Biomedical Engineering Reference
In-Depth Information
collapsing and elongating along the [1
1 0] direction. The surface is then exposed
to AsBr
3
which selectively etches the seed QDs away while the GaAs surface
remains relatively unscathed. Holes formed from the etched QDs are typically 5-
nm deep, approximately the nominal height of the seed QDs. The nanoholes are
used as a template for subsequent regrowth of InAs, but In-Ga intermixing and
Ga diffusion result in InGaAs QDMs. During regrowth, adatoms arriving at the
surface preferentially attach to the nanohole rim. By varying the deposition amount
and the substrate temperature, the number of constituent QDs making up a QDM
along the rim can be varied between 2 and 6; the latter is shown in the plane-view
schematic drawing in the bottom of Fig.
3.1
a. These QDMs are of high crystalline
quality as the reported photoluminescent (PL) intensity is similar to the original
QDs [
10
].
−
3.2.2
QDMs on Nanomound Templates
Nanomound templates can also be used to form lateral InGaAs QDMs as reported
by Lee et al. in 2006 [
11
]. This approach is best described as a hybrid process
between droplet homoepitaxy and standard SK heteroepitaxy as summarized in
Fig.
3.1
b. The procedure begins by the formation of a few monolayers (MLs) Ga
droplets at high substrate temperatures without the presence of arsenic. The large
lattice mismatch between the metallic Ga and the semiconducting GaAs substrate
means that Ga droplets readily form on the flat GaAs surface via the Volmer-Weber
growth mode. The substrate temperature is then lowered, typically to below 200
◦
C,
and the surface exposed to As
4
which then crystallizes the Ga droplets into GaAs
nanomounds with base length and height around 100 and 50 nm, respectively. These
nanomounds serve as a template for subsequent overgrowth in a similar fashion
to the nanohole approach described above. The substrate temperature, kept low
during crystallization to preserve the original mound geometry, is raised prior to
the deposition of InAs. By varying the deposition amount the number of InAs QDs
nucleated around the GaAs nanomounds can be varied between 2 and 6, similar to
the previous approach. GaAs nanomounds not only serve as a spatial template but
also act as a Ga reservoir since Ga atoms get incorporated into the growing In(Ga)As
QDs. These QDMs also have high crystalline quality as shown by strong PL.
A variant of the above hybrid DE-SK approach is also reported by Strom et
al. where, using different conditions, the starting Ga droplets are crystallized into
nanorings and subsequent overgrowth results in the formation of InAs QDs inside
and outside the rings with a significant amount of GaAs rings still remaining after
crystallization [
12
]. While the QDs which nucleate outside the nanorings are closely
spaced and likely be coupled, those nucleated inside are completely decoupled as
they are isolated from outside QDs by the thick GaAs nanorings.
