M. Seibt

Electronic and chemical properties of the c-Si/Al2O3 interface

Using aluminum oxide (Al2O3) films deposited by atomic layer deposition (ALD), the dominant passivation mechanisms at the c-Si/Al2O3 interface, as well as the chemical composition of the interface region, are investigated. The excellent surface passivation quality of thin Al2O3 films is predominantly assigned to a high negative fixed charge density of Qf = − (4 ± 1) × 1012 cm−2, which is located within 1nm of the Si/Al2O3 interface and is independent of the layer thickness. A deterioration of the passivation quality for ultrathin Al2O3 layers is explained by a strong increase in the interface state density, presumably due to an incomplete reaction of the trimethyl-aluminum (TMA) molecules during the first ALD cycles. A high oxygen-to-aluminum atomic ratio resulting from the incomplete adsorption of the TMA molecules is suggested as a possible source of the high negative charge density Qf at the Si/Al2O3 interface.

Room-temperature silicon light-emitting diodes based on dislocation luminescence

We demonstrate electroluminescence (EL) with an external efficiency of more than 0.1% at room temperature from glide dislocations in silicon. The key to this achievement is a considerable reduction of nonradiative carrier recombination at dislocations due to impurities and core defects by impurity gettering and hydrogen passivation, respectively, which is shown by means of deep-level transient spectroscopy. Time-resolved EL measurements reveal a response time below 1.8 μs, which is much faster, compared to the band-to-band luminescence of bulk silicon.

Photoluminescence of carbon nanodots: dipole emission centers and electron-phonon coupling.

Inorganic carbon nanomaterials, also called carbon nanodots, exhibit a strong photoluminescence with unusual properties and, thus, have been the focus of intense research. Nonetheless, the origin of their photoluminescence is still unclear and the subject of scientific debates. Here, we present a single particle comprehensive study of carbon nanodot photoluminescence, which combines emission and lifetime spectroscopy, defocused emission dipole imaging, azimuthally polarized excitation dipole scanning, nanocavity-based quantum yield measurements, high resolution transmission electron microscopy, and atomic force microscopy. We find that photoluminescent carbon nanodots behave as electric dipoles, both in absorption and emission, and that their emission originates from the recombination of photogenerated charges on defect centers involving a strong coupling between the electronic transition and collective vibrations of the lattice structure.

Nucleation mechanism of the seed of tetrapod ZnO nanostructures

Tetrapod zinc oxide (T-ZnO) nanorods have been synthesized by evaporation and recondensation of metallic Zn under ambient conditions. The total sizes of the T-ZnO nanostructures range from 300nmto15μm with leg diameters of about 30to650nm, depending on the deposition temperature. A detailed high-resolution electron microscopy analysis showed that the center core of T-ZnO nanorods consists of four hexagonal grains with a twinlike relation. The nucleation and growth mechanism has been generated on the basis of energy considerations during a phase transition from a fullerenelike ZnO cluster to a nanometer-sized tetrahedron, which is directly visible in our high-resolution transmission electron microscopy investigations.

Microstructure-controlled magnetic properties of the bulk glass-forming alloy Nd60Fe30Al10

We report a combination of analytical transmission electron microscopy, small angle neutron scattering, and studies of magnetic properties of the glass-forming alloy Nd60Fe30Al10. These investigations show the existence of an in situ formed finely dispersed nanocrystalline Nd-rich phase embedded in a Fe-rich glassy matrix of a bulk sample. The crystalline phase forms an extended network over the whole sample but its volume fraction is small compared to that of compact phase. Small angle neutron scattering data exhibit power law behavior with an exponent of −2.5 indicating the formation of a mass fractal. The microstructure observed may be related to phase separation in the undercooled liquid which induces a microstructure that can explain the hard magnetic behavior of such an intrinsic composite.

Structural and Electrical Properties of Metal Silicide Precipitates in Silicon

This paper summarizes current understanding of structural and electronic properties of metal silicide precipitates in silicon and their interrelation. Combined studies of high-resolution transmission electron microscopy and deep level transient spectroscopy together with numerical simulations show that the bounding dislocation of nickel silicide platelets is the key to understand their rapid growth and electrical properties. Different misfit relaxation phenomena govern the structural evolution of copper silicide precipitates from their early stages to the well-known colony growth. This evolution involves different types of secondary defects indicating that the deep band-like states observed throughout this process are associated with the silicide precipitates themselves.

Light‐Controlled Interconversion between a Self‐Assembled Triangle and a Rhombicuboctahedral Sphere

Stimuli-responsive structural reorganizations play an important role in biological processes, often in combination with kinetic control scenarios. In supramolecular mimics of such systems, light has been established as the perfect external trigger. Here, we report on the light-driven structural rearrangement of a small, self-assembled Pd3L6 ring based on photochromic dithienylethene (DTE) ligands into a rhombicuboctahedral Pd24L48 sphere measuring about 6.4 nm across. When the wavelength is changed, this interconversion can be fully reversed, as confirmed by NMR and UV/Vis spectroscopy as well as mass spectrometry. The sphere was visualized by AFM, TEM, and GISAXS measurements. Due to dissimilarities in the photoswitch conformations, the interconversion rates between the two assemblies are drastically different in the two directions.

Recombination properties of structurally well defined NiSi2 precipitates in silicon

We report first results on the recombination properties of structurally well defined NiSi2 precipitates in n‐type silicon. Under the conditions applied, precipitates form without the occurrence of punched out dislocations or any other secondary defects. We find that the minority‐carrier diffusion length (LD) measured by electron beam induced current (EBIC) is related to the precipitate density NV and LD ≂ 0.7 × NV−1/3. EBIC investigations of individual precipitates reveal contrasts up to 40% demonstrating NiSi2 particles to be efficient recombination centers.

Formation and Properties of Copper Silicide Precipitates in Silicon

We report results of a detailed study of structural and electrical properties of copper silicide precipitates in silicon. Using conventional and high-resolution transmission electron microscopy: we observe that metastable platelets surrounded by extrinsic stacking faults form upon quenching from high temperatures. By ripening experiments at low temperatures as well as by a variation of cooling rates it is shown how homogeneous copper precipitation merges into the heterogeneous precipitation mode of colony growth. The application of recently developed criteria for the interpretation of deep level transient spectra from extended defects allows to conclude that deep electronic states associated with the precipitates have bandlike character.

Impact of surface topography and laser pulse duration for laser ablation of solar cell front side passivating SiNx layers

Local contact openings in SiNx layers that passivate the front side of solar cells offer an attractive alternative to the current standard “fire-through” screen printing process for front grid fabrication. Additionally, this technology can be used for enabling a selective emitter. In the present paper, we investigate laser ablation of SiNx layers on planar and textured silicon surfaces for various laser wavelengths and pulse durations in the nanosecond (ns) to femtosecond (fs) range. We characterize the dark J-V characteristics of diodes with laser contact openings in the SiNx layer passivating the emitter. Our results show that on alkaline textured surfaces the ablation by a ns laser produces less damage than by an ultrashort pulse laser. The dark currents of alkaline textured diodes treated with picosecond (ps) or fs lasers are one order of magnitude higher than those of ns laser treated diodes. High ideality factors furthermore indicate crystal damage in the ∼500 nm deep space charge region of the diod...