Casey Schwarz

Effect of low dose γ-irradiation on DC performance of circular AlGaN/GaN high electron mobility transistors

The changes in direct current performance of circular-shaped AlGaN/GaN high electron mobility transistors (HEMTs) after 60Co γ-irradiation doses of 50, 300, 450, or 700 Gy were measured. The main effects on the HEMTs after irradiation were increases of both drain current and electron mobility. Compton electrons induced from the absorption of the γ-rays appear to generate donor type defects. Drain current dispersions of ∼5% were observed during gate lag measurements due to the formation of a virtual gate between the gate and drain resulting from the defects generated during γ-irradiation.

Electron beam induced current profiling of ZnO p-n homojunctions

Variable temperature electron beam induced current technique was employed for the profiling of ZnO p-n homojunctions and the extraction of minority electron diffusion length values in the Sb-doped p-type ZnO region. A thermally induced increase for diffusion length of minority electrons was determined to have an activation energy of ∼145meV. The latter parameter likely represents carrier delocalization energy and determines the increase of the diffusion length due to the reduction in recombination efficiency.

Infrared detectors based on semiconductor p-n junction of PbSe

P-n junctions based on physical vapor deposition of thin PbSe films and conductivity type inversion from n- to p-type are developed and characterized over a wide range of temperatures and bias voltages. Photosensitivity and diode characteristics in the thin film PbSe diode structures were found at temperatures up to 300 K. The values of the measured and estimated parameters of these structures demonstrate their high photodetector performance and the potential for development of IR detectors with optimal sensitivity at the highest possible operating temperature.

Degradation of dc characteristics of InAlN/GaN high electron mobility transistors by 5 MeV proton irradiation

The dc characteristics of InAlN/GaN high electron mobility transistors were measured before and after irradiation with 5 MeV protons at doses up to 2 × 1015 cm−2. The on/off ratio degraded by two orders of magnitude for the highest dose, while the subthreshold slope increased from 77 to 122 mV/decade under these conditions. There was little change in transconductance or gate or drain currents for doses up to 2 × 1013 cm−2, but for the highest dose the drain current and transconductance decreased by ∼40% while the reverse gate current increased by a factor of ∼6. The minority carrier diffusion length was around 1 μm independent of proton dose. The InAlN/GaN heterostructure is at least as radiation hard as its AlGaN/GaN counterpart.

Processing and properties of arsenic trisulfide chalcogenide glasses for direct laser writing of 3D microstructures

Arsenic trisulfide (As2S3) is a transparent material from ~620 nm to 11 μm with direct applications in sensors, photonic waveguides, and acousto-optics. As2S3 may be thermally deposited to form glassy films of molecular chalcogenide (ChG) clusters. It has been shown that linear and multi-photon exposure can be used to photo-pattern thermally deposited As2S3. Photo-exposure cross-links the film into a network solid. Treating the photo-patterned material with a polarsolvent removes the unexposed material leaving behind a structure that is a negative-tone replica of the photo-pattern. In this work, nano-structure arrays were photo-patterned in As2S3 films by multi-photon direct laser writing (DLW) and the resulting structure, morphology, and chemical composition were characterized and correlated with the conditions of the thermal deposition, patterned irradiation, and etch processing. Raman spectroscopy was used to characterize the chemical structure of the unexposed and photo-exposed material, and near infrared ellipsometry was used to measure the refractive index. Physical characterization including structure size and surface adhesion of nano-scale features is related to the processing conditions.

Neutron irradiation-induced enhancement of electronic carrier transport in ZnO

Irradiation of ZnO single crystals by thermal neutrons with a dose up to 7×1017 cm−2 and subsequent annealing at 400 °C for 1 h leads to a significant increase in majority carrier mobility and concentration in this material, with the corresponding decrease of its sheet resistance. Additionally, cathodoluminescence spectra taken before and after neutron irradiation are consistent with the growing carrier lifetime. The observed effects are attributed to irradiation-induced formation of electrically active species of interstitial Zn and improvement of lattice crystallinity due to annealing.

Fabrication and characterization of micro-structures created by direct laser writing in multi-layered chalcogenide glasses

Arsenic trisulfide (As2S3) is a chalcogenide (ChG) material with excellent infrared (IR) transparency (620 nm to 11 μm), low phonon energies, and large nonlinear refractive indices. These properties directly relate to commercial and industrial applications including sensors, photonic waveguides, and acousto-optics. Multi-photon exposure can be used to photopattern thermally deposited As2S3 ChG glassy films of molecular clusters. Immersing the photo-patterned cross-linked material into a polar-solvent removes the unexposed material leaving behind a structure that is a negative-tone replica of the photo-pattern. Nano-structure arrays that were photo-patterned in single-layered As2S3 films through multi-photon direct laser writing (DLW) resulted in the production of nano-beads as a consequence of a standing wave effect. To overcome this effect, an anti-reflective (AR) layer of arsenic triselenide (As2Se3) was thermally deposited between the silicon substrate and the As2S3 layer, creating a multi-layered film. The chemical composition of the unexposed and photo-exposed multi-layered film was examined through Raman spectroscopy. Nano-structure arrays were photopatterned in the multi-layered film and the resulting structure, morphology, and chemical composition were characterized, compared to results from the single-layered film, and correlated with the conditions of the thermal deposition, patterned irradiation, and etch processing.

Cathodoluminescence study of silver and gold lamellar gratings

Cathodo-luminescence spectroscopy is performed on silver and gold lamellar gratings of period 7.5 or 20 microns for a range of grating amplitudes from 0.1 to 4.6 microns. The overall emission spectrum consists of a 400 nm wide band centered at ~600 nm which depends little on the grating amplitude, metal, or e-beam energy. For the larger grating amplitudes the emission spectrum is periodically modulated as a function of wavelength. Both the strength of the emission envelop and the depth and phase of the modulation depend on grating orientation with respect to the light collection axis, the distance of the excitation spot from the grating, and the distance between the grating and the collection optics. The modulation can be explained as interference of surface emission from grating bars and grooves. The origin of the emission remains unclear, as mechanisms of electron collision with image charge, transition radiation, surface contamination, and inverse photo-electron effect all fail to explain the observed spectrum. This work is relevant to the interpretation of cathodoluminescence studies of surface plasmons on structured metals for nano-photonic applications.

Advances in infrared GRIN: a review of novel materials towards components and devices

Novel optical materials capable of advanced functionality in the infrared will enable optical designs that can offer lightweight or small footprint solutions in both planar and bulk optical systems. UCF’s Glass Processing and Characterization Laboratory (GPCL) with our collaborators have been evaluating compositional design and processing protocols for both bulk and film strategies employing multi-component chalcogenide glasses (ChGs). These materials can be processed with broad compositional flexibility that allows tailoring of their transmission window, physical and optical properties, which allows them to be engineered for compatibility with other homogeneous amorphous or crystalline optical components. This paper reviews progress in forming ChG-based GRIN materials from diverse processing methodologies, including solution-derived ChG layers, poled ChGs with gradient compositional and surface reactivity behavior, nanocomposite bulk ChGs and glass ceramics, and meta-lens structures realized through multiphoton lithography (MPL).

Fabrication and characterization of microstructures created in thermally deposited arsenic trisulfide by multiphoton lithography

Abstract. A detailed study of multiphoton lithography (MPL) in arsenic trisulfide (As2S3) films and the effects on nanoscale morphology, chemical networking, and the appearance of the resulting features by the chemical composition, deposition rate, etch processing, and inclusion of an antireflection (AR) layer of As2Se3 between the substrate and the As2S3 layer is reported. MPL was used to photo-pattern nanostructured arrays in single- and multilayer films. The variation in chemical composition for laser-exposed, UV-exposed, and unexposed films is correlated with the etch response, nanostructure formation, and deposition conditions. Reflection of the focused beam at the substrate back into the film produces standing wave interference that modulates the exposure with distance from the substrate and produces nanobead structures. The interference and the modulation can be controlled by the addition of an AR layer of As2Se3 deposited between the substrate and the As2S3 film. Relative to structures produced in a single-layer As2S3 film having no AR layer, photo-patterning in the multilayer As2S3-on-As2Se3 film yields pillar-shaped structures that are closer to the targeted shape and are narrower (120 versus 320 nm), more uniform, and better adhering to the substrate. Processing methods are demonstrated for fabricating large-area arrays with diffractive optical function.