The invention discloses a method of preparing a warp-free group-III
nitride composite substrate and a substrate placing device. A substrate with both sides polished is erected on the substrate placing device and placed in a
reaction chamber, a group-III
nitride film or
microstructure is grown on the both sides at the same time in an epitaxial manner to form a buffer layer, thick-film group-III
nitride is grown on the both sides at the same time, and the epitaxial thickness of one side of the AL surface of
sapphire is slightly greater than the epitaxial
layer thickness of one side of the O surface. The substrate placing device is a multi-piece
graphite frame, and comprises a base, a hole, rollers, and slots. The substrate can be rotated, and the uniformity in thickness of the grown films is ensured. Warping is inhibited, the quality of
crystal is improved, and the obtained
composite substrate can be used as a group-III nitride quasi-homogeneous epitaxial substrate to prepare related optoelectronic devices. According to the invention, the space of the
reaction chamber is fully utilized, the production cost is reduced, the process is simple and easy to control, and different substrates can be selected and a variety of equipment can be used to grow a variety of thick-film group-III nitride substrates.