Title: Investigation of Point Defects in AlGaN/GaN HEMT Structures
Deleterious effects of high concentration of point defects on the performance of AlGaN/GaN high electron mobility transistors (HEMTs) cannot be disregarded since they limit the device performance due to undesirable current collapses. Hence, influence of the point defects have been investigated by using the spectroscopic photo current-voltage (IV), the depth-resolved ultra-violet (UV) spectroscopic photo current voltage (IV) (DR-UV-SPIV), and time-resolved photocurrent (TRPC) spectroscopy measurements. Spectroscopic photo IV measurements revealed the existence of sub-bandgap defects as well as their nonhomogeneous distribution across the wafer. After confirmation of presence of the defects, we have discovered variations in the depth dependent distribution of electrically active defects across the wafer via DR-UV-SPIV measurement. It was found that while one sample has predominant defects close to the interface of the AlGaN/GaN HEMTs structure the other one has more defects deeper into the bulk. Finally, TRPC spectroscopy with a variable-wavelength sub-bandgap light excitations was performed to understand the different origins of the defects. Even though two samples exhibit the same characteristics for wavelength-dependency on photocurrent generation, dissimilar TRPC spectra was observed for the samples. We have showed that TRPC spectroscopy can be utilized to differentiate the traps which are originated from different reasons but display the similar de-trapping energy. As a conclusion, it was demonstrated that combination of these three spectroscopic measurements can be very useful diagnostic tool for the quick evaluation of the nature and distributions of the surface defects in AlGaN/GaN HEMTs wafer industry.
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