H. Groiss

Size control and midinfrared emission of epitaxial PbTe∕CdTe quantum dot precipitates grown by molecular beam epitaxy

Epitaxial quantum dots with symmetric and highly facetted shapes are fabricated by thermal annealing of two-dimensional (2D) PbTe epilayers embedded in a CdTe matrix. By varying the thickness of the initial 2D layers, the dot size can be effectively controlled between 5 and 25nm, and areal densities as high as 3×1011cm−2 can be achieved. The size control allows the tuning of the quantum dot luminescence over a wide spectral range between 2.2 and 3.7μm. As a result, ultrabroadband emission from a multilayered quantum dot stack is demonstrated, which is a precondition for the development of superluminescent diodes operating in the near infrared and midinfrared.

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.

Phase separation and exchange biasing in the ferromagnetic IV-VI semiconductor Ge1−xMnxTe

Ferromagnetic Ge1−xMnxTe grown by molecular beam epitaxy with Mn content of xMn≈0.5 is shown to exhibit a strong tendency for phase separation. At higher growth temperatures apart from the cubic Ge0.5Mn0.5Te, a hexagonal MnTe and a rhombohedral distorted Ge0.83Mn0.17Te phase is formed. This coexistence of antiferromagnetic MnTe and ferromagnetic Ge0.5Mn0.5Te results in magnetic exchange-bias effects.

Tuning the Localized Surface Plasmon Resonance in Cu2–xSe Nanocrystals by Postsynthetic Ligand Exchange

Nanoparticles exhibiting localized surface plasmon resonances (LSPR) are valuable tools traditionally used in a wide field of applications including sensing, imaging, biodiagnostics and medical therapy. Plasmonics in semiconductor nanocrystals is of special interest because of the tunability of the carrier densities in semiconductors, and the possibility to couple the plasmonic resonances to quantum confined excitonic transitions. Here, colloidal Cu2–xSe nanocrystals were synthesized, whose composition was shown by Rutherford backscattering analysis and electron dispersive X-ray spectroscopy, to exhibit Cu deficiency. The latter results in p-type doping causing LSPRs, in the present case around a wavelength of 1100 nm, closely matching the indirect band gap of Cu2–xSe. By partial exchange of the organic ligands to specific electron trapping or donating species the LSPR is fine-tuned to exhibit blue or red shifts, in total up to 200 nm. This tuning not only provides a convenient tool for post synthetic adjustments of LSPRs to specific target wavelength but the sensitive dependence of the resonance wavelength on surface charges makes these nanocrystals also interesting for sensing applications, to detect analytes dressed by functional groups.

The influence of a Si cap on self-organized SiGe islands and the underlying wetting layer

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 alloye...

Microphotoluminescence and perfect ordering of SiGe islands on pit-patterned Si(001) substrates

We show that both the morphology and the optoelectronic properties of SiGe islands growing in the pits of periodically pre-patterned Si(001) substrates are determined by the amount of Ge deposited per unit cell of the pattern. Pit-periods (p) ranging from 300 to 900 nm were investigated, and Ge growth was performed by molecular beam epitaxy (MBE) at temperatures of 690 and 760 °C. The ordered SiGe islands show photoluminescence (PL) emission, which becomes almost completely quenched, once a critical island volume is exceeded. By atomic force and transmission electron microscope images we identify the transition from pyramid-shaped to dome-shaped islands with increasing p. Eventually, the nucleation of dislocations in the islands leads to PL quenching. Below a critical Ge coverage a narrowing and a blue shift of the PL emission is observed, as compared to islands grown on a planar reference area of the same sample.

Lasing from Glassy Ge Quantum Dots in Crystalline Si.

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.Semiconductor light-emitters compatible with standard Si integration technology (SIT) are of particular interest for overcoming limitations in the operating speed of microelectronic devices. 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 defect-free Si matrix show extraordinary optical properties if partially amorphized by Ge-ion bombardment (GIB). In contrast to conventional SiGe nanostructures, these QDs exhibit dramatically shortened carrier lifetimes and negligible thermal quenching of the photoluminescence (PL) up to room temperature. Microdisk resonators with embedded GIB-QDs exhibit threshold behavior as well as a superlinear increase of the integrated PL intensity with concomitant line width narrowing as the pump power increases. These findings demonstrate light amplification by stimulated emission in a fully SIT-compatible group IV nanosystem.

Stranski-Krastanow growth of tensile strained Si islands on Ge (001)

Stranski-Krastanow island growth is demonstrated for tensile strained silicon epilayers on Ge (001) substrates over a wide range of growth temperatures. Small, Si-rich islands show sidewall faces near {1,1,10}, whereas larger islands are {113}-terminated truncated pyramids with an aspect ratio near 0.1. In contrast to compressively strained Ge on Si, we find for Si on Ge a significantly thicker wetting layer of >8 ML and coexistence of islands and dislocations.

Rebonding at coherent interfaces between rocksalt-PbTe/zinc-blende-CdTe

We study interfaces between highly ionic crystals with different crystal structure by means of first-principles total-energy calculations in the repeated-slab approximation, and compare the results with experimental data extracted from high-resolution transmission electron micrographs. Despite the same Bravais lattices and the electrostatic neutrality of each atomic plane, the (110) interface between rocksalt and zinc-blende crystals gives rise to the most remarkable effect, a lateral spatial offset between the two crystals due to rebonding across the interface. A strong variation is observed for the separation of the two polar (001) interfaces depending on their cation- or anion-termination. In general, the long-range electrostatic forces lead to opposite atomic displacements along the interface normal independently of the interface orientation.

Strain engineering in Si via closely stacked, site-controlled SiGe islands

The authors report on the fabrication and detailed structural characterization of ordered arrays of vertically stacked SiGe/Si(001) island pairs. By a proper choice of growth parameters, islands which have both large sizes and high Ge fraction are obtained in the upper layer. Finite element method calculations of the strain distribution reveal that (i) the Si spacer between a pair of islands can act as a lateral quantum dot molecule made of four nearby dots for electrons and (ii) the tensile strain in a Si cap deposited on top of the stack is significantly enhanced with respect to a single layer.