arXiv subjects

Robert Kudrawiec

Publications and source records attributed to Robert Kudrawiec.

18 recordsLinked to original sources

arXiv2026

Hydrostatic Pressure Driven Band Gap Tuning and Self-Trapped Exciton Formation in (4FPEA)$_2$SnBr$_{4}$ Halide Perovskite

Two-dimensional tin halide perovskites provide a highly tunable platform for exciton phonon coupling and local lattice distortions, enabled by their intrinsically soft lattice. We report a combined temperature and pressure dependent photoluminescence study of the layered perovskite (4FPEA)$_{2}$SnBr$_{4}$. At room temperature, its optical response is dominated by near band edge (NBE) excitons, which redshift linearly under hydrostatic pressure up to $\sim$3 GPa, indicating a rigid band edge behavior without phase transitions. Cooling reveals a broad, strongly Stokes shifted self-trapped exciton (STE) emission, evidencing a crossover from delocalized to self localized excitonic states. Strikingly, while NBE emission redshifts under pressure, STE emission exhibits an anomalous blueshift, reflecting pressure induced modification of the exciton phonon energy landscape. In contrast, the iodide analogue (4FPEA)$_{2}$SnI$_{4}$ shows no STE emission under identical conditions, highlighting the critical role of lattice rigidity and dielectric screening in stabilizing self-trapped excitons.

cond-mat.mtrl-sci↗

arXiv2025

Sensitivity of excitonic transitions to temperature in monolayers of TMD alloys

Two-dimensional transition metal dichalcogenides (TMDs) offer tunable optical and electronic properties, making them highly promising for next-generation optoelectronic devices. One effective approach to engineering these properties is through alloying, which enables continuous control over the bandgap energy and excitonic transitions. In this study, we perform temperature-dependent transmission spectroscopy on monolayers of Mo1-xWxS2 and Mo(S1-xSex)2 alloys, transferred onto the core of an optical fiber and measured within a cryostat over a temperature range of 20-320 K. The use of an all-fiber configuration allowed us to probe interband transitions A, B, and C with high stability and precision. We observe a systematic redshift of excitonic transitions with increasing Se content, and a blueshift when Mo is replaced with W. These spectral shifts correlate with alloy composition and enable the tuning of bandgap energies between 1.6 eV and 2.0 eV at room temperature. Furthermore, we analyze the temperature sensitivity of the excitonic transitions, revealing that Se incorporation enhances thermal response in a non-monotonic manner, while W substitution results in a more monotonic and stronger temperature dependence. The splitting between A and B transitions, associated with spin-orbit coupling, also varies with composition. Our findings underscore the potential of compositional engineering in 2D TMD alloys to achieve both spectral and thermal control of optical properties, relevant for the design of robust and tunable optoelectronic systems.

physics.app-ph↗

arXiv2024

Linear dichroism of the optical properties of SnS and SnSe van der Waals crystals

Tin monochalcogendies SnS and SnSe, belonging to a familiy of van der Waals crystals isoelectronic to black phosphorus, are know as enivornmetally-friendly materials promisng for thermoelecric conversion applications. However, they exhibit other desired functionalities, such as intrisic linear dichroism of the optical and electronic properties originating from strongly anisotropic orthorhombic crystal structure. This property makes them perfect candidats for polarization-sensitive photodetectors working in near infrared spectral range. We present a comprehensive study of the SnS and SnSe crystals by means of optical spectroscopy and photoemission spectroscopy, supported by ab initio calcualtions. The studies revealed the high sensitivity of the optical response of both materials to the incident light polarization, which we interpret in terms of the electronic band dispersion and orbital composition of the electronic bands, dictating the selection rules. From the photoemission investigation we determine the ionization potential, electron affinity and work function, which are parameters crucial for the design of devices based on semiconductor heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

Electronic and spectral properties of Ge1-xSnx quantum dots: an atomistic study

In this paper, we study theoretically the electron and spectral properties of Ge1-xSnx systems, including alloys, cubic- and spherical quantum dots. The single-particle electron and hole states are calculated within the sp3d5s* tight-binding approach and used in further modeling of the optical properties. We systematically study the interplay of Sn-driven indirect-direct band-gap transition and the quantum confinement effect in systems of reduced dimensionality. We demonstrate the regime of sizes and composition, where the ground state in Ge1-xSnx quantum dot is optically active. Finally, we calculate absorbance spectra in experimentally-relevant colloidal quantum dots and demonstrate a satisfactory agreement with experimental data.

cond-mat.mes-hall↗

arXiv2024

Material gain and eight-band k.p description for selected perovskites

In this work, we present a ready-to-use symmetry invariant expansion form of the eight-band k.p Hamiltonian for inorganic and organic metal halide perovskites (CsPbX$_3$ and MAPbX$_3$ with $X = \{$Cl, Br, I$\}$). We use the k.p model to calculate the electronic band structures for perovskite materials of cubic and pseudo-cubic phase. In order to find respective parameters, the band structures of considered materials were obtained within state-of-the-art density functional theory and used next as targets to adjust the k.p bands and determine the values of k.p parameters. The calculated band structures were used to obtain the material gain for bulk crystals (CsPbCl$_3$, CsPbBr$_3$, CsPbI$_3$, MAPbCl$_3$, MAPbBr$_3$ and MAPbI$_3$) which is compared with the material gain in well-established III-V semiconductors. It was found that for these perovskites a positive material gain appears at lower carrier density than for the reference materials (GaAs and InP). We demonstrate that from the point of view of the electronic band structure, the studied perovskites are very promising gain medium for lasers.

cond-mat.mtrl-sci↗

arXiv2024

Electronic and structural properties of group IV materials and their polytypes

Nanotechnology's impact on semiconductor industry advancement, particularly through the engineering of nanostructures like nanowires, opens new possibilities for material functionality due to the tunable physical properties of nanostructures compared to bulk materials. This paper presents a comprehensive study on group IV semiconductors and their binaries across four polytypes: 2H, 3C, 4H, and 6H, focusing on their optoelectronic application potential. Deep understanding of these polytypes is particularly relevant for nanowire-based technologies. Through first principles modeling, we examine the structural and electronic properties of these materials, emphasizing their band structure, stability, and the feasibility for light-emitting applications. We use a generalized Ising model to discuss materials stability and tendency for polytypism. We also determine relative band edge positions and employ a six $k\cdot p$ model for a detailed understanding of the materials' electronic properties. Due to the comprehensive nature of this study, we provide insight on the chemical trends present in all of the studied properties. Our theoretical predictions align well with existing experimental data, suggesting new avenues for nanostructure-based device development. The discussion extends to the implications of these findings for the fabrication of optoelectronic devices with the studied IV-IV materials, highlighting the challenges and opportunities for future research in nanowire synthesis and their application.

cond-mat.mtrl-sci↗

arXiv2024

The Defects Genome of 2D Janus Transition Metal Dichalcogenides

Two-dimensional (2D) Janus Transition Metal Dichalcogenides (TMDs) have attracted much interest due to their exciting quantum properties arising from their unique two-faced structure, broken-mirror symmetry, and consequent colossal polarisation field within the monolayer. While efforts have been made to achieve high-quality Janus monolayers, the existing methods rely on highly energetic processes that introduce unwanted grain-boundary and point defects with still unexplored effects on the material's structural and excitonic properties Through High-resolution scanning transmission electron microscopy (HRSTEM), density functional theory (DFT), and optical spectroscopy measurements; this work introduces the most encountered and energetically stable point defects. It establishes their impact on the material's optical properties. HRSTEM studies show that the most energetically stable point defects are single (Vs and Vse) and double chalcogen vacancy (Vs-Vse), interstitial defects (Mi), and metal impurities (MW) and establish their structural characteristics. DFT further establishes their formation energies and related localized bands within the forbidden band. Cryogenic excitonic studies on h-BN-encapsulated Janus monolayers offer a clear correlation between these structural defects and observed emission features, which closely align with the results of the theory. The overall results introduce the defect genome of Janus TMDs as an essential guideline for assessing their structural quality and device properties.

cond-mat.mtrl-sci↗

arXiv2022

Stress-Tuned Optical Transitions in Layered 1T-MX2 (M= Hf, Zr, Sn; X= S, Se) Crystals

Optical measurements under externally applied stresses allow us to study the materials' electronic structure by comparing the pressure evolution of optical peaks obtained from experiments and theoretical calculations. We examine the stress-induced changes in electronic structure for the thermodynamically stable 1T polytype of selected MX2 compounds (M=Hf, Zr, Sn; X=S, Se), using the density functional theory. We demonstrate that considered 1T-MX2 materials are semiconducting with indirect character of the band gap, irrespective to the employed pressure as predicted using modified Becke-Johnson potential. We determine energies of direct interband transitions between bands extrema and in band-nesting regions close to Fermi level. Generally, the studied transitions are optically active, exhibiting in-plane polarization of light. Finally, we quantify their energy trends under external hydrostatic, uniaxial, and biaxial stresses by determining the linear pressure coefficients. Generally, negative pressure coefficients are obtained implying the narrowing of the band gap. The semiconducting-to-metal transition are predicted under hydrostatic pressure. We discuss these trends in terms of orbital composition of involved electronic bands. In addition, we demonstrate that the measured pressure coefficients of HfS2 and HfSe2 absorption edges are in perfect agreement with our predictions. Comprehensive and easy-to-interpret tables containing the optical features are provided to form the basis for assignation of optical peaks in future measurements.

cond-mat.mtrl-sci↗

arXiv2022

Strong substrate strain effects in multilayered WS2 revealed by high-pressure optical measurements

The optical properties of two-dimensional materials can be effectively tuned by strain induced from a deformable substrate. In the present work we combine first-principles calculations based on density functional theory and the effective Bethe-Salpeter equation with high-pressure optical measurements in order to thoroughly describe the effect of strain and dielectric environment onto the electronic band structure and optical properties of a few-layered transition metal dichalcogenide. Our results show that WS2 remains fully adhered to the substrate at least up to a -0.6% in-plane compressive strain for a wide range of substrate materials. We provide a useful model to describe effect of strain on the optical gap energy. The corresponding experimentally-determined out-of-plane and in-plane stress gauge factors for WS2 monolayers are -8 and 24 meV/GPa, respectively. The exceptionally large in-plane gauge factor confirm transition metal dichalcogenides as very promising candidates for flexible functionalities. Finally, we discuss the pressure evolution of an optical transition closely-lying to the A exciton for bulk WS2 as well as the direct-to-indirect transition of the monolayer upon compression.

cond-mat.mtrl-sci↗

arXiv2022

The effect of isovalent doping on the electronic band structure of group IV semiconductors

The band gap engineering of group IV semiconductors has not been well explored theoretically and experimentally, except for SiGe. Recently, GeSn has attracted much attention due to the possibility of obtaining a direct band gap in this alloy, thereby making it suitable for light emitters. Other group IV alloys may also potentially exhibit material properties useful for device applications, expanding the space for band gap engineering in group IV. In this work the electronic band structure of all group IV semiconductor alloys is investigated. Twelve possible A:B alloys, where A is a semiconducting host (A = C, Si, and Ge) and B is an isovalent dopant (B = C, Si, Ge, Sn, and Pb), were studied in the dilute regime (0.8%) of the isovalent dopant in the entire Brillouin zone (BZ), and the chemical trends in the evolution of their electronic band structure were carefully analyzed. Density functional theory with state-of-the-art methods such as meta-GGA functionals and a spectral weight approach to band unfolding from large supercells was used to obtain dopant-related changes in the band structure, in particular the direct band gap at the Γ point and indirect band gaps at the L(X) points of the BZ. Analysis of contributions from geometry distortion and electronic interaction was also performed. Moreover, the obtained results are discussed in the context of obtaining a direct fundamental gap in Ge:B (B = C, Sn, and Pb) alloys, and intermediate band formation in C:B (B = Sn and Pb) and Ge:C. An increase in localization effects is also observed: a strong hole localization for alloys diluted with a dopant of a larger covalent radius and a strong electron localization for alloys with a dopant of smaller radius. Finally, it is shown that alloying Si and Ge with other elements from group IV is a promising way to enhance the functionality of group IV semiconductors.

cond-mat.mtrl-sci↗

arXiv2022

Invariant expansion of the 30-band k.p model and its parameters for III-V compounds

In this work we derive a ready-to-use symmetry invariant expansion of the full-zone 30-band k.p Hamiltonian for the Td point group. In order to find respective parameters, the band structures of III-V materials were calculated within a state-of-the-art Density Functional Theory (DFT) approach and used next as targets to adjust the k.p bands. A satisfactory agreement of the k.p model with the DFT band structures, for all the tested zinc blende III-V semiconductors, has been achieved. Values of many of the parameters have not been known so far. We also compare the fitted k.p parameters with the values calculated using momentum matrix elements obtained directly from the DFT.

cond-mat.mes-hall↗

arXiv2022

Spectroscopy and structural investigation of iron phosphorus trisulfide -- FePS$_3$

Lamellar structures of transition metal phosphorus trisulfides possess strong intralayer bonding, albeit adjacent layers are held by weak van der Waals interactions. Those compounds received enormous interest due to their unique combination of optical and long-range magnetic properties. Among them, iron phosphorus trisulfide (FePS$_3$) gathered special attention for being a semiconductor with an absorption edge in the near-infrared, as well as showing an Ising-like anti-ferromagnetism. We report a successful growth of centimeter size bulk FePS$_3$ crystals with a chemical yield above 70%, whose crystallographic structure and composition were carefully identified by advanced electron microscopy methodologies, including atomic resolution elemental mapping, along with photoelectron spectroscopy. The knowledge on the optical activity of FePS$_3$ is extended utilizing temperature-dependent absorption and photoacoustic spectroscopies, while measurements were corroborated with density-functional theory calculations. Temperature-dependent experiments showed a small and monotonic band-edge energy shift down to 115 K and exposed the interconnected importance of electron-phonon coupling. Most of all, the correlation between the optical behavior and the magnetic phase transition is revealed, which shows the practical utilization of temperature-dependent optical absorption to investigate magnetic interactions.

cond-mat.mtrl-sci↗

arXiv2021

Modified Band Alignment Method to Obtain Hybrid Functional Accuracy from Standard DFT: Application to Defects in Highly Mismatched III-V:Bi Alloys

This paper provides an accurate theoretical defect energy database for pure and Bi-containing III-V (III-V:Bi) materials and investigates efficient methods for high-throughput defect calculations based on corrections of results obtained with local and semi-local functionals. Point defects as well as nearest-neighbor and second-nearest-neighbor pair defects were investigated in charge states ranging from -5 to 5. Ga-V:Bi systems (GaP:Bi, GaAs:Bi, and GaSb:Bi) were thoroughly investigated with significantly slower, higher fidelity hybrid Heyd-Scuseria-Ernzerhof (HSE) and significantly faster, lower fidelity local density approximation (LDA) calculations. In both approaches spurious electrostatic interactions were corrected with the Freysoldt correction. The results were verified against available experimental results and used to assess the accuracy of a previous band alignment correction. Here, a modified band alignment method is proposed in order to better predict the HSE values from the LDA ones. The proposed method allows prediction of defect energies with values that approximate those from the HSE functional at the computational cost of LDA (about 20x faster for the systems studied here). Tests of selected point defects in In-V:Bi materials resulted in corrected LDA values having a mean absolute error (MAE)=0.175 eV for defect levels vs. HSE. The method was further verified on an external database of defects and impurities in CdX (X=S, Se, Te) systems, yielding a MAE=0.194 eV. These tests demonstrate the correction to be sufficient for qualitative and semi-quantitative predictions, and may suggest transferability to many semiconductor systems without significant loss in accuracy. Properties of the remaining In-V:Bi defects and all Al-V:Bi defects were predicted with the use of the modified band alignment method.

cond-mat.mtrl-sci↗

arXiv2020

Inhomogeneous broadening of optical transitions observed in photoluminescence and modulated reflectance of polar and non-polar InGaN quantum wells

In this work the broadening of interband transitions in InGaN/GaN quantum wells (QWs) resulting from structural inhomogeneities is analyzed. The role of polarization-induced electric field in the mechanism behind the inhomogeneous broadening observed in photoluminescence (PL) and electromodulated reflectance (ER) spectra of InGaN QWs dedicated to green/blue lasers is explained. Spectra of both polar and non-polar QWs are simulated within the random QW model distinguishing contributions of individual transitions taking into account QW inhomogeneities (QW width and indium content fluctuations). On this basis we interpret the ER and PL spectra measured for a polar multiple QW InGaN/GaN structure. The built-in electric field shifts the emission wavelength to red, and enhances the broadening of optical transitions. It is clearly shown that for polar QWs the Stokes shift can be easily overestimated if PL spectra are compared with ER spectra since the intensity of the fundamental transition observed in ER spectra significantly decreases with the increase in QW width. In this way ER signal related to excited states start to dominate. This effect is strongly enhanced by QW inhomogeneities.

cond-mat.mes-hall↗

arXiv2019

Phase Transition Induced Carrier Mass Enhancement in 2D Ruddlesden-Popper Perovskites

There is a variety of possible ways to tune the optical properties of 2D perovskites, though the mutual dependence between different tuning parameters hinders our fundamental understanding of their properties. In this work we attempt to address this issue for (C$_n$H$_{2n+1}$NH$_3$)$_2$PbI$_4$ (with n=4,6,8,10,12) using optical spectroscopy in high magnetic fields up to 67T. Our experimental results, supported by DFT calculations, clearly demonstrate that the exciton reduced mass increases by around 30% in the low temperature phase. This is reflected by a 2-3 fold decrease of the diamagnetic coefficient. Our studies show that the effective mass, which is an essential parameter for optoelectronic device operation, can be tuned by the variation of organic spacers and/or moderate cooling achievable using Peltier coolers. Moreover, we show that the complex absorption features visible in absorption/transmission spectra track each other in magnetic field providing strong evidence for the phonon related nature of the observed side bands.

cond-mat.mes-hall↗

arXiv2019

Electronic band structure of nitrogen diluted Ga(PAsN): Formation of the intermediate band, direct and indirect optical transitions, localization of states

The electronic band structure of Ga(PAsN) with a few percent of nitrogen is calculated in the whole composition of Ga(PAs) host using the state-of-the-art density functional methods including the modified Becke-Johnson functional to correctly reproduce the band gap, and band unfolding to reveal the character of the bands within the entire Brillouin zone. As expected, relatively small amounts of nitrogen introduced to Ga(PAs) lead to formation of an intermediate band below the conduction band which is consistent with the band anticrossing model, widely used to describe the electronic band structure of dilute nitrides. However, in this study calculations are performed in the whole Brillouin zone and reveal the significance of correct description of the band structure near the edges of Brillouin zone, especially for indirect band gap P-rich host alloy, which may not be properly captured with simpler models. The theoretical results are compared with experimental studies, confirming their reliability. The influence of nitrogen on the band structure is discussed in terms of application of Ga(PAsN) in optoelectronic devices such as intermediate band solar cells and light emitters. It is found that Ga(PAsN) with low N and As concentration has a band structure suitable for integration in Si tandem solar cells, since the lattice mismatch between Si and Ga(PAsN) is small in this case. Moreover, it is concluded that P-rich Ga(PAsN) alloys with low N concentration have a promising band structure for two colour emitters. Additionally, the effect of nitrogen incorporation on the carrier localization is studied and discussed.

cond-mat.mtrl-sci↗

arXiv2019

Strain and Band-Gap Engineering in Ge-Sn Alloys via P Doping

Ge with a quasi-direct band gap can be realized by strain engineering, alloying with Sn, or ultrahigh n-type doping. In this work, we use all three approaches together to fabricate direct-band-gap Ge-Sn alloys. The heavily doped n-type Ge-Sn is realized with CMOS-compatible nonequilibrium material processing. P is used to form highly doped n-type Ge-Sn layers and to modify the lattice parameter of P-doped Ge-Sn alloys. The strain engineering in heavily-P-doped Ge-Sn films is confirmed by x-ray diffraction and micro Raman spectroscopy. The change of the band gap in P-doped Ge-Sn alloy as a function of P concentration is theoretically predicted by density functional theory and experimentally verified by near-infrared spectroscopic ellipsometry. According to the shift of the absorption edge, it is shown that for an electron concentration greater than 1x10^20 cm-3 the band-gap renormalization is partially compensated by the Burstein-Moss effect. These results indicate that Ge-based materials have high potential for use in near-infrared optoelectronic devices, fully compatible with CMOS technology.

cond-mat.mtrl-sci↗

arXiv2018

Highly efficient optical transition between excited states in wide InGaN quantum wells

There is a lack of highly efficient light emitting devices (LEDs) operating in the green spectral regime. The devices based on (In,Al)GaN show extremely high efficiencies in violet and blue colors but fall short for longer emission wavelengths due to the quantum confined Stark effect (QCSE). In this paper we present a design of the active region based on wide InGaN quantum wells (QWs) which do not suffer from QCSE and profit from an enhancement in the internal quantum efficiency (IQE). The design exploits highly efficient optical transitions between excited states. It is shown that, counterintuitively, the devices with higher InGaN composition exhibit a higher enhancement in IQE. Experimental evidence is provided showing a gradual change in the nature of the optical transition with increasing thickness of the QW. Moreover, optical gain in long wavelength LDs incorporating standard and wide QWs is investigated to show the utilization of our concept.

physics.app-ph↗