Engineering of Impact Ionization Characteristics in GaAs/GaAsBi Multiple Quantum Well Avalanche Photodiodes

The presence of large bismuth (Bi) atoms has been shown to increase the spin-orbit splitting energy in bulk GaAsBi, reducing the hole ionization coefficient (β) and thereby reducing the excess noise seen in avalanche photodiodes. In this study, we show that even very thin layers of GaAsBi introduced as quantum wells (QWs) in a GaAs matrix exhibit a significant reduction of β while leaving the electron ionization coefficient, α, largely unchanged. The optical and avalanche multiplication properties of a series of GaAsBi/GaAs multiple quantum well (MQW) p-i-n structures with nominally 5 nm thick, 4.4% Bi GaAsBi QWs, varying from 5 to 63 periods and corresponding barrier widths of 101 to 4 nm were investigated. From photoluminescence, ω-2θ X-ray diffraction, and cross section transmission electron microscopy measurements, the material was found to be of high quality despite the strain introduced by the Bi in all except the samples with 54 and 63 QW periods. Photomultiplication measurements undertaken with different wavelengths showed that α in these MQW structures did not change appreciably with the number of QWs; however, β decreased significantly, especially at lower values, the noise factor, F, is reduced by 58% to 3.5 at a multiplication of 10, compared to a similar thickness bulk GaAs structure without any Bi. This result suggests that Bi-containing QWs could be introduced into the avalanching regions of APDs as a way of reducing their excess noise.