Admittance spectroscopy of solar cells based on GaPNAs layers

Admittance spectroscopy is used to study defect levels in the layers of a GaPNAs quaternary solid solution. Centers with an activation energy of 0.22 eV and a capture cross section of $\sim$ 2.4 $\cdot$ 10$^{-15}$ cm$^2$ are found in doped n-GaPNAs layers grown on GaP substrates. These centers correspond to already known Si$_{\mathrm{Ga}}$+$V_{\mathrm{P}}$ defects in $n$-GaP; annealing decreases their concentration by several times. A level with an activation energy of 0.23–0.24 eV and capture cross section of $\sim$ 9.0 $\cdot$ 10$^{-20}$ cm$^2$ is found in undoped GaPNAs layers grown on Si and GaP substrates. The concentration of these centers substantially decreases upon annealing, and, at annealing temperatures exceeding 600$^\circ$C, there is absolutely no response from these defects. For undoped GaPNAs layers grown on GaP substrates, a level with an activation energy of 0.18 eV and capture cross section of $\sim$ 1.1 $\cdot$ 10$^{-16}$ cm$^2$ is also found. The concentration of these centers remains unchanged upon annealing.