Hydrostatic-pressure coefficient of the direct band-gap energy of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ for $x=0-0.35\mathrm{}$

We have determined the pressure coefficient of the direct band-gap energy of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ ($0<~x<~0.35$), by measuring the photoluminescence and photoluminescence-excitation spectra of 200-Å-wide ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}/\mathrm{AlAs}$ quantum wells with $x$ values 0, 0.1, 0.18, 0.25, 0.3, and 0.35 at pressures up to 30 kbar. We have found that the pressure coefficient is independent of $x$ within experimental uncertainties for $x$ values in this range, clustering around 10.7 meV/kbar, a widely accepted value for bulk GaAs. This is in direct contrast to the only systematic measurement (at 300 K) in the literature, reported by Lifshitz et al. [Phys. Rev. B 20, 2398 (1979)], which showed a nonlinear, nonmonotonic dependence of the direct band-gap pressure coefficient on the $x$ values. An explanation for this difference is offered.