Substrate Effects on The Nucleation and Growth of GaSe Layers by Van Der Waals Epitaxy
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the property of interest without being limited by lattice and thermal expansion mismatch[ 1-5]. To accomplish this promise, layered materials are used which have strong intralayer bonding, but weak interlayer bonding. The crystallographic 2D metal chalcogenides, characterized by strong covalent bonding within a layer and weak van der Waals bonds between layers, have electrical properties ranging from superconductors (NbSe2 and NbS2), to metals (TaS2, TaSe2, WTe2) to semiconductors(MoS2, MoSe2, WS2, GaSe, InSe) and to wide bandgap materials (ZrS2 and HfS2). It has been found that oriented thin films of these layered materials can be deposited on various substrates with little or no interdiffusion. Thus, by combining the layered nature of the metal chalcogenides with other layered materials as well as crystallographic 3D materials(i.e., Si, GaAs, A12 0 3 , CaF 2 ) oriented films can be deposited, resulting in novel heterostructures with abrupt interfaces. A characteristic of van der Waals Epitaxy is that the depositing films maintain their own lattice from the first layer of growth, thus no strain is built up from the interface. However, even with the weak van der Waals bond, substrate/film interactions play a large role in the final structure. Among the advantageous substrate/film interactions is the rotational alignment of the film on the substrate. On the other hand, in the growth of a layered material on a three dimensional substrate, termination of the substrate dangling bonds must be accomplished before the growth of the layered material can occur [5]. Therefore, if the substrate surface is not uniformly terminated, defects will likely exist in the initial film layer. In addition, it is likely that defects (steps, vacancies, ect.) will act as nucleation sites for the depositing material. We have looked at the nucleation and growth of the layered material GaSe on GaAs (1 ll)A using a combination of complementary surface (RHEED and AFM) and bulk characterization (TEM) techniques. In this paper we report on the growth, structure and 101 Mat. Res. Soc. Symp. Proc. Vol. 382 01995 Materials Research Society
crystalline quality of the GaSe film as a function of the initial substrate condition and correlate features observed by RHEED with those observed by TEM and AFM. EXPERIMENTAL PROCEDURE Experimentally, VDWE is similar to molecular beam epitaxy (MBE) with the deposition taking place in a standard ultrahigh vacuum (UHV) chamber. In this research, GaSe (99.999% purity) was sublimed from a single Knudsen cell. The deposition rate was measured with a previously calibrated deposition thickness monitor and was maintained at a constant rate. The deposition chamber base pressure was maintained at 2 x 10-9 Torr. RHEED observations were made at an acceleration voltage of 20 kV and digitized directly through a CCD camera. GaAs(ll 1l)A (Ga terminated) and GaAs(001) substrates were used in this study. Following the deposition, the surface chemistry was determined using XPS. In all cases, stoichiometric GaSe was obtained. A de
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