Positively charged V N do not trap positrons, but V Ga are usually negatively charged and can therefore be observed using PAS. In GaN, the point defects most probable to appear are N vacancies (V N) in the case of p-type and Ga vacancies (V Ga) in n-type GaN. PAS with monoenergetic slow positrons has been used to study semiconductors, since the early 1980s. Positrons, due to their positive charge, are strongly repulsed from the positive ion cores and trapped inside negatively charged or neutral defects. The advantage of this method is its ability to determine the type of defects.
Positron annihilation spectroscopy (PAS) has been established as an efficient characterization method for investigating defects at atomic level, such as vacancies, interstitials, and dislocations. Since a common goal of researchers is to increase the quality of GaN films and to extend their usage in a larger variety of applications, it is mandatory to clearly understand the types of defects present in GaN materials grown in different conditions, as well as their formation mechanisms. The lattice mismatches between the wurtzite structure of GaN and the zinc-blende structure of α-Al 2O 3 (>14%, a Al 2 O 3 = 4.76 Å, a GaN = 3.18 Å), as well as the large difference in their thermal expansion coefficients (~25%), usually determine the high density of defects and residual strain in the resulting GaN films, which affects the quality of the final devices. Despite the two crystal structures being similar and the relatively low cost of the substrate, the usage of the GaN/Al 2O 3 still has some limitations. Due to the lack of commercial lattice-matched substrate, single-crystal sapphire can act as a substrate for epitaxial GaN film growth, with the best crystalline quality being obtained for the (0 0 0 1) oriented α-Al 2O 3. Substrates are of great importance, because they influence to a large extent the crystal orientation, surface morphology polarity, chemical composition, and elastic strains of the grown thin films.
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Since the first trials of obtaining crystalline GaN films failed because of the high density of nitrogen vacancies, a series of developments have been made, with much work being dedicated to the fabrication of large-area heteroepitaxial GaN films on a different material platform. The defect densities ρ d e = 6.13 × 10 10 cm −2, ρ d s = 1.36 × 10 10 cm −2, along with the defect correlation lengths L e = 155 nm and L s = 229 nm found in the 289 nm layer of GaN, account for the effective positron diffusion length L eff~60 nm. Within the epitaxial layers, defined by a relationship, regarding the GaN film, a strong correlation between defect densities, defect correlation lengths, and positron diffusion length was assessed. DBS studies were used to determine the effective positron diffusion length of the GaN film. The structure of the GaN film, AlN buffer, Al 2O 3 substrate and their growth relationships were determined through HR-TEM. The results were mathematically modeled to extract defect densities and defect correlation lengths in the GaN film. The heterostructure was evaluated by means of high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), and Doppler-Broadening Spectroscopy (DBS). This study assesses the characteristics (edge and screw dislocation density) of a commercially available GaN/AlN/Al 2O 3 wafer.
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