Resonance Raman Scattering on Wurtzite Gallium Arsenide Nanowires

Abstract

Advances in crystal growth allow for precise control of crystal phase in semiconductor nanowires. The arising novel materials, like wurtzite gallium arsenide (wz-GaAs), are promising options for new and improved, mainly optoelectronic devices. To do this, however, intensive investigation into their electroninc band structure is needed. This can for example be achieved by optical spectroscopy like photoluminescence spectroscopy (PL), photoluminescences excitation spectroscopy (PLE) or resonance Ramam scattering (RRS).
To date, many different values for the fundamental bandgap and higher electron and hole state energies have been reported and the band structure of wz-GaAs is still controversially discussed. In order to support the results gathered with an established PLE setup, it is desirable to conduct RRS measurements as well.
This work reports about the implementation of RRS into an existing PLE setup. Tests on wz-GaAs nanowires revealed a bandgap energy of 1.525 eV at low temperature (4 K) and of 1.46 eV at room temperature (295 K). Polarization dependent measurements suggest a Γ_{9v} → Γ_{7c} transition between the heavy hole band and the lowest conduction band. For transitions from the light hole band to the first conduction band a temperature dependence following the empirical Varshni equation has been found, yielding a light-hole heavy-hole splitting of about 50 meV at low temperature that rises to 90 meV at room temperature.

Popular Abstract

New materials for smarter devices
Computers are everywhere in today’s life. While the number of regular desktop computers might slowly decrease, other computing devices like notebooks and tablets, but lately especially smartphones and smartwatches increase in number and, of course, become faster and “smarter”. Without knowing it, the average consumer desires significant advances in solid state physics and nanotechnology, because these two fields mainly push the limits of device development.
All the electronic components of such devices are made of semiconductors. Other than conductors like metals, which are used to interconnect different components, these materials only conduct current under certain conditions. These conditions can again be controlled electronically by other semiconductor components, making up a big network that is eventually able to execute simple computations. It is obvious that these structures are very small and had to decrease in size over the years in order to fit more technology into the same smartphone shell. However, the size of such structures has a lower limit: Imagine for example a group of 100 broadleaves next to 100 conifers. One would say there is a broadleaf forest next to a conifer forest and the respective faunas would probably settle in these two habitats. When the trees are now shuffled in a way that there are always groups of let’s say five trees of the same kind, nobody would talk about a sequence of broadleaf forests and conifer forests but instead it would be seen as one mixed forest and be populated by the mixed forest fauna. When trying to scale down the regions of material A and material B in an electronic device to decrease its size, the properties of the materials will mix and result in a big region with electronic properties of the compound material AB, which corresponds to neither A nor B. As a consequence, the device will not work.
To still facilitate a further improvement of our technology the only way seems to be to look for different materials or material combinations which are suitable for electronic devices. Scientists, among others at NanoLund, have found that in gallium arsenide, which is well-researched and widely used in light emitting diodes and photovoltaics, the atoms are sometimes arranged in a different order than usual when it is produced in thin threads, the so-called nanowires. This kind of reordered materials, which are basically the same as the well-known ones but still have some differences in their properties, is extremely promising for advances in the development of electronic components.
The use in electronic devices of course requires detailed knowledge about the electronic properties of the material, which can be found by different methods of optical investigation. One of these methods is resonance Raman scattering. It needs a higher experimental effort and is sometimes a bit harder to interpret than photoluminescence excitation spectroscopy, which is another common method, but is in exchange capable of revealing all the desired information.
In this project an experimental setup for photoluminescence excitation spectroscopy is expanded to enable resonance Raman scattering, with the aim to allow for precise determination of the electronic properties of the “new” materials found in nanowires.

• author: Kubitza, Simon
• supervisor: Mats-Erik Pistol
• organization: Solid State Physics; Department of Physics
• course: PHYM01 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: semiconductors, bandstructure, Raman scattering, GaAs
• language: English
• id: 8893565
• date added to LUP: 2016-10-20 11:29:08
• date last changed: 2016-11-15 13:42:14