IN SITU LOW TEMPERATURE SCANNING TUNNELING MICROSCOPY AND SPECTROSCOPY STUDY OF THE TUNABLE ELECTRONIC STRUCTURE OF TOPOLOGICAL INSULATORS
Niv Levy1, Tong Zhang1,2, Jeonghoon Ha1,3, Joseph A. Stroscio1
1CNST, NIST, Gaithersburg, MD
2Maryland NanoCenter, UMD, College Park, MD
3 Dept of Physics and Astronomy, Seoul National University Seoul, Korea
Low temperature and high energy resolutions are usually key factors for discovering the new physics and phenomena, especially for tunneling spectroscopy measurements. To achieve this goal, we developed the next generation scanning tunneling microscope (STM) which operated in extreme conditions of ultra low temperature (ULT, 13 mK) , high magnetic field (15 T) and ultra-high vacuum (UHV). A unique dilution refrigerator (DR) was designed to obtain the extreme ULT environment. All STM instruments including the RF shielded room are vibration isolated by three separate floating stages. High signal-to-noise ratio of topography and spectroscopy were both achieved. An MBE system, tip characterization chamber and an auxiliary low temperature (5K) STM allow for in situ sample growth and preliminary characterization.
We are currently studying topological insulators (TI), whose spin helical Dirac surface states can be the host of many striking quantum phenomena. We have grown atomically flat Bi2Se3 and Sb2Te3 films on high dielectric constant SrTiO3 substrates using molecular beam epitaxy (MBE). The growth dynamics was characterized by real-time reflection high-energy electron diffraction (RHEED). The substrates were pre-patterned with Pt electrodes and mounted in specially designed sample holders, allowing us to in situ control the film carrier density with the back gate and removing the necessity of making ex situ post-processing contacts to the sample. As a result, we are able to continuously change and observe the electronic structure due to gating of the pristine films. Following initial characterization in the auxiliary LT-STM, similar samples will be studied in the ULT-STM.