Study On Thermal Annealing And Wet Etching Of GaN

Owing to the advantages of nitride materials, such as wide band-gap,high breakdown voltage,high electron saturated velocity,high resistant to radiation and good chemical stability, AlGaN/GaN high electron mobility transistors (HEMTs) are emerging as promising candidates for next-generation high-power and microwave power amplifiers. Despite the great progress that have been made in high-power and microwave characteristics of AlGaN/GaN HEMT, GaN-based HEMT devices also have many problems. The influence of thermal annealing on the properties of AlGaN/GaN heterostructures when it is used for high-temperature operation is one of the most important problems, and the research on the properties of etch-pits of GaN is also a hotspot. In this paper, in terms of the research of thermal annealing on GaN, the influence of the process of thermal annealing on the GaN based HEMT devices is investigated. In terms of the research on the properties of etch pits of GaN, the relationship between the etch-pits and threading dislocations is analyzed when etching is performed on the n-type and p-type GaN epitaxial layers through molten KOH, a new model is proposed based on the analysis of AFM surface morphology images and current distribution images.In this paper, at first, the influence of thermal annealing on the properties of AlGaN/GaN heterostructures is studied. For comparison, unintentionally doped GaN and Si-doped GaN are thermally annealled under the same condition, the influence of thermal annealing on the three kinds of material is analyzed. It is thought that strain relaxation of GaN and AlGaN are happened in the AlGaN/GaN heterostructures, lattice constant of AlGaN is increased, polarization of AlGaN barrier layer is decreased,decreasing the 2DEG density;Also the mobility of 2DEG is decreased, it may be that thermal annealing introduces the new defects in the AlGaN, and the edge dislocation density is increased or carrier compensation is happened in GaN, all of the variations could influence the performance of GaN-based HEMT devices.Secondly, the properties of etch pits of GaN is studied. N-type and p-type GaN are etched by the molten KOH. It is found that there are three kinds of etch-pit (?????and??), which are related to pure edge dislocation,pure screw dislocation and mixed dislocation, respectively. Also three dimensional images of the three kinds of etch-pit are shown in this paper.Finally, GaN epilayers etched by molten KOH have been investigated by conductive atomic force microscopy (C-AFM), it is found that conductive ability of different kinds of GaN are different. Meanwhile, it is found that these three types of etch pits reveal three different kinds of current distribution and the conductive ability of the different areas in the same kind of etch-pit are different. Furthermore, a new model is proposed to explain the different current distributions .Off-axis sidewalls are more electrically active than the (0001) axis on c-plane surface of GaN. For the etched samples, the c-plane must be Ga-plane due to the chemical stabilization of Ga face. And off-axis sidewalls are usually terminated with N atoms. A smaller barrier height for Schottky contacts on N-faces as compared to Ga-faces in GaN. By comparison with the planar surface, the stronger electron affinity of the N atoms leads to the decrease in Schottky barrier at the off-axis sidewalls. This results in an increased current at the off-axis sidewalls, which was proven by the C-AFM measurements.