Creation of unexplored tunnel junction by heterogeneous integration of InGaAs nanowires on germanium

Heteroepitaxy has inherent concerns regarding crystal defects originated from differences in lattice constant, thermal expansion coefficient, and crystal structure. The selection of III-V materials on group IV materials that can avoid these issues has however been limited for applications such as photonics, electronics, and photovoltaics. Here, we studied nanometer-scale direct integration of InGaAs nanowires (NWs) on Ge in terms of heterogenous integration and creation of functional materials with an as yet unexplored heterostructure. We revealed that changing the initial Ge into a (111)B-polar surce anabled vertical InGaAs NWs to be integrated for all In compositions examined. Moreover, the growth naturally formed a tunnel junction across the InGaAs/Ge interface that showed a rectification property with a huge current density of several kAcm^-2 and negative differential resistance with a peak-to-valley current ratio of 2.8. The described approach expands the range of material combinations for high-performance transistors, tandem solar cells, and three-dimensional integrations.