Optical and Electrical Properties of Single Self-Assembled Quantum Dots in Lateral Electric Fields

Diplomarbeit von Malte Huck

Abstract:

Chapter 1:

In this thesis we investigate the optical properties of self-assembled quantum dots exposed to a lateral electric field. As a result of the electric field the wave functions of electrons and holes inside the quantum dot are manipulated, which makes it possible to tune their energy levels and control the optical properties of the system. The possibility of tuning the emission energy of different few particle states using this method makes this system very promising for the use of a source of polarization entangled photons as discussed in the following sections.

In Section 1.1 the concept of entangled states is introduced together with a brief historical overview. The possibility of using the exciton–biexciton cascade of a self-assembled quantum dot for the generation of entangled photon pairs is presented in Section 1.2.

Chapter 2:

In this chapter we introduce the concept of quantum dots and demonstrate their optical emission properties. In Section 2.1 the quantum dot is introduced as a three-dimensional charge carrier trap. Several types of quantum dots are presented in an overview.

In Section 2.2 we discuss the physical effects that occur on the way from bulk semiconductor material to the three-dimensional charge carrier confinement in the case of quantum dots. The growth of self-assembled quantum dot samples is the topic of Section 2.3, where the technique of molecular beam epitaxy is introduced (Section 2.3.1). This technique is used to grow the semiconductor quantum dots via heteroepitaxy in the Stranski-Krastanov growth mode (Section 2.3.2).

Quantum dots are commonly referred to as artificial atoms due to their atomlike emission features. The origin for this expression is explained in Section 2.4 on the basis of the energetic structure of self-assembled quantum dots.

The optical properties of quantum dots are discussed in Section 2.5, beginning with an introduction to the experimental setup that has been used to investigate the quantum dots during this thesis (Section 2.5.1). Different optical excitation modes are presented in Section 2.5.2 and in Section 2.5.3 we discuss, how to achieve a low enough quantum dot density on the analyzed samples.

Section 2.5.4 deals with the photoluminescence of different exciton states and in Section 2.5.5 we present how these lines can be identified via power dependent measurements. Finally, the concept of initial charges in self-assembled quantum dots is presented in Section 2.5.6.

Chapter 3:

In order to investigate quantum dot emission under the influence of a lateral electric field, it is essential to electrically contact the sample. The fabrication of the electrical contacts and the characterization of the resulting back-to-back Schottky diode is the topic of this chapter.

In Section 3.1 the fabrication of the back-to-back Schottky diode is discussed, which allows the application of lateral electric fields of up to F _ 80 kV/cm. In Section 3.2 electric field simulations are presented. These calculations provide us with the necessary information of the orientation and the quantitative value of the electric field F in the quantum dot layer.

Characterization measurements of the photocurrent under various conditions are discussed in Section 3.3, which is composed of several subsections. Section 3.3.1 introduces the different processes that affect the behavior of charge carriers in the quantum dot sample under the influence of a lateral electric field. Experimental evidence of these effects is given in the following sections, where photocurrent measurements are presented. Section 3.3.2 discusses the temperature dependence of the photocurrent, while in Section 3.3.3 the excitation energy dependence is discussed. The influence of the excitation power on the photocurrent is discussed in Setion 3.3.4. Finally, in Section 3.3.5 the photoluminescence intensity of a quantum dot ensemble is compared with the corresponding photocurrent.

Chapter 4:

In this chapter we present the results of optical measurements performed on single self-assembled quantum dots subject to a lateral electric field. We begin by presenting a brief introduction to the most important experimental concepts in Section 4.1. In Section 4.2 the quantum confined Stark effect is introduced.

This phenomenon is responsible for the shift of the quantum dot emission lines under the electric field. A simple mathematical model is used to calculate the shift of the single exciton line. For the more complicated excitonic configurations calculations were performed by Dr. Filippo Troiani, showing the effect of the quantum confined Stark effect on several few particle states in the quantum dot.

An overview of lateral electric field dependent photoluminescence measurements is given in Section 4.3. Here, we introduce a model where the quantum dots are initially charged to explain observations in the low field regime. The Stark shifts of single emission lines are presented in Section 4.4 and attributed to biexciton and exciton emission. Comparison with the simulations presented in Section 4.2 shows a very good accordance between calculated and measured data. In Section 4.5 we discuss a screening mechanism that occurs due to charge accumulation underneath the diode contacts and influences the electric field dependent behavior of the emission lines. In Section 4.6 several shifting exciton–biexciton line pairs are presented. In order to obtain the polarizability of the electron–hole pair in the quantum dot, the exciton feature is fitted with a parabolic function. The resulting values are found to be in very good accordance with polarizabilities measured in previous works.

Table of Contents:

1.	Motivation: Entangled Photons	1
1.1	Entangled States	2
1.2	The Quantum Dot 2X–1X Cascade as Entangled Photon Source	4
1.3	Summary and Conclusions	7
2.	Semiconductor Quantum Dots: Artificial Atoms	9
2.1	Motivation	11
2.2	Low-Dimensional Semiconductor Nanostructures	14
2.3	MBE Growth of Self-Assembled InGaAs Quantum Dots	16
2.3.1	Molecular Beam Epitaxy	16
2.3.2	Stranski-Krastanov Growth Mode	18
2.3.3	Sample Design	20
2.4	Energetic Structure of Single Quantum Dots	21
2.5	Optical Properties	24
2.5.1	Micro-Photoluminescence Setup	25
2.5.2	Optical Excitation of Electron–Hole Pairs	28
2.5.3	From High to Low Quantum Dot Density Material	32
2.5.4	Photoluminescence of Few Particle States	35
2.5.5	Line Identification	38
2.5.6	Initially Charged Quantum Dots	40
2.6	Summary and Conclusions	42
3.	Fabrication and Characterization of Lateral Electric Field Devices	43
3.1	Fabrication	45
3.2	Electric Field Simulation	48
3.3	Electrical Characterization and Photocurrent Measurements	50
3.3.1	Electric Field Activated Carrier Loss Mechanisms	51
3.3.2	Temperature Dependence	56
3.3.3	Energy Dependence	61
3.3.4	Power Dependence — Responsivity	64
3.3.5	Field Dependent Photoluminescence Quenching	67
3.4	Summary and Conclusions	70
4.	Independent Control of Few Exciton States in Single Quantum Dots	73
4.1	Motivation	75
4.2	The Quantum Confined Stark Effect	76
4.3	Electric Field Dependent Micro-Photoluminescence — Overview	80
4.4	Stark Shifts of Single Lines	87
4.5	Screening Mechanism	90
4.6	Determination of the Polarizability	92
4.7	Peak Broadening and Lifetime Measurements	95
4.8	Summary and Conclusions	100
5.	Outlook	103

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