Photoelectrochemical fabrication of spectroscopic diffraction gratings, phase 2

This program was directed toward the production of Echelle diffraction gratings by a light-driven, electrochemical etching technique (photoelectrochemical etching). Etching is carried out in single crystal materials, and the differential rate of etching of the different crystallographic planes used to define the groove profiles. Etching of V-groove profiles was first discovered by us during the first phase of this project, which was initially conceived as a general exploration of photoelectrochemical etching techniques for grating fabrication. This highly controllable V-groove etching process was considered to be of high significance for producing low pitch Echelles, and provided the basis for a more extensive Phase 2 investigation

Process And System For The Photoelectrochemical Etching Of Silicon In An Anhydrous Environment

The photoelectrochemical oxidation and dissolution of silicon (Si) is performed in the absence of water and oxygen. Etch rates and photocurrents in an anhydrous HF-acetonitrile (MeCN) solution are directly proportional to light intensity up to at least 600 mW/cm2, producing a spatially selective etch rate of greater than 4 ?m/min. Four electron transfer reactions per silicon molecule occur with a quantum yield greater than 3.3 due to electron injection from high energy reaction intermediates. Further, the electrochemical oxidation of p-doped silicon in HF-MeCN results in the formation of porous silicon which electroluminescence in an aqueous solution. In an aprotic electrolyte, where tetrabutylammonium tetrafluoroborate (TBAFB) is used as both the supporting electrolyte and source of fluoride in MeCN, photo-induced etching of n-doped silicon occurs at quantum efficiency of 1.9. This indicates that the oxidation and dissolution mechanism of Si in MeCN can occur without protons.Georgia Tech Reserach Corporatio

Photoelectrochemical fabrication of spectroscopic diffraction gratings

Photoelectrochemical etching was demonstrated as a means of fabricating a variety of periodic structures in semiconductors. The semiconductor is used as an electrode in an electrochemical cell, and is in contact with a liquid electrolyte. When the crystal is held at a positive voltage and illuminated, etching occurs in only the illuminated regions to a depth proportional to the illumination intensity and exposure time. In Phase 1, it was determined that diffraction gratings could be produced in gallium arsenide crystals by this method, using either a scanned focused laser beam or by uniform illumination of a ruling mask defined in metal or photoresist on the crystal surface. The latter approach was determined to produce V-grooves if the mask is oriented along certain crystallographic directions. These V-grooves were produced with an exceedingly smooth crystal morphology due to the highly controllable nature of the process and the mild electrolytes involved. The results form the basis for photoelectrochemical fabrication of deep, low pitch Eschelle gratings for use in high orders in NASA spectrographic instrumentation such as the Space Telescope Imaging Spectrograph

Progress in Nanoporous Templates: Beyond Anodic Aluminum Oxide and Towards Functional Complex Materials

Successful synthesis of ordered porous, multi-component complex materials requires a series of coordinated processes, typically including fabrication of a master template, deposition of materials within the pores to form a negative structure, and a third deposition or etching process to create the final, functional template. Translating the utility and the simplicity of the ordered nanoporous geometry of binary oxide templates to those comprising complex functional oxides used in energy, electronic, and biology applications has been met with numerous critical challenges. This review surveys the current state of commonly used complex material nanoporous template synthesis techniques derived from the base anodic aluminum oxide (AAO) geometry

Porous nitride semiconductors reviewed

Porous nitride semiconductors are a fast-developing area of study, which open up a wide range of new properties and applications, including strain free optical reflectors, chemical sensors and as a pathway to device lift-off. This article reviews the current progress in porous nitrides formed through electrochemical and photoelectrochemical methods. Using a simple electrochemical cell, pores are formed by injecting holes into the surface layer in order to oxidise the material into a soluble form and releasing nitrogen gas. The process is controlled principally by the electric field that drives the injection of holes and hence the applied potential and doping density are the key parameters for controlling pore morphology, along with how and whether illumination is used. We describe the mechanisms responsible for this process in detail and outline the trends for changing pore size and pore shape. For example, larger applied potential creates a larger electric field and hence larger pores. These methods have been used to produce a wide variety of different structures. We present a layered porous structure created by the modulation of the applied potential. Alternatively, layered structures can be produced by growing alternate doped and non-intentionally doped layers. Electrochemical etching can then create pores only in the doped layers, as they are conductive. This process can be performed by etching laterally through access trenches that expose the doped material or through the etching of dislocations to create nanopipes that allow subsurface porosity to form. This process requires no prior processing steps. We combine this method with patterning of surface protective layers to influence where the resulting pores grow. Based on these various fabrication processes, significant progress has been made towards applications of porous GaN across optoelectronics, sensing and for improving material quality.The authors gratefully acknowledge funding from the EPSRC under grant numbers: EP/M010589/1, EP/L015455/1, and EP/R511675/1

Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and Technology, SST-105156.R1 2018

III-V Semiconductor Nanostructures for Photoelectochemical Water Splitting

The desire for the development of renewable energy technologies is ever growing to sustain global socio-economic growth and meet future technological developments due to declining fossil fuel reserves and growing environmental concerns of their by-products. Although photovoltaics is well established as a renewable technology to generate clean energy, it is intermittent in nature and hence storing solar energy for short and long-term applications is still challenging. Hydrogen generation via photoelectrochemical (PEC) water splitting is one of the promising routes to secure a sustainable, green, storable and portable form of energy. III-V semiconductors have gained intense research interest for PEC water splitting applications owing to their outstanding properties such as variable band gaps to capture the entire solar spectrum, high absorption coefficients and high crystalline quality. In addition, nanostructures possess several essential attributes towards achieving efficient water splitting such as enhanced light absorption, reduced carrier transfer length and large surface area. This thesis report on GaN and InP nanopillar (NP) photoelectrodes fabricated using a top-down approach for PEC water splitting. This work involves the fabrication of large area GaN and InP NPs using inductively coupled plasma (ICP) etching of the respective wafers masked by a self-organized random mask technique, followed by a study of their PEC performance. NP photoelectrodes exhibited a remarkable improvement in PEC performance compared to their planar counterparts due to the enhanced absorption and increased semiconductor/electrolyte interface area. The PEC performance of the GaN NP photoanodes was shown to be influenced by doping concentration, NP dimensions such as diameter and length, and band gap engineering of the GaN NPs. The PEC performance of the InP NPs was strongly dependent on the surface damage of NPs, which was eliminated by wet treatment of the NPs in sulfur-oleylamine (S-OA) solution. Finally, long-term photo-stability was demonstrated for both NP photoelectrodes

Structured Materials for Photoelectrochemical Water Splitting

Efficient and economical photoelectrochemical water splitting requires innovation on several fronts. Tandem solar absorbers could increase the overall efficiency of a water splitting device, but economic considerations motivate research that employs cheap materials combinations. The need to manage simultaneously light absorption, photogenerated carrier collection, ion transport, catalysis, and gas collection drives efforts toward structuring solar absorber and catalyst materials. This chapter divides the subject of structured solar materials into two principal sections. The first section investigates the motivations, benefits, and drawbacks of structuring materials for photoelectrochemical water splitting. We introduce the fundamental elements of light absorption, photogenerated carrier collection, photovoltage, electrochemical transport, and catalytic behavior. For each of these elements, we discuss the figures of merit, the critical length scales associated with each process and the way in which these length scales must be balanced for efficient generation of solar fuels. This discussion assumes a working knowledge of the fundamentals of semiconductor-liquid junctions; for more details the reader is encouraged to consult review articles. The second section of this chapter reviews recent approaches for generating structured semiconductor light absorbers and structured absorber-catalyst composites. This literature review emphasizes the insights gained in the last six years that are specifically related to photoelectrochemical water splitting, rather than to general photoelectrochemistry or photovoltaic applications. This chapter concludes with perspectives and an outlook for future efforts aimed at solar water splitting using structured materials. The realization of a practical, efficient, and useful water splitting device requires significant new developments in materials synthesis as well as deeper understanding of the relevant chemistry and physics. This chapter is intended to motivate such developments

Tree-Like Features Formed on Photoelectrochemically etched n-GaN surfaces ―Revelation of threading dislocations in GaN―

Electrochemical etching behavior of n-type GaN films grown on sapphire has been studied under UV (λ=325 nm) light illumination. As the cases for photoelectrochemical etching of n-type GaAs and InP, three different features appear on etched n-GaN surfaces depending on current density for etching; a high density (10^10 cm^<-2>) of tree-like protrusions at a lower c-urrent density, a relatively flat surface at an intermediate current density, and peeling of the film from the substrate at a higher current density. From the shape and the density of tree-like protrusions, in addition to the analogy of these results with those for n-type GaAs and InP, it is reasonable to conclude that tree-like protrusions formed at a low current density are due to threading dislocations involved in n-GaN films. Thus, the photoelectrochemical etching is found to become a convenient method to detect dislocations in n-type III nitride materials

Ga-based III-V semiconductor photoanodes for solar fuels and novel techniques to investigate their photocorrosion.

Solar energy is one of the most abundant renewable energy sources. However, the diurnal variation of the sun as well as seasonal and weather effects, limits the widespread global implementation of solar energy. Thus, Cost-effective energy storage is critical to overcome the intermittent nature of solar energy available on the earth. Photoelectrolysis of water to oxygen and hydrogen fuel is a promising large-scale solution to store intermittent solar energy in a dense and portable form. Photoelectrochemical (PEC) water-splitting, or artificial photosynthesis, research strives to develop a semiconductor photoelectrode with both high efficiency and long-term stability. Semiconductors of the III–V class are among the most promising materials for high efficiency solar fuels applications. However, they suffer from severe instability in acidic and alkaline electrolyte and fundamental understanding of the corrosion mechanism of III-V semiconductors is of significant importance for the solar fuels community. This dissertation is focused on study of photocorrosion of Gallium based III-V semiconductors. A thorough review of important in-situ analytical techniques for the investigation of materials stability is given. The review explains some of the main in-situ electrochemical characterization techniques, briefly explaining the principle of operation and the necessary modifications for in-situ operation, and highlighting key relevant work in applying the method for the investigation of material stability and interfacial properties for electrocatalysts and photoelectrode materials. Next, in this dissertation, the corrosion of n-GaP, a promising III–V material for tandem top subcells, was investigated in strongly acidic electrolyte using an in-situ UV-Vis spectroscopy technique to monitor dissolved Ga and P species as a function of applied bias and time. The changing faradaic efficiency of the electrochemical GaP oxidation reaction was calculated from this data and used to interpret the corrosion process in conjunction with SEM and XPS characterization. In addition, corrosion measurements were made with thin conformal coatings of TiO2 as a protective barrier layer on the GaP surface. Although the protective coating slowed the rate of GaP dissolution, the TiO2 layers produced herein contributed significant charge-transfer resistance and still showed similar trends in the corrosion faradaic efficiency vs. time as the bare n-GaP. Further, photocorrosion of n-GaAs, one of the most well-developed and efficient III-V semiconductors was studied in strongly acidic electrolyte. Three type of Ir, OER co-catalyst, were tested to investigate their affect on photocorrosion of n-GaAs. In-situ UV-Vis spectroscopy was utilized to monitor the corrosion faradaic efficiency and the results showed decreased dissolution faradaic efficiency to a small degree over the first 15 minutes for samples with thin layers of Ir. SEM and XPS characterization have also been used to understand the photocorrosion mechanism. To develop high efficiency and stable water splitting systems new semiconductor materials with appropriate band gap, band edge positions, charge carrier mobility and chemical stability are demanded. Synthesis of ternary III-V alloys enable us to tune the band gap of III-V semiconductor with changing the compositions according to the requirements of PEC systems. Herein, optical and electrical properties of a novel III-V ternary alloy GaSbxP(1-x), synthesized in Conn Center for Renewable Energy Research by Halide Vapor Phase Epitaxy (HVPE) is reported. The effect of Sb addition on the band gap of the semiconductor was studied utilizing diffuse reflectance spectroscopy and photoluminescence spectroscopy. Band gap of HVPE-grown GaSbxP(1-x) film, with x=0.03-0.06 is decreased due to Sb incorporation to the lattice of GaP indicating that it can be a promising photoabsorber for PEC systems. In addition, incorporation of Sb to the lattice of GaP was estimated using Vegard’s law and X-ray diffraction spectrum of samples. Finally, resistivity and Hall effect measurements were performed to study the electrical properties of GaSbxP(1-x) films