6/12/2023 0 Comments Vector td easy![]() To evaluate the mosaicity and TD density, rocking scans ( ω scans) of symmetric ( 000 ℓ ) ( ℓ = 2, 4, and 6) and asymmetric ( 12 3 ¯ 1 ) reflections as well as an azimuthal scan ( ϕ scan) of ( 12 3 ¯ 1 ) reflection were performed. The edge TDs, however, are usually considered to distribute at grain boundaries separated with an average in-plane correlation length ( L a corresponds to average grainsize), and the edge-TD density ( ρ a ) can be assessed by a modified expression ρ a = α ϕ ∕ ( 2.1 ∣ b a ∣ L a ), where α ϕ is the twist angle. Assuming the screw TDs distributed at random, the density of screw TDs ( ρ c ) can be estimated as ρ c = α ω 2 ∕ ( 4.35 ∣ b c ∣ 2 ), where α ω is the tilt angle. As for the (0001) oriented AlGaN layer, the tilt misorientation is mainly related to the screw ( c type) threading dislocations (TDs) with Burgers vector b c = , while the twist is mainly associated with the edge ( a type) TDs with b a = 1 ∕ 3 . ![]() The mosaicity is a measure of the grain misorientation described in terms of out-of-plane tilt and in-plane twist angles. For the less perfect crystals, such as AlGaN layers grown on c-plane sapphire, the broadening of XRD peaks is dominated by three factors, namely mosaicity, grain size and microstrain. The structural properties of the samples were characterized by high resolution x-ray diffraction (HRXRD) technique using a Philips X’Pert PRO x-ray Diffraction System. The effects of HT-GaN interlayer on the structural properties of AlGaN layer are also discussed. In this work, we demonstrate that the edge-type TD density can be efficiently suppressed by inserting a thin high-temperature (HT) GaN layer between the AlGaN layer and AlN ∕ sapphire template. It is then of interest to reduce the edge TDs in the AlGaN layer. 8 This is also the case for the AlGaN layers on AlN ∕ sapphire templates. 7 Since the nucleation energy of the edge TDs is much smaller than that of the screw TDs, the major TDs existing in GaN-based film grown on c-plane sapphire are, in general, of edge type. One problem, however, appears to be the high number of edge TDs propagating from the underlying AlN layer. 3,6 Due to their much better lattice- and thermal-expansion matching, the AlGaN layers grown on the AlN ∕ sapphire templates are easy to be free of cracks. The recent success in the growth of thick AlN film on c-plane sapphire, the so-called AlN ∕ sapphire template, provides a more encouraging solution to achieve high-crystalline quality Al x Ga 1 − x N ( x > 0.2 ) layers. To overcome this issue, much efforts have been made to accommodate the mismatches, and crack-free AlGaN layers with reduced TD density have been demonstrated by introducing low-temperature AlN interlayers 4 or AlN ∕ AlGaN superlattices. Such a problem is also found in the AlGaN grown on GaN ∕ sapphire template. It is found that the thick Al x Ga 1 − x N ( x > 0.2 ) layers grown on sapphire substrate usually contain cracks and/or high densities of threading dislocations (TDs), even with the use of nucleation layers. ![]() A major obstacle concerning the growth of such AlGaN layers is the large mismatches in the lattice constants and thermal expansion coefficients between the AlGaN and the commonly used substrate of sapphire. 1–3 For the fabrication of these devices, thick Al x Ga 1 − x N epitaxial layers with high Al composition ( x > 0.20 ) and high-crystalline quality are essential. III-nitride optoelectronic devices operating in the ultraviolet (UV) spectral ranges of UV-B (280–315 nm) and UV-C (100–280 nm) are now the focus of intense research interest.
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