Enhancement of Quantum Efficiency in Thin Photodiodes through Absorptive Resonance

The spectral response of resonantly enhanced photodiodes is analyzed theoretically and verified experimentally. Comprehensive design guidelines and formulas are given for device structures contain- ing a metal reflector, a contact layer, an optional grading layer, an absorbing layer, and a quarter-wave stack (QWS). The analysis shows, for instance, that the quantum efficiency of a Schottky photodiode with a 162-nm GaInAs absorbing layer can be enhanced 3.7 fold by using a 41-layer AlInAs/AlGaInAs QWS. The number of layers required could be much lower for other material systems and/or if the substrate is removed. Experimentally, 50% enhancement is demonstrated for a 475-nm thick absorbing layer at 1.52 µm by using a 16-layer QWS. The resonance width is ~4.4%. This type of structures should be very valuable for very small size ultrafast photodiodes and for optoelectronic integration.