National Institute Of Materials Physics - Romania

Despite the promise of high-k dielectrics, inherent limitations persist in transistor scaling and enhancing energy efficiency, including a fundamental threshold of 60 mV/dec for increasing drain current by an order of magnitude. Proposed solutions involve negative capacitance at the gate oxide to overcome this barrier using ferroelectric structures. Efforts to understand and regulate the switching dynamics and intricate electrostatic configurations of ferroelectric structures towards achieving negative capacitance regimes have intensified. While standalone ferroelectric capacitors cannot stabilize negative capacitance without external fields, multilayered thin films offer a promising solution. Typically, ferroelectric layers are paired with dielectrics/insulator, demonstrating steady-state negative capacitance, often at nanoscale or specific temperature domains. This study aims to stabilize negative capacitance in ferroelectric structures by inducing internal electric fields, aligning the system near coercivity, particularly in bilayer structures formed by two ferroelectric layers with slight differences in polarization values, such as p-n heterojunctions using Pb (Zr,Ti)O3 PZT) with different doping as Fe, Nb, Bi. Most of these structures exhibit evident amplification of capacitance compared to the equivalent series-connected capacitance, across a large temperature domain. The complex capacitance-frequency characteristic of these structures indicates a complex equivalent circuit. Analysis of these complex circuits compared with simple component layers concludes that at least one of the FE layers in these bilayer structures is in a negative capacitance (NC) state.

This study explores the fabrication and optimization of superstrate-structured antimony sulfide (Sb2S3) thin-film solar cells using RF magnetron sputtering of antimony (Sb) followed by sulfurization. The study systematically investigates the effects of varying absorber and buffer layer thicknesses on the photovoltaic performance of FTO/ CdS/Sb2S3/Spiro-OMeTAD/Au solar cell devices. Analytical techniques confirmed the structural and chemical properties of the Sb2S3 films obtained after Sb post-sulfurization, demonstrating improved crystallinity and a composition consistent with a primarily Sb2S3 phase. Optimizing the Sb2S3 absorber thickness to 100 nm resulted in a maximum power conversion efficiency of the champion device of 2.76%, with enhanced short-circuit current density (J(sc)) up to 14 mA/cm(2) and open-circuit voltage (V-oc) of up to 650 mV. The device exhibited semi-transparency up to 20% in the wavelength range of 380-740 nm, making it suitable for indoor and building-integrated photovoltaic applications. The results underscore the potential of magnetron-sputtered Sb2S3 for emerging transparent thin-film photovoltaics while highlighting the importance of thickness control and interface engineering for efficiency improvements.

Herein, Cu(II)-doped SrSnO3 perovskites (SrSn1-x Cu x O3, namely SSO:Cux) were prepared by a modified Pechini method and applied as supercapacitors (SCs) for the first time. The effect of dopant concentration (x = 1, 2.5, and 5 mol %) was investigated to fine-tune the structural and electronic properties to design potential candidates as SCs. The SSO:Cux samples were characterized by conventional XRD and synchrotron XRD (S-XRD) combined with Rietveld refinements and spectroscopic analyses, such as Raman, FTIR, UV-vis, EPR, and XANES/NEXAFS. The electrochemical performance of the SSO:Cux samples was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic cycling with potential limitation (GCPL). It was evidenced that incorporating 2.5 mol % Cu(II) into the SrSnO3 perovskite lattice (SrSn0.975Cu0.025O3, SSO:Cu2.5) led to a significant change in structural disorder and electronic properties, which play an essential role in creating a mixture of point defects such as reduced Sn3+ and Cu+ cations, and oxygen vacancies (VO). The SC device constructed with the SSO:Cu2.5 material showed a specific capacitance of 613 F g-1 at a scan rate of 1 mV s-1, with a remarkable specific energy density and specific energy power of 25.42 W h kg-1 and 32678.57 W kg-1, respectively, which are higher than those observed for any other available perovskite-based SCs. This performance was primarily attributed to the formation of mixed Sn4+/Sn3+ and Cu2+/Cu+ cations, which alter the structural and electronic properties of SrSnO3. Our findings indicate that an improved capability to store high energy and power may be achieved by fine-tuning the Cu(II) dopant concentration in the lattice and controlling the formation of undesired phases. This offers experimental guidance to design other Cu-doped perovskites as alternative materials for energy storage applications.

This study covers three complementary aspects; it focuses first on analyzing the turn-on and turn-off processes of the power CoolMOS transistor and investigating its switching times. Furthermore, it expands the scope of our previous works by validating the dynamic behaviour of the proposed model of the power CoolMOS transistor and it assesses the influence of the external gate resistance on the device's switching performance. Simulation results for the switching characteristics were verified through experimental measurements. An experimental test was carried out using a resistive load circuit with different external gate resistance values to analyze its impact on the device's switching behavior. The findings confirm the accuracy of the conclusions drawn from the theoretical and simulation analyses presented in this paper.

Due to the importance of buffer layers in interface engineering, the development of more variants and the rational design of materials have a significant influence on the performance of optoelectronic devices. This study provides a strategy to increase device performance by facilitating efficient charge transfer and defect passivation by combining the properties of eco-friendly materials (adenine, cytosine, guanine, thymine, and uracil) with the physicochemical properties of metal oxides. The aim of this paper was to investigate the interaction of zinc oxide (ZnO) nanostructures (seed, nanoparticles, and nanowires) with nucleobase layers and to discuss their potential applications as organic-inorganic interfacial bilayers. The impact is analyzed from structural, morphological, optical, and electrical points of view. Nucleobase-ZnO nanostructure layers present high optical transparency in the visible range. Electrical measurements confirmed that the high surface area of nanowires can enhance interactions with nucleobases, leading to better charge transfer. The results showed that these nucleobase-ZnO nanostructure layers are promising interface materials for enhancing optoelectronic device performance through interfacial charge transport and light management, while enabling the design of environmentally friendly devices.

Integrating two-dimensional transition-metal dichalcogenides with graphene is attractive for low-power memory and neuromorphic hardware, yet sequential wet transfer leaves polymer residues and high contact resistance. We demonstrate a complementary metal-oxide-semiconductor (CMOS)-compatible, transfer-free route in which an atomically thin amorphous MoS2 precursor is RF-sputtered directly onto chemical vapor-deposited few-layer graphene and crystallized by confined-space sulfurization at 800 degrees C. Grazing-incidence X-ray reflectivity, Raman spectroscopy, and X-ray photoelectron spectroscopy confirm the formation of residue-free, three-to-four-layer 2H-MoS2 (roughness: 0.8-0.9 nm) over 1.5 cm x 2 cm coupons. Lateral MoS2/graphene devices exhibit reproducible non-volatile resistive switching with a set transition (SET) near +6 V and an analogue ON/OFF approximate to 2.1, attributable to vacancy-induced Schottky-barrier modulation. The single-furnace magnetron sputtering + sulfurization sequence avoids toxic H2S, polymer transfer steps, and high-resistance contacts, offering a cost-effective pathway toward wafer-scale 2D memristors compatible with back-end CMOS temperatures.

Pnictogen chalcohalide semiconductors are emerging materials with broad potential in energy-related applications, including solar cells, photocatalysis, photodetectors, batteries, supercapacitors, thermoelectric and piezoelectric generators. Their compositional flexibility allows fine tuning of structural and optoelectronic properties. In this study, microcrystalline powders of Sb1-xBixSeI (x = 0-1) were synthesized from binary precursors by a solid-state method in evacuated quartz ampoules. Energy dispersive spectroscopy confirmed the successful substitution of Sb with Bi in Sb1-xBixSeI. The formation of solid solutions was also supported by Raman spectroscopy and X-ray diffraction (XRD). All materials exhibited needle-shaped crystal morphologies and orthorhombic crystal structure (Pnma), regardless of the Bi/Sb ratio. XRD patterns shifted toward smaller angles with increasing Bi content, indicating lattice expansion. Calculated lattice parameters (b and c) increased linearly with Bi incorporation, while the lattice parameter (a) remained constant. Raman spectra exhibited characteristic peaks at 182 cm- 1 for Bi-Se vibration and 209 cm- 1 for Sb-Se vibration, with intensity ratios reflecting Bi content. UV-Vis-NIR diffuse reflectance spectroscopy revealed a direct band gap that decreased from 1.7 eV (SbSeI) to 1.29 eV (BiSeI). Room-temperature photoluminescence measurements exhibited a single emission band, shifting from 1.75 eV to 1.41 eV with increasing Bi content. Ultraviolet photoelectron spectroscopy indicated a shift in the valence band maximum from 0.44 eV (SbSeI) to 1.1 eV (BiSeI). These findings highlight the tunability of Sb1-xBixSeI compounds, offering pathways for optimizing their properties for specific optoelectronic applications.

CuO is a p-type semiconductor that can be found useful in various applications, sensing, photocatalysis or photovoltaics. Better material performance can be achieved by doping. In our study the doping was done using Cr ions and implantation method. Thin film samples were characterised with X-ray photoelectron spectroscopy (XPS) technique to study chemical properties of the films' surface and to determine the in-depth compositional profile of the films before and after annealing of an implanted sample. Spectroscopic ellipsometry was used to extract the dielectric function of CuO thin films. Depolarization measurements are shown as a useful method to quickly study differences between similar samples. XPS measurements proved that before annealing there is a peak of Cr concentration in depth of the sample, which is no longer present after annealing. Measurement of film resistance as function of temperature in range of 150-300 degrees C resulted with 0.82 eV bandgap. Electronic structure obtained with density functional theory calculations (DFT) showed that with Cr doping the energy band gap narrows and the material should become metallic.

Superconducting bulk disks, of 20 mm in diameter and similar to 3.5-mm thickness of MgB2 were prepared by spark plasma sintering. Samples are co-added with 10 wt. % hexagonal BN (h-BN) or graphene (G) and other additives (B4C, Te, cubic BN, fullerene C-60, or Repa-C6H10O7Ge2 (GEP)), where h-BN and G are introduced in the composite to provide full machinability by chipping of the composite and the other additives to modify microstructure and superconducting characteristics. Measurements of trapped magnetic field B-tr for a fixed rate of the applied magnetic field decrease (0.00015 T/s) indicate that samples with G show less flux jumps, but a higher thermomagnetic stability is accompanied by lower values of B-tr than for samples with h-BN. The highest maximum B-tr at 12 K for samples added with h-BN or graphene was recorded for MgB2(Te)(0.01) + 10wt.% h-BN (3.48 T) and MgB2(B4C)(0.01) + 10wt.% G (2.73 T), respectively. These values of maximum trapped field were determined for an applied field of - 2.5 and - 1.8 T. Results suggest that machinable MgB2-based composites show potential for bulk superconducting magnet applications.