Au - Free Growth of InAs Nanowires

Full text: Lattice mismatch and defects at boundaries between different semiconductors are limiting factors hindering the fabrication of otherwise extremely appealing heterostructures. It seems that nanowires provide a way to circumvent many of such problems due to the small contact area between substrate and wire, which apparently can lead to almost defect-free structures even if the interface region itself may contain some defects. Nanowire growth makes material combinations possible, which have long been envisioned but are very difficult to realize with conventional heterostructure growth, like III-V compounds on top of silicon substrates. One such example is the growth of InAs nanowires, which - like many other nanowire materials - are usually grown in a vapor-liquid-solid like growth mode with Au seed particles. InAs is of considerable interest for the electronic devices due to its low effective electron mass. However, for the growth on Si, gold is rather unwanted since it acts as a deep trap in Si, and thus it is crucial to develop other growth schemes without the need for Au seed particles. We have achieved the controlled Au-free growth of InAs nanowires on Si, as well as on III-V substrates, using SiOx or organic layers to nucleate wire growth. Furthermore position controlled growth of InAs on Si has also been demonstrated in the literature. However, in contrast to Au-nucleated nanowire growth, so far only little is known about the details of this Au-free growth mechanism. In our work we studied the nucleation and growth mechanism of Au - free grown InAs nanowires. For the growth of these wires InAs and InP substrates are covered with a thin layer of SiOx and wires are grown using metal-organic vapor phase epitaxy (MOCVD). As the possibilities for in-situ observation of growth in MOCVD are very limited, a series growth studies were carried out. In particular, to clarify the origin of the Au-free InAs growth on SiOx covered InAs in-situ Low Energy Electron Microscopy (LEEM) and x-ray photo electron emission microscopy experiments were carried out. With these in-situ heating experiments it was determined that In droplets play a crucial role for the wire growth. By comparing the obtained results with such from commonly used growth mechanisms we determine the mechanism responsible for our wire growth and develope a corresponding model. Finally a comparison of our results to reports of Au free growth in the literature is made to evaluate how general the derived growth model is. (author)