An optical study of III-nitride semiconductor devices, their thermal properties and degradation mechanisms

• Main Author: Hodges, Christopher John
• Other Authors: Kuball, Martin
• Published: University of Bristol 2014
• Subjects: 541.377
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627918

Degradation processes in AIGaN/GaN high electron mobility transistors were investigated by optical methods. Temperatures within device channels, as well as electric fields are important for on-state degradation. Raman and photoluminescence (PL) thermography were used to investigate these temperatures and the thermal conductivity of GaN channels in both conventional GaN-on-SiC structures and novel AIGaN/GaN/AIGaNon- Si double heterostructure field effect transistors (DHFETs). The thin (150 nm) GaN channel layer in the DHFET had a lower thermal conductivity, at - 60 W m-I K- 1 than typical epilayers, which at - 2 pm thick have more than twice this value. This reduced thermal conductivity has implications for the design of devices employing thin GaN layers, especially when combined with the thick strain relief layers common on Si substrates, as the resulting high temperatures will affect their reliability by on- state thermal degradation processes. The depth resolution of Raman thermography on devices with typical GaN buffers usually limits results to one temperature, averaged through the buffer thickness. A method was developed to improve the depth resolution using a spatial filter and azimuthal polarisation; when combined with offset focal planes it was possible to obtain temperatures of the top and bottom of the GaN epilayer separately. Off-state degradation processes are more closely related to electric fields than self-heating; the generation of leakage current paths from the gate to the channel is particularly important. This leakage-path generation and associated localised electroluminescence (EL) emission was studied using EL imaging and spectroscopy combined with deep UV PL spectroscopy. The PL from the AIGaN barrier was reduced in regions associated with localised EL, indicating the formation of non-radiative recombination centres in the form of defects in the AIGaN. These non-radiative recombination centres were found to be generated over a larger area than the location of the gate leakage currents - these currents only start to flow when sufficient defects form to constitute a path.