Fabrication of Semiconductor Quantum Dot Molecules: Droplet Epitaxy and Local Oxidation Nanolithography Techniques - Quantum Dot Molecules - page 4

1.2 Fabrication of Vertical and Lateral QDMs by Droplet

Epitaxy Technique

The droplet epitaxy growth technique was first used at the beginning of the 1990s
by Koguchi and Ishige [ 35 , 36 ]. It was initially proposed as a new molecular beam
epitaxy (MBE) method for the fabrication of III-V semiconductor nanostructures
on II-VI semiconductor substrates with nearly equal lattice constant. It basically
consists in the initial deposition of atoms of Group-III element in absence of any
supply of Group-V element, creating liquid metal droplets on the substrate. These
metal droplets are, right afterwards, exposed to an atmosphere of Group-V element,
which induces their crystallization into III-V nanostructures. On GaAs surfaces,
and depending on the substrate temperature, this growth method yields different
types of nanostructures. Particularly, QDs [ 37 , 38 ] and quantum rings (QRs)
complexes [ 39 ] are formed at relatively low temperatures of 200-300 ◦ C, whereas
nanoholes surrounded by mounds [ 28 ] are obtained at high substrate temperatures
of approximately 500 ◦ C. This relatively high temperature “nanodrilling” process
has been recently emerged as a very promising in situ patterning technique. From a
purely formation point of view, such a nanohole etching process is not a surprising
result on GaAs, as long as it is well known that GaAs surfaces are unstable
under Ga-rich conditions and As desorption occurs at a temperature of 500 ◦ C[ 40 ].
Specifically, when depositing Ga droplets on the GaAs surface, As atoms from the
underlying GaAs may diffuse and escape to the vacuum chamber or incorporate
into the droplet surroundings, which explains the formation of GaAs mounds around
them even before any additional supply of As [ 28 ]. On the other hand, the remaining
Ga atoms at the interface region after As desorption merge into the Ga droplet. As
this process continues, there are more GaAs being dissolved below the droplet, more
Ga atoms from the GaAs substrate being incorporated into the droplet, and more
As atoms forming GaAs mounds on the surroundings. The Ga droplet acts then
as a “nanodrill” into the GaAs surface inducing the formation of nanoholes. This
etching process presents many advantages over other patterning techniques, as the
density of nanoholes obtained on the surface is controllable by the initial Ga supply
in the form of droplets, which can be as low as 10 7 cm − 2 . Also, as the kinetics of
this mechanism is strongly affected by the supply of As atoms that induces a fast Ga
droplet crystallization (i.e., the “nanodrilling” process is stopped), the nanohole size
can be accurately controlled. More importantly, due to its self-assembling nature
and the high substrate temperatures used, this in situ patterning mechanism avoids
any contamination or degradation of the substrate, thus ensuring an excellent optical
quality of the semiconductor material [ 30 ].

1.2.1 Vertical QDMs

The potential and versatility of the droplet epitaxy etching for subsequent InAs
preferential epitaxial growth is demonstrated in the following by the fabrication