Nanostructure and strain in InGaN/GaN superlattices grown in GaN nanowires

Abstract

The structural properties and the strain state of InGaN/GaN superlattices embedded in GaN nanowires were analyzed as a function of superlattice growth temperature, using complementary transmission electron microscopy techniques supplemented by optical analysis using photoluminescence and spatially resolved microphotoluminescence spectroscopy. A truncated pyramidal shape was observed for the 4 nm thick InGaN inclusions, where their (0001¯) central facet was delimited by six-fold {101¯l} facets towards the m-plane sidewalls of the nanowires. The defect content of the nanowires comprised multiple basal stacking faults localized at the GaN base/superlattice interface, causing the formation of zinc-blende cubic regions, and often single stacking faults at the GaN/InGaN bilayer interfaces. No misfit dislocations or cracks were detected in the heterostructure, implying a fully strained configuration. Geometrical phase analysis showed a rather uniform radial distribution of elastic strain in the (0001¯) facet of the InGaN inclusions. Depending on the superlattice growth temperature, the elastic strain energy is partitioned among the successive InGaN/GaN layers in the case of low-temperature growth, while at higher superlattice growth temperature the in-plane tensile misfit strain of the GaN barriers is accommodated through restrained diffusion of indium from the preceding InGaN layers. The corresponding In contents of the central facet were estimated at 0.42 and 0.25, respectively. However, in the latter case, successful reproduction of the experimental electron microscopy images by image simulations was only feasible, allowing for a much higher occupancy of indium adatoms at lattice sites of the semipolar facets, compared to the invariable 25% assigned to the polar facet. Thus, a high complexity in indium incorporation and strain allocation between the different crystallographic facets of the InGaN inclusions is anticipated and supported by the results of photoluminescence and spatially resolved microphotoluminescence spectroscopy.

Heterostructures of III-Nitride Semiconductors for Optical and Electronic Applications

“…The film also exhibits a peak at 3.41 eV, which corresponds to underlying GaN layer. A second peak at 2.94 eV might correspond to the thin InGaN strain-relieving layer with~6% of In [62,63]. The indium composition of the InGaN layer was calculated using Vegard's law and was found to be 22% in sample E, which is in correlation with the value as estimated by HRXRD.…”

Localization and defects in axial (In,Ga)N/GaN nanowire heterostructures investigated by spatially resolved luminescence spectroscopy

“…In any case, the bottom-up fabrication of NW based QW structures seems to induce larger inhomogeneities in the alloy composition of the insertions than for planar QWs. Along this line, Kehagias et al [11] have shown that for lower growth temperatures of the (In,Ga)N insertions, the inhomogeneity of the alloy is further increased and the width of the (In,Ga)N band for the NW ensemble can amount to 800 meV. A certain degree of localization might well be necessary to avoid the possibility of detrimental carrier recombination at the NW surface in line with what is discussed for dislocations in planar layers [48].…”

“…From previous studies, it is known that the large FWHM of the (In,Ga)N emission in NW ensembles partly results from a variation in the emission energy between individual NWs [4,7,11,17]. In [17], it is shown that the peak energy of the (In,Ga)N band for single NWs can vary over a range of about 400 meV.…”

Growth and electrical transport properties of InGaN/GaN heterostructures grown by PAMBE

“…The emission peak at around 3.41 eV is assigned to the emission from the under layer GaN film. The emission peak at around 2.94 eV is assigned to the recombination in the thin GaN under-layer in which a small amount ($6%) of indium atoms incorporate due to the diffusion [12,13]. From the PL peak position the composition of indium in InGaN alloy was calculated using standard bowing equation, given by…”