Jukka Tulkki

Behaviour of Raman resonance scattering across the K X-ray absorption edge

A generalisation of the Kramers-Heisenberg formula is used to demonstrate the evolution of inelastic X-ray scattering into fluorescence across the K absorption edge. As an illustration the results of a recent synchrotron radiation study of the inelastic scattering in the vicinity of the Mn K absorption edge in KMnO4 by Briand et al. (1981) are analysed and found to be in qualitative agreement with the theory.

Ultimate limit and temperature dependency of light-emitting diode efficiency

We discuss the ultimate limit of performance of semiconductor light-emitting diodes (LEDs) and its dependence on temperature. It is known that in high quality semiconductor materials it is, in principle, possible to reach wall plug efficiencies exceeding unity, which allows electroluminescent cooling in addition of very high efficiency light emission. Our simulation results suggest a few fairly simple measures that may further improve the external quantum efficiency (EQE) of LEDs toward the electroluminescent cooling limit. These include reducing the current density, modifying the LED structure by making thicker active regions and barrier layers, and doping of the active material. Our calculations also indicate that, contrary to the present understanding, operating LEDs at relatively high temperatures of 400–600 K may, in fact, improve the performance.

Self-organized InAs islands on (100) InP by metalorganic vapor-phase epitaxy

Abstract The effects of growth temperature, InAs deposition thickness and deposition rate on the areal density, size, uniformity and spatial distribution of self-organized InAs nanoscale islands grown on exact and vicinal (100) InP substrates by metalorganic vapor-phase epitaxy are investigated in detail by AFM. At 500°C, the island density is found to increase as the InAs deposition thickness is increased, while the average island size decreases slightly. At growth temperatures above 500°C, larger inhomogenous islands also appear. Decreasing the deposition rate increases the island density and substrate coverage. The unintentional As/P exchange is found to have a significant influence on island formation by producing excess material for the islands. Low-temperature photoluminescence from the recombination of carriers in the buried InAs islands is observed in the 1.4 – 1.8 μm spectral region.

The challenge of unity wall plug efficiency: The effects of internal heating on the efficiency of light emitting diodes

We develop a self-consistent model to describe the internal heating of high power light emitting diodes (LEDs) and use this model to simulate the operation of GaAs–AlGaAs double heterostructure LEDs. We account for the heating by nonradiative recombination processes in the simulations and solve self-consistently the steady state junction temperature. Based on the simulation results, we discuss the plausibility of unity conversion efficiency in LEDs and also the mechanisms underlying the efficiency droop. We show that the rise in the junction temperature limits the light output available from LEDs and further degrades the efficiency of operation at high operating currents. In addition to high power applications we study the optimal operating point and discuss the methods to increase the efficiency of LEDs toward the thermodynamical limits.

Frequency-dependent phonon mean free path in carbon nanotubes from nonequilibrium molecular dynamics

Owing to their long phonon mean free paths (MFPs) and high thermal conductivity, carbon nanotubes (CNTs) are ideal candidates for, e.g., removing heat from electronic devices. It is unknown, however, how the intrinsic phonon MFPs depend on vibrational frequency in nonequilibrium. We determine the spectrally resolved phonon MFPs in isotopically pure CNTs from the spectral phonon transmission function calculated using nonequilibrium molecular dynamics, fully accounting for the resistive phonon-phonon scattering processes through the anharmonic terms of the interatomic potential energy function. Our results show that the effective room temperature MFPs of low-frequency phonons (

Polarization doping and the efficiency of III-nitride optoelectronic devices

fl0.5

On crosstalk and noise in an optical amplifier with gain clamping by vertical laser field

THz) exceed

Diffusion injected multi-quantum well light-emitting diode structure

10\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}

Photon mass drag and the momentum of light in a medium

, while the MFP of high-frequency phonons (

Quantum theory of post-collision interaction in inner-shell photoionization

f\ensuremath{\gtrsim}20