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@PHDTHESIS{Paul:62537,
author = {Paul, Neelima},
othercontributors = {Voigtländer, Bert},
title = {{B}i surfactant mediated growth for fabrication of
{S}i,{G}e nanostructures and investigation of {S}i,{G}e
intermixing by {STM}},
address = {Aachen},
publisher = {Publikationsserver der RWTH Aachen University},
reportid = {RWTH-CONV-124101},
pages = {114, 2 S. : Ill., graph. Darst.},
year = {2007},
note = {Aachen, Techn. Hochsch., Diss., 2007},
abstract = {Heteroepitaxial growth of Ge on Si is a prototypical system
for studying strained Stranski-Krastanow growth.The
technological drawback is that it turns out to be difficult
to grow Ge on Si in a controlled layer-by-layer manner.
After a few smooth layers have been grown epitaxially, the
growth then proceeds by the formation of islands. This is
known as the Stranski-Krastanov growth mode. It has been
found, however, that deposition of a single monolayer of As,
Sb or Bi onto the silicon surface before growth of Ge
begins, can overcome the problem. The Ge then grows
epitaxially in a two-dimensional mode, while the surfactant
(As, Sb or Bi) atoms apparently rise up through the growing
Ge layers and segregate on the top of the film. This is
called surfactant mediated growth (SME). Surfactant mediated
growth of Ge on Si(111) has been well studied in the last
decade. Initial investigations used As and Sb as
surfactants. In fact, Ge p-channel MOSFETs have already been
fabricated using the Sb surfactant. However for the Bi
surfactant, the studies were not so thorough. In the thesis
work presented here, we show that Bi is more promising
surfactantmaterial than Sb. We demonstrate that by using Bi
as a terminating layer on Ge/Si surface, it is possible to
distinguish between Si and Ge in Scanning tunnelling
microscope (STM). Something which was very difficult in the
past. Using this property, it is possible to create Ge/Si
nanostructures in a controlled manner. Moreover, it is also
possible to study Ge/Si intermixing in surface layers in
some detail. Any attempt to utilize surfactant mediated
growth must be preceded by a thorough study of its effect on
the the system being investigated. Thus, the third chapter
of this thesis deals with an extensive study of the Bi
surfactant mediated growth of Ge on Si(111) surface as a
function of Ge coverage. The growth is investigated from the
single bilayer Ge coverage till the Ge coverage of about 15
BL when the further Ge deposition leads to two-dimensional
growth. In the fourth chapter, the unique property of Bi
terminating layer on Ge/Si surface to result in an STM
height contrast between Si and Ge is explained with possible
explanations given for the reason of this apparent height
contrast. The controlled fabrication of Ge/Si nanostructures
such as nanowires and nanorings is demonstrated. A study on
Ge-Si diffusion in the surface layers by a direct method
such as STM was impossible previously because of the similar
electronic structure of Ge and Si. Since with the Bi
terminating surface layer, one is able to distinguish
between Ge and Si, the study of intermixing between them is
also possible using STM. This method to distinguish between
Si and Ge allows one to study intermixing on the nanoscale
and to identify the fundamental diffusion processes giving
rise to the intermixing. In Chapter 5 we discuss how this
could prove useful especially as one could get a local probe
over a very narrow Ge -Si interface. On one hand it is
possible to study the displacement of the Si and Ge atoms
when factors like temperature and deposition rate are varied
during growth. On the other hand, a post growth study like
annealing already grown Ge-Si wires over a period of time
could also be performed. A new model is proposed to estimate
change in the Ge concentration in the surface layer with
time. The values of the activation energies of Ge/Si
exchange and Si/Ge exchange are estimated by fitting the
experimental data with the model. During step flow growth of
Ge wires along Si step edges, the step speed is seen to
strongly affect the Ge/Si intermixing. Ge wires grown at
faster step speeds appear higher than Ge wires grown at
slower step speeds. In chapter 5, the Ge/Si intermixing has
been studied on a surface having 1 ML Bi reconstruction. In
Chapter 6, we discuss the Ge-Si intermixing on surfaces with
different reconstruction, such as the 1/3 ML Bi
reconstruction and the Si reconstruction. The vertical Ge-Si
intermixing is more in the surface with the Si
reconstruction as compared to the surface with 1/3 ML Bi
reconstruction. This is due to the reason that the Bi layer
inhibits Ge atoms from exchanging with Si substrate atoms
during Ge growth. In the last chapter, an attempt has been
made to elucidate the need for utilizing two dimensional Bi
surfactant Ge/Si surfaces for industrial applications as
transistors by demonstrating the quick, efficient and
complete removal of Bi surfactant monolayer from thick Ge
layers by ion beam sputtering without damaging the
underlying Ge/Si layer.},
keywords = {Rastertunnelmikroskop (SWD) / Grenzflächenaktiver Stoff
(SWD) / Mischen (SWD) / Wachstum (SWD)},
cin = {130000 / 134110},
ddc = {530},
cid = {$I:(DE-82)130000_20140620$ / $I:(DE-82)134110_20140620$},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-opus-20963},
url = {https://publications.rwth-aachen.de/record/62537},
}
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