arXiv subjects

Roberto C. Myers

Publications and source records attributed to Roberto C. Myers.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear Photocurrent Spectroscopy and Polarization-Tunable Shift Current in the Layered Semiconductor $\mathrm{CuScP_2S_6}$

The class of layered metal thiophosphates and selenophosphates are emerging as promising candidates for non-linear photonic and optoelectronic applications. Here we report one-photon absorption (1PA) and two-photon absorption (2PA) induced photocurrents in $CuScP_2S_6$, establishing it as an optoelectronically-active non-centrosymmetric semiconductor. Spectrally-resolved 1PA and 2PA photoresponsivity paired with photoluminescence and differential reflectance measurements reveal a band gap of 2.35 eV, several higher energy interband transitions, and sub-band gap multiphoton excitonic transitions. Vertical graphite/$CuScP_2S_6$/graphite devices exhibit zero-bias photocurrent with a polarity controlled by both photon energy and optical polarization orientation within the ab-plane. This sign-dependence combined with a characteristic power-law transition from linear to square-root scaling are consistent with a shift-current origin to this bulk photovoltaic effect (BPVE). These findings establish $CuScP_2S_6$ as a multi-functional platform for polarization-sensitive non-linear optoelectronics.

cond-mat.mtrl-sci↗

arXiv2026

Dislocation-templated antiferromagnetic domains in epitaxial NiO

Controlling antiferromagnetic domains is essential for spintronics, yet deterministic manipulation remains challenging due to their lack of net magnetization. Here, we utilize scanning electron microscopy electron channeling contrast imaging (ECCI) to demonstrate a robust structural memory effect in epitaxial NiO/MgO(001), where antiferromagnetic twin-domain walls (DWs) are deterministically pinned at the nanoscale by interface dislocation networks. Thermal cycling across the Neel temperature reveals that while DW contrast completely vanishes in the paramagnetic phase, the features re-emerge at identical spatial locations upon cooling. Diffraction-vector-dependent ECCI demonstrates that domain contrast stems from localized rhombohedral magnetostrictive strain fields. By tracking a film thickness series from 23 to 60 nm, we resolve an explicit transition in oxide relaxation mechanics. A primary slip system initiates strain relaxation via interface misfit dislocations (MDs) tracking along <100>. At greater thicknesses, rising critical strain prompts threading segments to cross-slip onto secondary or higher-index planes, depositing a wavy network of zig-zag MD lines deviated toward <110>. Quantitative image analysis reveals that DW area fractions directly track the local density and spatial configuration of the evolving MD networks, providing a framework for defect-engineering antiferromagnetic textures using one-dimensional defects.

cond-mat.mtrl-sci↗

arXiv2026

Cryogenically Enhanced Laser-Induced Amorphous Phase Transitions in Crystalline Silicon

Amorphization of silicon is crucial to applications in photonics, microelectronics and solar cell technologies. Ultrafast lasers have been used to generate amorphous silicon from crystalline silicon using rapid nonthermal melting and solidification in room temperature. As material temperature can affect cooling rates significantly, adding temperature control in ultrafast laser modification of silicon may allow a new degree of freedom in ultrafast laser modification. In this work, we investigate the role of cryogenic temperature in governing ultrafast damage pathways via single-shot femtosecond laser irradiation of silicon from room temperature down to 24K at 1030nm. Across this temperature range, we observe a pronounced enhancement of amorphization at lower temperatures, revealed through optical microscopy, Raman spectroscopy, and Kelvin probe force microscopy (KPFM). Raman analysis identifies this ring as an amorphous surface layer, while complementary AFM and SEM imaging show temperature-dependent changes in surface morphology, including localized melt redistribution and refrozen material. To elucidate the physical origins of this behavior, we implement a carrier dependent two-temperature model (nTTM). The simulations reproduce the experimentally observed trends and indicate that reduced phonon population, modified absorption pathways, and altered lattice relaxation dynamics at cryogenic temperatures collectively promote amorphous freezing over recrystallization. This study represents the first detailed examination of silicon under ultrafast irradiation below the liquid-nitrogen regime and reveals temperature-governed mechanisms relevant for advanced silicon microstructuring.

cond-mat.mtrl-sci↗

arXiv2025

A common origin of photoplastic and electroplastic effects in ZnS

Dislocation motion--the atomic-scale mechanism of crystal plasticity--governs the strength and ductility of materials. In functional materials, external stimuli beyond mechanical stress can also affect dislocation glide. In the wide band gap semiconductor ZnS, optical illumination suppresses plasticity, whereas electric fields can enhance dislocation motion. Here, we show that the common underlying mechanism for these phenomena is the charged dislocations that respond to the changes in carrier concentration. Our prior theoretical work showed that locally charged dislocations in ZnS trap excess carriers, triggering core reconstructions that modify their mobility, with the positively charged Zn-rich core dislocations showing the most drastic change. Here, we validate this prediction experimentally by showing that either optical excitation or electronic doping selectively inhibits the glide of Zn-rich dislocations in epitaxially grown ZnS. First, imaging individual interface misfit dislocations under different optical excitation conditions shows that Zn-core glide is strongly reduced as optical power is increased, while the S-core dislocations show negligible sensitivity to light, marking the first, single misfit dislocation imaging of the photoplastic effect. Next, we show that a similar behavior is observed with direct electron (n-type) doping of ZnS epitaxial layers grown beyond the critical thickness. As the n-type dopant density is increased, the resulting Zn-core dislocation density is reduced by more than one order of magnitude, while the S-core density remains essentially unchanged, causing a sign reversal of the strain-anisotropy with n-type doping. These results demonstrate a common origin for the opto-electronic sensitivity of dislocations in ZnS and provide a pathway for the engineering of dislocation content in compound semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Photoluminescence excitation spectroscopy of quantum wire-like dislocation states in ZnS

Recent \textit{ab initio} calculations predict 1D dispersive electronic bands confined to the atomic scale cores of dislocations in the wide bandgap (3.84 eV) semiconductor ZnS. We test these predictions by correlating sub-bandgap optical transitions with the density of dislocations formed during strain relaxation in epitaxial ZnS grown on GaP. The densities for four predicted partial dislocations are quantified using scanning electron microscopy-based electron channeling contrast imaging. Room-temperature ellipsometry reveals absorption peaks that scale with dislocation density and align with theoretical predictions. Low-temperature photoluminescence spectra show deep emission peaks matching dislocation 1D band-to-band transitions. Photoluminescence excitation spectroscopy reveals six distinct emission lines with contrasting excitation dependence. Four peaks (2.78, 2.41, 2.20, 1.88 eV), assigned to dislocations, exhibit only modest suppression ($\leq$5$\times$) when excited below the ZnS bandgap, while two other peaks (3.11, 1.53~eV) are strongly quenched ($>$10$\times$). These findings support the existence of efficient, 1D band-to-band radiative transitions within quantum wire-like dislocation core states in ZnS, distinct from typical non-radiative deep-level defects in wide-gap semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Ferroelectric Epsilon-WO3 Nanoparticles and Its Bipolaron Driven Opto-electronic Properties at Room Temperature

A unique polymorph of binary tungsten trioxide, the epsilon phase of WO3, has non-centrosymmetric ferroelectric structure, typically stable below -43 degree C in bulk. We have stabilized the epsilon-WO3 at room temperature (RT) and nanostructured powders via flame spray pyrolysis synthesis. These nanopowders are drop cast into uniform thin films to enable RT measurement of ferroelectric and optoelectronic properties. We report ferroelectric hysteresis, nanoscale domains, and dipole switching measured via Piezo-response force microscopy (PFM). The epsilon-WO3 films also display optical second harmonic generation (SHG) and anticlockwise ferroelectric butterfly capacitance versus voltage hysteresis, further demonstrating the ferroelectric nature of epsilon-WO3. Remarkably, epsilon-WO3 shows ferroelectric polarization responses to optical stimuli and form bipolaron at RT, a spin-zero quasiparticle previously found only in cryogenic temperatures. The bipolaron formation and its interaction with electro-optical stimuli results in a single layer solid-state blue coloration, a ferrochromic effect. A mechanism of the ferrochromic effect is discussed. In summary, epsilon-WO3 appears to be a ferroelectric with the simplest structure, forming bosonic spin-zero bipolaron at RT, and it's dipoles respond to opto-electrical signals; therefore, this material holds significant promise for transforming the field of optoelectronics.

cond-mat.mtrl-sci↗

arXiv2024

Origin of the Anisotropic Beer-Lambert Law from Dichroism and Birefringence in $β$-Ga$_2$O$_3$

The anisotropic optical absorption edge of $β$-Ga$_2$O$_3$ follows a modified Beer-Lambert law having two effective absorption coefficients. The absorption coefficient of linearly polarized light reduces to the least absorbing direction beyond a critical penetration depth, which itself depends on polarization and wavelength. To understand this behavior, a Stokes vector analysis is performed to track the polarization state as a function of depth. The weakening of the absorption coefficient is associated with a gradual shift of linear polarization to the least absorbing crystallographic direction in the plane, which is along the a-exciton within the (010) plane or along the b-exciton in the (001) plane. We show that strong linear dichroism near the optical absorption edge causes this shift in $β$-Ga$_2$O$_3$, which arises from the anisotropy and spectral splitting of the physical absorbers i.e., excitons. The linear polarization shift is accompanied by a variation in the ellipticity due to the birefringence of $β$-Ga$_2$O$_3$. Analysis of the phase relationship between the incoming electric field to that at a certain depth reveals the phase speed as an effective refractive index, which varies along different crystallographic directions. The critical penetration depth is shown to be correlated with the depth at which the ellipticity is maximal. Thus, the anisotropic Beer-Lambert law arises from the interplay of both the dichroic and birefringent properties of $β$-Ga$_2$O$_3$.

cond-mat.mtrl-sci↗

arXiv2024

The Origin of Photoplasticity in ZnS

ZnS is a brittle material but shows extraordinary plasticity during mechanical tests performed in complete darkness. This phenomenon is known as the photoplastic effect, whose underlying mechanisms have long been unclear. We study the impact of light, via photoexcited charge carriers, on the dislocation core structure and mobility using first-principles calculations. We calculate the core structure and the charge-dependent Peierls barriers of the glide set of Shockley partial dislocations in ZnS. Our findings reveal that locally charged dislocations capture excess carriers in the system, leading to core reconstructions that alter the Peierls barrier, resulting in higher barriers and lower mobility for these dislocations. This altered and asymmetric mobility, depending on dislocation character (edge or mixed) and local stoichiometry (Zn or S rich), is responsible for the brittle behavior of ZnS under light exposure and will be reversed in complete darkness.

cond-mat.mtrl-sci↗

arXiv2024

Log-normal glide and the formation of misfit dislocation networks in heteroepitaxial ZnS on GaP

Scanning electron microscopy (SEM) based electron channeling contrast imaging (ECCI) is used to observe and quantify misfit dislocation (MD) networks formed at the heteroepitaxial interface between ZnS and GaP grown by molecular beam epitaxy (MBE). Below a critical thickness of 15-20 nm, no MDs are observed. However, crystallographic features with strong dipole contrast, consistent with unexpanded dislocation half-loops, are observed prior to the formation of visible interfacial MD segments and any notable strain relaxation. At higher film thicknesses (20 to 50 nm), interfacial MD lengths increase anisotropically in the two orthogonal in-plane <110> line directions, threading dislocation (TD) density increases, and a roughening transition is observed from atomically smooth two-dimensional (2D) to a multi-stepped three-dimensional (3D) morphology, providing evidence for step edge pinning via surface terminating dislocations. The ZnS strain relaxation, calculated from the total MD content observed via ECCI, matches the average strain relaxation measured by high-resolution x-ray diffraction (HRXRD). The MD lengths are found to follow a log-normal distribution, indicating that the combined MD nucleation and TD glide processes must have a normal distribution of activation energies. The estimated TD glide velocity ($v_{g}$) along [$\bar{1}$10] is almost twice that along [110], but in both directions shows a maximum as a function of film thickness, indicating an initial burst of plasticity followed by dislocation pinning.

cond-mat.mtrl-sci↗

arXiv2024

The anisotropic Beer-Lambert law in $β$-Ga$_{2}$O$_{3}$: Spectral and polarization dependent absorption and photoresponsivity

Due to its low symmetry, $β$-Ga$_{2}$O$_{3}$ exhibits a strongly anisotropic optical response. As a result, the absorption spectra change with the polarization state of the incoming photons. To understand this phenomenon, here we calculate the complete electromagnetic wave equation solutions as a function of linear polarization angle and photon energy for $β$-Ga$_{2}$O$_{3}$ using its previously measured complex dielectric function tensor. The significant off-diagonal terms in this tensor can result in a non-exponential decay in the photon flux, indicating that the Beer-Lambert law is not generally valid in this anisotropic material. However, for above-band-gap spectral regions which depend on crystallographic orientations (> 5.8 eV (001 plane),>5.2 eV (010 plane)) an effective absorption coefficient well approximates the photon flux decay with depth. On the other hand, near the optical absorption edge (4.9 - 5.8 eV (001 plane),4.65 - 5.2 eV (010 plane)) the photon flux decay exhibits a sum of two exponential decays, such that two effective absorption coefficients are necessary to model the loss behavior versus the absorption depth. This behavior manifests from the presence of dichroism in $β$-Ga$_{2}$O$_{3}$. A single effective absorption coefficient can only be recovered for this energy range by augmenting the isotropic Beer-Lambert law with a critical penetration depth and polarization dependence. Using these results, we calculate the polarization-dependent photoresponsivity spectra for light polarized along different crystallographic directions.

cond-mat.mtrl-sci↗

arXiv2023

Anisotropic excitonic photocurrent in $β$-Ga$_{2}$O$_{3}$

Polarization dependent photocurrent spectra are measured on a (001) $β$-Ga$_{2}$O$_{3}$ Schottky photodetector, where the linear polarization of light is rotated within the ab plane. Three spectral peaks at 4.92 eV, 5.15 eV, and 5.44 eV are observed that vary in intensity with the optical polarization direction. The peak transition energies are consistent with excitons previously reported in $β$-Ga$_{2}$O$_{3}$ due to interband transitions modified by the valence band p-orbital anisotropy and the electron-hole Coulombic attraction. The measured polarization-dependence of the photocurrent matches our predictions based on electromagnetic simulations of anisotropic absorption using the complex dielectric function tensor extracted from previous ellipsometry studies. These results illustrate the dominance of excitonic absorption and photocurrent in $β$-Ga$_{2}$O$_{3}$ both below and above the band gap, demonstrate a combined theoretical/experimental understanding of anisotropic photocarrier generation, and validate previous atomistic band structure calculations in this low-symmetry ultra-wide band gap semiconductor.

cond-mat.mtrl-sci↗

arXiv2022

Dislocations as natural quantum wires in Diamond

We study the electronic properties of the glide set of dislocations in diamond from first principles using hybrid exchange correlation functionals and find that the atomic-scale dislocation core states give rise to a prototypical one-dimensional (1D) band structure, i.e. natural quantum wires. The position and character of the core states varies strongly with local structure, where mixed dislocations with dangling bonds exhibit a 1D metallic band with a characteristic 1D density of states ($1/\sqrt{E})$. This 1D Fermi gas is spatially localized to single atomic diameter orbital chain along the dislocation core. When the dangling bonds within the core are reconstructed, the 1D metallic band disappears. In contrast, pure edge dislocations in diamond reveal a 1D semiconductor with a direct band gap of 3.0 eV. These calculations provide a possible explanation to the long standing observation of a blue luminescence band correlated with dislocations in diamond. This opens the door to using dislocations as 1D quantum phases with functional (electronic and optical) properties arising from the atomic-scale core states.

cond-mat.mtrl-sci↗

arXiv2021

Spectral measurement of the breakdown limit of $β-Ga_{2}O_{3}$ and tunnel ionization of self-trapped excitons and holes

$β-Ga_{2}O_{3}$ is an unusual semiconductor where large electric fields (~1-6 MV/cm) can be applied while still maintaining a dominant excitonic absorption peak below its ultra-wide bandgap. This provides a rare opportunity in the solid-state to examine exciton and carrier self-trapping dynamics in the strong-field limit at steady-state. Under sub-bandgap photon excitation, we observe a field-induced red-shift of the spectral photocurrent peak associated with exciton absorption and threshold-like increase in peak amplitude at high-field associated with self-trapped hole ionization. The field-dependent spectral response is quantitatively fit with an eXciton-modified Franz-Keldysh (XFK) effect model, which includes the electric-field dependent exciton binding energy due to the quadratic Stark effect. A saturation of the spectral red-shift with reverse bias is observed exactly at the onset of dielectric breakdown providing a spectral means to detect and quantify the local electric field and dielectric breakdown behavior. Additionally, the field-dependent responsivity provides insight to the photocurrent production pathway revealing the photocurrent contributions of self-trapped excitons (STXs) and self-trapped holes (STHs). Photocurrent and p-type transport in $β-Ga_{2}O_{3}$ are quantitatively explained by field-dependent tunnel ionization of excitons and self-trapped holes. We employ a quantum mechanical model of the field-dependent tunnel ionization of STX and STH to model the non-linear field-dependence of the photocurrent amplitude. Fitting to the data, we estimate an effective mass of valence band holes $(18.8 m_{0})$ and an ultrafast self-trapping time of holes (0.045 fs). This indicates that minority-hole transport in $β-Ga_{2}O_{3}$ can only arise through tunnel ionization of STH under strong fields.

cond-mat.mtrl-sci↗

arXiv2020

Deep-recessed $β$-Ga$_2$O$_3$ delta-doped field effect transistors with in situ epitaxial passivation

We introduce a deep-recessed gate architecture in $β$-Ga$_2$O$_3$ delta-doped field effect transistors for improvement in DC-RF dispersion and breakdown properties. The device design incorporates an unintentionally doped $β$-Ga$_2$O$_3$ layer as the passivation dielectric. To fabricate the device, the deep-recess geometry was developed using BCl$_3$ plasma based etching at ~5 W RIE to ensure minimal plasma damage. Etch damage incurred with plasma etching was mitigated by annealing in vacuum at temperatures above 600 $°$C. A gate-connected field-plate edge termination was implemented for efficient field management. Negligible surface dispersion with lower knee-walkout at high V$_\mathrm{DS}$, and better breakdown characteristics compared to their unpassivated counterparts were achieved. A three terminal off-state breakdown voltage of 315 V, corresponding to an average breakdown field of 2.3 MV/cm was measured. The device breakdown was limited by the field-plate/passivation edge and presents scope for further improvement. This demonstration of epitaxially passivated field effect transistors is a significant step for $β$-Ga$_2$O$_3$ technology since the structure simultaneously provides control of surface-related dispersion and excellent field management.

cond-mat.mtrl-sci↗

arXiv2020

Local Electric Field Measurement in GaN Diodes by exciton Franz-Keldysh Photocurrent Spectroscopy

The eXciton Franz-Keldysh (XFK) effect is observed in GaN p-n junction diodes via the spectral variation of photocurrent responsivity data that redshift and broaden with increasing reverse bias. Photocurrent spectra are quantitatively fit over a broad photon energy range to an XFK model using only a single fit parameter that determines the lineshape, the local bias ($V_{l}$), uniquely determining the local electric field maximum and depletion widths. As expected, the spectrally determined values of $V_{l}$ vary linearly with the applied bias ($V$) and reveal a large reduction in the local electric field due to electrostatic non-uniformity. The built-in bias ($V_{bi}$) is estimated by extrapolating $V_{l}$ at $V=0$, which compared with independent C-V measurements indicates an overall $\pm$0.31 V accuracy of $V_{l}$. This demonstrates sub-bandgap photocurrent spectroscopy as a local probe of electric field in wide bandgap diodes that can be used to map out regions of device breakdown (hot spots) for improving electrostatic design of high voltage devices.

physics.app-ph↗

arXiv2019

Ferromagnetic Epitaxial μ-Fe$_{2}$O$_{3}$ on β-Ga$_{2}$O$_{3}$: A New Monoclinic form of Fe$_{2}$O$_{3}$

Here we demonstrate a new monoclinic iron oxide phase (μ-Fe$_{2}$O$_{3}$), epitaxially stabilized by growth on (010) β-Ga$_{2}$O$_{3}$. Density functional theory (DFT) calculations find that the lattice parameters of freestanding μ-Fe$_{2}$O$_{3}$ are within ~1% of those of β-Ga$_{2}$O$_{3}$ and that its energy of formation is comparable to that of naturally abundant Fe$_{2}$O$_{3}$ polytypes. A superlattice of μ-Fe$_{2}$O$_{3}$/β-Ga$_{2}$O$_{3}$ is grown by plasma assisted molecular beam epitaxy, with resulting high-resolution x-ray diffraction (XRD) measurements indicating that the μ-Fe$_{2}$O$_{3}$ layers are lattice-matched to the substrate. The measured out-of-plane (b) lattice parameter of 3.12 $\pm$ 0.4 Å is in agreement with the predicted lattice constants and atomic-resolution scanning transmission electron microscopy (STEM) images confirm complete registry of the μ-Fe$_{2}$O$_{3}$ layers with β-Ga$_{2}$O$_{3}$. Finally, DFT modeling predicts that bulk μ-Fe$_{2}$O$_{3}$ is antiferromagnetic, while the interface region between μ-Fe$_{2}$O$_{3}$ and β-Ga$_{2}$O$_{3}$ leads to ferromagnetic coupling between interface Fe$^{3+}$ cations selectively occupying tetrahedral positions. Magnetic hysteresis persisting to room temperature is observed via SQUID measurements, consistent with the computationally predicted interface magnetism.

cond-mat.mtrl-sci↗

arXiv2019

Long lifetime of thermally-excited magnons in bulk yttrium iron garnet

Spin currents are generated within the bulk of magnetic materials due to heat flow, an effect called intrinsic spin-Seebeck. This bulk bosonic spin current consists of a diffusing thermal magnon cloud, parametrized by the magnon chemical potential ($μ_{m}$), with a diffusion length of several microns in yttrium iron garnet (YIG). Transient opto-thermal measurements of the spin-Seebeck effect (SSE) as a function of temperature reveal the time evolution of $μ_{m}$ due to intrinsic SSE in YIG. The interface SSE develops at times < 2 ns while the intrinsic SSE signal continues to evolve at times > 500 $μ$s, dominating the temperature dependence of SSE in bulk YIG. Time-dependent SSE data are fit to a multi-temperature model of coupled spin/heat transport using finite element method (FEM), where the magnon spin lifetime ($τ$) and magnon-phonon thermalization time ($τ_{mp}$) are used as fit parameters. From 300 K to 4 K, $τ_{mp}$ varies from 1 to 10 ns, whereas $τ$ varies from 2 to 60 $μ$s with the spin lifetime peaking at 90 K. At low temperature, a reduction in $τ$ is observed consistent with impurity relaxation reported in ferromagnetic resonance measurements. These results demonstrate that the thermal magnon cloud in YIG contains extremely low frequency magnons (~10 GHz) providing spectral insight to the microscopic scattering processes involved in magnon spin/heat diffusion.

cond-mat.mtrl-sci↗

arXiv2017

Scalable Nernst Thermoelectric Power using a Coiled Galfenol Wire

The Nernst thermopower usually is considered far too weak in most metals for waste heat recovery. However, its transverse orientation gives it an advantage over the Seebeck effect on non-flat surfaces. Here, we experimentally demonstrate the scalable generation of a Nernst voltage in an air-cooled metal wire coiled around a hot cylinder. In this geometry, a radial temperature gradient generates an azimuthal electric field in the coil. A Galfenol (Fe$_{0.85}$Ga$_{0.15}$) wire is wrapped around a cartridge heater, and the voltage drop across the wire is measured as a function of axial magnetic field. As expected, the Nernst voltage scales linearly with the length of the wire. Based on heat conduction and fluid dynamic equations, finite-element method is used to calculate the temperature gradient across the Galfenol wire and determine the Nernst coefficient. A giant Nernst coefficient of -2.6 $μ$V/KT at room temperature is estimated, in agreement with measurements on bulk Galfenol. We expect that the giant Nernst effect in Galfenol arises from its magnetostriction, presumably through enhanced magnon-phonon coupling. Our results demonstrate the feasibility of a transverse thermoelectric generator capable of scalable output power from non-flat heat sources.

cond-mat.mtrl-sci↗