Jani Oksanen

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.

Role of anharmonic phonon scattering in the spectrally decomposed thermal conductance at planar interfaces

A detailed understanding of the vibrational heat transfer mechanisms between solids is essential for the efficient thermal engineering and control of nanomaterials. We investigate the frequency dependence of anharmonic scattering and interfacial thermal conduction between two acoustically mismatched solids in planar contact by calculating the spectral decomposition of the heat current flowing through an interface between two materials. The calculations are based on analyzing the correlations of atomic vibrations using the data extracted from nonequilibrium molecular dynamics simulations. Inelastic effects arising from anharmonic interactions are shown to significantly facilitate heat transfer between two mass-mismatched face-centered-cubic lattices even at frequencies exceeding the cutoff frequency of the heavier material due to (i) enhanced dissipation of evanescent vibrational modes and (ii) frequency-doubling and frequency-halving three-phonon energy transfer processes at the interface. The results provide substantial insight into interfacial energy transfer mechanisms, especially at high temperatures, where inelastic effects become important and other computational methods are ineffective.

On the correlation of the Auger generated hot electron emission and efficiency droop in III-N light-emitting diodes

Recent experiments presented in by Iveland et al. [Phys. Rev. Lett. 110, 177406 (2013)] demonstrated that hot electron emission from cesiated p-contacts of III-nitride quantum-well (QW) light-emitting diodes (LEDs) coincides with the onset of the efficiency droop. We have carried out Monte Carlo simulations of hot-electron transport in realistic III-N LEDs. The simulations account for the hole population and all relevant electron scattering and recombination processes. We show that Auger recombination generates a significant hot electron population, which is temporarily trapped in the conduction band side-valleys, without decaying completely before reaching the p-contact. The leakage current due to electron overflow and thermal escape from the QWs is shown to have a minimal impact on the droop. We conclude that the experimentally observed hot electrons are created by Auger recombination in QWs, and that the Auger effect as the origin of the droop is the only consistent explanation for the experimental findings of Iveland et al., [Phys. Rev. Lett. 110, 177406 (2013)].

Enhanced light extraction from InGaN/GaN quantum wells with silver gratings

We demonstrate that an extraction enhancement by a factor of 2.8 can be obtained for a GaN quantum well structure using metallic nanostructures, compared to a flat semiconductor. The InGaN/GaN quantum well is inserted into a dielectric waveguide, naturally formed in the structure, and a silver grating is deposited on the surface and covered with a polymer film. The polymer layer greatly improves the extraction compared to a single metallic grating. The comparison of the experiments with simulations gives strong indications on the key role of weakly guided modes in the polymer layer diffracted by the grating.

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.

Effects of lateral current injection in GaN multi-quantum well light-emitting diodes

In conventional multi-quantum well (MQW) LEDs, typically only one QW emits light due to a very nonuniform carrier distribution. We show by numerical simulations that by adopting a genuinely two-dimensional LED structure enabling lateral hole current injection into the QWs it becomes possible to achieve a more even carrier distribution potentially enabling smaller droop and uniform light emission from all the QWs in the MQW stack. We also show that the uneven emission may be explained with standard current transport models as quasi-Fermi losses between successive quantum wells. We demonstrate our findings by studying three different LED structures and comparing our results to published experimental results.

The Green’s Function Description of Emission Enhancement in Grated LED Structures

Using scattering to improve light extraction from semiconductors is a widely adopted method to increase the efficiency of modern light-emitting devices. Recently, there has also been much interest in the potential emission enhancement provided by the strong coupling between surface plasmons and semiconductor emitters. In this study, we develop a Greens function-based model to describe the emission enhancement and modification in optical properties obtained as a result of scattering and plasmon engineering. The Greens function method is used to answer fundamental questions regarding luminescence enhancement in periodically grated GaN light-emitting structures. The Greens function approach is a very attractive analytical method to studying the emission properties of grated multilayer structures, providing insight beyond numerical solutions. Modeling results from reflectometry measurements of silver-grated GaN structures allows to explain experimentally observed interference features. A discussion regarding the role of periodic grating in enhancing emission from the structures is included.

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 (

Thermophotonic heat pump—a theoretical model and numerical simulations

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Aluminum-Induced Photoluminescence Red Shifts in Core–Shell GaAs/AlxGa1–xAs Nanowires

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