The preparation of bulk AlN single crystals with large-size and high-quality has become an urgent request to meet industrial demands. In this talk, the application potential of high performance and long lifetime UVC-LED devices based on AlN substrates will be summarized firstly. An overview of the general strategies and associated sublimation reactors to grow AlN crystals by PVT method will be given. Particular interests are focused on different AlN growth strategies for large-size AlN crystal growth, and the advantages and disadvantages of these growth strategies will also be addressed in great detail when applied to UVC-LED devices. Perspectives and huge technological challenges to grow large-size and high-quality bulk AlN crystals with reasonable cost for AlGaN-based devices will finally be summarized based on worldwide research efforts and our AlN growth results as well. Finally some high-quality and large-size freestanding AlN crystals/wafers grown by our proprietary sublimation reactor will be presented.