The buried interface (IF) formed by molecular-beam epitaxy overgrowth on differently prepared (Al,Ga)As templates has been analyzed by secondary ion mass spectrometry and capacitance/voltage profiling. For the regrowth of GaAs on GaAs the lowest IF contamination is achieved by a H-assisted oxide desorption at 450 degrees C. This process retains atomically flat surfaces and is highly efficient in reducing the carbon contamination and the interfacial carrier depletion compared to thermal oxide desorption at 580 degrees C. For the regrowth on AlxGa1-xAs templates, due to the high reactivity of Al, the carbon and oxygen contaminations increase with increasing Al-mole fraction. An efficient reduction of the LF contamination can be accomplished by reevaporating a sacrificial GaAs cap layer. Carrier deficits as low as 7 x 10(11) cm(-2) for the H-assisted oxide-desorbed GaAs homointerface and 7.5 x 10(11) cm(-2) for AlxGa1-xAs on Al0.5Ga0.5As after reevaporating the sacrificial GaAs cap could be achieved. The carrier deficit can be compensated by a tailored doping profile around the IF.