The cliff layer is a layer between the InGaAsP transition layer and the InP collection layer, which has a thin thickness and a high doping concentration. In both cases without cliff layer and with cliff layer, the energy band diagram and electric field distribution diagram of MUTC-PD are given. Compared with the case without a cliff layer, it can be seen that when there is a cliff layer, the potential energy gradient is greater, and the electric field intensity of the depletion absorption region is stronger. The larger potential energy gradient and electric field strength can cause the electrons to drift rapidly towards the collection layer. On the other hand, when there is no cliff layer, the band gap between the InGaAs absorption region and the InP collection region is obvious, and electrons are easy to accumulate, which seriously affects the rapid transmission of electrons. When there is a cliff layer, the transition of the energy band is smooth, and there is no obvious barrier spike, which plays a role in weakening the space charge effect. Therefore, the introduction of a cliff layer in the MUTC-PD structure has an important role in improving its performance.
The relationship between MUTC-PD photocurrent and incident optical power. It can be observed that the DC photocurrent changes linearly with the light intensity until the light intensity increases to 4106Wcm2. The slope value of the curve represents the magnitude of responsivity, and the calculated responsivity can reach 0.28A / W. When the responsivity begins to decrease, the corresponding photocurrent It is 158mA, which is DC saturation current. When the incident optical power exceeds 4: 5106Wcm2, the DC photocurrent reaches saturation. Studies in the literature have shown that under high-power light injection, current saturation and reduced responsivity are caused by space charge effects. Therefore, the saturation current characteristics of the photodetector can be estimated by analyzing its DC saturation current. It should be noted that the saturation current will be slightly lower than the DC saturation current, because the saturation current is affected to some extent by the voltage swing on the load.
The following is a detailed analysis of the internal causes of bandwidth reduction and current saturation under high-power light injection. (A) and (b) give the energy band diagram and electric field distribution of MUTC-PD under different incident optical powers. As shown in (a), as the incident light power increases, the energy band gradually shifts upward, and the potential energy gradient gradually decreases, which will be detrimental to the movement of electrons. From (b), it was found that under high-power illumination, the electric field strength of the absorber layer gradually collapsed, which could no longer effectively accelerate the movement of electrons toward the collection layer. Band shift and electric field collapse will uniformly cause the accumulation of photo-generated carriers in the absorption region and its boundary, resulting in space charge effect, resulting in reduced bandwidth and current saturation.
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