Utilizing Different Grating Structures to Investigate the Influence of Evanescent Wave in Quantum Well Infrared Photodetector

The quantum-well infrared photodetector(QWIP) is widely applied in contemporary technology. It is mainly applied in the field of astronomy, medical science, architecture, military and safety system. Therefore, it became a popular topic on how to make the fabrication of QWIP costless and simultaneously promote its absorption efficiency. In this paper, we attempt to improve the absorption efficiency of QWIP by applying the grating structure. Also, the relation between the etched depth and absorption efficiency will be observed. Consequently, we learned that when the deeper the active layer is etched, the better responsivity is shown. The grating structure can make the light diffract, meaning that the proportion of the electric field direction vertical to the substrate surface can be raised. By doing so, the photocurrent can be generated faster than the previous condition. While the light is diffracted into the grating slot, the evanescent wave will be generated, which can greatly enhance the absorption at the active layer. As for the present result, we observed the highest responsivity when the active layer is etched thoroughly. Yet there are still rooms for improvement. We deduced that it may be resulted from the higher surface recombination and excessively-etching of the active layer. Besides, we also investigated the responsivity under fixed etched depth and different grating pitch. The result shows that the optimal grating pitch is 2.0 micrometer. The detail reason will be discussed in the 4th chapter.