detection characterizations,these diodes exhibit a sharp wavelength cut-off at 365 nm,a high UV/visible rejection ratio of 1.2×104,and a high photoresponsivity of 0.35A/W,which are comparable with commercially available GaN p-n photodiode.A slight internal gain mechanism was observed,indicating the existence of localized states in the Mg-implanted GaN.With consideration of both the photoresponsivity's dependence on the reverse bias and the transient response characteristics,a physics model of electric-field-assisted photocarrier hopping is proposed.Ion-implantation based GaN p-i-n UV PDs could become fairly competitive for future mass production.3.The effect of pulsed laser annealing(PLA)on the structural and optical recovery in the high-dose Mg ion-implanted GaN has been investigated.The structural evolution and vibrational dynamics indicate an obvious structural recovery and partial strain release of Mg-implanted GaN during pulsed laser annealing process.The implantation induced disorders,vacancies and lattice distortion could be partially removed after PLA process.Reduction of lattice disorders and improvement of the optical recovery quality by higher laser fluences or more pulse times can be confirmed in transmission results.The improvement of crystal quality by laser irradiation has also been verified both in the Raman and XRD measurements.The enhanced donor-acceptor transition at 3.35 e V after pulsed laser irradiation is a sign of the effective activation of Mg from interstitial sites into substitution of Ga ions.Taking TEM results into consideration,the influence of various activation methods on Mg ion-implanted GaN is further analyzed.Based on this,an annealing activation scheme that integrates the two processes of conventional annealing and laser annealing is proposed to further manipulate p-type conductivity of Mg-implanted GaN.