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Top-Down Fabrication of High Quality Gallium Indium Phosphide Nanopillar/Disk Array Structures

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Top-Down Fabrication of High Quality Gallium Indium Phosphide Nanopillar/Disk Array Structures Accepted Manuscript - IEEE copyright Top-Down Fabrication of High Quality Gallium Indium Phosphide Nanopillar/disk Array Structures Dennis Visser Rinat Yapparov Eleonora De Luca Department of Applied Physics Department of Applied Physics Department of Applied Physics KTH Royal Institute of Technology KTH Royal Institute of Technology KTH Royal Institute of Technology Kista, Sweden Kista, Sweden Stockholm, Sweden [email protected] [email protected] [email protected] Marcin Swillo Yohan Désières Saulius Marcinkevičius Department of Applied Physics University of Grenoble Alpes Department of Applied Physics KTH Royal Institute of Technology CEA, LETI, MINATEC KTH Royal Institute of Technology Stockholm, Sweden Grenoble, France Kista, Sweden [email protected] [email protected] [email protected] Srinivasan Anand Department of Applied Physics KTH Royal Institute of Technology Kista, Sweden [email protected] In this work, top-down fabrication methods for fabricating supplying proper conditions (e.g., reactants, temperature and high optical quality gallium indium phosphide (GaInP) pressure). Challenges for this method are to control the quality nanopillar/disk arrays are investigated for optoelectronic of the material, geometry (size and shape) and correct applications. Time-resolved photoluminescence (TRPL) assembly location. Reported methods are, e.g., gold-seeded measurements are used to characterize the fabricated growth, self-seeded growth and selective area growth [17]. nanostructures and the results are compared to the properties Top-down methods rely on a starting bulk material, for which of a reference GaInP ‘slab’. Photoluminescence (PL) spectra the initial quality of the grown (Al)GaInP layer plays an and carrier lifetimes are characterized for the fabricated GaInP important role. A selective etching process is used on this structures embedded in a highly transparent film. layer to obtain the desired geometry and spacing of the final Additionally, using GaInP structures on a gallium arsenide structuring. Approaches used for this are: reactive ion etching (GaAs) substrate the effect of a sulphur-oleylamine based (RIE), inductively coupled RIE (ICP-RIE) and wet etching surface passivation procedure is investigated. This was done [7]. In this work, GaInP nanopillar/disk array structures were for the purpose of improving the PL intensities, increase fabricated by a combination of a colloidal lithography and an carrier lifetimes and prevent photodegradation by passivating ICP-RIE process in order to obtain nanostructures with a the surface states. controlled geometry and spacing, and high optical quality. These GaInP nanopillar/disk structures were designed to A. Introduction present low reflection, high absorption as well as high light III-V semiconductor materials show interesting properties extraction efficiency in the visible range, with possible for optoelectronic applications, e.g., high refractive index, applications in order to achieve anti-reflection and absorption direct bandgap, absorption properties and high carrier enhancement, coloring, sensing, or wavelength down mobility. For example, GaInP and AlGaInP have been conversion. The fabricated structures were embedded in a reported for a wide range of applications such as transistors highly transparent film in order to study their optical [1], diodes [2], lasers [3], light emitting diodes (LEDs) [4], properties, especially their photoluminescence properties. solar cells [5] and window layers in solar cells [6]. Top-down Additionally, a surface passivation procedure was [7-12] and bottom-up [13-16] fabrication methods for investigated for the structures still on the substrate in order to structuring GaInP have been reported to fabricate improve the optical quality of the fabricated structures. nanostructured layers in order to enhance light-matter interactions. Patterning of the initial layer can be beneficial for B. Fabrication several features, e.g., absorption enhancement, light GaInP nanopillar/disk arrays were fabricated from high extraction enhancement and/or improvement of carrier quality epitaxially grown Ga0.51In0.49P layers, on a GaAs extraction. However, processing of the layer may lead to a substrate, having a direct bandgap emission wavelength of degradation of the optical quality due to surface states and ~660 nm; where the structures were obtained from ENT E. A., process induced defects. Therefore, it is important to Poland. As a reference, microdisks were fabricated to mimic investigate the effect of the fabrication methods on the a bulk-like GaInP slab. A stack layer of GaInP/GaAs was used material properties of GaInP by characterizing the in order to obtain substrate-free GaInP structures embedded in optical/electrical quality of the structures. Important features a polydimethylsiloxane (PDMS) film. A fabrication process for these applications are the optical and electrical properties based on a combination of colloidal lithography (CL) or of this patterned material. Bottom-up methods rely on the optical lithography and inductively coupled plasma reactive assembly or growth of a (distorted) starting material, by ion etching (ICP-RIE) was used for obtaining the GaInP D. Visser et al., "Top-Down Fabrication of High Quality Gallium Indium Phosphide Nanopillar/disk Array Structures," 2019 IEEE 14th Nanotechnology Materials and Devices Conference (NMDC), Stockholm, Sweden, 2019, pp. 1-4, doi: 10.1109/NMDC47361.2019.9083990. https://ieeexplore.ieee.org/document/9083990 Accepted Manuscript - IEEE copyright Fig. 1. Scanning electron microscopy (SEM) images of the fabricated GaInP nano- and microstructures. Cross-section images of the ICP-RIE etched (a) nanopillar arrays (with mask and partially wet etched), (b) nanodisk arrays (with mask) and (c) microdisk, are shown. Tilted (30°) top view images are shown for the ICP-RIE etched (d) nanopillar arrays, (e) nanodisk arrays and (f) microdisk. structures; where CL was used for the nanopillar/disk subsequently peeled off from the substrate. The resulting structures and optical lithography for the (reference) nanopillar arrays have a height of 1 μm, hexagonal array microdisk. For the CL, a colloidal solution of silicon dioxide period of ~500 nm and a top-bottom diameter of ~150-350 (SiO2) nanospheres (Sigma Aldrich; diameter of ~500(±5%) nm. The nanodisk arrays have a height of 200 nm, hexagonal nm) was used for masking purposes. A thin SiO2 layer array period of ~500 nm and a diameter of ~350 nm. The (thickness of ~55 nm) was first deposited on the initial GaInP microdisk has a height of 1 μm and diameter of ~20 μm. layer by plasma-enhanced chemical vapor deposition Representative scanning electron microscopy (SEM) images (PECVD) in order to improve the surface wettability and to of the fabricated structures are shown in Fig. 1. For the serve as an additional hard mask. The SiO2 colloidal solution sculpting and passivation of the structures, a sulphur- was then deposited on the surface by a mild spin coating oleylamine based treatment was used (1.5% sulphur solution, process, resulting in close-packed hexagonal array patches temperature of 94 °C and treatment time of 3.5 hours) [18]. with a homogeneous coverage of several mm2; where the This resulted in a slight removal of the surface material and original colloid diameter determines the hexagonal array the formation of chemical bonds between sulphur and GaInP; period. The organized colloidal particles are then size reduced thereby acting as a passivation layer. These preliminary by a RIE process (CHF3 flow of 25 sccm, RF power of 100 studies were performed for the ICP-RIE etched GaInP W, pressure of 50 mTorr and an average diameter size structures still on the substrate. reduction rate of ~20 nm/min). For the microdisks, optical lithography patterning was used, where the pattern was C. Optical Characterization transferred from a patterned positive photoresist layer to a Lattice matched epitaxially grown GaInP layers have been 300-nm-thick SiO2 PECVD (masking) layer deposited on the reported to show good electrical and optical properties such as initial GaInP layer. An initial GaInP layer of 1 μm thickness their minority exciton mobility and diffusion length, was used for the fabrication of the nanopillar arrays and the absorption dynamics, and PL/Raman properties [19-24]. microdisks, whereas a 200 nm layer thickness was used for the Characterization of the original GaInP/GaAs substrate and the fabrication of the nanodisk arrays; where the thickness of the embedded GaInP ‘slab’ in PDMS by high-resolution X-ray layer determines the height of the structures. A Cl2/H2/CH4- diffraction (HR-XRD) analysis, energy-dispersive X-ray based chemistry was used for the ICP-RIE process in order to spectroscopy (EDS) and Raman spectroscopy, indicates a -3 etch the micro/nanostructures (Cl2 flow of 9 sccm, H2 flow of proper lattice matched layer of Δa/a=1.8·10 (where ‘a’ is the 5.5 sccm, CH4 flow of 7.5 sccm, ICP power of 1 kW, RF lattice parameter) with the composition Ga0.51In0.49P and a power of 100 W, temperature of 60 °C, set pressure of 4 mTorr structure factor (S) [23] of S=0.4; indicating a high optical and (calibrated) etch rate of ~170 nm/min). A selectivity quality (starting) material. PL measurements were performed between the GaInP:SiO2 mask of ~4-5 was observed, resulting on the embedded GaInP nanopillar/disk arrays and ‘slab’ in a slightly tapered side wall. A thin (100 nm) GaAs structures for two source wavelengths: 450 (blue) and 532 sacrificial layer was provided below the GaInP layer. (green) nm; where Fig. 2(a) shows representative data for the Substrate-free structures are then obtained by using a selective PL emission for a source wavelength of 532 nm. The PL peak wet etching process to etch away the GaAs layer. For the shows a full-width-at-half-maximum (FWHM) of ~15 nm and (partial) sacrificial layer etch, a wet etch chemistry based on an emitting wavelength of ~660 nm; where variations in the H3PO4/H2O2/H2O (ratio of 3:1:25 and etch rate of ~300 peak position (657-663 nm) are due to non-uniformity in the nm/min) is used.
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