arXiv subjects

Vanya Darakchieva

Publications and source records attributed to Vanya Darakchieva.

At least 19 recordsLinked to original sources

arXiv2025

Impact of AlN buffer thickness on electrical and thermal characteristics of AlGaN/GaN/AlN HEMTs

We investigate the influence of AlN buffer thickness on the structural, electrical, and thermal properties of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on semi-insulating SiC substrates by metal-organic chemical vapor deposition. X-ray diffraction and atomic force microscopy reveal that while thin AlN layers (120 nm) exhibit compressive strain and smooth step-flow surfaces, thicker single-layer buffers (550 nm) develop tensile strain and increased surface roughness. Multi-layer buffer structures up to 2 μm alleviate strain and maintain surface integrity. Low-temperature Hall measurements confirm that electron mobility decreases with increasing interface roughness, with the highest mobility observed in the structure with a thin AlN buffer. Transient thermoreflectance measurements show that thermal conductivity (ThC) of the AlN buffer increases with the thickness, reaching 188 W/m.K at 300 K for the 2 μm buffer layer, which is approximately 60% of the bulk AlN ThC value. These results highlight the importance of optimizing AlN buffer design to balance strain relaxation, thermal management, and carrier transport for high-performance GaN-based HEMTs.

cond-mat.mtrl-sci↗

arXiv2025

Thermal transport in GaN/AlN HEMTs on 4H-SiC: Role of layer thickness and hetero-interfaces

Thermal transport in high-electron-mobility-transistor (HEMT) structures grown on 4H-SiC substrates by metalorganic-vapour-phase epitaxy (MOCVD) is systematically investigated. The thermal conductivity of the GaN channel and AlN buffer layers is measured by thermoreflectance (TTR). A pronounced thickness dependence of thermal conductivity as a result of phonon-boundary scattering is observed at low temperatures, while this effect becomes significantly weaker at elevated temperatures. The thermal boundary resistance (TBR) at the AlN/4H-SiC and GaN/AlN interfaces is also examined, showing a substantial reduction and eventual saturation with increasing temperature, indicating elastic phonon transport as the dominant mechanism. Reliable simulations of the temperature profile across the structures based on the measured thermal metrics highlight the critical role of TBR in thin-channel device and the advantage of thicker channel and buffer layers for efficient heat dissipation in the HEMTs.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz frequency-domain 4x4 Mueller matrix ellipsometer instrument designed for high-frequency magnetic resonance measurements

We report a Mueller matrix ellipsometer design using dual continuously rotating anisotropic meta wave plates which determines the full set of Mueller matrix elements in the terahertz spectral range. The instrument operates in the frequency domain and employs a frequency tunable, solid state synthesizer based, continuous wave terahertz source with sub-MHz bandwidth. The implemented source permits operation within 82-125 GHz and 170-250 GHz, without and with an additional frequency doubler, respectively. The terahertz transparent meta wave plates consist of 3D-printed polymer based columnar thin film structures with subwavelength dimensions. The rotating wave plates produce sufficient modulation of the Stokes vector components of transmitted terahertz light to use the wave plates in polarization state generator and polarization state detector devices. Fast terahertz light detection rate of a quasi optical solid state detector permits acquisition with few microsecond temporal resolution and electronic sweeping control of the source frequency. We develop a fast frequency sweeping scheme while continuously rotating the terahertz wave plates. Subsequent sorting of measured data permits measurements of Mueller matrix elements at hundreds of different wavelengths. The Mueller matrix elements are obtained by forward numerical reduction of the measured data using an algorithm developed by Ruder et al.[Opt. Lett. 45, 3541 (2020)]. The instrument is combined with a magnetocryostat capable of reaching magnetic field strengths at the sample position of -8 T...8 T and sample temperatures from 4 K...400 K. Hence, the instrument is suitable to measure the full Mueller matrix of samples with magnetic resonances as demonstrated recently by Rindert et al. [Phys. Rev. B 110, 054413 (2024)]. We discuss design, calibration, and example applications.

physics.ins-det↗

arXiv2024

The paramagnetic Lyddane-Sachs-Teller relation

In this letter, we derive an expression for magnetic dipole transitions that is analogous to the Lyddane-Sachs-Teller relation for dielectric polar lattice vibrations. We thereby define transverse and longitudinal optical frequencies at which paramagnetic resonance and antiresonance occurs, respectively. The relation found here thus permits non-invasive optical analysis of static magnetization properties in paramagnetic materials. Validated through terahertz electron paramagnetic resonance ellipsometry and superconducting quantum interference device measurements on Iron-doped Gallium Nitride, our findings show very good agreement between theory and experiment. We term the excitations associated with the paramagnetic transitions as paramagnetic polaritons which may find use in future photonic applications

cond-mat.mtrl-sci↗

arXiv2024

The strain-stress relationships for coherent in-plane strain in heterostructures with monoclinic crystal systems: $β$-(Al$_x$Ga$_{1-x}$)$_2$O$_3$ on $(h0l)$ $β$-Ga$_2$O$_3$ as example

In this work we derive the state of strain or stress under symmetry conserving conditions in pseudomorphic lattices with monoclinic symmetry. We compare surface vectors across the template epitaxial layer interface and impose conditions of a stress free epitaxial layer. As a result, we demonstrate the existence, in theory, of exactly three possible unit cells which can establish onto a given template. We demonstrate this approach for a class of templates with $(h0l)$ planes and $β$-(Al$_x$Ga$_{1-x}$)$_2$O$_3$ on $(h0l)$ $β$-Ga$_2$O$_3$. We discuss the effects of composition $x$ and surface orientation onto the formation of three elastically stable unit cells, their strain and stress tensors, unit cell axes, unit cell volumes, lattice spacing, elastic potential energies, and stress free directions. The previous paradigm for epitaxial layer growth where the stress free direction is always perpendicular to the growing surface is not generally valid for low symmetry materials. In the example here, we find two possible competing domains with stress free direction oblique to the surface of the template for almost all planes $(h0l)$. We calculate the band-to-band transitions for $β$-(Al$_{0.1}$Ga$_{0.9}$)$_2$O$_3$ on $(h0l)$ $β$-Ga$_2$O$_3$ using the composition dependent deformation parameters and elastic coefficients reported prevoiously [Korlacki~\textit{et al.} Phys. Rev. Appl.~\textbf{18}, 064019 (2022)].

cond-mat.mtrl-sci↗

arXiv2024

Bloch equations in Terahertz magnetic-resonance ellipsometry

A generalized approach derived from Blochs equation of motion of nuclear magnetic moments is presented to model the frequency, magnetic field, spin density, and temperature dependencies in the electromagnetic permeability tensor for materials with magnetic resonances. The resulting tensor model predicts characteristic polarization signatures which can be observed, for example, in fully polarization-resolved Mueller matrix element spectra measured across magnetic resonances as a function of frequency, magnetic field, magnetic moment density, and temperature. When augmented with thermodynamic considerations and suitable Hamiltonian description of the magnetic eigenvalue spectrum, important parameters such as zero-frequency magnetization, spectral amplitude distribution, relaxation time constants, and geometrical orientation parameters of the magnetic moment density can be obtained from comparing the generalized model approach to experimental data. We demonstrate our approach by comparing model calculations with full Mueller matrix element spectra measured at oblique angle of incidence in the terahertz spectral range, across electron spin resonance quintuplet transitions observed in wurtzite-structure GaN doped with iron. Measurements were performed by ellipsometry, using a superconducting cryostat magnet at magnetic fields of 7.23 T and at temperatures of 20 K and 30 K. We detail the occurrence of linear and circular birefringence and dichroism associated with each of the zero-field split spin transitions in the S = 5/2 defect system. We derive the spectral dependence of the magnetic susceptibility function and obtain the temperature and magnetic field dependence of the spin Hamiltonian. Our model correctly predicts the complexity of the polarization signatures observed in the 15 independent elements of the normalized Mueller matrix for both positive and negative magnetic fields.

cond-mat.mtrl-sci↗

arXiv2024

Effective uniaxial dielectric function tensor and optical phonons in ($\bar{2}01$)-plane oriented $β$-Ga$_2$O$_3$ films with equally-distributed six-fold rotation domains

Monoclinic $β$-Ga$_2$O$_3$ films grown on $c$-plane sapphire have been shown to exhibit six $(\bar{2}01)$-plane oriented domains, which are equally-spaced-by-rotation around the surface normal and equally-sized-by-volume that render the film optical response effectively uniaxial. We derive and discuss an optical model suitable for ellipsometry data analysis of such films. We model mid- and far-infrared ellipsometry data from undoped and electrically insulating films with an effective uniaxial dielectric tensor based on projections of all phonon modes within the rotation domains parallel and perpendicular to the sample normal, i.e., to the reciprocal lattice vector $\mathbf{g}_{\bar{2}01}$. Two effective response functions are described by model, and found sufficient to calculate ellipsometry data that best-match measured ellipsometry data from a representative film. We propose to render either effective dielectric functions, or inverse effective dielectric functions, each separately for electric field directions parallel and perpendicular to $\mathbf{g}_{\bar{2}01}$, by sums of Lorentz oscillators, which permit to determine either sets of transverse optical phonon mode parameters, or sets of longitudinal optical phonon mode parameters, respectively. Transverse optical modes common to both dielectric functions can be traced back to single crystal modes with $B_{\mathrm{u}}$ character, while modes with $A_{\mathrm{u}}$ character only appear within the dielectric function for polarization perpendicular to the sample surface. The thereby obtained parameter sets reveal all phonon modes anticipated from averaging over the six-fold rotation domains of single crystal $β$-Ga$_2$O$_3$, but with slightly shifted transverse optical, and completely different longitudinal optical phonon modes.

cond-mat.mtrl-sci↗

arXiv2022

Polarity control by inversion domain suppression in N-polar III-nitride heterostructures

Nitrogen-polar III-nitride heterostructures offer advantages over metal-polar structures in high frequency and high power applications. However, polarity control in III-nitrides is difficult to achieve as a result of unintentional polarity inversion domains (IDs). Herein, we present a comprehensive structural investigation with both atomic detail and thermodynamic analysis of the polarity evolution in low- and high-temperature AlN layers on on-axis and 4$^{\circ}$ off-axis Carbon-face 4H-SiC (000$\bar{1}$) grown by hot-wall metal organic chemical vapor deposition. A polarity control strategy has been developed by variation of thermodynamic Al supersaturation and substrate misorientation angle in order to achieve desired growth mode and polarity. We demonstrate that IDs are totally suppressed for high-temperature AlN nucleation layers when step-flow growth mode is achieved at the off-axis. We employ this approach to demonstrate high quality N-polar epitaxial AlGaN/GaN/AlN heterostructures.

physics.app-ph↗

arXiv2022

Strain and composition dependencies of the near bandgap optical transitions in monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys with coherent biaxial in-plane strain on (010) Ga$_2$O$_3$

The bowing of the energy of the three lowest band-to-band transitions in $β$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ alloys was resolved using a combined density functional theory (DFT) and generalized spectroscopic ellipsometry (GSE) approach. The DFT calculations of the electronic band structure of both, $β$-Ga$_2$O$_3$ and $θ$-Al$_2$O$_3$, allow extracting of the linear portion of the energy shift in the alloys, and provide a method for quantifying the role of coherent strain present in the $β$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ thin films on (010) $β$-Ga$_2$O$_3$ substrates. The energies of band-to-band transitions were obtained using the spectroscopic ellipsometry eigenpolarization model approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)]. After subtracting the effects of strain which also induces additional bowing and after subtraction of the linear portion of the energy shift due to alloying, the bowing parameters associated with the three lowest band-to-band transitions in monoclinic $β$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ are found.

cond-mat.mtrl-sci↗

arXiv2022

Incorporation of Magnesium into GaN Regulated by Intentionally Large Amounts of Hydrogen during Growth by MOCVD

Metalorganic chemical vapor deposition (MOCVD) of GaN layers doped with Mg atoms to the recognized optimum level of [Mg] $\sim2 \times 10^{19}$ cm$^{-3}$ has been performed. In a sequence of MOCVD runs, operational conditions, including temperature and flow rate of precursors, have been maintained except for intentionally larger flows of hydrogen carrier gas fed into the reactor. By employing the largest hydrogen flow of 25 slm in this study, the performance of the as-grown Mg-doped GaN layers has been certified by a room-temperature hole concentration of $p\sim2 \times 10^{17}$ cm$^{-3}$ in the absence of any thermal activation treatment. Experimental evidence is delivered that the large amounts of hydrogen during the MOCVD growth can regulate the incorporation of the Mg atoms into GaN in a significant way so that MgH complex can co-exist with a dominant and evidently electrically active isolated MgGa acceptor.

physics.app-ph↗

arXiv2022

Mg-doping and free-hole properties of hot-wall MOCVD GaN

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range ($2.45\times{10}^{18}~cm^{-3}$ up to $1.10\times{10}^{20}~cm^{-3}$) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analysed in relation to the extended defects, revealed by scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance-voltage (C-V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in-situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art $p$-GaN layers with a low-resistivity and a high free-hole density (0.77 $Ω$.cm and $8.4\times{10}^{17}~cm^{-3}$, respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.

physics.app-ph↗

arXiv2022

Terahertz electron paramagnetic resonance generalized spectroscopic ellipsometry: The magnetic response of the nitrogen defect in 4H-SiC

We report on terahertz (THz) electron paramagnetic resonance generalized spectroscopic ellipsometry (THz-EPR-GSE). Measurements of the field and frequency dependencies of the magnetic response due to the spin transitions associated with the nitrogen defect in 4H-SiC are shown as an example. THz-EPR-GSE dispenses with the need of a cavity, permits independently scanning field and frequency parameters, and does not require field or frequency modulation. We investigate spin transitions of hexagonal ($h$) and cubic ($k$) coordinated nitrogen including coupling with its nuclear spin (I=1), and we propose a model approach for the magnetic susceptibility to account for the spin transitions. From the THz-EPR-GSE measurements we can fully determine the polarization properties of the spin transitions and we obtain $g$ and hyperfine splitting parameters using magnetic field and frequency dependent Lorentzian oscillator lineshape functions. We propose frequency-scanning THz-EPR-GSE as a new and versatile method to study properties of spins in solid state materials.

cond-mat.mtrl-sci↗

arXiv2021

Infrared dielectric functions and Brillouin zone center phonons of $α$-Ga$_2$O$_3$ compared to $α$-Al$_2$O$_3$

We determine the anisotropic dielectric functions of rhombohedral $α$-Ga$_2$O$_3$ by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for $α$-Al$_2$O$_3$, the binary parent compound for the emerging alloy of $α$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single mode or more complex mode behaviors.

cond-mat.mtrl-sci↗

arXiv2021

Electrical Tuning of Plasmonic Conducting Polymer Nanoantennas

Nanostructures of conventional metals offer manipulation of light at the nanoscale but are limited to static behavior due to their fixed material properties. To develop the next frontier of dynamic nanooptics and metasurfaces, we utilize the redox-tunable optical properties of conducting polymers, which were recently shown to be capable of sustaining plasmons in their most conducting oxidized state. Using nanodisks of poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf) as a model system, we present the first electrically tunable conducting polymer nanooptical antennas. In addition to repeated on/off switching of the polymeric nanoantennas, we demonstrate the possibility for gradual electrical tuning of their nanooptical response, which was found to be related to the modulation of both density and mobility of the mobile polaronic charge carriers in the polymer. The presented concept takes important steps towards electrically tunable metasurfaces with truly dynamic optical nanoantenna pixels, with not only varying farfield but also tunable nearfield. The work paves the way for applications ranging from tunable flat metaoptics to adaptable smart windows.

physics.optics↗

arXiv2021

Optical phonon modes, static and high frequency dielectric constants, and effective electron mass parameter in cubic In$_2$O$_3$

A complete set of all optical phonon modes predicted by symmetry for bixbyite structure indium oxide is reported here from a combination of far-infrared and infrared spectroscopic ellipsometry, as well as first principle calculations. Dielectric function spectra measured on high quality, marginally electrically conductive melt grown single bulk crystals are obtained on a wavelength-by-wavelength (a.k.a. point-by-point) basis and by numerical reduction of a subtle free charge carrier Drude model contribution. A four-parameter semi-quantum model is applied to determine all sixteen pairs of infrared-active transverse and longitudinal optical phonon modes, including the high-frequency dielectric constant, $\varepsilon_{\infty}=4.05\pm 0.05$. The Lyddane-Sachs-Teller relation then gives access to the static dielectric constant, $\varepsilon_{\mathrm{DC}}=10.55\pm 0.07$. All experimental results are in excellent agreement with our density functional theory calculations and with previously reported values, where existent. We also perform optical Hall effect measurements and determine for the unintentionally doped $n$-type sample a free electron density of $n=(2.81 \pm 0.01)\times 10^{17}$~cm$^{-3}$, mobility of $μ=(112 \pm 3)$~cm$^{2}$/(Vs), and an effective mass parameter of $(0.208\pm0.006)m_e$. Density and mobility parameters compare very well with results of electrical Hall effect measurements. Our effective mass parameter, which is measured independently of any other experimental technique, represents the bottom curvature of the $Γ$ point in In$_2$O$_3$ in agreement with previous extrapolations. We use terahertz spectroscopic ellipsometry to measure the quasi-static response of In$_2$O$_3$, and our model validates the static dielectric constant obtained from the Lyddane-Sachs-Teller relation.

cond-mat.mtrl-sci↗

arXiv2021

Redox-tunable structural colour images based on UV-patterned conducting polymers

Precise manipulation of light-matter interaction has enabled a wide variety of approaches to create bright and vivid structural colours. Techniques utilizing photonic crystals, Fabry-Pérot cavities, plasmonics, or high-refractive index dielectric metasurfaces have been studied for applications ranging from optical coatings to reflective displays. However, complicated fabrication procedures for sub-wavelength nanostructures, limited active areas, and inherent absence of tunability with these approaches significantly impede their further developments towards flexible, large-scale, and switchable devices compatible with facile and cost-effective production. Herein, we present a way to generate structural colours based on conducting polymer thin films prepared on metallic surfaces via vapour phase polymerization and ultraviolet (UV) light patterning. Varying the UV dose leads to synergistic variation of film absorption and thickness, which generates controllable colours from violet to red. Together with greyscale photomasks this enables fabrication of high-resolution colour images using single exposure steps. We further demonstrate spatiotemporal tuning of the structurally coloured surfaces and images via electrochemical modulation of the polymer redox state. The simple structure, facile fabrication, wide colour gamut, and dynamic colour tuning make this concept competitive for future multi-functional and smart displays.

physics.optics↗

arXiv2020

Tunable cavity-enhanced terahertz frequency-domain optical Hall effect

Presented here is the development and demonstration of a tunable cavity-enhanced terahertz frequency-domain optical Hall effect technique. The cavity consists of at least one fixed and one tunable Fabry-Pérot resonator. The approach is suitable for enhancement of the optical signatures produced by the optical Hall effect in semi-transparent conductive layer structures with plane parallel interfaces. The physical principle is the constructive interference of electric field components that undergo multiple optical Hall effect induced polarization rotations upon multiple light passages through the conductive layer stack. Tuning one of the cavity parameters, such as the external cavity thickness, permits shifting of the frequencies of the constructive interference, and enhancement of the optical signatures produced by the optical Hall effect can be obtained over large spectral regions. A cavity-tuning optical stage and gas flow cell are used as examples of instruments that exploit tuning an external cavity to enhance polarization changes in a reflected terahertz beam. Permanent magnets are used to provide the necessary external magnetic field. Conveniently, the highly reflective surface of a permanent magnet can be used to create the tunable external cavity. The signal enhancement allows the extraction of the free charge carrier properties of thin films, and can eliminate the need for expensive super-conducting magnets. Furthermore, the thickness of the external cavity establishes an additional independent measurement condition, similar to, for example, the magnetic field strength, terahertz frequency, and angle of incidence. A high electron mobility transistor structure and epitaxial graphene are studied as examples. We discuss the theoretical background, instrument design, data acquisition, and data analysis procedures.

physics.optics↗

arXiv2019

THz optical properties of polymethacrylates after thermal annealing

Polymer based stereolithographic additive manufacturing has been established for the rapid and low-cost fabrication of THz optical components due to its ability to construct complex 3D geometries with high resolution. For polymer based or integrated optics, thermal annealing processes are often used to optimize material properties. However, despite the growing interest in THz optics fabricated using stereolithography, the effects of thermal annealing on the THz dielectric properties of polymethacrylates compatible with stereolithography has not been studied yet. In this manuscript we report on the THz ellipsometric response of thermally annealed polymethacrylates prepared using UV polymerization. Our findings indicate that the investigated polymethacrylate maintain a stable optical response in THz spectral range from 650 to 950 GHz after thermal annealing at temperatures up to 70 degrees C for several hours.

physics.optics↗