Site-controlled and advanced epitaxial Ge/Si quantum dots: fabrication, properties, and applications.

In this review, we report on fabrication paths, challenges, and emerging solutions to integrate group-IV epitaxial quantum dots (QDs) as active light emitters into the existing standard Si technology. Their potential as laser gain material for the use of optical intra- and inter-chip interconnects as well as possibilities to combine a single-photon-source-based quantum cryptographic means with Si technology will be discussed. We propose that the mandatory addressability of the light emitters can be achieved by a combination of organized QD growth assisted by templated self-assembly, and advanced inter-QD defect engineering to boost the optical emissivity of group-IV QDs at room-temperature. Those two main parts, the site-controlled growth and the light emission enhancement in QDs through the introduction of single defects build the main body of the review. This leads us to a roadmap for the necessary further development of this emerging field of CMOS-compatible group-IV QD light emitters for on-chip applications.

Laser Level Scheme of Self-Interstitials in Epitaxial Ge Dots Encapsulated in Si.

Recently, it was shown that lasing from epitaxial Ge quantum dots (QDs) on Si substrates can be obtained if they are partially amorphized by Ge ion bombardment (GIB). Here, we present a model for the microscopic origin of the radiative transitions leading to enhanced photoluminescence (PL) from such GIB-QDs. We provide an energy level scheme for GIB-QDs in a crystalline Si matrix that is based on atomistic modeling with Monte Carlo (MC) analysis and density functional theory (DFT). The level scheme is consistent with a broad variety of PL experiments performed on as-grown and annealed GIB-QDs. Our results show that an extended point defect consisting of a split-[110] self-interstitial surrounded by a distorted crystal lattice of about 45 atoms leads to electronic states at the Γ-point of the Brillouin zone well below the conduction band minimum of crystalline Ge. Such defects in Ge QDs allow direct transitions of electrons localized at the split-interstitial with holes confined in the Ge QD. We identify the relevant growth and annealing parameters that will let GIB-QDs be employed as an efficient laser active medium.

Co-Immobilization of Proteins and DNA Origami Nanoplates to Produce High-Contrast Biomolecular Nanoarrays.

The biofunctionalization of nanopatterned surfaces with DNA origami nanostructures is an important topic in nanobiotechnology. An unexplored challenge is, however, to co-immobilize proteins with DNA origami at pre-determined substrate sites in high contrast relative to the nontarget areas. The immobilization should, in addition, preferably be achieved on a transparent substrate to allow ultrasensitive optical detection. If successful, specific co-binding would be a step towards stoichiometrically defined arrays with few to individual protein molecules per site. Here, we successfully immobilize with high specificity positively charged avidin proteins and negatively charged DNA origami nanoplates on 100 nm-wide carbon nanoislands while suppressing undesired adsorption to surrounding nontarget areas. The arrays on glass slides achieve unprecedented selectivity factors of up to 4000 and allow ultrasensitive fluorescence read-out. The co-immobilization onto the nanoislands leads to layered biomolecular architectures, which are functional because bound DNA origami influences the number of capturing sites on the nanopatches for other proteins. The novel hybrid DNA origami-protein nanoarrays allow the fabrication of versatile research platforms for applications in biosensing, biophysics, and cell biology, and, in addition, represent an important step towards single-molecule protein arrays.

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. 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.

Photoluminescence investigation of strictly ordered Ge dots grown on pit-patterned Si substrates.

We investigate the optical properties of ordered Ge quantum dots (QDs) by means of micro-photoluminescence spectroscopy (PL). These were grown on pit-patterned Si(001) substrates with a wide range of pit-periods and thus inter QD-distances (425-3400 nm). By exploiting almost arbitrary inter-QD distances achievable in this way we are able to choose the number of QDs that contribute to the PL emission in a range between 70 and less than three QDs. This well-defined system allows us to clarify, by PL-investigation, several points which are important for the understanding of the formation and optical properties of ordered QDs. We directly trace and quantify the amount of Ge transferred from the surrounding wetting layer (WL) to the QDs in the pits. Moreover, by exploiting different pit-shapes, we reveal the role of strain-induced activation energy barriers that have to be overcome for charge carriers generated outside the dots. These need to diffuse between the energy minimum of the WL in and between the pits, and the one in the QDs. In addition, we demonstrate that the WL in the pits is already severely intermixed with Si before upright QDs nucleate, which further enhances intermixing of ordered QDs as compared to QDs grown on planar substrates. Furthermore, we quantitatively determine the amount of Ge transferred by surface diffusion through the border region between planar and patterned substrate. This is important for the growth of ordered islands on patterned fields of finite size. We highlight that the Ge WL-facets in the pits act as PL emission centres, similar to upright QDs.

Evolution of epitaxial semiconductor nanodots and nanowires from supersaturated wetting layers.

In this tutorial we review recent progress in the design and growth of epitaxial semiconductor nanostructures in lattice-mismatched material systems. We focus on the Ge on Si model system after pointing out the similarities to III-V and other growth systems qualitatively as well as quantitatively. During material deposition, the first layers of the epitaxial film wet the surface before the formation of strain-driven three-dimensional nanostructures. In particular, we stress that the supersaturation of the wetting layer (WL), whose relevance is often neglected, plays a key role in determining the nucleation and growth of nanodots (NDs), nanodot-molecules and nanowires (NWs). At elevated growth temperatures the Ge reservoir in the planar, supersaturated WL is abruptly consumed and generates NDs with highly homogeneous sizes - a process mainly driven by elastic energy minimization. Furthermore, the careful control of the supersaturated Ge layer allows us to obtain perfectly site-controlled, ordered NDs or ND-molecules on pit-patterned substrates for a broad range of pit-periods. At low growth temperatures subtle interplays between surface energies of dominant crystal facets in the system drive the material transfer from the supersaturated WL into the elongating NWs growing horizontally, dislocation- and catalyst-free on the substrate surface. Due to the similarities in the formation of nanostructures in different epitaxial semiconductor systems we expect that the observation of the novel growth phenomena described in this Tutorial Review for Ge/Si should be relevant for other lattice-mismatched heterostructure systems, too.

Recipes for the fabrication of strictly ordered Ge islands on pit-patterned Si(001) substrates.

We identify the most important parameters for the growth of ordered SiGe islands on pit-patterned Si(001) substrates. From a multi-dimensional parameter space we link individual contributions to isolate their influence on ordered island growth. This includes the influences of: the pit size, pit depth and pit period on the Si buffer layer and subsequent Ge growth; the pit sidewall inclination on Ge island growth; the amount of Ge on island morphologies as well as the influences of the pit-size homogeneity, the pit period, the Ge growth temperature and rate on island formation. We highlight that the initial pit shape and pit size in combination with the growth conditions of the Si buffer layer should be adjusted to provide suitable preconditions for the growth of Ge islands with the desired size, composition and nucleation position. Furthermore, we demonstrate that the wetting layer between pits can play the role of a stabilizer that inhibits shape transformations of ordered islands. Thus, dislocation formation within islands can be delayed, uniform arrays of one island type can be fabricated and secondary island nucleation between pits can be impeded. These findings allow us to fabricate perfectly ordered and homogeneous Ge islands on one and the same sample, even if the pit period is varied from a few hundred nanometres to several micrometres.

Morphological evolution of Ge/Si(001) quantum dot rings formed at the rim of wet-etched pits.

We demonstrate the formation of Ge quantum dots in ring-like arrangements around predefined {111}-faceted pits in the Si(001) substrate. We report on the complex morphological evolution of the single quantum dots contributing to the rings by means of atomic force microscopy and demonstrate that by careful adjustment of the epitaxial growth parameters, such rings containing densely squeezed islands can be grown with large spatial distances of up to 5 µm without additional nucleation of randomly distributed quantum dots between the rings.

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.