在本研究中，我們利用新穎的方式氫原子蝕刻三維拓樸絕緣體碲化鉍塊材來製備雙層鉍。
　　從STM可觀察表面結構的變化，發現低曝氫量的蝕刻時，主要是對於台階邊緣和最外層的碲原子進行蝕刻。當增加曝氫量蝕刻後，可發現表面形成新的大面積平台，並可在觀察到雙層鉍的原子排列結構。而殘餘的氫原子也會與碲原子鍵結形成突起物存在於台階的邊緣上。
　　在電子特性上，拓樸絕緣體表面態轉變成金屬態，能帶結構則具有Rashba分裂的特性。在樣品表面成分組成上，發現隨著曝氫量蝕刻增加，表面Te含量減少，而使Bi金屬鍵結增加。
　　因此證實利用top-down的方式，可以製備出雙層鉍在碲化鉍的異質結構。

The study of physical properties and fabrication of heterointerface between Bismuth bilayer and Bismuth Telluride
Author’s Name：Yuan-Cheng Chi
Advisor’s Name：Rong-Jyun Huang
Department of Physics, National Cheng Kung University

SUMMARY
　In this research, two-dimensional (2D) bismuth bilayer (BL) structure are prepared on Bi2Te3 bulk by atomic hydrogen etching. After at least 12000L of hydrogen dosing, the new large-area terrace appeared at surface, and atomic arrangement structure of bismuth bilayer is observed. The transformation of surface morphology is observed using scanning tunneling microscopy (STM). On the electronic properties, the electronic feature of Bi2Te3 surface turn surface state of topological insulator into metallic-like state after hydrogen etching using scanning tunneling spectroscopy (STS). The Rashba-splitting property near the Fermi level is clearly observed by angle-resolved photoemission spectroscopy (ARPES). The reduction of Te amount and enhancement of Bi metal bonding on the surface composition after increasing the amount of hydrogen dosing are confirmed by X-ray photoelectron spectroscopy (XPS). Our results present the top-down approach to prepare the hetero-interface between bismuth bilayer and bismuth telluride.

Key words: Etching fabrication ; Bismuth telluride (Bi2Te3);Bismuth Bilayer (Bi-BL); Rashba effect; herto-interface

INTRODUCTION
　Topological insulators (TI), realized in materials with strong spin-orbital interaction, are gaining increasing attention in condensed matter physics. 3D TI has been well established experimentally by measuring the helical-like surface states (SS) bands dispersion for various Bi-based materials, like Bi2Te3, Bi2Se3, with angle-resolved photoemission spectroscopy (ARPES).
For 2D quantum spin Hall states (2D QSH), there has been only one case: HgTe/CdTe quantum wells, be performed in 2007. Another interesting system predicted to be in a 2D QSH phase is a single bilayer Bi(111) ultrathin film (Bi-BL). However, it is difficult to grow ultrathin Bi film without a suitable substrate, so the effect of the substrate is definitely important.
In the present paper, we find that Bi2Te3 (111) are appropriate substrate for Bi(111) growth on account of their small lattice mismatch. A simple top-down approach to prepare Bi-BL on Bi2Se3 were proposed by Roozbeh Shokri’s group. They suggested that terminated Se atoms can be remove by hydrogen gas, via the chemical reaction 2H + Se → H2Se, and producing a Bi enriched surface.
Based on lattice matching and the feasibility of top-down approach, we fabricated 2D bismuth bilayer (BL) on Bi2Te3 bulk by atomic hydrogen etching. We tried out the way and further studied its structural properties, etching mechanism by STM and electronic properties by ARPES and XPS.

MATERIALS AND METHODS
The Bi2Te3 bulk we used in our research come from Prof. Ming-Ci Chou (Dep. Of Materials and Optoelectronic Science, National Sun Yat-sen University). The STM and STS research were performed at MBE Lab.-STM group , National Cheng Kung University; all Work at ultra-high vacuum , about 3×10-8 pa. And the Bi2Te3 etching fabrication need to heat up Bi2Te3 to 80-120oC, We exposure the hydrogen gas at 1.0 ×10-4 pa (~ 7.5 ×10-7 torr), in different conditions, by unit of Langmuir (1L= 1.0 ×10-6 torr．sec). In different exposure conditions, research the morphology and electronic properties, make a comparison to the result. The ARPES and XPS experiments were performed at BL 21B and 24A, respectively, in NSRRC (Hsinchu, Taiwan), all the analysis also under the UHV condition.

RESULTS AND DISCUSSION
　In this research, two-dimensional (2D) bismuth bilayer (BL) structure are prepared on Bi2Te3 bulk by atomic hydrogen etching. And the Bi-BL/ Bi2Te3 hetro-interface structure finally formed after at least 12000L hydrogen atoms etching.
The transformation of surface morphology is observed using scanning tunneling microscopy (STM). At low dosing condition, the step edge and the outermost layer of tellurium are initially etched. After increasing the amount of hydrogen dosing, the new large-area terrace appeared at surface, and atomic arrangement structure of bismuth bilayer is observed. Furthermore, the residual hydrogen atoms are bonded to the tellurium atoms to form protrusions on the step edges. We use STM and XPS to confirm that the content of Te will decrease after annealing, but the quantity will decay into a constant. It shows Bi2Te3 surface binding will much more easier to break after annealing, and it provide an easier condition for hydrogen atoms etching.
On the electronic properties, the electronic feature of Bi2Te3 surface turn surface state of topological insulator into metallic-like state after hydrogen etching using scanning tunneling spectroscopy (STS). We used ARPES to research band structure of Bi2Te3 after 12000L etching condition, the band structure most same as the structure of epitaxy Bi-BL on Bi2Te3 in reference. The Rashba-splitting property near the Fermi level is clearly observed in our result, we guess that the protrusions may change the electronic structure of origin band structure of epitaxial Bi-BL/ Bi2Te3.
The reduction of Te amount and enhancement of Bi metal bonding on the surface composition after increasing the amount of hydrogen dosing are confirmed by X-ray photoelectron spectroscopy (XPS).

CONCLUSION
　 Our results present the top-down approach to prepare the hetero-interface between bismuth bilayer and bismuth telluride. The transformation of surface morphology is observed using scanning tunneling microscopy (STM) , to clarify the surface morphology changes under different circumstances. The electronic band structure was carried out more detail than the epitaxial Bi-BL/ Bi2Te3, we observed Rashba-splitting property near the Fermi level. XPS analysis carried out the main surface element lead by metallic type bonds after 12000L etching condition. According to the above experimental results, Bi-BL/ Bi2Te3 hetro-interface structure can easily prepared by this mechanism.

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