Controlled polarity inversion in GaAs/Ge/GaAs{111} heterostructures

We have fabricated the GaAs/Ge/GaAs heterostructures on the {111}-oriented substrates using molecular-beam epitaxy for quasi-phase matching applications in nonlinear optics. The nonlinear optical coefficient of GaAs is beyond that of conventional LiNbO3, enabling more efficient generation of entangled photon pairs via parametric downconversion. We show that GaAs films with either (111)A- or (111)B-orientation could be grown on the Ge/GaAs{111} substrates, regardless of the polarity of the initial substrates; the (111)A- and (111)B-oriented GaAs overlayers were grown when the surfaces of Ge interlayers on the GaAs{111} substrates were terminated with 1 monolayer (ML)-Ga and 1 ML-In, respectively. Both (111)A- and (111)B-oriented GaAs overlayers have atomically flat surfaces and are almost free of defects, such as rotational twins and stacking faults. The present results provide a promising way to improve the efficiency of nonlinear optical processes in quasi-phase matching devices.