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)
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 ).
Passivating the dangling bonds at the surfaces of 3D materials (e.g. S-GaAs(001)
), 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 .
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.
A. Koma, K. Sunouchi, and T. Miyajima, in Proc. 17th Int. Conf. on the
Physics of Semiconductors, San Francisco 1984 (Springer, New York, 1985),
A. Koma, K. Sunouchi, and T. Miyajima, Microelectronic Enngineering 2,
A. Koma, J. Cryst. Growth 201-202, 236 (1999).
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).
S. Tiefenbacher, H. Sehnert, C. Pettenkofer, and W. Jaegermann, surface science
318, L1161 (1994).
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).
K. Ueno, K. Saiki, T. Shimada, and A. Koma, J. Vac. Sci. Technol. A
8, 68 (1990).
K. Ueno and T. Shimada and K. Saiki and A. Koma, Appl. Phys. Lett. 55,
C. Hammond, A. Back, M. Lawrence, K. Nebesny, P. Lee, R. Schlaf, and N. R.
Armstrong, J. Vac. Sci. Technol. A 13, 1768 (1995).
A. Koma and K. Saiki and Y. Sato, Appl. Surf. Sci. 41/42, 451 (1989).
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.
T. Löher, Y. Tomm, C. Pettenkofer, and W. Jaegermann, Appl. Phys. Lett.
65, 555 (1994).
L. T. Vinh, M. Eddrief, J. E. Mahan, A. Vantomme, J. H. Song, and M. A. Nicolet,
J. Appl. Phys. 81, 7289 (1997).
T. Löher, Y. Tomm, A. Klein, C. Pettenkofer, and W. Laegermann, J. Appl. Phys.
80, 5718 (1996).
T. Löher, K. Ueno, and A. Koma, Appl. Surf. Sci. 130-132, 334 (1998).