Defect Control And Physical Properties Of AlN

AlN is a typical representative of the third generation semiconductor materials with wide bandgap and stable physico-chemical properties.AlN and GaN can form AlGaN alloy,which is one of the most promising materials to fabricate ultra-violet photoelectronic devices.Meanwhile,AlN based materials also have unique advantages in high frequency and high power electronic devices.However,bulk single crystal AlN substrate with large size is hard to be obtained.To achieve high quality AlN templates on sapphire or other heterogeneous substrates is the main research direction of researchers.Due to the large mismatch between epitaxial layer and heterogeneous substrates,high density dislocations exist in AlN based materials,which will reduce the performance and service life of AlN based devices grown on it.Moreover,the relationship between the refractive index of AlN and dislocation density is still not clear,which limits the design of AlN based photoelectronic devices.The aim of this work is to decrease the threading dislocation density in AlN crystal grown on heterogeneous substrate and research the optical properties of AlN with different dislocation density.We promote AlN crystal quality by regulating the sapphire surface,AlN growth interlayer condition and high temperature annealing.The mechanism on how the AlN crystal quality improved is analyzed.Moreover,the refractive index of AlN with different dislocation densities are studied.The main achievements are shown as follows:First,growth of low dislocation density AlN epilayer by controlling the surface and interlayer condition via Metal-organic Chemical Vapor Deposition?MOCVD?.In this research,flat surface of AlN is obtain by insert middle temperature AlN interlayer during the growth of AlN on sapphire substrate.Because the surface of interlayer is formed by AlN islands,the dislocation will bend and be eliminated when flat AlN grown on it.The number of interlayer is changed to improve AlN quality.Moreover,the interlayer can release the stress of AlN suffers from sapphire substrate,the surface root mean square roughness reaches to about 0.1 nm.To further improve the quality of AlN,PVD-AlN/Sapphire templates is introduced.We use PVD-AlN to improve the surface condition of sapphire and research the relationship between AlN epitaxy layer and the thickness of PVD-AlN.It is found that the?002?and?102?plane XRC full width at half maximum?FWHM?of AlN epitaxy layer have decreased 338 arcsec and 153 arcsec for the thickness of PVD-AlN decreases from 150 nm to 40 nm.The mechanism can be explanated that thinner PVD-AlN suffers higher compressive stress from sapphire substrate,which is beneficial for the bending and annihilation of dislocations in AlN epitaxy layer.The middle temperature interlayer method is used during the growth of AlN on PVD-AlN templates.The FWHM of AlN crystal's XRD rocking curve reaches to 216 arcsec for?002?plane and 482 arcsec for?102?plane.Second,the dislocation density of AlN is reduced by high temperature annealing and the mechanism is revealed.In this work,we improve the quality of AlN crystal by high temperature annealing successfully and study the mechanism.When the annealing temperature reaches to 1750?,the XRD rocking curve FWHM of AlN reaches to 98.3 arcsec for?002?plane and 234.7 arcsec for?102?plane,the corresponding screw and edge dislocation density are 2.10×10⁷ cm^-22 and 3.06×10⁸ cm^-2,respectively.The improvement of AlN crystal quality is attributed to the recrystallize process during high temperature annealing.The changes of surface morphology during high temperature annealing is studied.It is found that the surface of AlN become rougher with the annealing temperature rises at first.When the annealing temperature above a threshold?about 1700??,the surface of AlN become smoother with higher annealing temperature under AlN cap.Third,the relationship between the refractive index and the dislocation density of AlN is obtained.It is found that the refractive index of AlN decrease with the increase of dislocation density.With the increase of dislocation densities from 4.24×10⁸ cm^-22 to3.48×10⁹ cm^-2,the refractive index of AlN decreases from 2.2508 to 2.2102 at 280 nm.The mechanism of this phenomenon is studied in this part.Via the calculation and analysis,it is found that the distribution of the refractive index field around dislocation?which caused by the distribution of strain field?will influence the refractive index of AlN.