Application of the MOSFET device structure in characterizing imperfection centers in indium-doped silicon

C.T. Sah has published a review article demonstrating the application of high-frequency small signal capacitance and current transients of a space charge layer. Application of such transients is a powerful technique in characterizing deep level imperfection center concentrations, energy levels, thermal and optical emission rates and thermal capture cross sections. The MOSFET device structure is particularly convenient for low temperature measurements of shallow levels where deionization occurs and the substrate becomes highly resistive, seriously limiting capacitance transient techniques. Examples are given by results on indium-doped silicon, such as employed in extrinsic IR detectors. The emission time constant of holes from the neutral indium center has been found to depend on the indium doping. Measurements on lightly doped samples yield a value for the emission rate, {1}/{e _p}, of 6.0 msec at 77°K and a thermal activation energy of 0.15 eV. Measurements on heavily doped samples yield values of {1}/{e _p} of 20 μsec at 77°K and an activation energy of 0.117 eV. These results are consistent with the Poole-Frenkel effect describing field enhanced thermal emission of holes from the indium center. Measurements of the hole capture coefficient at 77°K yield values for c_p of 3.7 × 10 ^-7 cm ³/sec. These measurements have been made on heavily doped samples. The capture coefficient measured is the zero field or quasi-equilibrium value. The temperature dependence of the hole capture coefficient has been found to be T^-4. Small transients in the thermal emission rate measurements have been observed. These transients have thermal activation energies of around 0.08 eV and are associated with the 0.11 eV level as reported by Hughes Research Labs after accounting for barrier lowering by the Poole-Frenkel effect.