Li doping of GaAlAs

Abstract

A series of unintentionally doped Ga1−xAlxAs epitaxial layers grown by molecular beam epitaxy, having various Al compositions (0.15, 0.18, and 0.29), have been Li diffused at 300 °C. Capacitance–voltage techniques show that the initial n-type doping concentrations (of the order of 1016 cm−3) increase after diffusion by typically a factor 5 to 10 demonstrating that isolated Li interstitials do exist and behave as donors. However, secondary-ion mass spectroscopy measurements show that the material contains 1019–1020 Li cm−3, which indicates that a large fraction of the Li impurities is not electrically active. Thus Li also produces defects as revealed by the fact that free electrons are frozen in the diffused layers below 77 K. Search for the existence of Li associated DX centers deep defects related to the donor impurities has been performed by deep level transient spectroscopy. Only the DX centers present before diffusion, i.e., associated with residual donor impurities, are detected in the layer of 0.29 Al fraction. This implies that either Li donors do not induce the existence of DX centers or electron emission from Li DX centers occurs below 77 K.