+{\color{red} Source of APB defects ...}
+However, lateral 3C-SiC growth was also observed on low tilt angle off-axis substrates originating from intentionally induced dislocations \cite{powell91}.
+Additionally, 6H-SiC was observed on clean substrates even for a tilt angle as low as \unit[0.1]{$^{\circ}$} due to low surface mobilities that facilitate arriving molecules to reach surface steps.
+Thus, 3C nucleation is assumed as a result of migrating Si and C cointaining molecules interacting with surface disturbances by a yet unknown mechanism, in contrast to a model \cite{ueda90}, in which the competing 6H versus 3C growth depends on the density of surface steps.
+{\color{red} This can be employed to create 3C layers with reduced density of APB defects.}
+
+Lower growth temperatures, a clean growth ambient, in situ control of the growth process, layer-by-layer deposition and the possibility to achieve dopant profiles within atomic dimensions due to the reduced diffusion at low growth temperatures reveal MBE as a promising technique to produce SiC epitaxial layers.
+Using alternating supply of the gas beams Si$_2$H$_6$ and C$_2$H$_2$ in GSMBE, 3C-SiC epilayers were obtained on 6H-SiC substrates \cite{yoshinobu92}.
+On \hkl(000-1) substrates ...
+gas source ... 3C homoeptiaxy \cite{yoshinobu90}
+3C on 3C homoepitaxy by ALE \cite{fuyuki89,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.
+
+To summarize ... remaining obstacles are ... APB in 3C ... and micropipes in hexagonal SiC?