Anders Blom

Uncertainty of oscillator strengths derived from lifetimes and branching fractions

Abstract A widely used method for determination of transition probabilities and oscillator strengths is based on measurements of branching fractions and radiative lifetimes. In the present work the different sources of uncertainty in branching fraction measurements using Fourier transform spectroscopy and lifetime measurements using laser induced fluorescence are discussed. A detailed description is presented of how the uncertainties should be combined to provide a well-defined uncertainty of the derived quantity. Finally, an example shows how the individual uncertainties can be presented in an “uncertainty budget”.

Atomic-scale model for the contact resistance of the nickel-graphene interface

We perform first-principles calculations of electron transport across a nickel-graphene interface. Four different geometries are considered, where the contact area, graphene and nickel surface orientations and the passivation of the terminating graphene edge are varied. We find covalent bond formation between the graphene layer and the nickel surface, in agreement with other theoretical studies. We calculate the energy-dependent electron transmission for the four systems and find that the systems have very similar edge contact resistance, independent of the contact area between nickel and graphene, and in excellent agreement with recent experimental data. A simple model where graphene is bonded with a metal surface shows that the results are generic for covalently bonded graphene, and the minimum attainable edge contact resistance is twice the ideal edge quantum contact resistance of graphene.

Mechanism of terahertz lasing in SiGe/Si quantum wells

Intense terahertz (THz) stimulated emission from boron-doped SiGe/Si quantum well structures with internal strain has been observed recently. We present a theoretical calculation which shows the formation of resonant states, and explains the origin of the observed temperature dependence of the dc conductivity under low bias voltage. Thus, the mechanism of THz lasing is population inversion of the resonant state with respect to the localized impurity states. This is the same mechanism of lasing as in uniaxially stressed p-Ge THz lasers.

Donor states in modulation-doped Si/SiGe heterostructures

We present a unified approach for calculating the properties of shallow donors inside or outside heterostructure quantum wells. The method allows us to obtain not only the binding energies of all localized states of any symmetry, but also the energy width of the resonant states which may appear when a localized state becomes degenerate with the continuous quantum well subbands. The approach is nonvariational, and we are therefore also able to evaluate the wave functions. This is used to calculate the optical absorption spectrum, which is strongly nonisotropic due to the selection rules. The results obtained from calculations for Si/Si1-xGex quantum wells allow us to present the general behavior of the impurity states, as the donor position is varied from the center of the well to deep inside the barrier. The influence on the donor ground state from both the central-cell effect and the strain arising from the lattice mismatch is carefully considered. (Less)

Parametrization of the Zeeman effect for hydrogen-like spectra in high-temperature plasmas

We present a method for parametrizing the Zeeman effect in hydrogen-like systems in high-temperature plasmas, where the fine-structure is completely unresolved. The method is based on the observation that the different polarization components behave collectively like separate entities, with simple relations. The entire Zeeman pattern can then be reduced to just three components, whose dependence on the magnetic field and the temperature can be described by only three numerical parameters. This makes it possible to include the influence of the Zeeman effect directly into ion temperature diagnostic procedures with minimal increase in the required computational effort and without the need for pre-calculated correction factors. We have tabulated such parametrizations-which are accurate for a wide range of fields and temperatures, even for cases when the total line-shape is no longer Gaussian-for 44 commonly studied hydrogen-like transitions. The effects of non-statistical population distribution in the upper sub-levels are briefly discussed, and we also note a temperature-dependent wavelength shift of the centre positions of the transitions.

Atomistic simulation of a III-V p-i-n junction: Comparison of density functional and tight-binding approaches

We compare the results of calculations based on tight-binding and density functional theory (DFT) for the description of an ultra-narrow two-dimensional (2D) InAs system. We first investigate the electronic structure of the 2D system to understand the effect of different surface terminations and how they are modeled using tight-binding and DFT approaches. We next set up a gated 2D InAs p-i-n junction and calculate the transistor characteristics of the system using the two different approaches.

Towards realistic atomic-scale modeling of nanoscale devices

On the nanoscale, electrical currents behave radically different compared to on the microscale. As the active regions become comparable to or smaller than the mean-free path of the material, it becomes necessary to describe the electron transport by quantum-mechanical methods instead of using classical relations like Ohms law. Over the past decade, methods for computing electron tunneling currents in nanosized junctions have evolved steadily, and are now approaching a sophistication where they can provide real assistance in the development of novel semiconductor materials and devices. At the same time, the industrys demand for such solutions is rising rapidly to meet the challenges both above and under the 16 nm node. In this paper we provide an overview of the current state-of-the-art of the field of how to model electrical currents on the nanoscale, using atomic-scale simulations.

First-principles simulations of nanoscale transistors

We describe the transport characteristics of a 50 nm (gate length) 2D InAs tunnel field-effect n-i-n transistor in a double-gate fin-like geometry (fin width 2.3 nm) by means of atomic-scale simulations. In particular, we compare results from density functional theory (DFT) using the Meta-GGA exchange correlation potential with those from a tight-binding Hamiltonian. For the first time we show that the two methods give comparable results, proving the predictive power of atomic-scale simulations for this type of devices, and that it is possible to accurately study realistic ultrascaled devices with first-principles methods.

THE MONTE CARLO METHOD APPLIED TO CARRIER TRANSPORT IN Si/SiGe QUANTUM WELLS

We present a complete Monte Carlo simulation of the transport properties of a Si/SiGe quantum well. The scattering mechanisms, viz. intervalley phonons, acoustic phonons, interface roughness and impurity scattering (including resonant scattering), are considered in detail, and we derive analytic expressions for the scattering rates, in each case properly taking the quantized electron wave functions into account. The numerically obtained distribution function is used to discuss the influence of each scattering mechanism for different electric fields applied parallel to the interfaces and also different temperatures.

Configuration interaction applied to resonant states in semiconductors and semiconductor nanostructures

A new method for calculating the parameters of resonant states as well as the probability of resonant scattering, capture and emission is developed. It is based on the configuration interaction method, which has been first introduced by Fano in the problem of autoionization of He. The method has been applied to resonant states induced by (i) defects in the barrier of GaAs/GaAlAs quantum well structure and (ii) acceptors in Ge under external stress.