Growth by the CSVT (close-spaced vapor transport) technique and characterization of epitaxial GaAs layers on Ge substrates

Abstract

Epitaxial layers of GaAs on (100) GaAs substrates can be grown by close-spaced vapor transport using water vapor as the transporting agent. The parameters for the transport are [Formula: see text], ΔT′ = 45 °C, and δ = 0.03 cm (where [Formula: see text] is the temperature of the graphite heating the substrate; ΔT′, the temperature difference between the graphite heating the source and the one heating the substrate; and δ, the thickness of the spacer separating the GaAs source and the substrate). Mirrorlike epitaxial layers of GaAs are obtained with these parameters when water vapor, at a partial pressure of 4.58 Torr (1 Torr = 133.3 Pa), is introduced with H2 at the beginning of the temperature rise of the reactor. The dimensions of the epitaxial layer are only limited by the size of the reactor. Using the same growth conditions, it is not possible to obtain mirrorlike films of GaAs on (100) Ge substrates. Instead, the layers are dull grey (sample no. 1). It is however not a polycrystalline deposition since the pole figures, obtained by X-ray diffraction, reveal only four crystallographic orientations; {100} the main one, {221} the secondary one, and {021} + {112} two minor contributions. Mirrorlike films of GaAs on (100) Ge substrates of less than 1 cm2 have been obtained with [Formula: see text], ΔT′ = 25 °C, and δ = 0.03 cm. With these conditions, the growth rate is 0.25 ± 0.08 μm min−1. The time evolution of [Formula: see text] and ΔT′, from room temperature up to the equilibrium temperature also influences the surface morphology of GaAs films on Ge while this was not the case for GaAs films on GaAs substrates. When the Ge substrate is larger than 1 cm2, the centre of the film becomes textured but the edges remain mirrorlike (sample no. 2). Pole figures obtained for the center and the edges of sample no. 2 are similar. They are characterized by one large diffraction due to the {100} orientation. A few random crystallographic orientations and sometimes the {221} orientation, however, bearly emerge from the background of these pole figures. Also transmission electron microscopy does not reveal any major difference between the center and the edges of sample no. 2. The density of threading dislocations is the same for both regions, varying from 108 cm−2, close (2–3 μm) to the interface, to 107 cm−2 in the thickness of the film. No misfit dislocations were observed. Antiphase boundaries are present in both regions as well. The only difference between the centre and the edges of sample no. 2 involves microtwin bundles: in the center region, there are two microtwin bundles per micrometre of interface, extending up to 6 μm in the GaAs film while on the edges, there is one bundle per micrometre with an extension of only one micrometre into the epitaxial layer. Mirrorlike GaAs films can be obtained on (100) Ge substrates of at least 1 in (1 in = 2.5 cm) in diameter by increasing δ to 0.2 cm and by injecting water vapor in the reactor only when [Formula: see text] reached 650 °C; the other deposition parameters remain the same as for sample no. 2. In these conditions, the growth rate of GaAs is 0.075 ± 0.020 μm min−1. By using a transport model based on thermodynamics, it is demonstrated that the flux intensity of GaAs transported from the source to the substrate, as well as the eventual presence of GeO as a nucleation site for GaAs on Ge, are both important for the morphology of the epitaxial layer.