The effects of annealing on microstructural changes for aluminium nitride epitaxial grown on sapphire by transmission electron microscopy

Abstract

The performance of semiconductor devices depends strongly upon the microstructure of the materials. Therefore the microstructural control is intrinsically important for fabrication of high performance devices. In this research, the microstructures have been analysed in detail and the mechanisms of microstructural changes in aluminium nitride (AlN) epitaxial have been clarified for the establishment of the growth method. AlN heteroepitaxial layers were made by growing an AlN buffer layer on a (0001) sapphire substrate by the Metal Organic Vapor Phase Epitaxy (MOVPE) growth process. Annealing treatments were added before and after the deposition of an AlN buffer layer. The surface roughness of AlN was observed with an Atomic Force Microscope (AFM) and X-ray Rocking Curve (XRC). The cross section of AlN heteroepitaxial was observed by using Transmission Electron Microscope (TEM) at 200 Kv and High-Angle Annular Dark- Field (HAADF) images were observed with a Scanning Transmission Electron Microscope (STEM) at 300 kV. Thin foil specimens or lamella for the TEM observation were made using a Focused Ion Beam (FIB) mill with accelerating voltage of 15 kV~3 kV for a smooth finishing of lamella. Prior to the deposition of a medium temperature MT-AlN layer, the sapphire substrate was cleaned or preannealed at a high temperature, TAn under the atmosphere of H2. For annealing temperature, TAn less than 1250°C , the crystallinity improved but twisting domains appeared above the temperature. Threading dislocations (TDs) of type c and typea+ c with 108 cm-2 dislocation density was observed. However when the temperature was increased to 1350°C, threading dislocation were reduced. On the other hand, post deposition annealing at a high temperature between 1500ºC and 1700oC for 2 hours under the atmosphere of N2+CO was carried out. Cross sectional TEM revealed that after annealing at 1500oC, cone-shaped domains and threading dislocations remained. The morphology of domains and the changes in TEM image contrast strongly suggest that the domains are inversion domains. TDs of type-a and type- a+c were visible for g =01-10 under the two beam condition. However, after annealing at 1550oC, the cone shaped domains coalesced with each other to leave a single domain boundary running in a zigzag laterally at the center of AlN buffer layer that the upper layer has the Al- polarity while the lower layer has the N-polarity determined by the HAADF analysis. The inversion domain boundary become smooth and flatter with the rising annealing temperature. The surface of MT-AlN buffer was finely rugged before the annealing, but became coarser and smoother with annealing. The changes in the surface morphology indicates the occurrence of grain coalescence. The density of TDs was reduced to roughly 5×108 cm-2 after annealing at 1650oC. Conclusively, this research confirms that pre-deposition and post deposition annealing are an effective treatment to control the microstructure and to reduce the dislocation density for advancement of semiconductor devices.