Study On GaN-based Heterostructures Growth On Patterned And Planar Sapphire Substrates

As the third generation semiconductor material,GaN has a good application prospect in the field of optoelectronic devices and electronic devices because of its wide band gap,high breakdown voltage,small dielectric constant and good conductivity.AlGaN/GaN-based heterojunction materials are very attractive materials in terms of high frequency,high voltage and high power amplification.In order to improve the frequency characteristics of GaN based devices,we need to continue to study new heterostructures.Firstly,we choose AlN/GaN materials with larger spontaneous polarization and piezoelectric polarization effect to fabricate ultra-thin barrier heterostructures to improve the frequency characteristics.However,due to the large lattice mismatch between AlN and GaN,it is difficult to prepare high quality AlN/GaN heterojunction.We have grown AlN/GaN heterostructures on planar sapphire substrate and patterned sapphire substrate with magnetron sputtering AlN by metal organic chemical vapor deposition（MOCVD）.The effects of magnetron sputtered AlN thickness on the crystal quality of AlN/GaN heterostructure are compared.Among the AlN/GaN heterojunction samples on the planar sapphire substrate,the sample of magnetron sputtering AlN with a thickness of 40nm has the best quality.The high resolution X-ray diffraction（HRXRD）（002）FWHM of 40 nm sample is only 67arcsec.The screw dislocation density of the sample is 9.00×10⁶cm^-2.The total dislocation density is 3.95×10⁸cm^-2.The surface roughness of the sample is only 0.657 nm.Among the AlN/GaN heterojunction on the planar sapphire substrate,the quality of the sample with a thickness of 40 nm is the best.Among the AlN/GaN heterojunction samples of patterned sapphire substrate,the quality of25 nm is similar to that of 30 nm and their crystal qualities are better than others.The samples on patterned sapphire substrate are generally better than those on planar sapphire substrate.The HRXRD（002）and（102）FWHM of sample with 25 nm AlN are 180 and 198arcsec respectively.The total dislocation density is 2.23×10⁸cm^-2.The sample with a thickness of 40 nm AlN obtained the smallest root mean square roughnessrms value,which is 0.433 nm.When analyzing the results of non-contact Hall test of AlN/GaN heterojunction samples,it is found that the mobilities of the samples are small,but the two-dimensional electron densities are large while the sheet resistances are very small.This is due to the leakage of electric current from the buffer layer.This is due to the introduction of oxygen elements in the process of magnetron sputtering AlN.In addition,oxygen atoms in the sapphire substrate enter the GaN epitaxial layer and form O_N,O_Nis donor in GaN,the background carrier density in the GaN buffer layer is increased,causing leakage.In order to solve the above problems,we have prepared AlGaN/GaN heterostructure samples on sapphire substrate using the MOCVD,optimized the process conditions.In the growth process of GaN buffer layer,C decomposed from Ga source（trimethyl gallium）was used for self doping,and C entered GaN to replace N atoms form C_N,C_Nis acceptor in GaN,which compensated the donor in buffer layer and effectively improved the leakage.Among the samples of AlGaN/GaN heterojunction on the planar sapphire substrate,,the sample with a thickness of 40nm aluminum nitride has the best crystal quality and the largest mobility.The electron density of 20nm samples is the largest.The HRXRD rocking curves（002）of the three samples have very small FWHMs of 57,59,and 58 arcsec,respectively.So the screw dislocation densities are very small.Among the AlGaN/GaN heterojunction samples on the patterned substrate,the sample with a thickness of 25 nm AlN has the best crystal quality and electrical characteristics,and the mobility reaches 2292 cm²（v.s）^-1.The quality of the sample on patterned substrate sample is better than that on the planar sample.The mobility of AlGaN/GaN heterojunction samples is very high,indicating that we have improved the structure and used self-doping technology to reduce the background carrier density in the GaN buffer layer,improve the leakage.The self-doping method can effectively avoid the lattice damage caused by ion implantation doping C.