Jean-Pierre Landesman

Investigation of point defect generation in dry etched InP ridge waveguide structures

Waveguides engraved in InP by dry etching, reactive ion etching and inductively coupled plasma (ICP), were studied by cathodoluminescence. The dry etching processes were found to induce nonradiative recombination centers, which reduce the luminescence emission from the ridge structures. In addition, the ICP process introduced intrinsic defects, probably In vacancy related defects, which were generated at the dielectric cap/InP interface at the ridge top.

Defect formation during chlorine-based dry etching and their effects on the electronic and structural properties of InP/InAsP quantum wells

The general objective is the investigation of the defects formed by dry etching tools such as those involved in the fabrication of photonic devices with III–V semiconductors. Emphasis is put on plasma exposures with chlorine-based chemistries. In addition to identifying these defects and describing their effects on the electro-optic and structural properties, the long-term target would be to predict the impact on the parameters of importance for photonic devices, and possibly include these predictions in their design. The work is first centered on explaining the experimental methodology. This methodology starts with the design and growth of a quantum well structure on indium phosphide, including ternary indium arsenide/phosphide quantum wells with graded arsenic/phosphor composition. These samples have then been characterized by luminescence methods (photo- and cathodoluminescence), high-resolution transmission electron microscopy, and secondary ion mass spectrometry. As one of the parameters of importance in this study, the authors have also included the doping level. The samples have been exposed to the etching plasmas for “short” durations that do not remove completely the quantum wells, but change their optical signature. No masking layer with lithographic features was involved as this work is purely oriented to study the interaction between the plasma and the samples. A significant difference in the luminescence spectra of the as-grown undoped and doped samples is observed. A mechanism describing the effect of the built-in electric field appearing as a consequence of the doping profile is proposed. This mechanism involves quantum confined Stark effect and electric-field induced carrier escape from the quantum wells. In the following part, the effects of exposure to various chlorine-based plasmas were explored. Differences are again observed between the undoped and doped samples, especially for chemistries containing silicon tetrachloride. Secondary ion mass spectrometry indicates penetration of chlorine in the structures. Transmission electron microscopy is used to characterize the quantum well structure before and after plasma bombardment. By examining carefully the luminescence spectral properties, the authors could demonstrate the influence of the etching plasmas on the built-in electric field (in the case of doped samples), and relate it to some ionic species penetrating the structures. Etching plasmas involving both chlorine and nitrogen have also been studied. The etching rate for these chemistries is much slower than for some of the silicon tetrachloride based chemistries. Their effects on the samples are also very different, showing much reduced effect on the built-in electric field (for the doped samples), but significant blue-shifts of the luminescence peaks that the authors attributed to the penetration of nitrogen in the structures. Nitrogen, in interstitial locations, induces mechanical compressive stress that accounts for the blue-shifts. Finally, from the comparison between secondary ion mass spectrometry and luminescence spectra, the authors suggest some elements for a general mechanism involved in the etching by chloride-chemistries, in which a competition takes place between the species at the surface, active for the etching mechanism, and the species that penetrate the structure, lost for the etching process, but relevant in terms of impact on the electro-optic and structural features of the exposed materials.

Evidence of chlorine ion penetration in InP/InAsP quantum well structures during dry etching processes and effects of induced-defects on the electronic and structural behaviour

In this work, the overall point of interest is the occurrence of artefacts associated with dry etching processes on InP-based structures. By artefacts we mean creation of defects in the remaining material after etching, defects which might be deleterious to both performance of the photonic devices being fabricated, and reliability/lifetime of these devices. A specific sample structure was defined on InP with InAsxP1 − x quantum wells (QWs). These QWs are buried within 1 μm from the surface, for maximum sensitivity to reactive species produced in the etch plasma, and are designed with a gradual As/P composition, such that the luminescence peak produced by each QW is clearly identified. These samples thus possess a “built-in” marker including its own scale. We focused on chemistries with chlorine (SiCl4/H2/Ar and Cl2/N2), implemented in an inductively coupled plasma reactor. With such chemistries, etch rates of 0.5 μm/min can be reached. The samples are not really etched, but just exposed shortly to the plasma for the interaction to take place. Actually, we just etch at most a few tens of nanometers. Characterisation was carried out by spectrally-resolved cathodo-luminescence and photo-luminescence. We also measured secondary ion mass spectrometry profiles, which revealed the penetration of chlorine into the samples. High resolution transmission electron microscopy was used, to probe the crystal quality. By comparing doped and un-doped samples, we show that the chlorine observed after exposure consists at least partly in Cl− ions. The other important observation is some mechanical compressive stress, which is also a consequence of the local concentration of Cl impurities after exposure to the plasma

Optical Characterizations of VCSEL for Emission at 850 nm with Al Oxide Confinement Layers

In-plane micro-photoluminescence (μ-PL) and micro-reflectivity measurements have been performed at room temperature by optical excitation perpendicular to the surface of two different structures: a complete vertical surface-emitting laser (VCSEL) structure and a VCSEL without the upper p-type distributed Bragg reflector (P-DBR). The two structures were both laterally oxidized and measurements were made on the top of oxidized and unoxidized regions. We show that, since the photoluminescence (PL) spectra consist of the cumulative effect of InGaAs/AlGaAs multi-quantum wells (MQWs) luminescence and interferences in the DBR, the presence or not of the P-DBR and oxide layers can significantly modify the spectrum. μ-PL mapping performed on full VCSEL structures clearly shows oxidized and unoxidized regions that are not resolved with visible light optical microscopy. Finally, preliminary measurements of the degree of polarization (DOP) of the PL have been made on a complete VCSEL structure before and after an oxidation process. We obtain an image of DOP measured by polarization-resolved μ-PL. These measurements allow us to evaluate the main components of strain.

Composition profiles in InP/InAsP quantum well structures under the effect of reactives gases during dry etching processes — Luminescence and SIMS

We have investigated the effects of reactive gases used during the deep reactive ion etching process of InP-based photonic structures in an inductively-coupled plasma (ICP) reactor. Samples with a specific structure, including 9 InAsP/InP quantum wells (QW) with graded As/P composition, were designed. Different chlorine-based gas chemistries were tested. Characterization was performed using cathodo-Iuminescence (CL) and photo-luminescence (PL) at different temperatures, and secondary ion mass spectrometry (SIMS). The luminescence lines display a blue shift upon exposure to the reactive gases, and a strong spectral sharpening. We discuss the influence of Cl diffusion and thermal processes during etching on these modifications.

Integrated optics based on plasma processed dielectric materials

A first generation of dielectric rib waveguides has been designed and shaped to support a monomode confinement by using organosilicon materials elaborated by plasma enhanced chemical vapor deposition (PECVD). Optical losses have been measured around 5.5dB.cm-1 and 11.5dB.cm-1 for TE00 and TM00 modes, respectively. Then, synthesis of titanium dioxide thin films by PECVD has been investigated to improve optical and propagation properties. The first results on these films exhibit a refractive index around 1.99 at a 633 nm wavelength with a 380 nm optical gap. When an rf bias is applied to the substrate, the refractive index reaches 2.36, from -5V, without significant optical gap variation.

Photonics integrated circuits on plasma-polymer-HMDSO: Single-mode TE00-TM00 straight waveguides, S-Bends, Y-Junctions and Mach-Zehnder Interferometers

The authors present the design, realization and characterization of photonics integrated circuits made up of organosilicon (SiOx CyHz) materials called HexaMethylDiSilOxane plasma polymers (pp-HMDSO), elaborated by plasma enhanced chemical vapour deposition (PECVD). Such a versatile technique offers the noticeable advantage to allow the control of respectively; the value of the refractive indices, the thickness of each layer while entailing a lower stress, as the gas proportion of precursors (HMDSO) and plasma conditions are conveniently adjusted. The cladding and core layers of such waveguides have been elaborated into the same reactor thanks to the same precursors with relevant changes regarding the plasma parameters. Then, various integrated photonics devices have been realized, from planar and single-mode rib waveguides to more complex structures like S-bends, Y-junctions, and Mach Zehnder interferometers (MZI). Such structures prove to yield a good single-mode confinement for both polarisations. Moreover optical losses ascribed to pp-HMDSO structures have been respectively evaluated to 5.6 dB.cm1 and 11.5 dB.cm 1 for TE00 and TM00 optical modes. Hence, pp-HMDSO and PECVD appear as quite promising techniques for devising ultra-integrated components like microresonators, and many another functional devices

Cathodoluminescence characterization of InP based photonic structures made by dry etching

Several structures produced by dry etching of InP substrates are studied. RIE (Reactive Ion Etching) and ICP (Inductively Coupled Plasma) dry etching procedures were compared. The characterization was carried out by spectrum imaging Cathodoluminescence (CL). The main changes induced by both procedures in the InP substrates were studied. In particular, the creation of defects and the generation of residual stress were analysed

InP surface properties under ICP plasma etching using mixtures of chlorides and hydrides

InP wafers after etching in an ICP (Inductively-Coupled Plasma) reactor with different kinds of reactant gases have been carefully studied using surface sensitive techniques, in order to gain insight into the mechanisms that control the process. Two types of reactive gas systems have been investigated, namely Cl/sub 2//CH/sub 4//Ar mixtures on one side, and CH/sub 4//H/sub 2/ on the other. In both cases, the composition (flow rate) of the different components was varied. X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and micro-Raman were the techniques used. From the XPS data, information like the surface overall enrichment (in P or In depending on the nature of the etching gases), quantitative surface stoichiometry, and detailed chemical analysis could be derived. AFM images provided an estimate of the roughness increase, while micro-Raman results were used to get indications on the surface structural disordering associated with the etching process, as well as the changes induced in the electronic properties of the InP material (Surface Recombination velocity - SRV - and modifications of the free carrier densities).

InP etching by chlorine ICP plasma for photonic crystal applications: experiments and simulations

Experimental and modeling studies concerning the etching of InP by chlorine plasma have been achieved. Chlorine plasma discharge has been analyzed using Langmuir probe. The latter allows to measure the electron density and temperature as a function of ICP reactor parameters such as RF power, chlorine flow rate and pressure. These two parameters play a crucial role in the modeling of plasma etch process because they directly affect the dissociation and ionization rate of gas into reactive species that participate in the InP etch process. In this context, a global gas phase kinetic model of chlorine plasma discharge is developed. The model allows to calculate the electron density and temperature, ion and atomic chlorine fluxes as a function of ICP reactor parameters. The gas phase parameters calculated from the kinetic model are thus injected in the 2D InP etch model as input parameters to study how the plasma parameters affect the etch profile through the mask. This multi-scale approach based on the coupling between the gas phase model and the surface model may contribute in the optimization of the ICP etch process for InP photonic crystal devices