Abstract
Photoluminescence (PL) and optically detected resonances (ODR) where studied on semiconductor quantum wells and quantum dots. Magnetic fields of up to 33 T where applied to samples at temperatures between 0.25 K and 10 K.
In nonmagnetic quantum wells optically detected cyclotron resonance was used to determine basic properties such as effective mass and mobility of GaAs/AlGaAs quantum wells. In CdTe/CdMgTe quantum wells evidence for the singlet and triplet state of the negatively and positively charged exciton was found at high magnetic fields. In a highly n-type doped GaAs/AlGaAs quantum well, signatures of the fractional quantum hall effect were observed in PL and ODR data. Also shake up processes in a variety of quantum wells are discussed.
In magnetic quantum wells, cusps in the exciton shift are present at moderate magnetic fields which could be assigned to next nearest neighbor interactions between Mn2+ ion pairs and single ions. Resonances in InGaAs/GaAs quantum dots induced by far-infrared radiation have been observed optically. They were studied in quantum dots with different confinement potential and under a series of tilting angles between sample normal and magnetic field direction. The resonances could be assigned to trion formation due to cyclotron resonance in the wetting layer and transitions in the internal energy structure of the dots.
Also magnetic CdMnTe/ZnCdTe quantum dots with different Mn content were measured at magnetic fields up to 17 T. At low Mn concentrations a competition between the giant and intrinsic Zeeman splitting leads to a reduction of the polarization of the sample at high magnetic field which makes it possible to determine the Mn content by photoluminescence measurements.
Table of Contents
- 1 Introduction
- 1.1 Motivation
- 1.1.1 Motivation
- 1.1.2 Outline
- 1.1.3 Technical notes
- 1.2 From atoms to solid states
- 1.2.1 Hydrogen atom
- 1.2.2 Pauli exclusion principle
- 1.2.3 Molecules
- 1.2.4 Solid states
- 1.3 From bulk semiconductors to nanostructures
- 1.3.1 Bulk semiconductors
- 1.3.2 Quantum wells
- 1.3.3 Quantum wires
- 1.3.4 Quantum dots
- 1.4 Growth of nanostructures
- 1.4.1 Quantum wells
- 1.4.2 Self assembled quantum dots
- 1.5 Optical properties
- 1.5.1 Exciton and trions
- 1.6 Effects of the magnetic field
- 1.6.1 Diamagnetic shift
- 1.6.2 Landau levels
- 1.6.3 Cyclotron resonance
- 1.6.4 Zeeman splitting
- 1.6.5 Energy and spin structure of excitons and trions
- 1.6.6 Fock-Darwin spectrum
- 1.1 Motivation
- 2 Experiment and technique
- 2.1 Experimental setup
- 2.1.1 Cryostat and inserts
- 2.1.2 Lasers
- 2.1.3 Far infrared laser
- 2.1.4 Monochromator and CCD
- 2.2 LabView software
- 2.2.1 Main program
- 2.2.2 Devices
- 2.2.3 Measurement types
- 2.2.4 Measurement series
- 2.3 Optically detected resonance technique
- 2.3.1 Effect of the FIR radiation
- 2.3.2 Historic overview
- 2.4 Analysing software
- 2.4.1 False color maps
- 2.4.2 PL and modulation spectrum
- 2.4.3 Modulation signal
- 2.4.4 Fitting peak positions
- 2.5 Experimental dependencies
- 2.5.1 Exposure time
- 2.5.2 Repetition rates
- 2.5.3 Temperature dependence
- 2.5.4 Dependence on excitation power and wavelength
- 2.5.5 Dependence on FIR power and energy
- 2.5.6 Photomultiplier tube and FIR stability
- 2.1 Experimental setup
- 3 PL and ODR study on nonmagnetic quantum wells
- 3.1 ODR in nonmagnetic quantum wells
- 3.1.1 Optically detected cyclotron resonance
- 3.1.2 Quantum well with optically tuneable carrier type
- 3.2 Photoluminescence in high magnetic fields
- 3.2.1 Spin and energy structure of positively and negatively charged excitons in CdTe/CdMgTe quantum wells
- Introduction
- Experimental
- Photoluminescence and reflection spectra
- Excitons
- Schematics for trion energy and spin structure in magnetic field
- Identification of charged excitons in the p- and n-type regime
- Binding Energy
- Negatively charged triplet trion
- Positively charged triplet trion
- Polarization degree of trion emission
- Conclusion
- 3.2.1 Spin and energy structure of positively and negatively charged excitons in CdTe/CdMgTe quantum wells
- 3.3 High density 2DEG
- 3.3.1 Quantum hall regime in a modulation n-type doped AlGaAs/AlAs quantum well with high electron density
- 3.4 Shake-up processes
- 3.4.1 Shake-up in a high dense 2DEG
- 3.4.2 Shake-up in a low dense 2DEG
- 3.4.3 Conclusion
- 3.1 ODR in nonmagnetic quantum wells
- 4 ODR Study on nonmagnetic quantum dots
- 4.1 Experimental results
- 4.2 Conclusion and outlook
- 5 PL and ODR study on magnetic quantum wells
- 5.1 Basic properties of diluted magnetic semiconductors
- 5.1.1 Magnetization in DMS
- 5.1.2 Energy and spin transfer in DMS semiconductors
- 5.2 Intrinsic resonance in spin system of DMS ZnMnSe/ZnBeSe QWs
- 5.3 ODR on DMS quantum wells
- 5.3.1 Cyclotron resonance
- 5.3.2 Nonmonotonic behaviour of the ODR signal
- 5.4 Conclusion
- 5.1 Basic properties of diluted magnetic semiconductors
- 6 ODR study on magnetic quantum dots
- 6.1 Competition between intrinsic and exchange Zeeman splitting
- 6.2 Heating of the Mn spin system by far infrared radiation
- 6.3 Conclusion
- Appendix Optical transitions and spin selection rules for trions in QWs
- A.1 How to handle exciton and trion schematics: simple rules
- A.2 Selection rules for trion singlet states
- A2.1 Heavy-hole trions
- A2.2 Light-hole trions
- A.3 Selection rules for singlet and triplet states of T-
- A.4 Crossing of triplet and singlet states (S-T crossing): Hidden or visible?
- Publications
- Bibliography
- List of samples
- Nonmagnetic samples
- Magnetic samples
- List of Figures
- Index
- Acknowledgements
Objectives & Thematic Focus
This dissertation fundamentally investigates optically detected resonances (ODR) induced by far-infrared (FIR) radiation in both magnetic and nonmagnetic semiconductor quantum wells and quantum dots. The primary objective is to analyze how FIR radiation and magnetic fields influence the optical properties, such as spin structures, energy transfer mechanisms, and various resonances, within these nanostructures.
- Development and application of Optically Detected Resonance (ODR) technique.
- Experimental study of semiconductor nanostructures, specifically quantum wells and quantum dots.
- Analysis of the effects of far-infrared (FIR) radiation on electronic and spin states.
- Investigation of magnetic field phenomena, including Zeeman splitting and cyclotron resonance.
- Characterization of excitons and trions, focusing on their spin and energy structures.
- Research into diluted magnetic semiconductors (DMS) and their unique magneto-optical properties.
Excerpt from the Book
1.1 Motivation
In recent years it became possible to fabricate nanostructures in a controlled manner. Many commercial products that make use of nanostructures in the one or other way are available on the market and the potential of these systems is not slowing down.
Also in the field of chip and computer technology, the shrinking of transistors and other fundamental structures found in every integrated circuit come along with faster processing power. First processors which are grown with structures sizes as small as 45 nm become available now and further minimization will stop when quantum effects come into play at even smaller structures. This will hint the development of faster processors which work in the conventional way. Other techniques such as massive parallel processing need to be used in order to enhance processing power further.
But quantum effects can also be used to create a new kind of computer: The quantum computer. It is not a digital device based on bits, which can be either 0 or 1. In a quantum computer a bit can also represent all superpositions of 0 and 1, so its state can be somewhere in-between 0 and 1. Up to now only the basic principle was demonstrated with seven nuclei in a molecule that could be controlled by nuclear magnetic resonance [Van01]. For reasons of scaleability and incorporation into classical systems, efforts are made to realize a semiconductor quantum computer. A good system for the implementation of quantum dots (QD), zero dimensional structures which are sometimes compared to atoms, a possible candidate for the realization of a qu-bit is the spin in magnetic fields. This can either be the nuclear spins or electron spins, the latter is convenient to have in QDs. Critical for the creation of a semiconductor quantum computer are decoherence and dephasing times, which define how long information can be stored in a quantum bit before it gets lost. In our group it was demonstrated recently that spin states could be preserved for 3ms in a quantum dot ensemble [Gre06]. Due to the interaction of electrons with nuclei in the QD, information could be stored over several hours and read out optically [Gre07].
Besides the storing of information in a QD ensemble, it is also necessary to be able to manipulate the spin states. One possibility is the use of far infrared radiation which may couple to the electron spins and induce transitions between energy levels. While the influence of far infrared radiation was studied on two dimensional systems to some extend, only very few attempts have been made to investigate the impact on quantum dots.
Summary of Chapters
1 Introduction: Provides a comprehensive theoretical background on semiconductor physics, nanostructures, optical properties, and the effects of magnetic fields, essential for understanding the experimental work.
2 Experiment and technique: Details the sophisticated experimental setup and techniques employed, including the cryostat, various laser systems (including FIR), monochromator, CCD detection, and custom LabView software for data acquisition and analysis.
3 PL and ODR study on nonmagnetic quantum wells: Presents the core experimental findings regarding photoluminescence (PL) and optically detected resonance (ODR) in nonmagnetic quantum wells, exploring cyclotron resonance, tuneable carrier types, and shake-up processes under high magnetic fields.
4 ODR Study on nonmagnetic quantum dots: Focuses on the experimental results of ODR studies on self-assembled nonmagnetic quantum dots, discussing various resonances observed and concluding with an outlook on their potential.
5 PL and ODR study on magnetic quantum wells: Investigates the properties of diluted magnetic semiconductor (DMS) quantum wells, covering magnetization, energy and spin transfer, intrinsic resonances, and the nonmonotonic behavior of the ODR signal in these systems.
6 ODR study on magnetic quantum dots: Explores the intricate interplay of intrinsic and exchange Zeeman splitting in magnetic quantum dots, as well as the effects of FIR radiation on the manganese spin system.
Keywords
Optically detected resonance (ODR), Far infrared (FIR) radiation, Quantum wells, Quantum dots, Nanostructures, Semiconductors, Magnetic fields, Excitons, Trions, Spin splitting, Cyclotron resonance, Zeeman effect, Photoluminescence, Diluted magnetic semiconductors (DMS), LabView
Frequently Asked Questions
What is this work fundamentally about?
This work fundamentally explores the optical and spin properties of semiconductor nanostructures, specifically quantum wells and quantum dots, when subjected to far-infrared radiation and strong magnetic fields using optically detected resonance (ODR) techniques.
What are the central thematic areas?
The central thematic areas include nanotechnology, semiconductor physics, quantum mechanics, experimental spectroscopy (photoluminescence, ODR), and the study of spin phenomena in low-dimensional systems, including diluted magnetic semiconductors.
What is the primary objective or research question?
The primary objective is to experimentally investigate and understand the mechanisms behind optically detected resonances induced by far-infrared radiation in quantum wells and quantum dots, particularly focusing on how magnetic fields and material properties influence these resonances and related spin structures.
Which scientific method is used?
The scientific method used is experimental physics, involving the design and implementation of a sophisticated setup for magneto-optical spectroscopy, data acquisition via LabView, and detailed analysis of photoluminescence and modulation spectra to deduce physical properties and phenomena.
What is covered in the main part?
The main part of the thesis covers the experimental studies on nonmagnetic quantum wells and quantum dots (Chapter 3 & 4), and then on magnetic quantum wells and quantum dots (Chapter 5 & 6), detailing the observed ODR signals, their dependencies on experimental parameters, and interpretations of various quantum phenomena like cyclotron resonance, Zeeman splitting, and shake-up processes.
Which keywords characterize the work?
Key terms characterizing this work are Optically detected resonance (ODR), Far infrared (FIR) radiation, Quantum wells, Quantum dots, Nanostructures, Magnetic fields, Excitons, Trions, Spin splitting, Cyclotron resonance, Photoluminescence, and Diluted magnetic semiconductors (DMS).
What specific type of nanostructures are investigated?
The dissertation investigates both nonmagnetic (e.g., GaAs/AlGaAs) and magnetic (e.g., CdTe/CdMgTe, ZnMnSe/ZnBeSe) semiconductor quantum wells and self-assembled quantum dots.
How is far-infrared radiation utilized in the experiments?
Far-infrared (FIR) radiation is utilized as an excitation source to induce transitions between Landau levels or to directly couple to electron spins, allowing for the detection and analysis of specific resonances that are optically detected via changes in photoluminescence.
What are "shake-up processes" and why are they relevant?
Shake-up processes refer to recombination events where an electron-hole pair partially transfers its energy to another electron, leading to characteristic satellite peaks in the photoluminescence spectrum. They are relevant in high-density two-dimensional electron gas (2DEG) systems in quantum wells, providing insights into many-body interactions.
What role do diluted magnetic semiconductors play in this research?
Diluted magnetic semiconductors (DMS) are used to study additional spin-related phenomena due to the presence of magnetic ions (e.g., Mn2+). They allow for the investigation of intrinsic resonance in the spin system, exchange Zeeman splitting, and the heating of the Mn spin system by FIR radiation, offering unique insights into spin manipulation.
- Quote paper
- Michael Gerbracht (Author), 2008, Optically detected resonances induced by far infrared radiation in quantum wells and quantum dots, Munich, GRIN Verlag, https://www.grin.com/document/93673
