Ultra-thin films of selenide compound epilayers were deposited epitaxially by molecular beam epitaxy (MBE) on AlN(0001) / Si(111) templates and were studied structurally using high resolution transmission electron microscopy (HRTEM), image simulations, and geometrical phase analysis (GPA). The films comprised Bi₂Se₃, MoSe₂, and HfSe₂ epilayers, as well as composite heterostructures of these materials.  Wurtzite AlN is a wide band gap semiconductor that strongly favors an excellent interfacial quality with the selenides under the employed growth conditions contrary to direct deposition on silicon that leads to interfacial amorphization and extended defects in the film. The structural observations were combined with angle-resolved photoelectron spectroscopy measurements.

Bi₂Se₃ is a topological insulator (TI), and the deposited films exhibited a surface Dirac cone making them promising for novel spintronics and quantum computing applications. HRTEM, combined with GPA, showed an epitaxial well-ordered interface with (0001) AlN. The interfacial periodicity was manifested by a 3:4 plane matching. No interdiffusion or chemical reaction was observed at the interface. High quality and large scale 2D films with thicknesses of 3 and 5 quintuple layers (QLs) were deposited [1]. The films contained only vertical and in-plane 180^o rotational domain boundaries, as shown in Fig. 1.

In addition, high quality films of a few monolayers (MLs) of MoSe₂ and HfSe₂ compound semiconductors were deposited in extended scale by MBE directly on AlN(0001), showing promise for nanoelectronic device applications mediated by the van der Waals bonding [2]. In an alternative approach, Bi₂Se₃ was employed as buffer layer in order to maintain low growth temperatures that favor large scale manufacture. Furthermore, various combinations of alternating selenide layers were achieved, signifying a versatility towards advanced nanodevice possibilities and prospects for combined 2D semiconductor/TI applications. Cross sectional HRTEM, in conjunction with image simulations elucidated the interfaces between dissimilar materials. Variations in lattice spacings were obtained by GPA. Such a heterostructure is illustrated in Fig. 2.

[1] P. Tsipas, E. Xenogiannopoulou, S. Kassavetis, D. Tsoutsou, E. Golias, C. Bazioti, G. P. Dimitrakopulos, Ph. Komninou, H. Liang, M. Caymax, A. Dimoulas, ACS Nano, 8, 6614 (2014).

[2] E. Xenogiannopoulou, P. Tsipas, K. E. Aretouli, D. Tsoutsou, S. A. Giamini, C. Bazioti, G.P. Dimitrakopulos, Ph. Komninou, S. Brems, C. Hughebaert, I. P. Radu, A. Dimoulas, Nanoscale, 7, 7896 (2015).

Acknowledgement: Work partially supported by the ERC Advanced Grant SMARTGATE-291260- and the National program of excellence (ARISTEIA-745) through the project TOP-ELECTRONICS.

Figures:

Fig. 1. Cross sectional HRTEM image along the [11-20]AlN zone axis, showing 5 QLs of Bi2Se3 grown on AlN(0001), and extended defects: (a) rotation (180o) and (b) lamellar twins.

Fig. 2. Combined HRTEM/GPA image, viewed along the [11-20]AlN zone axis, illustrating a HfSe2/MoSe2/AlN(0001) heterostructure, capped by selenium that is projected along the [0001]Se zone axis. Color shows changes in the lattice constant along the growth direction.

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