Daniel Steiauf

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.

First-Principles Calculations of Luminescence Spectrum Line Shapes for Defects in Semiconductors: The Example of GaN and ZnO

We present a theoretical study of the broadening of defect luminescence bands due to vibronic coupling. Numerical proof is provided for the commonly used assumption that a multidimensional vibrational problem can be mapped onto an effective one-dimensional configuration coordinate diagram. Our approach is implemented based on density functional theory with a hybrid functional, resulting in luminescence line shapes for important defects in GaN and ZnO that show unprecedented agreement with experiment. We find clear trends concerning effective parameters that characterize luminescence bands of donor- and acceptor-type defects, thus facilitating their identification.

First-principles calculations of indirect Auger recombination in nitride semiconductors

Auger recombination is an important non-radiative carrier recombination mechanism in many classes of optoelectronic devices. The microscopic Auger processes can be either direct or indirect, mediated by an additional scattering mechanism such as the electron-phonon interaction. Phonon-assisted Auger recombination is particularly strong in nitride materials and affects the efficiency of nitride optoelectronic devices at high powers. Here we present a first-principles computational formalism for the study of direct and indirect Auger recombination in direct-band-gap semiconductors and apply it to the case of nitride materials. We show that direct Auger recombination is weak in the nitrides and cannot account for experimental measurements. On the other hand, carrier scattering by phonons and alloy disorder enables indirect Auger processes that can explain the observed loss in devices. We analyze the dominant phonon contributions to the Auger recom- bination rate and the influence of temperature and strain on the values of the Auger coefficients. Auger processes assisted by charged-defect scattering are much weaker than the phonon-assisted ones for realistic defect densities and not important for the device performance. The computational formalism is general and can be applied to the calculation of the Auger coefficient in other classes of optoelectronic materials.

First-principles study of vacancy-assisted impurity diffusion in ZnO

Group-III elements act as donors in ZnO when incorporated on the Zn site. Their incorporation and behavior upon annealing is governed by diffusion, which proceeds mainly through a vacancy-assisted process. We report first-principles calculations for the migration of Al, Ga, and In donors in ZnO, based on density functional theory using a hybrid functional. From the calculated migration barriers and formation energies, we determine diffusion activation energies and estimate annealing temperatures. Impurity-vacancy binding energies and migration barriers decrease from Al to In. Activation energies for vacancy-assisted diffusion are lowest for In and highest for Al.

First-principles characterization of native-defect-related optical transitions in ZnO

We investigate the electrical and optical properties of oxygen vacancies (VO), zinc vacancies (VZn), hydrogenated VZn, and isolated dangling bonds in ZnO using hybrid functional calculations. While the formation energy of VO is high in n-type ZnO, indicating that this center is unlikely to form, our results for optical absorption signals associated with VO are consistent with those observed in irradiated samples, and give rise to emission with a peak at less than 1 eV. Under realistic growth conditions, we find that VZn is the lowest-energy native defect in n-type ZnO, acting as an acceptor that is likely to compensate donor doping. Turning to optical transitions, we first examine NO as a case study, since N-related transitions have been identified in experiments on ZnO. We also examine how hydrogen, often unintentionally present in ZnO, forms stable complexes with VZn and modifies its optical properties. Compared with isolated VZn, VZn-H complexes have charge-state transition levels lower in the band gap...

Impact of electric-field dependent dielectric constants on two-dimensional electron gases in complex oxides

High-density two-dimensional electron gas (2DEG) can be formed at complex oxide interfaces such as SrTiO3/GdTiO3 and SrTiO3/LaAlO3. The electric field in the vicinity of the interface depends on the dielectric properties of the material as well as on the electron distribution. However, it is known that electric fields can strongly modify the dielectric constant of SrTiO3 as well as other complex oxides. Solving the electrostatic problem thus requires a self-consistent approach in which the dielectric constant varies according to the local magnitude of the field. We have implemented the field dependence of the dielectric constant in a Schrodinger-Poisson solver in order to study its effect on the electron distribution in a 2DEG. Using the SrTiO3/GdTiO3 interface as an example, we demonstrate that including the field dependence results in the 2DEG being confined closer to the interface compared to assuming a single field-independent value for the dielectric constant. Our conclusions also apply to SrTiO3/LaAlO3 as well as other similar interfaces.

Auger recombination in InAs: Role of spin-orbit coupling and phonons

Indium arsenide (InAs), with a low band gap of 0.35 eV, is used in long-wavelength photo-detectors, lasers, photovoltaic junctions and a host of other semiconductor devices. Very high Auger recombination coefficients (ranging from 10^-27~10^-26 cm⁶s^-1) have been measured in this material. Here, we present first-principles-based investigations of Auger recombination processes in InAs. For the direct process, we calculate an Auger coefficient of 1.6 × 10^-27 cm⁶s^-1; for the indirect phonon-assisted process, the coefficient is 1.7 × 10^-29 cm⁶s^-1. Our results elucidate the role of strong spin-orbit coupling: in InAs, the spin-orbit splitting of the valence band is close in magnitude to the band gap, allowing for efficient excitation of Auger holes and leading to a significant enhancement of the Auger recombination coefficient.