Real-time TEM observations of III-V nanowire growth by molecular beam epitaxy

Here, we present experimental observations of the growth of III-V nanowires using a transmission electron microscope (TEM) equipped with molecular-beam-epitaxy (MBE) sources. Nanowires are grown directly inside the microscope, and their growth is monitored in situ and in real-time with high spatial and temporal resolution. When GaAs nanowires are grown using the vapor-liquid-solid (VLS) method, the stable zincblende (ZB) phase coexists with metastable wurtzite (WZ) structure, resulting in nanowires having a mixed-phase structure. Remarkably, the valence and conduction bands of the two phases are misaligned so that small sections of one phase within the other effectively confine charge carriers. These heterostructures are defined as crystal-phase quantum dots (CPQDs) and have been proposed for several potential applications in photonics and quantum computing. The technological application of CPQDs has been severely limited by the poor understanding of the crystal-phase selection mechanism and the difficulty of controlling their formation. Only recently, thanks to in situ TEM, could we shed new light on the phase selection mechanism and demonstrate how to achieve structural control. In fact, in situ TEM provides unparalleled imaging resolution and allows one to capture in real-time the growth dynamics and the effects of changing growth parameters. By using in situ TEM we demonstrate that the phase selection can be controlled by simply tuning the contact angle. These findings are explained within a dedicated model based on surface energetics and provide a clear route for the crystal phase control in III-V nanowires.