Beryllium doping and diffusion in molecular-beam epitaxy of GaAs and Al<i>x</i>Ga1−<i>x</i>As

Abstract

p-type doping levels of up to 5×1019 cm−3 in GaAs and 3×1019 cm−3 in Al0.3Ga0.7As have been obtained in molecular-beam epitaxy (MBE) with Be doping. These doping levels are more than one order of magnitude higher than can be achieved with other acceptors excepting ionized Zn, and are obtained while maintaining surface morphologies comparable to those of undoped layers. Mobilities of the Be-doped GaAs MBE layers are comparable to those obtained in liquid-phase epitaxy (LPE) material. Mobilities of Be-doped Al0.3Ga0.7As MBE layers studied in this experiment are lower than those measured in LPE crystals and do not increase with decreasing doping level which indicates a high degree of donor compensation in these samples. Room-temperature photoluminescent efficiencies obtained from the p-MBE GaAs and AlxGa1-xAs layers grown under As-stabilized surface conditions also are significantly lower than those of comparably doped LPE material. This lower efficiency may be related to impurity contamination specific to the dopant source and is not observed during n-type (Sn-doped) GaAs layer growth under otherwise identical conditions. p (Be) -n (Ge) junctions have been evaluated and the results indicate that Be should be a suitable dopant for device fabrication. Doping profile measurements by the differential capacitance technique show that very abrupt (half-width ?350 Å) Be doping pulses can be realized. By observing the out diffusion of Be from these doping pulses during annealing of the diodes a value of D?1×10−15 cm2/sec for the diffusion coefficient of Be at 800° was established. It is concluded that, with the possible exception of ionized Zn, Be should emerge as the preferred acceptor in MBE growth of III–V compounds.