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Topological Insulator Thin Films: Growth and Characterisation
- 1. Topological Insulator Thin Films: Growth and Characterisation John Sandford O’Neill
- 2. Overview • Introduction – Topological Insulators (TIs) – UHV Experimental Techniques – TI Thin Films on I07 • Method and Results – Bi - Te - Cu(110) – Sn - InSb(111)A • Summary and Future Work
- 3. Topological Insulators • Recently-discovered insulating materials with conductive surface states • Surface states are topologically protected – they cannot be destroyed by defects • Spin-dependent current flow – Spintronics – Quantum Computing
- 4. Topological Insulators • Topologically protected surface states require strong spin-orbit coupling – Heavy metals – HgTe, Bi2Te3, Bi2Se3, Sb2Te3, stanene - Sn
- 5. Ultra-High Vacuum (UHV) Experimental Techniques • Film Growth: – Molecular Beam Epitaxy (MBE) • Characterisation: – Low-Energy Electron Diffraction (LEED) – Auger Electron Spectroscopy (AES) – Scanning Tunnelling Microscopy (STM) – X-Ray Photoelectron Spectroscopy (XPS) – X-Ray Reflectivity (XRR)
- 6. Molecular Beam Epitaxy (MBE) • Used to deposit epitaxial thin films • Knudsen effusion cells used to sublimate solid materials • Requires ultra-high vacuum (<10-9 mbar)
- 7. Low-Energy Electron Diffraction (LEED) Si(111) 7x7 • Principal technique for surface structure determination • Low energy electrons used to achieve surface specificity • Θ 1/a∝ • Requires comparison of LEED pattern before and after film growth
- 8. Auger Electron Spectroscopy (AES) • The Auger electron kinetic energy spectrum is dependent on energy levels in the atom →elemental identification
- 9. Scanning Tunnelling Microscopy (STM) • Images the surface topography to sub- nanometer resolution • Provides information about growth mode of thin films • Individual atoms can be imaged
- 11. X-Ray Reflectivity (XRR) • Specular reflection: angle of incidence = angle of reflection • Provides information about the layered structure X-Rays Detector
- 12. TI Thin Films on I07 • Structural studies of topological insulators • Correlate with electronic structure experiments BaF2(111) Bi2Te3 Bi-bilayer Te BaF2 + Bi2Te3 + Bi-bilayer + Bi- bilayer BaF2 + Bi2Te3 + Bi-bilayer BaF2 + Bi2Te3 BaF2
- 13. TI Thin Films on I07 STM LEED/Auger X-Ray Source Knudsen cells UHV Chamber EH2, I07
- 14. Bi - Te - Cu(110) • Surface cleaning: – Ar+ ion sputtering 1.5 keV 20 mins – Annealing to 550o C 20 mins • Auger spectroscopy – Difficult to completely remove Oxygen and Carbon contaminants from the surface • Te growth – Did not stick to the substrate • Bi growth – Very slow Cu(110) Te Bi-bilayer Carbon Oxygen
- 15. α-Sn - InSb(111)A • Surface cleaning – Ar+ ion sputtering 0.5 keV 20 mins – Annealing to 400o C 1 hour • LEED – 3x3 pattern? • Auger – Analyser not sensitive at higher energies required to detect In and Sb peaks InSb(111)A α-Sn InSb(111)A LEED: 35eV
- 16. α-Sn - InSb(111)A STM images: “clean” InSb(111)A
- 17. α-Sn - InSb(111)A STM images: “clean” InSb(111)A
- 21. α-Sn - InSb(111)A • Tin growth: – Sn Knudsen cell temp: 880o C – InSb temp: 200o C – XPS data taken at 10 minute intervals InSb(111)A α-Sn
- 24. α-Sn - InSb(111)A Mo Mo In Sb Carbon In Sb Sb In Sn Sn Sn
- 25. α-Sn - InSb(111)A After 70 minute Sn deposition After deposition and annealing to 300o C
- 26. Summary
- 27. Future Work • Improve Bi growth – Evaporator faulty? – Alignment in chamber? • Repeat Sn growth on InSb(111)A – Find conditions for ordered monolayer of α-Sn – Try different substrate temperatures – Try other substrates • InSb(111)B • InSb(100)
- 28. Acknowledgements • Pilar Ferrer-Escorihuela • Matt Forster • Chris Nicklin • Jonathan Rawle • Adam Warne • Tom Arnold
- 29. Any Questions?
Notas del editor
- Strange new materials which are insulating in the bulk but conducting on the edge or surface. They also have the property that the conducting states are topologically protected, so they cannot be disrupted or destroyed by defects or impurities in the material. But the most intriguing aspect of these states is that the direction of current flow is correlated with the spin of the electrons. So TIs are quite promising as materials for spintronic devices – where the spin of the electrons encodes information.
- TI idea: Create a system where the electrons in the centre of the material are bound to their parent atoms but electrons on the edge or surface are free to move and conduct electricity. A: Insulator. Electrons bound to localised orbitals. Energy gap separating the filled valence and empty conduction band. B: 2D quantum hall state. In the quantum hall effect a magnetic field is applied to a material. Because of the Lorentz force, electrons inside the material start moving in circular paths as given by the right hand rule. However, the electrons near the edge have their orbits interrupted by the surface - resulting in surface currents on the edge of the material, while the bulk remains insulating. So the aim with TIs is to create a material like those that exhibit the quantum hall effect but without needing an applied magnetic field. It was found that doing this would only be possible if the intrinsic angular momentum (i.e. the spin) of the conducting electrons was taken into account: with the spin-up electrons moving along the edge in one direction and the spin-down electrons moving in the opposite direction. This is due to the fundamental requirement for physical systems to be symmetric under time-reversal. Quite a few materials that have shown this property have been discovered and they’ve all been heavy-metal based compounds with strong spin-orbit coupling. So it seems that a high degree of spin orbit coupling is required for a material to be a topological insulator in order to generate the internal magnetic field
- These are the techniques I’ve used over the past couple of months and are considered some of the standard techniques for surface science. I hadn’t worked with UHV before this project so a significant part of my time was spent learning how to use all these techniques so I’ll run through the basics of them now.
- Molecular beam epitaxy is the method we use to deposit high-purity thin films. The materials that make up the thin film are placed into Knudsen effusion cells which heat the solid material until it begins to sublimate, forming a beam of atoms that is directed at the substrate. Due to the long mean-free path of atoms in UHV, the evaporated atoms do not interact with anything until they reach the substrate.
- LEED is really the principal technique for determining the structure of a surface. Basic schematic of a LEED device is shown on the right. Basically, low energy electrons are fired at a sample. These electrons are then diffracted by the planes in the crystal, forming a pattern of spots which can be viewed on a phosphorescent screen that correspond to the reciprocal lattice of the material on the surface. Low energy electrons are used so that the electrons do not penetrate too far into the material and therefore the technique is surface-sensitive. The idea of using LEED in thin film growth is to look at the pattern before and after deposition – if you know the structure of the substrate then you can infer the structure of the thin film from how the pattern changes.
- Auger spectroscopy relies on the Auger effect. This is where an incoming electron ionises an atom by knocking out one of the core electrons. Electron in a higher energy level drops down to the hole left by the ionised electron and this energy is given to another outer-shell electron and is ionised. This is in contrast to photoemission where the energy would be given out in the form of a photon.
- STM – pretty ubiquitous technique in nanoscience. Atomically sharp tip is scanned over a surface by highly-precise piezoelectric motors. When the tip is very close to the surface, a tunneling current of electrons between the tip and sample surface can be detected – it’s generally of the order of nanoamps. By keeping the current constant as the tip is scanned over the surface the topography can be mapped. This is useful to us when growing thin films as the growth mode can be ascertained – island growth is quite clear.
- X-Ray Photoelectron Spectroscopy or XPS relies on the photoelectric effect whereby photons are fired at a material which excites electrons to produce photoelectrons. The photoelectrons have characteristic kinetic energies corresponding to the binding energy of the orbitals and thus can be used for elemental identification. XPS also known as ESCA – electron spectroscopy for chemical analysis Photon in – electron out.
- Synchrotron technique – move sample and detector so that the sample is functioning like a mirror. Specular reflection. Various characteristics of the XRR curve can be used to infer parameters of a layered thin-film structure.
- My project was to build on work previously done on I07 involving thin films of bismuth telluride. Previous experiments had shown it was possible to grow the topological insulator bismuth telluride on top of a barium fluoride(111) substrate. X-Ray Reflectivity data showed that there was a Te wetting-layer between the substrate and the thin film. The aims of my project were to deposit a single Bi-bilayer, which is a topological insulator in its own right, directly on top of the Te wetting-layer with the goal of studying the growth in order to find the optimum conditions for deposition. So we’re interested in looking at the physical structure of these materials with the ultimate aim of making high-quality ordered thin films suitable for further investigation of electronic structure.
- X-Ray source which produces photons of energy 1500eV. Knudsen cells that I spoke about earlier.
- Aim was to grow a bismuth bilayer on top of Cu(110) with a Te buffer layer. Auger: had to purchase new lock-in amplifier and took a few weeks to get that working. Difficulty in obtaining a completely clean surface but the Auger spectra had low noise and very clearly showed the characteristic peaks of copper but there were still traces of Carbon and Oxygen contaminants. We had fairly limited success when it came to growing tellurium and bismuth. The Te did not seem to stick to the substrate at all and we found bismuth growth to be extremely slow. We would carry out deposition for several hours and end up with tiny signals on the XPS. So we decided to try something new and focus on another system of interest for the remainder of my project.
- Single layer of alpha-tin which has been given the name “stanene” as a combination of stannum and graphene and has been shown to be a topological insulator when strained in a particular way on top of certain substrates. With this system we aimed to grow a layer of alpha-tin on an indium antimonide substrate terminated with indium atoms, which is what the A refers to. The combined LEED/Auger didn’t seem to be sensitive to the higher energy electrons produced by indium and antimony
- Believing we had a relatively clean surface because of our LEED data it was time to look at the surface with the STM. We found an extremely corrugated surface in places but there were some flat areas where we were able to see atomic features. Further sputtering and annealing did not seem to change the fact we had a very bumpy surface.
- Some more scans of the atomic resolution images we obtained on the flat areas of the crystal.
- First XPS data from the indium antimonide after first few cycles of sputtering and annealing the crystal. Can identify the In and Sb peaks but a very strong signal from carbon on the surface. We decided to try annealing the sample for many hours (including once over the weekend) to see if we could get a better surface.
- This was the result, massively decreased carbon signal and the other peaks for In and Sb started to become clear.
- Here’s the two spectra on the same graph so you can see the improvement.
- We decided to start growing the Sn on the surface even though it seemed not exactly perfect. This was monitored with XPS in the region I’ve highlighted. Sn has an XPS doublet right in between the In and Sb doublets in this region.
- XPS data taken every 10 minutes up to 70 minutes
- Black line is just the substrate before any film growth, red line is after 70 minutes. We stopped growing when the Sn roughly appeared of similar intensity to the original substrate peaks which according to the literature should correspond to a film thickness of less than 8 angstroms. Unexpectedly, the substrate peaks did not decrease, which should happen as the surface is increasingly covered in Sn. This is possibly due to the fact we had such a corrugated surface, with the Sn perhaps filling in the troughs on the material.
- Full energy range before and after deposition. Extra peaks due to Sn clearly visible – so we had at least a few layers and it was time to investigate the surface with STM.
- Very disordered structure – far from the monolayer epitaxial growth that we wanted. So we tried annealing the substrate to 300 degrees to see if we could get any more order on the surface, and the answer is no. You can see in these images there isn’t much difference at all. So we didn’t have an ordered structure and annealing didn’t seem to have any effect in terms of ordering the material. But we certainly had a reasonably significant amount of tin on the surface. Not surprising given that the substrate was very corrugated and not perfectly clean.
- My project forms the start of work that will continue on I07 in trying to grow and structurally characterise these thin film materials.