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Dependence of the Au/AlGaN Schottky Characteristics on Al Content

In this work we report the characteristics of Au/AlGaN Schottky diodes as a function of Al content. The materials were grown by MOCVD and have different free electron concentration and alloy composition. A variety of electrical techniques such as current-voltage (I-V), capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements were used to characterize the diodes. Although the samples received the same treatment prior and during the Au evaporation process, their I-V and C-V characteristics seem to depend significantly on the Al content x. We obtained a lower value for the Schottky barrier height than that predicted by the Schottky rule for the sample with x = 0.3, along with a departure from unity of the ideality factor. Also we observed a hysteresis in the C-V characteristics in the same sample. The DLTS measurements showed the presence of a density of defects at the Au/AlGaN interface which increases with Al content. We believe that these defects, inherent to the material, cause the lowering of the barrier height and the observed hysteresis. They result from a roughness of the surface due to a high Al content. This also induces spatial fluctuations in the barrier height at the surface.