Selective Area Growth of High-Quality In-Plane InAs Nanowires and Nanowire Networks by Molecular-Beam Epitaxy on Ge Substrates
In-plane InAs nanowires and nanowire networks are promising platforms for electronics, optoelectronics, and topological quantum computing due to their small electron effective mass, narrow bandgap, high electron mobility, strong spin-orbit coupling, and large Landé g factor. However, their selective area growth on CMOS-compatible group-IV substrates remains challenging. Here, we report the selective area growth of high-quality in-plane InAs nanowires and nanowire networks on Ge(111) substrates by molecular-beam epitaxy. Conventional selective-area epitaxy fails to simultaneously achieve good selectivity and continuity. To overcome this, we developed a metal-sown, single-indium-source two-step growth method, which attains both selectivity and continuity but yields nanowires with rough surfaces and lengths below 10 μm. We then introduced an upgraded metal-sown, dual-indium-source two-step growth method, successfully fabricating in-plane InAs nanowires and nanowire networks with smooth surface morphology and lengths exceeding 60 μm. By optimizing the As beam equivalent pressure, overgrowth at network junctions is effectively suppressed, resulting in uniform nanowire networks. High-resolution transmission electron microscopy and Raman spectroscopy confirm the high-quality single-crystalline nature and pure zinc-blende structure of the nanowires and networks. This work establishes a foundation for fabricating high-quality in-plane InAs/superconductor hybrid nanowires and nanowire networks on Ge substrates.