As a result of quantum-confinement effects, the emission colour of semiconductor nanocrystals can be modified dramatically by simply changing their size1,2. Such spectral tunability, together with large photoluminescence quantum yields and high photostability, make nanocrystals attractive for use in a variety of light-emitting technologies—for example, displays, fluorescence tagging3, solid-state lighting and lasers4. An important limitation for such applications, however, is the difficulty of achieving electrical pumping, largely due to the presence of an insulating organic capping layer on the nanocrystals. Here, we describe an approach for indirect injection of electron–hole pairs (the electron–hole radiative recombination gives rise to light emission) into nanocrystals by non-contact, non-radiative energy transfer from a proximal quantum well that can in principle be pumped either electrically or optically. Our theoretical and experimental results indicate that this transfer is fast enough to compete with electron–hole recombination in the quantum well, and results in greater than 50 per cent energy-transfer efficiencies in the tested structures. Furthermore, the measured energy-transfer rates are sufficiently large to provide pumping in the stimulated emission regime, indicating the feasibility of nanocrystal-based optical amplifiers and lasers based on this approach.

Energy-transfer pumping of semiconductor nanocrystals using an epitaxial quantum well.

https://publikationen.bibliothek.kit.edu/1000010981/754615

Periodic nanostructures for photonics

This paper reviews current technological developments in polarization engineering and the control of the quantum-confined Stark effect (QCSE) for InxGa1- xN-based quantum-well active regions, which are generally employed in visible LEDs for solid-state lighting applications. First, the origin of the QCSE in III-N wurtzite semiconductors is introduced, and polarization-induced internal fields are discussed in order to provide contextual background. Next, the optical and electrical properties of InxGa1- xN-based quantum wells that are affected by the QCSE are described. Finally, several methods for controlling the QCSE of InxGa1- xN-based quantum wells are discussed in the context of performance metrics of visible light emitters, considering both pros and cons. These strategies include doping control, strain/polarization field/electronic band structure control, growth direction control, and crystalline structure control.

Control of Quantum-Confined Stark Effect in InGaN-Based Quantum Wells

The possibility that the Earth suffered episodes of global glaciation as recently as the Neoproterozoic period, between about 900 and 543 million years ago, has been widely discussed1,2,3. Termination of such ‘hard snowball Earth’ climate states has been proposed to proceed from accumulation of carbon dioxide in the atmosphere4. Many salient aspects of the snowball scenario depend critically on the threshold of atmospheric carbon dioxide concentrations needed to trigger deglaciation2,5. Here I present simulations with a general circulation model, using elevated carbon dioxide levels to estimate this deglaciation threshold. The model simulates several phenomena that are expected to be significant in a ‘snowball Earth’ scenario, but which have not been considered in previous studies with less sophisticated models, such as a reduction of vertical temperature gradients in winter, a reduction in summer tropopause height, the effect of snow cover and a reduction in cloud greenhouse effects. In my simulations, the system remains far short of deglaciation even at atmospheric carbon dioxide concentrations of 550 times the present levels (0.2 bar of CO2). I find that at much higher carbon dioxide levels, deglaciation is unlikely unless unknown feedback cycles that are not captured in the model come into effect.

High levels of atmospheric carbon dioxide necessary for the termination of global glaciation

We show the presence of the Goos–Hanchen effect when a monochromatic beam illuminates a photonic crystal inside a photonic bandgap.

https://hal.archives-ouvertes.fr/hal-00090728/document

Goos-Hänchen effect in the gaps of photonic crystals

A detailed study on the effects of Si-doping in the GaN barrier layers of InGaN-GaN multiquantum well (MQW) light-emitting diodes (LEDs) has been performed. Compared with unintentionally doped samples, X-ray diffraction results indicate that Si-doping in barrier layers can improve the crystal and interfacial qualities of the InGaN-GaN MQW LEDs. It was also found that the forward voltage is 3.5 and 4.52 V, the 20-mA luminous intensity is 36.1 and 25.1 mcd for LEDs with a Si-doped barrier and an unintentionally doped barrier, respectively. These results suggests that one can significantly improve the performance of InGaN-GaN MQW LEDs by introducing Si doping in the GaN barrier layers.

L.W. Wu

Papers

Influence of Si-doping on the characteristics of InGaN-GaN multiple quantum-well blue light emitting diodes