An exploration of deep level defects in compound semiconductors using optoelectronic modulation spectroscopy

The work is concerned with the detection and characterisation of defect states within semiconductor materials. OptoElectronic Modulation Spectroscopy (OEMS) has been adapted to isolate the electrical and optical responses of traps in semiconductor device and materials. Sub band-gap photons are used to permit penetration into the semiconductor to excite charges in deep defect states. In order to isolate the effect of back-plane charge traps in MESFET structures gate depletion region modulation of the channel current has been eliminated by using a closed loop control system to stabilise the gate depletion capacitance continuously throughout the measurement of the OEMS spectrum. Any change of the channel current will then be due only to the charges within the back-plane interface depletion region between the active layer and buffer/substrate. Depth defined OEMS has also been demonstrated and used to detect defects situated between two preset gate depletion regions. The location of the defect states is therefore determined and their response obtained by examining the difference between two spectra. This method is direct and easy to use to explore deep level states in defined regions within semiconductor materials and devices. An energy resolution of at least 30meV has been demonstrated. It has been possible to differentiate between transition involves the conduction and valence bands. The depth defined method can localise the responding states to an accuracy of 2nm and it can detect charges of occupation of about 10 sup 9 electrons/cm sup 2 /eV. These methods have been applied to GaAs, GaN and 4H-SiC MESFET structures and many defects have been observed. Some of the observations are new; in particular it has been possible to recognise the responses of extended crystal defects and to estimate their minimum spatial extent in 4H-SiC. A particular interesting aspect of the work was the observation and interpretation of periodic replica spectra in GaN and in GaAs prepar