Yee Rui Koh

Thermoelectric properties of epitaxial ScN films deposited by reactive magnetron sputtering onto MgO(001) substrates

Epitaxial ScN(001) thin films were grown on MgO(001) substrates by dc reactive magnetron sputtering. The deposition was performed in an Ar/N2 atmosphere at 2 × 10−3 Torr at a substrate temperature of 850 °C in a high vacuum chamber with a base pressure of 10−8 Torr. In spite of oxygen contamination of 1.6 ± 1 at. %, the electrical resistivity, electron mobility, and carrier concentration obtained from a typical film grown under these conditions by room temperature Hall measurements are 0.22 mΩ cm, 106 cm2 V−1 s−1, and 2.5 × 1020 cm−3, respectively. These films exhibit remarkable thermoelectric power factors of 3.3–3.5 × 10−3 W/mK2 in the temperature range of 600 K to 840 K. The cross-plane thermal conductivity is 8.3 W/mK at 800 K yielding an estimated ZT of 0.3. Theoretical modeling of the thermoelectric properties of ScN calculated using a mean-free-path of 23 nm at 300 K is in very good agreement with the experiment. These results also demonstrate that further optimization of the power factor of ScN is...

Anisotropic Effects on the Thermoelectric Properties of Highly Oriented Electrodeposited Bi2Te3 Films.

Highly oriented [1 1 0] Bi2Te3 films were obtained by pulsed electrodeposition. The structure, composition, and morphology of these films were characterized. The thermoelectric figure of merit (zT), both parallel and perpendicular to the substrate surface, were determined by measuring the Seebeck coefficient, electrical conductivity, and thermal conductivity in each direction. At 300 K, the in-plane and out-of-plane figure of merits of these Bi2Te3 films were (5.6 ± 1.2)·10−2 and (10.4 ± 2.6)·10−2, respectively.

Superdiffusive heat conduction in semiconductor alloys. II. Truncated Lévy formalism for experimental analysis

evy dynamics with fractal dimension α< 2. Here, we present a framework that enables full three-dimensional experimental analysis by retaining all essential physics of the quasiballistic BTE dynamics phenomenologically. A stochastic process with just two fitting parameters describes the transition from pure L´ evy superdiffusion as short length and time scales to regular Fourier diffusion. The model provides accurate fits to time domain thermoreflectance raw experimental data over the full modulation frequency range without requiring any “effective” thermal parameters and without any ap rioriknowledge of microscopic phonon scattering mechanisms. Identified α values for InGaAs and SiGe match ab initio BTE predictions within a few percent. Our results provide experimental evidence of fractal L´ evy heat conduction in semiconductor alloys. The formalism additionally indicates that the transient temperature inside the material differs significantly from Fourier theory and can lead to improved thermal characterization of nanoscale devices and material interfaces.

Compensation of native donor doping in ScN: Carrier concentration control and p-type ScN

Scandium nitride (ScN) is an emerging indirect bandgap rocksalt semiconductor that has attracted significant attention in recent years for its potential applications in thermoelectric energy conversion devices, as a semiconducting component in epitaxial metal/semiconductor superlattices and as a substrate material for high quality GaN growth. Due to the presence of oxygen impurities and native defects such as nitrogen vacancies, sputter-deposited ScN thin-films are highly degenerate n-type semiconductors with carrier concentrations in the (1–6)  × 1020 cm−3 range. In this letter, we show that magnesium nitride (MgxNy) acts as an efficient hole dopant in ScN and reduces the n-type carrier concentration, turning ScN into a p-type semiconductor at high doping levels. Employing a combination of high-resolution X-ray diffraction, transmission electron microscopy, and room temperature optical and temperature dependent electrical measurements, we demonstrate that p-type Sc1-xMgxN thin-film alloys (a) are substit...

Fractal Lévy Heat Transport in Nanoparticle Embedded Semiconductor Alloys

Materials with embedded nanoparticles are of considerable interest for thermoelectric applications. Here, we experimentally characterize the effect of nanoparticles on the recently discovered Lévy phonon transport in semiconductor alloys. The fractal space dimension α ≈ 1.55 of quasiballistic (superdiffusive) heat conduction in (ErAs)x:InGaAlAs is virtually independent of the Er content 0.001 < x < 0.1 but instead controlled by alloy scattering of the host matrix. The increased nanoparticle concentration does reduce the diffusive recovery length by an order of magnitude. The bulk conductivity drops by 3-fold, in close agreement with a Callaway model. Our results may provide helpful hints toward engineering superdiffusive heat transport similar to what has been achieved with light in Lévy glasses.

Full-field thermal imaging of quasiballistic crosstalk reduction in nanoscale devices

Understanding nanoscale thermal transport is of substantial importance for designing contemporary semiconductor technologies. Heat removal from small sources is well established to be severely impeded compared to diffusive predictions due to the ballistic nature of the dominant heat carriers. Experimental observations are commonly interpreted through a reduction of effective thermal conductivity, even though most measurements only probe a single aggregate thermal metric. Here, we employ thermoreflectance thermal imaging to directly visualise the 2D temperature field produced by localised heat sources on InGaAs with characteristic widths down to 100 nm. Besides displaying effective thermal performance reductions up to 50% at the active junctions in agreement with prior studies, our steady-state thermal images reveal that, remarkably, 1–3 μm adjacent to submicron devices the crosstalk is actually reduced by up to fourfold. Submicrosecond transient imaging additionally shows responses to be faster than conventionally predicted. A possible explanation based on hydrodynamic heat transport, and some open questions, are discussed.When thermal fields in semiconductors approach the submicron scale, non-diffusive heat transport is observed where Fourier based heat transport models fail. Here, the authors use thermal imaging to visualise these thermal field variations and in turn derive a hydrodynamic heat transport model.

Cooling power optimization for hybrid solid-state and liquid cooling in integrated circuit chips with hotspots

We report theoretical investigation and optimization of a hot-spot cooling method. This hybrid scheme contains a liquid cooling microchannel and superlattice hotspot cooler(s). This analysis of the hybrid method aims to solve the potential thermal management challenges for hotspots especially in 3D stacked multichip packaging. The goal is to reduce the overall cooling power and optimize the energy efficiency. Starting with a generic modeling of the superlattice cooler system, the cooling temperature as a function of the superlattice thickness and the driving current is found. The analytic results are then compared with full 3D numerical simulation. The role of spreading thermal resistance in the chip substrate was found to be important. The later part of this report is the integration of the microchannel with the hotspot cooler. The pumping power is modeled based on the microchannel design and fluid properties. The total cooling power, the sum of the electrical power to pump the liquid and the electrical power to drive the superlattice cooler, is found as a function of overall heat dissipation of the chip including hotspot(s). As the goal is to keep the hottest point on the chip below certain threshold (e.g. 85°C), the result shows a dramatic reduction of the required total cooling power, when hybrid cooling scheme - superlattice hotspot cooler in conjunction with microchannel cooler - is used. Above particular analysis is based on the specific microchannel, but this proposed scheme allows us a systematic study to reduce the pump power further.

Impact of Material Properties on Cooling COP of Integrated Thermoelectric Microcoolers

Thermoelectric (TE) microcooling is promising for removing hotspots in integrated circuit chips. The cooling coefficient-of-performance (COP) of the on-chip thin film or superlattice micro-cooler (SLC) is a metric for assessing the energy efficiency of the hot spot removal. The COP is key for lowering total power consumption and minimizing heat sinking requirements. Due to the moderate performance compared to vapor compression cycles, researchers have devoted considerable effort to improving the figure-of-merit (ZT) of the material over the past decade. However, the impact of each of the individual thermoelectric properties has not been studied separately. We report our study based on an analytical model and analysis results that show the intrinsic impact of electrical conductivity, the Seebeck coefficient, and thermal conductivity, while the device thickness and the drive current are optimized for maximizing cooling COP. The results show that the power factor of the TE materials is a more important parameter than thermal conductivity reduction for improving the cooling performance of the on chip SLC.Copyright

Phonon wave effects in the thermal transport of epitaxial TiN/(Al,Sc)N metal/semiconductor superlattices

Epitaxial single crystalline TiN/(Al,Sc)N metal/semiconductor superlattice metamaterials have generated significant interest in recent years for their potential applications in high temperature thermoelectric devices, optical hyperbolic metamaterials in the visible and near infrared-spectral range, and as candidates for solar-thermophotovoltaics and high temperature electronic materials. While significant progress in their structural, mechanical, and optical properties has been made, in-depth analysis and detailed understanding of their thermal transport mechanism remain to be addressed. In this article, we show that in short-period epitaxial, lattice-matched TiN/(Al,Sc)N metal/semiconductor superlattices, thermal transport is dominated by phonon wave effects as the wavelengths of phonons that carry significant amounts of heat become comparable to the superlattice period thickness. Due to the increasing contribution of such phonon wave-modes, the cross-plane thermal conductivity at short-periods increases...

Quasi-ballistic thermal transport in Al0.1Ga0.9N thin film semiconductors

We investigate thermal transport in high-quality Al0.1Ga0.9N thin films grown using plasma-assisted molecular beam epitaxy by time-domain thermoreflectance (TDTR) in the 100 K–500 K temperature range. The apparent thermal conductivity at 300 K and 500 K drops by 30% when the laser modulation frequency is increased from 0.8 MHz to 10 MHz. Tempered Levy analysis of the quasi-ballistic heat conduction reveals superdiffusion exponents α ≈ 1.70 ± 0.06 at room temperature and α ≈ 1.83 ± 0.16 at 500 K. We describe limitations in concurrent extraction of other model parameters and also discuss the impact of boundary scattering in the 100 K–200 K temperature range.