Experimental studies on the growth, characterization and topological properties Bi2Se3 thin film
Date Issued
2015
Date
2015
Author(s)
Lin, Yen-Cheng
Abstract
Topological insulators are a new kind of innovative materials that have an insulating behavior in the bulk but a conducting behavior at the surface. Up to date, most of the high quality bismuth selenide (Bi2Se3) thin films were grown by using molecular beam epitaxy (MBE). This method, however, is too expensive and it is desirable to have a simpler and more affordable way of making the samples. In this work, the physical vapor deposition (PVD) method was used to grow large area Bi2Se3 thin films with size of cm2, on sapphire (Al2O3) substrates. This method has the advantage that it is simple and inexpensive. For the PVD growth the source powder and the substrate were placed in a quartz tube that is put inside a furnace and a mechanical pump is connected to the quartz tube to reduce the pressure in the tube. During the growth, Bi2Se3 powder was placed at the center of furnace, Al2O3 substrate was placed 15 cm away from the source powder, the source temperature was kept at either 500 or 530 , the substrate temperature was kept at either 300 or 350 , and the environment pressure was fixed at 5X10^(-3)torr. Atomic force microscopy (AFM) measurements indicate that, depending on growth durations, the thickness of Bi2Se3 thin films varies from 8 nm to 60 nm, X-ray diffraction (XRD) indicate that the films were of good crystalline quality. The six-fold symmetry of the phi-scan indicates that the growth mode is domain matching epitaxy mode. The energy-dispersive X-ray spectroscopy (EDS) measurements confirmed that the ratio of Bi and Se were 2 to 3 and the electron spectroscopy for chemical analysis (ESCA) showed that the binding energies are that for Bi and Se. Transmission Electron Microscopy (TEM) measurements indicate that the film had a layer by layer structure and the thickness of each quintuple layer is 0.94 nm. For the electrical measurement, the film with thickness of 30 nm was made into a Hall pattern. The carrier concentration in this sample is found to be between 1.07X10^(19)/cm^3 and 1.09X10^(19)/cm^3 from T=2k to T=300K and metal insulator transition was found to occur at 4.5 K. The mobility of the sample is 940 cm2/Vs at 2 K, about 18 times larger than similar film grown by using pulsed laser deposition. Weak anti-localization (WAL) was found at low magnetic field in the magneto-resistivity measurement, showing that the sample has the property expected for a topological insulator. By fitting the data with the HLN formula we found that the surface conduction occur only in one surface of the sample and the dephasing length of the electron is 640 nm.
Subjects
Bi2Se3
thin film
Type
thesis
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