Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof-of-principle studies on quantum-well superlattice and quantum-wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/AdvMater_19_1043_2007.pdf

New Directions for Low-Dimensional Thermoelectric Materials**

A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

I. Introduction (Preface, Nanostructures in Si Inversion Layers, Nanostructures in GaAs-AlGaAs Heterostructures, Basic Properties). 
II. Diffusive and Quasi-Ballistic Transport (Classical Size Effects, Weak Localization, Conductance Fluctuations, Aharonov-Bohm Effect, Electron-Electron Interactions, Quantum Size Effects, Periodic Potential). 
III. Ballistic Transport (Conduction as a Transmission Problem, Quantum Point Contacts, Coherent Electron Focusing, Collimation, Junction Scattering, Tunneling). 
IV. Adiabatic Transport (Edge Channels and the Quantum Hall Effect, Selective Population and Detection of Edge Channels, Fractional Quantum Hall Effect, Aharonov-Bohm Effect in Strong Magnetic Fields, Magnetically Induced Band Structure).

/pdf/quantum-transport-in-semiconductor-nanostructures-1n3dkljf6x.pdf

Quantum Transport in Semiconductor Nanostructures

https://hal.archives-ouvertes.fr/hal-00120432/document

Raman Spectroscopy of Nanomaterials: How Spectra Relate to Disorder, Particle Size and Mechanical Properties

Publisher Summary Quantum transport is conveniently studied in a two-dimensional electron gas (2DEG) because of the combination of a large Fermi wavelength and large mean free path. Semiconductor nanostructures are unique in offering the possibility of studying quantum transport in an artificial potential landscape. This is the regime of ballistic transport in which scattering with impurities are neglected. The chapter presents a self-contained account of these three novel transport regimes in semiconductor nanostructures. The study of quantum transport in semiconductor nanostructures is motivated by more than scientific interest. The fabrication of nanostructures relies on sophisticated crystal growth and lithographic techniques that exist because of the industrial effort toward the miniaturization of transistors. Conventional transistors operate in the regime of classical diffusive transport, which breaks down on short length scales. The discovery of novel transport regimes in semiconductor nanostructures provides options for the development of innovative future devices.

/pdf/quantum-transport-in-semiconductor-nanostructures-3vpunw3uta.pdf

The lattice vibration of black phosphorus was investigated under hydrostatic high pressure by polarized Raman scattering. On applying pressure the crystal structure changes to the rhombohedral A7-structure from the layered orthorhombic structure at 52 kbar. At atmospheric pressure four phonon modes are observed at 362 cm -1 with A g -symmetry, 467 cm -1 A g , 194 cm -1 B 1 g and 439 cm -1 B 2 g . The Gruneisen parameters are 0.74 for the 362 cm -1 A g -mode, 0.10 for the 467 cm -1 A g -mode and 0.22 for the 439 cm -1 B 2 g -mode. The energies of the latter two modes change little with pressure, since in these modes the atoms displace to deform the rigid zigzag chain along the a -axis. In the A7-structure the A 1 g -mode was observed at 466 cm -1 and the E g -mode at 392 cm -1 . The comparison with the phonon modes in other group V materials was made.

Pressure Dependence of the Lattice Vibration in the Orthorhombic and Rhombohedral Structures of Black Phosphorus

GaAs/Al0.3Ga0.7As quantum wells (QWs) grown on (411)A-oriented GaAs substrates by molecular beam epitaxy (MBE) showed extremely flat interfaces over a macroscopic area (about 200 µm ) even for the case of no growth interruption, which is mainly due to the intrinsically large migration of Ga atoms and layer growth in the step-flow mode on the (411)A plane. Photoluminescence linewidths at 4.2K were almost the same as or better than the narrowest linewidths reported for GaAs/AlGaAs and GaAs/AlAs QWs grown on GaAs (100) substrates with growth interruption at each GaAs/AlGaAs(AlAs) interface. Only one sharp luminescence peak was observed for each QW on the (411)A substrates, in contrast with three luminescence peaks for the QWs on the (100) substrates, indicating that extremely flat and uniform interfaces over a macroscopic area of laser excitation (200 µm diameter) are realized in the GaAs/AlGaAs QWs grown on (411)A GaAs substrates.

https://iopscience.iop.org/article/10.1143/JJAP.32.L1728/pdf

Extremely Flat Interfaces in GaAs/AlGaAs Quantum Wells Grown on GaAs (411)A Substrates by Molecular Beam Epitaxy

The insistent two mode type behavior of the A 1 and high frequency F 2 bands in the tin contained glasses is reproduced by our vibrational calculations based on the outrigger-raft with some modifications High pressure Raman spectra suggest an instability of the tin tetrahedral bonding Comparative study of glassy and crystalline Ge(Se, or S) 2 by high pressure Raman scattering gives important informations about the strain field and the topological order in the glasses

Investigation of stability of (Ge, Sn)-(S, or Se)42 cluster vibrational spectra

It is shown that the well-known Erginsoy's formula for scattering of electrons by neutral impurity is not valid for the electron scattering by neutralized acceptors in germanium or silicon. An alternative formula based on the positron-hydrogen scattering picture is presented and compared with the cyclotron resonance observation of the electron scattering by gallium and indium atoms in germanium. From the temperature dependence of the scattering a unique determination of the effective Bohr radius of the neutral acceptor wave function is possible and the values of a ,* ca. 35A for gallium and 55A for indium have been obtained. In addition, a promising possibility of observing electron cyclotron resonance in highly doped p-type materials is pointed out, which is due to the smaller scattering cross section for acceptors than that for donors.

Electron Scattering by Neutralized Acceptors in Germanium : I.Gallium and Indium

GaAs/AlGaAs quantum wells (QWs) were grown on (411)A‐oriented GaAs substrates by molecular beam epitaxy (MBE). Photoluminescence linewidths at 4.2 K are almost the same as the narrowest linewidths reported so far for GaAs/AlGaAs QWs grown on (100)‐oriented GaAs substrates with the growth interruption at the heterointerfaces. Furthermore, only one sharp peak was observed for each QW on the (411) substrate over the whole area of the wafer (10 mm×10 mm), in contrast with three splitted luminescence peaks for one kind of GaAs/AlGaAs QW grown on the (100) substrates by MBE with growth interruption. This result implies that effectively atomically flat interfaces over a macroscopic area (about 10 mm×10 mm) has been realized for the first time in GaAs/Al0.3Ga0.7As QWs grown on (411)A GaAs substrates by MBE. This is possibly due to the large migration of Ga and Al atoms on the (411)A plane during MBE growth and the step‐flow growth mode on the atomically corrugated (411)A plane.

Kazuo Murase

Papers

Pressure Dependence of the Lattice Vibration in the Orthorhombic and Rhombohedral Structures of Black Phosphorus

Extremely Flat Interfaces in GaAs/AlGaAs Quantum Wells Grown on GaAs (411)A Substrates by Molecular Beam Epitaxy

Investigation of stability of (Ge, Sn)-(S, or Se)42 cluster vibrational spectra

Electron Scattering by Neutralized Acceptors in Germanium : I.Gallium and Indium

Extremely high uniformity of interfaces in GaAs/AlGaAs quantum wells grown on (411)A GaAs substrates by molecular beam epitaxy