Semiconductor light emitters compatible with standard Si integration technology (SIT) are of particular interest for overcoming limitations in the operating speed of microelectronic devices 1-3. Light sources based on group-IV elements would be SIT compatible but suffer from the poor optoelectronic properties of bulk Si and Ge. Here, we demonstrate that epitaxially grown Ge quantum dots (QDs) in a fully coherent Si matrix show extraordinary optical properties if partially amorphised by Ge-ion bombardment (GIB). The GIB-QDs exhibit a quasi-direct-band gap and show, in contrast to conventional SiGe nanostructures, almost no thermal quenching of the photoluminescence (PL) up to room-temperature (RT). Microdisk resonators with embedded GIB-QDs exhibit threshold-behaviour and super-linear increase of the integrated PL-intensity (IPL) with increasing excitation power Pexc which indicates light amplification by stimulated emission in a fully SIT-compatible group-IV nano-system.

We report on the fabrication and characterization of silicon-on-insulator (SOI) photonic crystal slabs (PCS) with commensurately embedded germanium quantum dot (QD) emitters for near-infrared light emission. Substrate pre-patterning defines preferential nucleation sites for the self-assembly of Ge QDs during epitaxial growth. Aligned two-dimensional photonic crystal slabs are then etched into the SOI layer. QD ordering enhances the photoluminescence output as compared to PCSs with randomly embedded QDs. Rigorously coupled wave analysis shows that coupling of the QD emitters to leaky modes of the PCS can be tuned via their location within the unit cell of the PCS.

During molecular beam epitaxial growth of Ge islands on Si(001) substrates containing pit-patterned areas, an efficient Ge surface diffusion from unpatterned to pit-patterned substrate regions results in position-dependent island growth rates. We exploit this effect to map out the evolution of strictly ordered Ge islands from prepyramids via pyramids and transition domes into domes by atomic force microscopy. A one-dimensional diffusion model is established which allows a quantitative determination of the Ge surface diffusion constant and the rates at which Ge atoms are incorporated into the growing islands. We find evidence that during the shape transitions from nonfaceted islands to {105}-faceted pyramids and from pyramids to domes, Ge is incorporated at higher rates than during the growth of pure pyramids or domes. From these data it follows that a Ge atom passes on average several thousand pits before it is incorporated into one. Within our model we show that secondary-island nucleation in the substrate regions between the pits is a consequence of a mismatch between Ge incorporation and deposition rates, which results in a growing wetting layer between the ordered island sites, the thickness of which eventually exceeds the critical one for spontaneous island nucleation. We experimentally demonstrate that by lowering the Ge deposition rate, perfectly ordered islands with large pit periods of around 1 mu m can be grown, in agreement with the model predictions.

The pumping intensity (I) dependence of the photoluminescence (PL) spectra of perfectly laterally two-dimensionally ordered SiGe quantum dots on Si(001) substrates was studied. The PL results from recombinations of holes localized in the SiGe quantum dots and electrons localized due to the strain field in the surrounding Si matrix. The analysis of the spectra revealed several distinct bands, attributed to phonon-assisted recombination and no-phonon recombination of the excitonic ground state and of the excited excitonic states, which all exhibit a linear I dependence of the PL intensity. At approximately I >= 3 W cm(-2), additional bands with a nearly quadratic I dependence appear in the PL spectra, resulting from biexcitonic transitions. These emerging PL contributions shift the composite no-phonon PL band of the SiGe quantum dots to higher energies. The experimentally obtained energies of the no-phonon transitions are in good agreement with the exciton and biexciton energies calculated using the envelope function approximation and the configuration interaction method.

For the prototypical SiGe/Si(001) Stranski-Krastanow (SK) growth system, the influence of intermixing caused by the deposition of a Si cap layer at temperatures Tcap between 300°C and 700°C is studied both for the SiGe wetting layer (WL) and the SiGe islands. Systematic growth experiments were carried out with an ultrahigh resolution of down to 0.005 monolayers (ML) of deposited Ge. The properties of the samples were investigated via photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy. We studied in detail the influence of Tcap in the three main coverage regions of SiGe SK growth, which are (i) the WL build-up regime, (ii) the island nucleation regime, where most of the Ge is supplied via material transfer from the WL, and (iii) the saturation regime, where the WL thickness remains initially stable. At Tcap = 300°C, we found that both the WL and the island are essentially preserved in composition and shape, whereas at 500°C the WL becomes heavily alloyed during capping, and at 700°C the islands also become alloyed. At Tcap = 500°C we found enhanced WL intermixing in the presence of dome-shaped islands, whereas at Tcap = 700°C the WL properties become dominated by the dissolution of pyramid-shaped islands upon capping. At Ge coverages above ≈6 ML, we found an unexpected thickening of the WL, almost independently of Tcap. This finding suggests that the density and the volume of the dome-shaped islands have an upper limit, beyond which excess Ge from the external source again becomes incorporated into the WL. Finally, we compared PL spectra with AFM-based evaluations of the integral island volumes in order to determine in a straightforward manner the average composition of the SiGe islands.

We use UV nanoimprint lithography for the pit-patterning of silicon substrates. Ordered silicon-germanium islands are grown inside these pits by molecular-beam epitaxy on arrays of 3×3 mm2 and characterized by atomic force microscopy (AFM) and photoluminescence (PL) measurements. AFM-based statistics reveals an extremely uniform size distribution of the islands in the patterned areas. These results are confirmed by very narrow and uniform PL peaks recorded at various positions across the patterned arrays.

A detailed study of the so far unexplored Ge island nucleation on Si (001) substrates patterned with {111} faceted pyramidal pits is reported. The pits are defined by an anisotropic wet-chemical etch through a SiNx hard mask. Due to the self-limiting of the wet etch, an extremely uniform pit pattern is achieved. On these substrates, Ge layers were grown by solid source molecular beam epitaxy at various growth temperatures T-Ge. For T-Ge = 550 degrees C, Ge fills the pits in the form of inverted {111} pyramids with a rounded apex and a (001)-oriented top surface. These islands have aspect ratios much larger than upright pyramids and domes that are usually obtained on substrates with cylindrical pits fabricated by lithographic techniques and reactive ion etching. Based on the experimentally determined shape, three-dimensional (3D) energy level calculations in an envelop function approach have been performed for the inverted pyramids. They show that, due to the orientation of the pyramid square base along < 110 > directions, the overlap between the hole ground states and electron states confined to the Si conduction band valleys perpendicular to the growth direction is much larger than for upright SiGe domes and {105} pyramids. For elevated growth temperatures around 700 degrees C, already a Ge coverage of five monolayers induces a strong Si transfer into the {111} pits, effectively converting them towards {1 1 10} faceted ones.

For randomly nucleated SiGe/Si(001) islands, a significantly stronger blue-shift of the PL spectra as a function of the excitation intensity is observed when compared to islands grown on patterned substrates side by side within the same run in a solid source molecular beam epitaxy chamber. We ascribe this different PL behavior to the much larger inhomogeneity of the Ge distribution in islands on planar substrates when compared to islands grown on pit-patterned ones, as observed previously. 3D band-structure calculations show that Ge-rich inclusions of approximately 5 nm diameter at the apex of the islands can account for the observed differences in the PL spectra. The existence of such inclusions can be regarded as a quantum dot in an island and is in agreement with recent nano-tomography experiments.

The Stranski-Krastanow growth of SiGe islands by deposition of SiGe alloys instead of pure Ge allows us to control both the Ge concentration and gradient in the islands. In contrast to the commonly found increasing Ge content with island height, growth conditions for islands with nearly constant and even decreasing Ge profile along the growth direction were found. Atomic force microscopy, transmission electron microscopy and high-resolution x-ray diffraction were employed to determine the islands' size, shape, lateral distance and Ge composition. Efficient photoluminescence is emitted from these islands. We show that for islands with higher Ge contents at the bottom than at the apex, transitions between heavy holes and electron Delta(xy) states in the compressive Si regions around the island's circumference dominate the photoluminescence spectra instead of the usually observed recombination between heavy holes and electrons in the Delta(z) valleys in the tensile Si above the island's apex. The relative importance of the Delta(xy) transitions is enhanced for lateral island distances less than 10 nm, where overlapping strain fields of neighbouring islands increase the compressive strain in the Si region between them. At intense photoexcitation, recombinations between electrons in the Delta(z) valleys and light holes within the islands appear in the photoluminescence spectra. These so far, for SiGe islands, unobserved transitions were identified by a quantitative modelling of the band structure within the islands and in the surrounding Si matrix based on full 3D simulations using the nextnano(3) package with the experimentally obtained island shape and composition as input parameters.