Effect of doping on the reliability of GaAs multiple quantum well avalanche photodiodes

The effect of various doping methods on the reliability of gallium arsenide/aluminum gallium (GaAs/AlGaAs) multiple quantum well (MQW) photodiode (APD) structures fabricated by molecular beam epitaxy is investigated. Reliability is examined by accelerated life tests by monitoring dark current and breakdown voltage. Median device lifetime and the activation energy of the degradation mechanism are computed for undoped, doped-barrier, and doped-well APD structures. Lifetimes for each device structure are examined via a statistically designed experiment. Analysis of variance (ANOVA) shows that dark current is affected primarily by device diameter, temperature and stressing time, and breakdown voltage depends on the diameter, stressing time, and APD type. It is concluded that the undoped APD has the highest reliability, followed by the doped-well and doped-barrier devices, respectively. To determine the source of the degradation mechanism for each device structure, failure analysis using the electron-beam induced current method is performed. This analysis reveals some degree of device degradation caused by ionic impurities in the passivation layer, and energy-dispersive spectrometry subsequently verifies the presence of ionic sodium as the primary contaminant. However since all device structures are similarly passivated, sodium contamination alone does not account for the observed variation between the differently doped APD's. This effect is explained by dopant migration during stressing, which is verified by free carrier concentration measurements using the capacitance-voltage (C-V) technique.