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Carrier Dynamics in Single Luminescent Silicon Quantum Dots Carrier Dynamics in Single Luminescent Silicon Quantum dots Fatemeh Sangghaleh PhD Thesis Information and Communication Technology KTH Royal Institute of Technology Stockholm, Sweden 2015 TRITA-ICT 2015:09 KTH School of Information and Communication Technology SE-164 40, ISBN 978-91-7595-665-7 Kista, SWEDEN 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 Abstract 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. iv Acknowledgement 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 and stimulating. 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 studies. 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 considerate friend. 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 vi 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 vii viii Abbreviations EBL Electron beam lithography EQE External quantum efficiency FWHM Full width at half maximum HF Hydrofluoric acid IQE Internal quantum efficiency NC Nanocrystal NW Nanowire PL Photoluminescence QD Quantum dot QE Quantum efficiency QY Quantum yield RIE Reactive ion etching RT Room temperature SEM Scanning electron microscope Si Silicon TEM Transmission electron microscope ix Contents Abstract ............................................................................................................ iii Acknowledgement
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