Significant Performance Improvement in AlGaN Solar-Blind Avalanche Photodiodes by Exploiting the Built-In Polarization Electric Field

We present improved AlGaN solar-blind avalanche photodiodes (APDs) with a separate absorption and multiplication (SAM) structure by introducing a polarization electric field with the same direction as reverse bias field in the multiplication region. This polarization electric field can be realized by reducing the Al composition of the p-AlGaN layer in a conventional p-i-n-i-n SAM-APD structure. After employing a reduced-Al-composition p-AlGaN instead of the commonly used p-AlGaN, the polarization enhanced APD exhibits a markedly lower avalanche breakdown voltage and a near two times higher avalanche gain up to 2.1 × 10⁴ compared with its conventional counterpart. In addition, X-ray diffraction and transmission electron microscopy results show that a moderate reduction of Al composition in the p-AlGaN layer does not degrade the crystalline quality of the polarization enhanced APD structure resulted from lattice mismatch, which guarantees the polarization enhanced effect.