Study On Defects In Nitride Semiconductors And Their Effects

Nitride semiconductor possess superior properties including adjustable and direct bandgap,large breakdown voltage,and high electron mobility.Thus,they are ideal for opto-electronic and power electronic devices.However,high density of defects in GaN devices,such as pits,dislocations,point defects,are common due to considerable lattice and thermal expansion mismatch between Nitride and foreign substrates.These defects severely restrict the performance of the device.In order to further optimize device performance and obtain long-life,high-reliability,high-power GaN-based devices,it is necessary to understand the impact of defects on the performance of nitrides.This paper focuses on the defect evolution and the effect of defects on the properties of nitride materials.The effect of pit and different types of dislocations on GaN carrier transport was studied by ultraviolet-assisted Kelvin probe force microscope(KPFM),and its physical mechanism was explained by energy band analysis.By the combination of photoluminescence,positron annihilation,secondary ion mass spectrometry and first-principles calculation,the type of point defects in AlN was identified,and effect of point defects on the optical properties of AlN were studied.The mechanism of the effect of temperature on the evolution of AlN point defects was revealed.The main innovative results achieved by this research are summarized as follows:(1)Surface potentials in the vicinity of V-pits(cone bottom)and U-pits(blunt bottom)on epitaxial GaN surface have been systematically studied using UV lightassisted Kelvin probe force microscopy(KPFM).The band structure models are established to understand variation of the surface potentials at the pits and planar surface with and without UV light.The photogenerated carrier behavior at the pit defects is studied.According to the surface potential results,the carrier distributions around the V-and U-pits are deduced,which are opposite between in dark and under UV light.In dark,the electron concentration at the bottom of V-pit(30n0)and U-pit(15n0)are higher than that at planar surface(n0).Under UV light,for V-pit,the electron concentration at the cone bottom(4.93×1011n0)is lower than that at the surrounding planar surface(5.68×1013n0).For U-pit,the electron concentration at the blunt bottom is 1.35×1012n0,which is lower than that at the surrounding planar surface(6.13×1013n0).According to the relationship between the non-equilibrium carrier concentration and the energy band,the uneven distribution of carriers and the increase of the recombination rate caused by pit defects is clarified.(2)The effect of different type dislocations on photoelectric properties of GaN is studied by UV-assisted KPFM combined with defect selective etching,which allows the information acquirement of dislocation type and optoelectrical behavior of a single dislocation simultaneously.The screw dislocations are found to be the main nonradiative recombination centers,and mainly responsible for leakage current in GaN based device,which shows higher surface potential and higher electron concentration due to the donor defects introduced during dislocation growth.Inversely,the edge and mixed dislocation can aggregate holes due to the acceptor-type defects incorporated during its growth.A potential barrier is generated,which will block photo-generated holes from injecting to the dislocations and suppressing non-radiative recombination at the dislocations.(3)The identification,evolution and optical properties of point defects in hightemperature annealing AlN were studied by photoluminescence spectroscopy(PL),secondary ion mass spectrometry(SIMS),positron annihilation and first-principles calculation.It proves that the near-ultraviolet defect peak in AlN is related to the carrier recombination between [VAl-(ON)n]-3+n and ON.The increase of O impurity concentration after high-temperature annealing is the key factor that causes the nearultraviolet defect peak enhancement.The increase of n in [VAl-(ON)n]-3+n is the reason for the increase of the complexity of the defect peak and the broadening of the defect peak to the high energy.The oxygen(O)impurities in high temperature annealing AlN increase due to the O in sapphire and annealing atmosphere.For Si and C impurities,the impurities' concentration on the surface of the sample decreases as the annealing temperature increases.At the AlN/sapphire interface,the C and Si impurities concentrated at the interface of MOCVD-AlN and PVD-AlN diffuse after high temperature annealing.The overall impurity concentration in the sample tends to be evenly distributed.