Accurate determination of the band offset between Zinc-Blende-Wurtzite GaAs

Abstract

Under certain conditions, the crystal phases in III-V semiconductor nanowires can be controlled, opening up the possibility to create polytype heterostructures with atomically sharp interfaces. The focus of this thesis lies on GaAs nanowires exhibiting the wurtzite (wz) and zinc-blende (zb) crystal structure. Since controlled growth of the wz phase of GaAs has only been achieved recently, knowledge is rather limited on the fundamental properties of this phase and even less is known about the character of the interface between wz- and zb-GaAs.
To shed more light on the wz-zb interface, which is assumed to have a type II band alignment, this project was focused on determining the band offset between zb- and wz-GaAs more accurately. This was realized by investigating nanowires with an axial segment of wz-GaAs embedded between zb-GaAs. The diameter of the nanowires, as well as the thickness of the passivating AlAs shell, were varied. These nanowires were investigated by photoluminescence (PL) and each PL-spectrum was correlated with scanning electron microscopy (SEM) images of the respective wire.
The PL-spectra show an emission peak which is red-shifting with decreasing excitation power density, which indicates a type II band alignment. When the measured type II transition energy is correlated with the diameter of the wire, a clear diameter dependence is visible. The measured transition energy decreases with increasing diameter, indicating radial band bending due to Fermi-level pinning at the GaAs surface. This diameter dependence is decreased by passivating the GaAs surface with an AlAs capping. Linear regression on the data yields 109 meV for the wz-zb band offset.
The PL spectra of capped nanowires do not show one broad shifting peak, but instead show multiple small peaks with a broad shifting envelope. This may be due to quantum states in the triangular wells formed at the wz-zb interface. If this is the case, the transition energies used for the determination of the band offset are actually that of the transition from the hole ground state to the electron ground state. This transition energy would correspond to a value larger than the one expected for the type II transition. Therefore the value for the band offset determined in this thesis may be underestimated and might hold as a lower limit.

Popular Abstract

Crystals are everywhere. They are in your ear rings, in your glass of iced soda and in the computer you are probably reading this on right now. Every electronic device is filled with very small crystals that can conduct current, but the same crystals are used in lights and solar cells. That is because that type of crystal has a very special property: it is a semiconductor.

Semiconductors, how do they work?
Semiconductors, like the name implies, can conduct current, but only occasionally. This is very useful, as we can control exactly when they conduct current and when they do not. This is why it is used in computers and other electronic devices: you want to be able to control when the current goes where.
To understand how semiconductors work we need to know about the electronic structure. Imagine having two boxes with covers on two shelves above each other: the bottom one is completely filled with marbles, while the top one is completely empty. It does not matter how you shake the boxes, no marbles will move: in the bottom box, the marbles are so closely packed that they can not move and the top one simply does not have any marbles to move.
Boxes, one filled with marbles and one empty, nicely stacked on a shelf. Now we lift one marble out of the bottom box and put it in the top box. When you shake the boxes now, you will suddenly have marbles moving. Even in the bottom one, the rest of the marbles will shift into the hole left behind by the lost marble.
Replace the marbles by electrons and you have the concept of semiconductors. Electrons who are bound to the atoms are in the bottom box, or valence band, as we call it. When the electrons have enough energy, usually by heat or light, they can break free from their atoms and go into the top box, or conduction band, where it can move freely. When the electron breaks free, the atom it belonged to will miss an electron. This works the same as the holes left by the marbles: electrons bound to other atoms can move around and fill up this hole.
The energy needed for an electron to break free is called the bandgap energy. It is called a "gap" because the electrons are not allowed to have an energy between being bound to the atom and breaking free. It is like the distance between the two boxes: marbles can not float between the boxes, just like electrons cannot float between the conduction band and the valence band.
You might wonder why it is useful to have two bands. After all, it is much more practical to keep all your marbles in a one box. But remember, when all electrons are in the valence band they cannot move and you can control exactly how many electrons can move by controlling the energy of the electrons. Take solar cells for example: the light of the sun lifts the electrons from the valence band to the conduction band where they can move to a contact and enter the electronic circuit that charges your phone or turns on your coffee machine. The holes in the valence band are collected at another contact where they get filled up by electrons who have already done their job in the electronic circuit. LED lights, that are slowly replacing all conventional lighting, work exactly the opposite. From a source electrons are directly put into the conduction band and another source takes away electrons from the valence band, creating holes. The electrons in the conduction band can sometimes fall down into one of the holes, losing the energy it had. This energy is emitted as light.

Gallium Arsenide
We all want faster computers and better solar panels, so a lot of research goes into making better semiconductors. Often this means using a better material. Gallium Arsenide is a semiconductor that has many desirable qualities: the electrons in the conduction band can move very fast and it is very suitable for making solar cells. The shelves for wurtzite are a little higher than for zinc-blende.
Instead of using a big piece of Gallium Arsenide, which can be expensive, you can instead use very small rods called nanowires. These nanowires are only 100 nm thick. In comparison: a human hair is 500 times thicker. When you make crystals that small, they start behaving slightly differently than when you use a big piece. In Gallium Arsenide, for example, you can get two different types of crystals. It is still made from the same atoms, but they are ordered slightly different. Although the differences between these crystal structures are very subtle, it has large consequences for the properties.
The first Gallium Arsenide crystal structure is called zinc-blende. This is the same structure as a large piece of Gallium Arsenide has and this has been researched very thoroughly. The other crystal structure is called wurtzite. This can only be found in these nanowires and very little is known about it.
If you look at the valence and conduction band of wurtzite and zinc-blende Gallium Arsenide, they look very similar. The bandgap energy, the energy needed to go from the valence to the conduction band, is approximately the same for both of them. However, it seems that the bands of wurtzite are moved slightly higher in energy. It is like the shelves on which the boxes stand are moved a few centimeters higher for wurtzite than for zinc-blende, while the distance between the boxes remains the same. So if you attach a piece of zinc blende to a piece of wurtzite, there is a step in the conduction and the valence bands. How large this step is, is still unclear.
Much like marbles in a box, electrons collect in the lowest point of the conduction band. So when you have a band with a step, they all collect on the lower side of the step. This would be in the zinc-blende crystal. For holes goes the exact opposite; they collect in the wurtzite crystal. Remember that electrons can fall back from the valence band into the conduction band, sending out light. Sometimes they can fall back, not strait down, but a little bit sideways. When this happens with electrons from the zinc-blende crystal to holes in the wurtzite crystal, the light that is emitted is exactly the height of the step subtracted from the bandgap energy. This way the height of the step can be measured. This is exactly what we have done. We have used the semiconducting property of Gallium Arsenide to find out the step between zinc-blende and wurtzite.

New physics
Wurtzite Gallium Arsenide can only be made in very small crystals and the production of these crystals has only been accomplished very recently. That is why very little is known about the wurtzite crystal. There is still much to learn about, for example, how it reacts in an electric field, or what happens when you put a current trough it. But with the step known, we can make all kinds of new interesting crystals. We can look at what happens in the wurtzite when we make the crystals even smaller. Or we can do very short pieces of wurtzite crystal between zinc-blende crystals. The research on these crystals is very new and exciting, and a lot still has to be discovered about wurtzite Gallium Arsenide crystals, but maybe, in the future, you will charge your phone with power generated by a wurtzite Gallium Arsenide solar cell.

• author: Geijselaers, Irene
• supervisor: Mats-Erik Pistol
• organization: Department of Physics; Solid State Physics
• course: FYSM60 20152
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 8879224
• date added to LUP: 2016-06-20 14:21:49
• date last changed: 2016-06-20 14:21:49