Deep level defects in proton radiated GaAs grown on metamorphic SiGe∕Si substrates

Abstract

The effect of 2MeV proton radiation on the introduction of deep levels in GaAs grown on compositionally graded SiGe∕Si substrates was investigated using deep level transient spectroscopy (DLTS). Systematic comparisons were made with identical layers grown on both GaAs and Ge substrates to directly assess the influence of threading dislocations on radiation-related deep levels for both n-type and p-type GaAs. DLTS revealed that for p+n structures, proton irradiation generates electron traps at Ec−0.14eV, Ec−0.25eV, Ec−0.54eV, and Ec−0.72eV in the n-GaAs base, and, for n+p structures, radiation-induced hole traps appear at Ev+0.18eV, Ev+0.23eV, Ev+0.27eV, and Ev+0.77eV in the p-type GaAs base, irrespective of substrate choice for both polarities. The primary influence of substituting SiGe∕Si substrates for conventional GaAs and Ge substrates is on the introduction rates of the individual traps as a function of proton radiation fluence. Substantially reduced concentrations are found for each radiation-induced hole trap observed in p-type GaAs, as well as for the Ec−0.54eV trap in n-GaAs for samples on SiGe∕Si, as a function of proton fluence. Calculated trap introduction rates reveal reductions by as much as ∼40% for certain hole traps in p-GaAs grown on SiGe∕Si. This increased radiation tolerance for GaAs grown on SiGe∕Si is attributed to interactions between the low density (∼106cm−2) of residual dislocations within the metamorphic GaAs∕SiGe∕Si structure and the radiation-induced point defects. Nevertheless, the fact that the impact of dislocations on radiation tolerance is far more dramatic for n+p GaAs structures compared to p+n structures, may have implications on future III-V∕Si space solar cell design optimization, since end-of-life versus beginning-of-life differences are critical factors for power profiling in high radiation environments.