Development and analyses of innovative thin films for photovoltaic applications

In solar cell current research, innovative solutions and materials are continuously requested for efficiency improvements. Si-based technology rules over 95% of the market, with silicon heterojunction (SHJ) solar cell reaching 26.7% record efficiency. Nonetheless, hydrogenated amorphous silicon (a-Si:H) layers employed in the structure still have challenges, resolvable with oxygen/nitrogen inclusion. In parallel, new technologies based on different materials still lack in the market due to stability issues or low efficiencies. However, a preliminary study of their properties creates a deeper knowledge exploitable in photovoltaic application. In this perspective, we investigated both innovative Si-based materials (nanocrystalline and amorphous silicon oxy-nitride and oxide thin films, nc-SiOxNy, a-SiOxNy and a-SiOx, respectively) and innovative materials (perovskite lanthanum-vanadium oxide LaVO3 thin films, indium gallium nitride InxGa1-xN and aluminium indium gallium nitride AlxInyGa1-x-yN layers) for solar cell concepts. Different deposition conditions have been employed to extract their influence on compositional, optical, and electrical properties. The study on nc-SiOxNy layers by conductive atomic force microscopy (c-AFM) and surface photovoltage (SPV) has allowed to clarify O, N, and B content, and annealing treatment role on microscopic transport properties. On a-SiOx and a-SiOxNy layers, by spectral ellipsometry, Fourier transform infrared spectroscopy, photoconductance decay and SPV, we can conclude that moderate insertions of O/N in a-Si:H lead to a decrease of optical parasitic absorption, preserving the passivation quality of the layers. The measurements by AFM and Kelvin probe force microscopy on LaVO3 have clearly shown that it is a poor charge-transport medium, thus not suitable for photovoltaic applications. The analysis on InGaN and AlGaInN by SPV measurements has shown how low In content, Si doping and no misfit dislocations in InGaN/GaN structure cause less recombination processes at the interface, whereas, the strain relaxation (tensile and compressive) with the formation of pinholes produces better interfaces in the AlGaInN/GaN samples

Micro- and Nanotechnology of Wide Bandgap Semiconductors

Owing to their unique characteristics, direct wide bandgap energy, large breakdown field, and excellent electron transport properties, including operation at high temperature environments and low sensitivity to ionizing radiation, gallium nitride (GaN) and related group III-nitride heterostructures proved to be enabling materials for advanced optoelectronic and electronic devices and systems. Today, they are widely used in high performing short wavelength light emitting diodes (LEDs) and laser diodes (LDs), high performing radar, wireless telecommunications, as well ‘green’ power electronics. Impressive progress in GaN technology over the last 25 years has been driven by a continuously growing need for more advanced systems, and still new challenges arise and need to be solved. Actually, lighting industry, RF defene industry, and 5G mmWave telecommunication systems are driving forces for further intense research in order to reach full potential of GaN-based semiconductors. In the literature, there is a number of review papers and publications reporting technology progress and indicating future trends. In this Special Issue of Electronics, eight papers are published, the majority of them focusing materials and process technology of GaN-based devices fabricated on native GaN substrates. The specific topics include: GaN single crystalline substrates for electronic devices by ammonothermal and HVPE methods, Selective – Area Metalorganic Vapour – Phase Epitaxy of GaN and AlGaN/GaN hetereostructures for HEMTs, Advances in Ion Implantation of GaN and Related Materials including high pressure processing (lattice reconstruction) of ion implanted GaN (Mg and Be) and III-Nitride Nanowires for electronic and optoelectronic devices

Towards Developing Mid-Infrared Photonics Using Mxenes

Recent research and development in the mid-infrared (IR) wavelength range (2-20 um) for a variety of applications, such as trace gas monitoring, thermal imaging, and free space communications have shown tremendous and fascinating progress. MXenes, which mainly refer to two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides, have drawn a lot of interest since their first investigation in 2011. MXenes project enormous potential for use in optoelectronics, photonics, catalysis, and energy harvesting fields proven by extensive experimental and theoretical studies over a decade. MXenes offers a novel 2D nano platform for cutting-edge optoelectronics devices due to their interesting mechanical, optical, and electrical capabilities, along with their elemental and chemical composition. We here discuss the key developments of MXene emphasizing the evolution of material synthesis methods over time and the resulting device applications. Photonic and optoelectronic device design and fabrication for mid-IR photonics are demonstrated by integrating MXene materials with various electrical and photonic platforms. Here, we show the potential of using Mxene in photonics for mid-IR applications and a pathway toward achieving next-generation devices for various applications.Comment: 50 Pages, 21 figure

Exploring graphitic carbon nitrides for (opto)electronic applications

Graphitische Karbonitride sind organische, kovalent gebundene, geschichtete und kristalline Halbleiter mit einer hohen thermischen und chemischen Stabilität. Diese Eigenschaften machen 2D Schichten der graphitischen Kristalle potentiell nützlich für das Ziel, Limitationen von organischen 0D Molekularen und 1D polymerischen Halbleitern zu überwinden. Trotz dieser interessanten Eigenschaften haben nur wenige Publikationen erfolgreich graphitische Karbonitride in optoelektronischen Bauteilen eingesetzt. Um die Vorteile dieser Materialien nutzbar zu machen, wurden bessere Synthesebedingungen gesucht. Die Verwendung von einem Iod-Eutektikum zeigt, dass Anionen mit einem größeren Radius als Bromid nicht für die Stabilisation von graphitischen Karbonitriden geeignet sind. Das Optimieren der Synthesebedingungen von Poly(triazin-imid)-LiBr resultiert in der Reduzierung von einem kohlenstoffreichen Zersetzungsprodukt bei vollständiger Kondensation. Das Untersuchen der elektronischen Struktur mit ab initio Berechnungen ergibt, dass der elektronische VB-CB-Übergang verboten ist. Dies resultiert daraus, dass die Zustände des obersten Valenzbandes nichtbindender Natur sind. Ein Band aus nichtbindenden Elektronen als oberstes Valenzband ist vor allem aus „lone-pair semiconductors“ aus der sechsten Hauptgruppe bekannt. In der Welt organischer Halbleiter wurde dieses Phänomen bisher nicht beobachtet. Die geringe makroskopische elektrische Leitfähigkeit der PTI-Filme wurde mit der Leitfähigkeit auf Nanoebene verglichen, woraus gefolgert werden kann, dass der Ladungsträgertransport durch den nanokristallinen Charakter an den Kristall-Kristall Übergängen gestört wird. Die elektronische Leitfähigkeit, Mobilität der Ladungsträger sowie die Ladungsträgerdichte wurden untersucht. Die Energie Niveaus legen nahe das Elektronentransport in der Präsenz von Sauerstoff möglich ist. Die erste Applikation eines kovalenten organischen Netzwerks in einer organischen lichtemittierenden Diode ist gezeigt worden.Graphitic carbon nitrides are organic covalently-bonded, layered, and crystalline semiconductors with high thermal and oxidative stability. These properties make 2D layers of graphitic carbon nitrides potentially useful in overcoming the limitations of 0D molecular and 1D polymer semiconductors. Only few reports have shown them being employed in optoelectronic applications. With the goal to find better reaction conditions that enable higher product quality from the ionothermal synthesis the size effect of anions is studied by using an iodide eutectic instead of bromide or chloride eutectic. The highest crystalline condensation product obtained is melem, revealing that the large iodide anion is not capable of stabilizing a graphitic structure. Studying the synthesis conditions of poly(triazine imide) (PTI), the best characterized graphitic carbon nitride in literature, it is revealed that the brown discoloration of the product is due to a carbon rich side product. Reduction of reaction temperature and increase of reaction time allows omittance of carbonisation. Analyzing the electronic structure with ab initio calculations one finds that the lowest energy electronic transition in PTI is forbidden due to a non-bonding uppermost valence band. A uppermost non-bonding valence band is most reminiscent of lone-pair semiconductors and unknown in the world of organic semiconductors making PTI the first organic lone-pair semiconductor. The low electrical conductivity of PTI derivatives is compared to nanoscale conductivity values. The results indicate that macroscopic conductivity is hampered by the nano-crystalline character due to charge carrier trapping at crystal interfaces. The effective mobility is in the range of amorphous organic semiconductors with an unexpectedly high carrier density. The energy levels in PTI-LiBr potentially enable environmentally stable n-transport. The first successful Application of a covalent organic framework in a organic light emitting diode is presented

Magnetron sputter deposition and nitridation of Ta3N5 thin films for photo-electrochemical water splitting

The reliance of human society on fossil fuels has created significant environmental consequences, as such it is imperative that renewable and sustainable sources of energy generation are developed. Hydrogen generation via solar water splitting is a compelling method of renewable energy production and could form the basis for a future hydrogen economy. Tantalum nitride (Ta3N5) is a compelling solar water splitting material due to its appreciable band gap of 2.1 eV and redox potentials that straddle those of water, which enables Ta3N5 to absorb light in the visible spectrum and perform the overall splitting of water into oxygen and hydrogen. However, Ta3N5 self-oxidizes under water splitting conditions which suppresses its performance. This can be avoided by separating Ta3N5 from H2O under water splitting conditions, or by performing the reduction of water with Ta3N5 instead of the oxidation reaction. This thesis describes the synthesis and characterisation of Ta3N5 thin films via sputter deposition and thermal nitridation, with the aim of developing Ta3N5 as a high performance photo-electrode material through the incorporation of chromium and aluminium as acceptor dopants to drive p-type properties. Films are characterised using SEM/EDS, XRD, UV-Vis, and SIMS to determine phase, elemental composition and opto-electric properties. Photo-response and majority carrier concentration were measured using Hall effect apparatus and electrochemical techniques. In this work, films were synthesized with the goal of directly depositing Ta3N5. Several phases within the tantalum nitride phase series were observed, with the metastable TaN group of phases being the most common. Ta3N5 was synthesized from sputtered tantalum oxide films and successfully annealed to produce Ta3N5. The difficulty in depositing Ta3N5 directly is addressed by describing a mechanism for the ammonolysis of Ta2O5 to Ta3N5. Acceptor doped Ta3N5 photo-electrodes were produced and characterised using this method. Chromium doped Ta3N5 films exhibited an exsolved chromium nitride phase in XRD results, and a solubility limit for chromium in Ta3N5 was determined to be ~6 at. %. For aluminium doped films, no exsolved phase was observed and no solubility limit determined under the conditions presented. Aluminium doped Ta3N5 films demonstrated improved photocathode response relative to the standard Ta3N5 films

Engineering Materials and Characterization Methods for Mass-produced Plasmonic Devices

University of Minnesota Ph.D. dissertation. JUne 2017. Major: Electrical Engineering. Advisor: Bethanie Stadler. 1 computer file (PDF); xiv, 123 pages.Over the last decade, plasmonic devices have seen considerable attention, and while there has been significant scientific advancement for plasmonic devices in the laboratory, there still are no industrially produced, high-tech devices which incorporate plasmonics on the market. Industry is in need of robust characterization methods for the development of near-field based devices en route to final product manufacturing lines as well as stable plasmonic materials that can easily be integrated into existing complex process flows. In this dissertation, original research that opens up doors for mass-produced plasmonic devices is presented. Engineered characterization methods include the development of a theoretical model for the prediction of scattering scanning near-field optical microscopy behavior of plasmonic devices, the use of this near-field characterization technique together with scanning electron microscopy cathodoluminescence to perform complete and convergent characterization of plasmonic excitation and coupled near-field emission. Engineered materials are centered on plasma-enhanced atomic layer deposited titanium nitride, discovering its chemistry and behavior under a variety of conditions, and demonstrating its fabricability as both two dimensional etched structures and three dimensional coatings of complex shapes

Surface characterization and luminescence properties of AlN doped with RE elements (Sm, Ho, Gd, Tm)

Rare‐ earth (RE)‐doped III‐nitride broad band‐gap semiconductors have attracted enormous interest as a foundation for optoelectronics devices, which combine the unique luminescence feature of Rare‐earth ions with the electronic properties of the semiconductors. Recent progress toward nitride‐based light emitting diode and light emitting due to electric current devices have been made using crystalline and amorphous AlN and GaN doped with a different lanthanide elements. The Rare‐earth ions’ electronic structures are differ from the other elements and are unique due to an incompletely filled 4Fn shell. The 4F‐orbital electrons lay inside the ion and are protected from the surroundings by the filled 5S2 and 5P6 electron orbitals. When these rare‐earths doped are excited by any external means, intense sharp‐line emission is observed due to intra‐4Fn shells transitions of the rare‐earth ion core. In the present work, sputtered deposited thin films of AlN doped with rare‐earth elements (Sm, Ho, Gd, Tm) are investigated for their structures, luminescence and spectroscopic properties. Thin films were deposited at various temperatures. X‐ray diffraction (XRD) analysis was performed for structural analysis and crystallite size calculation in crystalline films. Scanning electron microscopy was also used to confirm the information obtained from XRD. Luminescence and spectroscopic analysis were performed using photoluminescence tool and Fourier transform infra‐red. The effect of the temperature on the surface morphology and luminescence properties was also studied. Energy dispersive x‐ray analysis was performed on the films to find the constituents and impurities in the samples. Atomic force microscopy was also used for determination of surface roughening, and thermal gravimetric analysis was used to investigate loss of mass of the samples over a range of temperature. This work provides investigations of these materials for their use in photonic and microelectronic devices

Precursor-surface interactions revealed during plasma-enhanced atomic layer deposition of metal oxide thin films by in-situ spectroscopic ellipsometry

We find that a five-phase (substrate, mixed native oxide and roughness interface layer, metal oxide thin film layer, surface ligand layer, ambient) model with two-dynamic (metal oxide thin film layer thickness and surface ligand layer void fraction) parameters (dynamic dual box model) is sufficient to explain in-situ spectroscopic ellipsometry data measured within and across multiple cycles during plasma-enhanced atomic layer deposition of metal oxide thin films. We demonstrate our dynamic dual box model for analysis of in-situ spectroscopic ellipsometry data in the photon energy range of 0.7–3.4 eV measured with time resolution of few seconds over large numbers of cycles during the growth of titanium oxide (TiO2) and tungsten oxide (WO3) thin films, as examples. We observe cyclic surface roughening with fast kinetics and subsequent roughness reduction with slow kinetics, upon cyclic exposure to precursor materials, leading to oscillations of the metal thin film thickness with small but positive growth per cycle. We explain the cyclic surface roughening by precursor-surface interactions leading to defect creation, and subsequent surface restructuring. Atomic force microscopic images before and after growth, x-ray photoelectron spectroscopy, and x-ray diffraction investigations confirm structural and chemical properties of our thin films. Our proposed dynamic dual box model may be generally applicable to monitor and control metal oxide growth in atomic layer deposition, and we include data for SiO2 and Al2O3 as further examples