Perfect crystals grown from imperfect interfaces

Abstract

The fabrication of advanced devices increasingly requires materials with different properties to be combined in the form of monolithic heterostructures. In practice this means growing epitaxial semiconductor layers on substrates often greatly differing in lattice parameters and thermal expansion coefficients. With increasing layer thickness the relaxation of misfit and thermal strains may cause dislocations, substrate bowing and even layer cracking. Minimizing these drawbacks is therefore essential for heterostructures based on thick layers to be of any use for device fabrication. Here we prove by scanning X-ray nanodiffraction that mismatched Ge crystals epitaxially grown on deeply patterned Si substrates evolve into perfect structures away from the heavily dislocated interface. We show that relaxing thermal and misfit strains result just in lattice bending and tiny crystal tilts. We may thus expect a new concept in which continuous layers are replaced by quasi-continuous crystal arrays to lead to dramatically improved physical properties.

Engineered Coalescence by Annealing 3D Ge Microstructures into High-Quality Suspended Layers on Si

“…To this end, is necessary to form the coalesced film from a set of Ge pillars featuring fully pyramidal top faceting, as found for narrow Si pillars and higher growth temperature shown in Figure 2. In refs [7], [9] and [10] we indeed demonstrated that no threading-dislocations are present in the upper part of the crystals, where the merging induced by annealing begins. Any additional dislocations nucleating during coalescence would therefore have to reside in a cylindrical expanding bridge, which is still surrounded by free surfaces.…”

“…Here we see that dislocations are indeed present close to the bottom of the Ge crystals, whereas only twin defects are seen in a columnar distribution in the bridge region. Indeed, small angle grain boundaries must be expected to originate from the small random Ge crystal tilts, analyzed in-depth in ref [9]. These results show the superior potential of the present technique with respect to other coalescence methods widely exploited in group IV or III-V semiconductors, such as Epitaxial Lateral…”

Strain accommodation through facet matching in La1.85Sr0.15CuO4/Nd1.85Ce0.15CuO4 ramp-edge junctions

“…The former has become a powerful tool for selective analysis thanks to the development of high brilliance synchrotron sources that employ micro and nano-focused beams. 8 This allows nXRD to combine sub-micron lateral resolution with high k-space resolution [9][10][11][12][13][14][15][16][17] for the nondestructive study of the crystal structures of buried layers. HAADF-STEM complements the nXRD measurements by providing a local cross-section of Z contrast with atomic resolution.…”

Strain relaxation of GaAs/Ge crystals on patterned Si substrates

“…This broad diffuse scattering around the GaAs(004) and Ge(004) reflections may be related to varying lattice tilts across the diameter of the crystals as a result of elastic thermal strain release. 26 Figure 2(c) compares the HRXRD 2h/x scans measured around the (004) reflection for a GaAs/Ge layer on a planar Si(001) substrate and for GaAs/Ge crystals on Si pillars with widths ranging from 2 to 40 lm. Two intense diffraction peaks corresponding to GaAs and Ge were found for both kinds of substrates.…”