Studies Of Aluminum Nitride Crystals Grown By Physical Vapor Transport Method

AlN is a direct bandgap semiconductor with a bandgap approximately 6.02 e V.Since AlN have very similar lattice constants and almost the same thermal expansion coefficient with Ga N,it is an ideal substrate material for Ga N based materials.However,the wafer-size AlN substrates are absent due to the lack of large size and high quality AlN single crystal,which limits the applications of the Ga N based materials with high Al compositions.There are two methods to obtain large-sized AlN single crystal,one is self-seeded growth by using AlN seed,the other is heterogeneous seed growth by using SiC seed.The advantage of self-seeded growth is the high quality of the growing crystal,but the disadvantage is the slow diameter expansion speed,which makes it difficult to obtain large size crystal.The advantage of heterogeneous seed growth is that large size AlN crystal can be quickly obtained by using large size SiC seed,but the disadvantage is that the obtained crystal contains a high concentration of Si and C impurities and its crystal quality is poor.However,if using SiC seed to obtain large AlN single crystal by heterogeneous seed growth method in the early stage of the growth,and the obtained large AlN single crystal template is used as seed for self-seed crystal growth,then the advantages of the two methods can be fully utilized to obtain large AlN single crystal with high quality in a relative short time.Base on the considerations,this dissertation focused on the studies of correlation of the growth conditions and seed orientations with the characteristics of AlN crystals grown on the AlN self-seeds and the SiC hetero-seeds,together with some necessary preliminary preparations for these studies.The main results are as follows:First,optimization of some subsidary condition's necessary for AlN growth:?1?It is very effective to solve the problems of crucible cracking observed in conventional integral crucible and serious AlN source leakage in a split crucible with isolated bottom and lateral,by designing an integral Ta C crucible with a domed bottom.The new integral Ta C crucible with a hemispherical bottom has not only a long service life,but also greatly reduces the ratio of the weightless and evaporation during crystal growth,which is only 28%of that in the case of split crucible.?2?The purification process of AlN source was improved by raising the temperature and injecting H₂ into the reaction chamber during the purification process,and the impurity contents of C and O in AlN source were decreased to 82 ppm wt and 150 ppm wt respectively.?3?It is effective to suppressed the random nucleations at the early stage of crystal growth by optimizing the reverse temperature field at the initial heating stage.Thus the quality of AlN crystal that epitaxially grew on seed is improved.Second,the temperature range for AlN crystal growth was optimized.The results showed that when the temperature was lower than 1950?,AlN usually grows into clusters of whisker.While the temperature is between 1950?and 2350?,the shape of AlN crystal becomes block.While the temperature is higher than 2400?,metal Al will appear inside the crucible,and the quality of AlN crrstal is deteriorated and the crucible is seriously corroded.By designing the experiments of AlN crystal growth on the AlN seeds in orientations of?11-22?and?000-1?faces,it is found that the crystal orientation was maintained along[11-22]at temperature range from 2050?to 2300?,and the crystal quality was improved with increasing growth temperature;while on the seed of?000-1?face,there exists[10-1n]misoriented crystal grain mixed in[000-1]orientated matrix,when the growth temperature is lower than 2250?.If the growth temperature reaches to 2250?,the grown crystal maintained the direction of the seed without misoriented grains.Third,exploring the growth mechanism in SiC hetero-seed crystal growth:?1?Studies on SiC seed crystal orientation and growth temperature show that a temperature of approximately 1920?on the Si-face with a 4°?8°off-axis angle is the best condition for heterogeneous seed crystal growth of AlN.?2?It is found that a 4°off-axis angle of the Si-face was equivalent to increasing the temperature by approximately 70±10?to achieve a similar AlN growth rate.?3?It is found that the best AlN sublimation energy was approximately 400±35 k J/mol under a growth ambient containing Si and C impurities,which is 36%less than the sublimation energy?630 k J/mol?predicted by the theory under the condition without impurities.?4?The software simulation predicted that the temperature gradient of the axial temperature field can be reduced by moving the crucible down to the vertical center of the inductive coil or by narrowing the diameter of the opening of the insulating felt at the top of the crucible;the temperature gradient of the radial temperature field can be effectively reduced by moving the crucible to top region in the inductive barrel.The predictions were approved by our experimental results.?5?Based on the temperature field modulation rule,an optimal conditions for growing wafer size AlN crystal were found,and a AlN crystal with diameter of 50 mm and thickness of 1.35 mm at its center was obtained,whose?002?rocking curve full width at half maximum was 104 arcsec.