Femtosecond Photoemission Electron Microscopy of Indium Arsenide Nanowires

Abstract

PhotoEmission Electron Microscopy (PEEM) has proven to be a successful method for examining the electronic characteristics of nanostructures. The experiments relevant for the following report were conducted using a combination of PEEM and a femtosecond pulsed laser to determine the electrical characteristics of Indium Arsenide nanowire of crystal structure varying between wurtzite and zinc blend. Particular emphasis was put on analyzing the difference between electrical characteristics of the two crystal structures.

Several experiments were conducted on these nanowires to investigate different properties. The photoemission of the different segments were examined using light of different polarization, which showed that the different atomic structures acted quite differently under different polarizations.

A set of time-resolved pump probe experiments were conducted, which further indicated the same results as the polarization scans. That the wurtzite and zinc blende segments acted quite differently for different polarizations. The time-resolved experiments also provided knowledge about the electron relaxation times of the different crystal structure segments of the nanowires.

Popular Abstract

Everything is made of small particles called atoms which contains even smaller particles called electrons. When an electron moves, electricity is created. Electrons move easier through some materials than others. In metals, electrons can move around very freely while in for example rubber, the electrons are hard stuck to their atom which makes it hard for them to move around and create electricity. Materials in which electrons can move around easily are called conductors while materials in which the electrons are bound hard to the atoms are called isolators.

Except for conductors and isolators, there is a third kind of material called semiconductors, in which the electrons need some help to be able to move. Imagine an electron as a tired man. When he is in his bed, he doesn’t move anywhere. However, when the alarm goes off and he gets out of the bed and drinks a cup of coffee, he can walk around freely in his house. If he at some point stops drinking coffee, he will get tired again and go back to bed and stop moving. This is how an electron in a semiconductor act. It is stuck in a place where it can’t move, until it gets an energy boost which makes it jump up to a state where it can move freely. However, if the electron doesn’t keep getting energy boosts, it will fall back into its original state where it can’t move. The energy boosts corresponding to coffee are in the case of this experiment, light particles called photons.

If the man however drinks to many coffees, he will get hyperactive and run out of the house. This happens to electrons as well. If an electron absorbs too many photons, they are emitted from the material into vacuum. This is called photoemission. In this experiment the emitted electrons are used to understand things about the material. For example, how long does it take for the man to go back to bed if he doesn’t keep drinking coffee? That is, how long can the electron stay in the excited state before decaying back to its original state? This time is called electron relaxation time. How many coffees does it take for the man to get hyperactive and run out of the building? This means how many photons are needed for an electron to be emitted from the material?

For different persons in different houses these properties can change. For a man in a small house with an uncomfortable bed it might be enough with one coffee to cause him to run out the door. For another person in another house it might take many more. This means that these properties change between electrons in different materials and different crystal structures.

Semiconductors are already used widely in modern technology and new applications are discovered all the time. You probably use semiconductors every day. For example, LED lights consist of semiconductors and so does lots of modern transistors. This means that when you use your computer or turn on your bike lights you are probably using semiconductors.

The goal of the experiment is to use photons to investigate how electrons act in a certain kind of semiconducting nanowire called Indium Arsenide. What is interesting with these specific nanowires is that in different parts of the nanowire, the atoms are structured in different ways which can impact how the electrons act in this part of the nanowire. This is what we have measured in this experiment. How the electrons act differently in different parts of the nanowire. This information is interesting for determining what the nanowire can be used for.

• author: Johansson, Björn
• supervisor: Anders Mikkelsen; Lukas Wittenbecher
• organization: Synchrotron Radiation Research; Department of Physics
• course: FYSK02 20181
• year: 2018
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Indium Arsenide, PEEM, Femtosecond laser, Time-resolved PEEM, InAs, Wurtzite, zinc blende, pump-probe, Photoemission Electron Microscopy, pulsed laser.
• language: English
• id: 8954953
• date added to LUP: 2018-08-09 12:27:51
• date last changed: 2018-08-09 12:27:51