In this thesis the InAs(111)B surface and III-V semiconductor nanowires are investigated using scanning tunneling microscopy and spectroscopy. The morphology of InAs nanowires grown without gold particle is studied. Radial nanowire heterostructure such as InP core with InAs shell are analyzed and the wurtzite top facet is identified. Furthermore nanowire heterostructures with an InP core and InAs shell with induced stacking faults possibly giving rise to quantum dots, which could be used as quantum dot lasers or for quantum information processing, are investigated. A model is obtained based on morphology analysis and as top facet the wurtzite and are found. Furthermore stacking faults on top of a nanowire are seen.
The analysis of the InAs(111)B surface shows the hexagonal pattern. Defects are determined to occur due to missing In atoms in the first layer. Spectroscopy next to those defects indicated no influence on the local electronic structure.
Table of Contents
1 Introduction
2 Theory
2.1 Scanning Tunneling Microscopy
2.2 Scanning Tunneling Spectroscopy
2.3 Nanowires
2.4 Quantum dots
3 Experimental background
3.1 Setup
3.2 Sample preparation
3.3 Tip production
3.4 Vacuum conditions
4 Results and Discussion
4.1 Cleaning
4.1.1 Surface
4.1.2 Nanowires
4.2 Imaging
4.2.1 InAs(111)B surface
4.2.2 InAs nanowires grown without gold particle
4.2.3 InP with InAs quantum well nanowires
4.2.4 InP with InAs quantum dots nanowires
4.3 Spectroscopy
4.3.1 InAs(111)B surface
4.3.2 Defect induced influence on spectra
5 Conclusion and Outlook
Objectives and Topics
This thesis aims to investigate the surface morphology and electronic properties of III-V semiconductor nanowires and the InAs(111)B substrate using scanning tunneling microscopy (STM) and spectroscopy (STS). The research focuses on the characterization of heterostructures, specifically evaluating radial heterostructures like InP cores with InAs shells and those containing quantum dots, to understand their potential for future optoelectronic applications.
- Scanning Tunneling Microscopy (STM) principles and tip preparation.
- Surface cleaning procedures for semiconductor nanowires in ultra-high vacuum.
- Morphological analysis of InAs and InP/InAs heterostructure nanowires.
- Investigation of electronic density of states via Scanning Tunneling Spectroscopy.
- Characterization of defects and their influence on surface properties.
Excerpt from the Book
3.3 Tip production
The quality of recorded images depends crucially on the quality of the tip, especially for non-flat surfaces. For the investigation of nanowires and quantum dots on nanowires, the tip should therefore be atomically sharp in order not to get multiple features as depicted in figure 3.3. A good example for tip effects can also be seen in figure 4.7. A tip consisting of several minitips leads to a repetition of the recorded features or objects may occur broader than they are since the tip from which electrons tunnel might change during the scan.
To produce a sharp tip, a tungsten wire of 0,38 mm diameter is chemically etched, using a 8% sodium hydroxide (NaOH) solution as electrolyte while a voltage between the wire and a steel cathode is applied. After etching the tip is cleaned by rinsing with distilled water and isopropanol. The tungsten tips get contaminated during the etching process and a common way to remove those contaminations and to further sharpen the tip is sputtering with Argon ions [15]. Therefore Argon gas is introduced into the preparation chamber via the argon source at energies around 3 kV and gets ionized using 19 mA filament current. The duration of the sputtering process is two times 15 minutes at a pressure of about 2, 5 · 10−6 mbar.
Summary of Chapters
1 Introduction: Provides an overview of the significance of III-V semiconductor nanowires and outlines the experimental scope of the thesis.
2 Theory: Covers the physical principles of STM and STS, as well as the fundamental concepts regarding nanowire growth and quantum dot formation.
3 Experimental background: Describes the technical setup, sample preparation methods, tip production, and the vacuum conditions required for STM experiments.
4 Results and Discussion: Details the cleaning procedures, imaging results of surfaces and nanowires, and spectroscopic analysis of electronic states and defects.
5 Conclusion and Outlook: Summarizes the findings regarding morphology and electronic properties, and suggests future improvements for cleaning and identification of quantum dot regions.
Keywords
Scanning Tunneling Microscopy, Scanning Tunneling Spectroscopy, Nanowires, InAs, InP, Heterostructures, Quantum Dots, Surface Morphology, Ultra High Vacuum, Semiconductor, Band Gaps, Stacking Faults, Crystal Structure, Epitaxy, Atomic Resolution.
Frequently Asked Questions
What is the primary focus of this research?
The research focuses on the growth, morphology, and electronic characterization of III-V semiconductor nanowires, specifically examining InAs and InP/InAs heterostructures using STM and STS.
What are the central thematic areas?
The work covers experimental surface science, including vacuum techniques, surface cleaning, scanning probe microscopy imaging, and tunneling spectroscopy analysis.
What is the primary goal of this study?
The goal is to determine the morphology and electronic surface properties of different nanowire heterostructures to assess their suitability for future optoelectronic devices.
Which scientific method is employed?
The thesis utilizes Scanning Tunneling Microscopy (STM) for topographic imaging and Scanning Tunneling Spectroscopy (STS) for investigating the local density of electronic states.
What is covered in the main section?
The main section details the cleaning processes for different nanowire samples, the imaging of specific facets, and the analysis of spectroscopic data collected from surfaces and defect regions.
Which keywords characterize the work?
Key terms include Scanning Tunneling Microscopy, InAs/InP nanowires, heterostructures, quantum dots, and surface cleaning.
How do defects influence the electronic structure?
The thesis concludes that the observed surface defects (attributed to missing Indium atoms) do not show a significant influence on the local electronic structure based on the spectroscopic data.
Why are InP/InAs heterostructures investigated?
They are studied because the modulation of materials allows for the confinement of electrons and holes, potentially creating quantum dots which are essential for lasers and quantum information processing.
What role does the tip quality play in these experiments?
Tip quality is critical because atomic resolution imaging is sensitive to tip geometry; blunt or multiple tips can cause artifacts that obscure the true surface morphology.
What challenges were encountered during cleaning?
Cleaning nanowires proved to be more difficult than cleaning bulk surfaces, as excessive annealing can destroy the top facets or the InAs shell of the nanowires.
- Quote paper
- Stephan Pröller (Author), 2011, Scanning Probe Microscopy of InAs/InP Nanowires, Munich, GRIN Verlag, https://www.grin.com/document/178801
