Study Of The Point Defect Luminescence Mechanism In Ga N And ScVO₄

Semiconductors have gradually become an indispensable role in our daily life and industry from the invention of the first transistor to the wide application of the large-scale integrated circuit and various electronic devices.During the synthesis of semiconductors,point defects,including intrinsic defects and extrinsic defects involving the introduced impurity elements,can be formed.Different types of point defects may have different effects on the photoelectric properties of the matrix materials,forming luminescence centers.With the development of computational methods,the first-principles calculations have been able to provide more reliable electronic properties of points defects in semiconductors.The radiation and non-radiation recombination processes related to point defects in semiconductors have also been well described.However,the wavelength and intensity of point defect related luminescence observed macroscopically are involved in complex carrier dynamics.The conventional calculation of point defect is not enough to accurately describe the physical mechanism of experimental phenomena.As a result,we developed the methods based on first-principles calculations,by considering the competition of radiation and non-radiation recombination processes,which can simulate the quantum efficiency and time-resolved photoluminescence?TRPL?of point defects without fitting parameters.Red phosphor ScVO₄:Bi has potential application in white light-emitting diode?LED?,while Gallium nitride?GaN?,as the core material in blue LED,plays a crucial role in the field of luminescence.Aiming at understanding the luminescence phenomenon of point defects in these two materials,the following researches are carried out:1.We studied the tunable luminescence mechanism of Bi doped ScVO₄ by the first-principles calculations.Due to the low defect formation energies,some intrinsic defects are energetically preferable in the ScVO₄ matrix.However,these intrinsic defects have no significant effect on the emitting of Bi doped ScVO₄.In the considered chemical potential environment,the doped Bi atom tends to replace the Sc atom rather than the V atom.The calculated defect formation energy shows that Bi _Sc and Vac_O defects are easily combined to form a complex defect Bi _Sc+Vac_O.According to the calculated defect transition energies and the imaginary parts of the dielectric function,we deduce that the transition between“0”and“+1”charge states of complex defect Bi_Sc+Vac_O may be responsible for the red luminescence of Bi doped ScVO₄ sample observed in the experiment.The tunable luminescence can be experimentally achieved by introducing H₂ into Bi_Sc+Vac_O and controlling the content of H₂.Our results show that the change of luminescence properties is due to the passivation effect of H atom on Vac_O.2.Combining the first-principles calculations with the experimental Raman and Fourier transform infrared spectrum measurements,we have determined that C atom will occupy the N atom site in the semi-insulated GaN doped with C,forming the C_N defect with“-1”charge state.The C impurity atom will generate new local phonon modes in the high-frequency part of the phonon spectrum of GaN matrix,including the horizontal mode(774 cm^-1)and the vertical mode(766 cm^-1).3.We have calculated the quantum efficiency of yellow luminescence?YL?caused by point defects in GaN by ab initio density functional theory?DFT?.The electron and hole recombination processes related to deep impurity energy levels are described in detail.The transition between the“-1”and“0”charge states of point defect C_N and the transition between the“0”and“+1”charge states of complex defect C_N+O_N may be the source of YL in n-type GaN observed experimentally.Moreover,our calculations show that YL begins to quench when the temperature is higher than 480 K.This may be due to the fact that the hole captured by the impurity levels from the valence band will return to the valence band through overheating excitation,in line with the observed quenching result of YL in the experiment.4.We have simulated the TRPL process of YL in GaN,which takes into account the complex competition among multiple recombination channels in the time range from picosecond to millisecond.It can be proved that YL induced by C_N defect presents obvious double exponential decays under large injection.The fast decay lifetime of several hundred picoseconds comes from the competition between YL and the band edge recombination channel,while the short life of several microseconds comes from the decay of YL itself.Both of these decay processes have been confirmed in the TRPL experiments designed by ourselves.Our study has provided a good explanation to the complex optical physical processes caused by the deep defect levels in wide bandgap semiconductor material,and it will give a powerful theoretical guidance for identifying and designing of point defects in future experiments.