Fundamental Energy Gaps of AlAs and Alp from Photoluminescence Excitation Spectra

The measurement of photoluminescence excitation spectra is shown to be a powerful method to obtain information on the lower fundamental energy gaps in materials where suitable samples for transmission measurements cannot readily be prepared. In this work this method is used to derive accurate values for the lowest indirect and direct excitonic band gaps of the "exotic" compounds AlAs and AlP. The indirect ${\Gamma }_{15v}-X_{1c}$ excitonic edge was found as 2.229 ± 0.001 eV at 4 K, 2.223 ± 0.001 eV at 77 K and 2.153 ± 0.002 eV at 300 K. The $X$-point phonon energies observed for AlAs are $E_{\mathrm{LO}\mathrm{}\left(X\right)\mathrm{}}$, 50.0 ± 1.0 meV; $E_{\mathrm{TO}\mathrm{}\left(X\right)\mathrm{}}$, 41.5 ± 1.0 meV; $E_{\mathrm{LA}\mathrm{}\left(X\right)\mathrm{}}$, 27.5 ± 1.5 meV; and $E_{\mathrm{TA}\mathrm{}\left(X\right)\mathrm{}}$, 13.5 ± 1.0 meV, giving an $X$-point deviation from the Brout sum rule ${\Delta }_K\mathrm{}\left(X\right)=0.09\mathrm{}$ for AlAs. The lowest direct ${\Gamma }_{15c}-{\Gamma }_{1c}$ excitonic edge in AlAs was found to vary from 3.13 ± 0.01 eV at 4 K to 3.03 ± 0.01 eV at 300 K. For AlP, the corresponding values found experimentally for excitonic band gaps are 2.505 ± 0.01 eV at 4 K for the indirect ${\Gamma }_{15v}-X_{1c}$ transition and 3.63 ± 0.02 eV at 4 K for the lowest direct gap (probably ${\Gamma }_{15v}-{\Gamma }_{1c}$).