Role of deep-level trapping on the surface photovoltage of semi-insulating GaAs

Dual-beam (bias and probe) transient surface photovoltage (SPV) measurements were made on undoped semi-insulating GaAs over an extended temperature range. Above 270 K, SPV recovery transients following a bias pulse were shown to reflect near-surface conductivity changes; these are in turn controlled by surface-interface-state thermal emission. Owing to the absence of a strong surface electric field in this material, the emitted carriers are not immediately removed from the near-surface region. The recapturing of the emitted carriers is shown to be responsible for nonexponential conductivity and reciprocal-SPV transients. This behavior is considered to be characteristic of relaxation-type semiconductors with near-surface ungated structures. Below 150 K, the photoinduced transition of EL2 from its ground to metastable state EL2* was shown to change the effective electron and hole mobilities and augment the SPV signals immediately following the bias pulse. Thermally induced EL2* recovery above 120 K decreases the SPV signal from its maximum. This decay transient was analyzed and the decay rate fitted to a single exponential. An activation energy of 0.32 eV and a preexponential constant of 1.9 x 10(12) s(-1) were obtained, and attributed to the thermal recovery rate for EL2*.