P. D. Yoder

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

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.

Efficiency droop due to electron spill-over and limited hole injection in III-nitride visible light-emitting diodes employing lattice-matched InAlN electron blocking layers

Data and analysis are presented for the study of efficiency droop in visible III-nitride light-emitting diodes (LEDs) considering the effects of both electron spill-over out of active region and hole injection into the active region. Performance characteristics of blue LEDs with lattice-matched In0.18Al0.82N electron-blocking layers (EBLs) with different thicknesses were measured in order to exclude the effects of strain and doping efficiency of the EBL, and the quantum efficiencies were analyzed taking account of the electron spill-over current and the relative hole concentration. The results suggest that the highest efficiency in LEDs with a 15-nm In0.18Al0.82N EBL is due to relatively lower hole-blocking effect, hence higher hole injection than in LEDs with a 20-nm EBL, while providing a higher potential barrier for reduced electron spill-over than in LEDs with thinner EBLs. This study suggests that the EBL hole-blocking and electron-confinement effects should be considered in order to achieve higher l...

Performance of Deep Ultraviolet GaN Avalanche Photodiodes Grown by MOCVD

We report high-performance GaN ultraviolet (UV) p-i-n avalanche photodiodes (APDs) fabricated on bulk GaN substrates. The fabricated GaN p-i-n diodes demonstrated optical gains > 104 and low dark current densities operating at wavelengths from 280 to 360 nm. The result is among the highest III-N-based APD gains at the deep UV wavelength of 280 nm reported to date.

Ab initio analysis of the electron‐phonon interaction in silicon

A Harris functional approach is used to investigate the electron‐phonon interaction in silicon, within the rigid ion approximation. The necessary lattice dynamics are solved via the valence shell model. The electron‐phonon matrix elements for transitions between selected electronic states are calculated, and equivalent deformation potentials are presented and compared with results of other models. The resulting deformation potentials exhibit significant dispersion throughout much of the Brillouin zone, though remain nearly constant for intervalley transitions between states close to the conduction band minima. The overall value of the deformation potentials is somewhat higher than found in previous models and thus in better agreement with experiment.

A generalized Ramo–Shockley theorem for classical to quantum transport at arbitrary frequencies

We present a generalized Ramo–Shockley theorem to evaluate particle currents and energy currents at device contacts, in classical drift‐diffusion or hydrodynamic simulation techniques as well as for semiclassical Monte Carlo and quantum mechanical transport simulation. In contrast to the Ramo–Shockley theorem, our technique (1) is derived for conditions of extreme time dependence in the charge carriers and forces (including particle‐induced radiation), (2) explicitly accounts for particle generation and recombination processes such as photoexcitation, forward and inverse Auger processes, or Shockley–Read–Hall recombination, and (3) distinguishes clearly between the contributions of electrons, holes, and the displacement current. The resulting simple new formalism reduces to the standard Ramo–Shockley theorem as a special case.

Design and Analysis of 250-nm AlInN Laser Diodes on AlN Substrates Using Tapered Electron Blocking Layers

A theoretical investigation into the operation of AlInN ultraviolet laser (UV) diodes on AlN substrates is presented. 2-D optoelectronic simulation of a prototypical design predicts lasing at a target wavelength of 250 nm. Simulations indicate optical gain degradation attributable to a parasitic inversion layer, which forms as a result of polarization charge associated with homogeneous electron blocking layers. Appreciable improvement in optical gain is demonstrated in designs featuring inhomogeneous electron blocking layers, by virtue of a volumetric redistribution of polarization charge. Numerical simulations inspire confidence in AlInN as a viable alternative to AlGaN technologies for UV laser-diode operation.

Optimized terminal current calculation for Monte Carlo device simulation

We present a generalized Ramo-Shockley theorem (GRST) for the calculation of time-dependent terminal currents in multidimensional charge transport calculations and simulations. While analytically equivalent to existing boundary integration methods, this new domain integration technique is less sensitive to numerical error introduced by calculations of finite precision. Most significantly, we derive entirely new optimized formulas for the ensemble Monte Carlo estimation of steady-state terminal currents from the time-independent form of our GRST, which are in general not equivalent to the time-average of the true time-dependent terminal currents. We then demonstrate, both analytically and by means of example, how our new variance-minimizing terminal current estimators may be exploited to improve estimator accuracy in comparison to existing methods.

Design Strategies for InGaN-Based Green Lasers

A design parameter subspace is explored to suggest epitaxial layer structures which maximize gain spectral density at a target wavelength for green In¿Ga1-¿N-based single quantum well active regions. The dependence of the fundamental optical transition energy on the thickness and composition of barriers and wells is discussed, and the sensitivity of gain spectral density to design parameters, including the choice of buffer layer material, is investigated.

Strain-Dependence of Electron Transport in Bulk Si and Deep-Submicron MOSFETs

The strain-dependence of electron transport in bulk Si and deep-submicron MOSFETs is investigated by full-band Monte Carlo simulation. On the bulk level, the drift velocity at medium field strengths is still enhanced above Ge-contents of 20% in the substrate, where the low-field mobility is already saturated, while the saturation velocity remains unchanged under strain. In an n-MOSFET with a metallurgical channel length of 50nm, the saturation drain current is enhanced by up to 11%, but this maximum improvement is essentially already achieved at a Ge-content of 20% emphasizing the role of the low-field mobility as a key indicator of device performance in the deep-submicron regime.

First-principles Monte Carlo simulation of transport in Si

We have constructed a unique Monte Carlo simulator incorporating the full band structure of the semiconductor, a realistic phonon spectrum and anisotropic electron-phonon scattering rates generated by ab initio electron-phonon matrix elements. Our computational model provides us with a rigorous test of our ability to formulate and calculate semiclassical transport properties based on fundamental physical principles. By treating the relevant scattering mechanisms with a greater degree of sophistication, we have drastically reduced the number of adjustable parameters and thereby hope to gain some measure of confidence in the calculated high-energy tail of the distribution function.