Photocapacitance Studies of the Oxygen Donor in GaP. II. Capture Cross Sections

Pulsed-photocapacitance measurements prove that an oxygen center in GaP can deeply capture two electrons and also provide strong evidence that this center is the isolated O donor. Measurements of the rates of capture of electrons by O and recombination rates of electrons bound to O with holes are presented. These rates were determined using a modification of the photocapacitance method. The O donor was prepared in specific charge states, in a diode depletion layer, and the changes in the trapped charge population resulting from appropriate pulsing of the diode bias voltage were then studied by photocapacitance. In this way the capture and recombination processes of interest always occurred in neutral material. The capture cross sections for electrons at 296 K were found to be ${\sigma }_{n1\mathrm{}}\approx 2\times {10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$, ${\sigma }_{n2\mathrm{}}\approx 1\times {10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ for $n$-type material and ${\sigma }_{p1\mathrm{}}\approx 4\times {10}^{-21\mathrm{}}$ ${\mathrm{cm}}^2$, ${\sigma }_{p2\mathrm{}}>4\mathrm{}\times {10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ for $p$ type. (The subscripts refer to the one- and two-electron states.) The lifetimes of the bound electrons in $p$-type GaP with $p=(2.5\mathrm{}\pm 0.7)\times {10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ are ${\tau }_{p1\mathrm{}}\approx 46$ μsec and ${\tau }_{p2\mathrm{}}<5\mathrm{}$ nsec. The second electron recombines with a hole without any Auger effect, since the first electron remains on the center. The transition rate is very fast and must be nonradiative. Both of these facts are consistent with the electron having a level just above the valence band, as found previously. Using ${\sigma }_{n1\mathrm{}}$, a minority carrier lifetime of 13 nsec, and an O-donor density of 2.8 × ${10}^{16}$ ${\mathrm{cm}}^{-13\mathrm{}}$, the maximum O concentration reported earlier, we calculate that only about 1.5% of the recombination current is through O in $p$-type material.