Wei Liu

Bio: Wei Liu is an academic researcher from École Polytechnique Fédérale de Lausanne. The author has contributed to research in topics: Quantum dot & Cavity quantum electrodynamics. The author has an hindex of 8, co-authored 22 publications receiving 432 citations. Previous affiliations of Wei Liu include University of California, Los Angeles & Xiamen University.

Measurements of anisotropic thermoelectric properties in superlattices

Abstract: Thermoelectric properties, i.e., thermal conductivity, electrical conductivity, and the Seebeck coefficient, have been measured in the directions parallel (in-plane) and perpendicular to the interface of an n-type Si(80 A)/Ge(20 A) superlattice. A two-wire 3ω method is employed to measure the in-plane and cross-plane thermal conductivities. The cross-plane Seebeck coefficient is deduced by using a differential measurement between the superlattice and reference samples and the cross-plane electrical conductivity is determined through a modified transmission-line method. The in-plane thermal conductivity of the Si/Ge superlattice is 5–6 times higher than the cross-plane one, and the electrical conductivity shows a similar anisotropy. The anisotropy of the Seebeck coefficients is smaller in comparison to electrical and thermal conductivities in the temperature range from 150 to 300 K. However, the cross-plane Seebeck coefficient rises faster with increasing temperature than that of the in-plane direction.

GaN surface as the source of non-radiative defects in InGaN/GaN quantum wells

Abstract: Blue light-emitting diodes based on III-nitride semiconductors are nowadays widely used for solid-state lighting They exhibit impressive figures of merit like an internal quantum efficiency close to 100% This value is intriguing when considering the high dislocation density running throughout the InGaN/GaN quantum well (QW) active region This striking feature is currently ascribed to carrier localization occurring in the InGaN alloy, which hinders their diffusion toward dislocations However, it was recently reported that another source of defects, disconnected from dislocations, dramatically decreases the radiative efficiency of InGaN/GaN QWs Those defects, present at the surface, are usually trapped in an InGaN underlayer (UL), which is grown before the QW active region To get insight into the trapping mechanism, we varied the UL thickness, In content, and materials system (InGaN or InAlN) and studied the photoluminescence decay time at 300 K of a single InGaN/GaN QW Our data demonstrate that defects are incorporated proportionally to the indium content in the UL In addition, we show that those defects are created during the high-temperature growth of GaN and that they segregate at the surface even at low-temperature Eventually, we propose an intrinsic origin for these surface defects

Exciton dynamics at a single dislocation in GaN probed by picosecond time-resolved cathodoluminescence

Abstract: We investigate the dynamics of donor bound excitons (D°XA) at T = 10 K around an isolated single edge dislocation in homoepitaxial GaN, using a picosecond time-resolved cathodoluminescence (TR-CL) setup with high temporal and spatial resolutions. An ∼ 1.3 meV dipole-like energy shift of D°XA is observed around the dislocation, induced by the local strain fields. By simultaneously recording the variations of both the exciton lifetime and the CL intensity across the dislocation, we directly assess the dynamics of excitons around the defect. Our observations are well reproduced by a diffusion model. It allows us to deduce an exciton diffusion length of ∼24 nm as well as an effective area of the dislocation with a radius of ∼95 nm, where the recombination can be regarded as entirely non-radiative.

Cross-plane thermal conductivity of self-assembled Ge quantum dot superlattices

Abstract: We report the temperature dependent cross-plane thermal conductivities of Ge quantum dot superlattices measured by the 3\ensuremath{\omega} method. A large reduction in the thermal conductivity of the superlattices compared with that of bulk materials is observed. A simple model taking into account the relaxation time approximation, including phonon scattering on quantum dots, well explains the experimental data.

Carrier-density-dependent recombination dynamics of excitons and electron-hole plasma in m -plane InGaN/GaN quantum wells

Abstract: We study the carrier-density-dependent recombination dynamics in $m$-plane InGaN/GaN multiple quantum wells in the presence of $n$-type background doping by time-resolved photoluminescence. Based on Fermi's golden rule and Saha's equation, we decompose the radiative recombination channel into an excitonic and an electron-hole pair contribution, and extract the injected carrier-density-dependent bimolecular recombination coefficients. Contrary to the standard electron-hole picture, our results confirm the strong influence of excitons even at room temperature. Indeed, at 300 K, excitons represent up to 63 \ifmmode\pm\else\textpm\fi{} 6% of the photoexcited carriers. In addition, following the Shockley-Read-Hall model, we extract the electron and hole capture rates by deep levels and demonstrate that the increase in the effective lifetime with injected carrier density is due to asymmetric capture rates in presence of an $n$-type background doping. Thanks to the proper determination of the density-dependent recombination coefficients up to high injection densities, our method provides a way to evaluate the importance of Auger recombination.

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

Abstract: Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.

Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy

Abstract: The dimensionless thermoelectric figure of merit (ZT) of the n-type silicon germanium (SiGe) bulk alloy at high temperature has remained at about one for a few decades. Here we report that by using a nanostructure approach, a peak ZT of about 1.3 at 900 °C in an n-type nanostructured SiGe bulk alloy has been achieved. The enhancement of ZT comes mainly from a significant reduction in the thermal conductivity caused by the enhanced phonon scattering off the increased density of nanograin boundaries. The enhanced ZT will make such materials attractive in many applications such as solar, thermal, and waste heat conversion into electricity.

Highly Thermally Conductive Papers with Percolative Layered Boron Nitride Nanosheets

Abstract: In this work, we report a dielectric nanocomposite paper with layered boron nitride (BN) nanosheets wired by one-dimensional (1D) nanofibrillated cellulose (NFC) that has superior thermal and mechanical properties. These nanocomposite papers are fabricated from a filtration of BN and NFC suspensions, in which NFC is used as a stabilizer to stabilize BN nanosheets. In these nanocomposite papers, two-dimensional (2D) nanosheets form a thermally conductive network, while 1D NFC provides mechanical strength. A high thermal conductivity has been achieved along the BN paper surface (up to 145.7 W/m K for 50 wt % of BN), which is an order of magnitude higher than that in randomly distributed BN nanosheet composites and is even comparable to the thermal conductivity of aluminum alloys. Such a high thermal conductivity is mainly attributed to the structural alignment within the BN nanosheet papers; the effects of the interfacial thermal contact resistance are minimized by the fact that the heat transfer is in the ...

Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers

Abstract: Tailoring the thermal conductivity of nanostructured materials is a fundamental challenge for nano- and microelectronics heat management It is now demonstrated how to modify the thermal conductivity of SiGe by engineering nanodot inclusions in regions as short as 15 nm A similar approach could used on other materials, extending the range of thermal conductivities available