Optical characterisation of III-V alloys grown on Si by spectroscopic ellipsometry

The initial problem of plasma wave dynamics in the presence of a sharp density jump that divides the space into transparent and opaque regions is studied. A wave packet is assumed to be initially localized in the transparent region. The transient process of field penetration beyond the density barrier during the wave packet reflection from the density jump is investigated. Signal velocity beyond the barrier is defined as the speed at which some small, but finite, value of the field amplitude appears in the evanescent region. This velocity, which is proved to be always less than the speed of light, is determined analytically for the case of quasi-Gaussian wave packet. Further insight into the field dynamics in opaque region is gained by considering a steplike initial wave packet.

Defect states in HfO2 and HfSiON films deposited on Ge(100) substrates were studied by spectroscopic ellipsometry (SE) and x-ray absorption spectroscopy (XAS). In addition, structural and compositional changes in these films were examined via medium energy ion scattering (MEIS). SE and XAS experiments revealed two distinct band edge defect states, located at 1.7±0.1eV and at 2.7±0.1 below the conduction band edges of these films. The number of defect states in HfO2 increased noticeably following postdeposition annealing (PDA), whereas in HfSiON, it showed only small increases following the same treatment. MEIS measurements showed that Ge diffusion into HfO2 films was enhanced significantly by PDA as well; however, this effect was less pronounced in the HfSiON films. The suppression of defect state enhancement in HfSiON films was correlated with lower levels of Ge diffusion and increased structural stability with respect to HfO2.

In recent years, the growing demand for silicon based light sources has boosted the research field of III-V/IV hybrid lasers. Here, the C/L-band light emission (1.53 μm-1.63 μm) of InAs/In0.25Ga0.75As quantum dots (QDs) epitaxially grown on Ge substrate by solid-source molecular beam epitaxy (MBE) is reported. By hybrid III-V/IV epitaxial growth, ultra-thin and anti-phase domains (APD) free III-V materials are achieved on Ge substrate. Step-graded InGaAs metamorphic buffer layers are applied to reduce the strain in InAs QDs in order to extend the emission wavelength. At last, a high quality InAs/In0.25Ga0.75As QD structure on Ge(001) substrate is obtained, which has a strong C/L-band emission centered at the wavelength of 1.6 μm with a full-width-half-maximum (FWHM) of 57 meV at room temperature.

Animals use colour for a wide range of adaptive functions, ranging from cryptic colours that blend into their environments to bright, conspicuous signals that convey information, either to attract mates or to ward off predators and rivals. However, perhaps one of the most intriguing adaptations is how animals can make use of the absence of colour through transparency. Animal transparency has long been understood as a form of camouflage, allowing predators to see straight through their prey as if it were not there. However, transparency can take many different forms, both in terms of the degree of transparency, ranging from opaque through translucent to transparent and in the extent of coverage, with different combinations of transparent and opaque regions. Despite this variation, transparency has often either been regarded as a unique form of concealment or synonymised with background‐matching camouflage. Yet, empirical evidence is increasingly demonstrating how different forms of transparency may facilitate different defensive and communicative strategies. Here, we contextualise the potential functions of transparency into the wider framework of visual ecology, review the evidence for different strategies and highlight areas in need of continued research. We find that, despite its seemingly intuitive role in camouflage, transparency can fulfil many different functions, including facilitating several conceptually distinct forms of camouflage (e.g. background matching, disruption, masquerade and edge diffusion), mimicry (both Batesian and Müllerian) and enhancing communicative signals (such as aposematism, mate choice and territory defence). Yet, many ecological and behavioural questions remain to be addressed, and caution is needed when assessing or interpreting the function of transparency. Read the free Plain Language Summary for this article on the Journal blog.

Chapter 2 - Incoherent Synchrotron Spectra

Despite the record-high efficiency of GaAs solar cells, their terrestrial application is limited due to both the particularly high costs related to the required single-crystal substrates and epitaxial growth. A water-soluble lift-off layer could reduce costs by avoiding the need for toxic and dangerous etchants, substrate repolishing, and expensive process steps. Sr3Al2O6 (SAO) is a water-soluble cubic oxide, and SrTiO3 (STO) is a perovskite oxide, where a SAO ≈ 4 × a STO ≈ (2√2)a GaAs. Here, the pulsed laser-deposited epitaxial growth of SrTiO3/Sr3Al2O6 templates on STO and Ge substrates for epitaxial GaAs growth was investigated, where SAO works as a sacrificial layer and STO protects the hygroscopic SAO during substrate transfer between deposition chambers. We identified that the SAO film quality is strongly dependent on the growth temperature and the O2 partial pressure, where either a high T or a high P(O2) improves the quality. XRD spectra of the films with optimized deposition parameters showed an epitaxial STO/SAO stack aligned to the STO (100) substrate, and TEM analysis revealed that the grown films were epitaxially crystalline throughout the thickness. The STO/SAO growth on Ge substrates at a high T with no intentional O2 flow resulted in some nonepitaxial grains and surface pits, likely due to partial Ge oxidation. GaAs was grown by metalorganic vapor-phase epitaxy (MOVPE) on STO/SAO/STO templates. Lift-off after dissolving the sacrificial SAO in water resulted in free-standing ⟨001⟩ preferentially oriented polycrystalline GaAs.

A general procedure to fit optical constants, using a transfer function model with temperature-and/or-composition-dependent coefficients, is presented. The model is further inverted by a simple algorithm to retrieve temperature and composition information from optical measurements obtained by spectroscopic ellipsometry. The method was applied to fit: (1) the complex index of refraction of the system AlXGa1−xAs at 600 °C, for values of X between 0 and 1. (2) Two data bases of complex dielectric constants, for near-lattice-matched InGaAs and InAlAs, and around temperatures of 500 °C. The parameters of the model are determined with a least squares algorithm with recursive “whitening” of the error, which shows fast convergence to a near-optimal solution, even when handling a large number of parameters. The level of accuracy achieved makes this method an adequate sensor for temperature, composition, and thickness control during molecular beam epitaxy growth.

Ion-conducting polymers (ionomers) are an important component of the heterogenous electrodes (catalyst layers, CL) in proton exchange membrane fuel cells (PEMFCs). In the CL, the nano-thin ionomer layer facilitates mass transport to and from the catalyst sites. Emerging chemistries have the potential to improve this mass transport by inserting additional acid groups to each sidechain or by modifying the backbone. This work aims to understand the impact of ionomer chemistry on the structure-property relationships relevant to fuel cells. Ionomer thin films (<100 nm) cast on planar supports model the CL ionomer. Film nanomorphology is characterized via in situ grazing-incidence x-ray scattering (GIXS); application-relevant properties, including hydration and proton conductivity, are probed via spectroscopic ellipsometry and electrochemical impedance spectroscopy, respectively. While both sidechain and backbone modifications lead to improvements in proton conductivity, the sidechain-modified ionomers show greater hydration and nanophase separation, and the backbone-modified ionomers show reduced hydration and nanophase separation, demonstrating the interplay between morphology, hydration, and film function. These results show that chemistry is a useful tool to tune thin film properties and provide insight to guide future ionomer design.

Influences of the iron ion (Fe 3+)-doping on structural and optical properties of nanocrystalline TiO 2 thin films prepared by sol–gel spin coating

In recent years, the growing demand for silicon based light sources has boosted the research field of III-V/IV hybrid lasers. Here, the C/L-band light emission (1.53 μm-1.63 μm) of InAs/In0.25Ga0.75As quantum dots (QDs) epitaxially grown on Ge and U-shape Si (001) substrates by solid-source molecular beam epitaxy (MBE) are reported. By hybrid III-V/IV epitaxial growth, ultra-thin and anti-phase domains (APD) free III-V materials are achieved on Ge and U-shape Si substrates. Step-graded InGaAs metamorphic buffer layers are applied to reduce the strain in InAs QDs in order to extend the emission wavelength. At last, a high quality InAs/In0.25Ga0.75As QD structures on Ge and Si substrates are obtained, which exhibit strong C/L-band emission centered at the wavelength of 1.6 μm with a full-width-half-maximum (FWHM) of 57 meV at room temperature.

This study explores an innovative approach to enhance the esthetic appeal while having minimal impact on the functional performance of solar-charged electric vehicles. We propose replacing the standard antireflective coating on solar cells with a custom-designed notch filter. This optical filter ensures high transmission across the solar spectrum and creates a distinct color rendering effect in the visible range, thereby making the cells more visually appealing. We utilized niobium pentoxide and silicon dioxide for their excellent optical, mechanical, and corrosive properties to fabricate filters reflecting at specific wavelengths, producing vibrant blue, green, and red color renderings at 400, 550, and 632 nm, respectively. Theoretical relative photocurrent density losses of only ∼7%,∼10%, and ∼14% were observed for blue, green, and red colors, respectively, due to the presence of these filters when compared to a silicon solar cell with a standard antireflective coating. Using optilayer and matlab software, we precisely designed filters with just two to four layers, achieving simplicity and effectiveness. Gradual evolution optimization followed by thin layer removal optimization produced automated and consistent thickness-modulated multilayered optical filter designs over a wide range of user inputs. Our designs were fabricated using magnetron sputtering and validated through variable angle spectroscopic ellipsometry and reflectance spectroscopy, showing strong agreement with our simulations. With a minimal trade-off in the functionality and efficiency of solar cells, this method transforms standard solar cells into esthetically pleasing components, broadening their appeal and potential applications in consumer products.

As applications for extreme ultraviolet (EUV) radiation have been identified, the demand for better optics has also increased. Thorium and thorium oxide thin films (19 to 61 nm thick) were RF-sputtered and characterized using atomic force microscopy (AFM), spectroscopic ellipsometry, low-angle x-ray diffraction (LAXRD), x-ray photoelectron spectroscopy (XPS), and x-ray absorption near edge structure (XANES) in order to assess their capability as EUV reflectors. Their reflectance and absorption at different energies were also measured and analyzed at the Advanced Light Source in Berkeley. The reflectance of oxidized thorium is reported between 2 and 32 nm at 5, 10, and 15 degrees from grazing. The imaginary component of the complex index of refraction, β, is also reported between 12.5 and 18 nm. Thin films of thorium were found to reflect better between 6.5 and 9.4 nm at 5 degrees from grazing than all other known materials, including iridium, gold, nickel, uranium dioxide, and uranium nitride. The measured reflectance does not coincide with reflectance curves calculated from the Center for X-Ray Optics (CXRO) atomic scattering factor data. We observe large energy shifts of up to 20 eV, suggesting the need for better film characterization and possibly an update of the tabulated optical constants.

The optical properties of several commonly used single-crystal oxide substrates were explored by spectroscopic ellipsometry over a wide spectral range from 0.74 eV to 8.8 eV. The crystals examined are (100) SrTiO3, 0.7 % wt Nb-doped (100) SrTiO3,(100) (LaAlO3)0.29(SrAl0.5Ta0.5O3)0.7, (011) DyScO3, (100) MgAl2O4, (100) MgO, and (100) LaAlO3, all of which enable epitaxial growth of numerous perovskite-type and other optical thin films. An analytic form for the complex dielectric function was derived from ellipsometric data through a physically consistent modeling process. The obtained dielectric spectra were further utilized to calculate the complex index of refraction and absorption coefficient for each substrate material. The absorption spectra and optical band gap were analyzed using Tauc plots. The parameters for reconstructing the dielectric functions are given in detail, allowing for extensive applications of the results of this work.

Optical properties of AlN thin films correlated with sputtering conditions

High quality InGaAs/AlGaAs lasers grown on Ge substrates