Local Droplet Etching of Semiconductor Nanostructures

Andreas Graf, Christian Heyn, Jochen Kerbst, Vera Paulava, David Sonnenberg, and Wolfgang Hansen

The fabrication of single-crystalline semiconductor nanostructures by utilizing the molecular beam epitaxy (MBE) technique has a long tradition beginning in the early 1970th. The first structures were build of planar semiconductor heterostructures often with integrated two-dimensional electron systems. Later, the application of self-assembly mechanisms allowed the fabrication of nanostructures with further reduced dimensionality. Prominent examples are the Stranski-Krastanov growth mode providing for instance strain-induced InAs or Ge quantum dots (QDs), the droplet epitaxy which enables the generation of quantum dots and rings (QRs), and the vapour liquid solid (VLS) growth of nanowires. Here, the unique control of the MBE technique on the process parameters allows the highly reproductive creation of nanostructures with properties being tunable over a wide range.

All these methods are based on the incorporation of additional material and, thus, represent bottom-up technologies. The local droplet etching (LDE) represents a qualitatively different approach which extends conventional molecular beam epitaxy and accounts for self-assembled local removal of material from a semiconductor surface in a top-down fashion. LDE is fully compatible with conventional molecular beam epitaxy (MBE) equipment and does not require any lithographic steps. During LDE, liquid metallic droplets are utilized to form nanoholes in semiconductor surfaces by local drilling. The method was introduced by Wang et al. for etching of GaAs surfaces with Ga droplets [Zh. M. Wang et al., Appl. Phys. Lett. 90, 113120 (2007)]. Later, we have expanded the range of materials and demonstrated etching with Al, In, and GaIn droplets.