An OEMS study of trap states at the active layer interface with the substrate in ion implanted GaAs MESFETs

Using OptoElectronic Modulation Spectroscopy (OEMS) we are now able to measure, simultaneously and independently, charges which are trapped in the gate depletion region and in the back plane depletion region at the interface with the substrate. These methods have been applied to an ion implanted GaAs MESFET structure. Stabilised gate depletion thicknesses extending to pinch-off and temperatures in the range 50 K to 300 K have been used to study traps in the two depletion regions. Spectra show that there is broad hole trap between 0.7 eV and 0.85 eV and three electron traps. One electron trap is broad, between 0.87 eV and 1.23 eV; superimposed on it are two sharply defined electron trapping levels at 0.92 eV and 1.09 eV. All of these traps are predominantly within the back plane interface depletion region. The method has wide applicability to other materials systems.