Thomas Nuytten

GaSb quantum dot morphology for different growth temperatures and the dissolution effect of the GaAs capping layer

We compare the characteristics of GaSb quantum dots (QDs) grown by molecular beam epitaxy on GaAs at temperatures from 400°C to 490°C. The dot morphology, in terms of size, shape and density, as determined by atomic force microscopy on uncapped QDs, was found to be highly sensitive to the growth temperature. Photoluminescence spectra of capped QDs are also strongly dependent on growth temperature, but for samples with the highest dot density, where the QD luminescence would be expected to be the most intense, it is absent. We attribute this to dissolution of the dots by the capping layer. This explanation is confirmed by atomic force microscopy of a sample that is thinly capped at 490°C. Deposition of the capping layer at low temperature resolves this problem, resulting in strong QD photoluminescence from a sample with a high dot-density.

Multilayer MoS2 growth by metal and metal oxide sulfurization

We investigated the deposition of MoS2 multilayers on large area substrates. The pre-deposition of metal or metal oxide with subsequent sulfurization is a promising technique to achieve layered films. We distinguish a different reaction behavior in metal oxide and metallic films and investigate the effect of the temperature, the H2S/H2 gas mixture composition, and the role of the underlying substrate on the material quality. The results of the experiments suggest a MoS2 growth mechanism consisting of two subsequent process steps. At first, the reaction of the sulfur precursor with the metal or metal oxide occurs, requiring higher temperatures in the case of metallic film compared to metal oxide. At this stage, the basal planes assemble towards the diffusion direction of the reaction educts and products. After the sulfurization reaction, the material recrystallizes and the basal planes rearrange parallel to the substrate to minimize the surface energy. Therefore, substrates with low roughness show basal plane assembly parallel to the substrate. These results indicate that the substrate character has a significant impact on the assembly of low dimensional MoS2 films.

Synthesis of PEGylated Magnetic Nanoparticles With Different Core Sizes

Tailoring the properties of superparamagnetic nanoparticles (MNPs) is essential for various nano-based biological applications. Having control over the properties of the MNPs permits a maximum flexibility. Starting from monodisperse iron oxide MNPs produced by thermal decomposition, we report on the optimization and characterization of a first and second seed mediated growth step by varying the surfactant amount and by optimizing the heating steps. We demonstrate the ability to gradually increase the size of crystalline MNPs from 6 over 9 to 12 nm with an improving monodispersity as demonstrated by Transmission Electron Microscopy, Dynamic Light Scattering and X-ray diffraction. The magnetic properties of the MNPs, studied by Vibrating Sample Magnetometry, were in concert with their size increase. We also show the functionalization of these particles with polyethylene glycolated silanes, to render the MNPs stable in water. Different characterization techniques, namely Transmission Electron Microscopy, Dynamic Light Scattering, Fourier-transform InfraRed, Thermo gravimetric analysis and X-ray Photoelectron Spectroscopy, confirmed the successful engraftment of the silanes on the MNPs surface. In conclusion, the proposed route of step-wise synthesis in combination with silane functionalization allows fine tuning the physical properties of iron oxide MNPs for applications in an aqueous environment.

Nucleation and growth mechanisms of Al2O3 atomic layer deposition on synthetic polycrystalline MoS2

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of Al2O3 (using Al(CH3)3/H2O ALD) on MoS2 whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS2, we exclude the impact of surface contamination from a transfer process by direct Al2O3 deposition on synthetic MoS2 layers obtained by a high temperature sulfurization process. It is shown that Al2O3 ALD on the MoS2 surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS2 crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS2 top surface. During further deposition, the as-formed Al2O3 nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous Al2O3 film is obtained by lateral growth over the MoS2 crystal basal plane, with the point of layer closure determined by the grain size at the MoS2 top surface and the lateral growth rate. The created Al2O3/MoS2 interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS2 basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the Al2O3 layers. To conclude, our growth study reveals that the inherent reactivity of the MoS2 basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation.

Investigation of Properties Limiting Efficiency in Cu 2 ZnSnSe 4 -Based Solar Cells

We have investigated different nonidealities in Cu2ZnSnSe4-CdS-ZnO solar cells with 9.7% conversion efficiency, in order to determine what is limiting the efficiency of these devices. Several nonidealities could be observed. A barrier of about 300 meV is present for electron flow at the absorber-buffer heterojunction leading to a strong crossover behavior between dark and illuminated current-voltage curves. In addition, a barrier of about 130 meV is present at the Mo-absorber contact, which could be reduced to 15 meV by inclusion of a TiN interlayer. Admittance spectroscopy results on the devices with the TiN backside contact show a defect level with an activation energy of 170 meV. Using all parameters extracted by the different characterization methods for simulations of the two-diode model including injection and recombination currents, we come to the conclusion that our devices are limited by the large recombination current in the depletion region. Potential fluctuations are present in the devices as well, but they do not seem to have a special degrading effect on the devices, besides a probable reduction in minority carrier lifetime through enhanced recombination through the band tail defects.

Edge-enhanced Raman scattering in narrow sGe fin field-effect transistor channels

We report micro-Raman spectroscopy results on strained Ge narrow (20 nm) channels for finFET nanoelectronics technology. It is found that the Raman activity of the structures is strongly dependent on the relative orientation of the excitation laser polarization and the structure geometry. While the observation of the typical Ge Raman signatures is challenging for the antiparallel orientation, a dramatic enhancement of the signal is found for a parallel orientation. Simulations confirm that a significant concentration of the lights electromagnetic field in the vicinity of the edges of the structures is at the origin of the strong Raman enhancement. The edge enhancement of the Raman scattering is a promising tool for the non-destructive characterization of nanometer-scale semiconductor structures and devices.

Nanofocusing of light into semiconducting fin photonic crystals

This letter demonstrates experimentally and investigates theoretically the possibility for enhanced light coupling into periodic arrays of nanoscale semiconducting fins. Using Raman spectroscopy, we show that an electromagnetic field impinging upon such periodic structures can be confined into the semiconducting regions when the ratio W/λ0 of the fin width to the incident wavelength is sufficiently small and when the incident light polarization is parallel to the fin edges. As we demonstrate based on band structure calculations and finite-element simulations, this corresponds to the availability and excitation of a dielectric-band mode of the constituted photonic crystal waveguide, i.e., a mode guided inside the semiconducting fins. The understanding of this nanofocusing behavior opens the way to a plethora of applications including the optical metrology of deep-subwavelength non-planar semiconductor devices.

Anisotropic stress in narrow sGe fin field-effect transistor channels measured using nano-focused Raman spectroscopy

The continued importance of strain engineering in semiconductor technology demands fast and reliable stress metrology that is non-destructive and process line-compatible. Raman spectroscopy meets these requirements but the diffraction limit prevents its application in current and future technology nodes. We show that nano-focused Raman scattering overcomes these limitations and can be combined with oil-immersion to obtain quantitative anisotropic stress measurements. We demonstrate accurate stress characterization in strained Ge fin field-effect transistor channels without sample preparation or advanced microscopy. The detailed analysis of the enhanced Raman response from a periodic array of 20 nm-wide Ge fins provides direct access to the stress levels inside the nanoscale channel, and the results are validated using nano-beam diffraction measurements.

Advanced Raman Spectroscopy Using Nanofocusing of Light : Advanced Raman Spectroscopy Using Nanofocusing…

Raman spectroscopy is uniquely sensitive to crucial material properties like stress and composition, but is inherently diffraction-limited, impeding its application potential in nanostructured devices. Under correct polarization conditions, the Raman response from a periodic array of fins is dramatically enhanced inside the semiconductor material, re-enabling fast and non-destructive optical characterization of deep-subwavelength semiconductor patterns. In this paper, it is shown that the effect is not limited to the material system where it was first observed, and results of nanofocused Raman spectroscopy in a variety of semiconductormaterials are presented. The authors illustrate and discuss how the universality of the enhancement creates a unique potential for non-invasive and rapid stress and composition measurements at the nanoscale.

Temperature dependence of the photoluminescence of self-assembled InAs/GaAs quantum dots studied in high magnetic fields

We have investigated the photoluminescence (PL) of self-assembled InAs/GaAs quantum dots (QDs) in high magnetic fields of up to 50 T and as a function of temperature. Our data clearly indicate that two different mechanisms are at work. At low temperatures (T < 80 K), the zero-field PL is increasingly dominated by lower energy dots. High-field measurements however demonstrate that these dots are larger in size only in the growth direction. At temperatures above 100 K, a strong decrease of the PL peak energy shift with field is observed, while the zero-field PL is characterized by a redshift according to the changes in the bandgap. We discuss these contradictory observations in terms of a phenomenon that we call field-assisted enhancement of the QD barrier potential. Since this effect is much stronger for small high-energy QDs, the latter progressively dominate the PL emission when temperature and magnetic field are increased.