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Spin generation and detection in low-dimensional semiconductors
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Semiconductor spintronics and opto-spintronics have intrigued intense attention as they promise great advance of contemporary semiconductor information technology with integrated spin functionalities. Over the last few decades, the development of growth techniques and discovery of topological band structures have led to the explosion of a wide range of low-dimensional semiconductor materials, many of which have superior properties compared to their bulk ancestors. The limited dimension of materials imposes constraint on the motion of charge carriers and causes spin interactions of various forms, which have profound influence on the spin properties that are important for various spintronic and/or opto-spintronic applications.

In this context, semiconductor quantum dot (QD) structures (QDS) and 3D topological insulator (TI) have emerged as promising material systems that exhibit distinct spin properties: In QDS, carriers are restricted in all three dimensions. The 3D confinement quenches the spin-orbit interaction (SOI) mediated spin depolarization/dephasing processes and, as a consequence, leads to a prolonged spin relaxation time, which can be used for non-volatile storage or quantum bits in quantum information technology; Whereas, the surface state of 3D TI, on the contrary, has the electronic structure that is dominated by SOI such that the orientation of the electron spin is tied to its momentum. The strong SOI limits the spin relaxation time but can be utilized to generate spin polarized current that is free from backscattering. This thesis work focuses on these two prototypical materials to provide an in-depth understanding of the spin phenomena as well as to tailor their spin properties such that novel spintronic and/or opto-spintronic devices can be built on.

To employ QDS for storage of spin information, first and foremost is to be able to generate and detect spin polarization effectively and efficiently. For this purpose, we have carefully inspected both the spin injection and spin detection processes in various QDS. In this thesis work, spin polarized carriers or excitons are generated via optical orientation that converts the angular momentum of the absorbed photons to the photo-generated carriers or excitons. The as-generated spin polarized carriers/excitons then need to relax their energy before getting injected to the QDS. We have found that the spin injection process is influenced by the interactions with phonons (Paper 1) and disordered environment associated with the injection path (Paper 2). In the former case, we show that the longitudinal optical (LO) phonon contributes to accelerated relaxation of the carrier/exciton energy to the QDS ground state, which preserves the spin polarization. By engineering the energy of the QDS, we can take advantage of such LO-phonon assisted process and can avoid the spin injection loss due to the commonly observed phonon bottleneck effect. In the latter case, we discover that the surrounding media of the QDS is generally disordered, distributed by potential fluctuations caused by alloying or strain randomness. Exciton injection via such localized potential undergoes spin relaxation caused by an anisotropic exchange interaction (AEI), which leads to appreciable spin injection loss at low temperatures.

The AEI is also found to be responsible for the low spin detection efficiency observed in the undoped QDS reported in Paper 3. The AEI causes mixing and splitting of exciton spin states, which leads to not only a low PL polarization degree of the QDS but also a serious issue in generation of entangled photon pairs utilizing QDS. We show that the aforementioned spin injection (Paper 2) and spin detection (Paper 3) loss associated with the AEI can be effectively tuned in the QDS by the arrangement of the constituting QDs. The effect originates from the modification of the strain and shape anisotropy both inside and outside the QDS due to the collective interaction with the neighboring QDs, which introduces a new degree of freedom in electronic-structure engineering of the QDS.

In the doped QDS, we have found that the spin detection efficiency can additionally be affected by the exciton charge states and a hyperfine interaction (HFI) with the nuclear-spin bath. In Paper 4, we discover a dynamic charging process that the charged states of an InGaAs QD ensemble are altered with different excitation power densities and excitation photon energies. The charging effect leads to an anomalous spectral dependence of PL polarization such that the copolarized emission can be dynamically converted to the counter-polarized one. This finding thus calls for caution in the correlation between the optical and spin polarization in QDS with a complex charging environment. The effect of the HFI depends on the condition of nuclear spin polarization. In QDS with an unpolarized nuclear-spin bath, the HFI is a primary electron/exciton spin depolarization/dephasing source in QDS at low temperatures. In Paper 5, we show that the ensemble spin dephasing time of QDs at a cryogenic temperature correlates with the averaged size of QDs. The behavior can be accounted for by electron spin dephasing in a fluctuating nuclear field, which is experimentally verified for the first time. The results thus highlight the important role of the HFI in the electron spin dephasing in the QDs. On the other hand, finite nuclear spin polarization can be achieved through the dynamic nuclear polarization (DNP) process that transfers the angular momentum from the spin-polarized electron to nuclei. DNP is recognized to be important for spintronics and quantum information in nuclear spin-rich nanostructures. This is not only because of its role in suppressing the aforementioned electron spin dephasing, but also because it is behind the idea of exploring the long coherent nuclear spin as a quantum computing qubit. In Paper 6, we have investigated the effect of DNP in a series of QDS, where the strength and orientation of the nuclear field resulted from the DNP are identified and measured. We find that the DNP is built along a tilted axis that deviates from the commonly observed orientation along the QD growth axis and the nuclear field develops a substantial transverse component. This anomalous behavior of the DNP is found to arise from the nuclear quadrupole interaction with an oblique principal axis. The resulting tilting nuclear field can further lead to dephasing and depolarization of the electron spin that has previously been overlooked. The results uncover the detrimental effect rooted in the complex electrostatic environment of the nuclei inside the QDS and call for special care of the strain and alloying engineering of the nanostructures.

In the case of 3D TI, we aim at providing both the experimental and theoretical understanding of the surface spin photocurrent as well as innovations in future opto-spintronic applications utilizing the semiconductor-TI interface. As has been shown earlier, the circular polarized excitation light creates a spin photocurrent that is resistant to moderate scattering. In Paper 7, we present detailed studies of the dependence of the spin photocurrent on the incident angle of the excitation light in a prototypical 3D TI, Bi2Te3. We point out that the spin photocurrent, as a result of spin-selective optical transitions, is associated with both the in-plane and out-of-plane spin texture of the topological surface states. We focus on the contribution of the out-of-plane spin texture, which is less explored, and demonstrate, for the first time, spin injection from a conventional semiconductor, GaAs, to a 3D TI. In favor of this hybrid system, we show that the spin photocurrent contributed by the spin injection exceeds that from the TI alone and the magnitude and direction of the current can be controlled by applying a transverse magnetic field. In Paper 8, we give a tight-binding description of the microscopic origin of the spin photocurrent in Bi2Te3, where we have provided theoretical calculations of the spin photocurrent as a function of the excitation incidence angle, Fermi energy and different scattering potentials. The results explain the observation of the out-of-plane spin texture contribution reported in Paper 7, which should have been forbidden by symmetry, and provide a pathway for opto-spintronic applications based on a TI-semiconductor hybrid system.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. , p. 58
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1946
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-150938ISBN: 9789176852576 (print)OAI: oai:DiVA.org:liu-150938DiVA, id: diva2:1245675
Public defence
2018-09-28, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2018-09-06 Created: 2018-09-05 Last updated: 2018-09-06Bibliographically approved
List of papers
1. Effect of a Phonon Bottleneck on Exciton and Spin Generation in Self-Assembled In1-xGaxAs Quantum Dots
Open this publication in new window or tab >>Effect of a Phonon Bottleneck on Exciton and Spin Generation in Self-Assembled In1-xGaxAs Quantum Dots
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2018 (English)In: Physical Review Applied, ISSN 2331-7019, Vol. 9, no 4, article id 044037Article in journal (Refereed) Published
Abstract [en]

We provide direct experimental evidence for the effect of a phonon bottleneck on exciton and spin generation in self-assembled In0.5Ga0.5As quantum dots (QDs). With the aid of tunable laser spectroscopy, we resolve and identify efficient exciton generation channels in the QDs mediated by longitudinal-optical (LO) phonons from an otherwise inhomogeneously broadened QD emission background that suffers from the phonon bottleneck effect in exciton generation. Spin-generation efficiency is found to be enhanced under the LO-assisted excitation condition due to suppressed spin relaxation accompanying accelerated exciton generation. These findings underline the importance of fine-tuning QD energy levels that will benefit potential spin-optoelectronic applications of QDs by reducing spin loss due to the phonon bottleneck.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-147924 (URN)10.1103/PhysRevApplied.9.044037 (DOI)000430911800002 ()
Note

Funding Agencies|Linkoping University; Swedish Research Council [621-2011-4254, 2016-05091]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish Foundation for International Cooperation in Research and Higher Education (STINT) [JA2014-5698]; Japan Society for the Promotion of Science [16H06359]; Bilateral Joint Research Project

Available from: 2018-05-23 Created: 2018-05-23 Last updated: 2018-09-05
2. Understanding and optimizing spin injection in self-assembled InAs/GaAs quantum-dot molecular structures
Open this publication in new window or tab >>Understanding and optimizing spin injection in self-assembled InAs/GaAs quantum-dot molecular structures
2016 (English)In: Nano Reseach, ISSN 1998-0124, E-ISSN 1998-0000, Vol. 9, no 3, p. 602-611Article in journal (Refereed) Published
Abstract [en]

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to strong quenching of spin relaxation processes promoted by carrier and excitons motions. Unfortunately, spin injection efficiency in such nanostructures remains very low and the exact physical mechanism for the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and quantum-dot molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer immediately surrounding QDs and QMSs that in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We identify the microscopic origin of the observed severe spin loss during spin injection as being due to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We find that the AEI of the injected excitons and thus the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs, as the latter largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures, which in turn provides a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of the QMSs thereby overcoming a major bottleneck in spintronic device applications utilizing semiconductor QDs.

Place, publisher, year, edition, pages
Tsinghua University Press, 2016
Keywords
Spin injection, spin loss, quantum dot, quantum - dot molecular structure, InAs/GaAs, exciton, anisotropic exchange interaction, polarization
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-123983 (URN)10.1007/s12274-015-0940-6 (DOI)000371797000002 ()
Available from: 2016-01-15 Created: 2016-01-15 Last updated: 2018-09-05Bibliographically approved
3. Exciton Fine-Structure Splitting in Self-Assembled Lateral InAs/GaAs Quantum-Dot Molecular Structures
Open this publication in new window or tab >>Exciton Fine-Structure Splitting in Self-Assembled Lateral InAs/GaAs Quantum-Dot Molecular Structures
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2015 (English)In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 9, no 6, p. 5741-5749Article in journal (Refereed) Published
Abstract [en]

Fine-structure splitting (FSS) of excitons in semiconductor nanostructures is a key parameter that has significant implications in photon entanglement and polarization conversion between electron spins and photons, relevant to quantum information technology and spintronics. Here, we investigate exciton FSS in self-organized lateral InAs/GaAs quantum-dot molecular structures (QMSs) including laterally aligned double quantum dots (DQDs), quantum-dot clusters (QCs), and quantum rings (QRs), by employing polarization-resolved microphotoluminescence (μPL) spectroscopy. We find a clear trend in FSS between the studied QMSs depending on their geometric arrangements, from a large FSS in the DQDs to a smaller FSS in the QCs and QRs. This trend is accompanied by a corresponding difference in the optical polarization directions of the excitons between these QMSs, namely, the bright-exciton lines are linearly polarized preferably along or perpendicular to the [11̅0] crystallographic axis in the DQDs that also defines the alignment direction of the two constituting QDs, whereas in the QCs and QRs, the polarization directions are randomly oriented. We attribute the observed trend in the FSS to a significant reduction of the asymmetry in the lateral confinement potential of the excitons in the QRs and QCs as compared with the DQDs, as a result of a compensation between the effects of lateral shape anisotropy and piezoelectric field. Our work demonstrates that FSS strongly depends on the geometric arrangements of the QMSs, which effectively tune the degree of the compensation effects and are capable of reducing FSS even in a strained QD system to a limit similar to strain-free QDs. This approach provides a pathway in obtaining high-symmetry quantum emitters desirable for realizing photon entanglement and spintronic devices based on such nanostructures, utilizing an uninterrupted epitaxial growth procedure without special requirements for lattice-matched materials combinations, specific substrate orientations, and nanolithography.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2015
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-118007 (URN)10.1021/acsnano.5b01387 (DOI)000356988500013 ()25965972 (PubMedID)
Available from: 2015-05-20 Created: 2015-05-20 Last updated: 2018-09-05
4. Anomalous spectral dependence of optical polarization and its impact on spin detection in InGaAs/GaAs quantum dots
Open this publication in new window or tab >>Anomalous spectral dependence of optical polarization and its impact on spin detection in InGaAs/GaAs quantum dots
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2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 13, p. 132106-Article in journal (Refereed) Published
Abstract [en]

We show that circularly polarized emission light from InGaAs/GaAs quantum dot (QD) ensembles under optical spin injection from an adjacent GaAs layer can switch its helicity depending on emission wavelengths and optical excitation density. We attribute this anomalous behavior to simultaneous contributions from both positive and negative trions and a lower number of photo-excited holes than electrons being injected into the QDs due to trapping of holes at ionized acceptors and a lower hole mobility. Our results call for caution in reading out electron spin polarization by optical polarization of the QD ensembles and also provide a guideline in improving efficiency of spin light emitting devices that utilize QDs.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-112185 (URN)10.1063/1.4897306 (DOI)000343031700033 ()
Note

Funding Agencies|Linkoping University; Swedish Research Council [621-2011-4254]; Japan Society for Promotion of Science [22221007]

Available from: 2014-11-18 Created: 2014-11-18 Last updated: 2018-09-05
5. Size dependence of electron spin dephasing in InGaAs quantum dots
Open this publication in new window or tab >>Size dependence of electron spin dephasing in InGaAs quantum dots
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2015 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 106, no 9, p. 093109-Article in journal (Refereed) Published
Abstract [en]

We investigate ensemble electron spin dephasing in self-assembled InGaAs/GaAs quantum dots (QDs) of different lateral sizes by employing optical Hanle measurements. Using low excitation power, we are able to obtain a spin dephasing time T-2* (in the order of ns) of the resident electron after recombination of negative trions in the QDs. We show that T-2* is determined by the hyperfine field arising from the frozen fluctuation of nuclear spins, which scales with the size of QDs following the Merkulov-Efros-Rosen model. This scaling no longer holds in large QDs, most likely due to a breakdown in the lateral electron confinement. (C) 2015 AIP Publishing LLC.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-117236 (URN)10.1063/1.4914084 (DOI)000351069900054 ()
Note

Funding Agencies|Linkoping University through the Professor Contracts; Swedish Research Council [621-2011-4254, 2008-6582]; Japan Society for Promotion of Science [22221007]

Available from: 2015-04-22 Created: 2015-04-21 Last updated: 2018-09-05
6. Spin injection and helicity control of surface spin photocurrent in a three dimensional topological insulator
Open this publication in new window or tab >>Spin injection and helicity control of surface spin photocurrent in a three dimensional topological insulator
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2017 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 8, article id 15401Article in journal (Refereed) Published
Abstract [en]

A three-dimensional (3D) topological insulator (TI) is a unique quantum phase of matter with exotic physical properties and promising spintronic applications. However, surface spin current in a common 3D TI remains difficult to control and the out-of-plane spin texture is largely unexplored. Here, by means of surface spin photocurrent in Bi2Te3 TI devices driven by circular polarized light, we identify the subtle effect of the spin texture of the topological surface state including the hexagonal warping term on the surface current. By exploring the out-of-plane spin texture, we demonstrate spin injection from GaAs to TI and its significant contribution to the surface current, which can be manipulated by an external magnetic field. These discoveries pave the way to not only intriguing new physics but also enriched spin functionalities by integrating TI with conventional semiconductors, such that spin-enabled optoelectronic devices may be fabricated in such hybrid structures.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-138241 (URN)10.1038/ncomms15401 (DOI)000401908900001 ()28530227 (PubMedID)
Note

Funding Agencies|Linkoping University; Swedish Research Council [621-2011-4254, 2016-05091]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; Swedish Foundation for Strategic Research [EM11-0002]; Key Program of Natural Science Foundation of China [61334004]; Creative Research Group Project of Natural Science Foundation of China [61321492]; Natural Science Foundation of China [61404153]; Shanghai Pujiang Program [14PJ1410600]

Available from: 2017-06-14 Created: 2017-06-14 Last updated: 2018-09-05

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