Structure of short period In(Ga)N/GaN superlattices comprising ultra‐thin quantum wells

Abstract

InGaN quantum wells (QWs) grown along the polar c‐axis are currently the principal structural elements of III‐nitride optoelectronic device active regions. Further advancement of their operational wavelength range with high internal quantum efficiency in the green part of the spectrum may be facilitated through the use of short period In(Ga)N/GaN superlattices comprising QWs with a minimal number of monolayers (MLs). At the same time, ultra‐thin InN/GaN QWs have recently generated considerable interest, due to the theoretical prediction that they may exhibit topological insulator properties [1], thus opening up new prospects for applications in quantum computing and spintronics. In recent work, two‐dimensional electron gas properties were demonstrated in ML‐thick, nominally InN QWs, as well as a temperature‐independent behavior in the diagonal resistance, indicating the topological nature of the 2DES [2]. We have studied a series of In(Ga)N/GaN short period superlattice (SPS) heterostructures deposited at low growth temperatures. The examined samples comprised SPS with nominally 1, 2, and 4 ML QW thicknesses grown by molecular beam epitaxy (MBE) on (0001) GaN/sapphire MOVPE templates. High resolution transmission electron microscopy (HRTEM), and high resolution high‐angle annular dark field imaging in scanning TEM mode (HRSTEM) have been employed in order to ascertain the compositional homogeneity due to issues such as the desorption, clustering, and diffusion of indium into the GaN barriers. Such issues may be aggravated by the large elastic strain associated with the pseudomorphic growth of the ultrathin QWs. On the other hand, it is also possible that the stress due to the misfit may in fact stabilize the growth of InN at unusually high temperatures. Fig. 1 illustrates HRTEM and HRSTEM images of a 1 ML InN / 10 ML GaN SPS and of a 4 ML‐thick QW. Strain and compositional mappings were implemented by using peak finding with the peak‐pairs method, and geometrical phase analysis. Z‐contrast image calibration was performed using reference samples and image simulations. The results were compared with energetic calculations of pertinent supercells using both molecular dynamics with a modified Tersoff interatomic potential, and ab initio density functional theory. In both cases, deviation from the biaxial stress state of InN was identified for these QWs in agreement with the experimental observations.