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.

Direct-gap gain up to 850 cm(-1) at 0.74 eV is measured and modeled in optically pumped Ge-on-Si layers for photoexcited carrier densities of 2.0 × 10(20) cm(-3). The gain spectra are correlated to carrier density via plasma-frequency determinations from reflection spectra. Despite significant gain, optical amplification cannot take place, because the carriers also generate pump-induced absorption of ≈7000 cm(-1). Parallel studies of III-V direct-gap InGaAs layers validate our spectroscopy and modeling. Our self-consistent results contradict current explanations of lasing in Ge-on-Si cavities.

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The synthesis of graphene on cubic silicon carbide on silicon pseudosubstrates draws enormous interest due to the potential integration of the 2D material with the well-established silicon technology and processing. However, the control of transport properties over large scales on this platform, essential for integrated electronics and photonics applications, has lagged behind so far, due to limitations such as 3C-SiC/Si interface instability and nonuniform graphene coverage. We address these issues by obtaining an epitaxial graphene (EG) onto 3C-SiC on a highly resistive silicon substrate using an alloy-mediated, solidsource graphene synthesis. We report the transport properties of EG grown over large areas directly on 3C-SiC(100) and 3C-SiC(111) substrates, and we present the corresponding physical models. We observe that the carrier transport of EG/3C-SiC is dominated by the graphene−substrate interaction rather than the EG grain size, sharing the same conductivity and same inverse power law as EG on 4H-or 6H-SiC(0001) substrates although the grain sizes for the latter are vastly different. In addition, we show that the induced oxidation/silicates at the EG/ 3C-SiC interface generate a p-type charge in this graphene, particularly high for the EG/3C-SiC(001). When silicates are at the interface, the presence of a buffer layer in the EG/3C-SiC(111) system is found to reduce somewhat the charge transfer. This work also indicates that a renewed focus on the understanding and engineering of the EG interfaces could very well enable the long sought-after graphene-based electronics and photonics integrated on silicon.

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.

We report micron-sized Ge crystal arrays grown on deeply patterned Si substrates that yield a surge of the interband photoluminescence intensity by more than 2 orders of magnitude with respect to that typical for epitaxial layers directly grown on planar substrates. This finding is ascribed to the strongly modified internal quantum efficiency induced by controlling the nonradiative recombination at dislocations and to the improved light extraction offered by the array architecture. By spectrally resolving the interband and the dislocation-related luminescence, we address the parasitic activity of extended defects and its impact on the optical properties of the heterosystem. Such results are then exploited along with band gap engineering to design SiGe reflectors and Ge quantum wells that are effective in further amplifying the emission yield.

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.

Defect-free mismatched heterostructures on Si substrates are produced by an innovative strategy. The strain relaxation is engineered to occur elastically rather than plastically by combining suitable substrate patterning and vertical crystal growth with compositional grading. Its validity is proven both experimentally and theoretically for the pivotal case of SiGe/Si(001).