Physical mechanisms causing the efficiency droop in InGaN/GaN blue light-emitting diodes and remedies proposed for droop mitigation are classified and reviewed. Droop mechanisms taken into consideration are Auger recombination, reduced active volume effects, carrier delocalization, and carrier leakage. The latter can in turn be promoted by polarization charges, inefficient hole injection, asymmetry between electron and hole densities and transport properties, lateral current crowding, quantum-well overfly by ballistic electrons, defect-related tunneling, and saturation of radiative recombination. Reviewed droop remedies include increasing the thickness or number of the quantum wells, improving the lateral current uniformity, engineering the quantum barriers (including multi-layer and graded quantum barriers), using insertion or injection layers, engineering the electron-blocking layer (EBL) (including InAlN, graded, polarization-doped, and superlattice EBL), exploiting reversed polarization (by either inverted epitaxy or N-polar growth), and growing along semi- or non-polar orientations. Numerical device simulations of a reference device are used through the paper as a proof of concept for selected mechanisms and remedies

We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al Ga 1 N and In Ga 1 N. Calculations are made using a nonparabolic effective mass energy band model, Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steadystate velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides.Index Terms-Monte Carlo method, semiconductor materials, wide bandgap semiconductors.

This work presents nonlocal pseudopotential calculations based on realistic, effective atomic potentials of the wurtzite phase of GaN, InN, and AlN. A formulation formulation for the model effective atomic potentials has been introduced. For each of the constitutive atoms in these materials, the form of the effective potentials is optimized through an iterative scheme in which the band structures are recursively calculated and selected features are compared to experimental and/or ab initio results. The optimized forms of the effective atomic potentials are used to calculate the band structures of the binary compounds, GaN, InN, and AlN. The calculated band structures are in excellent overall agreement with the experimental/ab initio values, i.e., the energy gaps at high-symmetry points, valence-band ordering, and effective masses for electrons match to within 3%, with a few values within 5%. The values of the energy separation, effective masses, and nonparabolicity coefficients for several secondary valleys are tabulated as well in order to facilitate analytical Monte Carlo transport simulations.

A nonlocal semiempirical pseudopotential calculation of the electronic structure of wurtzite ZnO is proposed. The local and nonlocal components of the atomic effective potentials have been sequentially optimized and an excellent quantitative agreement has been achieved with a wide range of band features (energy gaps at high symmetry points, valence band ordering, in-plane and perpendicular components of the effective masses for electrons and holes at Γ), selected not only from available experimental and ab initio results, but also from new calculations performed with the code developed by the ABINIT project. The valence band description has been further improved through the inclusion of spin-orbit corrections. The complex dielectric function along the main crystallographic directions corresponding to the optimized electronic structure is also presented, along with extensive comparisons of all the computed quantities with the literature data.