Kavli Affiliate: Grace Xing
| Summary:
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 textitnon-alloyed textitas-deposited ohmic contacts to 71% n+AlGaN ($E_textgsim5.4$~eV) with a free carrier concentration of roughly $7times 10^19$~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 $ρ_textc=(4.4pm1.0)times10^-4$~$Ω$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 $(0.81pm0.02)$ eV.
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