arXiv subjects

Shivali Agrawal

Publications and source records attributed to Shivali Agrawal.

3 recordsLinked to original sources

arXiv2025

Edge emission from 265~nm UV-C LEDs grown by MBE on bulk AlN

UV-C LEDs pseudomorphically grown by MBE on bulk AlN substrates emitting at 265 nm are demonstrated. High current density up to 800 A/cm$^2$, 5 orders of on/off ratio, and low differential on-resistance of 2.6 m$Ω\cdot$cm$^2$ at the highest current density is achieved. The LED heterostructure has a high refractive index waveguide core surrounded by n- and p-cladding layers similar to a laser diode designed for mode confinement at 270 nm to facilitate edge emission and collection of photons. Edge-emitting devices are made by cleaving the fabricated LEDs along the $m$-plane of the wurtzite crystal. Electrical injection results in emission of high energy 4.7 eV photons that are collected from the cleaved edge of the LEDs corresponding to the optical bandgap of the AlGaN active region. The contribution of power dissipation across the n- and p-regions of the diode is discussed. The n-contact resistance to n-AlGaN is identified as the largest contributor to the series resistance of the LED in the present generation of devices.

physics.optics↗

arXiv2025

Low Resistance Non-Alloyed Ohmic Contacts to High Al Composition n-type AlGaN

Ohmic contacts to high (>70\%) Al content n-type Al$_x$Ga$_{1-x}$N ultra-wide bandgap semiconductor layers in nitride electronic and photonic devices are typically fabricated by a lift-off process and high temperature ($>700^\circ$C) thermal alloying. These conditions often result in significant structural deformations of the fabricated structures and impose a harsh thermal budget on all other aspects of the device. Here, we report the fabrication of \textit{non-alloyed} \textit{as-deposited} ohmic contacts to 71\% n+AlGaN ($E_\text{g}\sim5.4$~eV) with a free carrier concentration of roughly $7\times 10^{19}$~cm$^{-3}$ and a resistivity of 4 - 5.5 m$Ω$cm (among the lowest reported for Al$_{0.71}$Ga$_{0.29}$N) with linear $I-V$ characteristics and a contact resistivity of $ρ_\text{c}=(4.4\pm1.0)\times10^{-4}$~$Ω$cm$^2$ (measured at zero voltage). Contacts with this quality are formed by two separate fabrication schemes: (i) metal-first patterning, and (ii) lift-off with an oxygen asher descum prior to metal deposition. Given the low threading dislocation density in the single-crystal AlN substrate used for epitaxy, the smooth morphology of the contacted epitaxial surface, and the non-alloyed nature of the contacts, this contact resistivity is attributed purely to thermionic field emission through the metal-semiconductor junction. Contact resistivity extraction at low current injection enables us to model these results using a thermionic field-emission model of contact resistivity, yielding a barrier height for Ti/Al$_{0.71}$Ga$_{0.29}$N of $(0.81\pm0.02)$ eV.

cond-mat.mtrl-sci↗

arXiv2023

Ultrawide bandgap semiconductor heterojunction p-n diodes with distributed polarization doped p-type AlGaN layers on bulk AlN substrates

Ultrawide bandgap heterojunction p-n diodes with polarization-induced AlGaN p-type layers are demonstrated using plasma-assisted molecular beam epitaxy on bulk AlN substrates. Current-voltage characteristics show a turn on voltage of $V_{\text{bi}}\approx5.5$ V, a minimum room temperature ideality factor of $η\approx 1.63$, and more than 12 orders of current modulation at room temperature. Stable current operation of the ultrawide bandgap semiconductor diode is measured up to a temperature of 300$^\circ$C. The one-sided n$^{+}$-p heterojunction diode design enables a direct measurement of the spatial distribution of polarization-induced mobile hole density in the graded AlGaN layer from the capacitance-voltage profile. The measured average mobile hole density is $p \sim 5.7 \times 10^{17}$ cm$^{-3}$, in close agreement with what is theoretically expected from distributed polarization doping. Light emission peaked at 260 nm (4.78 eV) observed in electroluminescence corresponds to interband radiative recombination in the n$^{+}$ AlGaN layer. A much weaker deep-level emission band observed at 3.4 eV is attributed to cation-vacancy and silicon complexes in the heavily Si-doped AlGaN layer. These results demonstrate that distributed polarization doping enables ultrawide bandgap semiconductor heterojunction p-n diodes that have wide applications ranging from power electronics to deep-ultraviolet photonics. These devices can operate at high temperatures and in harsh environments.

physics.app-ph↗