W- Structured type-II superlattice based long and very-long wavelength infrared photodiodes

摘要

W-structured type-II superlattices (W-SLs) were initially developed to increase the gain of mid-wave infrared (MWIR)lasers. The design addressed the reduced optical transition matrix elements due to the spatial displacement betweenvalence and conduction band wavefunctions in the type-II superlattice (T2SL), and further improved the differentialoptical gain by providing a mostly two-dimensional density of states. As a result, W-SL and W interband cascade lasershave lower thresholds and higher pulsed and cw operating temperatures than any other III-V interband MWIR lasers.These same features give W-SLs desirable properties for IR detectors, and here we report for the first time oncharacteristics of W-SLs used for long-wave and very long-wave IR photodiodes. IR transmission measurements of Wand conventional T2SL photodiodes revealed absorption characteristics that are well described by theory, including lineshape and peak absorption coefficient values which are about a factor of 2 greater in the W-SLs. Similarly, the lowtemperature photoluminescence shows much higher and sharper emission intensity in the W-SLs. While the W-SLs havedemonstrated superior optical properties, as predicted, additional work is needed to achieve higher detector quantumefficiency. Results suggest that the excess carrier collection in the W-structures is reduced with respect to similar T2SLstructures, especially for the lowest energy state. Possible mechanisms of excess carrier loss, as well as new designs toimprove charge collection, in the W-SL, will be discussed.