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

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.

Imaging of three-dimensional (Si,Ge) nanostructures by off-axis electron holography.

Quantitative phase mapping in transmission electron microscopy is applied to image the three-dimensional (3D) morphology of (Si,Ge) islands grown on Si substrates. The phase shift of the transmitted electrons induced by the crystal inner potential was recorded by using off-axis electron holography. The analysis of the experimental data requires the knowledge of the mean inner potential (MIP) of the (Si,Ge) solid solution. The MIP was calculated using different models of isolated or bonded atoms, which are based on the interpolation of first principle data. The results are compared with structure modeling and related MIP calculations applying classical molecular dynamics (MD) simulations. For MD simulations bond order potentials were applied, which can take into consideration both electronic effects and elastic relaxations. The calculated mean inner potential is used to transform the phase shifts into thickness mapping for the reconstruction of the 3D island morphology. Both, phase shift due to dynamical electron diffraction and structural relaxation influence the resulting 3D reconstruction.

Quantitative high resolution transmission electron microscopy of nanostructured semiconductors.

Peak-finding procedures and the geometric phase method of quantitative high resolution electron microscopy (qHRTEM) were applied to determine the local strain and the chemical composition of nanostructured semiconductor materials. The growth of the structures investigated was induced by minimization of strain energy. The analysis of strain distribution is necessary for the understanding of the self-organized formation of nanostructures. The possibilities and limitations of the methods are discussed in detail by analysing HRTEM images of (Si,Ge) islands and of a double layer of stacked quantum dots of (In,Ga)As and Ga(Sb,As).