Carrier Dynamics in
Information and Communication Technology
KTH Royal Institute of Technology
Stockholm, Sweden 2015
KTH School of Information and
Communication Technology SE-164 40,
A dissertation submitted to KTH Royal Institute of Technology, Stockholm, Sweden, in
partial fulfillment of requirements for the degree of Teknologie Doktor (Doctor of
Philosophy). The defense will take place on October 23 2015 at 10.00 a. m. at Sal C, KTH-
Electrum, Kistagången 16, Kista.
© Fatemeh Sangghaleh, October 2015.
Printed by: Universitetsservice US-AB
Bulk silicon as an indirect bandgap semiconductor is a poor light emitter. In contrast,
silicon nanocrystals (Si NCs) exhibit strong emission even at room temperature,
discovered initially at 1990 for porous silicon by Leigh Canham. This can be
explained by the indirect to quasi-direct bandgap modification of nano-sized silicon
according to the already well-established model of quantum confinement.
In the absence of deep understanding of numerous fundamental optical properties
of Si NCs, it is essential to study their photoluminescence (PL) characteristics at the
single-dot level. This thesis presents new experimental results on various
photoluminescence mechanisms in single silicon quantum dots (Si QDs).
The visible and near infrared emission of Si NCs are believed to originate from the
band-to-band recombination of quantum confined excitons. However, the mechanism
of such process is not well understood yet. Through time-resolved PL decay
spectroscopy of well-separated single Si QDs, we first quantitatively established that
the PL decay character varies from dot-to-dot and the individual lifetime dispersion
results in the stretched exponential decays of ensembles. We then explained the
possible origin of such variations by studying radiative and non-radiative decay
channels in single Si QDs. For this aim the temperature dependence of the PL decay
were studied. We further demonstrated a model based on resonance tunneling of the
excited carriers to adjacent trap sites in single Si QDs which explains the well-known
thermal quenching effect.
Despite the long PL lifetime of Si NCs, which limits them for optoelectronics
applications, they are ideal candidates for biomedical imaging, diagnostic purposes,
and phosphorescence applications, due to the non-toxicity, biocompability and
material abundance of silicon. Therefore, measuring quantum efficiency of Si NCs is
of great importance, while a consistency in the reported values is still missing. By
direct measurements of the optical absorption cross-section for single Si QDs, we
estimated a more precise value of internal quantum efficiency (IQE) for single dots in
the current study. Moreover, we verified IQE of ligand-passivated Si NCs to be close
to 100%, due to the results obtained from spectrally-resolved PL decay studies. Thus,
ligand-passivated silicon nanocrystals appear to differ substantially from oxide-
encapsulated particles, where any value from 0 % to 100 % could be measured.
Therefore, further investigation on passivation parameters is strongly suggested to
optimize the efficiency of silicon nanocrystals systems.
My journey as a PhD student will be over soon. Looking back, I see years full of
curiosity, enthusiasm, learning, improvements, challenges, failures and success.
Today I’m a person with new perspectives about science, and research. During such
an amazing journey I received support and encouragements from great number of
people whom I would like to thank.
First and Foremost, I would like to express my sincerest gratitude to my advisor
Professor Jan Linnros, for his encouragement, support and deep knowledge in the
research topic which motivated and guided me through the right path. Thanks Jan for
giving me this opportunity to be part of nano-silicon group and such a fascinating
project. I appreciate all your time and ideas that made my PhD experience productive
I would also like to thank Professor Ulf Karlsson, the former head of the Material’s
platform, for his support and positive attitude. I was so motivated to be a better
researcher by your valuable advice and great encouragement. My passion for Swedish
language comes from the amazing people like you, who inspired me by bringing front
my progress continuously. Tack så jätte mycket, Ulf.
I would like to express my deep appreciation to Dr. Ilya Sychugov whom I regard
as my unofficial co-supervisor. Thanks Ilya for building positive group dynamics as
well as the scientific advice, knowledge and insightful discussions. You were the
primary source of my morning scientific questions. I learned from you, how to
“think” and “act” in a more balanced way for being a better experimentalist, and not
get disappointed by the first failures.
Benjamin, thank you for showing me how to get started with sample fabrication
and PL characterization and helping me to catch up with the ongoing project. I
learned a lot from you and I was so inspired by your different approach to research
and your curiosity upon scientific problems.
Fede, thank you for all the scientific adventures we had, from shaking and backing
the vacuum tube, and long hours of low temperature measurements, to one day
“Paris” walking after EMRS conference. You may continue our project with
spectacular results and great success.
Roody, thank you for not only be the best officemate but also my “PhD buddy”. I
will always remember our interesting fundamental discussions (from electron
entanglement to biological evolution impacts on human behaviors). Thanks for your
support during up and downs and for all the great times we had together. Without you
the late night and weekend works would be so boring and unbearable.
I would like to thank my colleagues in Nano-silicon group (former and present),
Aki-Kimmo, Miao, Yashar, Karolis, Rikard, Torsten, Apurba and Anna, for all the
fruitful discussions, mind refreshing coffee breaks, paper cake gatherings and
inspiring lunch breaks we had.
Many thanks go to all the present and former members in Materials physics
department. Prof. Mats Göthelid, Prof. Johan Åkermann, Prof. Oscar Tjernberg, and
Assoc. Prof. Jonas Weissenrieder for contributing in the nice working environment in
our unit. I have enjoyed the company of Karin, Fatjon, Magnus, Bastian, Alex, Luca,
Viktor, Hamid, Sohrab, Sareh, Karolina, Stefano, Zahra, Dunja, Marcelo, Olof,
Milad, Markus, Shun, and Tobias. Thank you all for keeping the warm and positive
atmosphere in the department. Thank you, Anneli for being so considerate, friendly
and for all your efforts to arrange amazing department activities.
A special thank goes to Madeleine Printzsköld, for her highly efficient and
professional work. Thanks Madeleine for your constant support and guide through all
the confusing paper work, contracts, documentations and travel arrangements.
I’m grateful to be around so many amazing people in Electrum. Thanks to Mohsen,
Carl, Dennis, Elena, Miguel, Aziza, Reyhaneh, Mounir, Anderson, Mahdi, Hossein,
Ahmad, Reza Sanatinia, and Reza Nikpars for the nice fika times and small inspiring
chats here and there in the building.
I appreciate Prof. Anders Hallen, my second supervisor, for his support and helps,
Prof. Anand Srinivasan, Prof. Saulius Marcinkevicius, Assoc. Prof. Sergei Popov, and
Assoc. Prof. Henry Radamsson for good advice. My deepest gratitude to Prof. Carl-
Mikael Zetterling, and Prof. Ana Rusu for helping me through the difficulties of PhD
Sincere thanks to Prof. Minuru Fujii for the inspiring discussions and our great
running session in Tossa. Thanks to Prof. Jan Valenta, for his valuable discussions.
I would like to thank my friends and classmates in short period of being part of the
Nanotechnology master program at KTH. Shabnam, Pooria, Sam and Arash for
boosting my days with talks and laughters. I would like to thank Shabnam M. and
Shabnam J. who tolerated (and survived?) all the crazy projects and adventures in my
path. Your friendship is invaluable! Many thanks, to Anders, for being a great and
Last but certainly not least, my family, especially my parents, for their endless love
and support, for their big hearts and always being there for me. Thanks to my siblings
Ali, Mahshid, Mojgan, Reza for their constant encouragement and support to follow
my dreams. So many thanks to Kamyar, Kiana, Aida and Mohammad, Ali N and
Hossein. I love you all and without you I was not who I’m and where I’m today.
Ayyub, Negar, Pedi, Saeid, Madeleine, Noella and Kaspian thanks for all your
support, especially at first difficult months in Sweden, without you life would be
hundred times tougher.
Chances are that I did not mention everybody. Thank you all for contributing to my
work and life in different ways during the past years and hopefully during the future.
Mahtab (Fatemeh) Sangghaleh
Stockholm, September 2015
EBL Electron beam lithography
EQE External quantum efficiency
FWHM Full width at half maximum
HF Hydrofluoric acid
IQE Internal quantum ef