Role of patterning in islands nucleation on semiconductor surfaces

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forecoming nanotechnology era. New frontiers in nanodevices technology rely on the precise positioning of the nucleation site and on controlling the shape and size of the dots. In this paper we will review some recent studies regarding the control of the nucleation process on semiconductor surfaces. After a brief review of the theory of free nucleation on surfaces and of the role of steps and defects, a few novel approaches to form ordered patterns on surfaces are explored: natural patterning induced by surface instabilities (as step bunching or step meandering), in situ substrate patterning by Scanning Tunneling Microscopy (STM), high resolution patterning by Focused Ion Beam (FIB). Growth of epitaxial layers of semiconductors (Ge/Si(100) or InAs/GaAs(100)) on these patterned surfaces has been studied by STM or Atomic Force Microscopy (AFM) unveiling the way in which the first atoms start to aggregate and identifying their exact nucleation site. Control of the dot size to match the patterning typical wavelength has been achieved by using surfactants on misoriented substrates. STM images acquired in real time allowed to identify the mechanism of Ge cluster formation on patterned Si(100), and to follow the island transition from pre-pyramid to pyramid. Nucleation of Ge islands on SiO2 layers has been obtained thanks to FIB tight patterning, achieving island densities of 3.5x10^10/cm^2.