Application Research On Raman Spectroscopy Properties Of ?-Nitride Semiconductors

? nitride semiconductors have excellent properties such as wide bandgap,strong polarization,high breakdown field strength and high electron saturation velocity,meanwhile,they have good thermal stability and chemical stability.During the past two and three decades,the research on ?-nitride semiconductor materials and devices has made considerable improvements,and achieved fruitful results.High efficiency nitride based blue LED was successfully commercialized since the problems of both epitaxial growth of high-quality GaN film and P type doping of GaN were solved.At the same time,many records on the power density,efficiency and bandwidth of semiconductor devices in the field of electronic devices have been broken by GaN-based heterostructures devices such as AlGaN/GaN heterostructure devices.However,there are still a large amount of problems in the field of ?-nitride semiconductor materials and devices.A plenty of problems in fundamental theory,material defects,device performance and reliability,device structure and so on need to be solved.To solve these problems,it is required not only breakthroughs in material growth and device processing technology,but also further accurate researches on the material properties and device performance through various semiconductor characterization techniques.Raman spectroscopy is an effective characterization method which can provide theinformation of stress,defect,impurity,alloy component,carrier cogncentration and temperature of the material at the same time.It has the advantages of high speed,no sample preparation,no damage and high spatial resolution.Therefore,Raman spectroscopy has become an important method both on the material characterization and device measurement.Raman spectroscopy can accurately characterize the change of material properties and device performance during the operation of GaN devices,which could be used to further study the influence of different factors on the device performance and provide a powerful technical help for obtaining high performance GaN materials and devices.Based on Raman spectroscopy,the properties of stress,defect,carrier concentration,temperature and alloy composition of GaN materials with different polarities are studied systematically in this dissertation.The major work and achievements are listed as follows:1.The spatial distribution of crystal quality in n-type GaN films grown on patterned sapphire substrates(PSS)was investigated by using Raman mapping technique.The dislocation annihilation mechanism of GaN films grown on patterned sapphire substrates was described by the analysis of the frequency shift and full width at half maximum(FWHM)of E2(high)modes of GaN.Many dislocations in the window region would bend towards the pattern of PSS during the lateral growth process and finally annihilate when they meet the pattern.The plane view CL and cross sectional TEM results further confirmed the phenomenon of dislocation bending and annihilation in the GaN grown on PSS.The results also proved that the accuracy of the dislocation annihilation mechanism analyzed by Raman mapping technique.2.The stress and dislocation distribution of hexagonal hillocks in N-polar GaN films was investigated by using Raman mapping technique.It was observed that the residual stress in the top surface region of the hexagonal hillock was much smaller than both the sidewall regions and the region around the hexagonal hillock according to the two-dimensional surface scanning of Raman spectra to a typical hexagonal hillock on the surface of N-polar GaN grown on sapphire substrate.CL results showed that the dislocation density in the region around the hexagonal hillock was obviously higher than the top surface region of the hexagonal hillock,which confirmed that the dislocation was bent and annihilated during the growth process and the partial residual stress was released.This was the reason leading to variations in the stress and dislocation distribution.3.The anisotropy of crystal quality and residual stress in a-plane GaN grown by Epitaxy Lateral Overgrowth were studied by using Raman spectroscopy.By comparing the intensity,frequency shift and FWHMs of the GaN E2(high)modes in the uncoalesced a-plane GaN stripes grown along different mask stripe directions,it was found that when the mask stripe was along the[0001]orientation,the crystal quality at the window region was remarkably improved which was due to the dislocation of the window region being bent toward the sidewall.The lateral growth region has better crystal quality and smaller residual stress when the mask stripe was along the[1100]orientation.CL mapping results further confirmed the different mechanisms of the dislocation bending during the lateral overgrowth along the different mask stripes.4.The distribution of free carrier concentration with depth in non-polar a-plane GaN was investigated by using Raman spectroscopy.It was found that the scattering frequency of the Ai(LO)phonon mode of non-polar GaN was the highest near the sapphire substrate according to the line scan to the cross section of the a-plane GaN sample.With the measured point closing to the top surface of the sample,the frequency of Ai(LO)mode becomes smaller gradually.The distribution of the free carrier concentration in the a-plane GaN material with depth was calculated by using the scattering frequency of GaN LOPC phonon modes.It was confirmed that the free carrier concentration near the substrate was higher.5.The polarized Raman spectra of semipolar(1122)plane GaN homoepitaxial substrate were investigated.By changing the polarization direction of the incident light,we obtain the relationship between the polarized angle of the incident light and the intensity of each phonon modes when the direction of polarization of the scattered light was ? and ?.The relationship between the scattering intensity I of the different phonon modes and the angle ? between the incident light polarization direction and the scattered light polarization direction is deduced.It was found that the experimental results of Q(TO)E1(TO),and Q(LO)were basically consistent with the theoretical calculations.However,the nonpolar E2(high)phonon mode,which should be forbidden when the scattered light polarization direction was ?,could still be observed due to the fact that the c-axis is neither parallel nor perpendicular to the surface in the(1122)plane GaN material.The variation of intensity with the polarization angle of the incident light was consistent with the trend when the scattered light polarization direction was ?.6.The temperature dependent Raman spectra of semipolar(1122)plane GaNmaterials were investigated.The first-order Raman spectra of the semipolar GaN samples in the range of 83 K to 563 K show that the frequencies of the phonon modes of the GaN materials were blue-shifted as the temperature increases.The FWHMs of the phonon modes also increased at the same time.The experimental results are fitted according to the theory models of temperature dependent Raman frequency shift and broadening.It was confirmed that the phenomenon of frequency shift of all phonon modes was caused by lattice thermal expansion and phonon decay.Meanwhile,the linewidth broadening of phonon modes was caused by impurities,defects and phonon decay.The anisotropy of phonon vibrations in GaN materials was further proved by the difference of the frequencies and FWHM of phonon modes such as E1(TO)and E2(high)of GaN materials under different geometrical configurations.