Free-Carrier and Exciton Recombination Radiation in GaAs

Photoluminescence studies were made at liquid-helium temperatures of epitaxial GaAs with ionized-impurity concentrations ≲${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Narrow linewidths at half-height were achieved by using material of high purity and perfection, by using low intensities of illumination, and by observing excitons bound to impurities which are almost entirely in one state of ionization. Impurity-exciton lines were ≈0.1 meV wide at 4.2°K, free-exciton lines ≈1 meV wide for $T\le 4.2°$K, and free-charge-carrier (band-gap) radiation ≲0.24 meV wide at 1.4°K. Free-carrier recombination radiation at 1.4°K establishes the band gap $E_G$ at 1.5202 eV ±0.3 meV. The exciton binding energy $G$ from the $n=1\mathrm{}$ exciton was found to be $G\left(1\right)=4.7\mathrm{}\pm 0.4$ meV, and from the $n=2\mathrm{}$ exciton to be $G\left(2\right)=3.6\mathrm{}\pm 0.6$ meV. The $G\left(1\right)\mathrm{}$ value agrees with that given by Wright and Galeener, and the $G\left(2\right)\mathrm{}$ value with that given by Sturge for $G\left(\infty \right)$. An ionized Zn-exciton complex has been observed, as recently predicted by Sharma and Rodriguez. Lines from excitons bound at ionized and neutral Zn and at ionized Se are, respectively, 31.2±0.4, 8.0±0.3, and 6.1±0.3 meV below $E_G$. The free-exciton and impurity-exciton binding energies are satisfied by $\frac{{m_h}^{*}}{{m_e}^{*}}\approx 5$, with impurity ionization energies of ≈26 meV for Zn and ≈6 meV for Se. The neutral Zn-exciton line and the ionized Se-exciton line have been resolved, respectively, as a triplet and doublet with separations ≈0.2 meV and widths ≈0.1 meV. This structure arises from exchange splitting of the states formed from the electrons and holes by $j-j$ coupling. Two lines arising from the excited state of the neutral Zn-exciton complex are identified at ±6 meV from the ground state. The ionized Zn-exciton complex is accompanied by an illumination-dependent phonon wing probably associated with a resonant vibration.