Direct observation of nanometer-scale gate leakage paths in AlGaN/GaN and InAlN/AlN/GaN HEMT structures

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This paper investigates the effects of MOVPE growth parameters on the gate leakage characteristics of InAlN HEMT structures and compares leakage current paths in AlGaN and InAlN HEMT structures on a nanometer scale. The gate leakage characteristics of AlGaN and InAlN HEMT were compared, and the large leakage current in InAlN HEMT was found to result from the high-density 2DEG-induced strong electric field accumulation in the barrier layer. Both the increasing growth rate and decreasing growth temperature in the InAIN layer of InAlN HEMT caused the deterioration of gate leakage characteristics and surface morphologies. Conductive AFM measurements were performed for AlGaN and InAlN HEMT structures, and a one-to-one clear correlation between surface pits and leakage paths was found for AlGaN HEMT. On the contrary, for the InAlN HEMT grown at a low temperature, high-density localized leakage paths were observed. We believe the observed leakage paths originate from nanometer-scale material fluctuations, which is most likely an indium composition fluctuation. Our results indicate that it is necessary to grow the InAlN barrier layer at 740 °C or higher and that the growth rate must be 1.25 nm min−1 or less to realize low-gate leakage in InAlN HEMT. A further reduction of the gate leakage current requires the reduction of the dislocation density and use of gate insulators.