E. Müller Gubler

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.

Thin SiGe virtual substrates for Ge heterostructures integration on silicon

The possibility to reduce the thickness of the SiGe virtual substrate, required for the integration of Ge heterostructures on Si, without heavily affecting the crystal quality is becoming fundamental in several applications. In this work, we present 1 μm thick Si1−xGex buffers (with x > 0.7) having different designs which could be suitable for applications requiring a thin virtual substrate. The rationale is to reduce the lattice mismatch at the interface with the Si substrate by introducing composition steps and/or partial grading. The relatively low growth temperature (475 °C) makes this approach appealing for complementary metal-oxide-semiconductor integration. For all the investigated designs, a reduction of the threading dislocation density compared to constant composition Si1−xGex layers was observed. The best buffer in terms of defects reduction was used as a virtual substrate for the deposition of a Ge/SiGe multiple quantum well structure. Room temperature optical absorption and photoluminescence ...

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.

Ge/SiGe quantum wells on Si(111): Growth, structural, and optical properties

The epitaxial growth of Ge/Si0.15Ge0.85 multiple quantum wells (MQWs) on Si(111) substrates is demonstrated. A 3 μm thick reverse, double-step virtual substrate with a final composition of Si0.10Ge0.90 has been employed. High resolution XRD, TEM, AFM and defect etching analysis has been used for the study of the structural properties of the buffer and of the QWs. The QW stack is characterized by a threading dislocation density of about 3 × 107 cm−2 and an interdiffusion layer at the well/barrier interface of 2.1 nm. The quantum confined energy levels of this system have been calculated using the k·p and effective mass approximation methods. The Ge/Si0.15Ge0.85 MQWs have been characterized through absorption and photoluminescence measurements. The optical spectra have been compared with those of Ge/Si0.15Ge0.85 QWs grown on Si(001) through a thick graded virtual substrate.

Si/SiGe nanoscale engineered thermoelectric materials for energy harvesting

Thermoelectric materials are one potential technology that could be used for energy harvesting. Here we report results from nanoscale Ge/SiGe heterostructure materials grown on Si substrates designed to enhance the thermoelectric performance at room temperature. The materials and devices are aimed at integrated energy harvesters for autonomous sensing applications. We report Seebeck coefficients up to 279.5±1.2 μV/K at room temperature with electrical conductivites of 77,200 S/m which produce a high power factor of 6.02±0.05 mWm-1K-2. Methods for microfabricating modules will be described along with techniques for accurate measurements of the electrical conductivity, Seebeck coefficient and thermal conductivity in micro- and nano-scale devices. The present thermoelectric performance is limited by a high threading dislocation density.