Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

This review provides numerical and graphical information about many (but by no means all) of the physical and electronic properties of GaAs that are useful to those engaged in experimental research and development on this material. The emphasis is on properties of GaAs itself, and the host of effects associated with the presence of specific impurities and defects is excluded from coverage. The geometry of the sphalerite lattice and of the first Brillouin zone of reciprocal space are used to pave the way for material concerning elastic moduli, speeds of sound, and phonon dispersion curves. A section on thermal properties includes material on the phase diagram and liquidus curve, thermal expansion coefficient as a function of temperature, specific heat and equivalent Debye temperature behavior, and thermal conduction. The discussion of optical properties focusses on dispersion of the dielectric constant from low frequencies [κ0(300)=12.85] through the reststrahlen range to the intrinsic edge, and on the ass...

Semiconducting and other major properties of gallium arsenide

Six years after their birth, terahertz quantum-cascade lasers can now deliver milliwatts or more of continuous-wave coherent radiation throughout the terahertz range — the spectral regime between millimetre and infrared wavelengths, which has long resisted development. This paper reviews the state-of-the-art and future prospects for these lasers, including efforts to increase their operating temperatures, deliver higher output powers and emit longer wavelengths.

Terahertz quantum-cascade lasers

We review recent advances in laser cell surgery, and investigate the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80-MHz repetition rate and with amplified pulses of 1-kHz repetition rate. Plasma formation in water, the evolution of the temperature distribution, thermoelastic stress generation, and stress-induced bubble formation are numerically simulated for NA=1.3, and the outcome is compared to experimental results. Mechanisms and the spatial resolution of femtosecond laser surgery are then compared to the features of continuous-wave (cw) microbeams. We find that free electrons are produced in a fairly large irradiance range below the optical breakdown threshold, with a deterministic relationship between free-electron density and irradiance. This provides a large ‘tuning range’ for the creation of spatially extremely confined chemical, thermal, and mechanical effects via free-electron generation. Dissection at 80-MHz repetition rate is performed in the low-density plasma regime at pulse energies well below the optical breakdown threshold and only slightly higher than used for nonlinear imaging. It is mediated by free-electron-induced chemical decomposition (bond breaking) in conjunction with multiphoton-induced chemistry, and hardly related to heating or thermoelastic stresses. When the energy is raised, accumulative heating occurs and long-lasting bubbles are produced by tissue dissociation into volatile fragments, which is usually unwanted. By contrast, dissection at 1-kHz repetition rate is performed using more than 10-fold larger pulse energies and relies on thermoelastically induced formation of minute transient cavities with lifetimes <100 ns. Both modes of femtosecond laser nanoprocessing can achieve a 2–3 fold better precision than cell surgery using cw irradiation, and enable manipulation at arbitrary locations.

Mechanisms of femtosecond laser nanosurgery of cells and tissues

Part 1 Band structure of semiconductors: the crystal Hamiltonian adiabatic approximation phonons the one-electron approximation Bloch functions nearly-free-electron model energy gaps spin-orbit coupling and orbital characteristics band structures chemical trends k.p perturbation and effective mass temperature dependence of energy gaps deformation potentials alloys. Part 2 Energy levels: the effective-mass approximation electron dynamics Zener-Bloch oscillations Landau levels plasma oscillations excitons hydrogenic impurities hydrogen molecule centres core effects deep-level impurities scattering states impurity bands. Part 3 Lattice scattering: general features energy and momentum conservation acoustic phonon scattering optical phonon scattering polar optical mode scattering piezoelectric scattering scattering-induced electron mass mobilities appendix - acoustic waves in the diamond lattice. Part 4 Impurity scattering: general features charged-impurity scattering neutral-impurity scattering central-cell contribution to charged-impurity scattering dipole scattering electron-hole scattering electron-electron scattering mobilities appendix - Debye screening length, average separation of impurities, alloy scattering. Part 5 Radiative transitions: transition rate photo-ionization and radiative capture cross-section wavefunctions direct interband transitions photo-deionization of a hydrogenic acceptor photo-ionization of a hydrogenic donor photo-ionization of quantum-defect impurities photo-ionization of deep-level impurities summary of photo-ionization cross-sections indirect transitions indirect interband transitions free-carrier absorption free-carrier scattering of light appendix - justification of effective-mass approximation in light scattering. Part 6 Non-radiative processes: electron-lattice coupling the configuration-co-ordinate diagram semi-classical thermal generation rate thermal broadening of radiative transitions thermal generation and capture rates electron-lattice coupling strengths selection rules for phonon-impurity coupling phonon-cascade capture the Auger effect impact ionization appendix - the multiphonon matrix element. Part 7 Quantum processes in a magnetic field: introduction collision-free situation collision-induced current scattering mechanisms transverse Shubnikov-de Haas oscillations longitudinal Shubnikov-de Haas oscillations magnetophenon oscillations. Part 8 Scattering in a degenerate gas: general equations elastic collisions acoustic phonon scattering energy relaxation time. Part 9 Dynamic screening: introduction polar optical modes plasma modes coupled modes the Lindhard dielectric function fluctuations screening regimes. Author index. Subject index.

Quantum Processes in Semiconductors

The inherent instability associated with a specific differential negative resistance in a solid is discussed for the two cases voltage-controlled and current-controlled differential negative resistance. In the case of voltage-controlled differential negative resistance, it is shown that domains of high electric field occur. These domains are generally mobile and their movement is discussed. In the case, of current-controlled differential negative resistance, high current filaments form. The sizes of the domains and filaments are governed by the size of the specimen and by the principle of least entropy production. The effect of an external circuit is to inhibit stable filament formation in the case of current-controlled differential negative resistance and impose conditions under which stable domain formation can be observed in the case of voltage-controlled differential negative resistance.

https://iopscience.iop.org/article/10.1088/0370-1328/82/6/315/pdf

Specific Negative Resistance in Solids

Approximate analytic expressions are obtained for the scattering rates and momentum-relaxation rates of an electron in a quasi-two-dimensional quantum well interacting with acoustic, optical and intervalley modes via the deformation potential and with longitudinal optical modes via the polar interaction. These analytic expressions are obtained using a momentum-conservation approximation. The threshold for optical phonon emission, unlike the case in the bulk, is abrupt. All scattering rates are energy-independent and are inversely proportional to L, the thickness of the well. The momentum-relaxation rate associated with the absorption of polar optical phonons, on the other hand, proves to be proportional to L. These properties are shown to lead to a negative differential resistance for pure polar mode scattering, and to the existence of a runaway field for deformation-potential scattering. The self-energy associated with the emission of polar optical phonons at absolute zero is shown to be divergent unless the polar interaction is screened, and some consequences of this for laser and other optical processes are pointed out. The description of scattering by perturbation theory breaks down in very narrow wells.

https://iopscience.iop.org/article/10.1088/0022-3719/15/28/021/pdf

The electron-phonon interaction in quasi-two-dimensional semiconductor quantum-well structures

The possibility of obtaining negative resistance effects in a new way in semiconductors is discussed. The principle of the method is to heat carriers in a high mobility sub-band with an electric field so that they transfer when they have a high enough `temperature' to a higher energy low mobility sub-band. The conditions required for negative resistance are discussed generally and more specific conditions are obtained for some simple cases of spherical and ellipsoidal bands by solving the Boltzmann equation. It is shown that the most favourable case is when the sub-bands are sufficiently separated in energy for the emission of optical phonons to be the dominant mechanism for energy relaxation in both sub-bands. Ge-Si alloys and some III-V compounds may have suitable sub-band structures in the conduction bands. The case of p-type uniaxially strained silicon appears to be marginal in the region where the current is proportional to the square root of the electric field. The electrical instability of a crystal with a differential negative resistance is briefly discussed and it is pointed out that some sort of `electrical domain' formation may establish itself and inhibit the observation of negative resistance. Side effects which can influence the condition for negative resistance such as specimen heating, which is advantageous, and impact ionization, which is deleterious, are also discussed.

https://iopscience.iop.org/article/10.1088/0370-1328/78/2/315/pdf

The Possibility of Negative Resistance Effects in Semiconductors

A simple model is used to highlight the main features of electron scattering in a quantum well via the Fr\"ohlich interaction with confined LO polar optical phonon modes. It is shown that the scalar potential field associated with strongly confined modes is different from that assumed by other authors, and consequently quite different results are obtained. One of the possible interface modes is shown to be mainly responsible for intrasubband scattering in thin wells, and as a result the scattering rate is proportional to the density of states and this increases with diminishing well width.

Brian K. Ridley

Papers

Quantum Processes in Semiconductors

Specific Negative Resistance in Solids

The electron-phonon interaction in quasi-two-dimensional semiconductor quantum-well structures

The Possibility of Negative Resistance Effects in Semiconductors

Electron scattering by confined LO polar phonons in a quantum well.