Tanja Etzelstorfer

Hexagonal silicon realized

Silicon, arguably the most important technological semiconductor, is predicted to exhibit a range of new and interesting properties when grown in the hexagonal crystal structure. To obtain pure hexagonal silicon is a great challenge because it naturally crystallizes in the cubic structure. Here, we demonstrate the fabrication of pure and stable hexagonal silicon evidenced by structural characterization. In our approach, we transfer the hexagonal crystal structure from a template hexagonal gallium phosphide nanowire to an epitaxially grown silicon shell, such that hexagonal silicon is formed. The typical ABABAB... stacking of the hexagonal structure is shown by aberration-corrected imaging in transmission electron microscopy. In addition, X-ray diffraction measurements show the high crystalline purity of the material. We show that this material is stable up to 9 GPa pressure. With this development, we open the way for exploring its optical, electrical, superconducting, and mechanical properties.

X-ray Nanodiffraction on a Single SiGe Quantum Dot inside a Functioning Field-Effect Transistor

For advanced electronic, optoelectronic, or mechanical nanoscale devices a detailed understanding of their structural properties and in particular the strain state within their active region is of utmost importance. We demonstrate that X-ray nanodiffraction represents an excellent tool to investigate the internal structure of such devices in a nondestructive way by using a focused synchotron X-ray beam with a diameter of 400 nm. We show results on the strain fields in and around a single SiGe island, which serves as stressor for the Si-channel in a fully functioning Si–metal–oxide semiconductor field-effect transistor.

The thermoelectric properties of Ge/SiGe modulation doped superlattices

The thermoelectric and physical properties of superlattices consisting of modulation doped Ge quantum wells inside Si1− y Ge y barriers are presented, which demonstrate enhancements in the thermoelectric figure of merit, ZT, and power factor at room temperature over bulk Ge, Si1− y Ge y , and Si/Ge superlattice materials. Mobility spectrum analysis along with low temperature measurements indicate that the high power factors are dominated by the high electrical conductivity from the modulation doping. Comparison of the results with modelling using the Boltzmann transport equation with scattering parameters obtained from Monte Carlo techniques indicates that a high threading dislocation density is also limiting the performance. The analysis suggests routes to higher thermoelectric performance at room temperature from Si-based materials that can be fabricated using micro- and nano-fabrication techniques.

Particle-assisted GaxIn1−xP nanowire growth for designed bandgap structures

Non-tapered vertically straight Ga(x)In(1-x)P nanowires were grown in a compositional range from Ga(0.2)In(0.8)P to pure GaP in particle-assisted mode by controlling the trimethylindium, trimethylgallium and hydrogen chloride flows in metal-organic vapor phase epitaxy. X-ray energy dispersive spectroscopy in transmission electron microscopy revealed homogeneous radial material composition in single nanowires, whereas variations in the material composition were found along the nanowires. High-resolution x-ray diffraction indicates a variation of the material composition on the order of about 19% measuring an entire sample area, i.e., including edge effects during growth. The non-capped nanowires emit room temperature photoluminescence strongly in the energy range of 1.43-2.16 eV, correlated with the bandgap expected from the material composition.

The cross-plane thermoelectric properties of p-Ge/Si0.5Ge0.5 superlattices

The electrical conductivity, Seebeck coefficients, and thermal conductivities of a range of p-type Ge/Si0.5 Ge 0.5 superlattices designed for thermoelectric generation and grown by low energy plasma enhanced chemical vapor deposition have been measured using a range of microfabricated test structures. For samples with barriers around 0.5 nm in thickness, the measured Seebeck coefficients were comparable to bulk p-SiGe at similar doping levels suggesting the holes see the material as a random bulk alloy rather than a superlattice. The Seebeck coefficients for Ge quantum wells of 2.85 ± 0.85 nm increased up to 533 ± 25 μV/K as the doping was reduced. The thermal conductivities are between 4.5 to 6.0 Wm−1K−1 which are lower than comparably doped bulk Si0.3 Ge 0.7 but higher than undoped Si/Ge superlattices. The highest measured figure of merit ZT was 0.080 ± 0.011 obtained for the widest quantum well studied. Analysis suggests that interface roughness is presently limiting the performance and a reduction in the strain between the quantum wells and barriers has the potential to improve the thermoelectric performance.

Scanning X-ray strain microscopy of inhomogeneously strained Ge micro-bridges.

A scanning X-ray strain microscopy technique using a micro-focused beam is demonstrated.

Ge/SiGe superlattices for thermoelectric energy conversion devices

Ge-rich multiple quantum well heterostructures have been investigated as engineered material for efficient thermoelectric generators monolithically integrated on silicon substrates. Thick Ge/SiGe multilayers on Si substrates designed for lateral thermoelectric devices have been grown and characterized in which electrical and thermal conduction occur parallel to the heterostructure interfaces. In this study, an overview of the investigated structures is presented together with results from X-ray scattering and transmission electron microscopy experiments. These analyses confirm the high quality of the material and the uniformity of the structure over the whole deposited thickness. Important parameters in terms of the optimization of the material quality which could affect thermoelectric properties, such as the interfaces roughness and the threading dislocation density, have also been evaluated. Preliminary electrical and Seebeck coefficient measurements indicate the viability of this material for the realization of thermoelectric devices.

Thermal Conductivity Measurement Methods for SiGe Thermoelectric Materials

A new technique to measure the thermal conductivity of thermoelectric materials at the microscale has been developed. The structure allows the electrical conductivity, thermal conductivity, and Seebeck coefficient to be measured on a single device. The thermal conductivity is particularly difficult to measure since it requires precise estimation of the heat flux injected into the material. The new technique is based on a differential method where the parasitic contributions of the supporting beams of a Hall bar are removed. The thermal measurements with integrated platinum thermometers on the device are cross-checked using thermal atomic force microscopy and validated by finite-element analysis simulations.

Structural investigations of the α12Si–Ge superstructure

X-ray diffraction-based structural analysis results of possible direct bandgap Si/Ge superlattices composed of monolayer thin deposits are presented, together with theoretical predictions and first optical measurements.

Structural investigations of the α12 Si-Ge superstructure.

X-ray diffraction-based structural analysis results of possible direct bandgap Si/Ge superlattices composed of monolayer thin deposits are presented, together with theoretical predictions and first optical measurements.