透過分子束磊晶系統在sapphire(0001)基板上成長鉍化碲拓樸絕緣體，並藉由X光繞射(XRD)看出鉍化碲具有強烈的C軸取向，由於樣品為易氧化材料，因此在蓋上硒/碲保護層上量測XRD與XPS，發現蓋上硒金屬後樣品原本的特性已經消失，進而產生較穩定的合金相。
樣品以ARPES量測證明薄膜確實為拓樸絕緣體，並且它的電子結構成長溫度調控。隨著溫度的上升至390oC，材料的費米能級從價帶往導帶靠近的趨勢，而過了390oC之後，其費米能級又往價帶靠近。之後我們以XAS實驗證明這費米能級的移動來自於反位缺陷的影響，在310oC至390oC為Bi-rich的情況，導致Bi原子取代Te原子，以至於材料以電洞傳輸為主，390oC之後則是Te-rich的情況，導致材料以電子傳輸為主。

Temperature dependent of the Microstructure and electrical properties of epitaxial 〖Bi〗_2 〖Te〗_3 films
Author: Yi-Fan Chen
Advisor: J.C.A Huang
Department of physics, National Cheng Kung University

SUMMARY

Utilizing a combined ARPES and XPS experimental study, we demonstrated that the electronic properties of Bi2Te3 thin films can be regulated by substrate temperature during MBE growth. Correlation between Bi/Te atomic ratio of Bi2Te3 thin films and EDP, i.e. the position of Dirac point, of individual film has been observed, using electronic measurement can verify the result of ARPES and XAS determinate the mechanism what happened in Bi2Te3 thin films.
INTRODUCTION
Topological insulators (TIs) are novel materials with electronic structure which are topologically distinct from regular insulators. A TI is a gapped insulator in the bulk, but has gapless conducting surface states with a Dirac cone electronic structure. These surface states arise from band inversion and create TI phase, and key ingredient is strong spin-orbit interactions due to heavy elements can inverse the band characters of valence and conduction band. The band inversion makes the surface states topologically nontrival, meaning that these states are protected and that these surface states have a helical Dirac type dispersion, where spin is tightly coupled to momentum called spin-momentum locking.
In this paper, we report the systematic angle-resolved photoemission spectroscopy (ARPES) study on MBE grown Bi2Te3 films with controlled substrate temperatures, using x-ray absorption spectroscopy (XAS) to analyze their bonding mechanism, and electronic measure to verify ARPES result. In this experiment , we observed a correlation between Bi/Te atomic ratio of Bi2Te3 thin films and EDP, i.e. the position of Dirac point, of individual film, which means that the Dirac point of individual film can be controlled by MBE growth parameters in a systematic way without any extrinsic doping. As demonstrated by the angle-resolved photoemission spectroscopy (ARPES), the Fermi energy (EF) or the position of Dirac point (EF-ED) of individual Bi2Te3 thin film can be tuned through regulating MBE growth parameters , so does electronic measurements , and in XAS measurement, we can know that the appearance come from anti-site defect.

Material and methods

For the growth of Bi2Te3 films on the Al2O3(0001) substrate. The c-plane sapphire substrate was cleaned by standard procedure before loaded into the growth chamber. To remove any possible contaminants from the substrate surface, the sapphire substrate was heated to 1000C for one hour. A streaky RHEED can be observed, therefore a single crystalline epi-ready substrate for the MBE growth of Bi2Te3 film. High-purity Bi (99.999%) and Te (99.999%) were evaporated by Knudsen cells and the fluxes were calibrated in situ with a quartz crystal micro-balance. The base pressure of the MBE system was below 2×10-10 torr and the growth pressure of the Bi2Te3 thin film was maintained at below 1×10-9 torr. The Bi and Te2 material fluxes were generated by effusion cells and calibrated in situ with a quartz crystal micro-balance.
The surface properties were monitored in-situ by reflection high energy diffraction (RHEED), additional structural characterization was performed by the combined studies of X-ray diffraction (XRD), thickness of the films were obtained from X-ray reflectivity (XRR), and the surface morphology were obtained by atomic force microscopy (AFM), respectively.
The stoichiometry of the films was characterized by x-ray photoelectron spectroscopy (XPS). And mechanism of Bi2Te3 thin films bonding measured by x-ray absorption spectroscopy (XAS) and electronic measurements is R-T and Hall effect .
The ARPES experiment was carried out at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan using BL21B1 U9-CGM Spectroscopy Beamline. The photoemission spectra were measured in a UHV chamber equipped with a Scienta R4000 hemispherical analyzer with ±7° collecting angle. The ex-situ sample preparation of Bi2Te3 thin film covered with 2 Å thickness of Te layer was annealed at 260 °C about 20 minutes in UHV environment to remove the capping layer before ARPES measurement. All spectra were taken at 83 K at a base pressure of 4.8x10-11 Torr and with the photon energy of 22 eV. The angle resolution was 0.1° and overall energy resolution was better than 8 meV. The Dirac point position of the surface state was determined by momentum distribution curves (MDCs).

Result and discussion

The Bi2Te3 epilayers were grown under various conditions. The Bi and Te effusion cell temperatures were chosen to letting the flux ratio Te2/Bi to be 15, i.e. Bi deposition rate = 1 Å /min and Te = 15 Å /min. Fig. 1 depicts XRD curves of several Bi2Te3 epilayers grown by MBE at various substrate temperatures Ts between 310C and 430C (nominal temperature). In these Ts ranges, the XRD curves of the Bi2Te3 samples clearly show features related to a single-crystal Bi2Te3 epilayer, which is (001)-oriented on the sapphire (0001) substrate. Bi2Te3 thin films were grown with Te2/Bi flux ratio of 15 at substrate temperature Ts ranging from 310C to 430C (nominal temperature). Particularly, at ΦTe2/Bi = 15, the sample grown at 310C exhibits a Dirac point at binding energy (EB) = 0.28 eV, while its EF is in conduction band. Here, the bulk states are observed as an intense spot right below the EF. The sample is n type. When increasing Ts to 370C and 390C, the DPs increase to EB = 0.26 eV and 0.21 eV, respectively. These results are similar to the reported result in literature . However, when the Ts exceeds 390C, an opposite trend was observed, i.e. EF moves further out from DP, the DP decreases again to EB = 0.25 eV. The ARPES spectra show a transition around Ts = 390C. These results suggest that the TI thin film can be changed from n-type state to insulating state (only a single Dirac cone surface state intersecting the Fermi energy) and back to n-type state again by increasing the substrate temperature Ts. And electronic measurement’s trend is closed to ARPES’s trend. From XAS experiment , we observed that anti-site defect(Bi substitute Te’s location ) of 390C is more than 310C, so does Dirac point would moving.

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