Al composition dependent properties of quaternary AlInGaN Schottky diodes

Pd Schottky barrier diodes were fabricated on undoped Al_xIn _0.02Ga_0.98-xN/GaN with x less than 20%. The material properties, which were characterized by photoluminescence, x-ray diffraction, and atomic force microscopy, indicated that the quaternary samples were coherently grown on GaN template with high crystalline quality. The flatband barrier height obtained by capacitance-voltage (C-V) measurement increased with increasing Al mole fraction (increasing the band gap of the quaternary) up to 2.06 eV, in agreement with the predictions of the Schottky-Mott theory. However, current-voltage (I-V) measurements revealed that the barrier height decreased from 1.32 to 1.12 eV, which was accompanied by an increase in ideality factor from 1.04 to 1.73. The large difference of barrier height between I-V and C-V measurements could not be quantitatively explained by the traditional electron transport mechanisms of Schottky diode, such as tunneling effect, image force effect, and barrier inhomogeneity theory. Strong polarization effect in strained Al_xIn_0.02Ga_0.98-xN/GaN heterostructure was proposed to account for the experimental results, since similar phenomena had been observed extensively in strained Al_yGa_1-yN/GaN heterojunction structures.