Friedhard Römer

Ultralow biased widely continuously tunable fabry-Perot filter

Optical filters capable of single control parameter-based wide tuning are implemented and studied. The surface micromachined Fabry-Perot filter consists of two InP-air-gap distributed Bragg reflectors and shows a wavelength tuning of more than 140 nm using only a single voltage of up to 3.2 V at currents below 0.2 mA. The membrane-based filter is designed to block all wavelengths in the whole range of 1250-1800 nm apart from its transmission wavelength.

On the uncertainty of the Auger recombination coefficient extracted from InGaN/GaN light-emitting diode efficiency droop measurements

III-nitride light-emitting diodes (LEDs) suffer from a severe efficiency reduction with increasing injection current (droop). Auger recombination is often seen as primary cause of this droop phenomenon. The corresponding Auger recombination coefficient C is typically obtained from efficiency measurements using mathematical models. However, C coefficients reported for InGaN active layers vary over two orders of magnitude. We here investigate this uncertainty and apply successively more accurate models to the same efficiency measurement, thereby revealing the strong sensitivity of the Auger coefficient to quantum well properties such as electron-hole ratio, electric field, and hot carrier escape.

Effect of Auger recombination and leakage on the droop in InGaN/GaN quantum well LEDs

We investigate the effect of the epitaxial structure and the acceptor doping profile on the efficiency droop in InGaN/GaN LEDs by the physics based simulation of experimental internal quantum efficiency (IQE) characteristics. The device geometry is an integral part of our simulation approach. We demonstrate that even for single quantum well LEDs the droop depends critically on the acceptor doping profile. The Auger recombination was found to increase stronger than with the third power of the carrier density and has been found to dominate the droop in the roll over zone of the IQE. The fitted Auger coefficients are in the range of the values predicted by atomistic simulations.

Modeling of ultrawidely tunable vertical cavity air-gap filters and VCSELs

Tunable vertical cavity devices including an air-gap integrated in the cavity have been designed, fabricated, and investigated. The ultrawide wavelength tuning is realized by micromechanical actuation of Bragg mirror membranes. Based on optical and mechanical model calculations, the air-gap filters and vertical cavity surface emitting lasers (VCSELs) are designed for investigating mainly the optical tuning efficiency. In our research, we focus on two different mirror material systems, dielectric Si/sub 3/N/sub 4//SiO/sub 2/ and InP/air-gap Bragg mirrors and on two tuning concepts, respectively. For the dielectric mirrors, continuous tuning is achieved by thermal actuation of the Si/sub 3/N/sub 4//SiO/sub 2/ mirror membranes, and for InP/air-gap mirrors, electrostatic actuation of the InP membranes is used. To verify the optical and mechanical simulations, InP/air-gap filters are characterized by measuring reflectance spectra and the tuning behavior. The measured results agree with the simulations used to optimize the micromechanical and optical characteristics of air-gap filters and VCSELs for optical communication applications.

Tuning efficiency and linewidth of electrostatically actuated multiple air-gap filters

We investigated the tuning efficiency of electrostatically actuated multiple air-gap filters fabricated in InP for dense wavelength division multiplex applications by comparing measured tuning curves with the results of optical and mechanical simulations. These filters exhibit a record tuning range of 127 nm at 7.3 V tuning voltage. The filters were measured in reflection using standard single mode fiber. The subsequent analysis is based on a one-dimensional electromechanical and optical model providing a reasonable estimation for the pull-in voltage. Optical simulations show that the filter linewidth does not affect the tuning efficiency.

Spectral and spatial properties of the spontaneous emission enhancement in photonic crystal cavities

Although the theory of photonic crystal cavities has been widely investigated, the interpretation of the experimental emission spectra still creates a challenge because the spontaneous emission enhancement is a spatially and spectrally varying property. We present a comprehensive simulation and analysis of the emission spectrum of photonic crystal cavities, considering the spatially and spectrally varying spontaneous emission enhancement, the material loss, and the coupling efficiency to the detection system. The simulations have been performed with a 3D finite-element Maxwell solver and an efficient mode expansion scheme. They have been compared to measured spectra and show very good agreement.

Luminescence and efficiency optimization of InGaN/GaN core-shell nanowire LEDs by numerical modelling

We present a computational study on the anisotropic luminescence and the efficiency of a core-shell type nanowire LED based on GaN with InGaN active quantum wells. The physical simulator used for analyzing this device integrates a multidimensional drift-diffusion transport solver and a k · p Schr¨odinger problem solver for quantization effects and luminescence. The solution of both problems is coupled to achieve self-consistency. Using this solver we investigate the effect of dimensions, design of quantum wells, and current injection on the efficiency and luminescence of the core-shell nanowire LED. The anisotropy of the luminescence and re-absorption is analyzed with respect to the external efficiency of the LED. From the results we derive strategies for design optimization.

Surface micromachined optical low-cost all-air-gap filters based on stress-optimized Si3N4 layers

A new surface micromachining approach based on a multiple Si3N4- and silicon-layer stack is presented. The fabrication process is implemented by plasma-enhanced chemical vapour deposition of stress-optimized films, reactive ion etching using SF6/CHF3/Ar, wet chemical etching of the sacrificial silicon layers by KOH and critical point drying. Using this approach, the fabrication of an optical all-air-gap vertical-cavity Fabry–Perot filter is demonstrated. The surface micromachined filter consists of two DBR mirrors, each having five 590 nm thick Si3N4 membranes separated by 390 nm wide air gaps. The distance between the mirrors (cavity) is 710 nm. The optical characterization and a white light interferometer measurement document the accuracy of the layer positioning and the performance of this low-cost approach. The filter shows the designed filter dip at 1490 nm, the full width at half maximum (FWHM) of the filter is 1.5 nm and the insertion loss is just 1.3 dB. The process is compatible with a variety of materials, e.g. III–V compounds, silicon, as well as organic materials, facilitating a huge application spectrum for sensors.

Auger recombination and leakage in InGaN/GaN quantum well LEDs

The efficiency of blue InGaN/GaN light emitting diodes (LEDs) for solid state lighting has been strongly increased in the past years. The decay of the internal quantum efficiency at current densities above 100 Acm−2 remains, though. This effect is known as droop and effectively limits the maximum current density. The physical mechanisms behind the droop are not yet fully understood. Theories for the origin of the droop include the Auger recombination process and direct carrier leakage. In this context we present an analysis of the contribution of these effects to the droop by means of physics based simulation. In contrast to lumped simulation models the physical simulation model presented in this work enables the exact matching of the internal quantum efficiency (IQE) characteristics by including the geometry, doping, and mole fraction profiles. The analysis presented in this work uses the IQE data of a fabricated blue single quantum well LED. Fitting this IQE curve with the physics based simulation enables the estimation of limits for the Auger coefficients as well as the contribution of the direct carrier leakage. Including Auger assisted carrier leakage facilitates fitting the IQE with reduced Auger coefficients closing the gap to atomistic simulations.

A novel low-cost tunable dielectric air-gap filter

Dense wavelength division multiplex (DWDM) systems is a promising technology for long-haul networks using the established fiber networks. Tunable devices such as optical filters, highly selective photodetectors, as well as lasers are considered to be key components for dynamic WDM systems. A novel low-cost tunable dielectric filter consisting of an air-gap cavity embedded by two DBRs is presented. A FWHM of 8 nm and a tunability of 15 nm/mA at 2 k/spl Omega/ heating resistance is obtained.