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Van der Waals epitaxy (VDWE)

Figure 1: Heterojunction of lattice mismatched layered crystals separated by a van der Waals gap (picture from C. Kreis, Ph.D. thesis).

Surfaces of quasi-two-dimensional layered crystals (2D materials) such as the transition metal dichalcogenides (TMDC's) display no dangling bonds and only weak van der Waals like forces act across the 2D/2D heterojunctions of layered materials [Fig. 1]. Due to the weak interaction, an epilayer grows from the beginning with its own lattice constant forming an  interface with only a small amount of defects. In 1984 Koma demonstrated for the first time heteroepitaxial growth of the layered crystals NbSe2 on 2H-MoS2. Since then this new kind of MBE with weak interaction at the heterojunction was called Van Der Waals Epitaxy (VDWE) [11,12,13]. The lattice matching condition is drastically relaxed for VDWE allowing a large variety of different heterostructures [13,10,14,15,16] even for highly lattice mismatched systems (e.g. Da = 58% for MoSe2 on mica [17]). Passivating the dangling bonds at the surfaces of 3D materials (e.g. S-GaAs(001) [21]), 2D/3D heterostructures (The abbreviation 2D/3D denotes 2D film on 3D substrate) were epitaxially grown [22,23,24]. Even the reverse case of crystalline 3D/2D heterostructures have been produced [25,26,27]. This offers the possibility to combine 3D materials with large lattice mismatch by introducing a buffer layer of a 2D crystal [28].
Compared to bulk materials grown with chemical vapor transport, van der Waals epitaxy of layered crystals offers the following advantages:

  • The relaxed lattice matching condition permits to combine almost any layered material. Since the electronic properties of especially the TMDC's vary considerably, a numerous amount of high quality semiconductor heterostructures, Schottky- and Josephson contacts can be produced.
     
  • Precise control of growth parameters allows to study the electronic structure of well defined low-dimensional systems such as monodisperse clusters and ultrathin films from one to several monolayers.
     
  • Investigating different stages of growth from seeding and formation of the first monolayer up to the growth of thick films, growth modes and growth models of layered crystals can be analyzed.
     
  • Specific doping of the epilayer by coevaporation of (non-magnetic or magnetic) dopants during deposition is possible.

References


[11]
A. Koma, K. Sunouchi, and T. Miyajima, in Proc. 17th Int. Conf. on the Physics of Semiconductors, San Francisco 1984 (Springer, New York, 1985), p. 1465.
[12]
A. Koma, K. Sunouchi, and T. Miyajima, Microelectronic Enngineering 2, 129 (1984).
[13]
A. Koma, J. Cryst. Growth 201-202, 236 (1999).
[14]
R. Schlaf, D. Louder, O. Lang, C. Pettenkofer, W. Jaegermann, K. W. Nebesny, P. A. Lee, B. A. Parkinson, and N. R. Armstrong, J. Vac. Sci. Technol. A 13, 1761 (1995).
[15]
S. Tiefenbacher, H. Sehnert, C. Pettenkofer, and W. Jaegermann, surface science 318, L1161 (1994).
[16]
S. Helveg, J. V. Lauritsen, E. Lægsgaard, I. Steensgaard, J. K. Nørskov, B. S. Clazsen, H. Topsøe, and F. Besenbacher, Phys. Rev. Lett. 84, 951 (2000).
[17]
K. Ueno, K. Saiki, T. Shimada, and A. Koma, J. Vac. Sci. Technol. A 8, 68 (1990).
[21]
K. Ueno and T. Shimada and K. Saiki and A. Koma, Appl. Phys. Lett. 55, 327 (1990).
[22]
C. Hammond, A. Back, M. Lawrence, K. Nebesny, P. Lee, R. Schlaf, and N. R. Armstrong, J. Vac. Sci. Technol. A 13, 1768 (1995).
[23]
A. Koma and K. Saiki and Y. Sato, Appl. Surf. Sci. 41/42, 451 (1989).
[24]
W. Jaegermann, A. Klein, and C. Pettenkofer, in Electron spectroscopies applied to low-dimensional structures, Vol. 24 of Physics and Chemistry of Materials with Low-Dimensional Structures (Kluwer Academic Publishers, Dordrecht / The Netherlands, 2000), Chap. Eletronic properties of van der Waals-epitaxy films and interfaces, p. 317.
[25]
T. Löher, Y. Tomm, C. Pettenkofer, and W. Jaegermann, Appl. Phys. Lett. 65, 555 (1994).
[26]
L. T. Vinh, M. Eddrief, J. E. Mahan, A. Vantomme, J. H. Song, and M. A. Nicolet, J. Appl. Phys. 81, 7289 (1997).
[27]
T. Löher, Y. Tomm, A. Klein, C. Pettenkofer, and W. Laegermann, J. Appl. Phys. 80, 5718 (1996).
[28]
T. Löher, K. Ueno, and A. Koma, Appl. Surf. Sci. 130-132, 334 (1998).

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