Photoluminescence Characteristics Of Vacancy-oxygen Complexes In AlN Crystal Grown By PVT Method

Aluminum nitride(AlN)is a compound semiconductor with ultra-wide direct bandgap(⁶.02 eV),high thermal conductivity(3.4 W·cm^-1·K^-1)and high critical electric field(1.2¹.8 MV·cm^-1),emerging as one of the most promising candidates for applications in deep ultraviolet(UV)optoelectronics and high-power electronics.In addition,AlN is the ideal substrate for epitaxial growth of AlGaN materials with high aluminum composition.At present,the performance of optoelectronic devices and/or electronic devices based on AlN is far from expectation.On the one hand,the large energy bandgap results in a high activation energy of n/p dopants,leading to a low effective doping efficiency.On the other hand,the large energy bandgap also results in wide accommodation energy range for mid-gap defects levels leading to below bandgap luminescence or electron trapping.In this thesis,the photoluminescence characteristics of defects in a non-polar AlN crystal grown by physical vapor transport(PVT)are studied.The material properties and elements distribution of the AlN crystal were characterized by X-ray diffraction(XRD),secondary ion mass spectroscopy(SIMS),X-ray photoelectron spectroscopy(XPS),atomic force microscopy(AFM)and Raman spectroscopy.By varying the wavelength and power of the excitation laser and temperature,we systematically studied the photoluminescence(PL)characteristics of the AlN crystal.The main research contents and conclusions are summarized as follow:The surface of the measured AlN crystal is non-polar m plane.Oxygen is the only impurity that can be detected in the AlN.The PL spectra of the AlN were studied using different excitation lasers with wavelengths of 633,514,266 and 193 nm.The excitation power and temperature dependent PL spectra were studied using a 266 nm laser.Nine PL emission bands,including ultraviolet(³.62,3.41 and 3.21 eV),infrared-red(1.42,1.58,1.84 and 2.03 eV),and blue(2.7 and 2.42eV),were identified using Gaussian peak fitting.They are related to V_A1-O_N complexes and V_A1,respectively.Blue emission bands come from the transitions between the conduction/valence band and the V_A1.Ultraviolet emission comes from the transitions between the conduction band and the V_A1-O_N center,and the infrared-red emission comes from the transition between the V_A1-O_N center and the valence band.The multi-peak characteristics of ultraviolet and infrared emission are related to the complexity of the geometry and composition of the V_A1-O_N complexes.There are two types of the V_A1-O_N complexes,i.e.,(V_Al–2O_N)¹–/0–/0 and(V_Al–O_N)^2–/1–,in both of which the O_N can be located along the c-axis or in the m-plane.The splitting energy between the two configurations is⁰.2 eV.The PL intensity of ultraviolet/infrared and blue emissions decrease with the increasing temperature.The temperature dependence of the PL intensity is analyzed by using an Arrhenius plot model and a Luminescence of localized-state ensemble model.This work identified the luminescence peaks related to oxygen-centers in AlN.The conclusions are important for a comprehensive understanding of the below-bandgap absorption and emissions,as well as of significance for improvement of the crystal quality of AlN.