Bernd Witzigmann

Germanium–tin (GeSn) microdisks have emerged as promising structures for CMOS‐compatible light sources. Their emission properties depend on the composition of the GeSn alloy, strain, and temperature. In particular, optical and electronic properties are sensitive to the distribution of mechanical strain introduced during the growth process and patterning steps. Understanding how mechanical constraints and structural configurations influence the optical mode confinement and band structure is crucial for optimizing GeSn‐based laser devices. In this work, we present a comprehensive computational investigation into the mechanical deformation and optical behavior of GeSn microdisks fabricated on silicon substrates combining photoelasticity, analysis of optical modes and band structure calculations.

This study investigates carrier transport and confinement within multiquantum well (MQW) AlGaN‐based UV light‐emitting diodes (LEDs), based on three different MQW structures that have a marker QW: 6‐MQW, 12‐MQW, and 21‐MQW. The main emission is at 233 nm, with one marker QW emitting at 250 nm included. Through electro‐optical simulations calibrated against experimental data, hole injection into and inter‐well transport in the MQW active region (AR), and its impact on the internal quantum efficiency is explored. Findings show that due to a relatively shallow hole confinement potential, inter‐well hole transport is efficient, leading to a homogenous hole distribution in 6‐MQW structures. In addition, barrier n‐doping is boosting radiative recombination due to higher electron density in the AR. The optimum doping level of 4 × 10 ¹⁸ /cm ³ improves charge balance and reduces leakage currents, aligning closely with experimental observations. This study emphasizes the importance of optimizing QW depth and doping to enhance UV‐LED performance, particularly in applications requiring efficient far‐UVC emission for germicidal purposes.

We demonstrate a novel band broadening model for the effects of alloy fluctuation on stimulated emission in aluminum gallium nitride (AlGaN) quantum wells (QWs). This model is used to study UV-C laser diode designs for different QW widths 3, 4.5, and 9 nm and varying Al mole fractions. We derive optimal parameters for target wavelengths of 270 and 240 nm. Using the 270 nm design as a case study, we analyze the impact of inhomogeneous broadening on the gain characteristic.

Distributed polarization doping (DPD) represents a promising pathway toward high aluminum () mole fraction aluminum gallium nitride () electronic devices. In order to design more complex high mole fraction devices such as transistors, accurate device modeling is a valuable tool. This article provides a theoretical analysis including special peculiarities of DPD heterojunction devices, material parameters appropriate for high mole fraction materials, and geometry, e.g., rotational symmetry. Moreover, the article studies the temperature‐dependent mobility of DPD devices and addresses the breakdown capabilities of high mole fraction materials.

The hole transport and the carrier distribution in AlGaN-based far-ultraviolett (UVC) light emitting diodes (LEDs) emitting around 233 nm was investigated. Temperature-dependent electroluminescence measurements on dual wavelength AlGaN multiple quantum well (MQW) LEDs show a strong shift in the spectral power distribution from 250 to 233 nm with decreasing temperature. Comparing experimental data with simulation shows that the hole mobility and the electron to hole mobility ratios have a significant influence on the carrier injection efficiency (CIE) and that the change in the spectral power distribution is originating from a change in the hole distribution in the MQWs. Poor hole injection and charge carrier confinement in the AlGaN MQW active region was identified as one of the main reasons for the low CIE in far-UVC LEDs.

Numerical simulations of carrier transport in aluminium gallium nitride based ultraviolet light emitting diodes (UV-LED) are performed in order to understand injection efficiency for light sources in the deep ultraviolet. With our simulator, calibrated with experimental data from a 265 nm UV-LED, quantum efficiencies have been analyzed. The maximum internal quantum efficiency (IQE) of 30% consists of the product from radiative recombination efficiency (RRE) of 60% and carrier injection efficiency (CIE) of 50%. It is found that poor hole injection into the active region and a surplus of electrons limit both efficiencies, and leads to significant electron leakage into the p-side. This leakage is bias dependent, and has a minimum at maximum IQE. Further simulations show that distributed polarization doping (DPD) could improve carrier injection efficiency.

Herein, the lasing threshold and gain characteristics of ultraviolet‐C optically pumped edge‐emitting lasers with thick single‐quantum‐well (SQW) active regions are investigated by the variable‐stripe length method. Positive net modal gain is observed in lasers with AlGaN‐based SQWs with thicknesses up to 12 nm. The lasers show a reduction of the threshold power density with increasing SQW thickness, with the lowest threshold of 1.3 MW cm⁻² achieved for a 9 nm SQW laser. The high gain and low threshold in lasers with thick quantum wells are attributed to lasing from excited states, where the polarization fields are screened by the carriers in the fundamental state and the barriers, thus recovering larger electron and hole wavefunctions overlap. These findings are supported by k · p simulations, and for a 9 nm SQW, the calculation predicts a contribution of the fundamental transition to the gain of 75% for non‐resonant optical excitation and below 1% for resonant optical or electrical excitation.

Simulations of optical gain in aluminum gallium nitride (AlGaN) quantum wells are extended to the high charge carrier density regime required for achieving gain at 275 nm for UV laser diodes. Coulomb interaction is modeled using the 2nd Born approximation. We demonstrate good agreement with experimental data obtained through optical pumping, and predict gain spectra for electrical pumping. Special consideration is given to the contribution of higher bands in wide quantum wells.

Aluminum nitride (AlN) exhibits large breakdown electric fields and high thermal conductivity which allows for excellent miniaturization and power density in high-power electronics. In this talk, we investigate normally-off vertical n-channel trench MISFETs, with the channel consisting of nominally undoped graded AlGaN. The graded AlGaN layer creates immobile volume charges, and the lack of impurities reduces impurity scattering. The n-doped drift layer is composed of AlN for optimum electric field management. Contacts are placed on AlGaN for low ohmic resistance. Using TCAD simulations, the physics of device operation is studied. For comparison, a conventional impurity-doped FET without PID is taken as reference device. The simulations encompass calculation of local strain, solution of the Poisson-equation and electron and hole continuity equation on a 2-dimensional cross-section of the device. Transfer characteristics, threshold voltage, on-resistance and electric field are discussed. Surface states and interface charges at the nonpolar trench sidewall are included in the study. Finally, technological implementation and experimental results are discussed.

Here we report on an experimental and theoretical investigation of the long-wavelength infrared (LWIR) photoresponse of photodetectors based on arrays of three million InP nanowires with axially embedded InAsP quantum discs. An ultra-thin top indium tin oxide contact combined with a novel photogating mechanism facilitates an improved LWIR normal incidence sensitivity in contrast to traditional planar quantum well photodetectors. The electronic structure of the quantum discs, including strain and defect-induced photogating effects, and optical transition matrix elements were calculated by an 8-band k·p simulation along with solving drift-diffusion equations to unravel the physics behind the generation of narrow linewidth intersubband signals observed from the quantum discs.