Deep level in the InAs/InAsSb superlattice revealed by forward-bias tunneling

What is it about?

In any semiconductor there is a fundamental gap in the energy levels that are available to be occupied by electrons. However, trap sites caused by defects or impurity atoms can introduce localized energy levels positioned within the gap. Such traps are important because they can act as steppingstones for charge carriers, which will add unwanted currents. We have measured the energy level of a significant trap within a p-n junction device that was built for detection of mid-wavelength infrared light. The semiconductor in this case is the superlattice InAs/InAsSb, consisting of very thin alternating layers of InAs and InAsSb. When a forward bias voltage is applied to the p-n junction, the traps cause a tunneling current that adds to the customary forward junction current. In our experiment, as we increase the forward voltage, a point is reached at which the tunneling current turns off due to the changing alignment of the energy bands adjacent to the junction. This allows us to determine the energy level of the traps.

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Why is it important?

For a good signal-to-noise ratio, infrared detectors require that the dark current (background current) be as low as possible. When deep levels are present, they can modify the properties of the semiconductor material, causing unwanted additional currents in fabricated devices. The InAs/InAsSb superlattice is now commonly used to build infrared detectors, but its family of typical defects is not well understood. Our results contribute to the understanding of this relatively new material, and they should lead to improvements in device performance. Additionally, the technique that we demonstrated for analyzing the forward-bias tunneling current should be applicable to a variety of semiconductor materials.

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