Evaluation of the Zn-O Complex and Oxygen-Donor Electron-Capture Cross Sections inp-Type GaP: Limits on the Quantum Efficiency of Red-Emitting (Zn,O)-Doped Material

The oxygen donor has been studied in a series of (Zn, O)-doped GaP samples. The infrared-luminescence intensity associated with this center was monitored over six decades of above band-gap photoexcitation intensity. A central feature of these measurements is a sublinear luminescent increase which extends over three decades of photoexcitation intensity in a range where the red emission associated with the Zn-O complex increases either linearly or superlinearly. The effect is observed in all measured zinc-doped crystals and the behavior is consistent with a model in which the oxygen center successively captures two electrons. In this model the infrared luminescence is associated with the deeply bound electron, while recombination at an oxygen center which has captured both electrons occurs through processes involving these electrons and either a bound or free hole. The experimental data yield recombination rates associated with two-electron processes of the same magnitude as the recombination rate of the deep electron. A value for the second electron energy level ≥ 400 meV is extracted from the measurements. The magnitude of the deep-electron-capture cross section of the oxygen center is evaluated on the basis of the excitation-intensity data and a linear dependence on hole concentration is found from deep-electron-thermalization data. These measurements lead to a value of ${\sigma }_0\sim \frac{{10}^{-17\mathrm{}}p}{{10}^{17}}$ ${\mathrm{cm}}^2$, where $p$ is the hole concentration per cubic centimeter. Similar experiments performed on the Zn-O red band, to attain a more accurate determination of the Zn-O complex electron-capture cross section, yielded a value of ${\sigma }_{\mathrm{Z}\mathrm{n}-\mathrm{O}}=2\mathrm{}\times {10}^{15}$ ${\mathrm{cm}}^2$. These two capture cross sections, the maximum values of the impurity-concentration ratio $\frac{N_{\mathrm{Z}\mathrm{n}-\mathrm{O}}}{N_{\mathrm{O}}}$ (obtained from pairing theory), and results of earlier work characterizing Zn-O red luminescence have been combined to obtain an upper limit for the internal quantum efficiency of the Zn-O system. The predicted maximum is ∼ 35% for material annealed at 600°C. Efficiencies in the 40% range are predicted for equilibrium pairing at 500°C. Our results are consistent with reported values of maximum-red-luminescence quantum efficiencies and minority-carrier lifetime (${\tau }_L$) evaluations. Measurements on a well-characterized sample with a net acceptor concentration of 2.5×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ yield parameter values of ${\tau }_L=13\mathrm{}$ nsec and $N_{\mathrm{Z}\mathrm{n}-\mathrm{O}}\simeq 4\times {10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$. On the basis of pairing theory the oxygen-donor concentration is ∼ 7×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$.