Growth Of High Al Fraction AlGaN And GaN Semiconductor Materials

The technology of GaN-based LED is the key to semiconductor lighting, GaN-based deep UV LED not only has high conversion efficiency to white light, but also has wide use in medical, cleaning, and other important applications. However, obtaining high-quality materials and high doping levels are difficult for AlGaN films with high Al fraction, and limit the development of devices.In the dissertation, the GaN-based UV LED is studied, the major work is focus on the growth of AlN, AlGaN with high Al fraction, doping of n-type and p-type AlGaN films and fabrication of UV LED devices. The major achievements are listed.1. The best thickness of LT-AlN buffer layer is obtained. It is found that too thick or too thin buffer layer is damaged to the growth of nitride semiconductor materials.The method of pulse MOCVD is used in growth high-quality AlN films. The FWHM of rocking curve of AlN (002) plane is only43arcsec, and that of(102) plane is only228arcsec, the materials quality has reached international advanced level.Through the growth of AlGaN films with different Al fraction based on AlN templates, the relation between strain and Al fraction is found. We also found that when the tensile strain and compress strain is in balance, the materials have high qualities.2. The quality of AlGaN films is improving by using AlGaN/AIN superlattices. The thickness of AlGaN/AlN SLs has important effect on the quality of AlGaN films. The SLs structure can not only stop the extending of dislocations, but also can control the strain of the films. The best thickness of SLs is7nm. The high-quality AlGaN film is obtained, which have259arcsec FWHM of (002) plane and885arcsec FWHM of (102) plane, the RMS of AlGaN film is only0.185nm.The relation between Al fraction of AlGaN and the ration of TMA/(TMA+TEG) is found in the work. We also find that the RMS of AlGaN films increase with increasing the Al fraction. The high-quality AlGaN films with different Al fraction are obtained, all the films have smooth surfaces.3. The n-type and p-type AlGaN films are successfully doping. The effect of Al fraction on the n-type doping and the change of n-type carriers concentration with SiH4flow rate are studied. The conditions for accurately controlling of the n-type doped level are obtained. The high level doping of p-type AlGaN is successfully achieved. Through optimization of the growth temperature, Mg flow rate and annealing temperature, the p-type AlGaN film with resistivity of0.71Q·cm is obtained, which is the optimal results for AlGaN films with0.2Al fraction.Using AlGaN/GaN SLs structure, we achieve high concentrations of p-type doping. Through optimization of SL thickness, we find10nm is the best thickness for p-type doping of SL structure. The resistance of p-type material is only0.034Q·cm, and the p-type carrier concentration is as high as1.26×1019cm-3, The doping level of SLs structure is high than the usual structure.4. AlxGa1-xN/AlYGa1-YN MQWs with different lighting wavelengths are successfully achieved. The effect of thickness of MQW and Si-doping of barrier on the luminescence properties of MQWs is studied.High quality UV LED with wavelength less than300nm is successfully grown. UV LED chips with transverse structure are successful fabricated. The testing results show that the output power of UV LED is a few milliwatts.5. Through optimization of growth conditions, the quality of GaN materials is effectively improved, the FWHM of (002) plane decreases from800arcsec to378arcsec, the FWHM of (102) decreases from1508arcsec to597arcsec.High-quality GaN materials are grown on high-quality AlN template, the surface of GaN is smooth. FWHM of GaN (102) plane is reduced to348arcsec, and that of (102) plane is reduced to70arcsec, which is the best results as far as we know.6. AlInN/GaN heterojunction materials are successfully grown on sapphire substrate. Pulse MOCVD method improves the AlInN film growth quality effectively, By optimization of the TMA flow rate and thickness of AlN, the mobility and2DEG density of AlInN/GaN heteroj unction are up to1033cm2/vs and1.96E+13/cm2respectively, which is close to the international research level.7. The quality of a-GaN grown on r-sapphire substrate is improved by using several superlattice. The study shows that the superlattice can eliminate the triangular defects. High-quality of a-GaN material is achieved, the value of FWHM is only695arcsec, which is in the international advanced level.In summary, this dissertation succeed in solving some key technical issues for the growth of deep UV LED, the issues include growth of high-quality AlN, AlGaN, doping of n-type and p-type AlGaN materials, growth of MQWs with different lighting wavelength and deep UV LED, fabrication of UV LED devices. Meanwhile, high quality of GaN materials, AlInN/GaN heteroj unction and a-GaN have been successfully obtained.