Steponas Ašmontas

Bio: Steponas Ašmontas is an academic researcher from Lithuanian Academy of Sciences. The author has contributed to research in topics: Microwave & Semiconductor. The author has an hindex of 12, co-authored 146 publications receiving 615 citations.

Silicon lens-coupled bow-tie InGaAs-based broadband terahertz sensor operating at room temperature

Abstract: A passive detection scheme for broadband, 10 GHz-252 THz, sensing at room temperature is demonstrated using a hemispherical silicon lens-coupled diode of an asymmetrically-shaped bow-tie geometrical form The device is fabricated from an MBE-grown In/sub 054/Ga/sub 046/As wafer as mesas of 3 /spl mu/m depth produced by wet etching The detector exhibits voltage sensitivity about 5 V/W below 1 THz

Cesium-Containing Triple Cation Perovskite Solar Cells

Abstract: Cesium-containing triple cation perovskites are attracting significant attention as suitable tandem partners for silicon solar cells. The perovskite layer of a solar cell must strongly absorb the visible light and be transparent to the infrared light. Optical transmittance measurements of perovskite layers containing different cesium concentrations (0–15%) were carried out on purpose to evaluate the utility of the layers for the fabrication of monolithic perovskite/silicon tandem solar cells. The transmittance of the layers weakly depended on cesium concentration in the infrared spectral range, and it was more than 0.55 at 997 nm wavelength. It was found that perovskite solar cells containing 10% of cesium concentration show maximum power conversion efficiency.

Detection of terahertz/sub-terahertz radiation by asymmetrically-shaped 2DEG layers

Abstract: A diode structure to detect THz/subTHz radiation based on an MBE-grown modulation-doped GaAs/Al0.25Ga0.75As structure is proposed. Devices have an asymmetrically-shaped geometrical form in the plane of the structure and are fabricated as mesas of 2 µm depth by wet etching. The incident terahertz/sub-terahertz radiation induces a voltage signal over the ends of the sample. Detection by non-uniform carrier heating effects under external illumination is explained and the device operation from 10 GHz up to 2.52 THz at room temperature is demonstrated.

Excitonic and impurity-related optical transitions in Be δ-doped GaAsAlAs multiple quantum wells: Fractional-dimensional space approach

Abstract: We have investigated the optical transitions in Be $\ensuremath{\delta}$-doped $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ multiple quantum wells with various width and doping levels. The fractional dimensionality model was extended to describe free-electron--acceptor (free hole-donor) transitions in a quantum well. The measured photoluminescence spectra from the samples were interpreted within the framework of this model, and acceptor-impurity induced effects in the photoluminescence line shapes from multiple quantum wells of different widths were demonstrated.

Giga- and terahertz frequency band detector based on an asymmetrically necked n-n+-GaAs planar structure

Abstract: We describe the concept of an asymmetrically shaped n-n+-planar GaAs diode whose operation is based on nonuniform free charge carrier heating effects. The detector was fabricated on thin elastic polyimide film and has been shown to have an operational bandwidth for detection ranging from 30 GHz up to 2.5 THz at room temperature. The voltage sensitivity of the detector is nearly independent of the frequency from 30 GHz up to 0.7 THz and is close to 0.3 V/W. In the upper section of the frequency range, 0.7–2.5 THz, the sensitivity is limited by the electron momentum relaxation time. The induced signal responds linearly to the incident power over the frequency range studied. It is shown that the performance of the detector can be explained well by a phenomenological theory.

Strain engineering in perovskite solar cells and its impacts on carrier dynamics.

Abstract: The mixed halide perovskites have emerged as outstanding light absorbers for efficient solar cells. Unfortunately, it reveals inhomogeneity in these polycrystalline films due to composition separation, which leads to local lattice mismatches and emergent residual strains consequently. Thus far, the understanding of these residual strains and their effects on photovoltaic device performance is absent. Herein we study the evolution of residual strain over the films by depth-dependent grazing incident X-ray diffraction measurements. We identify the gradient distribution of in-plane strain component perpendicular to the substrate. Moreover, we reveal its impacts on the carrier dynamics over corresponding solar cells, which is stemmed from the strain induced energy bands bending of the perovskite absorber as indicated by first-principles calculations. Eventually, we modulate the status of residual strains in a controllable manner, which leads to enhanced PCEs up to 20.7% (certified) in devices via rational strain engineering. The residual strains in the mixed halide perovskite thin films and their effects on the solar cell devices are less understood. Here Zhu et al. study the impact of the gradient in-plane strain on the carrier dynamics of the strained perovskite films and optimize the device efficiency.

The Monte Carlo method for semiconductor device simulation

Abstract: There can be little doubt that the Monte Carlo method for semiconductor device simulation has enormous power as a research tool. It represents a detailed physical model of the semiconductor material(s), and provides a high degree of insight into the microscopic transport processes. However, if the authority ascribed to Monte Carlo models of devices at 1/spl mu/m feature size is to be maintained for devices below O.1/spl mu/m, modelling of the fundamental physics must be further improved. And if the Monte Carlo method is to be successful as a semiconductor device design tool, the device model must be made more realistic. Success in the industrial sector depends on this, but also on achieving fast run-times optimisation - where the scope and need for ingenuity is now greatest.

A review of terahertz sources

Abstract: Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.

Rational design of high-responsivity detectors of terahertz radiation based on distributed self-mixing in silicon field-effect transistors

Abstract: In search of novel detectors of electromagnetic radiation at terahertz frequencies, field-effect transistors (FETs) have recently gained much attention. The current literature studies them with respect to the excitation of plasma waves in the two-dimensional channel. Circuit aspects have been taken into account only to a limited degree. In this paper, we focus on embedding silicon FETs in a proper circuitry to optimize their responsivity to terahertz radiation. This includes impedance-matched antenna coupling and amplification of the rectified signal. Special attention is given to the investigation of high-frequency short-circuiting of the gate and drain contacts by a capacitive shunt, a common approach of high-frequency electronics to induce resistive mixing in transistors. We theoretically study the effect of shunting in the framework of the Dyakonov–Shur plasma-wave theory, with the following key results. In the quasistatic limit, the capacitive shunt induces the longitudinal high-frequency field neede...