107 Search Results

Two-dimensional (2D) 𝑀⁢𝐴₂⁢𝑍₄-family monolayers (MLs) have emerged as promising semiconductors due to their element tunability and rich electronic and optoelectronic properties. In this work, using first-principles calculations, we investigate the electronic, mechanical, transport, and optoelectronic properties of WGe_2⁢N₄ ML with a small indirect bandgap. Various nanodevices based on WGe₂⁢N₄ ML are studied, including pn-junction diodes, pin-junction field-effect transistors (FETs), and phototransistors. The present results reveal that the WGe_2⁢N₄ ML exhibits high rigidity, thermal stability, and remarkable light absorption. These nanodevices demonstrate excellent performance: (1) the pn-junction diode shows a high rectification ratio and a near-Shockley-limit ideality factor, (2) the pin-junctionmore » FET exhibits significant gate voltage modulation capability with subthreshold swing as low as 71 mV/dec (close to the theoretical limit calculated based on the Boltzmann distribution), and (3) the phototransistor displays strong optoelectronic responses in the visible and ultraviolet regions. Furthermore, these findings establish WGe₂⁢N₄ ML as a versatile platform for developing high-performance, multifunctional nanoelectronic and optoelectronic devices, significantly expanding the application potential of 𝑀⁢𝐴_2⁢𝑍₄-family materials.« less

Device-quality tensile-strained Ge (ε-Ge) grown on a large bandgap semiconductor with superior electrical and optical carrier confinement is essential for group-IV-based optoelectronics. Properties of ε-Ge active layers synthesized on In_0.24Ga_0.76As buffers with two different surface terminations─arsenic-rich and indium-rich─were experimentally demonstrated, highlighting the factors not considered in theoretical calculations. High-resolution X-ray diffraction and Raman spectroscopy analyses of these ε-Ge/In_0.24Ga_0.76As heterostructures confirmed the fully strained (1.6%) and partially relaxed (0.82%) nature of the ε-Ge bonded with arsenic-terminated (Ge_{As-terminated}) and indium-terminated (Ge_{In-terminated}) In_0.24Ga_0.76As stressors, respectively. High-resolution cross-sectional transmission electron microscopy showed a coherent, sharp, and fully strained ε-Ge/In_0.24Ga_0.76As heterointerface in the Ge_{As-terminated} heterostructure,more » whereas microtwin defects were present in the Ge_{In-terminated} heterostructure. These heterostructures were further characterized by evaluating the minority carrier lifetimes, high for Ge_{As-terminated} (525 ns) and low for Ge_{In-terminated} (69 ns), using the photoconductive decay technique. Moreover, band alignment was constructed using X-ray photoelectron spectroscopy, where the Ge_{As-terminated} heterostructure revealed that both holes and electrons were confined within the ε-Ge active layer as a type-I band alignment with ΔE_{V, As-terminated} = 0.22 eV and ΔE_{C,As-terminated} = 0.38 eV. On the other hand, the Ge_{In-terminated} heterostructure exhibited a type-II band alignment with ΔE_{V,In-terminated} = – 0.02 eV and ΔE_{C,In-terminated} = 0.53 eV. Furthermore, the magnetotransport properties revealed high mobility (321 cm²/(V s)) with single-electron transport in Ge_{As-terminated} heterostructure and low mobility (3.34 cm²/(V s)) with multihole transport in the Ge_{In-terminated} heterostructure. Therefore, preferring the ε-Ge on the arsenic-rich surface of In_0.24Ga_0.76As stressor over the indium-rich surface during material synthesis offers device-quality materials with high carrier lifetime and superior carrier confinement, which can provide an opportunity to fabricate efficient group-IV-based optoelectronic devices.« less

Charge carrier dynamics in three-dimensional (3D) perovskites is critical to understand for enhancing device performance since the photoabsorption mechanism in perovskites influences key functional devices such as photodetectors and solar cells. Temperature-dependent optoelectronic transport measurements were conducted on our triple cation formulation for the first time from 4 K to 300 K to investigate the role of an interfacial Ti underlayer, beneath the conventional Au collector electrode. The photocurrent was 10X larger with the use of a Ti interfacial layer compared to only Au, where device measurements were made using a broadband white light source at room temperature. As temperaturemore » increased from 4 K, the photocurrent increased in both cases, consistent with the semiconducting nature of the triple-cation absorber. Besides computing the responsivity as a function of power and temperature, time-domain measurements with ON/OFF pulses of incident white light, showed the switching time constants to be in the tens to few hundred milliseconds range, and largely temperature-invariant for the two contacts examined. Finally, we constructed solar cells with the same triple cation absorber, in an n-i-p architecture with a Spiro-OMeTAD hole transport layer, but the collector was composed of both types of contacts. Exposing our devices to moisture-rich conditions of up to 70% relative humidity showed the Au/Ti contacted devices to be more robust. Here, our experimental results demonstrate that the addition of a Ti interlayer improves collector efficiency through the photoabsorption process while also potentially stabilizing the solar cells, compared to the bare Au, in moisture-rich environments« less

Hyperbolic plasmons are a highly desired property in optoelectronics and biomolecular sensing. The necessary condition to realize hyperbolic plasmons is a significant anisotropy of the principal components of the dielectric function, such that at a certain frequency range, one component is negative and the other is positive, i.e., one component is metallic, and the other one is dielectric. Here, we study the effect of anisotropy in ReS₂, and our theory shows that ReS₂ can host hyperbolic plasmons in the ultraviolet frequency range. The operating frequency range of the hyperbolic plasmons can be tuned with the number of ReS₂ layers. However,more » we note that the significantly large imaginary part of the macroscopic dielectric response in all layered variants of ReS₂ can result in substantial losses for the hyperbolic plasmons, as in the case with other known hyperbolic materials, with the exception of MoOCl₂. We also note that ReS₂ hosts ultraviolet hyperbolic plasmons while ZrSiSe, WTe₂, and CuS nanocrystals host infrared plasmons, providing a novel platform for optoelectronics in the ultraviolet range.« less

Achieving deterministic control over the properties of low-dimensional materials with nanoscale precision is a long-sought goal. Mastering this capability has a transformative effect on the design of multifunctional electrical and optical devices. Here, we present an ion-assisted synthetic technique that enables precise control over the material composition and energy landscape of two-dimensional (2D) atomic crystals. Our method transforms binary transition-metal dichalcogenides, like MoSe2, into ternary MoS2αSe2(1-α) alloys with systematically adjustable compositions, α. By piecewise assembly of the lateral, compositionally modulated MoS2αSe2(1-α) segments within 2D atomic layers, we present a synthetic pathway toward the realization of multicompositional designer materials. Our techniquemore » enables the fabrication of advanced 2D structures with arbitrary boundaries, dimensions as small as 30 nm, and fully customizable energy landscapes. Our optical characterizations further showcase the potential for implementing tailored optoelectronics in these engineered 2D crystals.« less

Wire medium (WM) resonators have emerged as a promising realization for plasma haloscopes—devices designed to detect axions, a potential component of dark matter. Key factors influencing the detection probability include cavity volume, resonance quality factor, and form factor. While the form factor has been explored for resonant frequency tuning, its optimization for axion detection remains unexplored. Here, in this work, we present an approach to significantly enhancing the form factor of WM plasma haloscopes. By shifting the metal walls of the resonator by a quarter wavelength, we effectively convert an electric wall boundary condition into a magnetic wall one, allowingmore » for an almost uniform mode. Theoretical analysis and numerical simulations confirm that this modification improves the electric field profile and boosts the form factor, while also slightly enhancing the quality factor. We validate these findings through experimental results from two prototype resonators: one with a standard geometry and another with a quarter-wave air gap between the WM and the walls. Additionally, our method provides a simple way to control the field profile within WM cavities, which can be explored for further applications.« less

Superconducting radio-frequency (SRF) niobium cavities are critical for modern particle accelerators, as well as for advancing superconducting quantum systems and enabling ultrasensitive searches for new physics. In this work, we report a systematic observation of an anomalous frequency dip in Nb cavities, which occurs at temperatures just below the critical temperature (𝑇𝑐 ), indicative of enhanced superconducting properties at 𝑇 ≪ 𝑇_𝑐. The magnitude of this dip is strongly correlated with the rf surface resistance, impurity distribution near the surface, and 𝑇𝑐 . Additionally, we report measurements of the coherence peak in the ac conductivity of two Nb SRF cavitiesmore » processed using distinct methods. By comparing recent theories developed to model this experimental data, we show that the frequency-dip feature, larger coherence peak height, and reduction in the temperature-dependent surface resistance with rf current occur at minimal but finite levels of disorder.« less

Halide vapor phase epitaxy shows promise for low-cost photovoltaic device manufacturing because of its high growth rates and lower cost elemental precursors but previously has not been used to deposit epitaxial Ge. Here, we demonstrate Ge deposition by generating GeCl₂ in situ from solid Ge and HCl in a N₂ ambient. To achieve Ge growth, we inject AsH₃ and PH₃ as sources of active hydrogen to the growth surface to create a driving force for growth. We do not observe Ge growth unless a supply of hydrogen is added, consistent with thermodynamic calculations. Furthermore, we show the hydrogen source mustmore » crack readily on the substrate surface to enable growth; relatively stable sources such as H₂ do not cause growth. Unintentional group V doping is one drawback of using AsH₃ and PH₃ to drive the Ge reaction. We observed As or P concentrations in the Ge films ranging from 4 x 10¹⁷ to 1 x 10¹⁸ atoms/cm³, concentrations that can drastically influence device characteristics. However, we note there are numerous other "helper molecule" options that can provide active hydrogen without doping or etching the material. This work provides a path forward for Ge deposition for optoelectronic devices from an elemental source.« less

Two-dimensional lead-halide perovskites provide a more robust alternative to three-dimensional perovskites in solar energy and optoelectronic applications due to increased chemical stability afforded by interlayer ligands. At the same time, the ligands create barriers for interlayer charge transport, reducing device performance. Using a recently developed ab initio simulation methodology, we demonstrate that ligand fluorination can enhance both hole and electron mobility by 1–2 orders of magnitude. The simulations show that the enhancement arises primarily from improved structural order and reduced thermal atomic fluctuations in the system rather than increased interlayer electronic coupling. Arising from stronger hydrogen bonding and dipolar interactions,more » the higher structural stability decreases the reorganization energy that enters the Marcus formula and increases the charge transfer rate. The detailed atomistic insights into the electron and hole transfer in layered perovskites indicate that the use of interlayer ligands that make the overall structure more robust is beneficial simultaneously for chemical stability and charge transport, providing an important guideline for the design of new, efficient materials.« less