In this study, electrical characteristics of MoTe₂ field-effect transistors (FETs) are investigated as a function of channel thickness. The conductivity type in FETs, fabricated from exfoliated MoTe₂ crystals, switched from p-type to ambipolar to n-type conduction with increasing MoTe₂ channel thickness from 10.6 nm to 56.7 nm. This change in flake-thickness-dependent conducting behavior of MoTe₂ FETs can be attributed to modulation of the Schottky barrier height and related bandgap alignment. Change in polarity as a function of channel thickness variation is also used for ammonia (NH₃) sensing, which confirms the p- and n-type behavior of MoTe₂ devices.

Single-crystalline MoSe ₂ and MoTe ₂ platelets were grown by Chemical Vapor Transport (CVT), followed by exfoliation, device fabrication, optical and electrical characterization. We observed that for the field-effect-transistor (FET) channel thickness in range of 5.5 nm to 8.5 nm, MoTe ₂ shows p-type, whereas MoSe ₂ with channel thickness range of 1.6 nm to 10.5 nm, shows n-type conductivity behavior. At room temperature, both MoSe ₂ and MoTe ₂ FETs have high ON/OFF current ratio and low contact resistance. Controlling charge carrier type and mobility in MoSe ₂ and MoTe ₂ layers can pave a way for utilizing these materials for heterojunction nanoelctronic devices with superior performance.

The X-ray Integral Field Unit (X-IFU) on the NewAthena X-ray Observatory will fly a 1,504 pixel transitionedge sensor (TES) microcalorimeter array to study the Hot and Energetic X-ray Universe. For the first time, we have developed and measured a flight-like X-IFU detector that matches the pixel design and array configuration of the planned flight model, including the now baselined 50μm × 30μm TES pixels as well as flight-like electrical and mechanical wafer interfaces. This detector has undergone detailed characterization at NASA/GSFC in preparation to serve as the Development Model (DM) array for the X-IFU instrument. We measured roughly 150 pixels distributed across the $\sim 1.5$ cm diameter hexagonal detector array to verify target adherence and spatial uniformity of key TES pixel parameters and performance metrics, including transition temperature, transition shape, normal-state resistance, conductance to thermal bath, complex impedance, and spectral resolution. The average spectral resolution at 6.9 keV is 2.01 ± 0.31 eV, or 1.93 ± 0.11 eV when removing 16 pixels with degraded performance from fabrication defects. For a smaller subset of pixels we also verified the pixel crosstalk and gain sensitivities meet requirements.

X-ray microcalorimeter space missions require an anti-coincidence detector to mitigate background events from Galactic Cosmic Rays (GCRs). These high-energy particles mimic an X-ray signal by depositing energy in the microcalorimeter absorber, with the amount proportional to their track length. An anti-coincidence detector, positioned below the main microcalorimeter array, identifies these background events and should respond quickly to a GCR signal, have a low threshold and high dynamic range, and use a similar readout scheme as the main detector array. For the proposed Line Emission Mapper (LEM) X-ray probe concept, a W and Al-based athermal phonon sensor design, originally developed for the Cryogenic Dark Matter Search, was adapted to cover a large Si substrate ( 14 sq. cm) with 12 parallel networks of these sensors. This initial prototype for the LEM mission was developed in collaboration with SLAC and Northwestern University, and tested at NASA Goddard Space Flight Center. The prototype demonstrated a combination of low threshold, high dynamic range, and millimeter-scale position discrimination, far exceeding all mission requirements. At the same time, detector modeling efforts were undertaken to understand the rich data sets produced by the prototype. In this work, we expand upon these modeling capabilities to explore new anti-coincidence particle detection designs enabled by the high performance of the LEM prototype. Using G4CMP, an add-on framework to Geant4 for phonon and charge transport in cryogenic detectors, we model the athermal phonon signals generated by GCR events and their collection as quasiparticles in the sensor network. We consider alternative sensor network layouts to maximize GCR event position discrimination, which could reduce false-coincidence events by localizing event rejection in the main array.

We have investigated the dependence of superconducting tran sition parameters on the bilayer properties in Mo/Au transition edge sensors (TESs). We present measurements of the normalized partial derivatives of TES resistance with respect to temperature (α) and current (β), in several fabricated wafers with differing bilayer properties, but in a single TES design characterized at the same relative point in the transition. The results show α increases and β decreases approximately linearly with TES temperature T and normal state resistance Rn. To study these dependencies further, we measured the temperature dependence of the critical current Ic(T), and found it decreases approxi mately exponentially at high temperature. The slope of Ic(T) on a logarithmic scale correlates with Rn but is independent of the transition temperature. We examine to what extent these f indings can explain the observed T and Rn dependence of α and β. These results have implications for our understanding of the fundamental physics of the transition in these devices, and the reproducibility of TES performance resulting from small changes in bilayer fabrication.

The X-IFU instrument for ESA's upcoming flagship mission Athena will use Mo/Au bilayer transition-edge sensors (TES) with Au/Bi X-ray absorbers to form a kilo-pixel microcalorimeter array. Such detectors require careful calibration to maintain their best possible performance, but such calibrations can change over long periods of time when exposed to certain environmental conditions. We have previously reported the effects of humidity and elevated temperature on prototype array characteristics. In that study, storage of a prototype array at 60 $^{\circ }$C for 10 days under vacuum produced an unexpected shift in the TES critical temperature T $_{c}$ (+13 to +19%) and normal resistance R$_{n}$ (+4%). In this work, we continue investigating shifts in array characteristics after long-duration temperature exposures so that safe storage and handling conditions for X-IFU flight arrays can be set. A prototype array exposed to 35 $^{\circ }$ C under vacuum for 26 days gradually increased in TES normal resistance. The energy scale of a second prototype array was measured before aging and then recovered to within 0.5 eV following a similar aging induced R$_{n}$ increase. Aging of the TES was studied directly by applying the same thermal conditions exclusively to Mo/Au bilayers on Si substrate. These measurements allow a better understanding of the cause of the observed shift by decoupling the TES bilayer from the remaining device structures such as the SiNx membrane and the absorber. Four-point resistance measurements were used to readout the bilayer samples at 4.2 K. Bilayers aged in a vacuum generally saw no change or a slight increase in resistance, while bilayers aged in nitrogen gas decreased in resistance. Finally, we conclude with a comparative overview of all test results on prototype arrays and TES bilayers, elaborate on possible mechanisms for the observed shifts in characteristics, and make recommendations for maintaining the long-term stability of our devices.