Silicon nanocrystals (SiNCs) have become a heavily researched material over the
past several years. Researchers envision that this material can be used in many diverse
applications such as electronic devices, non-toxic biological tags, optical devices such as
LEDs, lasers or displays, thermoelectrics, and photovoltaic (PV) applications. For many
of these proposed applications one needs to properly control the NC size and the surface
chemistry via passivation. Current passivation techniques allow for the creation of highly
efficient SiNC optical emitters, however the emission of these NCs are fixed in the red-
NIR range. To resolve this issue several novel in-flight passivation techniques were
A novel dual-plasma setup which allows for the in-flight passivation of SiNCs
through a thermal or LPCVD based nitridation process was developed first. FTIR and
XPS analysis were used to study the surface chemistry on of the nitride passivated NCs
while TEM was used to investigate whether or not a “shell” was grown on the surface.
PL measurements and thermal stability tests were performed on the nitride passivated
NCs to gain a further understanding of the stability (in both air as well as other ambients)
of the NCs and their surface chemistry.
Tunable full color emission from SiNCs was developed for the dual-plasma
reactor utilizing CF4 as both an etching and passivating source. F radicals generated in
the etching plasma remove Si from the surface of the NC, while at the same time CF2
radicals lead to the formation of a fluorocarbon passivation layer on the NC surface. By
controlling the parameters of the reactor (CF4 flow rate, power), the NC size and thus its
color can be controlled. Red to green luminescence was observed from SiNCs and is
believed to be due to the quantum confinement effect. The blue emission observed from
the NCs is appears to be related to oxide related surface states. Despite the defects, high
QY was observed from these CF4-etched NCs.
The fluorocarbon passivation layer, although stable, prevents further
functionalization of the NCs. To counteract this problem another silicon-based dry etch
chemistry, SF6 was investigated. Full-color emission was observed from SF6 etched
NCs, with QY 2X higher than that of CF4-etched NCs. A maximum QY of nearly 55% at 700 nm was observed after several weeks in air, comparable to that observed with alkyl
passivation. The native oxidation of the bare oxidized and SF6-etched NCs were also
studied. Results show that the NC oxidation follows the Cabrera-Mott mechanism for
low temperature oxidation.
Inorganic-NC based LED structures were then investigated. Fabrication
processes for the inorganic hole and electron transport layers were developed by RF
sputtering and atomic layer deposition (ALD). Thorough characterization was performed
on the metal-oxide films (ZnO, TiO2, NiO) to verify their stoichiometry as well as study their optical and electrical properties. Novel inorganic-NC device structures were
fabricated. Inorganic NC devices which use a metal-oxide HTL but no ETL, emit light,
however their emission is so weak. The addition of an ETL increases the light output by
a factor of 4, but the device reproducibility is poor. To improve efficiency two insulating
matrix layers were investigated. In both cases, the film deposited on the top of the NC is
rough, porous, discontinuous, and potentially full of traps – certainly not the ideal film
for a device. Therefore, more work is needed, specifically on the NC layer to improve
the structure of the as-deposited NC film, but efficient device structures appear to be
University of Minnesota Ph.D. dissertation. October 2009. Major: Electrical Engineering. Advisors: Stephen A. Campbell and Uwe Kortshagen. 1 computer file (PDF);xviii, 237 pages. Ill. (some col.)
Liptak, Richard William.
In-flight gas phase passivation of silicon nanocrystals for novel inorganic-silicon nanocrystal based electroluminescent devices..
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