-The cleaned substrate surface shows a $(1\times 1)$ pattern at \unit[1000]{$^{\circ}$C}, which turns into a $(3\times 2)$ pattern when Si$_2$H$_6$ is introduced and it is maintained after the supply is stopped.
-A more detailed investigation showed the formation of a preceeding $(2\times 1)$ pattern within the exposure to the Si containing gas \cite{yoshinobu90}.
-The $(3\times 2)$ superstructure contains approximately 1.7 monolayers of Si atoms.
-The insertion of C$_2$H$_6$ leads to a reconstruction of the surface into the initial $(1\times 1)$ pattern and the formation of crystalline 3C-SiC with a smooth and mirror-like surface after an appropriate number of cycles.
-The growth rate ... higher, due to physically adsorbed Si, which depends on Si supply ...
-Not really ALE ... 1.7 monolayers per cycle ... now real ALE \cite{fuyuki93,hara93}
-6H on 6H ... \cite{tanaka94}
-Problem of gas source ... strong adsorption and incorporation of atomic decomposited hydrogen of the gas phase reactants at low temperatures.
-Growth rate lower than desorption rate of hydrogen ...
-Solid source MBE may be the key to avoid such problems ...
-Realized on and off-axis 3C on 4H and ... \cite{fissel95,fissel95_apl} ...
-Nonstoichiometric reconstruction plays a relevenat role ... handled by Si/C flux ratio ... \cite{fissel96,righi03} ...
-change in adlayer thickness and, consequently, in the surface super structure leading to growth of another polytype \cite{fissel95} ...
-Possibility to grow heterostructures (band gap engineering) by careful control of the Si/C ratio and Si excess.
+The cleaned substrate surface shows a C terminated $(2\times 2)$ pattern at \unit[1000]{$^{\circ}$C}, which turns into a $(3\times 2)$ pattern when Si$_2$H$_6$ is introduced and it is maintained after the supply is stopped.
+A more detailed investigation showed the formation of a preceeding $(2\times 1)$ and $(5\times 2)$ pattern within the exposure to the Si containing gas \cite{yoshinobu90,fuyuki93}.
+The $(3\times 2)$ superstructure contains approximately 1.7 monolayers of Si atoms, crystallizing into 3C-SiC with a smooth and mirror-like surface after C$_2$H$_6$ is inserted accompanied by a reconstruction of the surface into the initial C terminated $(2\times 2)$ pattern.
+A minimal growth rate of 2.3 monolayers per cycle exceeding the value of 1.7 is due to physically adsorbed Si atoms not contributing to the superstructure.
+To realize single monolayer growth precise control of the gas supply to form the $(2\times 1)$ structure is required.
+However, accurate layer-by-layer growth is achieved under certain conditions, which facilitate the spontaneous desorption of an additional layer of one atom species by supply of the other species \cite{hara93}.
+Homoepitaxial growth of the 6H polytype has been realized on off-oriented substrates utilizing simultaneous supply of the source gases \cite{tanaka94}.
+Depending on the gas flow ratio either 3C island formation or step flow growth of the 6H polytype occurs, which is explained by a model including aspects of enhanced surface mobilities of adatoms on a $(3\times 3)$ reconstructed surface.
+Due to the strong adsorption of atomic hydrogen \cite{allendorf91} decomposited of the gas phase reactants at low temperatures, however, there seems to be no benefit of GSMBE compared to CVD.
+Next to lattice imperfections, incorporated hydrogen effects the surface mobility of the adsorbed species \cite{eaglesham93} setting a minimum limit for the growth temperature, which would preferably be further decreased in order to obtain sharp doping profiles.
+Thus, growth rates must be adjusted to be lower than the desorption rate of hydrogen, which leads to very low deposition rates at low temperatures.
+Solid source MBE (SSMBE), supplying the atomic species to be deposited by evaporation of a solid, presumably constitutes the preffered method in order to avoid the problems mentioned above.
+Although, in the first experiments, temperatures still above \unit[1100]{$^{\circ}$C} were necessary to epitaxially grow 3C-SiC films on 6H-SiC substrates \cite{kaneda87}, subsequent attempts succeeded in growing mixtures of twinned 3C-SiC and 6H-SiC films on off-axis \hkl(0001) 6H-SiC wafers at temperatures between \unit[800]{$^{\circ}$C} and \unit[1000]{$^{\circ}$C} \cite{fissel95,fissel95_apl}.
+In the latter approach, as in GSMBE, excess Si atoms, which are controlled by the Si/C flux ratio, result in the formation of a Si adlayer and the formation of a non-stoichiometric, reconstructed surface superstructure, which influences the mobility of adatoms and, thus, has a decisive influence on the growth mode, polytype and crystallinity \cite{fissel95,fissel96,righi03}.
+Therefore, carefully controlling the Si/C ratio could be exploited to obtain definite heterostructures of different SiC polytypes providing the possibility for band gap engineering in SiC materials.