Converse piezoelectric strain in undoped and Fe-doped AlGaN/GaN heterostructure field effect transistors studied by Raman scattering

Converse piezoelectric strain in undoped and Fe-doped AlGaN/GaN heterostructure field effect transistors ( HFETs), i.e. the strain induced by applying bias to a transistor, was studied using micro-Raman scattering spectroscopy as a function of applied source-drain voltage for different GaN buffer doping levels and substrate types. By monitoring the phonon frequency shifts and line width of the E-2 and A(1)(LO) phonon modes of GaN, a considerable piezoelectric strain/stress was found in undoped devices, which exhibited a saturation above 40 V bias. This saturation voltage was used to quantify the deep acceptor concentration in the GaN buffer layer. Using experimental Raman data and numerical modelling of the electric field distribution in the device, it was furthermore established that Fe doping causes confinement of the strain/stress to the vicinity of the AlGaN/GaN interface, i.e. near the electron channel, with potential implications for device reliability. It was concluded that varying the structure and doping in the buffer layer has the potential to modify the converse piezoelectric strain and hence affect reliability issues in AlGaN/GaN HFETs.