Anna Marzegalli

Scaling Hetero-Epitaxy from Layers to Three-Dimensional Crystals

Laying It on Thick The growth of one layered material onto a second lies at the heart of many electronic devices. However, if there is a lattice mismatch between the two materials, strains develop in the overgrowth material leading to bowing and cracking. Falub et al. (p. 1330; see the cover) patterned Si substrates into a series of pillars onto which they grew a germanium layer. The germanium initially coated the top of each silicon pillar but then widened as the layer thickened, leading to thick, crack-free germanium films. A space-filling array of self-limited three-dimensional epitaxial crystals averts wafer bowing, layer cracking, and dislocation propagation. Quantum structures made from epitaxial semiconductor layers have revolutionized our understanding of low-dimensional systems and are used for ultrafast transistors, semiconductor lasers, and detectors. Strain induced by different lattice parameters and thermal properties offers additional degrees of freedom for tailoring materials, but often at the expense of dislocation generation, wafer bowing, and cracks. We eliminated these drawbacks by fast, low-temperature epitaxial growth of Ge and SiGe crystals onto micrometer-scale tall pillars etched into Si(001) substrates. Faceted crystals were shown to be strain- and defect-free by x-ray diffraction, electron microscopy, and defect etching. They formed space-filling arrays up to tens of micrometers in height by a mechanism of self-limited lateral growth. The mechanism is explained by reduced surface diffusion and flux shielding by nearest-neighbor crystals.

Unexpected Dominance of Vertical Dislocations in High-Misfit Ge/Si(001) Films and Their Elimination by Deep Substrate Patterning

An innovative strategy in dislocation analysis, based on comparison between continuous and tessellated film, demonstrates that vertical dislocations, extending straight up to the surface, easily dominate in thick Ge layers on Si(001) substrates. The complete elimination of dislocations is achieved by growing self-aligned and self-limited Ge microcrystals with fully faceted growth fronts, as demonstrated by AFM extensive etch-pit counts.

Perfect crystals grown from imperfect interfaces

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.

Photodetection in Hybrid Single-Layer Graphene/Fully Coherent Germanium Island Nanostructures Selectively Grown on Silicon Nanotip Patterns

Dislocation networks are one of the most principle sources deteriorating the performances of devices based on lattice-mismatched heteroepitaxial systems. We demonstrate here a technique enabling fully coherent germanium (Ge) islands selectively grown on nanotip-patterned Si(001) substrates. The silicon (Si)-tip-patterned substrate, fabricated by complementary metal oxide semiconductor compatible nanotechnology, features ∼50-nm-wide Si areas emerging from a SiO2 matrix and arranged in an ordered lattice. Molecular beam epitaxy growths result in Ge nanoislands with high selectivity and having homogeneous shape and size. The ∼850 °C growth temperature required for ensuring selective growth has been shown to lead to the formation of Ge islands of high crystalline quality without extensive Si intermixing (with 91 atom % Ge). Nanotip-patterned wafers result in geometric, kinetic-diffusion-barrier intermixing hindrance, confining the major intermixing to the pedestal region of Ge islands, where kinetic diffusion barriers are, however, high. Theoretical calculations suggest that the thin Si/Ge layer at the interface plays, nevertheless, a significant role in realizing our fully coherent Ge nanoislands free from extended defects especially dislocations. Single-layer graphene/Ge/Si-tip Schottky junctions were fabricated, and thanks to the absence of extended defects in Ge islands, they demonstrate high-performance photodetection characteristics with responsivity of ∼45 mA W(-1) and an Ion/Ioff ratio of ∼10(3).

Formation of strain-induced Si-rich and Ge-rich nanowires at misfit dislocations in SiGe: A model supported by photoluminescence data

Molecular dynamics simulations with the Tersoff potential of the strain distribution around 60° misfit dislocation in a heteroepitaxial SiGe film confirm that highly compressed and expanded, cylindrical nanometer-sized regions appear on opposite sides of the (111) glide plane. Such a configuration is suggested to generate opposite chemical potential gradients for Si and Ge diffusion and, as verified by a Monte Carlo simulation, in the formation of Si-rich and Ge-rich nanowires along the dislocation core. This model is supported by photoluminescence measurements as a function of annealing temperature and time.

Onset of vertical threading dislocations in Si1-xGex/Si (001) at a critical Ge concentration

We show that the Ge concentration in Si1−xGex alloys grown under strong out-of-equilibrium conditions determines the character of the population of threading dislocations (TDs). Above a critical value x ∼ 0.25 vertical TDs dominate over the common slanted ones. This is demonstrated by exploiting a statistically relevant analysis of TD orientation in micrometer-sized Si1−xGex crystals, deposited on deeply patterned Si(001) substrates. Experiments involving an abrupt change of composition in the middle of the crystals clarify the role of misfit-strain versus chemical composition in favoring the vertical orientation of TDs. A scheme invoking vacancy-mediated climb mechanism is proposed to rationalize the observed behavior.

Vertical and lateral ordering of Ge islands grown on Si(001): theory and experiments

A set of recent results concerning lateral and vertical ordering of Ge islands grown on Si(001) is reviewed. Experimental data generated by chemical vapour deposition and analysed by atomic force microscopy and photoelectron spectroscopy are compared with computer simulations and modelling based on atomistic approaches and continuum theory. In particular, we show that it is possible to probe experimentally the detailed strain field generated by buried Ge islands at the surface of the Si capping layer. The observed arrangement of small Ge islands grown over the capping layer is demonstrated to be very close to the one predicted by a simple model where the local chemical potential is inferred from the strain field at the atomic scale, as given by Tersoff-potential molecular dynamics simulations. Moreover, we review recent experimental evidence for lateral ordering, triggered by partial Si capping, in the first layer of Ge islands on Si(001). Theoretical support is given by showing that when two islands lie in close proximity the elastic field is likely to generate a flow of atoms leading to an effective gliding motion along opposite directions of both islands, eventually stopped by the presence of further neighbouring islands.

Imaging Structure and Composition Homogeneity of 300 mm SiGe Virtual Substrates for Advanced CMOS Applications by Scanning X-ray Diffraction Microscopy

Advanced semiconductor heterostructures are at the very heart of many modern technologies, including aggressively scaled complementary metal oxide semiconductor transistors for high performance computing and laser diodes for low power solid state lighting applications. The control of structural and compositional homogeneity of these semiconductor heterostructures is the key to success to further develop these state-of-the-art technologies. In this article, we report on the lateral distribution of tilt, composition, and strain across step-graded SiGe strain relaxed buffer layers on 300 mm Si(001) wafers treated with and without chemical-mechanical polishing. By using the advanced synchrotron based scanning X-ray diffraction microscopy technique K-Map together with micro-Raman spectroscopy and Atomic Force Microscopy, we are able to establish a partial correlation between real space morphology and structural properties of the sample resolved at the micrometer scale. In particular, we demonstrate that the lattice plane bending of the commonly observed cross-hatch pattern is caused by dislocations. Our results show a strong local correlation between the strain field and composition distribution, indicating that the adatom surface diffusion during growth is driven by strain field fluctuations induced by the underlying dislocation network. Finally, it is revealed that a superficial chemical-mechanical polishing of cross-hatched surfaces does not lead to any significant change of tilt, composition, and strain variation compared to that of as-grown samples.

InAs/GaAs Sharply Defined Axial Heterostructures in Self-Assisted Nanowires

We present the fabrication of axial InAs/GaAs nanowire heterostructures on silicon with atomically sharp interfaces by molecular beam epitaxy. Our method exploits the crystallization at low temperature, by As supply, of In droplets deposited on the top of GaAs NWs grown by the self-assisted (self-catalyzed) mode. Extensive characterization based on transmission electron microscopy sets an upper limit for the InAs/GaAs interface thickness within few bilayers (≤1.5 nm). A detailed study of elastic/plastic strain relaxation at the interface is also presented, highlighting the role of nanowire lateral free surfaces.

Stability of shuffle and glide dislocation segments with increasing misfit in Ge∕Si1−xGex(001) epitaxial layers

Using molecular dynamics simulations, based on Tersoff potentials, we show that at typical experimental temperatures high compressive strain regimes suppress the formation of partial glide dislocations, while enhancing the gliding of the shuffle segments. Despite being qualitative in nature, these results suggest that strain relaxation in thin virtual substrates at high misfit may occur with a different modality than in thick graded layers, as indicated by preliminary experimental results by low-energy plasma enhanced chemical vapor deposition.