Friedhard Roemer

Substrate Modes of (Al,In)GaN Semiconductor Laser Diodes on SiC and GaN Substrates

In semiconductor laser diodes layers with high refractive index can act as parasitic waveguides and cause severe losses to the optical mode propagating in the longitudinal direction. For (Al,In)GaN laser diodes, the parasitic modes are typically caused by the SiC or GaN substrate or buffer layers, hence the name substrate modes. A set of four different experiments shows the effect of substrate modes in the near-field (the most direct evidence of substrate modes), as side lobes in far-field, oscillations of the optical gain spectra, and as dependency of threshold current on n-cladding thickness. We derive several basic properties of the substrate modes by simple estimates. For a quantitative analysis we employ a 2-D finite element electromagnetic simulation tool. We simulate periodic variations in the cavity gain spectrum that explain the measurements in terms of absolute value and oscillation amplitude. We show that it is necessary to include the refractive index dispersion in order to get the correct period of the gain oscillations. Furthermore, we use the simulations to optimize the laser diode design with respect to substrate mode losses within the constraints given, e.g., by growth conditions

Analysis of surface recombination in nanowire array solar cells

In this work, microscopic three-dimensional (3D) simulations were performed on nanowire array solar cells to study the impact of surface recombination on the photovoltaic performance. Both axially and radially arranged p-n junction in III-V based structures were taken into consideration. From the cases with surface recombination velocity varying from 1e3cm/s to 1e6cm/s, the core-shell nanowire was found to provide better tolerance for surface recombination. The difference of surface recombination within the axial and core-shell structures is explained by analyzing the relevent minority carrier density, followed by a discussion on the impact of surface recombination on the performance of nanowires as photovoltaic devices.

Simulation and design of optical gain in In(Al)GaN/GaN short wavelength lasers

In this contribution, microscopic simulation of optical gain in GaN-based short-wavelength lasers is presented. The model is used to perform a design study of different active regions, and to discuss the impact of inhomogeneous broadening, carrier-induced screening of the piezo charges, and well thickness on material gain and laser threshold current. As a reference, the model parameters are calibrated with temperature dependent Hakki-Paoli measurements of spectral gain. Excellent agreement between measurement and simulation is achieved, which gives the design studies a quantitative character.

Potential of micromachined photonics: miniaturization, scaling, and applications in continuously tunable vertical air-cavity filters

In technology and nature, tailored scaling represents a principle of success which allows the effectiveness of physical effects to be enhanced. For our optical microsystems, we state that appropriate miniaturization increases the mechanical stability and the effectiveness of spectral tuning by electrostatic and thermal actuation since the relative significance of the fundamental physical forces involved considerably changes with scaling. These basic physical principles are rigorously applied in micromachined 1.55μm vertical-resonator-based filters, capable of wide, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanical actuation of one or several air-gaps which are part of a vertical resonator including two ultra-highly reflective DBR mirrors of strong refractive index contrast: (I) Δn=2.17 for InP/air-gap DBRs (3.5 periods) using GaInAs sacrificial layers and (II)Δn=0.5 for Si3N4/SiO2 DBR’s (12 periods) with a polymer sacrificial layer to implement the air-cavity. In semiconductor multiple air-gap filters, a continuous tuning of >9% of the absolute wavelength is obtained. Varying the reverse voltage (U=0 .. 3.2V) between the membranes (electrostatic actuation), a tuning range up to 142nm was obtained. The correlation of the wavelength and the applied voltage is accurately reproducible without any hysteresis. The extremely wide tuning range and the very small voltage required are record values to the best of our knowledge. Principles of III/V semiconductor micromachining and the detailed technological fabrication process of our filters are focused.

Novel low-cost and simple fabrication technology for tunable dielectric active and passive optical air-gap devices

A novel low cost technology for fabrication of micro-opto-electro-mechanical devices based on plasma enhanced chemical vapor deposition (PECVD) of dielectric materials is presented. Applying surface micromachining, we produce suspended dielectric membranes and cantilevers by involving a common photo resist as sacrificial layer. The intrinsic stress in the layers is adjusted using an interlacing of high (13.56MHz) and low (130kHz) plasma excitation frequencies in the PECVD. A diffraction image method and microstructures are used for the homogeneous stress evaluation. The stress of silicon nitride can be varied in a wide range between +850MPa compressive and −300MPa tensile and no dependence of the frequency on silicon dioxide intrinsic stress is noticed. Depending on lateral design and gradient stress variation, Fabry-Perot filter membranes with radius of curvature (ROC) between −1.7mm and 51mm as well as cavity lengths between 2.3μm and 13.5μm are implemented. Thus, convex, concave and plane membranes are produced. Furthermore, a thermally tuned air-gap Fabry-Perot filter with 8nm FWHM and a tunability of 15nm/mA is fabricated. Strategies of combining these filters with organic laser materials are developed. For this purpose, molecular glasses capable of amplified spontaneous emission (ASE) are chosen, e.g. the molecular glass 4-Spiro which shows an amplified spontaneous emission line at a low threshold of 3.2μJ/cm2 pump laser power density.

Temperature-dependent investigation of carrier transport, injection, and densities in AlGaAs-based multi-quantum-well active layers for vertical-cavity surface-emitting lasers

Abstract. The electro-optical efficiency of vertical-cavity surface-emitting lasers (VCSELs) strongly depends on the efficient carrier injection into the quantum wells (QWs) in the laser active region. Carrier injection degrades with increasing temperature, which limits VCSEL performance in high-power applications where self-heating imposes high-operating temperatures. In a numerical model, we investigate the transport of charge carriers in an 808-nm AlGaAs multi-quantum-well structure with special attention to the temperature dependence of carrier injection into the QWs. Experimental reference data were extracted from oxide-confined, top-emitting VCSELs. The transport simulations follow a drift-diffusion-model complemented by an energy-resolved carrier-capture model. The QW gain was calculated in the screened Hartree–Fock approximation. With the combination of the gain and transport model, we explain experimental reference data for the injection efficiency and threshold current. The degradation of the injection efficiency with increasing temperature is not only due to increased thermionic escape of carriers from the QWs, but also to state filling in the QWs initiated from higher threshold carrier densities. With a full opto-electro-thermal VCSEL model, we demonstrate how changes in VCSEL properties affecting the threshold carrier density, like mirror design or optical confinement, have consequences on the thermal behavior of the injection and the VCSEL performance.

Continuously tunable air gap micro-cavity devices for optical communication systems

We present ultra-widely tunable micro-cavity devices realized by micro-opto-electro-mechanical system (MOEMS) technology. We modeled, fabricated and characterized 1.55μm micromachined optical filter and VCSEL devices capable of wide, monotonic and kink-free tuning by a single control parameter. Our vertical cavity devices comprise single or multiple horizontal air-gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air-gaps are implemented. Due to the high refractive index contrast between air (n=1) and InP (n=3.17) only 3 periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500nm. Unlike InGaAsP/InP or dielectric mirrors they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of outmost importance for the fabrication of MOEMS devices. By controlling the stress we are able to fabricate InP membranes which are extremely thin (357nm thickness) and at the same time flat (radius of curvature above 5mm). Micromechanical single parametric actuation is achieved by both, thermal and electrostatic actuation. Filter devices with a record tuning over 127nm with 7.3V are presented.

Ultrawide continuously tunable 1.55-μm vertical-air-cavity filters and VCSELs based on micromachined electrostatic actuation

We study 1.55micrometers filter and VCSEL devices capable of wide and continuous tuning based on a single control parameter. Ultra-high reflective DBR mirrors are realized with a low number of DBR periods using high refractive index contrast: (I) (Delta) n=2.17 for InP/airgap DBRs (3.5 periods) and (II) (Delta) n=0.5 for Si3N4/SiO2 DBRs (12 periods) with a polymer sacrificial layer to implement the air-cavity. Corresponding fabrication technologies are presented in detail. In both cases spectral tuning (>100nm, theoretically) is obtained by micomachined actuation of the included air-cavity. Large stopband widths and very large tuning efficiencies are obtained by model calculations. For VCSELs a trade-off between lasing efficiency and tuning efficiency is obtained. Experimental results show very good optical properties: high mirror reflectance and clear single-line filter transmission. The first tunable dielectric filter based on polymer sacrificial layers is presented: (Delta) (lambda) /(Delta) U= -7nm/V at 1mA. The potential of the airgap concept: the filter transmission or the laser emission wavelength can be continuously tuned over more than 100nm, thus, the whole spectral gain profile can be addressed by a single control parameter.

Efficiency optimization and analysis of 808nm VCSELs with a full electro-thermal-optical numerical model

A high electro-optical conversion efficiency of a VCSEL (Vertical-Cavity Surface-Emitting Lasers) is one of the key requirements for their application in high power systems for heating, illumination and pumping applications. The substantial amount of degrees of freedom in the epitaxial and structural design of a VCSEL demands numerical guidance in form of technology computer aided design (TCAD) modeling for a straight forward and successful optimization of the devices. We set up a full electro-thermal optical model for the simulation of VCSEL devices. The electro-thermal part of the simulation follows a drift-diffusion model complemented by a customized, energy resolved, semi-classical carrier capture theory in the QW regions. Optical modes, eigensolutions of the vectorial electromagnetic wave equation, stem from a finite element vectorial solver. The electro-thermal and optical models are linked via the photon-rate equation using QW gain spectra (screened Hartree-Fock approximation) and iterated to self-consistency in a Gummel-type iteration scheme. For comparison and calibration, experimental reference data was extracted from oxide-confined, top-emitting VCSEL devices with an emission wavelength of 808 nm. Our simulations are in good agreement with the electro-optical characteristics of the experimental reference. With the calibrated, microscopic model, routes of design adjustment for efficiency optimization are explored. Exemplarily, the maximum VCSEL efficiency of the simulated reference design increases by 10% (absolute) when free hole absorption is switched off. Accordingly, with the combination of an electro-thermal and optical description, a balancing of the tradeoffs of pDBR doping towards reduced free carrier absorption results in a noteworthy efficiency improvement which is validated with experimental data.

Continuously tunable InP-based multiple air-gap MOEMS filters with ultrawide tuning range

Continuously tunable Fabry-Perot filters manufactured using multiple air-gap MOEMS technology are studied and presented. The InP/air-gap filters optimized for optical telecommunication systems using the third optical telecommunication window (1550nm) exhibit a wide tuning range of 142nm and an extremely wide stop-band of 550nm (1250nm-1800nm). The tuning is continuously adjustable requiring ultra-low actuation voltages between 0V (1599nm) and 3.2V (1457nm). The filters are based on a relatively simple vertical structure which is fabricated by few surface micro machining steps. No mirror alignment or subsequent micro mounting are necessary facilitating a compact batch process production.