We demonstrate the growth of thin films of molybdenum ditelluride and molybdenum diselenide on sapphire substrates by molecular beam epitaxy. In situ structural and chemical analyses reveal stoichiometric layered film growth with atomically smooth surface morphologies. Film growth along the (001) direction is confirmed by X-ray diffraction, and the crystalline nature of growth in the 2H phase is evident from Raman spectroscopy. Transmission electron microscopy is used to confirm the layered film structure and hexagonal arrangement of surface atoms. Temperature-dependent electrical measurements show an insulating behavior that agrees well with a two-dimensional variable-range hopping model, suggesting that transport in these films is dominated by localized charge-carrier states.

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Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

We report on low temperature transport studies of Bi2Te3 topological insulator thin films grown on Si(111)-(7 × 7) surface by molecular beam epitaxy. A sharp increase in the magnetoresistance with magnetic field at low temperature indicates the existence of weak anti-localization. The measured weak anti-localization effect agrees well with the Hikami-Larkin-Nagaoka model, and the extracted phase coherence length shows a power-law dependence with temperature indicating the existence of a two-dimensional system. An insulating ground state has also been observed at low temperature showing a logarithmic divergence of the resistance that appears to be the influence of electron-electron interaction in a two-dimensional system.

Reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), vibrating sample magnetometry, and other physical property measurements are used to investigate the structure, morphology, magnetic, and magnetotransport properties of (001)-oriented Cr2Te3 thin films grown on Al2O3(0001) and Si(111)-(7×7) surfaces by molecular beam epitaxy. Streaky RHEED patterns indicate flat smooth film growth on both substrates. STM studies show the hexagonal arrangements of surface atoms. Determination of the lattice parameter from the atomically resolved STM image is consistent with the bulk crystal structures. Magnetic measurements show the film is ferromagnetic, having a Curie temperature of about 180 K, and a spin glass-like behavior was observed below 35 K. Magnetotransport measurements show the metallic nature of the film with a perpendicular magnetic anisotropy along the c-axis.