Strain relaxation mechanisms in AlGaN based light emitting diodes emitting in the ultraviolet B spectral range (UVB‐LEDs) grown on different AlN/sapphire templates are analyzed by combining in situ reflectivity and curvature data with transmission electron microscopy. In particular, the impact of dislocation density, surface morphology, and lattice constant of the AlN/sapphire templates is studied. For nonannealed AlN/templates with threading dislocation densities (TDDs) of 4 × 10⁹ and 3 × 10⁹ cm⁻² and different surface morphologies strain relaxation takes place mostly by conventional ways, such as inclination of threading dislocation lines and formation of horizontal dislocation bands. In contrast, a TDD reduction down to 1 × 10⁹ cm⁻² as well as a reduction of the lattice constant of high temperature annealed AlN template leads to drastic changes in the structure of subsequently grown AlGaN layers, e.g., to transformation to helical dislocations and enhanced surface enlargement by formation of macrofacets. For the growth of strongly compressively strained AlGaN layers for UVB‐LEDs the relaxation mechanism is strongly influenced by the absolute values of TDD and the lattice constant of the AlN templates and is less influenced by their surface morphology.

中文翻译：

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AlN模板对UVB-LED结构MOVPE生长期间应变松弛机制的影响

通过将原位反射率和曲率数据与透射电子显微镜相结合，分析了在不同AlN /蓝宝石模板上生长的，在紫外线B光谱范围内发射的AlGaN基发光二极管中的应变松弛机制。特别地，研究了AlN /蓝宝石模板的位错密度，表面形态和晶格常数的影响。对于螺纹位错密度（TDD）为4×10 ⁹和3×10 ⁹ cm ^-2且表面形态不同的非退火AlN /模板，应变松弛主要通过常规方式进行，例如螺纹位错线的倾斜和水平位错的形成乐队。相比之下，TDD减小到1×10 ⁹ cm^-2以及高温退火AlN模板的晶格常数降低会导致随后生长的AlGaN层结构发生剧烈变化，例如，转变为螺旋位错，并通过形成宏观面增强表面扩大。对于用于UVB-LED的高压缩应变AlGaN层的生长，弛豫机理受TDD的绝对值和AlN模板的晶格常数的强烈影响，而受其表面形态的影响则较小。