Epitaxial Growth Of High Quality AlN Films On Si Substrates By MOCVD

The third generation of semiconductor materials is an important part of China's 13 th Five-year plan.As the third generation of semiconductor material,AlN is a group ?-nitride semiconductor materials with excellent properties,like wide band gap,high surface acoustic wave velocity,and good thermal conductivity,and has become the first choice in lots of fields such as ultraviolet(UV)and deep ultraviolet electronic devices,surface acoustic wave devices and integrated circuit insulation.By mow,AlN films are usually grown on sapphire and SiC substrates,however,the sapphire is insulating with very low thermal,and the SiC is too expensive.To circumvent these issues,Si substrates seem to be a very promising material because the Si substrates have the advantages of mature preparation,large-size,low manufacturing cost and excellent thermal conductivity.But,it is not easy to epitaxially grow high quality AlN on Si by metal organic chemical vapor deposition(MOCVD).Apart from the low mobility of Al adatom on the growth surface and the parasitic reactions between TMAl and NH3,another main difficulties are the large lattice mismatch and thermal expansion mismatch between AlN and Si.Facing the problems above,a systematic research on the growth techniques and the design of AlN epitaxial layer structures for high quality AlN epitaxial films on Si substrates by MOCVD were carried out,the main achievements are summarized as follows:(1)The key process conditions for the growth of AlN by MOCVD were optimized,and the growth characteristics of AlN epitaxial films on Si substrates were obtained.Firstly,the effect of Al pre-deposition on the growth of AlN on Si substrates was studied and the temperature of the Al pre-deposition was optimized.And then the influence of the growth temperature,V/? ratio,and the atmosphere in the reaction chamber on the crystal quality,surface morphology,growth rate and growth mode of AlN epitaxial films were discussed in detail.(2)A two-step growth of AlN epitaxial films on Si substrates was proposed and its mechanism was studied.The first AlN layer was grown at high V/? ratio,which releases the mismatch stress at first and provides more nucleation centers for the subsequent AlN layers.Afterwards,the decrease in V/? ratio in the second AlN layer enhances the surface diffusion length of the Al adatoms,resulting in the healing of AlN films.At the same time,by adjusting the thickness of the first AlN layer and the growth conditions of the second AlN layer,the quality of the AlN epitaxial films was improved effectively.The full-width at half-maximum(FWHM)of 0.47° for AlN(0002)rocking curves,the root-mean-square(RMS)surface roughness of 0.78 nm,and the warpage of the AlN films is 6.4 nm,which is lower than that of the single-step AlN epitaxial films.(3)The quality of the AlN epitaxial films was enhanced via the low temperature AlN interlayer and the optimization of temperature and thickness of the low temperature AlN interlayer.The results show that the optimized low temperature AlN interlayer could further reduce the dislocation density in the AlN epitaxial films,and finally high quality AlN epitaxial films with FWHM of 0.41° for AlN(0002)rocking curves,and the RMS surface roughness of 0.52 nm were obtained.To sum up,this thesis focuses on the influence of different growth conditions on the AlN epitaxial films,and the reasonable AlN layer structure is designed,which provide guidance and reference for obtaining high quality AlN epitaxial film on Si substrates.