Spectroscopic characterization of high-quality polycrystalline Bi-Te films grown by thermal evaporation

Rafał M. Rapacz ,  Katarzyna Balin ,  Krzysztof Smuda ,  Anna Nowak ,  Jacek Szade 

University of Silesia, August Chełkowski Institute of Physics, Department of Solid State Physics, Uniwersytecka 4, Katowice 40-007, Poland

Abstract

Topological insulators form a group of materials exhibiting unusual metallic properties of the surface with an insulating gap in the bulk. One of the 3D topological insulator is Bi2Te3. Which is concerned as promising material for multiple applicationsin future electronics, particularly in spintronics.

The synthesis high-quality thin films of topological insulators on various substrates including silicon is an important technological challenge. Although the synthesis of monocrystalline films of Bi2Te3 was achieved we focused in our work on growth of polycrystalline Bi-Te films. The important question from the point of view of applications and future industrial technology is to find out if the polycrystalline films show the satisfactory properties and what are the conditions for the growth of high-quality polycrystalline samples.

In our studies we concentrated on the growth of high-purity Bi-Te films with varying stoichiometry and on their complex characterization. A particular attention has been devoted to the films with the Bi/Te ratio close to 2/3 as in the standard 3D topological insulator Bi2Te3. Additionally, we tested electronic structure and structural properties of Bi-Te films with different Bi/Te ratio as well as characterized those properties for pure Bi and Te films of the same thickness.

The Bi-Te films were obtained by thermal evaporation method in ultrahigh vacuum cluster system consisting of a Molecular Beam Epitaxy (MBE) growth chamber, Reflection High Energy Electron Diffractometer (RHEED), X-Ray Photoelectron Spectroscopy (XPS) Ultraviolet Photoelectron Spectroscopy (UPS) and Scanning Probe Microscopy (AFM and STM). Further characterization was realized ex-situ with the use of X-Rey Reflectometry technique (XRR) and X-Ray Diffractometry (XRD).

The Bi-Te films were deposited on the silicon (100) substrate with which was prepared in-situ using Ar-ion beam sputtering and annealing at temperature of 970K for 1h. The Bi-Te films  were grown in the co-deposition mode from the effusion cells on monocrystalline Si (100) substrate kept at about 400K. The deposition rate of tellurium and bismuth were matched to obtain the appropriate stoichiometry (established rate for Te was 1.4 times higher than Bi) and were controlled by the quartz weight. The deposition was held under ultra-high vacuum conditions, the pressure during the growth did not exceed the level of 5∙10-9 mbar. The thicknesses of deposited films was of about 20-30nm.

In-situ RHEED measurements were preformed immediately after the deposition process. The diffraction pattern obtained after the growth process showed a set of rings indicating the formation of polycrystalline films. The RHEED patterns showed in some cases the diffused spots indicating texturization of the films. The ex-situ XRD studies indicated polycrystalline structure of grown films with relatively small size of the crystallites. XRR confirmed the assumed value of the thickness of the Bi-Te films.

Analysis of the XPS spectra of Bi-Te films indicated that only the deposited elements are the components of the film -  no oxygen, carbon or other contamination were visible in the XPS survey spectra even after several days in the vacuum chamber.The chemical state of bismuth and tellurium was obtained from the  detailed analysis of the Bi 4f and Te 3d multiplets.  The valence band was studied for the excitation by x-rays, He I and He II radiation. Moreover, by changing the geometry of photoelectron analysis we were able to tune the surface sensitivity of the spectra. The photoemission data enabled us to draw conclusions about the influence of growth conditions on the electronic structure and film structure in the vicinity of the surface – the crucial element of a topological insulator.

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Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 10, by Rafał M. Rapacz
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-15 22:24
Revised:   2013-04-15 22:27
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