Microstructural and Compositional Investigations of Indium Gallium Nitride/Gallium Nitride and Aluminum Gallium Nitride/Gallium Nitride Structures for Light Emitting Diodes

The microstructure of In0.2Ga0.8N/GaN MQWs grown on both (0001) AlN/6H-SiC substrates and on (0001) GaN substrates were systematically investigated with the emphasis on these studies focused on threading dislocations in the substrates and in the epitaxial layers and their relationship with V-defects in the MQWs. Through a variety of techniques, we characterized the propagation of threading dislocations from the substrates into the epitaxial GaN layers. As expected, the threading dislocation density in homoepitaxial GaN layers (4x106 cm-2) was more than two orders of magnitude lower than that in GaN films grown on (0001) AlN/6H-SiC substrates (6x109 cm-2). In addition, site-specific comparisons and high-resolution TEM images confirmed that no new threading dislocations were generated during GaN homoepitaxial growth. Similarly, the V-defect density in In0.2Ga0.8N/GaN MQWs grown on homoepitaxial GaN layers (7x106 cm -2) was more than two orders of magnitude lower than that in the same MQW structures grown on GaN on (0001) AlN/6H-SiC (1x1010 cm-2). Site-specific comparisons before and after growth In 0.1Ga0.9N/GaN MQWs confirmed a one-to-one correspondence between V-defects and threading dislocations. However, site-specific comparisons of In0.2Ga0.8N/GaN MQWs revealed additional V-defects beyond those that could be matched to underlying threading dislocations. These results indicate that the driving force for V-defect formation increases with increasing In composition, which could be associated with strain or the kinetics of In incorporation.;AlxGa1--xN is also an important semiconductor material for optoelectronic applications because, by varying the ratio of Al to Ga, the band gap can be tuned between 3.4 eV (GaN) and 6.2 eV (AN). In addition, high mobility transistors that utilize the formation of two-dimensional electron gas at the interface of AlxGa1--xN/GaN are also widely studied. The crystalline quality and compositional homogeneity of the AlxGa1--xN thin films used in different devices need to be understood and characterized as these can affect device performance. In this research a systematic study of the Al distribution in AlxGa1--xN (0.18 ≤ x ≤ 0.51) films of 30 nm to 50 nm thickness grown on both (0001) AlN/SiC substrates and (0001) AlN/sapphire substrates via MOCVD was conducted. Atom probe tomography reconstructions show consistent Al concentration among different tips fabricated from the same samples, as well as consistently abrupt interfaces between AlxGa 1--xN layers and the underlying GaN layers. Frequency distributions of Al within the APT reconstructions of AlxGa1--xN (0.18 ≤ x ≤ 0.51) films (∼80 nm field of view) indicate a homogeneous Al distribution within the layer. A similar distribution analysis of a much larger area (∼300 nm along [0001] axis) in sideways-oriented tips of Al 0.51Ga0.49N (30 nm)/GaN (1 microm) grown on a (0001) AlN/sapphire substrate suggests some degree of Al inhomogeneity. A possible reason for the observed Al inhomogeneity is the increase in sampling volume for the analysis. Nevertheless, artifacts due to degradation of x-y spatial resolution can also be responsible. Furthermore, both STEM images and SADPs of the samples with the three highest Al concentrations indicate uniform Al distribution without clustering or superlattice ordering. Although some studies based on TEM analyses report superlattice peaks within AlGaN layers, there were significant differences between our growth conditions and those in the reported studies. Also, a small fraction of ordered material would make the observation of inhomogeneity challenging in our AlxGa1--xN films, which are much thinner (30 nm ∼ 50 nm) than the reported films (200 nm ∼ 10 microm). (Abstract shortened by UMI.);Due to the small dimensions of InGaN (3 nm)/GaN (6 nm) MQWs, atom probe tomography (APT) was chosen to study the composition of the layers. Firstly, we optimized the experimental conditions for obtaining accurate, stoichiometric concentration profiles of GaN (and InGaN) films using APT via consistent evaporation conditions. We found that In0.2Ga0.8N/GaN MQWs grown on (0001) GaN contain a homogeneous In composition (10 at%) across six QWs and laterally within the QWs (on a scale of ∼100nm), regardless of the type or presence of an InxGa1--xN buffer layer. The interface roughness between InGaN and GaN was also found to be independent of the underlying InxGa1--xN buffer layers. We attribute the similar roughness values to a smoothening effect during growth of the GaN barrier layers. Our results thus suggest that differences in LED efficiency are not attributable to compositional inhomogeneity or interfacial roughness.