Optimum growth conditions for the epitaxy of GaAs on Ge by close-spaced vapor transport

GaAs epitaxial layers are grown by close-spaced vapor transport (CSVT) on (100)Ge substrates and (100)Ge substrates misoriented 1.5° and 3° toward (011). Water vapor is used as the transport agent. When the temperatures of the GaAs source (T 1 ) and of the Ge substrate (T 2 ) are 800 and 750 °C, respectively, the growth rate is about 3 μm h −1 . When an optimum source–substrate temperature evolution is followed, it is possible to grow specular layers of GaAs/Ge that contain only a small number (< 10 5 cm −2 ) of threading dislocations. All antiphase boundaries (APBs) annihilate close to the interface (from about 230 nm for (100)Ge substrates to about 65 nm for vicinal (3° off) (100)Ge substrates). The GaAs growth occurs via the coalescence of 3D nuclei that are formed on an arsenic prelayer n-type GaAs layers are always obtained. By encapsulating the Ge substrate, it is possible to drastically decrease the autodoping resulting from the transport of Ge by water vapor in the same growth conditions as those prevailing for GaAs. After encapsulation, uncompensated doping densities N D – N A in the order of 5 × 10 16 cm −3 are easily obtained for GaAs/Ge films grown from undoped semi-insulating GaAs sources. These GaAs/Ge layers can be used as bases for solar cells.