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

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 A 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.