Physics of Si-Related DX center in AlxGa1-xAs and GaAs

Abstract

The existence of the two energy levels associated with the Si?related DX center in Al?Ga???As, besides alloy broadening effect and motion of substitutional donor during emission of electrons, has been established conclusively. The existence of these two levels due to DX center in GaAs rules out the only alloy broadening effect. It is conjectured that the origin of these two levels may arise due to the splitting of ground state of DX level through a Coulombic contribution to the intervalley scattering in L band. A new spectroscopic technique is presented for determining capture cross?section activation energy directly. In this technique, capture transient due to large capture barrier during capture of electrons from the conduction band by defects is analyzed instead of emission transient in conventional DLTS. This method has been successfully applied for determining capture barrier for Si?related DX centers in MBE?grown Al?Ga???As for different AlAs mole fractions. In conclusion, we have unambiguously and directly demonstrated that there are two levels in the gap: an acceptor level (DX? ? e? + DX?) and a donor level (DX²? ? e? + DX?) in Al?Ga???As. In this system the acceptor level (?/0) lies inverted below the donor level (0/+). The second electron, which is required to form the DX²? state, is more strongly bound than the first electron, which is required to form the DX? state. This inverted ordering of the energy levels reveals an effective negative correlation energy with the existence of a thermodynamically unstable DX? state. In this work, we have done a detailed study of transient growth of photoconductivity due to Si?related DX center in Al?Ga???As. In our detailed analysis of photoionization kinetics of DX center, we have observed a two?step photoionization which provides us a desired proof for the existence of metastable intermediate state. The two?step photoionization can be fitted with rate equations with negative?U initial conditions and it has been possible phenomenologically to describe the photoionization kinetics at different temperatures and different light intensities. We believe that these results provide further support of the negative?U model of DX center and its acceptor?like nature. In conclusion, we have shown that migration of Si atom in Al?Ga???As during capture of photo?excited electrons from the conduction band causes the structural relaxation which gives rise to stretched exponential relaxation of persistent photoconductivity. Local atomic alloy disorder in Al?Ga???As causes the distribution in capture activation energies. The width of the distribution of these activation energies determined from the temperature dependence of stretching parameter ? is also verified by the direct measurement of capture activation energy by a novel spectroscopic technique based on deep?level transient spectroscopy. In conclusion, we have confirmed the presence of DX?related localized level in the conduction band of DX center by deep?level transient spectroscopy of Si?implanted GaAs. We have observed two distinct peaks: a low?temperature peak with an activation energy of 270 meV and a high?temperature peak with an activation energy of 315 meV. The difference of these two activation energies (45 meV) is almost equal to the splitting of L donor state due to intervalley scattering. The difference between emission activation energy of DX center in GaAs and Al?Ga???As has been explained in terms of a strong perturbation by the Al atom on the energy level of the defect orbital.