420
14 Nanostructures
Fig. 14.32 Lithography
and etching techniques for
the fabrication of
semiconductor structures
14.4.3 Self-Assembled Preparation
The preparation methods for QDs split into top-down (lithography and etching) and bottom-up (selfassembly) methods. The latter achieve typically smaller sizes and require less effort (at least concerning
the machinery).
Artificial Patterning
Using artificial patterning, based on lithography and etching (Fig. 14.32), quantum dots of arbitrary
shape can be made (Fig. 14.33). Due to defects introduced by high-energy ions during reactive ion
etching the quantum efficiency of such structures is very low when they are very small. Using wetchemical etching techniques the damage can be significantly lowered but not completely avoided.
Since the QDs have to compete with other structures that can be made structurally perfect, this is not
acceptable.
Template Growth
Template growth is another technique for the formation of nanostructures. Here, a mesoscopic structure
is fabricated by conventional means. The nanostructure is created using size-reduction mechanisms,
e.g., faceting, (Fig. 14.34). This method can potentially suffer from low template density, irregularities
of the template, and problems of reproducibility.
Colloids
Another successful route to nanocrystals is the doping of glasses with subsequent annealing (color
filters). When nanocrystals are prepared in a sol-gel process, the nanoparticles are present as a colloid
in wet solution. With the help of suitable stabilizing agents they are prevented from sticking to each other
14 Nanostructures
Fig. 14.32 Lithography
and etching techniques for
the fabrication of
semiconductor structures
14.4.3 Self-Assembled Preparation
The preparation methods for QDs split into top-down (lithography and etching) and bottom-up (selfassembly) methods. The latter achieve typically smaller sizes and require less effort (at least concerning
the machinery).
Artificial Patterning
Using artificial patterning, based on lithography and etching (Fig. 14.32), quantum dots of arbitrary
shape can be made (Fig. 14.33). Due to defects introduced by high-energy ions during reactive ion
etching the quantum efficiency of such structures is very low when they are very small. Using wetchemical etching techniques the damage can be significantly lowered but not completely avoided.
Since the QDs have to compete with other structures that can be made structurally perfect, this is not
acceptable.
Template Growth
Template growth is another technique for the formation of nanostructures. Here, a mesoscopic structure
is fabricated by conventional means. The nanostructure is created using size-reduction mechanisms,
e.g., faceting, (Fig. 14.34). This method can potentially suffer from low template density, irregularities
of the template, and problems of reproducibility.
Colloids
Another successful route to nanocrystals is the doping of glasses with subsequent annealing (color
filters). When nanocrystals are prepared in a sol-gel process, the nanoparticles are present as a colloid
in wet solution. With the help of suitable stabilizing agents they are prevented from sticking to each other