Free-exciton transitions in the optical absorption spectra ofGaAs1−xPx

Optical-absorption data are presented on thin (∼ 2 μm) $\mathrm{Ga}{\mathrm{As}}_{1-x}{\mathrm{P}}_x$ samples in the composition range $0.29\lesssim x\lesssim 0.43$, showing a distinct free-exciton absorption peak. In a particularly lightly doped $x\approx 0.37$ sample ($n_d\lesssim 7\times {10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$), the $n=1,2\mathrm{}$ free-exciton transitions are resolved at 4.2°K. At 4.2°K the $n=1\mathrm{}$ peak shows a broadening ≲ 1 meV, confirming that the band edges in semiconductor alloys are well defined in spite of alloy (As-P) disorder. The observed energy separation ($\Delta E\sim 3.6$ meV) for the $n=1,2\mathrm{}$ exciton states gives the value $E_{\mathrm{ex}}=4.8\mathrm{}$ meV (compared to 4.2 meV in GaAs) for the exciton binding energy in $\mathrm{Ga}{\mathrm{As}}_{1-x}{\mathrm{P}}_x$ ($x=0.37\mathrm{}$). Using the absorption data for these samples, we obtain reliable values for the direct band gap, the band-edge bowing parameter, and the direct-indirect crossover in $\mathrm{Ga}{\mathrm{As}}_{1-x}{\mathrm{P}}_x$ ($x_c\approx 0.45,77$ °K).