Manuel Cardona

Bio: Manuel Cardona is an academic researcher from Max Planck Society. The author has contributed to research in topics: Raman spectroscopy & Raman scattering. The author has an hindex of 86, co-authored 979 publications receiving 33140 citations. Previous affiliations of Manuel Cardona include National Technical University.

Infrared Spectrum and Structure of Hydrogenated Amorphous Silicon

Abstract: The ir absorption spectrum of hydrogenated amorphous silicon is discussed in the context of structural models for this material. It is shown that the integrated strength of the bond stretching bands in hydrogenated amorphous silicon cannot be used to determine the hydrogen concentration because the local effective charge for ir absorption is a function of hydrogen concentration and sample preparation. The effective charge for the wagging—rocking—rolling vibrations at 640 cm−1, however, is independent of hydrogen concentration and sample preparation. Hence the integrated strength of this band can be used to measure the hydrogen concentration provided the proportionality constant is determined empirically. Changes in the ir absorption bands as a function of isochronal annealing temperature show that two different types of bonds contribute to the bond stretching band at 2100 cm−1. The bond bending bands at 890 and 840 cm−1 are associated with one of the bonds producing the 2100 cm−1 stretching band. The relative strengths of the 890 and 840 cm−1 bands to the 2100 cm−1 band depend strongly on the film preparation method: the glow discharge produced films show much stronger bending bands than those produced by rf sputtering. A structural model for hydrogenated amorphous silicon is presented in which a Maxwell-Garnett-type local field correction, which is dependent on the dipole location within a microvoid and the shape of the microvoid, is used to explain the data. Das IR-Absorptionsspektrum von hydrogenisiertem, amorphem Silizium wird im Zusammenhang mit Strukturmodellen fur dieses Material untersucht. Es wird gezeigt, das die integrale Starke der ‚bond-stretching’-Banden in hydrogenisiertem, amorphem Silizium nicht fur die Bestimmung der Wasserstoffkonzentration benutzt werden kann, weil die lokale effektive Ladung fur die IR-Absorption eine Funktion der Wasserstoffkonzentration und Probenpraparation ist. Die effektive Ladung fur die ‚wagging—rocking—rolling’-Schwingungen bei 640 cm−1 ist jedoch unabhangig von der Wasserstoffkonzentration und Probenpraparation. Somit kann die integrale Starke dieser Bande benutzt werden, um die Wasserstoffkonzentration zu messen, vorausgesetzt, das die Proportionalitatskonstante empirisch bestimmt wird. Anderungen in den IR-Absorptionsbanden in Abhangigkeit von der isochronen Temperungstemperatur zeigen, das zwei unterschiedliche Bindungsarten zur ‚bond-stretching’-Bande bei 2100 cm−1 beitragen. Die ‚bond-bending’-Banden bei 890 und 840 cm−1 sind mit einer der Bindungen verknupft, die die 2100 cm−1-Stretching-Bande hervorrufen. Die relativen Starken der 890- und 840-cm−1-Banden zur 2100 cm−1-Bande hangen stark von der Schichtherstellungsmethode ab: die durch Glimmentladung hergestellten Schichten zeigen viel starkere Bindungsbanden als die durch HF-Sputtern hergestellten. Ein Strukturmodell fur hydrogenisiertes, amorphes Silizium wird angegeben, in dem eine lokale Feld-korrektur vom Maxwell-Garnett-Typ benutzt wird, die von der Dipollokalisierung im Innern eines Mikrohohlraumes und der Form des Mikrohohlraums abhangt, um die Werte zu erklaren.

Infrared and Raman spectra of the IV-VI compounds SnS and SnSe

Abstract: The results of Raman scattering and infrared reflectivity measurements on the IV-VI layer-type semiconductors SnS and SnSe are presented. The infrared-active TO, the associated LO-phonon frequencies, and the dielectric constants for all three principal polarizations have been determined from a Kramers-Kronig analysis of the reflectivity data. The symmetries of the zone-center phonons observed in the different polarization configurations are in agreement with the group-theoretical analysis of the ${D}_{2h}^{16}$ space group of these compounds. Despite the center of inversion symmetry in this structure, some infrared- and Raman-active modes are found to be nearly degenerate, suggesting the importance of the layerlike character in these compounds as in the isomorphic GeS and GeSe. A comparison of the phonon frequencies of the corresponding modes in the spectra of SnS and SnSe, or GeS and GeSe, indicates that the frequencies vary as a power (-2.2) of the lattice constant.

Thermal-conductivity measurements of GaAs/AlAs superlattices using a picosecond optical pump-and-probe technique

Abstract: We present measurements of the lattice thermal conductivity ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ normal to the interfaces of $(\mathrm{GaAs}{)}_{n}/(\mathrm{AlAs}{)}_{n}$ superlattices with n between 1 and 40 monolayers. The conductivity was measured by an optical pump-and-probe technique in the temperature range of 100 to 375 K. In the experiment, an Al film is deposited onto a superlattice sample, and the rate at which this film cools by conduction into the superlattice is determined. We find a general decrease in ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ with a reduction of the superlattice period. At 300 K, ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ of the $(\mathrm{GaAs}{)}_{40}/(\mathrm{AlAs}{)}_{40}$ superlattice is approximately three times less than $\ensuremath{\kappa}$ of bulk GaAs, and ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ of the $(\mathrm{GaAs}{)}_{1}/(\mathrm{AlAs}{)}_{1}$ superlattice is an order of magnitude less than $\ensuremath{\kappa}$ of bulk GaAs. We discuss the decrease in ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ compared to the bulk constituents in terms of extrinsic and intrinsic scattering mechanisms.

Pressure dependence of the lattice dynamics of ZnO: An ab initio approach

Abstract: We have performed first-principles calculations of the electronic structure of ZnO, and applied them to the determination of structural and lattice-dynamical properties and their dependence on pressure. The dynamical matrices have been obtained for the wurtzite, zinc-blende, and rocksalt modifications with several lattice parameters optimized for pressures up to 12 GPa. These matrices are employed to calculate the one-phonon densities of states ~DOS! and the two-phonon DOS associated with either sums or differences of phonons. These results provide the essential tools to analyze the effect of isotope-induced mass disorder and anharmonicity on phonon linewidths, which we discuss here and compare with experimental data from Raman spectroscopy, including first- and second-order spectra. Agreement of calculated properties with experimental results improves considerably when the renormalization due to anharmonicity is subtracted from the experimental data.

Isotope effects on the optical spectra of semiconductors

Abstract: Since the end of the cold war, macroscopic amounts of separated stable isotopes of most elements have been available ``off the shelf'' at affordable prices. Using these materials, single crystals of many semiconductors have been grown and the dependence of their physical properties on isotopic composition has been investigated. The most conspicuous effects observed have to do with the dependence of phonon frequencies and linewidths on isotopic composition. These affect the electronic properties of solids through the mechanism of electron-phonon interaction, in particular, in the corresponding optical excitation spectra and energy gaps. This review contains a brief introduction to the history, availability, and characterization of stable isotopes, including their many applications in science and technology. It is followed by a concise discussion of the effects of isotopic composition on the vibrational spectra, including the influence of average isotopic masses and isotopic disorder on the phonons. The final sections deal with the effects of electron-phonon interaction on energy gaps, the concomitant effects on the luminescence spectra of free and bound excitons, with particular emphasis on silicon, and the effects of isotopic composition of the host material on the optical transitions between the bound states of hydrogenic impurities.

Spintronics: Fundamentals and applications

Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

Phonons and related crystal properties from density-functional perturbation theory

Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.

Band parameters for III–V compound semiconductors and their alloys

Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Thermal properties of graphene and nanostructured carbon materials

Abstract: Recent years have seen a rapid growth of interest by the scientific and engineering communities in the thermal properties of materials. Heat removal has become a crucial issue for continuing progress in the electronic industry, and thermal conduction in low-dimensional structures has revealed truly intriguing features. Carbon allotropes and their derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range--of over five orders of magnitude--from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. Here, I review the thermal properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. Special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe the prospects of applications of graphene and carbon materials for thermal management of electronics.

Enhanced thermoelectric performance of rough silicon nanowires

Abstract: Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30-40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2-4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20-300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.