Negative differential resistance in AlAs/NiAl/AlAs heterostructures: Evidence for size quantization in metals

Abstract
We report on the first electron transport measurements in (Al,Ga)As/NiAl/(Al,Ga)As semiconductor-metal-semiconductor double heterostructures grown entirely by molecular beam epitaxy. For sufficiently thin NiAl films, a voltage-controlled negative differential resistance region is observed in the axial current-voltage characteristics of our AlAs/NiAl/AlAs double-barrier structures. Room-temperature peak-to-valley ratios as high as 2 have been obtained for a 33 Å NiAl layer. The general characteristics of this phenomenon are remarkably similar to the negative differential resistance observed in all-semiconductor resonant tunneling structures. We believe that this effect can be attributed to electron size quantization in the thin metal film.