Composition-dependent interfacial abruptness in Au-catalyzed Si(1-x)Ge(x)/Si/Si(1-x)Ge(x) nanowire heterostructures.

As MOSFETs are scaled down, power dissipation remains the most challenging bottleneck for nanoelectronic devices. To circumvent this challenge, alternative devices such as tunnel field effect transistors are potential candidates, where the carriers are injected by a much less energetically costly quantum band to band tunneling mechanism. In this context, axial nanowire heterointerfaces with well-controlled interfacial abruptness offer an ideal structure. We demonstrate here the effect of tuning the Ge concentration in a Si1-xGex part of the nanowire on the Si/Si1-xGex and Si1-xGex/Si interfacial abruptness in axial Si-Si1-xGex nanowire heterostructures grown by the Au-catalyzed vapor-liquid-solid method. The two heterointerfaces are always asymmetric irrespective of the Ge concentration or nanowire diameter. For a fixed diameter, the value of interface abruptness decreases with increasing the Ge content for the Si/Si1-xGex interface but shows no strong Ge dependence at the Si1-xGex/Si interface where it features a linear correlation with the nanowire diameter. To rationalize these findings, a kinetic model for the layer-by-layer growth of nanowire heterostructures from a ternary Au-Ge-Si alloy is established that predicts a discrepancy in Ge concentration in the layer and the catalyst droplet. The Ge concentration in each layer is predicted to be dependent on the composition of the preceding layer. The most abrupt heterointerface (∼5 nm) is achieved by growing Si1-xGex with x = 0.85 on Si in a 25 nm diameter nanowire.