Emmanouil Kioupakis

Indirect Auger recombination as a cause of efficiency droop in nitride light-emitting diodes

InGaN-based light-emitting diodes(LEDs) exhibit a significant efficiency loss (droop) when operating at high injected carrier densities, the origin of which remains an open issue. Using atomistic first-principles calculations, we show that this efficiency droop is caused by indirect Auger recombination, mediated by electron-phonon coupling and alloy scattering. By identifying the origin of the droop, our results provide a guide to addressing the efficiency issues in nitride LEDs and the development of efficient solid-state lighting.

Interplay of polarization fields and Auger recombination in the efficiency droop of nitride light-emitting diodes

We use theoretical modeling to investigate the effect of polarization fields and non-radiative Auger recombination on the efficiency-droop and green-gap problems of polar and nonpolar nitride light-emitting diodes. The dependence of radiative and nonradiative recombination rates on electron-hole wave-function overlap is analyzed. Device designs that minimize the polarization fields lead to higher efficiency, not because the internal quantum efficiency is improved at a given carrier density but because they can be operated at a lower carrier density for a given current density.

Anisotropic Spin Transport and Strong Visible-Light Absorbance in Few-Layer SnSe and GeSe

SnSe and GeSe are layered compound semiconductors that can be exfoliated to form two-dimensional materials. In this work, we use predictive calculations based on density functional and many-body perturbation theory to study the electronic and optical properties of single-layer, double-layer, and bulk SnSe and GeSe. The fundamental band gap is direct in single-layer and double-layer GeSe, but indirect in single-layer and double-layer SnSe. Moreover, the interplay of spin-orbit coupling and lack of inversion symmetry in the monolayer structures results in anisotropic spin splitting of the energy bands, with potential applications in directionally dependent spin transport. We also show that single-layer and double-layer SnSe and GeSe exhibit unusually strong optical absorbance in the visible range. Our results suggest that single-layer and double-layer SnSe and GeSe are promising materials for ultrathin-film photovoltaic applications with theoretical upper bounds to the conversion efficiency that approach the efficiency records realized in organic and dye-sensitized solar cells.

Spatially resolved electronic and vibronic properties of single diamondoid molecules

Diamondoids are a unique form of carbon nanostructure best described as hydrogen-terminated diamond molecules. Their diamond-cage structures and tetrahedral sp3 hybrid bonding create new possibilities for tuning electronic bandgaps, optical properties, thermal transport and mechanical strength at the nanoscale. The recently discovered higher diamondoids have thus generated much excitement in regards to their potential versatility as nanoscale devices. Despite this excitement, however, very little is known about the properties of isolated diamondoids on metal surfaces, a very relevant system for molecular electronics. For example, it is unclear how the microscopic characteristics of molecular orbitals and local electron-vibrational coupling affect electron conduction, emission and energy transfer in the diamondoids. Here, we report the first single-molecule study of tetramantane diamondoids on Au(111) using scanning tunnelling microscopy and spectroscopy. We find that the diamondoid electronic structure and electron-vibrational coupling exhibit unique and unexpected spatial correlations characterized by pronounced nodal structure across the molecular surfaces. Ab initio pseudopotential density functional calculations reveal that much of the observed electronic and vibronic properties of diamondoids are determined by surface hydrogen terminations, a feature having important implications for designing future diamondoid-based molecular devices.

Quasiparticle band structures and thermoelectric transport properties of p-type SnSe

We used density functional and many-body perturbation theory to calculate the quasiparticle band structures and electronic transport parameters of p-type SnSe both for the low-temperature Pnma and high-temperature Cmcm phases. The Pnma phase has an indirect band gap of 0.829 eV, while the Cmcm has a direct band gap of 0.464 eV. Both phases exhibit multiple local band extrema within an energy range comparable to the thermal energy of carriers from the global extrema. We calculated the electronic transport coefficients as a function of doping concentration and temperature for single-crystal and polycrystalline materials to understand the previous experimental measurements. The electronic transport coefficients are highly anisotropic and are strongly affected by bipolar transport effects at high temperature. Our results indicate that SnSe exhibits optimal thermoelectric performance at high temperature when doped in the 1019–1020 cm−3 range.

Temperature and carrier-density dependence of Auger and radiative recombination in nitride optoelectronic devices

Nitride light-emitting diodes are a promising solution for efficient solid-state lighting, but their performance at high power is affected by the efficiency-droop problem. Previous experimental and theoretical work has identified Auger recombination, a three-particle nonradiative carrier recombination mechanism, as the likely cause of the droop. In this work, we use first-principles calculations to elucidate the dependence of the radiative and Auger recombination rates on temperature, carrier density and quantum-well confinement. Our calculated data for the temperature dependence of the recombination coefficients are in good agreement with experiment and provide further validation on the role of Auger recombination in the efficiency reduction. Polarization fields and phase-space filling negatively impact device efficiency because they increase the operating carrier density at a given current density and increase the fraction of carriers lost to Auger recombination.

Fundamental limits on optical transparency of transparent conducting oxides: Free-carrier absorption in SnO2

Transparent conducting oxides combine high electrical conductivity with transparency to visible light. However, the large concentration of free electrons introduces a source of absorption that limits the transparency. Here, we evaluate the importance of phonon-assisted free-carrier absorption in SnO2 completely from first principles. Our results show that absorption is modest in the visible and much stronger in the UV and infrared. We also provide insight into the mechanisms that govern absorption in different wavelength regimes.

Phonon-assisted optical absorption in silicon from first principles.

The phonon-assisted interband optical absorption spectrum of silicon is calculated at the quasiparticle level entirely from first principles. We make use of the Wannier interpolation formalism to determine the quasiparticle energies, as well as the optical transition and electron-phonon coupling matrix elements, on fine grids in the Brillouin zone. The calculated spectrum near the onset of indirect absorption is in very good agreement with experimental measurements for a range of temperatures. Moreover, our method can accurately determine the optical absorption spectrum of silicon in the visible range, an important process for optoelectronic and photovoltaic applications that cannot be addressed with simple models. The computational formalism is quite general and can be used to understand the phonon-assisted absorption processes in general.

Determination of Internal Loss in Nitride Lasers from First Principles

Nitride laser diodes are pivotal for optical data storage and laser projection but their performance is limited by large internal absorption losses. Here we quantify the internal loss due to free-carrier absorption using first-principles calculations and show that indirect free-carrier absorption is an important loss mechanism in nitride lasers. The dominant loss process is the phonon-assisted absorption by holes in the p-type layers. Parameters for the absorption coefficients have been extracted for use in device modeling. This work constitutes an important step towards the understanding of the efficiency problems in lasers and may assist the design of future devices.

Visible-Wavelength Polarized-Light Emission with Small-Diameter InN Nanowires

Group III nitrides are widely used in commercial visible-wavelength optoelectronic devices, but materials issues such as dislocations, composition fluctuations, and strain negatively impact their efficiency. Nitride nanostructures are a promising solution to overcome these issues and to improve device performance. We used first-principles calculations based on many-body perturbation theory to study the electronic and optical properties of small-diameter InN nanowires. We show that quantum confinement in 1 nm wide InN nanowires shifts optical emission to the visible range at green/cyan wavelengths and inverts the order of the top valence bands, leading to linearly polarized visible-light emission. Quantum confinement on this scale also leads to large exciton binding energies of 1.4 eV and electronic band gaps in excess of 3.7 eV. Our results indicate that strong quantum confinement in InN nanostructures is a promising approach to developing efficient visible-wavelength light emitters.