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Avalanche Photodiodes (APDs) that target a wavelength of 1550 nm, have several applications ranging from optical communications to imaging to single photon detection. The DE-JTO will use an array of APDs to image the wavefront of its 1550 nm laser. The distinctive feature of an APD is high sensitivity due to the gain achieved by impact ionization of carriers. Because impact ionization is a stochastic process, it introduces excess noise that limits the signal to noise ratio of an APD. However, the excess noise may be reduced by engineering the k (=β/α) value of the device, where β and α are the impact ionization coefficients of holes and electrons, respectively. k can be engineered by band diagram engineering [1], band structure engineering [2], or dead space effect [3,4]. Combinations of these are also used [5]. Band diagram engineering enables the implementation of Capasso’s channeling APD [1]. In this design, electrons and holes are spatially separated in different channels with distinct materials and bandgaps. These channel materials are designed to minimize the impact ionization of one carrier and promote the other, thereby optimizing the k and excess noise. The two limitations of the Capasso design are (1) the leakage current due to doping the channels and (2) excess noise due to dual carrier injection. Firstly, to spatially separate the carriers between narrow and wide bandgap materials, type I band alignment with doping is suggested by Capasso. However, type II band alignment, due to the valance band offset, may inherently provide the field required for the spatial separation of carriers. And type II alignment avoids the doping that could lead to leakage currents. Secondly, channeling APD is a planar configuration leading to dual carrier injection that increases the excess noise. Using a window injection layer defined by lithography, a channeling APD with single carrier injection is designed.
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