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
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
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
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
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
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
Objectives and Scope
This thesis investigates the electronic and spin properties of semiconductor nanostructures—specifically quantum wells and quantum dots—using the Optically Detected Resonance (ODR) technique under far-infrared (FIR) radiation and high magnetic fields. The primary goal is to probe internal energy transitions, charge carrier dynamics, and spin configurations in both nonmagnetic and diluted magnetic semiconductor (DMS) systems to understand the interplay between carrier spin, magnetic ions, and external fields.
- Analysis of optically detected cyclotron resonance in nonmagnetic quantum wells and dots.
- Investigation of spin and energy structures of charged excitons (trions) in high magnetic fields.
- Study of many-body effects, such as shake-up processes in high-density electron gases.
- Examination of magnetic ion spin systems in DMS nanostructures and their interaction with FIR radiation.
- Optimization of experimental setups for high-resolution optical spectroscopy at low temperatures.
Excerpt from the Book
1.1.1 Motivation
Science related to nanostructures has become famous at the beginning of this decade. Today the word ’Nanotechnology‘ appears almost daily, even in newspapers and magazines not related to science. It is believed that nanotechnology is able to solve problems in many fields, from material science to biology and medicine. It is rated as technology of the future but it is widely unknown that nanostructures have been present in nature for very long time. The most common example is the ability of the Lotus (Nelumbo) plant to keep their leafs clean from dust. It has been found out, that the surface is not perfectly even as one might expect but shows a microscopic structure so that only 2-3% of the surface is thought by water droplets [Sol07]. Dust on the surface sticks to water and is washed of the leaf surface.
Another example are fullerene, carbon molecules consisting of 60 carbon atoms looking like a football (Fig. 1.1) which have been discovered in 1985 [Kro85]. When people tried to produce these molecules they discovered that it can also be found in the smoke of a candle.
Summary of Chapters
1 Introduction: Provides the scientific context of nanostructures, discusses the shift from bulk materials to low-dimensional systems, and outlines the theoretical background of excitons and magnetic field effects.
2 Experiment and technique: Describes the cryogenic experimental apparatus, the FIR laser system, and the custom LabView software developed to automate measurement series and data analysis.
3 PL and ODR study on nonmagnetic quantum wells: Focuses on experimental results for nonmagnetic samples, including cyclotron resonance and the spin-energy structure of charged excitons (trions) under high magnetic fields.
4 ODR Study on nonmagnetic quantum dots: Presents findings on self-assembled quantum dots, examining the influence of FIR radiation on transitions within confined electron shells.
5 PL and ODR study on magnetic quantum wells: Investigates diluted magnetic semiconductor (DMS) structures, focusing on the interplay between carrier spin and Mn ion magnetization.
6 ODR study on magnetic quantum dots: Explores magnetic quantum dots, focusing on the competition between intrinsic and giant Zeeman splitting and the effects of FIR-induced heating on the Mn spin system.
Keywords
Optically Detected Resonance, ODR, Quantum Wells, Quantum Dots, Photoluminescence, Zeeman Splitting, Cyclotron Resonance, Diluted Magnetic Semiconductors, Trions, Excitons, Nanostructures, Landau Levels, FIR radiation, Spin Physics, Semiconductor Physics
Frequently Asked Questions
What is the core focus of this research?
The research focuses on investigating the fundamental physical properties of semiconductor nanostructures using the Optically Detected Resonance (ODR) technique.
What types of nanostructures are examined?
The study examines nonmagnetic and magnetic (diluted magnetic) semiconductor quantum wells and self-assembled quantum dots.
What is the primary objective of the work?
The primary objective is to analyze internal energy transitions, charge carrier dynamics, and spin configurations in these low-dimensional systems under extreme conditions like high magnetic fields.
Which methodology is employed?
The work utilizes photoluminescence spectroscopy combined with far-infrared (FIR) radiation to induce and detect transitions, supported by automated LabView software for complex data collection.
What does the main body of the work cover?
The main body covers theoretical backgrounds, experimental hardware setups, and detailed analysis of findings in both nonmagnetic and magnetic quantum systems.
Which keywords characterize this work?
Key terms include ODR, quantum wells, quantum dots, photoluminescence, Zeeman splitting, and diluted magnetic semiconductors.
How does the FIR radiation influence the samples in this study?
FIR radiation probes internal transitions by heating the carrier system or resonant coupling to Landau levels, allowing the identification of specific energetic states through changes in the photoluminescence spectrum.
What is the significance of the "shake-up" processes discussed?
Shake-up processes represent many-body interactions where carrier recombination is coupled to the excitation of other electrons to higher Landau levels, providing insight into the electronic structure of highly doped quantum wells.
- Arbeit zitieren
- Michael Gerbracht (Autor:in), 2008, Optically detected resonances induced by far infrared radiation in quantum wells and quantum dots, München, GRIN Verlag, https://www.grin.com/document/93673
