Defect Energy Levels in GaAsBi and GaAs Grown at Low Temperatures*

GaAs$_{1-x}$Bi$_x$ alloys have the potential to extend conventional III-V semiconductor devices to longer infrared wavelengths. The bandgap energy decreases as the Bi fraction is increased, but with a small increase in lattice constant, thus reducing lattice mismatch constraints for GaAsBi/GaAs heterostructures. However, Bi is incorporated into GaAs films grown by molecular beam expitaxy (MBE) only at T$_G$ $<$400 $^{\circ}$C, making defects a concern. DLTS measurements show that trap concentrations in Si-doped (n-type) GaAs layers grown at standard temperatures are $<$4x10$^{13}$ cm$^{-3}$. They increase to 2x10$^{16}$ cm$^{-3}$ when T$_G$ is 390 $^{\circ}$C and to $\sim$10$^{18}$ cm$^{-3}$ when T$_G$ is 330 $^{\circ}$C, where the energy level of the dominant defect is E$_C$-0.40 eV. When only 0.3\% Bi is incorporated into n-type GaAs at 330 $^{\circ}$C, formation of the E$_C$-0.40 eV trap is suppressed. Other electron traps, including the dominant traps having energy levels at E$_C$-0.66 eV and E$_C$-0.80 eV, are present in similar concentrations in both GaAs and GaAsBi layers grown at 330 $^{\circ}$C and, therefore, result from the low growth temperature. The dominant traps are both point defect complexes involving an arsenic atom on a gallium lattice site (AsGa).