Characterization of GaN-based MESFETs by comparing electroluminescence, photoionization and cathodoluminescence spectroscopy

In this work we present a methodological comparison of Photoionization, Electroluminescence (EL) and Cathodoluminescence (CL) investigations of GaN based MESFETs, in order to study the influence of electronic traps on the optical and electrical properties of the devices. The I-V electrical characteristics clearly showed the presence of traps as shown by the development of a knee voltage. Photoionization measurements showed four distinct energy levels the majority carriers were trapped at. The capture cross-section versus energy showed the transitions were located at 1.75 eV, 2.32 eV, 2.67 eV and 3.15 eV. CL studies show the presence of the near band edge (NBE), the yellow luminescence (YL) and a donor acceptor pair emissions; a broad band centered at about 2.8 eV was also found and correlated to the deep levels revealed by photoionization measurements. As for the emission at 2.8 eV, CL showed it originated mainly in the semi-insulating GaN layer below the conductive channel, in agreement with photoionization data