III-nitride semiconductors have received much attention for its optical and electrical applications. One challenge is to achieve the sub-300 nm ultraviolet diodes. High Al-content AlGaN-based technology grown on single crystalline AlN buffer layer is one of the most promising ways. However, due to the poor point defects control, high dislocation density, doping difficulties, and alloy fluctuations of AlGaN, it is hard to control the electron conduction which leads to the low efficiency of this technology. To solve this problem, it is essential to grow high quality AlGaN films. Additionally, N-polar AlN could enhance emission, injection, and collection efficiency with low buffer leak current. To check the quality of AlGaN/AlN, TEM (Transmission Electron Microscopy) is utilized, which provides high resolution atomic images with the information of crystal structures, grain boundaries, dislocations, and point defects. With the x-ray detector, local element concentrations can be calculated from EDX (Energy Dispersive X-ray). In this study, we characterized the structure and quality of various AlGaN and N-polar AlN samples by TEM and other techniques and proposed explanations for the phenomena we observed.