Local Strain Relaxation by A-type Dislocation Clusters in InxGa1-xN/GaN Film with Indium Compositions

A photovoltaic thermal hybrid solar cell is being developed for an energy conversion efficiency higher than the Shockley-Queisser limit, consisting of two junctions with different bandgaps operating on top of a thermal collector at 450 C under solar radiation focused by a concentrator. InxGa1-xN thin films are used due to their wide bandgap range and their stability at high temperatures. The target compositions are x ~ 0.60 and 0.15. Good quality InxGa1-xN layers with x ~0.60 has been achieved by molecular beam epitaxy.[1] This presentation deals with the microstructure of the lower indium content films. InGaN with 7% and 12% indium have been grown by metal organic vapor phase epitaxy. The early stages of misfit strain relaxation in this composition range appear as dislocation clusters that look like baskets.(Fig. 1) The nature of the basket defects has been studied under different diffraction contrast conditions. The threading dislocation surrounding the baskets have Burgers vectors lying on the basal plane, and therefore are not visible under g = 0002.(Fig. 2) The exact Burgers vectors are determined by large-angle convergent-beam electron diffraction (LACBED) in plan-view. The sample was tilted close to the [10-14] zone. The Bragg line intersecting a dislocation splits into several sections depending on the Burgers vector of the dislocation and on the nature of the Bragg line. Three LACBED images are recorded for each dislocation to completely determine the three independent components of the Burgers vector using Cherns and Preston rules.[2] The trace of the dislocation and the Burgers vector of each dislocation are labeled in Fig 3. These a-type dislocations appear in pairs with opposite Burgers vectors and are associated with a missing plane in the basket, contributing to the relaxation of the compressive misfit strain. The remaining strain in the basket can be calculated by counting the number of dislocations around a basket and taking the size of the basket into account: