Spin generation and detection in low-dimensional semiconductors
Sammanfattning: 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.
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