arXiv subjects

Surajit Chakraborty

Publications and source records attributed to Surajit Chakraborty.

6 recordsLinked to original sources

arXiv2026

Enhancement-Mode Vertical $\beta$-Ga$_2$O$_3$ U-Trench MOSFET with MOCVD Regrown n$^+$ Contact Layers and Nitrogen-Implanted Current Blocking Layer

In this work, an implantation-free ohmic contact technology based on selectively MOCVD-regrown Si-doped n$^+$ layers is demonstrated for enhancement-mode vertical $\beta$-Ga$_2$O$_3$ U-trench MOSFETs. The regrown n$^+$ contact structure eliminates the need for implantation-based ohmic contact formation while maintaining excellent electrical characteristics. A multi-energy nitrogen-ion-implanted current blocking layer (CBL) followed by 1100~$^\circ$C activation annealing in N$_2$ ambient for 30 min was employed to achieve normally-OFF operation. Transmission line model measurements yielded a low specific contact resistivity of $2.65 \times 10^{-7}~\Omega\cdot$cm$^2$. The fabricated devices exhibited a threshold voltage of approximately 5~V, an ON/OFF current ratio of $1.15\times10^{6}$, a peak current density of 158A/cm$^2$, and a specific ON-resistance of 120.9~m$\Omega\cdot$cm$^2$. Three-terminal OFF-state breakdown voltages ranging from 920 to 980~V were achieved at $V_{GS}=0$~V. Multi-finger MOSFETs show current scaling to 0.25 A. These results demonstrate that selectively MOCVD-regrown n$^+$ contact layers provide a promising implantation-free approach for realizing high-performance vertical $\beta$-Ga$_2$O$_3$ power MOSFETs.

cond-mat.mtrl-sci↗

arXiv2025

Neutron-Assisted Breakdown Enhancement in $β$-Ga$_2$O$_3$ Schottky Diodes

This study demonstrates a substantial enhancement of breakdown voltage in $β$-Ga$_2$O$_3$ Schottky diodes through an approach that combines fast neutron irradiation with controlled post-irradiation electro-thermal annealing. Devices irradiated with 1 MeV neutrons at a high fluence of 1E15 n/cm^2 exhibited substantial degradation, including a drastic reduction in on-current and an increase in on-resistance. Electrothermal testing, conducted through simultaneous current-voltage (J-V) measurements and thermal annealing, resulted in significant recovery. After four cycles of electro-thermal testing, the devices demonstrated significant improvements in performance, with a substantial recovery of on-current and a reduction in on-resistance compared to the post-radiation condition, approaching pre-radiation levels. Most recovery occurred during the first two cycles, with diminishing improvements in later cycles, indicating that most thermally recoverable traps were mitigated early. Capacitance-voltage (C-V) measurements revealed a substantial reduction in carrier concentration, decreasing from 3.2E16 cm^-3 pre-radiation to 5.5E15 cm^-3 after the first electro-thermal testing cycle, indicating an over 82% reduction. Following the third cycle, the carrier concentration partially recovered to 9.9E15 cm^-3, reflecting a carrier removal rate of ~22 cm^-1. The breakdown voltage exhibited a remarkable enhancement, increasing from approximately 300 V to 1.28 kV (a ~325% improvement) after the first electro-thermal testing, attributed to the reduction in carrier concentration by compensating radiation-induced traps. Subsequent testing reduced breakdown voltage slightly to 940 V due to partial recovery of carrier concentration, but it remained significantly higher than pre-radiation levels.

physics.app-ph↗

arXiv2025

Stress Response of Jammed Solids: Prestress and Screening

Unlike classical elasticity, where stresses arise from deformations relative to a stress-free reference configuration, rigidity in amorphous systems is maintained by disordered force networks that generate internal prestress. Previously, we introduced a ''stress-only'' formulation, where mechanical equilibrium resembles Gauss's law in a rank-2 tensor electrostatics with vector charges, and demonstrated that the mechanical response of jammed solids is described by the dielectric response of this gauge-theoretic formulation. Here, we extend this framework by incorporating scale-dependent screening that captures both dielectric and Debye-type behaviour. This introduces a characteristic length scale in stress correlations as well as in the response to external forces. Through numerical simulations of soft-sphere packings, we show that this length scale is set by the particle size, thus providing a natural ultraviolet cutoff while preserving long-wavelength emergent elasticity. We show that this lengthscale remains finite for all pressures, with no evidence for an emergent Debye-like screening near the frictionless unjamming transition. We demonstrate that although individual realisations show strong fluctuations, disorder averaging at fixed macroscopic conditions yields a robust dielectric-like response that persists up to unjamming. Finally, we also provide a physical interpretation of the gauge field within the electrostatic mapping: relative grain displacements in response to localised external perturbations correspond to difference in the gauge field, linking the field-theoretic description to particle-level mechanics.

cond-mat.soft↗

arXiv2024

Instabilities govern the low-frequency vibrational spectrum of amorphous solids

Amorphous solids exhibit an excess of low-frequency vibrational modes beyond the Debye prediction, contributing to their anomalous mechanical and thermal properties. Although a $ω^4$ power-law scaling is often proposed for the distribution of these modes, the precise exponent remains a subject of debate. In this study, we demonstrate that boundary-condition-induced instabilities play a key role in this variability. We identify two distinct types of elastic branches that differ in the nature of their energy landscape: Fictitious branches, where shear minima cannot be reached through elastic deformation alone and require plastic instabilities, and True branches, where elastic deformation can access these minima. Configurations on Fictitious branches show a vibrational density of states (VDoS) scaling as $D(ω) \sim ω^3$, while those on True elastic branches under simple and pure shear deformations exhibit a scaling of $D(ω) \sim ω^{5.5}$. Ensemble averaging over both types of branches results in a VDoS scaling of $D(ω) \sim ω^4$. Additionally, solids relaxed to their shear minima, with no residual shear stress, display a steeper scaling of $D(ω) \sim ω^{6.5}$ in both two and three dimensions. We propose two limiting behaviors for amorphous solids: if the system size is increased without addressing instabilities, the low-frequency VDoS scales with an exponent close to $3$. Conversely, by removing residual shear stress before considering large system sizes, the VDoS scales as $D(ω) \sim ω^{6.5}$.

cond-mat.soft↗

arXiv2024

Long-Range Correlations in Elastic Moduli and Local Stresses at the Unjamming Transition

We explore the behavior of spatially heterogeneous elastic moduli as well as the correlations between local moduli in model solids with short-range repulsive potentials. We show through numerical simulations that local elastic moduli exhibit long-range correlations, similar to correlations in the local stresses. Specifically, the correlations in local shear moduli exhibit anisotropic behavior at large lengthscales characterized by pinch-point singularities in Fourier space, displaying a structural pattern akin to shear stress correlations. Focussing on two-dimensional jammed solids approaching the unjamming transition, we show that stress correlations exhibit universal properties, characterized by a quadratic $p^2$ dependence of the correlations as the pressure $p$ approaches zero, independent of the details of the model. In contrast, the modulus correlations exhibit a power-law dependence with different exponents depending on the specific interaction potential. Furthermore, we illustrate that while affine responses lack long-range correlations, the total modulus, which encompasses non-affine behavior, exhibits long-range correlations.

cond-mat.soft↗

arXiv2023

Enhanced Vibrational Stability in Glass Droplets

We show through simulations of amorphous solids prepared in open boundary conditions that they possess significantly fewer low-frequency vibrational modes compared to their periodic boundary counterparts. Specifically, using measurements of the vibrational density of states, we find that the $D(ω) \sim ω^4$ law changes to $D(ω) \sim ω^δ$ with $δ\approx 5$ in two dimensions and $δ\approx 4.5$ in three dimensions. Crucially, this enhanced stability is achieved when utilizing slow annealing protocols to generate solid configurations. We perform an anharmonic analysis of the minima corresponding to the lowest-frequency modes in such open-boundary systems and discuss their correlation with the density of states. A study of various system sizes further reveals that small systems display a higher degree of localization in vibrations. Lastly, we confine open-boundary solids in order to introduce macroscopic stresses in the system which are absent in the unconfined system, and find that the $D(ω) \sim ω^4$ behavior is recovered.

cond-mat.soft↗