Photothermal investigation for optimizing a lattice strain relaxation condition of InGaAs/GaAsP superlattice photovoltaic structures from a nonradiative transition point of view

Abstract

The carrier collection efficiencies of InGaAs/GaAsP superlattice (SL) photovoltaic structures were optimized by choosing adequate manufacturing parameters, such as the composition and thickness of the quantum wells (QWs) and barrier layers. However, no insights have been observed from the viewpoint of the nonradiative transition of photoexcited carriers. In this study, piezoelectric photothermal (PPT) and photoluminescence (PL) measurements were performed as a function of temperature from 100 to 340 K. Using a piezoelectric transducer, the PPT signal detected the heat generated by nonradiative recombination (NR). The indium composition of the QW layer was fixed at 0.3, and the phosphorus composition x[P] in the barrier layer was changed from 0.4 to 0.6. The observed temperature dependences of the PPT and PL signal intensities were analyzed using a rate equation for the photoexcited carriers in e1 and hh1 quantized levels. Four carrier dissipating processes, namely, radiative recombination, NR, thermal escape from the QW thermal excitation (TE), and tunneling after thermal excitation (TATE), were considered for both electrons and holes. Thermal activation energies were included in the NR, TE, and TATE processes. Because nonradiative and radiative transition components cause PPT and PL signals, curve fitting of the temperature behavior enabled us to determine the activation energies. We then found that the activation energy of the NR process reached a maximum at x[P] = 0.45. No such maxima were observed for the TE and TATE process. This result was explained by a trade-off between the strain valance condition over the entire range of the SL structure and the local residual strain at the interfaces between the QW, interlayer, and barrier layer when x[P] increased. Because no software can theoretically calculate the activation energy of the NR process, we demonstrated the usefulness of the present PPT experimental methodology for investigating carrier transport properties.