In situ and real-time SXRD study of the growth and the solid-liquid-solid transition of bismuth on an insulating substrate

In situ and real-time SXRD study of the growth and the solid-liquid-solid transition of bismuth on an insulating substrate.

M. Jankowski

The European Synchrotron- ESRF, 71 Avenue des Martyrs, Grenoble, France

Nanostructured ultrathin bismuth films have recently attracted a lot of interest as they reveal exotic functional properties that do not exist in the bulk [1-4]. Especially the spin-momentum locked surface states of topological insulating (TI) bismuth films make them very attractive candidates for spintronic devices. To develop and optimize TIs towards applications, thin films of high quality are a necessity, as otherwise the exotic electronic properties are hampered by bulk conduction.  

In this contribution we report on the controlled growth of bismuth on the α-Al2O3(0001) surface, investigated by surface x-ray diffraction and x-ray reflectivity using synchrotron radiation. At temperatures as low as 40 K, unanticipated pseudo cubic Bi(110) films are grown with the thickness between few up to few tens of nanometre [5]. The roughness at the film-vacuum and film-substrate interfaces can be decreased by mild heating, followed by a phase transition of Bi(110) towards Bi(111) at 400 K. From 450 K onwards the high quality and ultrasmooth Bi(111) films are formed. Growth at temperatures around the transition temperature results in competing growth of Bi(110) and Bi(111) thin-film domains. 

In addition, we have found two new scenarios of solid-liquid-solid transition of bismuth films [6,7], depending on the initial growth conditions.  The solid- liquid transition of (111) films is observed at 548 K and the liquid recrystallizes to the (110) crystallographic phase at 440 K, one hundred degrees below the Bi melting point (supercooling). Ex-situ AFM investigations revealed the formation of large sub-microcrystals. The second scenario routes towards the creation of Bi(110) nanodots,  very promising for their electronic properties.  The deposition of bismuth just below the transition temperature of (110) to (111) phase leads to the formation of well-defined nanodots, which growth was followed in–situ and real- time as a function of temperature.  Similar to the (111) films, the nanodots go through the solid-liquid-solid phase transition during slow annealing and subsequent cooling, which finally leads to their unexpected recrystallization on the surface.

The presented results show a variety of bismuth nucleation scenarios on an insulating substrate, which is very beneficial for an understanding of fundamental processes at the metal-oxide interface and has potential practical applications.

References:

[1] M.-Y. Yao et al, Sci. Rep. 6 (2016) 21326.  [2] H. Du et al., Nat. Commun. 7 (2016) 10814.  [3] Y. Lu et al., Nano Lett. 15 (2015) 80–87. [4] I.K. Drozdov et al., Nat. Phys. 10 (2014) 664–669. [5] M. Jankowski et al., Nanotechnology 28 (2017) 155602. [6] Y. Li et al., ACS Nano 10 (2016) 2386–2391. [7] Y. Li et al., Nat. Commun. 8 (2017) 14462.