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Switchable Resonant Hyperpolarization Transfer from Phosphorus-31 Nuclei to Silicon-29 Nuclei in Natural Silicon

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Switchable Resonant Hyperpolarization Transfer from Phosphorus-31 Nuclei to Silicon-29 Nuclei in Natural Silicon Switchable resonant hyperpolarization transfer from phosphorus-31 nuclei to silicon-29 nuclei in natural silicon by Phillip Dluhy B.Sc., Loyola University Chicago, 2012 Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in the Department of Physics Faculty of Science c Phillip Dluhy 2015 SIMON FRASER UNIVERSITY Summer 2015 All rights reserved. However, in accordance with the Copyright Act of Canada, this work may be reproduced without authorization under the conditions for “Fair Dealing.” Therefore, limited reproduction of this work for the purposes of private study, research, criticism, review and news reporting is likely to be in accordance with the law, particularly if cited appropriately. Approval Name: Phillip Dluhy Degree: Master of Science (Physics) Title: Switchable resonant hyperpolarization transfer from phosphorus-31 nuclei to silicon-29 nuclei in natural silicon Examining Committee: Chair: Dr. Malcolm Kennett Associate Professor Dr. Michael L. W. Thewalt Senior Supervisor Professor Dr. David Broun Supervisor Associate Professor Dr. Simon Watkins Internal Examiner Professor Department of Physics Date Defended: 01 May 2015 ii Abstract Silicon has been the backbone of the microelectronics industry for decades. As spin-based technologies continue their rapid development, silicon is emerging as a material of primary interest for a number of these applications. There are several techniques that currently 1 29 exist for polarizing the spin- 2 Si nuclei, which account for 4.7% of the isotopic makeup of natural silicon (the other two stable isotopes, 28Si and 30Si, have zero nuclear spin). Polar- ized 29Si nuclei may find use in quantum computing (QC) implementations and magnetic resonance (MR) imaging. Both of these applications benefit from the extremely long T1 and 29 29 T2 of the Si nuclear spins. However, the lack of interactions between the Si nuclei and their surroundings that allow for these long relaxation times also means that it is difficult to find a source of spin-polarization that effectively couples to the 29Si spin ensemble. We identify and exploit a field-dependent, frequency-matched resonant transfer process between 31P donor and 29Si nuclear spins in natural silicon to efficiently hyperpolarize the bulk 29Si to over 6%. This all-optical technique requires no microwave irradiation, and the coupling can be switched off to recover the ultra-long nuclear spin relaxation lifetimes of 29Si. This switchable hyperpolarization technique significantly enhances the usefulness of 29Si spins in QC and MR imaging applications. Keywords: Silicon; silicon-29; phosphorus-31 donors; hyperpolarization; donor bound ex- citons; magnetic resonance detection iii Acknowledgements I have many, many people to thank for their expertise, advise, friendship, and emotional support throughout this degree. First of all, I must express my deep gratitude to my senior supervisor, Mike Thewalt, for supporting me financially and contributing so much of his knowledge and prowess to this project. His experimental intuition and knowledge of the physical systems under question are extraordinary and give me something to strive towards. It is also my pleasure to acknowledge Stephanie Simmons and to give her credit for the genesis of this idea and the mentorship she has provided me from abroad as we carried out the experiments here at SFU. Her enthusiasm and drive are unmatched, and when she was able to visit, she never failed to inspire me. Thanks to both of you for mentoring me and helping me learn from my (many) mistakes–this has been an invaluable experience. I was lucky enough to be ‘stuck’ in the grotto with a fantastic and supportive team of scientists – Jeff, Kevin, Kamyar, and Julian – I have had such a wonderful time working with all of you. A special thank you needs to go out to Jeff for all the emotional support he provided, and the huge amount of physics I was able to understand and more truly appreciate through our numerous exhilarating conversations. That spark of understanding keeps me passionate about physics. Finally, I need to thank my Mom, Dad, and brother for always believing in me and for helping shape who I have become throughout my life. I love you all so much. Thank you for pouring everything you have into me. And to everyone else, from my amazing life-long friends and support system in Chicago to all the magical people I have met in the beautiful city of Vancouver, thank you for everything – you know who you are and everything you have done. iv Table of Contents Approval ii Abstract iii Acknowledgements iv Table of Contents v List of Figures vii 1 Introduction 1 1.1 29Si Nuclear Polarization . 1 1.2 Applications of 29Si Polarization Methods . 2 1.2.1 Quantum Computation . 2 1.2.2 Direct MRI of Micro- and Nanoparticles . 4 1.2.3 29Si Hyperpolarization Via Switchable Dipole-Dipole Coupling . 5 2 Theoretical Framework 6 2.1 The Silicon Crystal . 6 2.2 Phosphorus Donor Impurities . 8 2.2.1 Donor Bound Excitons . 10 2.2.2 Relaxation Processes in 31P....................... 14 2.3 Spin Polarization . 15 2.3.1 Spin Temperature . 15 2.3.2 Nuclear Hyperpolarization Via the Overhauser Effect . 16 2.4 The 29Si Isotope . 18 2.4.1 Spin Diffusion in 29Si........................... 19 3 Experimental Method for 29Si Hyperpolarization 21 3.1 31P and 29Si Nuclear Larmor Frequencies . 21 3.2 Nuclear Magnetic Resonance Detection Methods . 23 3.2.1 NMR in the Rotating Reference Frame . 23 3.2.2 Experimental Pulsed NMR Detection . 25 v 3.3 31P Hyperpolarization . 26 3.4 Experimental Realization . 28 3.4.1 Cryostat and Magnet . 30 3.4.2 Optical Excitation . 30 3.4.3 NMR Probe . 31 3.4.4 NMR Detection . 31 3.4.5 Photoconductive Spectrum . 34 3.4.6 Rabi Cycle . 36 3.4.7 The Experimental Parameters . 37 4 Results and Discussion 39 4.1 Resonant Spin Transfer . 39 4.1.1 Temperature Dependence . 40 4.1.2 Width and Shape . 41 4.2 Variation of Optical Excitation Conditions . 42 4.2.1 Negative Spin Temperature . 43 4.2.2 Hot Spin Temperature . 43 4.2.3 Cold Spin Temperature . 44 4.2.4 Insights into the 31P-29Si Polarization Mechanism . 44 4.2.5 Optical Intensity Dependence . 45 4.3 Total 29Si Hyperpolarization . 45 4.3.1 Equilibrium Polarization . 46 4.3.2 29Si Hyperpolarization Build Up . 47 4.4 Improving the 29Si Hyperpolarization . 47 4.4.1 Improving Hyperpolarization Transfer . 48 4.4.2 Increasing the Number of Hyperpolarized 31P . 48 5 Conclusion and Future Directions 50 5.1 Future Directions . 51 5.2 Considerations for Applications in Quantum Computation and Magnetic Res- onance Imaging . 53 5.2.1 Quantum Computation . 53 5.2.2 Magnetic Resonance Imaging . 53 Bibliography 55 vi List of Figures Figure 2.1 The Si unit . 7 Figure 2.2 The silicon band structure along the [001] directions . 8 Figure 2.3 Diagram of the 31P hyperfine energy levels . 11 Figure 2.4 Diagram of the 12 allowed 31P D0X absorption transitions . 12 Figure 2.5 31PD0 spin-lattice relaxation channels . 14 Figure 3.1 29Si and neutral 31P nuclear Larmor frequencies . 22 Figure 3.2 Nutation in the rotating frame . 24 Figure 3.3 NMR FID and FFT at 4.2 K and 1.4 K . 27 Figure 3.4 Diagram illustrating the 31P populations during polarization . 29 Figure 3.5 29Si hyperpolarization and detection set up . 33 Figure 3.6 31P D0X photoconductive spectrum set up . 35 Figure 3.7 Photoconductive spectrum of D0X transitions 1-6 . 36 Figure 3.8 Rabi experiment illustration . 37 Figure 3.9 Rabi cycle data . 38 Figure 4.1 Resonant transfer data . 40 Figure 4.2 Anisotropic hyperfine interaction between 31P donor electrons and 29Si.................................... 42 Figure 4.3 Effect of different energies of laser excitation . 43 Figure 4.4 Laser intensity versus polarization . 45 Figure 4.5 Equilibrium polarization build up . 47 Figure 4.6 29Si hyperpolarization dependence on exposure time . 48 Figure 5.1 29Si hyperpolarization using a quantum SWAP operation . 52 vii Chapter 1 Introduction Due to the incredible success of the microelectronics industry, the silicon manufacturing process has become extraordinarily well developed. Because of this, silicon is a quintessen- tial material when considering any future applications of semiconductors in science and technology. The size of transistors in computer processors has steadily decreased and quan- tum effects have become more important in these devices [1]. Related to this, scientists have been considering the possibility of using the quantum properties of silicon, as well as the dopant impurities it may contain, as a primary platform for future technologies [1, 2]. This thesis work focuses on the silicon-29 (29Si) isotope, the only stable isotope of Si that possesses a non-zero nuclear spin. In this work we develop a technique to hyperpolarize 29Si nuclear spins, inducing a nuclear polarization far larger than the polarization achiev- able under easily realized thermal equilibrium conditions. This chapter will give a short background of 29Si hyperpolarization techniques that have already been developed and will discuss the two primary applications this procedure is likely to influence the most. 1.1 29Si Nuclear Polarization Scientists first began working on techniques of nuclear hyperpolarization for applications in high energy particle physics and low energy nuclear physics [3]. These methods became known as dynamic nuclear polarization (DNP) experiments. Even though the nuclear polar- izations required (>20%) [4] for use as polarized targets in high energy physics experiments were never achievable in silicon [3], these techniques were incorporated into studies of the fundamental dynamics of semiconductors. In silicon, DNP has been used to study the inter- action of conduction electrons [5, 6], different species of impurities and lattice defects [7, 8], and static magnetic fields [9] with the host 29Si nuclear spins in the Si lattice.
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