β-Ga2O3 is a wide bandgap semiconductor with potential for surpassing current-generation high-power device performance and cost-effectiveness, due to its unique properties and availability of large high-quality substrates. However, β-Ga2O3 power electronics are still relatively immature, and commercial realization of reliable high-power devices will require intimate knowledge of performance-limiting extended defects. While several defects have been characterized in bulk substrates, less attention has been given to defects in homoepitaxially grown β-Ga2O3, despite its importance in producing high quality active layers for power devices. In this work, we characterize the bulk electronic properties and extended structural defects in (010) β-Ga2O3 homoepitaxially grown via hydride vapor phase epitaxy (HVPE) using photoemission, transmission electron microscopy (TEM), and complementary spectroscopy and microscopy techniques. We observe two types of linear, surface defects aligned along the [001] crystal axis. One defect consists of a micrometer-sized particle and a tail of protruding material, while the other is a groove in the surface. The large particle is a Ga-rich phase that is likely present early in the HVPE growth that disrupts the surface, while the groove defect appears purely structural in nature. Defect etching and TEM analysis reveal that the linear defects are associated with different dislocation structures, which can explain the different local conductivity measured at each. Our results emphasize that proper surface processing of the bulk substrate is still necessary for obtaining higher quality epitaxial growth for large area power devices.

Overcoming the Shockley-Queisser limit in photovoltaic systems is an ongoing focus. Processes such as impact ionization that produce carrier multiplication in bulk materials are typically inefficient due to the requirements of energy and momentum conservation. Transition metal dichalcogenides, which are of interest for numerous reasons, have, additionally, recently been shown to exhibit very efficient carrier multiplication (CM) in thin films of 2H-MoTe2 and 2H-WTe2 formed by chemical vapor deposition (CVD). The photoconductivity in the thin films exhibited ultrafast (<1 ps) dynamics, atypical of most indirect semiconductors, that may be hard to translate to increased photovoltaic efficiency. To provide a window to the impact of material quality on this intriguing CM effect, we used optical pump-terahertz probe (OPTP) techniques to investigate carrier lifetimes and the efficiency for carrier multiplication for bulk single crystals of 2H-MoTe2 produced by chemical vapor transport (CVT). Efficient carrier multiplication is observed in this bulk single crystal, and photocarrier lifetimes are increased by orders of magnitude although there is still evidence of photocarrier localization in the THz frequency response.

Advances in THz methods and applications require the detection of weak THz pulsed signals. One solution to this problem is to amplify weak signals using optically biased electro-optical (EO) techniques. Several different EO amplification schemes are compared. The simplest of these involves using a non-polarizing beam splitter followed by two quarter-wave plates set to small positive and negative angles, respectively. The signals are then passed through polarizers perpendicular to the polarization of the 800 nm EO detector crystal gating beam. As long as sufficient intensity is present in the gate pulse for linear photodiode detection, amplification of up to 10 times with dynamic ranges for single sweep scans of >104 is obtained.

A 15-laboratory round-robin inter-comparison of ultrafast Terahertz and Microwave time-domain measurements will be presented. Detailed time-to-frequency domain spectral analyses to extract photo-generated carrier conductivity/mobility of high-mobility Perovskite film samples were examined. Experiments comparing non-contact Terahertz to Hall conductivity measurements conducted at NIST for bulk semiconductors (Float Zone Si, n/p-doped Si, GaAs, ZnTe, GaP, etc.) will be briefly shown. An overview of ultrafast terahertz studies of conducting polymers, 2D monolayer and exciton "multiplication" in multilayer transition metal dichalcogenide films (MoTe2, MoS2) will be given.

Polymer passivation has been leveraged to improve photodetection in two-dimensional transition metal dichalcogenide field-effect transistors. The relative passivation effects of common polymers, however, are not well understood. In this work, the interface of monolayer MoS2 and three common polymers, parylene N (Pa-N), polymethyl methacrylate (PMMA), and polyvinylidene difluoride trifluoroethylene (PVDF-TrFE), is assessed with multiple spectroscopic methods. Raman and photoluminescence spectroscopy demonstrate that Pa-N and PMMA provide an n doping effect, which increases photoconductivity and photogenerated charge in terahertz domain and time-resolved spectroscopy. Terahertz time-resolved spectroscopy shows significantly longer carrier lifetime for MoS2 coated with PVDF-TrFE compared to other polymers. These results suggest that PVDF-TrFE provides a unique benefit for photodetection applications.

The development of direct absorbing kinetic inductance detectors (KIDs) for broadband far-infrared (FIR) observations designed to meet the needs of present and future telescopes is presented. This development was initiated to investigate the potential for upgrading the High-resolution Airborne Wideband Camera Plus (HAWC+) instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA) with KIDs; but also has applications generally for FIR detectors. These detectors consist of backside-illuminated KIDs on a silicon substrate where the hybrid TiN/Al inductor forms a resistive wire grid with broadband absorption. Detectors can be configured to absorb in either one or two polarizations with a pixel filling factor of around 80%. A novel two-layer metamaterial anti-reflection coating, and a sub-quarter-wavelength backshort allow for greater than 85% detection efficiency over the ultra-wide 1 to 6 THz bandwidth. These detectors require no focal plane focusing optics such as feedhorns or microlenses, do not require fragile membranes, and utilize proven and straightforward fabrication methods. The optical and microwave design of these detectors is presented. Additionally, the performance of test devices is quantified. This includes measurements of the AR coating effectiveness, detector noise equivalent powers, and detector internal quality factors under the relevant loading levels for the HAWC+ instrument. This information is used to assess the potential benefit of upgrading the HAWC+ instrument with these new detectors and to determine the applicability of this technology for other future FIR detectors.

Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current‐voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter‐laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br) 3 halide perovskite thin‐film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance‐free JV‐curve with a potential power conversion efficiency of 24.6 %. For grainsizes above ≈20 nm, intra‐grain charge transport is characterized by terahertz sum mobilities of ≈32 cm 2 V −1 s −1 . Drift‐diffusion simulations indicate that these intra‐grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best‐realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented.

Ultrafast Time-Domain and Time-Resolved Terahertz (TDS/TRTS) spectroscopic methods are utilized to explore photoconductive properties of five conjugated donor-acceptor co-polymers (PCDTPT, CDTBTZ, IDTBT, PBDTTPD and N2200) dispersed in room temperature toluene, a nonpolar dielectric solvent. TRTS provides an effective non-contact handle to interrogate localized relative conductivity of dispersed co-polymers. Conductivity dynamics are contrasted among disparate polymer structures and between their photoconductive peak and thermalized time-delayed signatures. Photoconductivity kinetics, relative levels of real/imaginary conductivity, and carrier properties are summarized with higher energy photoexcitation increasing the real photoconductivity by up to three-fold. Molecular orbital pictures and donor-acceptor twisting mode frequencies ascertained through ab initio calculations (via Density Functional Theory) support descriptions of the relative efficacies of charge transfer from the perspective of the localized THz method.

Two anionic asymmetric Fe-Fe hydrogenase model compounds containing a single cyano (CN) and five carboxyl (CO) ligands, [Et4N][Fe2(μ-S2C3H6)(CO)5(CN)1] and [Et4N][Fe2(μ-S2C2H4)(CO)5(CN)1], dissolved in room-temperature acetonitrile, are examined. The molecular asymmetry affects the redox potentials of the central iron atoms, thus changing the photophysics and possible catalytic properties of the compounds. Femtosecond ultraviolet excitation with mid-infrared probe spectroscopy of the model compounds was employed to better understand the ultrafast dynamics of the enzyme-active site. Continuous ultraviolet lamp excitation with Fourier transform infrared (FTIR) spectroscopy was also used to explore stable product formation on the second timescale. For both model compounds, two timescales are observed; a 20-30 ps decay and the formation of a long-lived photoproduct. The picosecond decay is assigned to vibrational cooling and rotational dynamics, while the residual spectra remain for up to 300 ps, suggesting the formation of new photoproducts. Static FTIR spectroscopy yielded a different stable photoproduct than that observed on the ultrafast timescale. Density functional theory calculations simulated photoproducts for CO-loss and CN-loss isomers, and the resulting photoproduct spectra suggest that the picosecond transients arise from a complex mixture of isomerization after CO-loss, while dimerization and formation of a CN-containing Fe-CO-Fe bridged species are also considered.

A comprehensive analysis of the compositional heterogeneity and carrier dynamics in novel rubidium-doped 3D/2D perovskites is investigated, showing a PCE over 20% and improved stability at ≈50% relative humidity without encapsulation.

Room-temperature time-domain terahertz (TDS) and time-resolved terahertz (TRTS) spectroscopic methods are employed to measure carrier mobility and charge generation efficiency in thin-film semiconductor polymers. Interrogation of the dependence on excitation and probe polarizations yields insight into the underlying material properties that guide charge transport. We apply terahertz polarization anisotropy probes to analyze charge conduction in preparations of the copolymer PCDTPT, consisting of alternating cyclopenta-dithiophene (donor) and thiadiazolo-pyridine (acceptor) units. Comparisons are made among films of different ordering and morphologies, including aligned films prepared by blade coating, a near-isotropic dropcast film, and isotropic liquid dispersion. They are further contrasted with their population dynamics ascertained through transient absorption and the traditional photoconductive polymer poly-3-hexylthiophene. Polarization anisotropy is observed as preferential charge conduction along the backbone propagation direction of PCDTPT, with various factors disproportionately influencing directional mobility and charge pair yield. PCDTPT exhibits unexpectedly strong conductivity when isolated in toluene dispersion. Quantitative comparisons yield a better understanding of polaron/free-charge relaxation and transfer mechanisms and illustrate dynamics among photoexcited charge carriers and their motion and diffusion through different material morphologies.

Below bandgap two-photon excitation of bulk silicon and gallium arsenide samples is used to evenly generate low density carriers (<;10 13 cm -3 ) throughout the entire sample thickness. Analysis of the frequency-dependent THz conductivity as a function of density allows determination of mobility at lower densities than previously examined using Hall contact measurements.

Efficient doping of 2D materials, including carrier type, concentration and mobility, is challenging but essential for enabling their future electronic and photonic applications. We are developing substitutional n- and p- doping of InSe semiconductor by introducing Sn and Zn, respectively, in the Bridgman bulk crystal growth. Electrical transport properties of undoped vs. n- and p- doped InSe crystals are compared by conducting Hall measurements on bulk crystals and FET transport measurements on exfoliated thin layers. Undoped InSe is intrinsically n-type in both bulk and thin-film forms, with [n]~3.5E14 cm-3 and mu values of up to 1,400 cm2 V-1 s-1 for thick layers at 300K. Carrier concentration in Sn-doped thick layers increases approximately two-fold, while the corresponding mobility reduces ~2 times at 300 K. Zn-doped InSe shows p- behavior for bulk InSe with [p]~7.9E13 cm-3 and mu~43 cm2 V-1 s-1 at 300 K, which reverts to ambipolar/n- type behavior for thin layers in FET devices.

Time-Resolved Terahertz methods with polarized pump-probe analysis are employed to extract conduction and mobility anisotropy of conjugated donor-acceptor polymers as films and liquid dispersions. Nano-grooved substrates with bladed films yield high in-chain conduction anisotropy compared to near isotropic drop-cast films and polymers in toluene.

Perovskite photovoltaics have made extraordinary progress in power conversion efficiency (PCE) and stability due to process and formulation development. Perovskite cell performance benefits from the addition of alkali metal cations, such as cesium (Cs+) and potassium (K+) in mixed‐ion systems, but the underlying reasons are not fully understood. Herein, the solidification of perovskite layers is studied, incorporating 5%, 10%, to 20% of Cs+ and K+ using in situ grazing incidence wide‐angle X‐ray scattering. It is found that K+‐doped solutions yield nonperovskite 4H phase rather than the 3C perovskite phase. For Cs+‐doped formulations, both 4H and 3C phases are present at 5% Cs+, whereas the 3C perovskite phase is formed in 10% Cs+‐doped formulations, with undesirable halide segregation occurring at 20% Cs+. Postdeposition thermal annealing converts the intermediate 4H phase to the desirable 3C perovskite phase. Importantly, perovskite layers containing 5% of Cs+ or K+ exhibit a reduced concentration of trap states and enhanced carrier mobility and lifetime. By carefully adjusting Cs+ or K+ concentration to 5%, perovskite cells are demonstrated with a ≈5% higher‐average PCE than cells utilizing higher cation concentrations. Herein, unique insights into the crystallization pathways toward perovskite phase engineering and improved cell performance are provided.

In this study Terahertz (THz) spectra of several commonly used active pharmaceutical ingredients are reported. Results showed that all the API's had unique bands associated with the structures allowing to analyse both qualitatively and quantitatively. The liq. nitrogen temperature and solid state and gas phase theoretical analysis of meloxicam have also been carried out. In addition, the transparency of the drug packaging materials like plastic blisters, cartons were checked to see the technique's ability for in situ nondestructive penetrating analysis.

Two asymmetrically structured model compounds for the hydrogen-generating [Fe-Fe]-hydrogenase active site were investigated to determine the ultrafast photodynamics, structural intermediates, and photoproducts compared to more common symmetric di-iron species. The bidentate-ligand-containing compounds studied were Fe2(μ-S2C3H6)(CO)4(bipy), 1, and Fe2(μ-S2C3H6)(CO)4(phen), 2, in dilute room temperature acetonitrile solution and low-temperature 2Me-THF matrix isolation using static FTIR difference and time-resolved infrared spectroscopic methods (TRIR). Ultraviolet-visible spectra were also compared to time-dependent density functional theory (TD-DFT) to ascertain the orbital origins of long wavelength electronic absorption features. The spectroscopic evidence supports the conclusions that only a propyl-bridge flip occurs in low-temperature matrix, while early time CO ejection leads to the formation of solvated isomeric species on the 25 ps time scale in room temperature solution.

Low density charge mobility from below bandgap, two-photon photoexcitation of bulk silicon (Si) is interrogated using time-resolved terahertz spectroscopy (TRTS). Total charge mobility is measured as a function of excitation frequency and fluence (charge carrier density), cut angle, and innate doping levels. Frequency dependent complex photoconductivities are extracted using the Drude model to obtain average and DC-limit mobility and carrier scattering times. These dynamic parameters are compared to values from contact-based Hall, above bandgap photoexcitation, and comparable gallium arsenide (GaAs) measurements. Mobilities are shown to increase beyond Hall values at low carrier densities and are modestly higher with increasing dopant density. The former occurs in part from below bandgap photoexcitation exhibiting abnormally small (faster) scattering times, while both reflect unique conduction characteristics at lowest (> 2x10(12)cm(-3) ) carrier densities achieved through photodoping.