Failure mechanism of avalanche photodiodes in the presence of water vapor

Lower cost optoelectronic modules (transmitters and receivers) could be produced if components could operate reliably in a nonhermetic package. Experiments (operation in humid ambient as a function of temperature, relative humidity (RH), and voltage) were carried out on InP-based avalanche photodiodes (APDs). The objectives were to reveal any moisture-related failure modes, to understand the failure mechanisms, to determine the relevant acceleration factors, and to propose design or material modifications to prevent the failures. Leakage current vs. time was measured for nine cells of 20 devices each. All devices were examined by optical microscopy, and representative samples were studied by scanning electron microscopy. A failure mechanism related to corrosion of the passivating SiN and the underlying semiconductor was identified. Based on the voltage and RH dependence of the degradation rate, a model for the failure process was developed. The mechanism involves RH-dependent transport of charge across the surface of the SiN and Poole-Frenkel current flow through the SiN. This process eventually produces a short circuit.