© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 4002–4007 4002 www.advmat.de www.MaterialsViews.com COMMUNICATION wileyonlinelibrary.com By Yao-Yi Li, Guang Wang, Xie-Gang Zhu, Min-Hao Liu, Cun Ye, Xi Chen, Ya-Yu Wang, Ke He, Li-Li Wang, Xu-Cun Ma, Hai-Jun Zhang, Xi Dai, Zhong Fang, Xin-Cheng Xie, Ying Liu, Xiao-Liang Qi, Jin-Feng Jia,* Shou-Cheng Zhang, and Qi-Kun Xue Intrinsic Topological Insulator Bi 2 Te 3 Thin Films on Si and Their Thickness Limit [∗] Y.-Y. Li, G. Wang, X.-G. Zhu, M.-H. Liu, C. Ye, Prof. X. Chen, Prof. Y.-Y. Wang, Prof. J.-F. Jia, Prof. Q.-K. Xue Key Lab for Atomic, Molecular and Nanoscience Department of Physics Tsinghua University Beijing, 100084 (P. R. China) E-mail: jjf@mail.tsinghua.edu.cn Dr. K. He, Dr. L.-L. Wang, Prof. X.-C. Ma, H.-J. Zhang, Prof. X. Dai, Prof. Z. Fang, Prof. X.-C. Xie, Prof. Q.-K. Xue Institute of Physics The Chinese Academy of Sciences Beijing, 100190 (P. R. China) Prof. Y. Liu Department of Physics The Pennsylvania State University Pennsylvania, 16802 (USA) Dr. X.-L. Qi, Prof. S.-C. Zhang Department of Physics Stanford University Stanford, California, 94305-4045 (USA) DOI: 10.1002/adma.201000368 Layer-by-layer molecular beam epitaxy growth of high quality Bi 2 Te 3 films has been achieved on Si(111) substrate. The Te-rich growth dynamics is found crucial for high quality stoichiometric Bi 2 Te 3 with few defects. In situ angle resolved photoemission spectroscopy (ARPES) measurement reveals that the as-grown Bi 2 Te 3 films without any doping are an intrinsic topological insulator with its Fermi level intersecting only the metallic surface states, which is different from available bulk crystal of Bi 2 Te 3 . Experimentally, we find that the single-Dirac-cone sur- face state develops at a thickness of two quintuple layers (2 QL). Theoretically, we show that the interaction between the surface states from both sides of the film, which is determined by the penetration depth of the topological surface state wavefunc- tions, sets this lower thickness limit. The success in growing high quality films by state-of-art molecular beam epitaxy tech- nique opens a new avenue for engineering of topological mate- rials based on well-developed Si technology, and is of significant importance for potential applications of topological insulators. Traditionally, Bi 2 Te 3 is known for having the highest figure-of- merit coefficient ZT ≈ 1 among bulk thermoelectric materials. [1–3] Recent theory and experiment reveal that stoichiometric Bi 2 Te 3 is also a topological insulator (TI) with time-reversal-symmetry protected surface states that reside in its bulk insulating gap. [4–6] The metallic surface states consist of a single Dirac cone at the Γ point and are predicted to exhibit a number of striking elec- tromagnetic properties, [7–11] which have recently attracted great attention. [4–20] Experimentally, angle-resolved photoemission spectroscopy (ARPES) is the only direct way to determine if a sample is a TI by mapping the band structure and see if the Fermi level (E F ) only intersects the metallic surface state. [5,6,11,16,17] However, the reported topological insulators such as bulk Bi 2 Te 3 and Bi 2 Se 3 all suffer from a great amount of unwanted bulk car- riers, [5,6,11,16–19] and an intense bulk electron pocket appears at the Fermi level due to the presence of vacancies and anti-site defects. [5,6,11] With the bulk electronic states at E F , it is difficult to characterize the pristine topological transport property and to use them to develop topological devices that rely only on the behaviors of Dirac fermions. Therefore, a major obstacle in the rapidly developing field of TIs is the extreme difficulty in growing intrinsic TI materials. Here we define intrinsic by refer- ence to intrinsic semiconductors: its Fermi level lies in between the bulk conduction band minimum (CBM) and the bulk valence band maximum (VBM) and only intersects the metallic surface state. The Bi 2 Te 3 crystals studied in recent experiments were grown by melting stoichiometric mixture of Bi and Te in a crucible. [5,6,11] Local composition fluctuation, which results in a high background carrier density, [5,6] cannot be avoided with this technique. To compensate the background carriers and remove the bulk states from the Fermi level, the materials had to be heavily doped by 0.67% Sn. [5] The situation makes sys- tematic doping control and device gating difficult. Thin films have several advantages: band engineering can be achieved by bipolar or gradient doping, and tunneling junctions, as well as heterostructures and superlattices. Growth of Bi 2 Te 3 and Bi 2 Se 3 films by molecular beam epitaxy (MBE) and other techniques has been investigated previously. [21–23] With these methods, nominally stoichiometric single crystalline films could easily be prepared. However, discussion of their topological properties was not possible in those studies since the band structure of the films is unknown. So, it’s difficult to judge that the quality of the films is better than that of bulk counterpart and they are the topological insulator without doping. The main purpose of this work is to establish the MBE growth conditions by which intrinsic topological insulator thin films of Bi 2 Te 3 can be readily obtained. We do this by a systematic study of growth dynamics under various Te 2 /Bi flux ratios and Si sub- strate temperatures with reflection high-energy electron diffrac- tion (RHEED). We identify unique Te-rich growth dynamics for preparing intrinsic topological insulator films by the character- istic RHEED intensity oscillations of layer-by-layer MBE growth. The high quality of the grown films is also confirmed by in situ scanning tunneling microscopy (STM). Remarkably, our in situ